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!
