P channel MOSFET: driver required? - Page 1
Forum section: electronics / projects

author: Bas
author_rank: Newbie
timestamp: August 22, 2017, 11:57:19 am
content: Hi everyone,I'm working on a board which uses eight separate P channel MOSFETs (IRF9530) as high side switches. The max current the FETs switch is around 2.5 A, and the high side voltage is just 5 V. The max gate charge of these FETs is 38 nC and the gate-source capacitance is around 800 pF. The frequency of switching is around 8 kHz.As a prototype, a microcontroller (ATmega) is used to directly switch the MOSFETs. Only one MOSFET is switched on at the same instant.This is all working without issue, but I was wondering whether using the outputs of the ATmega to drive the MOSFET gates directly is a good plan, or a high side gate driver is necessary. Can anyone share some experience about this or suggestions? Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: August 22, 2017, 12:25:02 pm
content: It should be OK if only one output ever switches at a time (switch-off is just as much a problem as switch-on) and you have gate resistors to limit the transient current charging/discharging that 38nC to less than the ATmega's maximum I/O pin output current limit (+/-40mA for ATmega328P). However the worst case losses in the MOSFETs may be excessive. Modelling the switching losses is far from simple.

author: nugglix
author_rank: Regular Contributor
timestamp: August 22, 2017, 12:33:45 pm
content: Given my datasheet (see below) the Vgs is -4V (for 250µA).So given your 5V supply I doubt that the 2.5A can be reached safely.Better choose a real logic-level MOSFET.

author: madires
author_rank: Super Contributor
timestamp: August 22, 2017, 12:51:15 pm
content: I second a logic level MOSFET, maybe an IRLML 6401 or 6402.

author: Bas
author_rank: Newbie
timestamp: August 22, 2017, 12:56:44 pm
content: Quote from: Ian.M on August 22, 2017, 12:25:02 pmYes, only one MOSFET is switching at a time, switch on and off of different outputs is sequential and not at the same instant.Quote from: nugglix on August 22, 2017, 12:33:45 pmQuote from: madires on August 22, 2017, 12:51:15 pmI understand, this is a prototype on stripboard and I'm planning to look for another MOSFET anyway which is surface mount for a PCB prototype. Would the FDD4243_F085 be a good option in your opinion? It specifies r_ds = 48 m? at V_GS = -4.5 V and I_D = -5.5 A. Input capacitance is a bit higher, but still around 1 nF. Gate charge is lower, at max 29 nC.I see the IRLML 6401 and 6402 are surface mount as well, SOT-23. That would be great to have small size, -4.3 A continuous I_D at 25 degrees C and max V_DS = 12 V should be sufficient. Would the SOT-23 package be a problem with power dissipation? Quick calculation for 2.5 A and r_DS = 0.05 ? gives 0.31 W. Specification gives 0.01 W/deg C, so around 30 degrees C temp rise. Would that be an issue to avoid somehow?

author: nugglix
author_rank: Regular Contributor
timestamp: August 22, 2017, 01:40:29 pm
content: V_GS = -4.5 VSo even higher than the IRF... go think about it.

author: Bas
author_rank: Newbie
timestamp: August 22, 2017, 02:05:13 pm
content: Quote from: nugglix on August 22, 2017, 01:40:29 pmYes but that's achieved, right? When pulling the gate to ground, V_GS = -5 V. The threshold voltage for turn on is max -3.0 V. For the IRML6401 that's max. -0.95 V.

author: madires
author_rank: Super Contributor
timestamp: August 22, 2017, 02:35:58 pm
content: Quote from: Bas on August 22, 2017, 12:56:44 pmShould be fine too and the D-PAK would help with the heat dissipation. But with just 2.5A load current it's a tad oversized.Quote from: Bas on August 22, 2017, 12:56:44 pmYou've mentioned something about 8kHz switching. If you PWM the MOSFET that will reduce the average power loss accordingly. Unless there are some temperature contraints I don't see any problem.

author: Audioguru
author_rank: Super Contributor
timestamp: August 22, 2017, 02:55:00 pm
content: The IRFbarely(0.25mA) turns on when its Vgs is 4V. Some of them need 10V to fully turn on.The FDD is barely turned on with 3V and all are fully turned on at 4.5V.

author: Bas
author_rank: Newbie
timestamp: August 22, 2017, 03:49:50 pm
content: Quote from: madires on August 22, 2017, 02:35:58 pmYes, there are 8 MOSFETs which are sequentially switched at 8 kHz. So each is on only for 1/8 of the time. I did not use that in my calculation, that would bring it to around 0.039 W dissipated, so only 4 deg C. I'd like to use the IRLML as it has a small package, I'm looking at the graphs in the datasheet and it looks like at V_GS = -1.7 V they should already support -10 A source-drain current since the max V_GS threshold is only -0.95 V.

author: KL27x
author_rank: Super Contributor
timestamp: August 22, 2017, 08:20:57 pm
content: Agree with all said.If max duty cycle is 1/8, and at 8kHz, you can easily do that with SOT23 with room to hide a few elephants.Your FET is a fine choice if you're building one. If you are making thousands of them, it is pretty horrible on the most important spec. Price.DMG2301L-7