Mosfet output low under load - Page 1
Forum section: electronics / beginners

author: magEle
author_rank: Newbie
timestamp: March 18, 2017, 09:12:19 pm
content: I have been working on a project to control the lights in my room, where all of it is controlled by a pwm signal from a Arduino uno to logic level mosfets that control the lights. (lights in this case being regular led strips).And when i tested it on my desk with just little bits of led strip everything worked great, all of the leds dimmed like they were supposed to.but when i connected the biggest piece of led strip to it the mosfet´s output wouldn´t go higher that 9-10 volts. (This led strip needs 3.6A at 12V)i have a pretty big heat sink on it, it peaks at 70 C with the big led strip connected to it.I started with 30N06L, then i tried to change the mosfet to a IRF520 but the problem was still there so i figured that i was doing something wrong.i have a hard time understanding what is happening here because both of the mosfets are rated over 3.6A.https://cdn.sparkfun.com/datasheets/Components/General/FQP30N06L.pdfHere is a schematic of the part of the circuit that i am having problems with.(The IC is supposed to be the entire UNO board, i am not using just the atmel)

author: rx8pilot
author_rank: Super Contributor
timestamp: March 18, 2017, 09:24:14 pm
content: The 5v from the UNO is not enough to turn the IRF520 all the way on. You need a logic level FET. I assume the MOSFET gets rather hot during operation?

author: magEle
author_rank: Newbie
timestamp: March 18, 2017, 09:33:04 pm
content: Yes i know, but as i said i have the same problem with the 30N06L.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 18, 2017, 09:45:30 pm
content: Is your UNO 3.3v?

author: MK14
author_rank: Super Contributor
timestamp: March 18, 2017, 09:56:23 pm
content: I don't understand.The Mosfet (as shown) can only bring the voltage down to 0 (ground).(Assuming the Mosfets are working ok) Not reaching 12V would suggest your 12V power supply is not strong/good enough to reach 12V (or the Mosfet is remaining partially on because it is faulty or something, Unlikely).I'm talking about the OUTPUT voltage. Maybe you are talking about the voltage ACROSS the Leds, in which case, ignore me. It would mean (as said above), the Mosfets are not being properly switched on (or faulty).Is the 12V supply remaining at 12V when this happens ?

author: rx8pilot
author_rank: Super Contributor
timestamp: March 18, 2017, 10:03:21 pm
content: It appears the source is grounded and the UNO would pull the gate up to whatever voltage it is running on 3.3v or 5v (not sure). If 3.3v - it may have the FET in the linear region.

author: MK14
author_rank: Super Contributor
timestamp: March 18, 2017, 10:09:48 pm
content: Quote from: rx8pilot on March 18, 2017, 10:03:21 pm(after re-reading the opening post) I agree with you. Something is very wrong, because they seem to be saying it is getting to 70 deg C, even with a big heatsink on it.I was taking the Output to be the low side of the bulb, with the high side of the bulb to be the +12V.As you said it is not turning fully on (too low a drive voltage on the gate) or the PWM frequency/waveforms are too fast (no fast mosfet drivers), and the Fets gate capacitance is meaning it doesn't have time to fully turn on and off, and so is getting very hot with the switching losses.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 19, 2017, 12:36:08 am
content: The irfp260 is in the same family of mosfet, the data sheet for it says its a standard mosfet. Are you sure your using the correct part?

author: magEle
author_rank: Newbie
timestamp: March 19, 2017, 07:28:33 am
content: Quote from: MK14 on March 18, 2017, 10:09:48 pmThat sounds plausible. What would be the fix for this though? could i lower the freq of the arduinos pwm for example?I still think its wierd that it works fine with a small load

author: danadak
author_rank: Super Contributor
timestamp: March 19, 2017, 12:22:33 pm
content: The 520 Vgs to get Rdson is 10V, the 30N06 is 5V, so if your UPis 5V only the latter will achieve a hard on low Rds state.The bigger issue I see is you are trying to drive the MOSFETs gateswith a UP I/O pin, which cannot achieve rapid turnon/tunroff ofthe Gate charge, eg. high capacitance associated with MOSFETgate. Hence it is in the linear region for long period of time anddissipating power. Give a serious consideration to using a gatedriver. Note when you do there will be very rapid dI/dT that willchallenge your ground layout. Many vendors have ap notes on thistopic. If you do not address this you can trigger erratic effects inUP and other logic.Regards, Dana.

author: MK14
author_rank: Super Contributor
timestamp: March 19, 2017, 06:52:56 pm
content: Quote from: magEle on March 19, 2017, 07:28:33 amWhat frequency are you trying to PWM the Leds with ?The MCU pin can only control a rather small current, such as (approx) 10mA (would need exact MCU model no, and which pin exactly, to look up datasheet value). So even a low frequency, such as 1 KHz, is probably far too high, without using a Mosfet (gate) driver (there are other ways of doing it, such as using bipolar transistors, to drive the fets gate).You could try a much lower PWM freq to see if it helps, such as 50 Hz.The reason very low loads seemed to work ok (when you tried them), could be because they need much less current, so the linear mode of the Fet (during switching transitions), was able to supply it with little/no voltage drop.What other posts have said about the gate voltage being too low for the FET. May also be part or fully what your problem is, as well.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 19, 2017, 11:21:02 pm
content: OP - do you happen to have access to an oscilloscope by chance?