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[DPRG] PWM Controller

Subject: [DPRG] PWM Controller
From: Chuck McManis cmcmanis at mcmanis.com
Date: Sun Mar 9 21:25:00 CDT 2008

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.

That being said, there are times when you might want a very fast 
Shottky diode there, the primary one being that you are switching the 
MOSFET at a very high rate of speed >1Mhz and you time it takes the 
parasitic diode to conduct you'd rather not wait. Or if you are 
running very nearly at the power limit of the MOSFET in question a 
small amount of power dissipation is avoided by conducting the back 
EMF through a flyback diode. Its important to note however that 
unless that flyback diode has a lower forward threshold voltage and a 
faster switching time to the parasitic diode, it won't ever actually 
conduct any current.

--Chuck


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