Friday, March 4, 2011

5-6 Wrong FET Specs and Poor heat dissipation.

So basically the previous four instances of me screwing up have snowballed up into one massive snafu ending in one, charred H Bridge.  At the risk of sounding redundant, reading the datasheets is EXTREMELY important.  For example, when I looked up the MOSFETs I was going to use, I read that they were rated for 150A, which was the peak current for 12V on the supposedly accurate fisher price motor curve graphs.  So I figured that it was fine, didn't care about any of the other confusing terms on the datasheet, and went ahead and soldered it all together.  This, however, is not the case.  If you don't RTFM, you won't learn things such as, while the FET's silicon may be able to handle 150A, the actual leads of the package that it is inside of will actually melt at 75A.  So that's pretty misleading, but whatever.

Next is the issue of heat dissipation.  Now I'm not the best at math, but basically what's required is to use the Rtheta factors on the data sheet to calculate the amount of heat the package can dissipate without melting.  As I've stated before, since I know I'm not the best at explaining things, here's a link (this will be a common thing, me linking you to helpful sites) that will help a great deal and walk you through the steps of calculating how much it current a FET can ACTUALLY handle.

I've found it to be very helpful, and realized that the FETs I was using could only handle about 20A before melting.  Read through it a couple of times to absorb everything.

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