HiperLCS2: Better understanding of a low load problem.
i have a project with a LCS7265C witch should be able to handle 135W output power @24V this works fine. We wound a transformer for the power supply with the two chamber integrated resonance inductance. At this time, we have a problem with the no load condition and we thought about the frequency range of the controller. With a higher range we think the power supply can handle the low loads better. Our controller is fine, because we know it from a measured bode-plot.
Now my question, could i change the center frequency with the resistor on the pin "PP". In the datasheet are some definitions for this resistor with fixed values for some frequency ranges, but i put a potentiometer on this pin and increase the resistance and i have the feeling that it helps for the low load condition. Is it possible, that i could change the value linearly and the frequency range changes with it or do I have to insert the exact value ?
Btw. the secondary IC have the H002 option.
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Yes you can adjust the center frequency by adjusting the PP resistor. BUT to answer your question, no frequency ranges are in discrete steps so you cannot linearly change the frequency range. Also please note that the PP pin resistor is only checked during start-up sequence so it will not change anything if it was adjusted after start up. And another thing, if you are using an H002 secondary controller, you will have to stick with a frequency mode 1 setting, if you want to change your frequency range you will also have to change your secondary controller accordingly or else there will be a mismatch between primary and secondary IC which will prevent successful start up.
What is the particular issue that you are seeing during No Load? If it is a high input power consumption, you might want to check if you are operating in burst mode. If not, you will need to adjust your IS pin resistors to achieve burst mode operation during light loads which will improve your no load consumption.