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UCC24612: Adaptive Blank time specification

YoonJin Kim
Intellectual 1100 points

Part Number: UCC24612

Hi team,

I have question related to Adaptive Blanking Time of UCC24612.

Following the datasheet, there is Adaptive Blank Time function which prevents false turn-on of the device occured by DCM ringing. It seems the blank time is decided to have the value which is calculated by (N-1) SR_OFF time * 70% but in Samsung board, there is false turn-on before the blank time ends. Details are as below:

 

 

TOFF(N-1) = 4.772us so, blank time would be calculated as 70%*TOFF(N-1)=3.34us but as you can see from the below waveform, the SR turned ON with blank time 2.21us which is much less than calculated value 3.34us. Also, it doesn’t make sense comparing with the spec of tOFF_MAX, 2.65us.

I think I might have misconfigured the oscilloscope point 1,2 to measure the blank time or real waveform doesn't follow the datasheet. Could you tell me the exact criteria for measuring blank time in the waveform below? or any idea why measured value is different from the datasheet?

 

  • 0
    TI__Intellectual 1100 points

    Adding the waveform captured from customer board,CH1: VGS, CH2: VD

    I've refered the figure20 from datasheet about the Adaptive off-blanking opeation.

  • 0 in reply to YoonJin Kim
    TI__Guru* 86286 points

    Hello YoonJin,

    The off time blanking occurs after Vds has become greater than 0 V when a flyback converter has gone into DCM.

    This is so the ringing does not falsely turn on the gate driver when the secondary rectifier just has ringing on it.

    Please see the description below.

    The blanking off time is supposed to be 70% of the previous cycles off time.  The circled portion of the waveform below shows high frequency ringing below ground.  This is most likely resetting the off time.  I would add an RC snubber across the drain to source of the FET to dampen this out.  This will let the off timer to work correctly.  There is information on the data sheet on how to setup this RC snubber.

    Regards,