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active clamped forward converter using LM5025C and self driven synchronous rectifier

Siddartha koganti
Prodigy 70 points
Other Parts Discussed in Thread: LM5025C

Hi,

We are designing a active clamp forward converter using lm5025C and a self driven synchronous rectifier at the output end. We are facing an issue of body diode conduction of the free wheeling mosfet at high input voltage. our input ranges from 18 to 75V and output is 3.3V DC and 12 A .  We are driving the synchronous mosfets using auxiliary winding. Is the issue because of insufficient magnetizing inductance at the primary side which is not able to supply the full range of the freewheeling mosfet requirement. Or is it about the clamp capacitor. System is operating at a frequency of 250KHz. The present value of  inductance is 140uH. and clamp capacitor value of 6.8nF.

Thanks in advance

  • 0
    TI__Intellectual 2230 points
    Hi Siddartha,

    Would you capture the waveform on the Vgs and Vds of freewheeling FET along with the Vds of primary main FET when the body diode conduction , it will be better to have some measurements such amplitude and duty cycle for the waveforms.

    Thanks
    Oliver
  • 0 in reply to Oliver Pan91
    Prodigy 70 points

    Oliver,

    I am attaching the wave forms of the Vgs and Vds of freewheeling mosfet and Vds of the main mosfet at 75V input voltage full load of 12 A.

    The green, blue and yellow waveforms are Vds of freewheeling mosfet , Vds of main mosfet, Vgs of the freewheeling mosfet

    the duty cycle at 75 V is around 14 percent and the freewheeling mosfet conducts for the remaining 85 percent.

    have included the diode conduction timing in the last waveform. please help about this issue.

  • 0 in reply to Siddartha koganti
    TI__Intellectual 2230 points
    Hi Siddartha,

    Thanks for the waveforms, it is really helpful. It is obviously that the body diode conduction causing by the insufficient gate drive voltage of freewheeling FET, this is because the clamp capacitor is discharge to Vin before the end of PWM cycle. Just try to increase the clamp capacitance such that the resonant time constant is much greater than the maximum off-time, to keep the Vgs voltage of freewheeling FET.

    Best Regards
    Oliver