Monday, April 8, 2013

Why would a DC power supply have RMS current readback?

During a conversation with a colleague at work one day the topic of having RMS current readback on DC power supplies came up. It is a measurement capability we have on a number of our system DC power supplies. He posed the question: Why the reason for having such a capability? I actually had not been involved with the original investigations identifying what reasons this was added so I instead had to rely on my intuition. That’s not always a good thing but it did help me out this time at least!

He had argued that since you are feeding a fixed DC voltage into the device you are powering, the power consumption is going to be a product of the DC (average) voltage and DC (average) current, regardless of whether the current is purely DC, or if it is dynamic, having a substantial amount of AC content. This is true, as I have illustrated in figure 1, comparing purely DC and pulsed currents being drawn by a load. For purely DC current the DC and RMS values are the same. In comparison, for a pulsed current the RMS value is greater the DC value. Regardless, the RMS current value does not factor into the overall power consumption of the DUT here. The power consumption is still the product of the DC voltage and current.

Figure 1: Comparing power consumption of a DC powered DUT drawing constant and pulsed currents

So why provide an RMS current measurement? Well there can be times when this can prove useful, even when the DUT is powered by a fixed DC voltage. Consider the scenario depicted in Figure 2.

Figure 2: Properly sizing a protection fuse on a DC powered device

Many products incorporate fuses to protect from over-current and subsequent damage, usually brought on due to misuse or component failure. Fuses are rated by their RMS current handling, not the DC current. In the case of the pulsed loading the RMS current is twice the DC current and the resulting power in the fuse is four times that for a constant current.  If the fuse was selected based on the DC current value it would most certainly fail well below the required operating level!

My colleague conceded that this fuse example was a legitimate case where RMS current measurement would indeed be useful. Maybe it was not a frivolous capability after all. No doubt sizing fuses is just one of many reasons why RMS measurement on DC products can be useful!

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