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[DPRG] PWM Controller

Subject: [DPRG] PWM Controller
From: Triffid Hunter triffid_hunter at funkmunch.net
Date: Sun Mar 9 21:34:32 CDT 2008

On Sun, 9 Mar 2008, Chuck McManis wrote:

> At 06:21 PM 3/9/2008, Triffid Hunter wrote:
>>  Don't forget a flyback diode across the motor! Electrically, it's a big
>>  inductor in series with a small resistor and a voltage source, and that
>>  inductance will easily destroy your mosfet when it switches off without
>>  something to limit the voltage to a safe level.
>
> Given the nature of how a power MOSFET is designed it always includes a 
> parasitic reverse biased diode between its drain and source pins. Because it 
> is built out of the transistors that make up the MOSFET it has the current 
> rating as the MOSFET as well. What that means is that a flyback diode is 
> unnecessary
> to protect the MOSFET.

The inherent diode won't protect it from a too-large d-s voltage caused by 
turning off an inductor in a boost-like arrangement, unless you like 
relying on reverse breakdown avalanche.

In a H-bridge, the high side mosfets' diodes protect the low side mosfets 
and vice versa, however this is a single mosfet controlling a motor in one 
direction only, so the motor's inductance could easily put a few hundred 
volts on the drain and the inherent diode would remain reverse biased and 
provide no protection.

Also, the inherent diode tends to have fairly poor characteristics (high 
forward drop and reverse recovery) which make it less than optimal even 
at frequencies as low as 50khz. Not sure about motor controllers, but 
they're useless for switchmode power converters.

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