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Mosfet paralleling reliability in real life - more smaller ones vs fewer bigger
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Miyuki:
Hi folks
I have a question about reliability of power supply with higher number of paralleled transistors

For example 2 Switch forward converter with planned 2x 5 in parallel switched at moderate frequency 200kHz
If current will be equally shared losses are calculated ~10W per device 4.5:5.5 Conductive:Switching losses ratio

Question is if then I can trust that this many devices have enough similar parameters to work reasonably and over longer time period
Yes then here is also some layout issue with switching parallel at this speed, but there little extra trace inductance at Drain side can be beneficial
MagicSmoker:
Five MOSFETs in parallel doesn't strike me as terribly economical unless you are relying on passive convection cooling and need to get the heat flux (ie - W/cm2) down.

Paralleling MOSFETs isn't usually a big deal as long as you increasingly derate the current for each additional switch and use an individual gate resistor for each one, however at 200kHz you might have a problem with differing loop impedances which will lead to uneven dynamic current distribution. Adding a bit of inductive impedance in series with the source of each switch can be helpful here; putting the L in series with the drain is less effective because you don't get the benefit of negative feedback from degeneration.

That said, it is always preferable to minimize the number of switches in parallel, and (5) seems a bit much unless this is a step-up application and the input voltage is low (e.g. - 12V to 340VDC to supply the H-bridge in an inverter).
Miyuki:
But five small is actually cheaper than one big and I can use fullpak ones with simpler and cheaper mounting
Plus I need to switch relative high current at short duty, about 20-30A it makes problem with single device and high switching losses
rx8pilot:
This YT channel is gold.

https://youtu.be/wjSM2oYY3OY
MagicSmoker:

--- Quote from: Miyuki on September 20, 2019, 06:10:27 pm ---But five small is actually cheaper than one big and I can use fullpak ones with simpler and cheaper mounting
Plus I need to switch relative high current at short duty, about 20-30A it makes problem with single device and high switching losses

--- End quote ---

30A isn't particularly challenging for a single device or, perhaps, two at most. What is the maximum supply voltage?

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