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UCC27201: How to drive high side mosfet of H bridge Buck-Boost converter ~100%duty, high side bootstrap capacitor voltage drop problem~

JapaneseStudent
Prodigy 250 points
Part Number: UCC27201
Other Parts Discussed in Thread: LM5175, LM5113

I’m sorry for my poor English.

I am studying H-bridge Buck-Boost converter. H-bridge Buck-Boost converter has 3 modes; Buck mode, Buck-Boost mode, Boost mode. When in Boost mode, SW1=ON, SW2=OFF, SW3=SW4=ON or OFF．So, SW1 needs Duty100%.

But when the drive method of the high side mosfet is set to the bootstrap method, the power of the bootstrap capacitor is exhausted, and 100% duty is not achieved. I used ucc27201(gate driver). When trying to operate at 100% duty, Undervoltage Lockout (UVLO) worked and Sometimes the duty ratio was not 100%. Undervoltage Lockout makes boost mode work. But Undervoltage Lockout causes noise. 

How can I achieve 100% duty? Is Undervoltage Lockout  the best method? Or Should I add another circuit like a charge pump circuit?

my application is Vin, Vout=3~20V, Iin, Iout=0~3A. I am using digital control with MCU.I will use lm 5113(Gate Driver) in the future. I am not considering pmos for the convenience of my application.

  • 0
    TI__Genius 9790 points
    Thanks for your interest in TI here. I've contacted the appropriate product group. You should hear from them soon.
  • 0 in reply to Yunsheng Qu
    Prodigy 250 points
    Thank you very much.
  • 0 in reply to JapaneseStudent
    TI__Genius 12990 points
    Hi JapaneseStudent,

    Thanks for your question.

    At 100% duty cycle the bootstrap capacitor will not be replenished and will eventually discharge to the point where the HO output can no longer keep the high side MOSFET turned on.
    -If you need to operate at 100% then you will have to generate a separate bias rail for the bootstrap capacitor


    You can 1) keep the bootstrap capacitor charged by using a small high frequency transformer to generate the 12V rail for the Bootstrap capacitor (or up to 20Vmax for VHB - VHS). This allows the driver to keep the top side MOSFET ON for as long as you like.
    -transformer requirements - drive its primary from a 12V supply and rectify the output.
    You may have to put some small resistive load or a zener diode on the output to keep the HB output within reasonable limits.
    You could find a suitable transformer if you look for gate driver transformers on the web.



    You can 2) of course use the 12V rectified transformer output to drive the MOSFET directly.
    -This would require that you can generate a carrier signal and be able to turn it on and off using the HI signal.


    You can also refer to this e2e post:
    e2e.ti.com/.../413229


    Thanks
  • 0 in reply to Jeffrey Mueller
    Prodigy 250 points

    Thank you very much. I want to listen to other people's opinions, so I will continue to ask this questions.

  • 0 in reply to JapaneseStudent
    TI__Genius 12990 points

    Hi JapaneseStudent,

    Sounds good, I can ask the rest of my team this question and see if they come up with anything different. I will also keep this thread open for you to seek more opinions.

    Thanks 

  • 0
    TI__Mastermind 21335 points

    Hello Japanese Student,

    I am an applications engineer at Texas Instruments and will work to answer your questions. I reviewed a previous post by Jeff, who provided good advice on using the UCC27201 driver with 100% duty cycle on the high side drive.

    As was mentioned, to provide DC drive for the high side, a small floating bias supply needs to be provided for the HB-HS high side driver bias to sustain constant on time. TI has a device well suited for the bias which is the SN6505. You may need to add some components to make sure the HB-HS bias stays within its operating range such as some load resistance, or limiting zener diode.

  • 0 in reply to Richard Herring
    Prodigy 250 points

    Because I am a beginner, your information is very helpful. Thank you.

  • 0 in reply to Richard Herring
    Prodigy 10 points
    Hi Richard,
    I noticed the lm5175 is also H-bridge topology as Japanese Student.
    How does it achieve SW1 or SW3 always ON(100% duty) in boost or buck mode?
  • 0 in reply to user3487469
    TI__Mastermind 21335 points
    Hello,
    I am an application engineer with the driver product line in TI, and assisted with the question on the UCC27201 question about high side driver DC operation. For the LM5175, I see why you asked the question, as the high side switches do need to operate in DC depending on the operating mode.
    I would have to refer you to someone who is more familiar with the LM5175 regarding that device operation details.

    Regards
    Richard Herring
  • 0 in reply to Richard Herring
    Prodigy 250 points

    Dear ,  Richard

    I'm sorry for my late reply, and thank you for your reply. The LM 5175 is a great suggestion. But for my convenience, I can not use the LM 5175. I have to use a MCU. 

    I am very interested in how the LM 5175 achieves Duty 100%. But maybe it's a trade secret. But if there are little hints, I want it. For example, whether 100% duty is realized with a transformer or whether it is realized by charge charge.

  • 0 in reply to JapaneseStudent
    TI__Mastermind 21335 points

    Hello Japanese Student,

    Thank you for the continued interest in the UCC27201, for my suggestion on 100% high side duty cycle we will recommend the high side floating dedicated bias supply. For the LM5175, I will refer you to another product line who is more familiar with this part.

    Regards,

    Richard Herring

  • 0
    Prodigy 250 points

    Everyone

    I thought about a circuit to achieve 100% duty as below. Do the following circuits operate correctly?

     I made Vin + 5V by adding a charge pump circuit.  LM5113's recommended voltage between the bootstrap voltages is 4 to 5.5 V. So, I clamped with a 5.1 Zener diode. I am concerned about the transient situation.

    In creating this circuit I referred to “Providing Continuous Gate Drive Using a Charge Pump, TI"

    Thanks,

  • 0 in reply to JapaneseStudent
    TI__Mastermind 21335 points

    Hello Japanese Student,

    I am an application engineer with TI, and have responded to previous posts about this application.

    In principal it looks like the charge pump should work. But you may want to confirm the effects of the diode drops from VIN and to HB. It looks like when PWM is 0 you will have 99.3V on the cathode of D2/anode of D1. When PWM is 5V you will have 104.2V at that same node to charge HB. It may take a slightly higher PWM signal.

    For the transient conditions there may need to be some limiting resistance either in the PWM source waveform or in series with D1.

    Regards,

    Richard Herring

  • 0 in reply to Richard Herring
    Prodigy 250 points
    Dear Mr./Ms. Richard Herring
    Thank you for your reply. I will consider your opinion.
    Best Regards,
    JapaneseStudent