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LM5155: (LM5155 in DCM ) the switching frequency waveform is abnormal (Some Ton waveforms are missing)

Part Number: LM5155

Hi TIer

The customer designed a flyback circuit using LM5155. The input voltage is 16V. Its transformer turns ratio is 4:54, the transformer inductance is 10uH, and the switching frequency is 100khz. At the output end, a DAC and a transistor are used to adjust the feedback resistor to achieve 10- 160V  (250mA max) output.

Currently, the normal voltage cannot be output under heavy load conditions of 160V 250mA. Check the inductor current (Yellow) and switch voltage (Green) waveforms as follows:

Some of my confusions:

1. Why is there some pulse current in this DCM mode? Does this waveform look normal?

2. Judging from the switching waveform, there should not be an event that turns on TON in DCM mode, so is the pulse of Ton normal, or does LM5155 not support DCM mode (because the calculation table tool only has CCM mode)?

3. Can you describe the corresponding relationship between Ton time and number and output power? Thank you so much

  • Hi Yuxi,

    I suppose CH1 is the primary MOSFET current and CH2 is the gate driver of the primary MOSFET.

    1.The pulse are very short turn on of the MOSFET.

    2.LM5155 supports DCM mode.

    I think the short pulse is due to the cycle-by-cycle peak current limit. You may increase RF and CF as described in datasheet 9.3.7 Current Limit and Minimum On-time (CS Pin).

    Best Regards,

    Feng Ji