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Original question:

UCC28070: UCC28070 simplis model

UCC28070: Several questions

Naizeng Wang
Expert 5470 points
Part Number: UCC28070

Hi team,

Here are some questions about UCC28070.

(1) Why we should limit the max duty cycle?

(2) The out A and out B are average current of inductor calculated by synthesizer. Right?

(3) The programming range of the PKLMT voltage extends to upwards of 4 V to permit the full use of the 3-V average current sense signal range; however, note that the linearity of the current amplifiers begins to compress above 3.6 V. Why the peak average current is 3V?

(4)For soft start to be effective and avoid overshoot on VOUT, the SS ramp must be slower than the voltage-loop control response. Choose CSS ≥ CVZ to ensure this. What's Cvz?

Thanks

  • 0
    TI__Genius 9140 points

    Hello Wang，

    1.for boost PFC, if there is no Dmax limit, then it is possible that the mosfet is turning on for a long time , and boost inductor become saturate that cause a very high current through mosfet. So we need a DMAX to protect the mosfet and inductor.

    since UCC28070 uses current transformer to do the current sampling, in order to avoid CT to go to saturation, we also need Dmax.

    2.The controller samples mosfet on current and use synthesizer to form the discharge current and in this way the controller know the average current.

    3.Here peak current at 4V, the ripple+ average current is rough equal to 3V.  with different pfc inductor ripple parameter, the total of the peak current should not be precious equal to 4V.

    4.Czv is the cap on VAO(Czv is in series with a resistor to ground.)