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UCC256303: UCC256303 startup issue

Rock Su
Intellectual 2220 points
Part Number: UCC256303
Other Parts Discussed in Thread: UCC25630-1EVM-291

Dear colleague,

The customer has encountered the below issue:

1. Now they have a UCC256303 for a project. Now there is a problem with the startup.
When starting, the resonant current is too large to trigger OCP protection, resulting in a continuous restart.

2. The following waveform is the instantaneous waveform of the startup. After the completion of charge boot, the chip starts to start with a frequency of about 770K, and the frequency of five pulses is about 120K. I do not know if this startup process is normal.

3. Attachment is schematic.

4747.Schematic.pdf

  • 0
    TI__Guru 73455 points

    For operation please refer to the datasheet Figure 24 and Figure 40 to help you understand if the operation of your design is correct.

    For high startup current, it looks you use single resonant capacitor which requires sometime to establish to its half input voltage and during the transient time, the current is usually high. So you need to design your current protection to tolerate this transient. Or alternatively, you can use two resonant capacitors to let them form as half bridge. Please refer to UCC25630-1EVM-291 (C7 and C8). In your design you have C31 and C37, total 55nF. If you split them into half, 27nF each, while connect them like C7 and C8 in the EVM, you should have much less initial power current.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    Hi 

    Thanks for your reply!

    I have splited the resonant capacitor into half like the design as  UCC25630-1EVM-291.And the startup current reduced to the  half value。Another

    issue as below waveform，CH2 is FB signal。When starting ，why FB signal is pulled to low and Lo MOS gate driver stoped for a while。

    Thanks！

  • 0 in reply to user5943494
    TI__Guru 73455 points

    FB pull down is due to your feedback so you need to probe your feedback start from Vout to see what Vout is doing.

    If you see LO turns off along with FB pull down, you need probe each of UCC256303 pins SS, BLK, RVCC, ISNS, VCR BW to see any pin in question then find out why to resolve the LO shutdown.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    FB pull down at the moment HO fist switching pulse after charge boot。 It should be no feedback from Vout at the moment。 I checked all signals indcluding

    SS, BLK, RVCC, ISNS, VCR BW，Vout， No abnormal behaviors was found when LO turns off。The system should restar when protection happens at this time.

  • 0 in reply to user5943494
    TI__Guru 73455 points

    You use a +12V to supply the opto as well as other circuit of feedback. You need to check these circuit to identify which is pulling down FB as well as possible other errors.

    I suggest you to get an EVM to help your trouble shooting as you can compare each signal from your board to the EVM and resolve accordingly.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    Thanks for your suggestion，EVM maybe the best choice to solve the problem。

  • 0 in reply to user5943494
    TI__Guru 73455 points

    I will close this thread and if you have additional questions, please start a new thread.