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UCC28950: designing an 850W ac/dc converter

Bart
Expert 6460 points
Part Number: UCC28950

Team,

we are working to develop an 850W 230ac/24Vdc @ 30A converter with UCC28950. 

Our design seems not to be working when connected to the ac supply - at 50% output voltage and at no load it seems to be fine, at 80% of output voltage level, the transistors are failing.

We can share offline the schematic and the datasheet of the used transformer, as well as the SP1 calculations.

Thanks for your help.

TI Customer

  • 0
    TI__Mastermind 25570 points
    Hello Bartosz

    Please send me the schematic, transformer files, SP1 calculations and PCB Gerber files to colingillmor@ti.com

    I'll review them and we can start to debug the system.

    As a first step - it is usually possible to run the power stage from a very low input voltage. For example, the UCC28950 EVM will run from almost 0V right up to 400V. You may need to supply a separate VCC supply to the UCC28950 Obviously, in this condition Vout is below its target until Vin reaches close to around 300V or so but the main thing is that running at a low input voltage - maybe around 20 to 50V gives you a chance to debug the system without it destroying itself. In this condition, the controller will run out to Dmax giving you an opportunity to verify that the gate drives at the MOSFETs are working correctly and that the transformer voltage and current waveforms are 'reasonable'.

    Please feel free to send me any waveforms you may take.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    TI__Expert 6460 points
    Hi Colin,

    thank you. The schematic and other design materials have been shared offline.

    Kind regards,
    Bartosz
  • 0 in reply to Bart
    TI__Expert 6460 points

    Hello Colin

    any feedback from your side?

    We changed SR Mosfets to diodes to eliminate potencial issues with rectification. If we start mains from 0V and slowly increase it, output voltage is growing up. When we have aprox 130Vac, we acheive nominal output voltage and everything looks ok (photo 1). Blue and red show signals on diodes, yellow - output voltage, green is 12V (Vcc for UCC28950). When we increase voltage a little bit above 130Vac, the transformer starts to generate some strange voices and signals goes to photo 2. We tested it without load and with 24V/21W bulb as load and it looks the same. It is not on the schematic, but we always connect 4 capacitors 4700uF/50V to output. What can be a reason of this? Photos 3-5 show h bridge mosfet gates sygnals (without mains). I have problem to measure it with mains due to common ground.

    1682.waveforms.zip

  • 0 in reply to Bart
    TI__Mastermind 25570 points
    Hello Bartoz

    This looks like the controller is operating at Dmax until the output voltage reaches its setpoint when Vac is 130V. This is exactly what I would expect it to do (picture ex1)
    If the input voltage increased a little more then the duty cycle will reduce as the controller seeks to keep Vo at its setpoint. If there is little or no load then the output inductor current will go discontinuous and the duty cycle will reduce further and eventually the Ton time will drop below the time set by the resistor at the TMIN pin. When this happens the controller operates in a burst mode. I'd suspect that this is what is happening in your system.
    The other possibility is that there is a problem with the current in the transformer - perhaps the transformer is saturating. If this happens you would see cycle-by-cycle (CbC) current limiting and this too would decrease the duty cycle.

    I would suggest you check the following -
    1/ look at the SS/EN pin. in normal operation - including burst operation - it is held at a constant voltage, around 4.5v. If the CbC current limiting is operating then the controller can go into a hiccup mode and the SS pin ramps down to 500mV and then recovers in a soft start cycle - Fig 42 in the DS
    2/ look at the CS pin and send me the image please. This will tell us more about the transformer condition.
    3/ if the currents at the CS pin look ok, then I'd suggest that you try to increase the load and see if the burst duration increases or if the controller comes out of burst mode.

    Please let me know how you get on - we can continue on the e2e forum or else by email - it's up to you.

    Regards
    Colin