----- Original Message ----- From: "Mark E. Skeels" To: "Microcontroller discussion list - Public." Sent: Wednesday, May 19, 2010 6:59 PM Subject: Re: [EE] Using a p-channel power mosfet for supply polarity reversalprotection > Russell McMahon wrote: > > [SNIP] >> All that is correct. >> Except that the MOSFET DOES turn on when the MOSFET body diode is >> forward biased. >> ie MOSFET is in quadrant 3, with DS polarity the opposite of normal >> BUT the Vgs voltage the correct polarity for turn on ie gate more >> negative than source for a P channel device. >> >> >> > Last time I thought about "quadrant" was in relation to triacs...... > > To clarify, > > * you are saying that, in my circuit, the mosfet D-S junction is on > IN ADDITION to (in spite of?) the body diode conducting? (thus > giving the possible advantage of lower voltage drop/power > dissipation than the body diode alone can provide) > * you are NOT saying that the fact of diode conduction is THE SAME > as the D-S junction conducting? > > OK, so you say the mosfet _IS_ on. I think it is too. I think that Russell means (I'd like to know which myself) that the D-S junction is on BECAUSE the body diode is conducting, as opposed to the above in addition to. For example, reverse supply: D = 16V G=0V S= 14.4(body diode 1.6V drop from D) At least that's the way it looks to me on the circuit shown, and the only way I could see the MOSFET being 'on' (bias = -14.4V)? -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist