Lab Power Supply - Re-designed Pre-Regulator Testing

The new pre-regulator design turned out to be a good move overall in that I can now reach the full output power of the supply and the pre-regulator noise on the output has gone. It wasn't straight forward however and I still haven't sorted out how to make it work better in current limit mode.

Unfortunately I didn't capture too many pictures so you have to take my word for it!

Output Instability

After the usual small construction errors I got the circuit basically running  This looked pretty good and then I enabled a 1A load. Even with just 1A the output went into oscillation. Also the transformer would make a distinctive hum also. At this point I was really stuck - was the pre-regulator inducing a noise in the Gate bias supply? Was the pre-regulator affecting the voltage when it is sampling the current output voltage? I had a million thought - none good.

I fiddled with different voltages and noted that it didn't seem to occur as much at higher output voltages. It was worse when I increased the load however. At some point I managed to get a scope trace of the capacitor voltage and the output at the same time and saw that when the capacitor voltage fell to within a few volts of the output I got oscillation!

I experimented with boosting the pre-regulator voltage higher above the output and this worked but again when I try and run the supply at a higher load and or at a voltage closer to the maximum it would oscillate. The voltage difference wasn't small either - 4V. I thought a MOSFET would have a much lower dropout than that. Also the amount I had to boost the pre-regulator meant it again would not work at higher loads close to 14V.

I tried experimenting with the output capacitor. I increased the capacitor and found the instability went away (at least until the voltage difference was much smaller). Then I increased it to 2uF (from 0.1uf) and now the instability went away entirely.

I checked the transient performance and it was still well within spec - less than 20us to recover from 0 - 80% load. I call this a win!

I also experimented with changing the capacitor across the base-collector junction of the transistor that turns on the MOSFET to try and reduce the transformer noise. If I increase this the transformer makes less of a hum but then the MOSFETs get a lot hotter. I decided to leave it as it is (1nF).

Tuning

So then I went back to tuning the pre-regulator voltage based on the load and the output voltage. I realized that making one of the resistors in my summing amplifier bigger wasn't working the way I expected so I went back to making them equal. 

I experimented with the zener diode used to set how high the pre-regulator is above the output (with no load) and found 3.3V worked pretty well. This kept the pre-regulator close to the output and boosted enough to stop it getting caught short when the load ramped up quickly. 

The snapshot below shows the AC waveform (yellow), the bulk-capacitor voltage (blue), the ouput (pink) and the output of the comparator (that triggers the SCR to turn off the MOSFETs and stop charging the capacitor) in green. The capacitor is maintained a modest voltage above the output and the charging is consistent.

In this trace the output has been set for 14V and the load set for 3.8A. Here you can see it is charging for much longer into the cycle. The output is still very clean. The ripple is now over 5V.

I also looked at how the pre-regulator handled current limiting. In the trace below the load was set for 3.8A pulsing at 50Hz but the power supply was set for 1A max. You can see the  pre-regulator voltage is getting very low by the end of the 50Hz cycle.

The wheels really fall off when the output is low at the start of a couple of AC cycles. Then the capacitor voltage runs down to the minimum and when the cycles lose sync (right hand side) you can see the output spike back up to the set voltage again.

 While this looks pretty ugly I think it is ok. Current limiting is not a normal mode of operation but a safety feature. The output isn't overshooting the set point so there is no risk of damaging something connected to the output. I experimented with adding a peak-detector but I couldn't get this to work satisfactorily. Right now I don't see this as an issue.

Even at 3.99A (highest my dummy load will go) the output was clean.

And here is the current pre-regulator schematic