Wednesday, September 10, 2014

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)

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