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UCC28780: Can it run in the continuous current mode? How to set the switching frequency?

Jiewen Hu
Prodigy 80 points
Part Number: UCC28780

I'm designing the active clamp flyback converter with UCC 28780. It seems that this controller works well in the CRM mode, but what if I want it to run in the continuous current mode (CCM)? I want to reduce the current ripple of the transformer, as a result, the magnetizing current is always positive. In this case, can the controller produce the PWM signal? 

My second question is, I want to design the converter running at 1 MHz switching frequency, what components should I set up to design the switching frequency?

Thank you and looking forward to your response. 

 

  • 0
    TI__Guru 73825 points

    UCC28780 cannot run in CCM as it is designed only run in critical conduction mode at heavy load and along with burst at light load.

    Its switching frequency is determined mainly on transformer primary inductance at given input voltage and load condition. An easy way is to use MathCAD tool or excel design tool to make calculation. These tools can be downloaded from ti.com, more specific through this link: http://www.ti.com/product/UCC28780/toolssoftware 

     

  • 0 in reply to Hong Huang
    Prodigy 80 points

    Hi Hong,

    Thank you so much for your replying. I have one more question. For some controller chips from Ti, such as UCC 28780, they require AC input and are powered from an auxiliary winding for high power density and low losses purpose. I noticed that this AC source will be rectified by a diode and capacitance. If I can not add another winding to the transformer, can I use a DC power supply or LDO instead to power the controller?

    Thanks and looking forward to your reply.

  • 0 in reply to Jiewen Hu
    TI__Guru 73825 points
    You can use a DC source + LDO to bias VDD pin. But note the aux winding on the transformer primary side has more functions than biasing VDD pin so the aux winding has to be there in order to make UCC28780 work properly. So you cannot skip this aux winding even you use an external DC source to bias the VDD pin.
  • 0 in reply to Hong Huang
    Prodigy 80 points
    I see. Thank you so much!