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UCC5350: SCH review

Allen Lu
Intellectual 2120 points
Part Number: UCC5350
Other Parts Discussed in Thread: SN6501

Tool/software:

hi team，

 can you help to review the sch of the UCC5350MCDR here ？Customers reported that the UCC5350MCDR driver was powered by bootstrap mode with a power supply of 15V. When testing the output waveform on the secondary side, it was found to be normal at the beginning, but the output was turned off after a while (the primary side continued to transmit the wave). we used a multimeter to measure the secondary side power supply voltage and found that it was only have 11V. The bootstrap diode voltage drop was 1.7V. then we changed the power supply voltage to 16.5V, but the problem still occurred. What is the possible reason? 

 tks for the comments here.

  • +1
    TI__Mastermind 19736 points

    Hi Allen,

    An oscilloscope measurement would help provide insight into the numerical specifics of this supply droop for this schematic review.

    A bootstrap capacitor's charge does not last forever. The charge starts draining as soon as the bootstrap diode is reversed biased. Furthermore, the UCC5350 will turn itself off once it detects that its power supply has fallen below 12V or so.

    You have 110.1uF of bootstrap capacitance, which is a lot! It sounds like your expectation of how long the bootstrap can last is too high. A bootstrap needs to be periodically rechanged, and it works best for use in a half-bridge with a relatively high complimentary switching frequency.

    If you need a DC capable high-side supply, you should consider using SN6501 and a small transformer or UCC14141, which is an integrated isolated supply IC.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    TI__Intellectual 2120 points

    hi Sean,

     tks for the comments here, after check with customer ,it major related to the 15v power issue. 

     regarding to the bootstrap mode of ucc5350, customer just want to save cost of the iso-pwoer device here and thats why customer use 110.1uF of bootstrap capacitance here. 

     so would you help to comment whether the bootstarp mode work here? 

     and why you mentioned "A bootstrap needs to be periodically rechanged". do you mean the capacitor will failure during long period working?

  • 0 in reply to Allen Lu
    TI__Mastermind 19736 points

    Hi Allen,

    It sounds like the switching speed here is too slow and the bootstrap capacitor is not going to work as well as an isolated supply.

    A bootstrap capacitor needs to be recharged, like a battery. It only gets charged when the low-side switch is closed. Once the low-side switch is open, the bootstrap capacitor's charge starts draining and eventually the voltage runs out. 

    The capacitor itself won't fail, it will just run out of charge and the power goes out for your high-side gate driver.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    TI__Intellectual 2120 points

    hi Sean,

     as you mentioned as below, if working at a high frequency, does it means the a big bootstrap capacitor will not runs out the voltage and it will keep the "battery" full and work like a iso power here?

    Sean Cashin said:
    It only gets charged when the low-side switch is closed. Once the low-side switch is open, the bootstrap capacitor's charge starts draining and eventually the voltage runs out. 
  • 0 in reply to Allen Lu
    TI__Mastermind 19736 points

    Yes, a high frequency means that the low-side switch will close more frequently, and there will be a shorter interval to recharging. This is the best application for a bootstrap circuit.

    Best regards,

    Sean