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Switch mode ps mosfet

Sammy180

I am looking to replace a high voltage mosfet on a switch mode power supply but am unable to find an exact replacement (2yr lead time). I have been able to source simular mosfets but they differ in gate capacitance and drain-source on resistance. How can I modify the circuit to accomadate these changes. Can I simply change resistors to make the rc an resistance value match. Any advice would be apritiated as I'm still a novice in this area.

  • Need to give part numbers and maybe a schematic.

  • Sometimes it's not so important if you just want the unit to work.

    Higher capacitance often means slower switching which means higher switching losses and more strain on the driver. I would expect a higher Cg MOSFET to have a lower Rds if it's any good - so perhaps it will have lower conduction losses. You should also make sure the rated drain-source withstand voltage and drain current is at least enough to support the application, sometimes it's also important to consider other parameters (avalanche robustness). If you want it to just work, the circuit may be accommodating enough for it to work, even if it is not optimal.

    If you have a schematic, that may make it easier - however, there are some things which may be hard to predict. For example, if the unit was EMC compliant, changing the part may lead to operation that creates more noise than before (e.g. if it switches faster). Otherwise, it may be below minimum efficiency standards if changing the part causes overall efficiency loss (and may require better heatsinking).

    I hope you understand and have resolved the reason behind the first failure before replacing it - otherwise you may just be setting yourself up to keep replacing it ...

    - Gough

  • The on resistance isn't much of a concern - try to at least have lower on resistance than the existing MOSFET, this way you won't be making more heat than the existing design. The gate capacitance may or may not be an issue depending on the switching frequency - if the switching frequency is quite high, the extra gate capacitance may require too much drive current from the driver and cause it to fail. See if you can get something similar. Also be very sure to check the new MOSFETs Safe Operating Area (SOA) against the original MOSFET. Just because a FET has impressive sounding ratings doesn't mean it will be capable of that in the conditions the power supply creates - if your new FET's SOA is wider or at least matches the old FET, you should be good to go. 

  • in reply to OriginalCircuit

    Thank you for your reply.

    So my smps runs at 500khz. Is there a good rule of thumb what chharge time should be say no more than 1/100th of the time period. In my case the tome period is 2 us so make sure my charge time is under 20ns ?

    -sammy