How do I protect my DUT against my power supply sense lines becoming disconnected, misconnected, or shorted?

The remote sense lines are a vital part of any good system power supply. As shown in Figure 1, by using a second, separate pair of leads for sensing, the output voltage is now regulated right at the DUT rather than at the output terminals on the power supply. Any voltage drops in the force leads are compensated for; assuring the highest possible voltage accuracy is achieved right at the DUT.

Figure 1: Remotely sensing and regulating output voltage at the DUT

Of course for this to work correctly the sense leads need to have a good connection at the DUT. However, what if the sense leads become disconnected, misconnected, or shorted?

One might think if one or both of the sense leads became disconnected, the sensed voltage would then become zero, causing the output voltage on the force leads to climb up out of control until the over voltage protect (OVP) trips. This turns out not to be the case, as a co-contributor here, Gary had pointed out in a previous posting “What happens if remote sense leads open?” (Click here to review). Basically a passive protection mechanism called sense protect maintains a backup connection between the sense line and corresponding output terminal inside the power supply in the event of a sense line becoming disconnected.

While sense protect is an indispensable feature to help protect your DUT by preventing runaway over-voltage, if a sense lead is open the voltage at your DUT is still not as accurate as it should be due to uncompensated voltage drops in the force leads. This can lead to miscalibrated DUTs and you would not even know that it is happening. To address this some system power supplies include an active open sense lead fault detection system. As one example our 663xx Mobile Communications DC Sources check the sense lead connections during each output enable and will issue a fault protect and shut down the output if one or both sense leads become disconnected. It will also let you know which of the sense leads are disconnected. It can be enabled and disabled as needed. I had written about this in a previous posting “Open sense lead detection, additional protection for remote voltage sensing” (Click here to review).

Taking sense protection further, we have incorporated a system we refer to as sense fault detect (SFD) in our N6900A and N7900A Advanced Power System (APS). It can be enabled or disabled. When enabled it continually monitors the sense lead connections at all times. If it detects a sense fault it sets a corresponding bit in the questionable status group register as well as turn on status annunciator on the front panel to alert the user, but does not disable the output. Through the expression signal routing system a “smart trigger” can be configured as shown in Figure 2 to provide a protect shutdown on the event of a sense fault detection.  In all, sense fault detect on APS provides a higher level of protection and flexibility.

Figure 2: Configuring a custom opens sense fault protect on the N6900/N7900 APS

What happens if the sense leads become shorted? Unlike open sense leads, in this case the output voltage can rise uncontrolled. The safeguard for this relies on the over voltage protect system. The same thing happens if the sense leads are reversed. The power supply will think the output voltage is too low and keep increasing the output voltage in an attempt to correct it. Again the safeguard for this relies on the over voltage protect system. The N6900/N7900 APS does actually distinguish the difference when the sense leads are reversed by generating a negative OVP (OV-) fault, giving the user more insight on what the fault is to better help in rectifying the problem.

Remote voltage sensing provides a great benefit by being able to accurately control the voltage right at the DUT. Along with the appropriate safeguards against sense lead misconnections you get all the benefit without any of the corresponding risks!

Remote sense protect and sense fault detect were just two of many topics about in my seminar “Protect your device against power related damage during test” I gave last month. As it was recorded it is available on demand if you have interest in learning more about this topic. You can access the sign up from the following link: (Click here for description and to register)

Open sense lead detection, additional protection for remote voltage sensing

A higher level of voltage accuracy is usually always needed for powering electronic devices under test (DUTs). Many devices provide guaranteed specifications for operating at minimum, nominal, and maximum voltages, so the voltage needs to accurate as to not require unacceptable amounts of guard banding of the voltage settings.

One very significant factor that affects the accuracy of the voltage at the DUT is the voltage drop in the wiring between the output terminals of the power supply and the actual DUT fixture, due to wiring’s inherent resistance, as shown in Figure 1.

 A standard feature of most all system DC power supplies is remote voltage sensing. Instead of the voltage being regulated at the output terminals of the DC power supply’s output terminal, it is instead sensed and regulated at the DUT itself, compensating for the voltage drop in the wiring. Additional details of this are documented in an earlier posting: “Use remote sense to regulate voltage at your load”

While remote voltage sensing addresses the problem of voltage drop in wiring affecting the voltage accuracy at the DUT, it then raises the concern of what happens if one of the sense lines becomes disconnected. Will the DC power supply voltage climb up to it maximum potential causing my DUT to be damaged?  Although this is a very legitimate concern, often the voltage is usually kept within a reasonable range of the setting by a feature referred to as “open sense lead protection”. A deeper dive on the issue of open sense lines and open sense lead protection are discussed at our posting: “What happens if remote sense leads open?”

Even with open sense lead protection and the voltage being kept within a reasonable range of the setting, this can be a concern for some customers who are relying on a high level of DC voltage accuracy at the DUT for test and calibration purposes. One categorical example of this is battery powered devices, where ADC circuits that need to precisely monitor the battery input voltage have to be accurately calibrated. If the voltage from the DC power supply has significant error, the DUT will be miss-calibrated.

One issue with open sense lead protection is it is a passive protection mechanism. It is simply a back up that takes over when a sense line is open. There is no way of knowing the sense lead is open. No error flag is set or fault condition tripped. The voltage being read back is the same as that is being regulated by the voltage sensing error amplifier, which is the same as the set voltage, so all looks fine from a read-back perspective. This is where open sense lead detection takes over. Open sense lead detection is a system that actively checks to see if the sense lines are doing their job. If not it lets the test system know there is a fault.

Open sense detection is not a common feature for most system DC power supplies. As one example we do employ it in our 663xx series Mobile Communications DC Sources as these are used for powering, testing and calibrating battery powered wireless devices. In the case of an open sense line condition it generates a fault condition and it keeps the output of the DC source powered down. It also provides status information on which of the sense lines are open as well.

What happens if remote sense leads open?

Remote sense is a feature on many power supplies that allows the power supply to regulate its output voltage right at your load (“remotely”) instead of at the power supply output terminals. Use remote sense when you want to compensate for load lead voltage drop caused by load current flowing in your load leads. This is accomplished by using a pair of remote sense leads that are in addition to your load leads. See an example in Figure 1. The power supply uses the voltage across the remote sense lead terminals to sense (measure) the voltage at the load terminals and regulate the voltage directly across the load by adjusting the output terminal voltage. Refer to this post I wrote last year on remote sense:
http://powersupplyblog.tm.agilent.com/2011/08/use-remote-sense-to-regulate-voltage-at.html

Remote sense leads could be accidentally left open, or once connected, one or both leads could inadvertently become open. I have had users of our power supplies testing very expensive devices under test (DUTs) ask me what would happen to the output voltage if a sense lead wired in a system opened; they were worried about subjecting their very expensive DUT to excessive voltage.

To understand why this is an important consideration, it is necessary to better understand the role of the sense leads. To regulate its output voltage, a power supply uses internal circuitry that acts as a feedback loop. The voltage is set to a particular value and the feedback loop monitors (measures) the voltage across the sense terminals and compares it to the setting. If it is too low, the loop circuitry increases the output voltage. If it is too high, the loop circuitry decreases the output voltage. So the actions of this loop result in the output voltage settling (being regulated) at a value such that the sense lead voltage equals the voltage set point.

If one or both of the sense leads is open, the feedback loop is broken and incorrect voltage information is sent to the loop. With an open sense lead, the sense voltage is typically near zero. The loop thinks the output voltage is too low and responds by increasing the output voltage. But this does not result in a corresponding increase in the sense lead voltage since the wire is broken so the loop increases the output voltage more. This continues until the value is increased to the maximum amount possible, which is usually somewhat higher than the maximum rated voltage of the power supply and very much beyond the desired set point. This could easily damage the DUT!

The scenario described in the previous paragraph is what would happen if no action was taken to prevent a runaway output voltage due to an open sense lead. Agilent power supplies have an internal circuit, called open sense protection, that prevents the output voltage from rising significantly above the set voltage if one or both of the remote sense leads is open. In fact, with one or both sense leads open, the output voltage of most Agilent power supplies will rise only 1 or 2 percent above the setting. Additionally, some Agilent power supplies can detect an open sense lead and respond by shutting down the output and alerting the user by changing a bit in a status register.

Note that this open sense protect circuitry is in addition to and independent from the over-voltage protection (OVP) circuitry common on most Agilent power supplies. OVP is a setting that is separate from the output voltage setting. If the actual output voltage exceeds the OVP setting, the OVP will shut down the output to protect the DUT.