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UCC256402: How to set startup frequency of UCC256402

user5910734
Prodigy 30 points
Part Number: UCC256402

About UCC256402, I have several questions listed below:

1.How to set the startup frequency? The startup sometimes works OK and sometimes the current may become very large and trig ocp.  Figure 1 is the normal startup waveform and figure 2 is the abnormal one. They are test under the same conditon.

CH1: low_drive   CH4:up_drive     CH3:current_Cr

2.If one MOS of the half bridge LLC has been broken due to some reason, does UCC256402 have any protection to prevent the other MOS from shoot through? 

figure1 

 

figure2

  • 0
    TI__Mastermind 47325 points

    During power on the frequency is controlled by soft start - you can change soft start capacitor CSS to adjust soft start time.

    If one MOSFET broken, there is no way to be sure the other MOSFET to be good, especially if the broken is due to over stressed voltage. If the damage is due to over current, the other MOSFET is less chance to damage, but still likely. The MOSFET cannot be protected ok if the other is damaged catastrophically.

     

  • 0 in reply to Hong Huang
    Prodigy 30 points

    For the first question, my understanding is that the startup current can be reduced by adjusting the soft start time?

    I have one more question, what's the max frequency limit of UCC256402. Can the designed max switching frequency exceed 200kHz?

  • 0 in reply to user5910734
    TI__Mastermind 47325 points

    You can refer to section 7.6 in the datasheet to find the maximum switching frequency. Also refer to section 8.1 for description. In nature, the maximum switching frequency is close to 2MHz, based on these details.

  • 0 in reply to Hong Huang
    Prodigy 30 points

    Thanks. But the startup problem is still not fixed. I have tried to set the initial voltage to 0.3V or even disable the ss initial function to rise up the startup efficiency. Sometimes the soft start is ok, but sometimes the soft start failed under the same Vin and load condition.

  • 0 in reply to user5910734
    TI__Mastermind 47325 points

    If you see inconsistent behavior, usually something not made properly. Which needs to track down such as solder connections, not good connections, etc., these are not something can be supported through E2E. You have to track your circuit by probe, such as sensing, VDD, etc, to find out.

    I suggest you get an EVM so you can have a reference point to compare your waveform to the EVM waveform.