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Original question:

UCC35706 resonant forward convertor failure

UCC35705: PWM generator not working correctly

user6098850
Prodigy 30 points
Part Number: UCC35705

Hi.

I am trying to make a PWM generator with UCC35705.The design tool of TI is used to calculate the frequency and duty cycle.

The working mode is selected in Mode 1.At present, I have connected a set of 10uF/50V electrolytic capacitors and 104 non-polar capacitors in parallel between VDD and GND.

The chip can successfully generate PWM waves and the duty cycle and frequency are the same as my calculation results.

However, after nearly 5 seconds of power on, the capacitor starts to generate a lot of heat, and the PWM disappears and the chip stops working.

Please help me check whether my design is correct. Can this chip be made into an open-loop system as my design?

Attached is the schematic diagram.

thank you.

Echo Wang0243.TI.pdf

  • 0
    TI__Intellectual 2640 points

    Hi Echo! 

    Thanks so much for reaching out and for your interest in the UCC35705! Which capacitor are you noticing heating up? 

    Best, 

    Aidan

  • 0 in reply to Aidan Davidson1
    Prodigy 30 points

    C5 and C14 already show in the schematic diagram above。

  • 0 in reply to user6098850
    TI__Guru**** 189401 points

    Hi, Echo,

    Both those capacitors are just bulk electrolytic decoupling capacitors. The only reasons they might overheat are excessive ripple voltage/current, bad caps, or reverse placement in your circuit.

  • 0 in reply to Don Dapkus
    Prodigy 30 points

    Hi.

    Thank you very much for your reply！

    In the experiment last week, I replaced the two capacitors C5 and C14 with 50V/10UF, which has solved the problem of capacitance heating I mentioned earlier.

    But now the chip is still working in the configuration shown in my schematic and automatically fails after a few minutes.

    In the process of debugging the chip, PWM frequency and duty ratio can be normally controlled by adjusting R14.

    The specific manifestation of chip failure is that the output port outputs 90mV and no PWM continues to occur.

    All power supply can be measured at the correct voltage by the multimeter.

    The updated schematic diagram is as follows. Please also help me find the cause of chip failure in the open-loop system.

    Thank you so much!

    Best regards,

    Echo Wang.

    Gas - electricity Interconnection Integrated Energy Laboratory

    NanJing Normal University

    NanJing,JiangSu CN4khzpwm.PDF

  • 0 in reply to user6098850
    TI__Intellectual 2640 points

    Hi Echo! 

    I'm glad to hear that your heating issue has been resolved, but there is still a larger issue here. You mention that the chip automatically fails after a few minutes and stops outputting PWM pulses. Can you check the VFF pin voltage? If it is less than 1 V the device will shut down the output and the oscillator. 

    Thanks! 

    Aidan