GGG posted:MTH has always used relays to switch polarity and FET to control motor current, but continue to speculate. It is fun to do analysis without facts
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What is interesting and is a change on PS-3 O boards is there is a Brake motor FET. I was told it is to reduce heat on the board.
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Did whoever told you it was to "reduce heat" actually call it a "Brake motor FET"? Or is that your term because it is wired into the motor drive circuit similar to the smoke fan configuration?
In pulse modulated circuits with inductive load such as a motor drive (but most commonly in switch-mode power supplies) it is now standard practice to use "synchronous rectification" to improve efficiency and hence reduce heat. So the familiar re-circulation diode associated with driving relay coils, motor windings, or power supply coils is now replaced-by or augmented-with a FET. A FET is a far more efficient switch than a diode with negligible voltage drop as compared to the tenths of Volts in a diode. Thus, the voltage x current switching losses attributed to the diode become much smaller. The trick is to turn the FET "on" at exactly the same time the original diode turns on - hence the term synchronous rectification.
If the circuit still worked (no melt down or over-voltage blowouts) with this new FET removed, then the original diode may still be there and the FET is augmenting (in parallel with) the rectification. You would have seen the benefit (reduced power loss) at higher power levels.
There's nothing magical about synchronous rectification. But you do need circuitry to manage when to turn on the switch which, as with any technique, can get tricky at higher frequencies (kHz, MHz) as found modern power converter designs. At slow frequencies (like 60 Hz) you could use an electro-mechanical relay to close/open when you want conduct/block current.
