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Topic: How come my MOSFET was a "smoking", I thought it was rated high enough. (Read 5 times) previous topic - next topic

apples

ahh, so instead of being the 120deg as mentioned before with no heatsink, with the heatsink it would only be 38deg then?

Grumpy_Mike


oric_dan

No one seems to have mentioned it, but your driver ckt is completely wrong.

You do not bias MOSFETs the same way you bias BJTs. A Darlington ckt is
wired as it is to increase the base-current drive into the 2nd BJT, ie IC1
becomes IB2.

What you need to do with n-MOSFETs is make the gate-source voltage large
[ie, typically 10V plus]. With your ckt, you are actually losing gate-source
voltage for your 2nd MOSFET.

1. as others mentioned, try a logic-level MOSFET, eg IRL540, where you'll
    get a good turnon with Vgs=5V.

2. go look up how to design a proper MOSFET driver.

MarkT

OK that driving circuit is plain wrong.  The first device (for which you've used an NPN symbol, not an n-channel FET) is in source-follower mode - this is no good at all, it won't pull up the second devices gate more than a volt or two.

The first device must be a common-source stage, with a drain load resistor to the +12V rail (something like 1k perhaps).  This will level convert from 5V to 12V (and invert the logic).  This can then drive the second devices gate at suitable levels to actually turn it on.

[ I won't respond to messages, use the forum please ]

apples

yeah righto , totally confused now, lol.

This is where I get my parts from as the hve a shop in town.  http://www.jaycar.com.au/index.asp

Can some one find me a suitable mosfet, the one you mentioned the IRL540 is not listed (I do not think) . 


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