I'm taking a refining pass on my DC Bench Power Supply design. It generally works quite well on veroboard, so I thought I'd move on to SMD PCBs and refine the design while working out a few of the finer points. This project is a "re-gut" of a classic HP 721A DC Bench Power Supply, presently looking like this on the "breadboard". The small 35mm square PCB on the right is the voltage error amp.
As I took a closer look at the turn-on transient, I noticed some semi-random high-frequency (10s of MHz) spikes (maybe 5V peak) on the output terminals during turn on.
The schematic looks like this. The V.unreg voltage source represents the transformer secondary full-wave rectified into a 500uF filter capacitor, all stock on the 721A.
Based on a few observations, I've concluded these are due to switch bounce in the old-fashioned panel toggle switch that directly applies mains (120V) to the transformer:
- The frequency is at least an order of magnitude greater than the bandwidth of my op amp, and maybe 50 times its cutoff frequency (~100kHz), as configured. Also, regulation is rock-solid after the rails come up.
- The spikes occur well before the bias power (+/- 15V) rails come up and even before the DC bus comes up.
- The same sort of spikes are observed on the "original" circa 1957 circuit (I have seven of these 721As, long story :)
- The spikes appear at irregular intervals, and sometimes don't appear at all. They certainly don't appear at consistent locations accross multiple turn-on events. Their amplitude is also inconsistent, sometimes zero, sometimes a volt, sometimes 5V and more.
I'm not entirely sure how the spikes get through the pass device before it turns on, but I'm assuming its C to E capacitance is enough to couple them to the output.
I'm looking for advice on how to deal with these spikes. Here are some alternatives that occur to me:
Quit trying to get a clean turn-on with a mechanical switch on a supply with "always on" outputs. Just accept I'll need to disconnect any sensitive loads before cycling the power and move on.
Design a snubber for the power switch that minimizes these spikes at their source, somehow avoiding sending watts of power down the drain continuously just to clean up the turn-on characteristics.
Work some other kind of magic a more experienced hand here can point me toward :)
Get fancy and put an RC-delayed zero-sensing TRIAC or something like that between the switch and the transformer.
Add an output switch that would probably also have to be fancy, maybe a MOSFET controlled by a delay circuit to avoid that switch's bounce. This one would be somewhat contrary to the spirit of the project, which is to make it look stock but have modern innards :)
Anyone have any ideas how I can mitigate the spikes without going to active component-adding extremes?