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UC2843: gate drive waveform

Given Ding
Genius 9215 points
Part Number: UC2843

Hi Team, 

Here are two questions when use UC2843 in flyback topology. 48V ( 34V-60V) input, 9.5V/2A output. 

1. we found when Ton is large, there are two steps on OUT pin. Please kindly check the reason and if the 2rd step will drops lower. 

Vin=34V  Ch1=OUT, Ch2=RTCT, Ch4=Comp 

Vin=55V, OUT pin signal is normal. Ton is smaller compared with above waveform 

2. At same circuit,  use AUX winding to power the Vcc pin. During power off, there is oscillation on Vcc. Please kindly check the reason. 

Ch4=Vcc 

If change driver resistor from 100ohm to smaller value, such as 50ohm, there is no such oscillation. 

BRs

Given 

  • 0
    TI__Mastermind 29835 points

    Hi Given,

    Interesting waveforms. In the first one that shows the step at D=22%, is that the waveform measured at the OUT pin or taken at the VGS of the MOSFET? With the 100-ohm gate resistor installed, measure the same waveform on both sides of the gate resistor and also measure the VCC on the same plot. You need to determine if this dip is caused by VCC, the controller gate drive OUT or a partial breakdown of the MOSFET VGS? I'm not sure why the issue is not appearing when the duty cycle is reduced but if you measure the same wavefoms as I described previously, perhaps the cause will become apparent?

    The third plot shows a slight oscillation on VCC (at UVLO off) during shutdown - the oscillation is apparent when the gate resistor is 100-ohm and is not apparent when the gate resistor is reduced top 50-ohm. If you are not seeing VGS oscillations, 100-ohm is a very large gate resistor - why so large? Measure OUT, VDS and both sides of the gate resistor.

    Also, in the first two plots, the COMP shows an oscillation equal to the switching frequency. Please verify your voltage amp is properly compensated and the currecnt sense signal has proper filtering to show a smooth linear ramp.

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    TI__Mastermind 29835 points

    Hi Given,

    I have not heard back since the last reply. I assume the issue you were having is resolved and I am closing this thread. Thanks for using the TI e2e forum and if you need additional support, please open a new thread.

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    TI__Genius 9215 points

    Hi Steve,

    Sorry for late reply. I'm asking customer to take the waveform you want to see. Once there is updates, will inform you. Thanks.

    BRs

    Given

  • 0 in reply to Steven Mappus1
    TI__Mastermind 29835 points

    Hi Given,

     

    Clearly we can see from the waveforms that the 100Ω gate resistor is too large and the 50Ω is also too large. Why are such large gate resistors being used?  All the gate current is gone before the voltage on VGS can reach VCC. We want a little bit of gate resistance (<10Ω for turn in and possible 0Ω for turn-off) to control ringing at the turn-on and turn-off edges but not so much resistance that the drive current can't effectively charge/discharge the VGS capacitance. I would recommend using the circuit shown below where R1 controls the turn-on edge and R1//R2 controls the turn-off edge.

    Also please see SLUA618a for more details regarding gate drive circuits.

     

    Regards,

     

    Steve