Could you suggest 2 Mosfets? - Page 1
Forum section: electronics / beginners

author: Tricka90
author_rank: Regular Contributor
timestamp: November 07, 2013, 09:04:11 pm
content: I need a N and a P Mosfet. They must bethe most economic and popular as possible(I should find them on the local electronic components store).I'm going to use them in the following configuration to drive the RL load with a 1kHz PWM signal from a PIC.So the N-Mosfet should totally saturate when the gate voltage is around 4-5 V (when the PWM signal is ON) and should have a response quick enough to operate at 1kHz.The P-Mosfet should also have a quick response and should have a max drain current of at least 2A (that's the current which the RL load will absorb).Do you have any advice? Thanks a lot to anyone who will help :-)

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 07, 2013, 09:29:43 pm
content: Dunno where local is for you, but here are a couple:http://www.radioshack.com/search/index.jsp?kwCatId=13384949&kw=n%20mosfet&origkw=n+mosfet&sr=1http://www.radioshack.com/search/index.jsp?kwCatId=13384949&kw=p%20mosfet&origkw=p+mosfet&sr=1Radio Shack, sorted by popularity.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 07, 2013, 10:24:41 pm
content: QuoteI think you might want something like figure 20 in this document.You can use BC807 and BC817 for the transistors instead of the N Channel fet.If you want to use your original circuit, ( I wouldn't) then you need a logical level fet for the N channel and normal for the P channel.Make sure the rds is low for the p channel and that it can handle the 2 amps heat wise as well as current wise.You didn't specify what packages you wanted to use, smd or through hole.

author: Tricka90
author_rank: Regular Contributor
timestamp: November 08, 2013, 07:56:10 am
content: Quote from: HackedFridgeMagnet on November 07, 2013, 10:24:41 pmExcuse me, could I ask you why you wouldn't? And why do you think should be better to use transistors instead of mosfets for this application?I need through hole packageBy the way, the NPN transistor you suggested can only supply a maximum collector current of 500mA. I need at least 2A.

author: flolic
author_rank: Frequent Contributor
timestamp: November 08, 2013, 08:18:02 am
content: Consider using logic level mosfet.

author: dr_p
author_rank: Frequent Contributor
timestamp: November 08, 2013, 08:34:51 am
content: you need RL to be common ground? cause if you don't care, just place RL instead of the 1K from 12V and switch the ground. Elliminate the second transistor as well.You can use any N-ch TO220 FET and drive it with 5V, but: if it's not a logic level (VGs(th)=1...3V) but instead a regular FET (VGs(th)=2...4V), then ant 5V it will not be fully ON, but in the linear region. So RDSon will be high and the FET gets hot plus the load doesn't get the full 12V because there's a voltage drop on the MOSFET.So: - for low currents (~ couple hundreed mA) you can live with the MOSFET not being fully ON, so you can use any MOSFET that can handle let's say twice the current you want. There will be some heat dissipated, but probably not too much. If you have to use a heatsink at these low currents then you're probably doing it wrong.- for high currents you have to use logic level MOESFETS that are fully ON at 5V, because the effect above will be much accentuated.mind you:Quote

author: Tricka90
author_rank: Regular Contributor
timestamp: November 08, 2013, 08:57:04 am
content: Thank you a lot for all the useful informationsYes I absolutely need RL to be common ground so I must use two mosfets.Thank you for teaching me about the parameter Vgs. Please can I ask you what other parameter should I check to make sure the mosfet has a response quick enough to switch at a certain frequency? (in this case 1kHz)

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: November 08, 2013, 10:04:14 am
content: QuoteI was just suggesting changing the level shifter to a transistor and adding a push pull bjt pair so the gate charges faster.the circuit i think still has the p channel fet, which is your main drive transistor.Quote2n2222 and 2N2907 are through hole.Quotethe 2 amps only goes through the p channel fet. The transistor I suggested is ok and is cheap.This should work up to say 100khz.A large Gate capacitance will slow the switching of a FET and therefore waste energy as heat, which can then cause problems.