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Original question:

LM5023: USA

LM5023: USA

Abhiman Hande47
Prodigy 60 points
Part Number: LM5023

Hi

We are noticing an additional pulse on output pin of our LM5023 based flyback circuit. This occurs only when the AC input is around 50Vrms (70Vdc input) or lower. Refer waveform below.

Ch1: Out (pin 7) of LM5023, Ch2: Switch node, Ch5: Vgs of primary side flyback FET (after driver)

In the figure, we can see a second pulse from LM5023 and corresponding Vgs pulse after driver's propagation delay resulting in hard switching of flyback FET.

Here is a second figure, we can see a second pulse from LM5023 resulting in hard switching of flyback FET (turns on at ~200V across Vds). The FET gets hot under this condition.

We need help in understanding why this pulse occurs and solutions to eliminate this problem.

Thanks

Abhiman

  • 0 in reply to Mike O'
    Prodigy 60 points

    Hi Mike

    We will try adding more filtering cap on the QR pin. But I don't think there is noise on QR pin causing this issue.

    Here is a waveform showing that the rogue Vgs pulse occurs first from IC resulting in drop in voltage levels at QR and switch node. I do not see noise on QR pin or current sense pin prior to occurrence of this pulse.

    Ch1: Out (pin 7) of LM5023, Ch2: Switch node, Ch5: QR (pin 1) of LM5023, Ch6: CS (pin 5) of LM5023.

    Thanks

    Abhiman

  • 0 in reply to Abhiman Hande47
    TI__Guru* 85926 points
    Hello Abhiman,

    I was reviewing your wave form at the beginning of the wave the waveform CH 1 is high and goes low briefly and then turns on again. On the QR signal I see a small pulse and then the gate drive turns on again. You need to zoom in on this signal to try to figure out what is going on. For some reason the QR signal is being pulled low.

    Your switch node (CH2) shows ringing down to ground. This signal should be transformer coupled to your aux winding and is more than likely triggering QR to turn on. Study the switch node 3 to 12 switch node. This should not be ringing down to ground like this. You need to figure out why the switch node is doing that. I believe if you remove this issue that extra pulse you are seeing should be resolved.

    You might have a layout issue and you have excessive ground noise. There is a good layout example in the data sheet and guidelines in section 10.1 and 10.2. that can help you evaluate the layout.
    1. You need to keep the LM5023 ground as close to the bulk capacitor as you can get it.
    2. Keep your traces as short as possible to reduce inductance and ringing.
    Regards,

    Mike