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UCC3895: phase shift full bridge output power down hold time can not meet the requirement

Howard Zou
Genius 12365 points
Part Number: UCC3895

Hi, my customer is using UCC3895 to build a phase shift full bridge converter from 400V to 12V(208A), the 400V DC is from a PFC converts 220VAC to 400VDC.

When the 220VAC is unplugged, the 12V output needs to keep at least 12ms, most of the good devices can reach 15ms-17ms, 12 out of 1200 pieces can only reach 10.5ms. Within the same board, if they switch only UCC3895 to a newer one, the problem can be fixed.

They've also monitored the 400VDC bus, with good UCC3895, it will shut down when 400V bus decrease to about 330V, with bad device, it will shut down when the bus decrease to about 380V.

They also tested the maximum output. With bad device, the maximum output can reach 12V/240A when the duty cycle reaches the maximum, increasing the load will make the circuit work in constant output power mode. With good device, the maximum output can reach 12V/290A when the duty cycle reaches the maximum.

Do you have any idea what would cause the problem?

  • 0
    TI__Mastermind 25570 points

    Hello Howard

    There is no specific input voltage UVLO function on the UCC3895. The only UVLO relates to the VCC on the device which must remain above the UVLO(off) threshold of 9V typ. I would suggest that the customer check the behaviour of the VCC rail to the device to confirm that it is not dropping unexpectedly and that it is also not noisy. Noise or a sudden drop on the VCC rail could possibly be triggering the device UVLO and causing it to shutdown. This UVLO threshold will of course vary from device to device so replacing the controller with a different device may indeed cure the problem on that specific PSU.

    If the VCC rail looks ok then it may be worthwhile returning some of the failing devices to TI so that their electrical characteristics may be double checked.

    Let me know how you get on.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points
    Hello Howard

    Here are a few other things to consider ...

    You say that the device will 'shut down' when the bus decreases - does this mean that the OUTx signals stop switching completely.
    Check the VREF pin, the reference output will drop to 0V if the device goes into UVLO. Can you check if VREF is ok immediately after the PSU 'shuts down' or does VREF stay in regulation.
    If VREF stays in regulation, then the system may be going into current limit. I presume that it is the input current that is being measured. The customer should look at the CS pin and see if it is reaching the 2.5V over current threshold which initiates an immediate halt of all switching. This threshold can vary from device to device of course. It would also be worthwhile to take a screenshot of the CS pin waveform to see if there is any evidence of transformer saturation during the holdup time.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    TI__Genius 12365 points

    The picture below: dark blue- VCC, light blue- VREF, green- output. We can see that the output voltage decrease is not caused by UVLO

    the picture below: dark blue- CS pin, green- output voltage, light blue- OUTA. We can see that the output voltage decrease is not caused by CS.

    The voltage should decrease, but the holding time from device to device varies greatly. I plan to send the failure parts to the CQE lab, but I doubt whether it can help me determine the root cause. If any ideas, please let me know.

  • 0 in reply to Howard Zou
    TI__Mastermind 25570 points

    Hello Howard

    Thanks for the added information and I would agree with your conclusions

    Can you look at these signals please

    The Waveform at the SS/EN pin - it should remain at a DC level of about 4.6V.

    the waveform at the COMP pin -

    Can you look at the device duty cycle at three points - normal operation, at the start of the Vo down ramp when Vo has dropped to about 11V from its initial 12V and at the end of the Vo down ramp when it reaches about 2.5V. The purpose is to see if the controller is running at Dmax throughout the shutdown or if the duty cycle is dropping as Vout drops. the easiest way to do this is to measure the voltage at the primary of the power transformer or else the voltage across the output inductor.

    Can you send me your schematic - if you don't want to post it here then you may send it to me directly at colingillmor@ti.com 

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points
    We have taken this off line for further work - this thread is closed