I'm replacing a bunch of burnt mosfets in a linear amplifier for a brushless motor.

  • 3 phases
  • Each phase: 3 pairs of totem IRFP240 / IRFP9240 in parallell. (no R in series)

My main concern is matching the new triple-pairs.

Which would be the most appropriate matching setup for the purpose. Unit is driving 80V/10A motor. I currently use the following matching rigg, but since MOSFETs isn't my domain I'm abit uncertain of the validity of the matching in accordance with the application.


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Do your matching at a much higher current that better matches the real motor operation .Consider individual source resistors if your drive has a habit of blowing up . \$\endgroup\$ – Autistic Oct 27 '15 at 2:01
  • \$\begingroup\$ Thanks, that was what I thought. Problem: increasing current adds increasing temperature to MOSFETs adding a variable. To match as a higher and same current AND under the same temperature, the match-rigging becomes complicated. Ideas? (without PID-regulated fan...) \$\endgroup\$ – user247245 Oct 27 '15 at 9:27
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    \$\begingroup\$ High current under very low duty cycle (<1 %)? \$\endgroup\$ – user247245 Oct 27 '15 at 10:04

1st -- matching probably isn't critical for this application. If you buy the FETs from the same source, and they are from the same date code, they will likely be closer than you can measure in a reasonable way.

2nd -- if you are matching them, you probably want to match RDSON -- the on resistance. Connect the gate to (say) +10 V, and measure the resistance between source and drain. Likely it will be << 1 ohm, and hard to measure in a straightforward way -- if you are familiar with Kelvin measurements, you could do it that way.

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