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UCC2893: UCC2893, mixing replacement MOSFETs

Joshua Schroeder
Prodigy 80 points
Part Number: UCC2893

I have a UCC2893 based ZVS active Clamp supply that uses the same part number MOSFET in both the Main and AUX positions. 
Part shortages have driven us to verify similar MOSFETs that may be used as drop in replacements.   The assumption during verification testing and analysis was that the same MOSFET would be used in both the MAIN and AUX positions in the circuit.  

What concerns might there be with using two different MOSFETs for the AUX and MAIN MOSFETs in a ZVS active clamp supply?

  • 0
    TI__Mastermind 27555 points

    Hi Joshua,

    Whether the two MOSFETs are the same or different, changing them from what is currently used in your existing design should be low-risk, assuming the new MOSFETs are properly rated for VDS, ID, power and are in similar packages. 

    Your active clamp converter can most likely achieve ZVS down to some minimum load condition. You should characterize your design with the existing MOSFETs for ZVS operation corners against line/load conditions. When you change the MOSFETs, the loss of ZVS is the one thing I expect could change and it could get better or worse - we don't know yet? Establishing the conditions for ZVS is dependent upon the circuit having enough resonant current to fully discharge the resonant capacitance within the programmed, set dead time. Nothing in your power stage is changing that will change the amount of available resonant current but the resonant capacitance is strongly dependent upon MOSFET COSS, which is highly non-liner and difficult to accurately compare from data sheets alone. When the resonant capacitance changes it could mean:

    1. The set dead time may need to be adjusted to get back to the same ZVS conditions you had with the pervious MOSFETs
    2. There is simply not enough resonant current to fully discharge the COSS of your new MOSFETs and in this case you might see loss of ZVS at a different line/load point compared to what you measured previous
    3. There could be too much resonant current if the new MOSFETs have lower COSS and in this case you would need to reduce the dead time to accommodate.

    Hope this helps you understand what to watch for? Thanks for connecting through E2E.

    Regards,

    Steve M