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Electronics => Electronics => Topic started by: airman00 on July 26, 2009, 07:53:19 AM

Title: Why don't the MOSFET leads burn?
Post by: airman00 on July 26, 2009, 07:53:19 AM

I'm designing this 20amp H bridge and I need really wide PCB traces - .7" wide actually. My question is that there are MOSFETs that are rated for something like 40 amps, can they really handle 40 amps all on their own? I mean won't the MOSFET's leads burn out once that high current runs through it? Is it some special metal?

Title: Re: Why don't the MOSFET leads burn?
Post by: wil.hamilton on July 26, 2009, 08:35:49 AM

I doubt they're made of special metal.  The reason your traces have to be really wide is because they are very thin.  Think about it, a normal copper wire may not be very wide, but its the same thickness in all directions, a trace on a PCB has width but very little depth, they're almost paper thin.  Thats why you need super wide traces, but the wire (in this case connectors) dont have to be super wide.

Honestly, I could be wrong here, but this is what my intuition is telling me.

Title: Re: Why don't the MOSFET leads burn?
Post by: Soeren on July 28, 2009, 05:50:48 AM

Hi,

Quote from: airman00 on July 26, 2009, 07:53:19 AM

[...] there are MOSFETs that are rated for something like 40 amps, can they really handle 40 amps all on their own?

Sure, just like a 150A device can handle 150A.
Current doesn't burn anything... Power does!  (At least if you don't cool it).

40A through a resistance of say 0.001 Ohm is just 1.6W (and the resistance is probably much lower - haven't got time to calc it right now).

Title: Re: Why don't the MOSFET leads burn?
Post by: Truckstop on August 04, 2009, 05:44:25 AM

7 inch wide tracers? wow. if you don't have room or want smaller tracers you could. and i know this can be messy but, use smaller tracers and add solder to them to make them thicker. wil's explanation is awesome about why they need to be so big, if cross connection is an issue you can green pen the tracers or dip/ spray the board with something to prevent further connection with a foreign object. just an idea if you have a 12 - 120 inverter laying around crack it open and look at some of the tracers, you'll see what i mean.

Edit:  Spell Check

Title: Re: Why don't the MOSFET leads burn?
Post by: Admin on August 04, 2009, 08:14:42 AM

Its all about cross-sectional area, not width :P

Also, PCB traces use copper, but your MOSFET uses something else. So, different resistance . . . copper probably has a lower resistance, though . . .

Title: Re: Why don't the MOSFET leads burn?
Post by: wil.hamilton on August 04, 2009, 08:19:48 AM

Quote from: Admin on August 04, 2009, 08:14:42 AM

Its all about cross-sectional area, not width :P

yea, thats what i was getting at when i said traces had to be wide b/c they were so thin