Decoupling of ac and dc signals

Check the clock. In DC-DC converters you can simply look carefully at the SW node. It should be stable otherwise please read up some more on PCB routing and input decoupling in previous chapters, and then return to this point. When designing AC-DC converters, I used to look very hard at the signal coming out of the IC meant for driving the Gate of the switch (Pin called OUT on the 3842). 

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