MOSFET Breakdown Voltage

[~MJS]

I plan to use the circuit below to measure the breakdown voltage of a MOSFET.

The oscilloscope I am using is capable of 10 mV / div. According to IR's spec, they measure their V_(BR)DSS at 250 uA. So, I thought making R_sense= 100 Ohm would be a good choice, because this would correspond to 25 mV.

Any opinions?

Also, I am still not sure whether I will need the R_limit or not.

FYI: In this first pass, I am going to test the theory on Si-MOSFETs. I plan on applying at least 300V to the drain via a variac, step-up transformer, and full-bridge rectifier.

 

[dick_freebird]

If it's DC you could as easily use cheap multimeters at
sub-mV accuracy.

If you want to use the FET ever again you want Rlimit,
because catastrophic damage takes less time than you,
or a DC supply's current compliance limit, can react to a
snapback.

Be sure your 'scope input channel really can take 300V,
otherwise it could be an expensive discovery.

 

[Mr.Cool]

AT LEAST 300V? you probably want a 1000V differential probe then, the tektronic kind that has built in overvoltage protection. so you can set it to measure the small voltage drop on 50V scale even though you apply high voltage across the device.

at 300V and desire 250uA.. Rllimit = 1.2 MEG. and to get the high voltage rating you are probably going to have to use 4 x 300 kOhm in series. note the resistance total will be different due to tolerance.

Mr.Cool

 

[carkiller]

There is an easier way witout the need for a dso or 1kv diff probe. short gate to source, use a current limiting resistor like earlier responders said, put dvm across fet hook resistor/fet to 1kv PS or greater than evpected BV ds of fet, raise voltage of PS until fet voltage stops rising. measure current thru resistor if you like but modern power fets have Esb protection meaning they can withstand a specified amout of energy in avalanche (just like a zener diode) but check the dats sheet for the fet you are using! It is considered bad design practice to depend upon the avalanche energy capability of mosfets in any switching supply if you care about reliability BTW.

 

[m.mokhtar]

connect a 250 ua current source in series with the mosfet and measure the voltage across the mosfet ; but the current source must be able to provide that current at a high voltage (more thanthe breakdown voltage)