We are using FQU30N06L for a brushless control board, the MOSFETs marked F37 or F45 work fine as they have to do at 55 VDC, others marked G21 do not work because they start to turn on at 35 VDC. Someone had the same problem? Ciao Bruno.
I would look for a better MOSFET.
IRFP240 IRFP9240 work great for me.
Bruno didn't tell us anything about the operating conditions, the circuit, or the PCB-layout implementation. All of these things are or can be important, depending on the circumstance. But I'll make one generalization: Noting the use of FQU30N06L parts, the L suffix being the relevant factor, these are the low-gate-voltage logic-level version of the part. These have an unusually-low gate voltage of 2.2 volts at 1.0 amp (ordinary FQU30N06 parts show nearly twice the voltage, 4.3 volts at 1A). My point is that low gate-voltage MOSFETs are not well suited for many switching applications, especially in a half-bridge or H-bridge, because they're much harder to turn off quickly.
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