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UCC256402: PFC+LLC Circuit PFC Circuit Unidentified Error

Cihangir Öksüz
Prodigy 85 points
Part Number: UCC256402
Other Parts Discussed in Thread: UCC256403

Hello everyone,

I designed an AC-DC converter in PFC+LLC Half bridge topology with 700 W maximum power and 61 V output voltage. Information about PFC is in the following title:

The system is currently powered by an external PSU during testing. In my last test I saw 390 VDC on the PFC output. When I soldered the LLC half Bridge side and connected it to the grid without loading the system and without making any changes, my pfc mosfet and shunt resistor burned out. The test procedure is as follows: First, the system is connected to the grid, then the integrated circuits are fed. I don't understand how the system reacts like this. Why do you think it might?

Kind regards
Cihangir

  • 0
    TI__Genius 16155 points

    Hi Cihangir,

    Could you power the LLC with the external supply and see if its running okay? 

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 85 points

    Hello Manikanta,

    No high voltage source for me to try this. Is it wrong to check the schematic design if I share it easily? Other than that, should I use the HV pin and the Vcc pin at the same time?

    Kind regards
    Cihangir

  • 0 in reply to Cihangir Öksüz
    TI__Genius 16155 points

    Hi Cihangir,

    Could you provide me more details of the testing procedure( probing points and types of probes used)?

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 85 points

    Hello Mantikanta,

    I was planning to check out the UCC256403 with the first of the design. Later I continued the design with the UCC256402 as I might have problems due to stock and now I realized that there are some that I missed. Since the UCC256403 does not have "Integrated High Voltage Startup" and has "Burst Soft On and Soft Off" features, I could connect the BW and HV pins directly to ground. After switching to the UCC256402, I supplied the start voltage to the HV pin before the PFC. But since I didn't put a bias winding in my transformer design, I thought I could connect the BW pin to ground again.

    However, during the test I mentioned above, I said that I tried the whole system, I had an external power supply and I could not understand how my PFC mosfet was lit without activating the supply. I think I can make sense of it now. Can you confirm me if I'm right? When I connected the system to the mains, 325DC reached the HV pin, voltage occurred on the Vcc pin of the UCC256402. In my PCB design, I connected the Vcc pins of the two circuits together in common. Then, in this case, my PFC integration was opened. I set the BLK voltage limit of the LLC part to 300 V. In this case, when the half bridge mosfets tried to be switched before the BLK voltage reached 390 V, the system drew high current and my PFC MOSFET burned out.

    If the system was damaged when I connected it to the mains, my bulk capacitors at the PFC output would not be charged at all. In other words, my bulk capacitors were charged at the PFC output until the appropriate voltage was formed on the VCC pin of the UCC256402.

    If my guess is correct, I will need to make some revisions.

    1) Is it okay if I connect the BW pin to ground on the UCC256402?
    2) I would appreciate if you share with me the points I need to revise in the control chart I sent you so that I can use the UCC256402

    I sent you the design files that I am using now via DM.

    Kind regards,
    Cihangir

  • 0 in reply to Cihangir Öksüz
    TI__Genius 16155 points

    Hi Cihangir,

    What do you mean by this?

    Cihangir Öksüz said:
    my PFC integration was opened

    Also could you tell me what probe you used while measuring DC voltage (+325VDC point in your schematic.) You have to use differential probe for measuring this. 

    I have two recommendations for your schematic:

    Connecting BW pin to aux winding is recommended. However, even if you don't connect the BW pin to aux winding, controller will still work as the HV startup regulates the VCC voltage. But this will heat up the IC. 

    Also Connect RVCC of the LLC controller to the VCC pin of PFC controller. This will make sure there wont be any surge currents in the converter during startup. By doing his, you are making sure that PFC DC output is stabilized and then LLC output slowly ramps up. 

    Regards

    Manikanta P