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[SOLVED] Different FETs for Top and Bottom FET in synchronous Buck converter

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Selvam_Perumal

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Hi,

I am designing a buck converter for 28V input to 5V @40A.

In synchronous buck converter,Top FET will have higher switching losses and less conductive loss(Because of less duty cycle).

But the bottom FET will higher conduction loss and negligible switching loss.
Can we use 2 different FETs.
FET with lower switching loss for Top FET.
FET with lower conduction loss for Bottom FET.

Thanks,
Selvam
 

asdf44

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Sure it makes sense and I've seen it done on large stepdown buck applications like this.

Practically speaking you could choose a fet family that has multiple Rdson choices in the same package then you can optimize after you build it. Or provide footprints for 2 or 3 bottom fets in parallel.


EPC's 48V to 5V reference design has asymmetric fets (Gan fets):
https://epc-co.com/epc/Products/DemoBoards/EPC9118.aspx

And EPC's very nice loss calculation tool calculates looses for the top and bottom fet seperately.
https://epc-co.com/epc/DesignSupport/ToolsandCalculators/GaNFETSelectionToolforBuckConverters.aspx
 
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treez

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Yes, and its the reverse recovery loss of the bottom fet's diode that can also cause problems....asdf44 offers a solution to this with GAN fets because they dont have an internal diode...and so you can put an external schottky with them and reduce reverse recovery current spikes, which heat both top and bottom fets.
Another problem is turning the top fet on too fast, and the bottom fet then gets turned on briefly due to current through its capacitive junctions...causing brief shoot through.
 
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