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UC3846: The Calculated Frequency And Deadtime Did Not Match The Measured Values

AlpB
Prodigy 10 points
Part Number: UC3846

Hello,

I'm currently work on H-Bridge control and I want to observe frequency by making changes on different resistance and capacitance values in UC3846. However, I noticed there is differences between the values calculated based on the formulas in the UC3846 datasheet and the values I measured. I connected 3.6k to CR pin and 8nF to CT pin. Calculated oscillator frequency is 76.38 kHz and each channel frequency is half of the result. When i connected the PWM outputs to the my oscilloscope , i just saw that each channel has  31.76 kHz frequency.Then i tried to change resistor to 4.1k and  then frequency changed to 28.11kHz. Is there something wrong with my calculations?

Thank you for your assistance!

  • +1
    TI__Guru 51435 points

    Hi,

    There are possibly two reasons.

    First, how did you get 8nF? A capacitor tolerance can be over 20% variation. If you parallel capacitors the tolerance can be further worse.

    Second, IC frequency calculation can only provide a typical number, the formula has its own tolerance. The data sheet shows over 10% variations.

    The difference should come from the above two. 

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thanks for your reply.I measured the capacitance value using an LCR meter, and it was 7.8nF. Therefore, I thought the tolerance value would stay within the required range. Additionally, when calculating the oscillator frequency of PWM controllers, should I always consider a difference of around 10%? Because it seems %10 variation is not mentioned  in datasheet.

  • +1 in reply to AlpB
    TI__Guru 51435 points

    Hi,

    The specs table shows 43 kHz with about +/- 10%.

    Your LCR meter likely using sine wave to get the capacitor value. While using on the IC it is a unidirectional voltage and if using ceramic there also dc offset capacitance drop. For a particular part number it is better to test with the ic to get the frequency you need then keep using that part number. 

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Thank you, your answers helped me a lot!