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LM5122-Q1: Output mosfet instead of schottky diode

Zampieri Claudio
Prodigy 20 points
Part Number: LM5122-Q1
Other Parts Discussed in Thread: LM5122, LM5156,

Hi, I have a question about the topology of the Synchronous Boost Controller. I was wondering why there is a mosfet instead of the typical fast recovery diode before the load. 

We would excpect an improvement in the power losses, but we don't see it in our simulations. We observe that there are big overshoot of current flowing from the output capacitors to the switching mosfet, during the turn on of the switching mosfet. If we use a schottcky diode instead of the mosfet then we see just a little current overshoot.

 Thanks for helping

  • 0
    TI__Expert 7450 points

    Hi Zampieri,

    Thank you for your interest in LM5122. You are right that synchronous rectifier could improve the power dissipation. But it will not be obvious if the conduction loss is too small. So based on your application, you may or may not need the synchronous controller. We also have non-sync controller like LM5156 that you can consider. As for the overshoot, it is better to show the waveform so that I can understand the issue correctly.

    Thanks,

    Yinsong

  • 0 in reply to Yinsong Cai1
    Prodigy 20 points

    Hi Yinsong Cai1, thanks for your kind answer. We have some observation to tell.

    We noticed in our simulation that the peak of current is due to the Recovery Reverse effect of the Mosfet,   used instead of the diode in the synchronous application. We observe this peak of current also if we use a Schottky diode. We attach a figure of our simulation.

     

    We observe that in the WeBench simulation of the LM5122-Q1 there is not any peak. And we don’t see any snubber circuit for stopping this peak. Could you please tell us how did you cancel this peak?

    Thanks, regards

    Claudio Zampieri

  • 0 in reply to Zampieri Claudio
    TI__Expert 7450 points

    Hi Claudio Zampieri,

    I'm not sure how accurate is the Webench simulation but in general, the deadtime is minimized to reduce the reverse recovery. Are the gate signals from LM5122 already?

    Thanks,

    Yinsong