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UCC27211: problem on the Vhs pin

Part Number: UCC27211

Hello everyone,

I use the UCC27211 in my project. In particular, I make the electrical circuit as in the datasheet in the half bridge configuration. In this moment, the Vhi and Vli pins are connected to ground and the Vhs pin is not connected to the transformer. I use 9V for UCC27211 and variable boot voltage from 6V to 24V for half bridge mosfet.

My problem is on Vhs pin because I always measure a constant voltage (close to the supply voltage). But if Vhi and Vli are at logical level voltage, in Vhs the voltage it should be zero. It's right?

If I disconnect the high side mosfet, the voltage on Vhs is always present.

Please, can anyone help me? 

Thank you

Best Regards,

Donatello

  • In reply to Donatello Amicone:

    Hi Donatello,

    Sorry to hear you're still having issues. Richard will get back to you next week. Thanks for your patience!

    Best regards,

    Don Dapkus

    Gate Driver Applications Engineering Manager

    Dallas, TX USA

  • In reply to Donatello Amicone:

    Hello Donatello,

    Thank you for you continued work with the UCC27211. I have some questions about the waveforms in the scope plots, to be able to give advice on making improvements.

    I see the scale is 10V/div on the yellow gate signal high side waveform. Is this waveform with the scope connected to HO and ground of the converter? The driver high side output is HO with reference to HS, so looking at HO to ground includes the HS switching signal.

    Can you provide waveforms with LO to ground, HO to HS with differential probe, and HS to ground? If you do not have a differential probe, then can you record the LO to ground, HO to ground and HS to ground; use the same scale and scope ground reference for HO and HS so the difference in HO to HS is clear.

    It is possible the small pulses you see on HO (yellow waveform) exists on the HS to ground, and maybe not the HO-HS gate drive signal.

    Also to confirm the timing, can you provide scope plots of the LI and HI signal inputs, and the HO and LO signals on the same plot?

    Regards,

    Richard Herring

    Richard Herring

  • In reply to Richard Herring:

    Hello Richard,
    I checked the two waveforms: LO to ground and HO to HS. This signal have no small pulses and are clear. At the moment, I do not have problems with UCC27211. Maybe the small pulses between HS to ground (or HO to ground) are produced in resonance by the transformer. I tried to mitigate the small pulses but they are difficult to eliminate, I hope these do not damage the high side of the UCC27211. The ON / OFF gate signals of the MOSFETs are never overlap using a good dead time.
    The half bridge currently works correctly, a heating (about 60°C) only for the high side MOSFET while the low side MOSFET is almost cold. The UCC27211 is quite hot (about 45°C) but I can lower the temperature by connecting the thermal pad to the ground plane. These days, I will try to determine the cause of the higher temperature of the high side mosfet.

    Regards,
    Donatello
  • In reply to Donatello Amicone:

    Hello Donatello,
    I am glad to hear the half bridge is working as expected. From what I recall on the waveforms, it is likely the HS pulses are from resonance on the half bridge when the output current is at lower levels.
    Connecting the thermal pad of the UCC27211 with improve the thermal performance substantially, so we recommend connecting the thermal pad to a ground referenced power pad for reduced thermal resistance.
    For the high side MOSFET there can be a couple of reasons for the higher thermal rise. I do not see if there are resistors in series with the gate of the MOSFET to the driver outputs. Higher gate resistance will reduce ringing and HS switch node peak overshoot, but can add to switching losses in the MOSFET's. You can experiment with reducing the gate resistance if in place.
    Another cause of loss during switching during higher load operation, when the inductor current is always over 0, is the body diode recovery time of the low side MOSFET will result in a short period when the high side Mosfet is turning on but the diode on the low side FET has not turned off yet. Mosfets with faster body diode recovery times can reduce these losses.

    Please confirm if this helps with the issues, and select the green icon on the thread.

    Richard Herring

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