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Original question:

UC3842: Why the curent sampling resistor always burnted out?

UC3842: About the switch frequency

aggil liang
Expert 2660 points

Part Number: UC3842

Hi 

We use UC3842D8 to design a 60W/（15V output） power supply. We design the switch frequency is about 100K, but when we test (only 15V output is loaded),the frequency is 20~30Khz, especially 3kHz with no load. 

Below is the schematic and some test waveform. I have two question that need to confirm:

1. Is the switch frequency fixed by the RT & CT , anyway different input or load ? 

2.Does the schematic need to improve ? 

UC3842 sch.pdf

  • 0
    TI__Genius 13660 points

    Hi aggil liang,

    1. Yes, the switching frequency is fixed once RRT & CCT is set, no matter what input and output.

    2. The schematic seems reasonable.

    From the Vgs waveform,  the measure method is not right.   I advise to capture the OUTPUT pin signal, which is easier to find out the switching frequency and use a smaller time scale and higher resolution.

    In the waveform of 15V/0.8A and 15/0.16A, we can see that the IC is operating at intermittent mode, maybe due to same protection (UVLO or OCP), and the pulse in a package still works at a high switching frequency.

    Could you capture the waveform of VCC pin, Isense pin, output pin, and output voltage?

    Regards,

    Teng

  • 0 in reply to Teng Feng
    Expert 2660 points

    Hi Teng

     thanks. The waveform is the output freewheel diode. 

  • 0 in reply to aggil liang
    TI__Genius 13660 points

    Hi aggil,

    They are with the same frequency.

    Could you help provide the waveform that I mentioned for further analysis?

    Thanks,

    Teng 

  • 0 in reply to Teng Feng
    TI__Genius 13660 points

    Hi aggil, I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out) – thanks, Teng