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!

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!

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.