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500W offline Full Bridge SMPS with general purpose diodes

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Advanced Member level 5
Jun 13, 2021
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assuming the fets are fets - do they really need anti-parallel diodes ?

certainly the ones in the link are likely to go poof at 500kHz, if they ever carry any fwd current and are then forcibly commutated off ...
Thanks, the FETs are IRF740L

These FETs have internal diodes which have trr = 570ns.
...even these look bad in that respect.

JTLYK , This Full Bridge has a output inductor ripple frequency of 250kHz. As such, i call this a 250kHz Full Bridge...i appreciate that this is not standard terminology

If the output inductor has 250kHz ripple, then the fets are switching at 125kHz

The slow mosfet diodes are fine as long as standard PWM is used

however the turn on losses in the IRF740 may overwhelm the fets ....
--- Updated ---

p.s. turning these fets on faster - will up the dv/dt in the ckt and likely interfere with the control chip and signal sensing - esp if thru a single layer of insulation to an earthed heatsink - this almost always catches out designers who try to go to HF with std pwm converters ....

there is also the effect on the output diodes if the mosfets are turned on too quickly ....
--- Updated ---

Calc losses for these fets @125kHz, 275VDC bus, 2A load, just about 5W each @ 80 deg C junc, 50nS turn on time ( current rise time - not Vds fall time ).

Turn off losses are assumed to be < 0.5W ( fast turn off, < 30nS gate fall time )
--- Updated ---

Will be extra for any snubbers needed for fets and o/p diodes ....
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Any diodes other than schottky won't carry relevant share of total current when paralleled with Si MOSFET and are more or less useless.


We can see here for the mosfet - top, and the diode, bottom, that in the range of interest - say 0 - 2A the mosfet will indeed carry the lions share of current ...
This says something about the experience of the designer who placed the diodes in the 1st place
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