I have design a flyback circuit based on the LM5156H controller. Input voltage is fixed at 48V, output voltage is 5V/1A.
Looking at the switching waveform of the transformer, I noticed that the pulse width of the main FET is always changing very fast from cycle to cycle, even though the output load is steady at around 1A. When the pulse (duty cycle) becomes too low, the transformer doesn't charge enough and the voltage collapses before the start of the next cycle.
I measured the flyback node that is switching, the current sense regulator and the gate signal of the main FET. Here how it looks:
Here's a zoom of a certain section:
I already tried change the sense resistor to 0.1Ohm, changes the sense resistor filter from 332Ohm+330pF to other values like 100Ohm and capacitors up to 1nF with no help.
I also tried changing the compensation resistors, as well as disabling the Spread spectrum feature, but none of these changes helped.
Can you advise what can cause the duty cycle to vary this much between cycle to cycle? And what necessary changes do I need to make to ensure that the duty cycle is stable and the transformer gets enough current so the voltage will not collapse?
Thank you,
Barak
