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LM5026: Gate voltage ringing in main MOSFET at turn off

Gonzalo Bustamante
Prodigy 20 points
Part Number: LM5026

Hello,

I'm debugging an active clamp forward converter using LM5026, with the following parameters:

  • Vin = 54V
  • Vout = 48 V
  • Pout = 250W
  • Inductor 240 uH
  • f_sw=130k
  • Cclamp = 30n
  • N mosfet : Infineon IPB117N20NFD
  • P mosfet:  ON Semi.  FQD4P40

Transformer parameters:

  • n = 2.5
  • Magnetizing inductance 140uH
  • Leakage inductance ~0.6 uH

The problem is that a voltage ringing appears at the gate of the N mosfet during toff. The oscillations are so high that it seems that the transistor is switched on several times during the oscillations, leading to current oscillations at the clamping capacitor. This has become an EMI issue at radiated emissions, as the oscillations have a frequency of approximately 100 MHz.

I attach an oscillator image, CH1 is Vds and CH2 is Vgs of the main mosfet. 

Any help will be appreciated,

Thank you,

  • 0
    TI__Mastermind 47715 points

    Hi,

    Can you try to increase the dead time to see if the ringing can be reduced or eliminated? If not, please provide your schematics and also indicate where the waveforms are measured.

  • 0 in reply to Hong Huang
    Prodigy 20 points

    Hello,

    I have increased the deadtime from 100ns to 150sn and it makes no difference, the ringing remains the same. By the way, the oscillations occur during dead time, before the P mosfet is turned ON.I attach the schematics, the points where the waveforms are measured are labelled with CH1 and CH2 

    6862.Sheet1.pdf

    Thank you very much,

  • 0 in reply to Gonzalo Bustamante
    TI__Mastermind 47715 points

    Hi,

    CH2 is N-ch MOSFET Vgs which shows high ringing to cause CH1 Vds ringing. You need to find a way to attenuate the Vgs turn off ringing. One way is add some resistance in series with the diode on the N-ch MOSFET to slow down its off speed to help reuce the ringing. Another way is to examine your board layout which it might have too much trace inductance so to cause the ringing.