Why does the response time of OCP vary on the power supply I am using and what can I do about it? Part 2

In the first part of this posting (click here to review) I highlighted what kind of response time is important for effective over current protection of typical DUTs and what the actual response characteristic is for a typical over current protect (OCP) system in a test system DC power supply. For reference I am including the example of OCP response time from the first part again, shown in Figure 1.

Figure 1: Example OCP system response time vs. overdrive level

Here in Figure 1 the response time of the OCP system of a Keysight N7951A 20V, 50A power supply was characterized using the companion 14585A software. It compares response times of 6A and 12A loading when the current limit is set to 5A. Including the programmed OCP delay time of 5 milliseconds it was found that the actual total response time was 7 milliseconds for 12A loading and 113 milliseconds for 6A loading.  As can be seen, for reasons previously explained, the response time clearly depends on the amount of overdrive beyond the current limit setting.

As the time to cause over current damage depends on the amount of current in excess of what the DUT can tolerate, with greater current causing damage more quickly, the slower response at lower overloads is generally not an issue.  If however you are still looking how you might further improve on OCP response speed for more effective protection, there are some things that you can do.

The first thing that can be done is to avoid using a power supply that has a full output current rating that is far greater than what the DUT actually draws. In this way the overdrive from an overload will be a greater percentage of the full output current rating. This will normally cause the current limit circuit to respond more quickly.

A second thing that can be done is to evaluate different models of power supplies to determine how quickly their various current limit circuits and OCP systems respond in based on your desired needs for protecting your DUT. For various reasons different models of power supplies will have different response times. As previously discussed in my first part, the slow response at low levels of overdrive is determined by the response of the current limit circuit.

One more alternative that can provide exceptionally fast response time is to have an OCP system that operates independently of a current limit circuit, much like how an over voltage protect (OVP) system works. Here the output level is simply compared against the protect level and, once exceeded, the power supply output is shut down to provide near-instantaneous protection. The problem here is this is not available on virtually any DC power supplies and would normally require building custom hardware that senses the fault condition and locally disconnects the output of the power supply from the DUT. However, one instance where it is possible to provide this kind of near-instantaneous over current protection is through the programmable signal routing system (i.e. programmable trigger system) in the Keysight N6900A and N7900A Advanced Power System (APS) DC power supplies. Configuring this triggering is illustrated in Figure 2.

Figure 2: Configuring a fast-acting OCP for the N6900A/N7900A Advanced Power System

In Figure 2 the N7909A software utility was used to graphically configure and download a fast-acting OCP level trigger into an N7951A Advanced Power System. Although this trigger is software defined it runs locally within the N7951A’s firmware at hardware speeds. The N7909A SW utility also generates the SCPI command set which can be incorporated into a test program.

Figure 3: Example custom-configured OCP system response time vs. overdrive level

Figure 3 captures the performance of this custom-configured OCP system running within the N7951A. As the OCP threshold and overdrive levels are the same this can be directly compared to the performance shown in Figure 1, using the conventional, current limit based OCP within the N7951A. A 5 millisecond OCP delay was included, as before. However, unlike before, there is now virtually no extra delay due to a current limit control circuit as the custom-configured OCP system is totally independent of it. Also, unlike before, it can now be seen the same fast response is achieved regardless of having just a small amount or a large amount of overdrive.

Because OCP systems rely on being initiated from the current limit control circuit, the OCP response time also includes the current limit response time. For most all over current protection needs this is usually plenty adequate.  If a faster-responding OCP is called for minimizing the size of the power supply and evaluating the performance of the OCP is beneficial. However, an OCP that operates independently of the current limit will ultimately be far faster responding, such as that which can be achieved either with custom hardware or making use of a programmable signal routing and triggering system like that found in the Keysight N6900A and N7900A Advanced Power Systems.

Why does the response time of OCP vary on the power supply I am using and what can I do about it? Part 1

In a previous posting of mine “Providing effective protection of your DUT against over voltage damage during test”(click here to review), an important consideration for effective protection was to factor in the response time of the over voltage protect (OVP) system. Due to the nature of over voltage damage, the OVP must be reasonably fast. The response time can typically be just a few tens of microseconds for a reasonably fast OVP system on a higher performance system power supply to hundreds of microseconds on a more basic performance system power supply. This response time usually does not vary greatly with the amount of over voltage being experienced.

Just as with voltage, system power supplies usually incorporate over current protect (OCP) systems as well. But unlike over voltage damage, which is almost instantaneous once that threshold is reached, over current takes more time to cause damage. It also varies in some proportion to the current level; lower currents taking a lot longer to cause damage. The I²t rating of an electrical fuse is one example that illustrates this effect.

Correspondingly, like OVP, power supply OCP systems also have a response time. And also like OVP, the test engineer needs to take this response time into consideration for effective protection of the DUT.  However, unlike OVP, the response time of an OCP system is quite a bit different. The response time of an OCP system is illustrated in Figure 1.

Figure 1: Example OCP system response time vs. overdrive level

Here in Figure 1 the response time of the OCP system of a Keysight N7951A 20V, 50A power supply was characterized using the companion 14585A software. It compares response times of 6A and 12A loading when the current limit is set to 5A. Including the programmed OCP delay time of 5 milliseconds it was found that the actual total response time was 7 milliseconds for 12A loading and 113 milliseconds for 6A loading.

This is quite different than the response time of an OVP system. Even if the OCP delay time was set to zero, the response is still on the order of milliseconds instead of microseconds for the OVP system. And when the amount of overdrive is small, as is the case for the 6A loading, providing just 1A of overdrive, the total response time is much greater. Why is that?

Unlike the OVP system, which operates totally independent of the voltage limit control system, the OCP system is triggered off the current limit control system. Thus the total response time includes the response time of the current limit as well. The behavior of a current limit is quite different than a simple “go/no go” threshold detector as well. A limit system, or circuit, needs to regulate the power supply’s output at a certain level, making it a feedback control system. Because of this stability of this system is important, both with crossing over from constant voltage operation as well as maintaining a stable output current after crossing over. This leads to the slower and overdrive dependent response characteristics that are typical of current limit systems.

So what can be done about the slower response of an OCP system? Well, early on in this posting I talked about the nature of over current damage. Generally over current damage is much slower by nature and the over drive dependent response time is in keeping with time dependent nature of over current damage. The important thing is understand what the OCP response characteristic is like and what amount of over current your DUT is able to sustain, and you should be able to make effective use of the over current protection capabilities of your system power supply.

If however you are still looking how you might further improve on OCP response speed, look for my follow up to this in my next posting!

Providing effective protection of your DUT against over voltage damage during test

The two most common ways DUTs can be electrically damaged during test are from current-related events or voltage-related events that mange to over-stress the DUT. Sometimes the cause can be an issue with the DUT itself. Other times it can be an issue stemming from the test system. The most common voltage-related damage to a DUT is an over voltage event, beyond a maximum level the DUT can safely tolerate. While there are a number of things that can cause this, most invariably it was an issue with the test system power supply, either from inadvertently being set too high or from an internal failure.

To protect against accidental over voltage damage, test system power supplies incorporate an over voltage protect (OVP) system that quickly shuts down the output upon detecting the voltage has gone above a preset threshold value. More details about OVP have been written about here in a previous posting “Overvoltage protection: some background and history”(click here to review).

The critical thing about over voltage damage is, in most all cases, that it is virtually immediate once the voltage threshold where damage to the DUT occurs is exceeded. It is therefore imperative that you optimize the test set up and settings in order to provide effective protection of your DUT against over voltage damage during test. To start with, the OVP trip threshold needs to be set at a reasonable amount below the threshold where DUT damage occurs and at the same time be set to a reasonable amount above the maximum expected DUT operating voltage. This is depicted in Figure 1.

Figure 1: OVP set point

However, to understand what are “reasonable amounts” above the maximum operating voltage and below the DUT damage voltage levels you need to take into account the dynamic response characteristics of the power supply output and OVP system, as depicted in Figure 2.

Figure 2: Power supply output and OVP dynamic response characteristics

It is important to have adequate margin above the maximum operating voltage to account for transient voltages due to the DUT drawing current from the power supply and resulting voltage response of the power supply in correcting for this loading, in order to prevent false OVP tripping. It is likewise important to adequate margin below the DUT damage threshold as it takes a small amount of time, in the range of 10’s to 100’s of microseconds, for the OVP system to start shutting down the power supply’s output once the OVP trip point has been crossed. At the same time the power supply typically has a maximum rate the output voltage can slew in. In practice these “reasonable amounts” typically need to be a few tenths to several tenths of a volt as a minimum.

Generally these margins are not difficult to manage, except when the DUT’s operating voltage is very small or the DUT operating current is very large producing a correspondingly large voltage drop in the power supply wiring. This is because the OVP is traditionally sensed on power supply’s output power terminals, so that it provides protection regardless of what the status and condition of the remote voltage sense wiring connection is. To improve on this we also provide OVP sensing on the remote sensing wires as an alternative to, or in addition to, the traditional sensing on the output power terminals. More details about this are described in another posting here “Protect your DUT: Use sense leads for over voltage protection (OVP)”(click here to review).

By following these suggestions you should be able to effectively protect your DUT against over voltage damage during test as well!

What can cause a power supply output voltage to exceed its setting?

We have done a number of posts on power supply protection topics covering both voltage and current issues:

Safeguarding your power-sensitive DUTs from an over power condition

How does power supply overvoltage protection work? 

Protect your DUT from over-current in more ways than one

What is a power supply’s over current protect (OCP) and how does it work?

Overvoltage protection: some background and history

Protect your DUT: use sense leads for overvoltage protection (OVP)

Types of current limits for over-current protection on DC power supplies

Protect your DUT with power supply features including a watchdog timer

And just last week, on August 20, 2014, my colleague and fellow Watt’s Up? blog contributor, Ed Brorein, presented a live webcast called “Protect Your Device Against Power-Related Damage During Test” which was recorded and can be accessed here. Before he presented the seminar, Ed mentioned it here.

Many of these posts talked about how the power supply responds to an overvoltage or overcurrent condition. Today I want to talk about what causes an overvoltage condition. I’m defining an overvoltage condition as a condition that causes the power supply output voltage to exceed its setting. Let’s take a look at some of the things that can cause this to happen.

Causes of power supply output voltage exceeding its setting

User-caused miswires
These miswires should be found and corrected during test setup verification before a device under test (DUT) is connected to the power supply. Possible miswires and their effect on the power supply output voltage are:

• Shorted sense leads – the output voltage will rapidly rise above the setting. Keysight power supplies will prevent the output from rising above the overvoltage protection (OVP) setting.
• Reversed sense leads – on most power supplies, the output voltage will rapidly rise above the setting and on Keysight supplies, it will be stopped by the OVP circuit. On our N6900/N7900 Advanced Power System (APS) power supplies, this condition is caught sooner: OV- is triggered when the output reaches about 10% of the rated voltage, so the output does not have to rise to the setting and above.
• Open sense leads – If your power supply does not have protection for open sense leads, it is possible for your output to rapidly rise above the setting if one or both sense leads are open. Keysight power supplies have built-in sense protect resistors which limit the output voltage rise to about 1% above the setting. The voltage will continue to be regulated there. In addition to limiting the output to about 1% above the setting with an open sense lead, Keysight N6900/N7900 APS power supplies have a feature called open sense lead detection. When enabled, open sense lead detection will cause a sense fault (SF) status about 50 us after open sense leads are detected. This status does not turn off the output, but it can be configured to turn off the output using the advanced signal routing capability.
• Special note about N7900 power supplies (not N6900): these models have output disconnect relays that open upon a protection fault. These mechanical relays take about 20 ms to open. Before they open, the output downprogrammer circuit is activated for about 2 ms and draws about 10% of rated output current to reduce the output voltage. The N7976A and N7977A (both higher voltage models) also have solid state relays in series with the mechanical relays. Upon a protection fault on these 2 models, the downprogrammer activates for 2 ms followed immediately by the solid state relays opening and then the mechanical relays open about 20 ms later.

Inadvertent wiring failure

• Sense leads inadvertently become shorted – power supply response is the same as mentioned above under shorted sense leads
• Sense leads inadvertently become open – power supply response is the same as mentioned above under open sense leads
• Sense leads should never become inadvertently reversed, nevertheless, the power supply response is the same as mentioned above under reversed sense leads

Power supply fault (circuit failure)

Note that Keysight’s overall power supply failure rate is very low. Since the below mentioned failures are a subset of all failures, they are very rare. This means that failures that cause the output to go to a higher-than-desired value are a small percent of a small percent, and while not impossible, they are extremely unlikely events.

• Power element fails (shorts)
• Series regulator – when a series regulator power element shorts, the output very quickly rises above the rated voltage of the power supply. The only way to limit this is to trip OVP and either fire an SCR across the output to bring the voltage back down or open output relays. For example, the Keysight N678xA models use a series regulator. When OVP trips on N678xA models, output relays are opened to protect the DUT. Solid state relays very quickly open first followed by mechanical relays about 6 ms later.
• Switching regulator – when a Keysight switching regulator power element shorts, the output will go toward zero volts instead of rising since Keysight switching regulators use power transformers and no power can be transferred through the transformer without the switching elements turning on and off. For example, all N6700 and N6900/N7900 series models use switching regulators except the N678xA models (series regulators).
• Note that if a power element fails open using either power regulation scheme, the output voltage will fall, not rise, so this condition is not a concern when looking at excessive output voltage possibilities.
• Regulation circuit failure (bias supply, DAC, amplifier, digital comparison processor, etc.)
• There are various circuits that could fail and cause the output voltage to rise in an uncontrolled manner. Keysight power supplies have OVP designed to respond to these failures. In series regulators, an SCR across the output can fire to reduce the voltage or output relays can open. In switching regulators, the pulse width modulator is turned off to prevent power from flowing to the output, downprogrammers are activated to pull any excessive voltage down, and output relays are opened (when present) to disconnect the output from the DUT.
• Multiple parallel failures – if both a regulating circuit fails that causes the output to rise AND the OVP circuit fails, there would be nothing to prevent the output voltage from rising above the setting. While this is possible, it requires just the right combination of multiple circuit failures and is therefore extremely unlikely.

Output response to load current transients

• It is possible for the output voltage to temporarily rise above the setting for short transients in response to fast load current changes (especially unloading). If the voltage excursion is high enough and long enough, it is possible that the OVP will activate and respond as outlined above.

External power source

• It is possible for an external source of power (such as a battery, charged capacitor, inductor with changing current, or another power supply) to cause the voltage to go above the setting. The OVP will respond to this condition as outlined above. If the external power source can provide more current than the rating of the power supply and an SCR circuit is used in the power supply, it is prudent to put a fuse in series with the external source of power to prevent damage to the power supply SCR and/or output circuit from excessive current.

So you can see that there are a number of ways in which the output voltage can rise above the setting. Luckily, Keysight design engineers are aware of these possibilities and have lots of experience adding protection circuits to prevent damage to your DUT!

Upcoming Seminar on Protecting Your Device against Power-Related Damage during Test

Here on “Watt’s Up?” we have provided a good number of posts about various protection features incorporated into system power supplies to protect your device against power-related damage during test. Just recently my colleague Gary posted “How Does Power Supply Over-Voltage Work?” (Click here to review) Here he reviews inner workings of different OVP implementations.  I recently posted “Safeguarding Your Power-Sensitive DUTs against an Over-Power Condition” (Click here to review) Here I go over a method to protect your DUT against excess power when other power supply features like over current protection may be less than ideal.

The reason why we frequently share power-related protection topics here is protecting your DUT is extremely important, there are a lot of different capabilities incorporated in system power supplies for this purpose, and there are a lot of practical considerations when putting them to use.  

Hopefully a number of you have found our posts on protection-related topics of help. Because this is a very important topic and there is so much more you should know about it I will be giving a live web-based seminar “Protecting Your Device against Power-Related Damage during Test” on August 20^th, just a few weeks away from today. I will be going over a number of protection-related topics which we have not yet covered here on “Watt’s Up?”.  One of my objectives is to provide a more holistic view of the many ways a system power supply is able to better safeguard against power-related damage as well as what is practical to expect when using these various capabilities incorporated in the power supply.

You can register online at the following (Click here for description and registration page) In case you are not able to attend the live event on August 20 you will be able to register and listen to seminar afterward as well, as it will be recorded.

So if protecting your device against power-related damage is important to you I hope you are able to attend the seminar!

How does power supply overvoltage protection work?

In past posts, I’ve written about what overvoltage protection (OVP) is (click here), where it is sensed (click here), and its history (click here). Today I want to cover a little about how it works inside the power supply.

As a quick review, OVP is a built-in power supply feature that protects the device under test (DUT) from excessive voltage by shutting down the power supply output if it senses voltage that exceeds the OVP setting. Depending on the power supply design, the voltage may be sensed at the output terminals or at the sense terminals.

Most of Agilent’s older power supplies sense OVP at the output terminals and use a simple analog comparator circuit to determine when the output exceeds the OVP threshold set by the user. The OVP threshold is translated into an overvoltage reference voltage (OVref) that could come from a simple divider with a potentiometer for adjustment (uncalibrated and rather crude) or from a more sophisticated calibrated digital-to-analog converter (DAC) voltage. When the comparator sees the scaled output voltage exceed the OVref voltage, the overvoltage trip (OVtrip) signal is generated which shuts down the power supply output and, on some designs, fires an SCR across the output. See Figure 1 for a simplified representation of this arrangement.

Some of our newer designs look for an overvoltage condition on the sense terminals for better accuracy. In this scheme, the sense voltage feeds one comparator input through a differential amplifier while the other comparator input is driven by the user-set calibrated OVref voltage. See Figure 2. An output terminal OVP as described above must also be used as a backup with these designs (not shown in Figure 2) because some OV conditions are not caught when sensing OV on the sense terminals. For example, if the sense leads are shorted together, the output voltage will go up uncontrolled yet the sense voltage will remain at zero volts.

Some other OVP designs use a calibrated analog-to-digital converter (ADC) on either the output terminal voltage or the sense terminal voltage and compare the measured digital data to the user’s threshold setting. See Figure 3. To avoid nuisance OVP shutdowns, this scheme frequently requires several analog-to-digital conversions in a row exceed the threshold (for example, 4). This adds a minor delay to the OVP response time. With fast ADC conversion rates, the OVP response can still be just a few tens of microseconds and it is worth spending a little extra time to gain immunity against nuisance tripping. For example, the Agilent N6781A uses this technique. Since it does an ADC conversion every 5 us and requires 4 consecutive conversions exceed the OVP threshold to cause a shutdown, it will trip in less than 30 us.

So you can see that there are various ways to implement overvoltage protection. In all cases, rest assured that your DUT is protected against excessive voltage when using Agilent power supplies!

Safeguarding your power-sensitive DUTs from an over power condition

Today’s system DC power supplies incorporate quite a variety of features to protect both the device under test (DUT) as well as the power supply itself from damage due to a fault condition or setting mishap. Over voltage protect (OVP) and over current protect (OCP) are two core protection features that are found on most all system DC power supplies to help protect against power-related damage.

OVP helps assure the DUT is protected against power-related damage in the event voltage rises above an acceptable range of operation. As over voltage damage is almost instantaneous the OVP level is set at reasonable margin below this level to be effective, yet is suitably higher than maximum expected DUT operating voltage so that any transient voltages do not cause false tripping. Causes of OV conditions are often external to the DUT.

OCP helps assure the DUT is protected against power-related damage in the event it fails in some fashion causing excess current, such as an internal short or some other type of failure. The DUT can also draw excess current from consuming excess power due to overloading or internal problem causing inefficient operation and excessive internal power dissipation.

OVP and OCP are depicted in Figure 1 below for an example DUT that operates at a set voltage level of 48V, within a few percent, and uses about 450W of power. In this case the OVP and OCP levels are set at about 10% higher to safeguard the DUT.

Figure 1: OVP and OCP settings to safeguard an example DUT

However, not all DUTs operate over as limited a range as depicted in Figure 1. Consider for example many, if not most all DC to DC converters operate over a wide range of voltage while using relatively constant power. Similarly many devices incorporate DC to DC converters to give them an extended range of input voltage operation. To illustrate with an example, consider a DC to DC converter that operates from 24 to 48 volts and runs at 225W is shown in Figure 2. DC to DC converters operate very efficiency so they dissipate a small amount of power and the rest is transferred to the load. If there is a problem with the DC to DC converter causing it to run inefficiently it could be quickly damaged due to overheating. While the fixed OCP level depicted here will also adequately protect it for over power at 24 volts, as can be seen it does not work well to protect the DUT for over power at higher voltage levels.

Figure 2: Example DC to DC converter input V and I operating range

A preferable alternative would instead be to have an over power protection limit, as depicted in Figure 3. This would provide an adequate safeguard regardless of input voltage setting.

Figure 3: Example DC to DC converter input V and I operating range with over power protect

As an over power level setting is not a feature that is commonly found in system DC power supplies, this would then mean having to change the OCP level for each voltage setting change, which may not be convenient or desirable, or in some cases practical to do. However, in the Agilent N6900A and N7900A Advance Power System DC power supplies it is possible to continually sense the output power level in the configurable smart triggering system. This can in turn be used to create a logical expression to use the output power level to trigger an output protect shutdown. This is depicted in Figure 4, using the N7906A software utility to graphically configure this logical expression and then download it into the Advance Power System DC power supply. As the smart triggering system operates at hardware speeds within the instrument it is fast-responding, an important consideration for implementing protection mechanisms.

Figure 4: N7906A Software utility graphically configuring an over power protect shutdown

A glitch delay was also added to prevent false triggers due to temporary peaks of power being drawn by the DUT during transient events. While the output power level is being used here to trigger a fault shutdown it could have been just as easily used to trigger a variety of other actions as well.

Why have programmable series resistance on a power supply’s output?

A feature we’ve included on our 663xxA Mobile Communications DC Sources, our N6781A 2-quadrant Source Measure Module, and most recently our N69xxA and N79xxA Advanced Power System (APS) is the ability to program in a value for a resistance that exists in series with the output voltage. So why do we offer this?

 Batteries are not ideal voltage sources. They have a significant amount of equivalent series resistance (ESR) on their output. Because of this, the battery’s output has a voltage drop that is proportional to the current drawn by the DUT that is being powered. An example of this is shown in the oscilloscope capture in Figure 1, where a GPRS mobile handset is drawing pulsed transmit current from its battery.

Figure 1: Battery voltage and current powering a GPRS handset during transmit

In comparison, due to control feedback, a conventional DC power supply has extremely low output impedance. At and near DC, for all practical purposes, the DC output resistance is zero. At the same time, during fast load current transition edges, many conventional DC power supplies can have fairly slow transient voltage response, leading to significant transient overshoots and undershoots with slow recovery during these transitions, as can be seen in the oscilloscope capture in Figure 2.

Figure 2: Example general purpose bench power supply powering a GPRS handset during transmit

It’s not hard to see that the general purpose bench power supply voltage response is nothing close to that of the battery’s voltage response and recognize that it will likely have a significant impact on the performance of the GPRS handset. Just considering the performance of the battery management, the battery voltage drop during loading and rise during charging, due to the battery’s resistance, will impact discharge and charge management performance.

We include programmable resistance in the above mentioned DC power supplies as they are battery simulators.  By being able to program a series output resistance these power supplies are able to better simulate the voltage response of a battery, as shown in Figure 3.

Figure 3: N6781A battery simulator DC source powering a GPRS handset during transmit

While the 663xxA and N6781A are fairly low power meant to simulate batteries for handheld mobile devices, The N69xxA and N79xxA APS units are 1 and 2 KW power supplies meant to simulate much larger batteries used in things like satellites, robotics, regenerative energy systems, and a number of other higher power devices. Figure 4 shows the voltage response of an N7951A 1 KW APS unit programmed to 20 milliohms output impedance, having a +/- 10 amp peak sine wave load current applied to its output.

Figure 4: N7951A 1 KW APS DC source voltage response to sine wave load

Programmable series output resistance is one more way a specialized DC source helps improve performance and test results, in this case doing a better job simulating the battery that ultimately powers the device under test.

Overvoltage protection: some background and history

In my previous post, I talked about some of the differences between sensing an overvoltage condition on the output terminals of a power supply and sensing on the sense terminals. In this post, I want to cover some background and history about overvoltage protection (OVP).

OVP is a feature on a power supply that is used to prevent excessive voltage from being applied to sensitive devices that are being powered by the power supply. If the voltage at the output terminals exceeds the OVP setting, the output of the power supply shuts down, thereby protecting the device from excessive voltage. OVP is always active; you cannot turn it off. If you do not want it to activate, you should set it to a value that is much higher than the maximum voltage you expect at the output of your power supply.

An overvoltage condition can occur due to a variety of reasons:

·         Operator error - an operator can mistakenly set a voltage higher than desired

·         Internal circuit failure – an electronic circuit inside the power supply can fail causing the output voltage to rise to an undesired value

·         External power source – an external source of power, such as another power supply or battery in parallel with the output, could produce voltage that is higher than desired

Some power supply OVP designs include a silicon-controlled rectifier (SCR) across the output that would be quickly turned on if an overvoltage condition was detected. The SCR essentially puts a short circuit across the output to prevent the output voltage from going to a high value and staying there. The SCR circuit is sometimes called a “crowbar” circuit since it acts like taking a large piece of metal, such as a crowbar, and placing it across the power supply output terminals to protect the device under test (DUT) from excessive voltage.

Turning on an SCR across the output of a power supply as a response to an overvoltage condition originated as a result of older linear power supply designs. Linear regulators use a series pass transistor (click here for a post about linear regulators). If the series pass transistor fails shorted, all of the unregulated rail voltage inside the power supply appears across the output terminals. This voltage is higher than the maximum rated voltage of the power supply and can easily damage a DUT. When the OVP is activated, a signal is sent to turn off the series pass transistor. However, if that transistor failed shorted, the turn-off signal will be of no use. In this situation, the only way to protect the DUT is to trigger an SCR across the output to essentially short the output. Of course, the SCR circuit is designed to have a large enough capacity to handle the rail voltage and then the current that will flow when it is tripped. If a series pass transistor fails shorted, the AC input line fuse will sometimes blow when the SCR shorts which will completely disable the power supply protecting the DUT.

More recent power supply designs use switching regulation technology (click here for a post on switching regulators). Switching regulators have multiple power transistors that can fail. However, unlike the linear regulator design, when a switching transistor fails, it does not create a path between the rail voltage and the output terminals. So it is unlikely that a failed switching transistor will cause an OVP. And when an OVP activates for another reason in a switching regulator, all of the switching transistors are told to turn off, preventing any power from flowing to the output. As a result, there is no need for an SCR across the output for added protection against an overvoltage.

Decades ago, when OVP first started to be used on our power supplies (we were Hewlett-Packard back then), the OVP setting was fixed. It was internally set to maybe 10% or 20% above the maximum rated output of the power supply. Later, we provided the power supply user with the ability to crudely control the setting of the OVP by turning a potentiometer accessible through a hole in the front panel (see pictures below). The OVP range was typically adjustable from about 20% to 120% of the maximum rated output voltage of the power supply. When this feature first became available, it was offered as an add-on option for some power supply models. Later still, the front panel manually-adjustable OVP became standard on most high-performance power supplies. With advances in electronics, the OVP adjustability was moved deeper inside the supply and controlled with a DAC through front panel button presses or over an interface such as GPIB. Today, OVP is included in nearly every power supply, is set electronically, and is often a calibrated parameter to improve overall accuracy.

Protect your DUT: use sense leads for overvoltage protection (OVP)

Earlier this week, one of our military customers providing DC power to a very expensive device during test asked about the availability of a special option on one of our power supplies. He wanted the option that changed the location of the overvoltage protection (OVP) sensing terminals from the output terminals of the power supply to the sense terminals of the power supply. Since his device under test (DUT) is located quite a distance away from the power supply, he is using remote sensing to regulate the power supply voltage right at his device under test. (Click here for a post about remote sense.) And since the DUT is very expensive and sensitive to excessive voltage, he needs to protect the input of the DUT from excessive voltage as measured right at the DUT input terminals.

The power supply he is using, an Agilent N6752A installed in an N6700B mainframe, normally uses the output terminals as the sensing location for the overvoltage protection. (Click here for a post that includes a description of OVP.) OVP is used to prevent excessive voltage from being applied to sensitive devices. If the voltage at the output terminals exceeds the OVP setting, the output of the power supply shuts down. Since this customer is very interested in preventing excessive voltage from being applied to his expensive DUT, sensing for an overvoltage condition right at the DUT is important. For the N6752A, Agilent offers a special option (J01) that adds the ability to do OVP sensing with the sense leads. See Figure 1. With the J01 option added to his N6752A, the customer’s DUT is protected against excessive voltage.

You may be wondering why the standard OVP would sense at the output terminals instead of at the sense terminals. For decades, we have been making power supplies that sense OVP at the output terminals. Probably the biggest reason for sensing at the output terminals is because that approach provides more reliable protection than sensing at the sense leads even though it is less accurate. The output terminals are the power-producing terminals. If the sense leads become inadvertently shorted, the voltage at the output terminals would rise uncontrolled beyond the maximum rated output of the power supply. This uncontrolled high voltage could easily damage any device connected to the power supply’s output leads! So sensing for an overvoltage condition at the output terminals actually makes sense. It may not be the most accurate way to protect the DUT, but it is the most reliable given all of the things that can go wrong, such as a wiring error or an internal fault in the power supply.

The J01 option is available for only certain N67xx power modules. It adds the ability to sense for an overvoltage condition on the sense leads. This option does not remove the existing output terminal overvoltage sensing feature; it is in addition to it. Additionally, the J01 option is a tracking OVP option. You set a voltage value that is an offset from the programmed output voltage value. The J01 tracking overvoltage threshold tracks the real-time programming changes to the voltage setting and uses the remote sense leads to monitor the voltage.

Addressing the challenge of sequencing multiple bias supplies on and off

A challenge test engineers are perennially faced with is how to best sequence the bias voltages powering their DUT, when their DUT requires several bias voltages. Many DUTs are sensitive to sequencing and an improper sequence may lead to the DUT hanging up, or worse, suffer damage as a consequence. Not only is sequencing an issue when powering the DUT on, but it can also be an issue when powering the DUT down as well. In addition to sequencing, the slew rates of the various bias voltages can likewise be important to the DUT correctly powering on.

Simply relying on the timing of output-on and output-off commands sent from the test system controller to all the system DC power supplies individually tends to be far too imprecise, especially for critical sequencing timing requirements. The actual turn-on time of a typical system DC power supply can be many tens of milliseconds, and will vary considerably between different models of power supplies. The turn-on and turn-off times of each will need to be carefully characterized in order to know when a command for a specific bias voltage needs to be sent in relation to the other bias voltages. It is very likely the sequence of commands sent for outputs to turn on or off may be in a different sequence to the outputs actually changing, due to delay differences between different DC power supplies! An even bigger problem however is most system controllers are PCs which may randomly experience a large delay in sending out a command, if a higher level service request interrupts and pre-empts execution of the test program.

An alternative approach often taken is adding some custom hardware to control output sequencing. This can assure correct sequencing, but adds a lot of complexity, is usually inflexible, and may introduce other issues and compromises.

At Agilent we added system features to our N6700 series multiple output modular DC power system that support correct power-on and power-off sequencing. The output-on and output-off controls for the individual outputs get grouped together. The N6700 platform knows and compensates for the actual delays of all the various DC power output modules so that the desired delay value entered will be what is accurately achieved. Figure 1 shows setting up an N6705A to achieve a desired turn-on sequence of DC outputs for powering up a PC mother board. Figure 2 shows the actual result. A more detailed description of this PC motherboard example is given in our application note: “Biasing Multiple Input Voltage Devices in R&D”. While the N6705B DC Power Analyzer mainframe is regarded as being primarily for R&D, which this app note is referencing, the low profile rack-mountable N6700 series mainframes have these very same features and suit automated test systems in manufacturing and other environments.

Figure 1: Setting Output Delays

Figure 2: Output Turn-on Sequence Results

Just like setting up the power-on sequence, separate delays for power-off can also be entered, as seen in the set up screen shown in Figure 1, for the expected shut down of the DUT. However, what if there is an emergency shut down due to an abnormal condition and you still want to assure a certain power-off sequence? A colleague worked out the procedure for setting up the N6700 series DC power system to provide an orderly shutdown of the outputs, in the event of a problem on one of the outputs. In this example it happens to be an overvoltage condition on one of the outputs, but any of a number of fault conditions can be acted on to initiate an orderly shutdown. Details of this procedure are provided in another application note; “Avoid DUT Damage by Sequencing Multiple Power Inputs Off Upon a Fault Event”.

So when faced with the challenge of having to properly sequence multiple DC bias voltages powering your DUT, reconsider trying to engineer a solution to accomplish this. Instead, look for features that provide this kind of capability in the system DC power supplies you are looking to use, already built-in. It makes a lot of sense having sequencing built into the power supplies and it will make your life a lot easier!

Protecting your DUT using a power supply’s remote inhibit and fault indicator features

Paramount in most any good electronic test system is the need to adequately protect the device under test (DUT), as well as the test equipment, from inadvertent damage due to possible faults with the yet-untested DUT, accidental misconnections, misapplication of power, and a large number of other unanticipated events that can occur. It is no surprise that a lot of these unanticipated events by nature are related to the powering of the DUT. For this reason good system DC power supplies incorporate a number of features designed to protect both the DUT, as well as the power supply, in the event of an unanticipated fault occurring.  Two related protection features incorporated into our DC system power supplies are the remote inhibit and the discrete fault indicator (RI/DFI). These features provide real-time protection enabling immediate shutting down the power supply, as well as enabling the power supply to take immediate action, on the event of detecting the occurrence of an unanticipated event or fault.

The remote inhibit is a digital input control while the discrete fault indicator is a digital output control signal, incorporated into the digital I/O port on our system DC power supplies. An example of a digital I/O port is illustrated in Figure 1. When the digital I/O port is configured for fault/inhibit (also called RI/DFI) pins 1 and 2 are the open collector and emitter of an isolated transistor, to serve as a digital output control, and pin3 and 4 are the digital input and common for the inhibit control input. The remote inhibit and the fault indicator can be used independently as well as in combination, for protecting the DUT.

Figure 1: Multi-function digital I/O port on Agilent 6600A series system DC power supplies

As the name implies, the remote inhibit is a digital control input, when activated, immediately disables the DC power supply’s output. One way this is commonly used is to connect an emergency shutdown switch that can be conveniently activated in the event of a problem. This may be a large pushbutton, or it may be a switch incorporated into a fixture safety cover. This arrangement is shown in Figure 2.

Figure 2: Remote inhibit using external switch

The fault indicator (i.e. FLT, FI, or DFI) digital output signal originates from the system DC power supply’s status system. The status system is a configurable logic system within the power supply having a number of registers that keep track of its status for operational, questionable, and standard events. Many of these events can be logically OR’ed together as needed to provide a fault output signal when particular, typically unanticipated, events occurs with the power supply. Items tracked by questionable status group register, like over voltage and over current, for example, are commonly selected and used for generating a fault output signal. An overview of the power supply status register system was discussed by a colleague in a previous posting. If you are interested in learning more; click here.

The fault indicator output can in turn be used to control an external activity for protecting the DUT, such as opening a disconnect relay to isolate the DUT, as one example, as depicted in Figure 3.

Figure 3: Fault output controlling an external disconnect relay

For DUTs that require multiple bias voltage inputs it is usually desirable that if a fault is detected on one bias input, that the other bias inputs are immediately shut down in conjunction with the one detecting a fault. The fault outputs and remote inhibit inputs on several DC power supplies can be used in combination by chaining them together, as depicted in Figure 4, to accomplish this task, to safeguard the DUT.

Figure 4: Chaining fault indicators and remote inhibits on multiple DC power supplies

The remote inhibit and fault indicator digital control signals on system DC power supplies provide a number of ways to disable power and take other actions for safeguarding the DUT. Their action is immediate, not requiring communication to, and intervention from, the test system controller. At the same time the system DC power supply generates status signals and can issue a service request (SRQ) to the test system controller so that it is notified of a problem condition and take appropriate correction action as well. The remote inhibit and fault indicator digital control signals are just two of many features found in many good system DC power supplies to assure the DUT is always adequately protected during test!

Protect your DUT with power supply features including a watchdog timer

The two biggest threats of damage to your device under test (DUT) from a power supply perspective are excessive voltage and excessive current. There are various protection features built into quality power supplies that will protect your DUT from exposure to these destructive forces. There are also some other not-so-common features that can prove to be invaluable in certain applications.

Soft limits
The first line of defense against too much voltage or current can be using soft limits (when available). These are maximum values for voltage and current you can set that later prevent someone from setting output voltage or current values that exceed your soft limit settings. If someone attempts to set a higher value (either from the front panel or over the programming interface), the power supply will ignore the request and generate an error. While this feature is useful to prevent accidentally setting voltages or currents that are too high, it cannot protect the DUT if the voltage or current actually exceeds a value due to another reason. Over-voltage protection and over-current protection must be used for these cases.

Over-voltage protection
Over-voltage protection (OVP) is a feature that uses an OVP setting (separate from the output voltage setting). If the actual output voltage reaches or exceeds the OVP setting, the power supply shuts down its output, protecting the DUT from excessive voltage. The figure below shows a power supply output voltage heading toward 20 V with an OVP setting of 15 V. The output shuts down when the voltage reaches 15 V.

Some power supplies have an SCR (silicon-controlled rectifier) across their output that gets turned on when the OVP trips essentially shorting the output as quickly as possible. Again, the idea here is to protect the DUT from excessive voltage by limiting the voltage magnitude and exposure time as much as possible. The SCR circuit is sometimes called a “crowbar” circuit since it acts like taking a large piece of metal, such as a crowbar, and placing it across the power supply output terminals.

Over-current protection
Over-current protection (OCP) is a feature that uses the constant current (CC) setting. If the actual output current reaches or exceeds the constant current setting causing the power supply to go into CC mode, the power supply shuts down its output, protecting the DUT from excessive current. The figure below shows a power supply output current heading toward 3 A with a CC setting of 1 A and OCP turned on. The power supply takes just a few hundred microseconds to register the over-current condition and then shut down the output. The CC and OCP circuits are not perfect, so you can see the current exceed the CC setting of 1 A, but it does so for only a brief time.

The OCP feature can be turned on or off and works in conjunction with the CC setting. The CC setting prevents the output current from exceeding the setting, but it does not shut down the output if the CC value is reached. If OCP is turned off and CC occurs, the power supply will continue producing current at the CC value basically forever. This could damage some DUTs as the undesired current flows continuously through the DUT. If OCP is turned on and CC occurs, the power supply will shut down its output, eliminating the current flowing to the DUT.

Note that there are times when briefly entering CC mode is expected and an OCP shutdown would be a problem. For example, if the load on the power supply has a large input capacitor, and the output voltage is set to go from zero to the programmed value, the cap will draw a large inrush current that could temporarily cause the power supply to go into CC mode while charging the cap. This short time in CC mode may be expected and considered acceptable, so there is another feature associated with the OCP setting that is a delay time. Upon a programmed voltage change (such as from zero to the programmed value as mentioned above), the OCP circuit will temporarily ignore the CC status just for the delay time, therefore avoiding nuisance OCP tripping.

Remote inhibit
Remote inhibit (or remote shutdown) is a feature that allows an external signal, such as a switch opening or closing, to shutdown the output of the power supply. This can be used for protection in a variety of ways. For example, you might wire this input to an emergency shutdown switch in your test system that an operator would use if a dangerous condition was observed such as smoke coming from your DUT. Or, the remote inhibit could be used to protect the test system operator by being connected to a micro switch on a safety cover for the DUT. If dangerous voltages are present on the DUT when operating, the micro switch could disable DUT power when the cover is open.

Watchdog timer
The watchdog timer is a unique feature on some Agilent power supplies, such as the N6700 series. This feature looks for any interface bus activity (LAN, GPIB, or USB) and if no bus activity is detected by the power supply for a time that you set, the power supply output shuts down. This feature was inspired by one of our customers testing new chip designs. The engineer was running long-term reliability testing including heating and cooling of the chips. These tests would run for weeks or even months. A computer program was used to control the N6700 power supplies that were responsible for heating and cooling the chips. If the program hung up, it was possible to burn up the chips. So the engineer expressed an interest in having the power supply shut down its own outputs if no commands were received by the power supply for a length of time indicating that the program has stopped working properly. The watchdog timer allows you to set delay times from 1 to 3600 seconds.

Other protection features that protect the power supply itself
There are some protection features that indirectly protect your DUT by protecting the power supply itself, such as over-temperature (OT) protection. If the power supply detects an internal temperature that exceeds a predetermined limit, it will shut down its output. The temperature may rise due to an unusually high ambient temperature, or perhaps due to a blocked or incapacitated cooling fan. Shutting down the output in response to high temperature will prevent other power supply components from failing that could lead to a more catastrophic condition.

One other way in which a power supply protects itself is with an internal reverse protection diode across its output terminals. As part of the internal design, there is often a polarized electrolytic capacitor across the output terminals of a power supply. If a reverse voltage from an external power source was applied across the output terminals, the cap (or other internal circuitry) could easily be damaged. The design includes a diode across the output terminals with its cathode connected to the positive terminal and its anode connected to the negative terminal. The diode will conduct if a reverse voltage from an external source is applied across the output terminals, thereby preventing the reverse voltage from rising above a diode drop and damaging other internal components.

What Is Going On When My Power Supply Displays “UNR”?

Most everyone is familiar with the very traditional Constant Voltage (CV) and Constant Current (CC) operating modes incorporated in most any lab bench or system power supply. All but the most very basic power supplies provide display indicators or annunciators to indicate whether it is in CV or CC mode. However, moderately more sophisticated power supplies provide additional indicators or annunciators to provide increased insight and more information about their operating status. One annunciator you may encounter is seeing “UNR” flash on, either momentarily or continuously. It’s fairly obvious that this means that the power supply is unregulated; it is failing to maintain a Constant Voltage or Constant Current. But what is really going on when the power supply displays UNR and what things might cause this?
To gain better insight about CV, CC and UNR operating modes it is helpful to visualize what is going on with an IV graph of the power supply output in combination with the load line of the external device being powered. I wrote a two part post about voltage and current levels and limits which you may find useful to review. If you like you can access it from these links levels and limits part 1 and levels and limits part 2. This posting builds nicely on these earlier postings. A conventional single quadrant power supply IV graph with resistive load line is depicted in Figure 1. As the load resistance varies from infinity to zero the power supply’s output goes through the full range of CV mode through CC mode operation. With a passive load like a resistor you are unlikely to encounter UNREG mode, unless perhaps something goes wrong in the power supply itself.
Figure 1: Single quadrant power supply IV characteristic with a resistive load

However, with active load devices you have a pretty high chance of encountering UNR mode operation, depending where the actual voltage and current values end up at in comparison to the power supply’s voltage and current settings. One common application where UNR can be easily encountered is charging a battery (our external active load device) with a power supply. Two different scenarios are depicted in Figure 2. For scenario 1, when the battery voltage is less than the power supply’s output, the point where the power supply’s IV characteristic curve and the battery’s load line (a CV characteristic) intersect, the power supply is in CC mode, happily supplying a regulated charge current into the battery. However, for scenario 2 the battery’s voltage is greater than the power supply’s CV setting (for example, you have your automobile battery charger set to 6 volts when you connect it to a 12 volt battery). Providing the power supply is not able to sink current the battery forces the power supply’s output voltage up along the graph’s voltage axis to the battery’s voltage level. Operating along this whole range of voltage greater than the power supply’s output voltage setting puts the power supply into its UNR mode of operation.
Figure 2: Single quadrant power supply IV characteristic with a battery load

A danger here is more sophisticated power supplies usually incorporate Over Voltage Protection (OVP). One kind of OVP is a crowbar which is an SCR designed to short the output to quickly bring down the output voltage to protect the (possibly expensive) device being powered. When connected to a battery if an OVP crowbar is tripped, damage to the power supply or battery could occur due to batteries being able to deliver a fairly unlimited level of current. It is worth knowing what kind of OVP there is in a power supply before attempting to charge a battery with it. Better yet is to use a power supply or charger specifically designed to properly monitor and charge a given type of battery. The designers take these things into consideration so you don’t have to!
I have digressed here a little on yet another mode, OVP, but it’s all worth knowing when working with power supplies! Can you think of other scenarios that might drive a power supply into UNR? (Hint: How about the other end of the power supply IV characteristic, where it meets the horizontal current axis?)