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UC3875: Slope compensation in CMC

Vasiliy Aleksandrovich
Intellectual 290 points
Part Number: UC3875
Other Parts Discussed in Thread: UC3879, UCC3895, UCC28951

Hello. I am now working on a PSFB with CMC and trying to run CMC configuration on UC3875. It's not many information about CMC with slope compensation on datsheet, so i want to clarify how to configure it properly. DS says to grounding SLOPE pin, and connecting CS directly to the RAMP pin. How can i now achive slope compensation? Should i connect FREQSET pin via resistor to the RAMP pin? How to calculate the value of this resistor?

  • 0
    TI__Expert 3350 points

    Hi Vasiliy,

    According to the datasheet description, the Ramp generator was disabled by grounding the slope pin for current mode. I am worry that you have to add external circuit for the slope compensation. You can ref to the chapter 8.3.9 of UCC2843 datasheet to design the outside circuit for slope compensation.

    thanks

  • 0 in reply to Bliss Zhou
    Intellectual 290 points

    Hi Bliss Zhou,

    Thank you very much for quick response, but I am afraid I still didn't get it. In UC3875 generator has a little bit different configuration compare to UCC2843 (BTW can't find chapter 8.3.9 in DS). Both, Rt and Ct goes to ground according to the DS. In UCC2843 Rt goes to Vref and Ct goes to gnd. Next thing is that 3875 has a high-speed oscillator and ramp generator witch one is disabled by grounding the SLOPE pin, right? So high speed oscillator still running and at Ct we should have a ramp, isn't it? So, idea is to take that ramp and via summing resistor add this signal to filtered current rump. Such technique is used in UC3879 (U-154 THE NEW UC3879 PHASE-SHIFTED PWM CONTROLLER SIMPLIFIES THE DESIGN OF ZERO VOLTAGE TRANSITION FULL-BRIDGE CONVERTERS p.4 fig.6). What did I missed?

  • 0 in reply to Vasiliy Aleksandrovich
    TI__Expert 3350 points

    Vasiliy

    Sorry for my  late reply.  You know UC3875 is old part. Actually, I did not test and debug this part before and I don't have EVM to test now. But, I think it is not hard to find way for Slope compensation using outside circuit. If you hope we discuss together, could you share your waveform to us.

    BTW, I will check with my colleague for the way of  Slope compensation. If he have good idea, I will update to you.

    thanks  

  • 0 in reply to Bliss Zhou
    TI__Expert 3350 points

    Vasiliy,

    UC3875 is an old part, Are you willing to select UCC3895 and UCC28951? UCC3895 and UCC28951 can be slope compensation easy.

    thanks

  • 0 in reply to Bliss Zhou
    Intellectual 290 points

    Bliss Zhou, hello again!

    Unfortunately i can't share waveforms now, due to the virus. I simply can't get to my lab, so i can work only with schematic for now ;( I know, that this new PWM controllers, are much more user-friendly, but i stuck with that particular model due to my task, and can't use another. Furthermore, we already have first version of PCB. Anyway UC3875 still has "ACTIVE" status, so i thought it's not so hopeless. But application notes for UC3875 are poor compare to modern one, indeed.

    Thank you for trying to help! 

  • 0 in reply to Vasiliy Aleksandrovich
    TI__Expert 3350 points

    Hi Vasiliy,

    I had discussed with my colleague this weekend. please review the updated.

    The UC3875 was designed mainly for voltage mode control applications – the SLOPE pin is mainly used to vary the slope of the RAMP pin to provide voltage feed forward. The reason the SLOPE pin cannot be used in Peak Current Mode control is that there cannot be any significant capacitive load on the current sense signal at the RAMP pin. we recommend you use the UC2895 or the UCC28951.

    In addition, There is a way for the UC3875 slope compensation. But it is quite complicated.

  • 0 in reply to Bliss Zhou
    Intellectual 290 points

    Thank you, sir, for finding time and providing such a useful answer. It's will help me alot. I will have tried to get such a waveforms on my oscilloscope, as soon as i get to my lab. BTW, wich tool did you used to simulate this schematic? Is it LTspice?