How can MOSFET pin (legs) handle the current?

Discussion in 'General Electronics Chat' started by mahela007, Oct 18, 2011.

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  1. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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  2. crutschow

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    Mar 14, 2008
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    Why are you worried about the MOSFET pins? The pins in the MOSFET package are designed to be large enough to readily carry the rated MOSFET current. Your only concern is that the wires or traces attached to the MOSFET source and drain are large enough to carry the current without significant voltage drop.
     
  3. mahela007

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    Jul 25, 2008
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    because the pins seem smaller in cross section than 22guage copper wire. the 22guage wire has a current limit of 7A.

    Anyway, I was also wondering how on attaches a MOSFET like this to other components. It is simply a matter of getting a PCB with wide enough traces?
     
  4. Wendy

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    Mar 24, 2008
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    Actually it is a very good question. The short answer is they can't always. I have worked on designs where I had to regularly replace the MOSFETs because the leads were too small. Generally when this happens you parallel MOSFETs (which is what they did), but it only goes far.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    additional heating of the mosfet due to the lead size is important and can be an issue. Typically you would make the circuit board have "oversized" traces in that area if possible to try and sink some of that heat.

    The ampacity rating of wire is typically based on a certain small temp rise over a certain ambient. Pins on the mosfet may be allowed to rise much hotter than wire and often do. I'm currently using a TO-263-7 mosfet and running 50 Amps through it. Granted it has multiple source leads but the lead size DOES effect device temperature (slightly). Its also only for a short distance so its not nearly as bad as you would think. I'm sure that if size wasn't an issue IC designers would LOVE to increase pin size to reduce heating.

    Fuses also show this issue. The element in a fuse is MUCH smaller that the wire its protecting and does cause the fuse body to heat up significantly. But that's how they work and the element MUST be that small or it wouldn't blow when needed.
     
  6. #12

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    Nov 30, 2010
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    I think strantor has been working with this lately. Conversations on this site about 75 amp internal wiring limit mosfets, but rated at 100 amps. You always consider the specifications when designing stuff, but you can't always trust the specifications!

    I've seen a lot of toasted circuit boards because some designer believed the datasheet and didn't allow for extra heat dissipation around a driver chip.

    You end up using every trick you can think of to get rid of the heat. Use PC board with double thick copper, even double sided copper, attach heat sink or heat pipe to the MOSFET, put 2 MOSFETS in parallel with each other, possibly even add a fan.
     
  7. colinb

    Active Member

    Jun 15, 2011
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    These current limits probably assume the wire is insulated and may have no easy thermal conductive path to a better thermal conductor. On the other hand, the MOSFET leads can easily conduct thermal energy into the MOSFET's case and its heat sink.
     
  8. mahela007

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    Jul 25, 2008
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    So would you guys say 14Amps is reasonable for a To-220 package? (through the drain / source pins)
     
  9. tgotwalt1158

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    Feb 28, 2011
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    When designing the devices such like MOSFETs, the designer's task is to keep balance between transistor power capabilities, thermal withstanding capabilities and physical size. The short foot print of pins is compensated by cooling the case by back metal plate which could further be cooled by attachment of extra heat sink material usually a piece of aluminum. Further, the Drain Source Voltage are selected much higher such that drain current from pin leads would be at least acceptable or acceptable with the heat sinking. The analogy of fuse does'nt work here since the melting point of fuse material is determined by the amount of dissipated energy which is IxIxR not ExI. Also, the the density of pins is kept much higher for the same reason as compared to fuse wire or winding wire.
     
  10. jesse_a_b

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    Mar 16, 2013
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    If you look at the wire gauge chart, the fusing current after 10 seconds for gage 19 wire is 70A.

    But that's sad considering the datasheet said my mosfet could carry a continuous 150A...
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Welcome to AAC.

    Congratulations, you have practiced the arcane art of necromancy, the revival of a long dead thread. Likely the OP (Original Poster) has solved his problem in the years that has passed, or thrown it away, or something.

    Try to keep it current please.
     
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