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UCC24612: FET selection

Masazumi Ishii
Genius 3945 points
Part Number: UCC24612

Hello.

We are considering using UCC 256301.
We are considering UCC 24612-2 by synchronous rectification on the secondary side.

We have elected FETs,


Unlike flyback, the secondary side resonance current waveform has a small current immediately after ON.

When a FET with a low ON resistance (for example, 8 mΩ) is used,

When MINIMUM ON TIME ends and VTHVGOFF is less than 9 mV, the gate turns OFF.

Therefore, I think that FET with low ON resistance can not be used.
Is this thought ok?

For details, see the attachment.

thank　you.UCC24612.pdf

  • 0
    TI__Mastermind 40580 points
    Hello Ishii-san,

    You are correct. With LLC current shape, too-low of FET ON-resistance can result in early turn-off of the SR.
    You can try using a MOSFET with higher Rds(on) to overcome this problem and see if the efficiency is acceptable.

    Or, another possible solution is to delay hard turn-on of the existing SR FET by using a series-resistor to the FET gate to slow down the rise time of Vgs and prevent saturation. Choose a gate resistor value high enough to keep Rds(on) higher during the rising part of the LLC current until the current is high enough to keep Vds < -9mV when the FET does become saturated. Then, add a parallel diode across the gate resistor to keep the turn-off drive fast.

    I hope these ideas help you to successfully implement the SR solution with your LLC application.

    Regards,
    Ulrich