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Active clamp forward TPS23754 startup repeatedly in light load

Di Wang
Expert 1740 points
Other Parts Discussed in Thread: TPS23754

Hi,

I designed a 12V/25W PD. The schematic is almost TPS23754-383 EVM. I used a adapter as input and APD was raised to disable the hotswap. I found the active clamp forward tps23754 startup repeatedly if the load was lower than 0.3A. I've replaced the synchronous FETs with two schottky diodes. 

sch _ 06_POE.pdf

Below are the waveforms on SW (CH1), Vout(CH3) and CS(CH2). Could give suggestion on this issue? Thanks.

Load=0.1A : Vout repeatedly restarts.

Load=0.2A :  Vout repeatedly restarts.

Load=0.3A, no restart.

  • 0
    Expert 1740 points
    Hi,
    There is a LED on T2P pin. I found when load>0.3A, T2P was active low, at this moment, the operation was fine. When load<0.3, T2P was not active, the restart issue appeared.
  • 0 in reply to Di Wang
    TI__Mastermind 41790 points
    Hi Di,

    It looks like the converter is in hiccup mode. This usually occurs when there is a fault. Can you double check that there is no shorts; or that the components and connections are correct on your board? Thanks!

    Regards,
    Darwin
  • 0 in reply to Darwin Fernandez
    Expert 1740 points
    Hi Darwin
    It is not hiccup. The operation is fine with larger than 0.3A load, even up to 2A. The restart only happens with less than 0.3A load.
    I found T2P is active low only with larger than 0.3A load.
  • 0 in reply to Di Wang
    TI__Mastermind 41790 points
    Hi Di,

    The adapter voltage is 48V correct? Or are you operating lower?
    After looking at the schematic. It looks like you have a very large clamp capacitor (2.2uF). Any particular reason that is? Typically it will be 47nF (250V) cap. This may be making the converter unstable.
    I also notice the opto feedback has a difference CTR than the EVM. This might also make it unstable. For TLP291 you should probably use the Y+ version.

    These may help narrow down the issue.