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UCC27211: About capacitor between HB pin and HS pin

Kaji@PAN
Mastermind 7795 points
Part Number: UCC27211
Other Parts Discussed in Thread: UCC27282

Hi,

I am considering a circuit that uses the UCC27211 device.
I ask about the capacitance of the capacitor connected between the HB and HS pins.
The data sheet states that for this capacitance, a capacitance of 0.022uF to 0.1uF should be used.
However, since I want to drive the High Side Switch on for a longer time, I want to use the capacitance of 4.7uF.
Is it okay to use 4.7uF?

Best regards,

  • 0
    TI__Mastermind 23155 points

    Hello Kaji,

    It is OK to use larger HB-HS capacitance than is listed in the datasheet typical value range. There are however important considerations using large boot capacitance values. It takes a longer time to charge the boot capacitor initially when the converter starts the 1st cycles, when the HB capacitor is charged initially, the charge is supplied by the VDD capacitor so the VDD capacitor needs to be sized to minimize the voltage drop. We usually recommend VDD cap 10x the boot capacitance.

    In this application is the switching frequency always low, which requires the long HO on time?

    I would recommend calculating the required boot capacitance using the design procedure in the UCC27282 datasheet section 8.2.2. The datasheet can be found here:

    http://www.ti.com/lit/ds/symlink/ucc27282.pdf

    The process is based on VDD and the switching frequency, and takes into account the quiescent current of the HO driver. For the UCC27211 use an IHB of 0.1mA to calculate the boot capacitance.

    If a lower HB capacitance can be used, we recommend that to allow faster charging of the HB capacitance on start up.

    Confirm if this addresses your questions, or you can post additional questions on this thread.

    Regards,

  • 0 in reply to Richard Herring
    Mastermind 7795 points

    Thank you for your reply.
    I would like to ask you an additional question.

    I understand that the device of UCC27211 can use 4.7uF.
    Is this understanding correct?

    It has been pointed out that the problem that the voltage drop of the VDD capacitor becomes large at startup, but the problem that the inrush current of the bootstrap diode becomes large at startup is not pointed out.
    The larger the capacitor, the larger the current flowing in the device's built-in bootstrap diode.
    How do you deal with diodes?
    Is there a margin in the current capacity of the diode?

    Best regards,

  • 0 in reply to Kaji@PAN
    TI__Mastermind 23155 points

    Hello Kaji,

    The UCC27211 can have a larger boot capacitor than the typical suggested range.

    The boot diode is capable of high peak currents, and this high current should only happen on the initial cycle(s) when charging the boot capacitor from 0V.

    I do suggest you look at the boot capacitor value based on my previous post, to see if you need this large value capacitance.

    I assume your application will be running at low frequency (<40kHz), can you confirm your operating frequency?

    We would not suggest, or do you need a large boot capacitance with higher switching frequencies such as in power convertors.

    If you desire, I can provide the required capacitance if you can provide me the following: VDD voltage, MOSFET part number (# number of devices in parallel), switching frequency, and HO expected maximum duty cycle.

    Regards,

  • 0 in reply to Richard Herring
    Mastermind 7795 points

    Could you please activate this thread for additional questions?

    I got the following answer from you.
    We would not suggest, or do you need a large boot capacitance with higher switching frequencies such as in power convertors.

    Please provide details on why you are not recommended for the above content.
    Is it because the large capacitance has a large ESR?
    In that case, can it be solved by connecting an additional small capacitance with low ESR in parallel?

    I have already shipped the product, so I would like to proceed with the current circuit constants. Boot capacitance 4.7uF, operating frequency 10kHz.

    Best regards,

  • 0
    Mastermind 7795 points

    Part Number: UCC27211

    Hi,

    I previously got the following answer in this thread.
    “I don't recommend if you have a high switching frequency like a power converter and you need a large boot capacitance.”

    Please provide details on why the above content is not recommended.
    Isn't it recommended to use a large capacitance because the ESR becomes large?
    If so, can this be solved by adding an additional small capacitor with low ESR in parallel?

    I have already shipped the product, so I would like to proceed with the current circuit constants.
    Current constants: boot capacitance 4.7uF, operating frequency 10kHz.

    Best regards,

  • 0 in reply to Kaji@PAN
    TI__Mastermind 23155 points

    Hello Kaji,

    With your application with switching frequency of 10kHz, the use of a larger boot capacitance will be OK.

    The concern I mention before, if you have higher switching frequency, as in a power converter, with a large boot capacitance it will take more time for the boot capacitor to charge on the initial cycles. Also with higher switching frequencies you should not need a larger boot capacitance since the HO on times will not be long duration.

    Confirm if this addresses your question, or you can post additional questions on this thread.

    Regards,