Friday, December 21, 2012

Two-quadrant power supplies are better than one!

Back in October, I posted an explanation about what was a bipolar (four-quadrant) power supply (see post here: http://powersupplyblog.tm.agilent.com/2012/10/what-is-bipolar-four-quadrant-power.html). That post covered two-quadrant supplies as well. Last week, while in Lorton, Virginia, I had an opportunity to meet with some of our U.S. Army customers  - engineers working at Fort Belvoir. Many of the engineers worked in the Counter Measures Research Laboratory (CMRL). While they are very careful to not reveal any details about the specifics of the work they do, one of the engineers shared a story with me about two-quadrant operation that is worth repeating.

The story was told while I was providing a demonstration of one of our power supplies, the N6705B DC Power Analyzer (see Figure 1). I was explaining to a group of engineers that some of the 34 power modules that can be installed in the N6705B are two-quadrant power supplies: they can source current and also sink current at one voltage polarity. Other power modules are four-quadrant power supplies: they can source and sink current, and provide positive or negative voltage. This explanation inspired one of the engineers to tell the group that the N6705B helped him solve a problem!


A battery operated device (he did not mention what it was) came into his lab because it was not functioning properly: it had some type of intermittent problem. In an attempt to reproduce the problem, he removed the battery and connected the device’s power input terminals to a power supply on his lab bench. But even after running the device for long periods of time and through all of its operating modes, he was unable to reproduce the intermittent problem.

One of his colleagues suggested he try connecting the device to a two-quadrant power supply installed in the N6705B they owned. The original power supply he was using was a one-quadrant supply – it could source power, but could not absorb power. The battery that normally powers the device can source and sink (absorb) power, so perhaps a power supply that more closely mimicked the behavior of the battery could help uncover the problem. Well, this worked! With the device connected to the two-quadrant power supply in the N6705B, the intermittent problem showed up again proving that it was related to the battery being able to source and sink power – a power supply with similar characteristics was needed. Apparently, the device has a mode in which it momentarily forces current back out of the battery input terminals. That current is normally absorbed by the battery. And during that time, this intermittent problem must show up. During test, a single-quadrant power supply is unable to absorb the power and therefore does not reveal the problem. A two-quadrant power supply can sink the momentary current, and the problem was back, enabling the engineer to track it down and eliminate it! See Figure 2 for an example of the output characteristic of a two-quadrant power supply.

This example demonstrates the importance of choosing a power supply with the right output characteristics for your test. When testing a device or circuit with a power supply, the closer that power supply’s behavior is to the actual power used with the device or circuit, the more you will reveal about the actual performance of your device or circuit.  There are applications in which a two-quadrant power supply will better replicate a battery’s behavior than a single-quadrant power supply, even if you don’t expect the battery to absorb power during test. One CMRL engineer experienced this firsthand.

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