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Cheap 300W cc/cv buck converter suddenly quadruples current draw as cc limit is


Julian Bowen:
Hi all,
I'm trying understand what is happening with a newly acquired cheap chinese 300W buck converter, which has cc and cv adjustment.
I am powering it from a 20A 12v atx power supply, and the voltage can be adjusted from 1.3v upto 11.2v in a linear fashion, and voltage appears stable under load.  The odd behaviour starts when I start to reduce the cc pot, all starts ok as the current limit is reduced and the output voltage starts to reduce, then the inductor starts to resonate, the current draw from the power supply almost quadruples and the atx power supply shuts down.
I've logged a few efficiencies, over a variety of voltages and different loads (11w to 90w) and the numbers are similar each time.  The effieciency is in the region of 90%, with a 2-3 watt difference between input and output then a slight decrease in the current limit and the output remains unchanged and the input increases dramatically. One logged change went from 2w difference to a 48w difference.  Can anyone explain this?


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