mosfet direction? - Page 1
Forum section: electronics / beginners

author: mercrazy
author_rank: Newbie
timestamp: November 26, 2017, 02:29:23 am
content: 1. is it ok to connect a mosfet source and drain in either direction as long as i take the body diode direction into account?will they conduct the same in either direction?2. on a pnp, if the gate is always tied to ground and only to ground, when the source or drain voltage goes from high to low, will it be turned off and stop conducting?thanks

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 26, 2017, 02:36:29 am
content: Quote from: mercrazy on November 26, 2017, 02:29:23 am1) Yes this is perfectly o.k. In Ideal Diode applications the channel is turned on in parallel with the body diode to reduce the conduction losses.2) If by pnp, you really mean P. Channel. If you ground the gate and raise either the Drain or the Source above the MOSFETs gate threshold voltage, typically 2 to 5V, then the Channel will conduct.PNP and NPN are used to describe Bipolar Junction Transistors.Regards,Jay_Diddy_B

author: mercrazy
author_rank: Newbie
timestamp: November 26, 2017, 02:42:31 am
content: sorry, yes i meant Pchannel.so the channel resistance will be the same too?the conduction losses you mentioned is why i ask.i'm only using this to replace a schottky to eliminate the voltage drop.i barely have enough overhead voltage.thanks

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 26, 2017, 03:19:08 am
content: Hi,The channel conduction is the same in both direction. So you will get to RDS(on) if there is enough gate voltage.This model illustrates the circuit action:I have chosen the IRF9Z24 to illustrate the circuit action. There are much better MOSFETs for this application.These are the results:You can see the MOSFET changing from body diode conduction to channel conduction.R2 is to discharge the Gate.R1 is a representation of the load.Regards,Jay_Diddy_B

author: Peabody
author_rank: Super Contributor
timestamp: November 26, 2017, 04:17:11 am
content: Quote from: mercrazy on November 26, 2017, 02:42:31 amThis configuration is commonly used to provide reverse battery protection. As you say, it effectively functions as a diode but without the voltage drop. But note that it is installed backward from the orientation needed to use it as a switch. The body diode is forward biased under correct polarity, so there would be no way to shut off the current even if the p-channel is shut down. To use the transistor as a switch in "normal" orientation, the body diode would be facing the other way, so when the p-channel is off, no current will flow.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 26, 2017, 03:45:51 pm
content: Hi,Quote from: Peabody on November 26, 2017, 04:17:11 amThis circuit does not function as a diode. It functions as a switch. Once the MOSFET is turned ON, the circuit no longer behaves like a diode.In an Ideal Diode circuit, a servo loop holds the voltage across the MOSFET at around 20mV. It does not fully enhance (turn on) the MOSFET. It does this so it can tell when the Ideal Diode is reverse biased, then it turn off the MOSFET.This circuit is good for polarity protection, but as a diode.Jay_Diddy_B

author: mercrazy
author_rank: Newbie
timestamp: November 26, 2017, 09:27:48 pm
content: R2 is to discharge the Gate.Regards,Jay_Diddy_Bremember, i'm a complete newb on this stuff.what do you mean by R2 is to discharge gate?if it's tied to ground, i thought it would always be discharged?thanks