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UC3825: Design Doubt

Prashant Surana
Intellectual 840 points
Part Number: UC3825

Hi TI e2e Members

I am using UC3825 for single switch forward converter application. I am using IC in current control mode. I am having some doubts regarding this IC. 

link to datasheet : www.ti.com/.../uc3825.pdf

From the page number 1 of datasheet, I can see that EA out and Ramp(+1.25V offset)  are compared to get PWM pulses. But in oscillator section(page 3), peak voltage 2.8V is given. So how this peak voltage of 2.8V is significant here? I am not able to understand actual parameters which controls duty.

With Regards

Prashant Surana

  • 0
    TI__Mastermind 26695 points

    Hi Prashant,

    Thank you for your interest in UC3825. The oscillator section refers to the internal oscillator on the chip which controls the switching frequency. The duty cycle is determined by comparing the ramp signal and the error amplifier output.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Intellectual 840 points
    Hi Ben
    This is applicable to current mode control. Am I right?
    And What is the Value of Current sense resister I should Use, can you provide any guideline for that?
    Thanks
  • 0 in reply to Prashant Surana
    TI__Mastermind 26695 points

    Hi Prashant,

    In both voltage mode control and current mode control the duty cycle is determined by comparing the ramp signal and the output from the error amplifier. The difference is how the ramp signal is derived. In voltage mode, this is done by charging/discharge capacitor Ct (which is connected to both the Ct pin and the Ramp pin). In current mode, the ramp signal is derived from the current information (sensed through Rsense). Page 5 of the datasheet illustrates this. 

    You will need to keep the peak voltage on the sense pin below the shutdown threshold. The size of your sense resistor will depend on the peak primary side current. If you already know your design values for input, output, transformer magnetizing inductance, I would recommend using a tool called power stage designer. You can input your design values and get a close approximation for your peak primary current which you can use to select an appropriate Rsense

    Best Regards,

    Ben Lough