MOSFET Switch Thermal Considerations...

Thread Starter

liquidair

Joined Oct 1, 2009
112
Hi guys-

Using MOSFETs as a switch, I'd just like to make sure my thinking is correct so I don't blow anything up. I am switching off and on relays, and the MOSFET will never see more than 500mA. I want to make sure I protect the part thermally but I don't see how the MOSFET will actually dissapate any power other than I^2 * Rds(on). Am I missing something and is a heatsink advisable if this stays well below 1W?

Thank you in advance!

Brian
 

SgtWookie

Joined Jul 17, 2007
22,210
Why don't you post a schematic of your proposed circuit?

It will help a good deal if you include part numbers.

This page is well worth the time spent reading:
http://mcmanis.com/chuck/robotics/projects/esc2/FET-power.html

Don't know what you're planning on using for MOSFETs, but check out these 4-pin DIP's rated for 2.4A, 60V, Rds(on)=100m @ 5v when Id=1.5a
[eta]
Forgot the link! here it is:
http://search.digikey.com/us/en/products/IRLD024PBF/IRLD024PBF-ND/812492
Datasheet:
http://www.vishay.com/docs/91308/sihld24.pdf
 
Last edited:

Thread Starter

liquidair

Joined Oct 1, 2009
112
I thought I posted a reply, but it never showed up.

The switching circuit is super generic like that found here under P Channel MOSFET switch:
http://www.electronics-tutorials.ws/transistor/tran_7.html

The MOSFET I am looking at is here:
http://www.fairchildsemi.com/ds/FQ/FQD11P06.pdf

I have no perticular fondness for this MOSFET just that it is IPAK. I read and did the calculations on the link you provided and it looks like I can switch 1A without issue (assuming a 40 deg. C ambient temp, which I doubt it will see) without a heatsink. Great link Sgt Wookie!
 

SgtWookie

Joined Jul 17, 2007
22,210
OK; I don't know what your supply voltage is. If it's <25v, you can probably get away with that switching scheme. If you're closer to 30v than that, you will really need some way to limit Vdss so that it doesn't approach ±30V; as the MOSFET will be destroyed if that happens.

I don't know for certain how you plan on switching the MOSFET on and off. The gate must never be allowed to "float" (that is, not connected to anything) even for a short period of time. If you are going to be using a hardware switch, then connect a 10k resistor from the source terminal to the gate. This will keep the MOSFET turned off by default.
 
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