Current Sense Transformer
I am designing a circuit to provide current feedback to the control circuit. I want isolation from the output of the inverter which is at 320V and is off the line!
So, let's start with materials, iron powder is out because it gets really hot from the high frequency current. So, we need a ferrite core that will not saturate. I will be using a toroidal core with 100 turns of #20 wire.
Here are the specifications:
--> 10 kHz minimum frequency
--> Up to 100 amps peak
--> 1 turn primary, 100 turns secondary
--> 320 volts peak
Next is the burden resistor value.
100A/100=1A
1A*2.2Ω=2.2V
100 amps is redundant, it is just an absolute maximum rating.
60A/100=0.6A
0.6A*2.2Ω=1.32V
60 amps is a more reasonable estimate which gives 1.32V output.
The wattage of the burden resistor is also important.
1A*2.2V=2.2W
0.6A*1.32V=0.792W
A 2 watt resistor should be sufficent for this application since we are only looking at an expected 60 amps peak under normal operating conditions.
<Edit>
I just read that a burden resistor should not be used for more than 55% of it's power capacity due to changes in value as heat builds up. I may go for a 3-4 watt resistor because this is a very important component and its failure could cause undesireable consequences. I know that this is overkill but a higher wattage resistor is not that expensive and I just want to be 100% sure that this will give me accurate results.
</Edit>
I will use a comparator to turn the current waveform into a square wave for the phase comparator. Another comparator will form a peak current detector to trip an SR latch and shut down several different parts of the curcuit. Current feedback will also be used to limit voltage at the buck converter to keep the output power constant under different operating conditions such as loading.
So, I guess my main question is how to pick a ferrite core material and size for this application?
Alex Sears