P-channel mosfet driver suggestion - Page 1
Forum section: electronics / projects

author: psam
author_rank: Newbie
timestamp: March 28, 2018, 02:55:46 pm
content: Hello,First i am newbie in electronics. I want to replace automotive car relay with P-channel mosfet as high side driver, so what kind of npn transistor to use as Q1 and what are needed resistor resistance values? Control signal to turn on mosfet is +12v. I want to use as less parts as possible. Maybe there is p channel mosfets with built in drivers in to220 package, or better to use optocoupler, or any other options that i dont know? Max current at load will be at 5A.

author: KhronX
author_rank: Frequent Contributor
timestamp: March 28, 2018, 03:00:05 pm
content: How fast do you need / want the FET to switch on / off?

author: psam
author_rank: Newbie
timestamp: March 28, 2018, 03:05:38 pm
content: speed is not priority here, just as typical car relay, even half second is ok.

author: KhronX
author_rank: Frequent Contributor
timestamp: March 28, 2018, 03:14:14 pm
content: You won't need any specific transistors. Just about any NPN should be fine, with a couple of 10k resistors.Similar arrangements can be found in just about any / all laptops, on the input power selection / routing (charger input, battery etc).

author: psam
author_rank: Newbie
timestamp: March 28, 2018, 03:41:39 pm
content: maybe you can suggest to220 package pnp transistor in place of mosfet to use as switch, this will elminate npn transistor in shematic, only is there transistor that will hold 3-5 amps without heat sink?

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 28, 2018, 03:42:38 pm
content: If speed is not a concern, you won't need much current (for the rated gate charge of the PMOS), so I think the resistor in series with the base of the NPN transistor should be as high as practical - probably in the order of 10k or above - so that this transistor is properly protected from overvoltage. The +12V inside a vehicle is not very clean so don't hesitate to protect your circuit as appropriate.

author: KhronX
author_rank: Frequent Contributor
timestamp: March 28, 2018, 04:01:47 pm
content: I'd stick with a PMOS as the pass element. PNP's will have a certain voltage drop at saturation, which will likely result inmorepower / heat that needs to be dissipated, than what would be the case for a PMOS with a few dozen (or hundreds) of milliohms on-resistance.Not to mention, no drive current needed

author: sourcecharge
author_rank: Regular Contributor
timestamp: March 28, 2018, 05:03:06 pm
content: Make it more simple:get rid of the transistor and the resistor on it's base.Put the switch between the gate of the mosfet to ground.Turn the switch to the orientation necessary.The resistance on the gate can be high due to the manual switch. >10kohmsIf bounce is a problem, try using different higher resistances, or use a de-bounce circuit.Hope that helps.

author: psam
author_rank: Newbie
timestamp: March 28, 2018, 05:24:19 pm
content: Thanks for answers.Switching is controlled by external 12v signal from the same power supply, so no manual switching. Ended with PN2222 as driver for pmos, and two 10k resistors, everything works fine.