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UCC27200A: shorted between HB and VDD

Howard Zou
Genius 12365 points
Part Number: UCC27200A

Hi,

my customer is using UCC27200A in their DC/DC push pull circuit. The DC BUS is 48V from battery. There are several times that they find the device HB and VDD shorted, which means that the diode is broken. The break happens during the start up of the push pull circuit.

we measured the voltage waveform with good device at operating, CH1 HB to GND, CH2 HS to GND.

rising edge

falling edge

During the circuit start up, the voltage waveform is as below: the peak voltage can reach 78V. But it should still be within the recommended operating condition.

The break happens during the start up of the push pull circuit.

1. what's the breakthrough voltage of the internal diode?

2. what may cause the shortage between HB and VDD, how to solve it?

  • 0
    TI__Guru**** 189399 points
    Hi Howard,

    Can you email me the schematic and layout?
  • 0
    TI__Mastermind 20950 points

    Hello Howard,

    I am an applications engineer supporting this device, and will work to solve your concerns.

    As Don requested, can you send the schematics and layout of the circuit, to help determine how to solve the failure?

    For the VDD to HB diode, the VHB has an absolute maximum rating of 120V to VSS, with a recommended rating of 115V to VSS. So the ~58V recorded in the scope plots is not a concern.

    Can you record scope plots of the HB, HS and HO during the start up event, and trigger on the highest voltages? Also can you confirm the VDD voltage level?

    Regards,

    Richard Herring

  • 0 in reply to Richard Herring
    TI__Genius 12365 points

    Richard,
    thank you.
    After evaluating the schematic. It seems that the boot cap value is too large for them. I'll tell the customer to use the appropriate boot cap.
    I have one more question.
    Will it help if we add an external diode between VDD and HB? If yes, what kind of diode is preferred? (voltage rating, current rating)

  • 0 in reply to Howard Zou
    TI__Mastermind 20950 points

    Hello Howard,

    Adding an external diode can help and is usually recommended if the expected boot diode average current may be high due to high frequency operation and high gate charge. Meaning the average current of the HB bias may be higher than most applications.

    We recommend an ultrafast recovery diode with a voltage rating meeting or exceeding the driver voltage rating. A 1A 100 to 200V diode is typically used. A good candidate is the  MUR120.

    Regards

    Richard Herring