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LM5023: Startup circuit issue

Muneeb Khan
Prodigy 30 points
Part Number: LM5023

Hi all!
I am having issue with high voltage startup circuit. I am using BSP135 D-Mode MOSFET with 10K(2512) drain resistor. The issue is R88 is smoking hot, i have checked the voltages on VSD pin and it fluctuates between 3-4V. I starting the LM5023 with 0 load with input voltages of 230VAC. Can somebody points me what's wrong here? The circuit is copied from datasheet and it supposed to work. Is there any specific tests to confirm LM5023 is fine or damaged?
Looking forward for your response, i have attached the circuit and datasheet of flyback transformer. 7508112325.pdf

  • 0
    TI__Mastermind 40490 points
    Hello Muneeb,

    Please check your connections of the auxiliary winding. According to the schematic diagram shown, it appears that the polarity is backwards for a flyback topology. This means that every time the Q7 MOSFET turns on, it applies a high voltage to the QR input, leading to OVP shutdown, which then endlessly recycles to try to restart. That would overheat R88, I imagine. Try reversing the connections to that winding.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 30 points
    Hi Ulrich!

    Thank You for your response and pointing me the issue. I have reversed the auxiliary winding terminals as you suggested but still the issue is same. I have noticed VCC voltages are not going up to 12.5V but constant at 9.12V so its obvious here the controller isn't starting. I can also feel the LM5023 is getting hot. Any more suggestion to debug this? Any specific test to be sure LM5023 is healthy?
  • 0 in reply to Muneeb Khan
    TI__Mastermind 40490 points
    Hi Muneeb,

    If the LM5023 is getting hot, I think that is already a good indication that the IC is damaged. This condition would be from excess voltage on VCC (absolute maximum limit = 16V) when trying to start up earlier with reversed AUX winding connections. With a Pri:Aux turns-ratio of ~7.7:1, 16V would be exceeded when the bulk voltage exceeds ~123Vdc, which is when AC input > 87Vac. It is unknown exactly what VCC voltage above 16V would permanently damage the IC, but it is a good bet that a nominal 115Vac input and higher, rectified and reflected to the Aux, would overstress VCC.

    I recommend to replace the IC with a new part and try again.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 30 points
    Thank You Ulrich for the quick reply. Well it looks like the IC is damaged. I will get the new part and will update here. Thank You again.