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TPS5102: pre-bias start up

Part Number: TPS5102
Other Parts Discussed in Thread: TPS40040


Is this device supported the pre-bias startup?

I know this device is very old device. However our customer has been using this device for a long time. They ask us some question.

If this device supports that function, how this device operate (means behavior) when the load current is increased (DCM to CCM).
(The duty of low side fet is increased to until (1-D)T immediately? or the duty is increased slowly?)


Koji Hamamoto

  • hello Koji,

    Pre-bias function has no relationship with loading current increaing(DCM to CCM).

    This part support Skip mode at light loading to increase efficiency. At skip mode, low side FET just give a short pulse instead of full pulse, so frequency is lower than CCM mode. when loading is increased(DCM to CCM), the duty is increasd slowly.  

    best regards.


  • Hi Edwin,

    Thank you for your reply.

    I am sorry for getting you confused.
    What I wanted to ask is about the synchronous and asynchronous operation at pre-bias state.(not CCM and DCM )
    When it startup with pre-bias operation the converter will operate at asynchronous operation because the low side fet turn off for not sinking the current.
    So that,the device begin to operate at asynchronous then will shift to the synchronous operation after startup.(In fact we measured and we saw the device looks operating at such behavior.)

    Is my understanding correct?
    If my understanding is correct, does this device have pre-bias function?
    And if it so,how this device work while this device shift to Sync from Async?
    The duty cycle is increased gradually? or the duty cycle will be high until to (1-D)*T immediately?

    Koji Hamamoto
  • Hello Koji,

    I'm sorry and I'm more confused.

    Generally, asynchronous or synchronous is two kinds of rectification. synchronous rectification use MOSFET, and asynchorous rectification use diode. they don't have relationship with pre-bias fuction.

    can you upload the waveform and say what's issue?

    best regards.


  • Hi Edwin,

    (I am sorry for getting you confused...)

    My question is does this device have a pre-bias startup function? Or not?

    Another question is how this device behave at the pre-bias startup ? The duty increase gradually ? or increase immediately?

    There are no explanations on the datasheet about pre-bias startup. However the behavior on measurement seems this device has the pre-bias function.

    That is why I asked about this question.

    Also regarding synchronous and asynchronous it is not configuration.

    (synchrnous = two FET (high side and low side). asynchrounous = high side is FET and low side is diode) It is about a behavior of switching.

    Please see the following waveform.That is a waveform of TPS40040 at pre-bias start. (There are some explanation about pre-bias startup function.)

    The high side is a short on duty and the low side is also a short on duty. At the time of high side is Off and low side is Off this region is an asynchronous behavior similally.

    Because the body diode of low side fet is conducted. (Also there are no sink current from output at pre-bias startup.) That is why I mentioned synchronous and asynchronous.

    Please correct me if my understanding is incorrect.


    Koji Hamamoto

  • Hello Koji,

    Thanks for your detail information, now I'm very clear.

    This is an old part, I couldn't find more data and block information. So we only analysis its pre-bias behavior according to waveform during soft start. Ithink it's also a better way to know the pre-bias behavior. and it's only way so far. I'm sorry that I can't provide pre-bias data,

    You have measured that it has pre-bias function. please help to capture high side driver and low side driver and Vout voltage during soft start to see how to rectificate current.

    best regards,


  • Hi Edwin,

    Thank you for your support.
    I had a discussion with our customer about this.
    They understood this situations. Please close this.

    Koji Hamamoto