EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: lk on September 02, 2013, 06:33:29 am
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Greetings,
I got an update from microchip for one of their power mosfets http://ww1.microchip.com/downloads/en/DeviceDoc/20005159B.pdf (http://ww1.microchip.com/downloads/en/DeviceDoc/20005159B.pdf), the low Rds ON made me curious. And then i noticed something, the the bottom pad is connected to the drain of the device, and this is a puzzle to me, I realize there is a perfect good explanation for it, it just escapes me.
In my little limited world i would have been made more sense to connect the pad to the source, as usually the N-channel mosfets are connected after a load. And it would therefore be able to dissipate to a large ground plane, rather to what you would need to create for it on the drain side.
The only thing i can come up with is to protect other "random" devices on the board from the changes in temperature. If i look at a device like the lm7805 in TO-220, ground is on the tap, so it could in principle dissipate to the ground plane, perhaps there has been a change in best practices between the creation of the two devices
Anyway im just curious :)
And to bad that the MCP87130 is only available in one of those no lead packages, it would have been nice in a S08 or something, i have seen Daves mess after trying to solder one of these small leadless packages :P
-lk
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The tab connection depends on the way the silicon is doped. The bottom of the silicon is bonded to the output tab, and then bond wires are used to wire to other parts of the device as needed. it all comes down to pricing, and if it is cheaper to make the tab the input then so be it.
-kizzap
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Consider where the voltage drop occurs when the device is passing a current. For bi-polar transistors this is obviously the collector junction, for MOSFETs it's less obvious but presumably it will be on the drain side of the channel. V * I = power dissipation. Makes sense to make the collector/drain from the largest part of the silicon structure with the best heatsinking capability.
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Hi Andy and Kizzap,
Thank you both for your answers, and im trying to come up with a clever thing to say in response, but the only thing that comes up in my head is a busy tone :)
-lk