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Original question:

UCC28951-Q1: UCC28951-Q1

UCC28951-Q1: Resonant frequency

Suresh Bhat1
Prodigy 90 points
Part Number: UCC28951-Q1
Other Parts Discussed in Thread: UCC28951

According to the datasheet, resonant frequency is dependent on shim inductance. But for my application, the power transformer has enough magnetizing inductance because of which I am not going with shim inductor and diodes D5 and D11. 
So for calculation I am using main transformer inductance and mosfet output capacitance. Please confirm whether my method is correct

  • 0
    TI__Mastermind 31580 points

    Hi Suresh,

    Our Applications engineer that normally supports the UCC28951 is OOO due to illness for a few days. I will need to do some checking before I can respond on this, please give me a day to see what I can find.

    Thanks

    Peter

  • 0 in reply to Peter Meaney
    TI__Mastermind 31580 points

    Hi Suresh,

    From looking at the datasheet the shim inductance is used to setup the resonant tank with the output capacitance of the mosfets in the primary circuit of the bridge.

    If the shim inductor is not used then you will be dependent on the energy stored in the leakage inductance of the transformer to provide the energy to discharge the switch node voltage to ensure ZVS.

    Note that the magnetising inductance of the transformer is not part of this ZVS process. The magentising inductance will set the peak to peak value of the magentising current. In a phase shifted full bridge, PSFB, the peak value of the magnetising current is usually not sufficient to ensure ZVS as the load is reduced.

    The energy stored in the switch is 1/2CV^2, to ensure ZVS we need to discharge the switch node so we need the neregy stored in the leakage + Shim inductance, 1/2LI^2 to be greater than 1/2CV^2 to ensure ZVS. The peak value of the current I has two component, one base on the peak value of the magnetising current and the other that follows the reflected load current.

    You may want to leave a position in your first PCB for the shim inductor and the clamp diodes.

    Regards

    Peter

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 90 points

    Hi Peter,

    Can I conclude that if my power transformer has got enough leakage inductance, say more than what my calculation says I can use leakage inductance to calculate resonant frequency?

    Regards

    Suresh

  • 0 in reply to Suresh Bhat1
    TI__Mastermind 31580 points

    Hi Suresh,

    That should be ok to use the leakage inductance for the ZVS but the leakage inductance will also cause high frequency voltage spikes on the secondary side SR's so please consider snubber circuits around those devices.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 90 points

    Hi Peter

    Thanks for the confirmation.

    I had considered to use snubbers. Thanks for the suggestions.

    Regards

    Suresh