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!

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.