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UCC21530: UCC21530 VDD to HV DC-Link Voltage gap issue

Chao Fu Chen
Intellectual 920 points
Part Number: UCC21530

Hi Team,

My customer mentioned that he supply the 25V in VDD ,and the HV DC-Link did not have high voltage to start.

But they can see the HV DC-Link have around 22V to 23V voltage.

And he just want to know Why have this voltage in there ? Is our IC have any internal circuit or logic part to let it doing ?

If possible , they wonder to get the detail revision block diagram to explain this issue what thing let it happen ?

Eddie

  • 0
    TI__Expert 5149 points

    Hi Eddie,

    Thanks for reaching out!

    Would it be possible to get the customers schematic? It is hard to look at possible links between VDD and HV DC-Link without seeing all the external connections. 

    Is the HV DC voltage present as soon as the VDD voltage is present, or has UCC21530 started switching?

    Finally, if the customer removes our part from the circuit do they still see the voltage on the DC link?

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    TI__Intellectual 920 points

    Hi Daniel,

    Please see my attachment and help to check this issue.

    This is important for us in this customer's project.

    If you see any wrong in here, please let me know it. Thanks!

  • 0 in reply to Chao Fu Chen
    TI__Expert 5149 points

    Chao,

    Thanks for providing the schematic.  don't see any problems here that would explain why the customer is seeing a voltage on the HV link.

    Could you confirm, is the customer seeing this issue in simulation, or in the actual application?

    What is the voltage on the switch node?

    What FET is the customer using? Can you provide the part number?

    Can the customer check if this voltage is present after removing our part? This would help determine if this issue is related to our part or not.

    Thanks,

    Daniel

  • 0 in reply to Daniel Norwood
    TI__Intellectual 920 points

    Hi Daniel,

    The customer feedback as below;

    1.Yes , this issue can see on the simulation and actual application.

    2.The switching node voltage is 24.25V

    3.The mosfet part number is NVHL080N120SC1-D

    4. Yes , they try to remove the UCC21530, and the HV DC LINK point have 3 ~5V voltage when it start up.

    then will down to 0V ,but the UCC21530 didn't remove it the HV DC LINK will keep in 23.46V in there.

    Eddie

  • 0
    TI__Expert 5149 points

    Hi Eddie,

    Thanks for the additional information. I have been able to replicate this in a simple simulation myself. It looks like current is following the path marked in blue.

    When VSSA is floating internal connections pull it up to VDDA as there is no path to charge the bootstrap capacitor. This means the switch node has a voltage just under VDD, and the HV DC link has a voltage just under that due to the reverse diode of the FET.

    As to why VSSA follows VDDA, sinse VSSA is floating there is almost no current through the device. This means that any internal supply circuits essentially serve as large pullup resistors, pulling VSSA up to VDDA.

    Functionally, this should cause no problem for the system. As soon as the bootstrap capacitor is given a chance to charge this effect will disappear. One option if the customer wants to remove this voltage is to turn on the lowside fet for a short time to allow the bootstrap capacitor to charge before the HV DC link is turned on.

    Please let me know if this answers your question by clicking the green button, or if I can help further please let me know.

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    TI__Intellectual 920 points

    Hi Daniel,

    As you mentioned this HV DC link Voltage come from the bootstrap capacitor charge to have in there. Am i right ?

    But I ask my customer, he said that he remove the bootstrap capacitor still can see the HV DC link have the Voltage on there.

    Could you have idea where is the power come from ?

    Eddie 

  • +1 in reply to Chao Fu Chen
    TI__Expert 5149 points

    Hi Eddie,

    I am sorry for the confusion, I believe a small amount of power is going through the UCC21530 from VDDA to VSSA. The bootstrap capacitor does not cause this voltage, but it is essential in controlling it, as the bootstrap capacitor needs to charge to keep the switch node from floating.This problem only occurs when the switch node is floating, as this means the VSS pin is floating. When the VSS pin is floating it can get pulled up to the VDD voltage.

    Does this make sense? Is this voltage causing problems in the customer application, or do they just want to understand where the voltage is coming from?

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    TI__Intellectual 920 points

    Hi Daniel,

    Thanks your comment . And I have an idea in here.

    As you said the bootstrap capacitor is charge the HV DC Link will remove this voltage come for VDDA to VSSA.

    If we try to let the bootstrap capacitor value to change the bigger and let it discharge slowly in next step.

    When it charge again then it will keep the power continue. Is it have any side effect ?

    Eddie

  • +1 in reply to Chao Fu Chen
    TI__Expert 5149 points

    Hi Eddie,

    I believe you understand correctly, as soon as there is a voltage across VDD to VSS then VSS will no longer be floating, so this issue will not be present.

    You could adjust the bootstrap capacitor value, as the bootstrap capacitor has to be large enough to keep the voltage above UVLO under normal operation. But I think this is a different issue, as the unexpected voltage on the switch node is only present before the circuit starts switching. It should not affect normally operation.

    I really don't think there is a need for a fix here. Is the customer seeing a problem with this voltage?

    Regards,

    Daniel

  • 0 in reply to Daniel Norwood
    TI__Intellectual 920 points

    Hi Daniel,

    Thanks your big support in this case and strong comment for us to clarify this application  issue.

    Finally customer decide to do another aux power for VSSA side to remove bootstrap cap.

    Because this  power stage not always turn ON , when the main high power voltage trough in here.

    It will start up , so the bootstrap cap will long time did not  in charge. It is hard to used our solution (low side  mos turn off or big capacitor)

    Just let you know the final decision from customer side. Still appreciate you help Daniel, Thanks! 

    Eddie