# Power supply current

Discussion in 'General Electronics Chat' started by Distort10n, Apr 20, 2008.

1. ### Distort10n Thread Starter Active Member

Dec 25, 2006
429
1
Does anyone know what a DC power supply will report in terms of current? That is, if I have an op-amp configured as an AC amplifier the current from the supply look like a half wave rectified waveform.
The DC supply will display a current, but is this an average current or an RMS current?
I am trying to calculate efficiency of a Class D amplifier. Calculating output power to the load is easy: hook up the scope and use the poor man's method (CHA - CHB) to determine the differential signal's amplitude across a fixed resistance (8Ω).
I was hoping that the DC supply would display the average current so I could simply multiply it by the DC supply voltage to cacluate power delivered (Pin).
So far, I am off by 30%.

2. ### Audioguru New Member

Dec 20, 2007
9,411
896
If an amplifier feeds a sine-wave into a load then the positive power supply has half-wave current during the positive swing and the negative power supply has half-wave current during the negative swing.

If a single supply feeds the amplifier and it has a coupling capacitor to the load then the power supply current is a full-wave signal.

You can average the current by filtering it.

3. ### Distort10n Thread Starter Active Member

Dec 25, 2006
429
1
The amplifier in question is the SSM2211 from Analog Devices. It is a single supply (5V) amplifier that can be configured as a bridge-tied load or single-ended.

In the BTL configuration, the power supply current waveform will look like a full wave rectified waveform In the single-ended configuration, the power supply current waveform will look like a half-wave rectified waveform.

The question is, what is the display on the power supply itself telling me it is measuring? I can calculate it, but I was hoping the display is already showing me an average.

4. ### Caveman Active Member

Apr 15, 2008
471
0
The current that your bench supply is showing is the average current being drawn from it. In the BTL configuration, one output will be pulling power from your switching power supply positive to pull its side up, and the other will be attempting to short its output to ground which will pull no significant current.
When the state changes, it will swap. However, the power supply will always see positive currents. And the display averages them.

5. ### Distort10n Thread Starter Active Member

Dec 25, 2006
429
1
Well that is a pisser. If it is showing me the average current then all I need to do is multiply that average by the supply voltage to calculate power delivered by the supply.

Problem is, my efficiency calculations are way off for a Class D amplifier. I have an RC filter on the output so I can hook up a scope in order to use the math function to calculate the differential voltage.

I am convinced that my calculations for power delivered to the load are correct...comon man, ((Vp/sqrt2)^2)/Rload.

I need to go back figure out how to do this. Without the RC filter, the output waveform will be switching.