Testing to see if SMPS FET is always in its Safe Operating Area.

[eem2am]

I wish to do a check on our 41W, isolated flyback power supply to see if the switching FET is always operated within its SOA.


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Vin = 410V Max and 370V min.
Switching frequency = 67KHz.

The switching FET is within the PWM Controller IC.

It is the STR2A153D from Sanken, and they are soon going to send us the SOA plot for the FET on this chip.
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I am trying to perform a check to see whether or not we are conforming to the switching FET’s “Safe Operating Area”. (SOA)

I believe I can do this on the oscilloscope by taking simultaneous drain voltage and current measurements……and then accumulating these scope images whilst putting the flyback through its most strenuous paces.

After doing this , I believe I can examine the accumulated scope shots so as to see if we are conforming to the SOA.

Do you know how I can actually do this, or if my method is correct.?

I have a Yokogawa DL9140L oscilloscope, but the user manual does not tell how to do this.

 

[mtwieg]

Sure a scope is the right way to verify the design, but you'll need probes that can handle decent voltage, and won't couple too much EMI. Also a small current probe would probably be very useful. Measuring drain current may be difficult without modifying the leakage inductance and overshoot voltage, so that may be tricky. You probably want to measure the current on the source with a small current sense resistor or CT.

 

[dick_freebird]

If your 'scope will do X-Y mode, piece of cake.

If it will export data to .CSV, you can X-Y plot it too.

Getting a believable current reading that doesn't mess
up operation, that's the trick.

And DC SOA may be more conservative than pulsed
SOA. Be sure you are looking at the right basis of
ratings.