Hi,
I am designing an inductance bridge for iron cored inductors of around 10H 100mA
I need a constant DC current of upto 100mA, to do this I have a stabilized 100V PSU, that feeds the top of the inductor.
The circuit is a standard current sink using a high voltage Mosfet.
Looking at the drain of the Mosfet, I can see quite large voltage spikes, that seem random, but quite large.
If I replace the inductor with a resistor, all is calm and there is no noise to speak of.
So it does seem to be the fact that this is an inductor. I have tried putting the inductor in a diecast box, to see if it was some form of pick up, but this made no difference.
Any ideas?
John
Hi MJ,
The symptoms you describe sound like the onset of break-down, possibly due to a combination of voltage, temperature, and current.
You do not state what the inductor characteristics are, particularly the inductor resistance, and you do not state the value of the resistor that you replaced the inductor with, so I will assume that the inductor resistance is zero and that the substitute resistor had a high enough resistance to drop a significant voltage and thus lower the voltage on the drain of the NMOSFET. The later could possibly explain why the symptoms disappeared when you replaced the inductor with the resistor.
100V is quite high, as is 100mA at that voltage and would mean that the IFRH5015 would be dissipating 0.1A * 100V =10W.
The IFRH5015 would require a substantial heatsink to keep it's junction temperature below the specification sheet limit of 150 Deg C at 10W dissipation.
But the main concern is the IFRH5015 secondary break down characteristic. It seems that you would be exceeding the limits as you can see from the safe operating area graph extracted from the IFRH5015 datasheet.
If my assumptions about the inductor are essentially true, you would need an NMOSFET with a much better SOA and I would advise a VDSmax of 250V upwards. Also, if my theory is correct the IFRH5015 would be damaged and should not be used. It is very unwise to use a device on its limits, especially as there are other factors, like junction temperature, rate of current and voltage change, etc, that need to be factored into the data sheet figures.
In general, I would also advise checking your circuit to ensure there is no arcing, especially from the NMOSFET case to heatsink, and ensure that any insulating washer, and thermal compound, is OK for your voltage.
I would be inclined to reduce the voltage to 50V, fit a new IFRH5015, fit a heatsink, if one is not already fitted, and see if the spikes vanish.
spec
DATASHEET
https://www.infineon.com/dgdl/irfh5015pbf.pdf?fileId=5546d462533600a40153561abe001e9c