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Flyback SMPS issue

Gajanan Mahangade
Prodigy 30 points

Part Number: LM5021-2

Hi,

I have developed flyback SMPS of rating 12V, 0.5V using switching IC LM5021-2. But output voltage is not coming.

In flyback, Can we use output voltage as bias supply to LM5021 instead of saperate bias winding?

kindly help to resolve this issue

regards,

GajananWBDesign7.pdf

  • 0
    TI__Genius 13660 points
    Hi Gajanan,

    It cannot realize the isolation between primary and second side if we use output voltage voltage.

    You can try to find the reason why there is no output voltage as follows.
    First, you need to observe output pin to confirm that whether LM5021 does not work or it enters protection mode.
    If it does not work, then ensure that Start-up voltage at VIN pin exceeds 20V as VCC Regulator enable threshold typically is 20V. Then ensure that VIin pin voltage exceeds 8V after start-up, which come from bias winding.
    If it has output signal at output pin,maybe it enters protection mode, then find which protection is activated, such as CS pin over current protection (CS pin voltage exceeds 500mV).
    Last, I found that Rled is 2.94Kohn, you can try to reduce this resistor to 1.3 Kohm.
    Regards,
    Teng
  • 0 in reply to Teng Feng
    TI__Genius 13660 points
    Hi Gajanan,

    Below is from our expert.

    I noticed that the schematic diagram “WBDesign7.pdf” shows the AUX diode as a 40-V schottky.
    However, with 264Vrms input theT1 turns ratio will impose 375Vpk*12/58 = 78V + ~15V bias = ~93V on it.
    I suspect that this diode will fail for any input voltage higher than about 102Vrms.
    This may be the reason that the circuit cannot get started.
    I have no idea why the Webench program would recommend a 40-V Schottky for this component. Probably a bug in the design algorithm.

    So I suggest to use an AUX diode with 120V maximum RMS voltage or larger.

    Regards,
    Teng