I have an 18V 1.5A mains transformer (outputs around 30V peak with no load at 50Hz) followed by a full-bridge rectifier and a filter capacitor:
(there would be a schematic here if SE allowed me more than 2 images)
Now I need a negative supply (which will be regulated to -2V with LM337 and draw around 10mA, so it should provide at least -5V). For this purpose I added a negative charge pump:
At first it seems to work fine, the voltage at the output of the charge pump goes to around -27V after a few periods, however as the current is drawn it starts to sag and slowly approach zero volts. When I was trying to figure out what is wrong, I noticed that at the same time the voltage at the main filter capacitor rises from 30V to about 60V, which is undesirable. It seems I got double the input voltage, but not where I wanted it. To illustrate, this is a graph from LTspice:
Is there any way to prevent this behavior? To keep the filter capacitor at 30V maximum while keeping full-wave rectification (i.e. the 18V 1.5A capability) and at the same time supplying a workable negative voltage for a small load via a charge pump? I don't want to use an inverting DC-DC switcher in order to avoid extra noise.
(Sorry if I misused a term or two, I am a hobbyist still learning this stuff.)
Last minute observation: I noticed this behavior only happens if I draw more current from the negative rail than from the positive rail, so I guess one way around the problem would be to design the rest of the circuit so that more current is always drawn from the positive rail. However, I don't like the idea of positive supply rail potentially shooting from 30V upwards damaging other components, and I would like to prevent it altogether if possible.