Author Topic: Flawed method of testing MOSFETs?  (Read 594 times)

Offline John123

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Flawed method of testing MOSFETs?
« on: March 01, 2020, 03:22:20 PM »
Would what this guy is doing be classed as a flawed method of testing power MOSFETs? https://www.youtube.com/watch?v=YWXgMJz9IBU

To summarize, he's connected a HV ferrite cored transformer to some kind of single ended oscillator driver (not sure but could be a 555 timer) and is swapping MOSFET's on the fly to determine their worthiness based on how big of an arc he can make when used to drive the primary coil, but in a few tests he had to retune the circuit to get any kind of spark.

Is this just a case of parasitic capacitance being changed with every different fet thus the harmonic sweet spot for best arc size is being shifted around? He deems any fet which doesn't work as "not fast switching". Obviously on resistance and the breakdown/avalanche rating of the fet probably has something to do with the final output voltage (it's being run with no snubbing at 15v so avalanche ratings are probably going to clamp and limit some of them).

I'm not trying to be mean to the guy as I like his channel, but to me it doesn't seem like an accurate test.

Offline johnf

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Re: Flawed method of testing MOSFETs?
« Reply #1 on: March 01, 2020, 07:14:43 PM »
Oh Dear
His method is only valid for his poor layout and driver characteristics
You can measure gate capacitance at zero VGS with a cap meter to get an idea of gate charge. This then tells you how much drive is required @ "X" frequency.
However the correct way of measuring the important parameters is usually given in the datasheet for the device, and no datasheet shows his method!!!

Offline John123

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Re: Flawed method of testing MOSFETs?
« Reply #2 on: March 02, 2020, 06:35:13 PM »
Oh Dear
His method is only valid for his poor layout and driver characteristics
You can measure gate capacitance at zero VGS with a cap meter to get an idea of gate charge. This then tells you how much drive is required @ "X" frequency.
However the correct way of measuring the important parameters is usually given in the datasheet for the device, and no datasheet shows his method!!!
Ah, so it's just pure luck in regards to the size of arcs he's getting between each fet?
« Last Edit: March 02, 2020, 07:16:37 PM by John123 »

Offline johnf

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Re: Flawed method of testing MOSFETs?
« Reply #3 on: March 02, 2020, 07:03:23 PM »
No
I think it more that a particular fet fits the circuit and stray C and L that his circuit presents

Offline John123

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Re: Flawed method of testing MOSFETs?
« Reply #4 on: March 02, 2020, 07:26:17 PM »
A bit after 7:48 he kills a fet, could this be down to those flying leads draped over the HV transformer?

This must be what it's like when you guys interact with me, I probably don't know as much as I think I do either.

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Re: Flawed method of testing MOSFETs?
« Reply #4 on: March 02, 2020, 07:26:17 PM »

 


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