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UC28025 application

Red Yen
Expert 4360 points
Other Parts Discussed in Thread: UC28025

Hi Team,

This is Red, power FAE from Taiwan,

My customer was using UC28025 in 1000W charger application, the schematic as attached

I some questions in datasheet page 8 Figure1 typical application,

 

  1. What is the function of below red circle and how to design it?
  2. Overshoot problem, the setting output voltage is 73.5V, in no load condition, output voltage will increase to 110V, how to solve it?
  3. Over 800W condition, power will unstable to brunt out the MOS, how to prevent it?

 

Thank you so much

 

 

 

UC28025.doc
  • 0
    TI__Expert 4375 points

    Red,

     

    Regarding question 1. This part of the circuit marked in your word file is adding slope compensation to the current sense signal that is used for current mode control. The 1kOhm resistor is just a small filter resistor. The 8.2k resistor is selected based on how much slope compensation you want to add. For the push-pull circuit shown in figure 1 the 8.2k resistor is selected using the following equation.

    V_ramp*fosc*Lout*n/(D_max*Vout*M)*Rflt, where

    V_ramp is the oscillator valley yo peak voltage, 1.8V

    fosc, is the switching frequency, 1.5Khz

    Lout is the output inductor, 0.8uH

    n is the transformer turns ratio, 5

    D_max is the maximum duty cycle, 0.45

    Vout is the output voltage, 5V

    M is the ratio between the added ramp and the down slope of the inductor current, usually between 0.5 and 1.

    Rflt is the 1k resistor.

     

    The 10nF cap is for DC blocking and is selected to ensure a low impedance at the switching frequency. The 150pF is a small by-pass cap for noise.

     

    We really do not have enough information to help with answering questions 2 and 3, above. It would probably require local debug in the lab to resolve.

     

    Regards,

     

    Richard.