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UCC28782: What is the principle of frequency dither of UCC28782

Shuai Fan
Genius 13451 points
Part Number: UCC28782

Hi expert!

We are promoting UCC28782 to customer. They have some questions about this IC's function. Could you please help to explain the below question?

1. What is the  principle of frequency dither of UCC28782?

2. Whether it will lost ZVS in some cycle because of  frequency dither operation?

Best regards

Shuai

  • 0
    TI__Mastermind 40230 points

    Hello Shuai, 

    Thank you for your interest in the UCC28782 Active-Clamp Flyback controller. 

    Section 8.4.3 of the UCC28782 data sheet discusses the details of the frequency dithering function. 
    Basically, when Vvs > 2.5V during the demagnetization time, a time-varying signal of 96mV peak-to-peak is superimposed on the nominal Vcst voltage (refer to Figure 8-23). 
    This time-varying signal is triangular in shape and repeats at a frequency of about 23kHz.  
    The peak-to-peak Vcst dithering amplitude is reduced to 36mV in the case where Vvs < 2.4V during the demagnetization time, such as in USB-PD applications when Vout is low. 

    Frequency dithering is a common PWM modulation technique that reduces the quasi-peak and average amplitudes of conducted-EMI noise measured at the LISN. 
    By varying the current-sense threshold (Vcst) on a periodic basis, the cycle-by-cycle peak current is varying, which in turn causes the switching frequency to vary as well. 
    Therefore, the quasi-peak and average amplitudes of those switching frequencies are lower than if dithering was not implemented.
    Note: in the UCC28782, the dithering function cannot be disabled. 

    The second paragraph of Section 8.4.3 states that ZVS will not be lost due to the dithering operation. 

    Regards,
    Ulrich