This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3478: non-monotonic gate causing too high switching loss

Part Number: LM3478
Other Parts Discussed in Thread: CSD19534Q5A, TPS2819,

I am getting a non-monotonic rise on my gate signal causing very high switching losses which is resulting in damaging my FET, I have tried multiple different FETS ranging from (Qg of 55nC, RDs(on) of 2.8mΩ) to (Qg of 6.9nC and RDs(on) of 32mΩ) and in-between, I experience this issue with all of them.

Power level do effect the edge although the glitch is still present at very low power levels. 

The circuit is a 24V to 48V 60W boost converter running at 700Khz

Thanks

  • Unsure if the trace uploaded correctly 

  • Hi James,

    Thank you for using LM3478.  The waveform indicates that your layout has significant trace inductance along your gate drive signal parth.  Add a gate resistor (try 2 to 5 Ohm) should help remove such ringing.   If you can, please improve your layout by routing the gate trace and there return trace closely side by side.  

    Thanks,

    Youhao Xi, Applications Engineering

  • Hi Youhao

    Thank you for your reply, Yes this was my first though, so i did a re-route where i made the gate as short as possible (5mm) and it has 3 Layers of uninstructed 0V planes below it. I also experimented with different value resistors to try and remove it, Please see attached the gate signal for a 10OHM gate resistor.

    The effect happens at the switching of my drain as if i had high capacitance coupling, although i am using incredibly low Crss value FETs around the 10pF and my routing physically is not able to couple that much. My Design initially started off by using the csd19534q5a FET, which from what i can tell has quite high performance for the price point. 

    My next move is to experiment with a TPS2819 to re-drive the gate signal, as well as slowly moving my component over to your eval board layout to see if i can see a similar issue on that.

    Thank you for the support on this.

    James

      

    CH1: 10Ohm resistor gate signal taken on P6243 1Ghz FET Probe

    CH1: drain signal taken on P6243 500Mhz passive Probe

    CH3: gate signal taken on P6138 500Mhz passive Probe

    CH2: source signal taken on P6243 1Ghz FET Probe

    It also varies with load with the pattern shown below, which i cannot explain.

    48V 130mA

    48V 140mA

    48V 150mA

    48V 160mA

    48V 170mA 

  • Hi  James,

    Thank you for the waveforms.  If you measure the signal correctly (like using a probe as below), then you still have oscillations in the gate drive.  I don;t think you should employ an external driver.  You may raise your gate resistor to say 20, or even 30 Ohm, to see if you can get the ringing damped out.

    Thanks,

    Youhao

  • Hi Youhao

    Thank you for your message, All my gate signal are taken on a P6243 1Ghz Fet probe with a ground reference close to power ground of the IC so i believe my scoping is accurate, with a 30ohm i am still seeing this 

      

    Thanks

    James

  • I just received the LM3478EVAL step-up board and out of the box with 5V input this is the gate signal, which has the glitch at the FETs threshold voltage, same as mine.

  • Hi James,

    Just to confirm, did you see the FET get excessively hot before it's damaged?

    Thanks,

    Yinsong