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UCC28951: UCC28951

Harshal Pawar17
Intellectual 260 points
Part Number: UCC28951

Hi

I am working on a step down converter using PSFB topology. The IC ucc28951q1 suits my purpose. My initial work of designing control card is completed. Now when i try to test the control card i observe these following

1. I observe gate pulses on channel A, B, C & D with around 50% duty cycle. My question is will these gate pulses appear even if there are no feedbacks ( output voltage and input current)?

2 . When I externaly give DC supply to pin 3 through a voltage divider and triangular wave with  2V peak amplitude and frequency being my switching frequency, I am not observing any changes in the duty cycle. Also if I fiddle with these external signals I expect my duty cycle to vary, but I am not observing any changes. I want to know If I am going wrong in testing the control card, or there is any mistake in my calculations?

I refered 600W PSFB DC-DC converter for my design 

 

  • 0
    TI__Mastermind 25570 points
    Hello Harshal

    1/ The UCC28951-Q1 will deliver OUTx signals when VCC is applied so what you are seeing is correct.

    2/ Assuming that you are using the controller in Peak Current Mode (not in Voltage Mode) then the best way to check operation is to connect the CS pin to an external DC source through a high impedance resistor and to put a capacitor from CS to ground. You will have to experiment to get the best values but you could start with 1nF and about 25k ohms fed from a 10V source. The 10V source will charge the capacitor to about 2V in 5us and the internal pull-down on CS will discharge the capacitor a the end of the switching cycle. This should give you a triangular ramp at the CS pin which will mimic the real CS signal. You can then adjust the voltage at Pin 3 to control the duty cycle. There is an 850mV offset on the PWM comparator input so a voltage of 850mV should give you 0 duty cycle and 2.8V should give you maximum Duty cycle.
    Please note that the internal pull down on the CS pin is not mentioned in the data sheet. Also please note that if the CS signal reaches 2V then the internal Cycle-By-Cycle comparator will trip and this may start a hiccup cycle. However this simple technique should allow you to check your control card.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Intellectual 260 points
    Thanks Colin, I will try this and get back to you soon with my observations