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UCC28740: PMP41009

cks Chen
Prodigy 100 points
Part Number: UCC28740
Other Parts Discussed in Thread: PMP41009

Hello, I have designed an AC-DC flyback converter with an input of 85–900 VAC and an output of 24 VDC, adopting a two-switch cascaded flyback topology that is consistent with the design of UCC28740 chip and PMP41009. My questions are as follows:

  1. Is the design of the transformer in the picture correct?
  2. The duty cycle of my MOSFET is very small. I replaced the parameters of the relevant components in the voltage feedback loop, but there was no change. Only by increasing the parameter values of the RC circuit between the control chip CS pin and the sampling resistor in the current sampling circuit can the duty cycle be increased, and changing other parameters has no effect. The output voltage of the flyback converter is normal under no-load and light-load conditions, with the duty cycle remaining at a very low level (appearing as a flat line). However, under heavy-load conditions, the duty cycle is still very small and the output voltage drops to around 15V. What could be the possible reasons for this phenomenon?a2890dc90a3d393213f5de076df3eb6a.jpg
  • 0
    TI__Mastermind 42940 points

    As mentioned, Pout = 46.31W and Vout = 24V is desired at full load condition, that means Iout = 1.9298A.

    And I_load_CC is mentioned as 1.875A, so under full load condition converter will enter into CC mode.

    So i would suggest to increase CC limit and that will be done using CS pin resistors only, and so you find that changing CS pin resistor, duty cycle is increased since CC limit is changed. 

  • 0 回复 Vivian Gao
    Prodigy 100 points

    So, what should I do now? Should I increase the resistance of the CS pin? What about the capacitance value? Should I increase both of them? However, increasing the RC values will result in loud transformer humming. Could you please give me some advice?

  • 0 回复 Vivian Gao
    Prodigy 100 points

    I'm not operating at full load — only at half load.

  • 0 回复 cks Chen
    TI__Mastermind 42940 points

    Decrease value of Rcs to have CV mode upto full load. Capacitance at CS pin is just for filter. Refer to 10.1.5 of datasheet---(A small filter capacitor may be placed on CS to GND, with short traces, to filter any ringing that may be present at light load conditions when driving MOSFETs with large gate capacitance. This capacitor may not be required in all designs.)

  • 0 回复 Vivian Gao
    Prodigy 100 points

    The value of RCS has dropped to 0.35 ohms. Do we need to lower it further?

  • 0 回复 cks Chen
    TI__Mastermind 42940 points

    Use value of Rcs as per equation 14 given in datasheet.

  • 0 回复 Vivian Gao
    Prodigy 100 points

    Hello, I haven’t been able to find the waveform diagram of the MOSFET Vgs for the flyback converter designed with the UCC28740 chip. It seems that only the Vds waveform diagram is available. Could you please help me find the Vgs waveform diagram?

  • 0 回复 cks Chen
    TI__Mastermind 42940 points

    yes, test report doesn't have VGS waveform and i don't have PCB to test and provide you. What is issue you are facing ?

  • 0 回复 Vivian Gao
    Prodigy 100 points

    Hello, my problem is that the duty cycle of my MOSFET is incorrect. My design requires it to work properly within the 85-900VAC input range, but the current duty cycle is too small. Adjusting the voltage feedback section has no effect; only changing the current sampling resistor of the CS pin on the control chip and the relevant parameter values of the RC circuit can alter the duty cycle. However, after making these changes, the output voltage is too high under no-load conditions, and the duty cycle does not increase significantly either. So is there any solution to this problem? I am using a SiC MOSFET—does this have any impact?The first image shows the Vgs and Vds waveforms of the MOSFET with 220VAC input under no-load condition, and the second image displays the Vgs and Vds waveforms of the MOSFET with 220VAC input under half-load condition. The upper trace is Vds and the lower trace is Vgs.

  • 0 回复 Vivian Gao
    Prodigy 100 points

    My design is almost the same as that of PMP41009, with the only major difference being the addition of a driver chip in the driving circuit to drive the SiC MOSFETs. Could this be the cause of the problem? However, the waveform output by the control chip DRV is exactly the same as and perfectly matches the Vgs waveform of the MOSFETs. So I really have no idea where the problem lies right now. Could you help me figure it out? Thank you.