load switching with mosfet power loss calculation - Page 1
Forum section: electronics / beginners

author: waspinator
author_rank: Contributor
timestamp: June 05, 2016, 02:55:29 am
content: I'm trying to turn on a 24V rail powering a 2.4A device using an N-channel MOSFET (IRLB8743). I'm having an issue understanding the basics of how it's workingThe MOSFET has an RDSof 3.2mOhms, at a VGSof 5V. The load is 10Ohms.I tried hooking it up as shown.The current going from the 24V power source to ground is about 0.22A, or 10X less than I expect it to be, and the MOSFET gets to about 100 degrees Celsius in about a minute while attached to a 9.6 °C/W heat sink.On Semi's application notefor MOSFETs in Load Switch Applications says that PLOSS= ILOAD2* RDS(on).I'm not exactly sure how to get ILOAD, but if just using Ohm's law it should be I=V/R or 24/10 = 2.4A. I'm actually only getting 10X less than that though. But even if ILOAD= 2.4A, than PLOSSwould be 5.76 * 3.2mOhms = 18.4mW. That seems like a very small amount of power, so why is the MOSFET getting so hot?What am I not understanding here?

author: waspinator
author_rank: Contributor
timestamp: June 05, 2016, 03:38:07 am
content: Quote from: blueskull on June 05, 2016, 03:09:18 amcan you point me to a place where I can learn more about driving MOSFETs? The places I've seen explaining how to use MOSFETS seem to suggest that applying VGSis enough to close the "switch". The datasheet for example says that it's RDSvalue is 3.2mOhm at a VGSof 5V, and that it's VGS(th)is 1.8V.Would my best bet be to get agate driver? Any suggestions on which one would work well with a FQP30N06L or IRLB8743?

author: waspinator
author_rank: Contributor
timestamp: June 05, 2016, 04:17:51 am
content: oh I see. So all I need to do is put the load above the transistor and it should work. I didn't realize that the effective VGSbecomes so much higher in the high side configuration. I don't see why you would ever use a MOSFET in the high side configuration, but is that the only time you would use a MOSFET driver? (other than when the VGSis naturally very high)this video cleared it up for me I think.Thanks for pointing me in the right direction.

author: danadak
author_rank: Super Contributor
timestamp: June 05, 2016, 11:19:36 am
content: http://www.onsemi.com/pub_link/Collateral/AND9093-D.PDFhttp://www.ti.com/lit/an/slva652/slva652.pdfhttps://www.renesas.com/en-us/doc/products/transistor/apn/rej05g0001_pmf.pdfThese might help.Regards, Dana.

author: waspinator
author_rank: Contributor
timestamp: June 05, 2016, 04:19:39 pm
content: Hmm, so if I wanted to control power going to two devices that have their grounds connected to each other, would low side switching be a concern?For example switching a Raspberry Pi at 5V and a Arduino at 3.3V, and then connecting them together through a USB cable.When both MOSFETS are off would the Arduino's ground be actually higher than 3.3V and have current flowing backwards?

author: Ian.M
author_rank: Super Contributor
timestamp: June 05, 2016, 04:35:23 pm
content: That obviously wont work and may actually damage something if there are any other ground referenced connections. If you need simple high side switching of moderate currents at voltages less than 20V, use P channel MOSFETs with a gate driver or level shifter to drive them.

author: waspinator
author_rank: Contributor
timestamp: June 05, 2016, 04:41:40 pm
content: so would this kind of configuration work?Edit: I think I had the P-MOSFETS connected backwards

author: Ian.M
author_rank: Super Contributor
timestamp: June 05, 2016, 04:48:27 pm
content: No. You've got your MOSFETs backwards and their body diodes will always conduct.For a P-MOSFET high side switch the source goes the supply and the drain to the load. Also, to turn them off, the gate must be taken right up to the supply rail so Vgs=0.

author: waspinator
author_rank: Contributor
timestamp: June 05, 2016, 11:41:04 pm
content: Could I control a 52V rail using 5V logic?Something like this wouldn't work for high voltage (>25V) rails because it would exceed the maximum VGSthe FQP27P06?But would something like this work? The Vgs would be -21V, which would be below the +-25V limit