Re: MOSFETS in parallel in switchmode improves effciency?
- From: Vladimir Vassilevsky <antispam_bogus@xxxxxxxxxxx>
- Date: Tue, 24 Feb 2009 20:18:14 -0600
Jan Panteltje wrote:
I was playing with my PIC (LOL) switch mode, and wanted some more current.
Build a small gate driver for higher gate voltages so I could use the IRFZ44A MOSFETs,
I have a bunch of those, 24 mOhm, 50 A.
Got more power, but was thinking what happens if I parallel some.
Now for a given current say 10A, the power in one is I^2 x Rdson.
But if I parallel 2, then the voltage drop is also half, and power is U^2 / Rdson.
that means each get 1/4 of the power, half the voltage at half the current.
Incredible. The laws of Ohm and Joule at work.
That means half the losses due to Rdson, and increased efficiency.
As far as gate capacitance goes, many of those low side driver chips have 2 drivers in
a package, so in a SEPIC type of design with only 1 MOSFET to ground, you can use the driver
to drive 2 separate MOSFETs in parallel.
Not only does it generate half the heat, you can also better spread that heat by mounting
the MOSFETs a bit apart on the heatsink.
So, do I see this right, does using 2 MOSFETs not only increase current allowed,
but also increases efficiency by halving the losses in the switch for the same current?
Sometimes we have to parallel up to four MOSFETs (current ~ 100A).
4 parallel, 1/4 the losses...
90 % effciency coming up, the rest in the coils?
Keep in mind that the capacitive switching losses are increasing almost proportionally to the number of FETs. So, at some point, the Rds losses are getting equal to the other losses.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
.
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- MOSFETS in parallel in switchmode improves effciency?
- From: Jan Panteltje
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