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!

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!