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UCC27712: Jitter in mosfet gate-source voltage

Part Number: UCC27712

Hello,

We are using UCC27712D gate driver,IPP051N15N5 mosfet in interleaved synchronous boost converter.

Switching frequency is 100kHz, duty cycle:20%., Gate resistance: 2.2ohm, Gate source resistance: 4.7k ohm, Cvdd: 2x4.7uF, Rbias:10ohm, gate drive voltage : 13V

attached is the gate - source voltage of the low side mosfet. 

Figure 1; Gate voltage waveform(yellow) is getting distorted once boost action starts happening and load current flowing. Green color waveform is input PWM waveform generated by microcontroller present on the same board.

Input voltage is around 20V and input current is near to 1.8A. 

input PWM waveform and mosfet gate-source waveform 

Figure1

We have tried using external pulse generator to provide input  PWM waveform to the gate driver 

Mosfet Gate - Source voltage waveform from external pulse generator to gate driver input

Figure2

Figure 2; Yellow waveform is Mosfet gate-source voltage and red waveform is pulse provided externally using function generator to gate driver.

Jitter in mosfet gate source voltage increases when load current increases.

Kindly suggest the reason behind the unexpected waveform in Mosfet gate-source voltages.

 

  • Hi Akash,

    There's a lot of noise on the micro controller output PWM. I think there might be might be noise coupling onto the trace. How long is the trace? Where on the trace are you measuring? RC filters on the HI/LI input trace might help with the high frequency components.

    Can you measure the gate driver pins LO and HO on the oscilloscope?

    Best Regards,
    Ethan Galloway

  • Hi Ethan,

    Currently we are switching only 1 leg of the interleaved boost converter. We have switched off the PWM pulse of the top MOSFET hence letting the body diode to conduct during the discharge phase. So HO output will not appear. We are only switching the bottom MOSFET.

     Attaching below the waveforms of VGS of the MOSFET (Yellow) and LO pin voltage (Red).

    To answer your other queries, we are measuring the PWM pulse at the LI pin. We tried with various combinations of RC filter values including the suggested value in the datasheet (R = 50 Ω and C = 220 pF) but the noise reduced only slightly. The trace length from the microcontroller pin to the input RC filter of the gate driver is 16.217mm.

  • Hello Pranav,

    Thank you for the scope plots and description of your test conditions. When you are operating in a condition where the body diode of the Mosfet is conducting, you can have very high dV/dt and ringing because you have to force off the body diode and the recovery time is usually not very fast. This sets up a conflict when you turn on the other Mosfet where high current will develop while the body diode is turning off.

    Can you comment on what is the external gate resistance in the gate drive circuit, and how long the traces are from the driver pins to the Mosfet gate and source? The high dV/dt on the Mosfet Vds will couple charge into the Vgs, this can lead to high ringing on the Vgs if there are not low inductance trace connections. Higher gate resistance can help reduce the Vgs ringing. Can you try increasing the gate resistance to see if that reduces the Vgs and driver output ringing?

    Regards,