Re: MOSFETS in parallel in switchmode improves effciency?

Nobody <nobody@xxxxxxxxxxx> wrote:

On Tue, 24 Feb 2009 13:57:31 -0800, Klaus Kragelund wrote:

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?

4 parallel, 1/4 the losses...
90 % effciency coming up, the rest in the coils?

I once played around doing this in a small 1W supply. Did the
paralleling of MOSFETs with the same assumptions you had, but the loss
was not reduced. Also did a simulation, showed more loss with the dual
solution, but never had time to chase the answer. Sorry, not much
help, but an interesting discussion

Intuitively the RDS on losses should be lower, but you cannot be sure
they turn on a the same time, so swithing losses are probably the same
or worse.

One thing that springs to mind is whether you had sufficient gate
current. A second MOSFET on the same driver means that you need to move
twice the gate charge; if current is limited, that would increase the
switching time.

That is a major problem when using Mosfets. For switching power
applications you should choose the Mosfet with the smallest gate
capacitance (i.e. the one that just fits the bill). Oversizing is
overkill -literally-.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
"If it doesn't fit, use a bigger hammer!"
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