# power supply regulation

• posted

Hi,

I am using the voltage error * gain to set dutycycle for regulating the voltage output of a power supply, under load we are getting a couple volts lower output voltage than the desired output voltage, but other than that overall the regulation seems to be working well. What is a good way to regulate up to the last couple of volts? I was thinking adding an intergral term to the dutycycle calculation (summed voltage error over time * gain) to make it a PI loop, but is there other techniques that work well or better than the standard PID loop technique for power supply regulation?

cheers, Jamie

• posted

A PI control is fairly simple to do and works very well for this sort of situation. You may want to do something like this:

+-!!-+ ! ! ---/\\/\\-------+----/\\/\\-+-!!--+- ! ! ---!-\\ ! ! >----------+---- GND-----!+/

The diodes restrict the range over which the "I" part of things can swing.

• posted

PI is good. You can also feed-forward a correction based on load current and/or unregulated input voltage.

John

• posted

Those diodes for anti-windup are a good idea, but you have to design the loop carefully. The wave-a-dead-chicken school of anti-windup design will often cause nonlinear oscillations during transients, and so make things worse rather than better. Back around 1981 I was building PLLs for satcom frequency references, and when I put in anti-windup diodes I had some pretty amusing looking settling transients until I figured out what was going on.

Cheers,

Phil Hobbs

• posted

When I used to design control loops for steamship throttle controls, I'd use a nonlinear function generator to minimize open-loop error, and a P+I correction with carefully restricted range, so the the thing wouldn't run away too far if the tach failed or something.

I once almost ripped a 900' LASH ship off the dock at Avondale Shipyards, turning a pot too far. I did "lights out" a tanker that was off-loading in Binecia.

John

• posted

If the DC regulation doesn't meet designed values, look for noise influences or sources of leakage.

DC regulation is highly predictable and repeatable, unit-to-unit. It is often tailored for controlled droop in parallel situations.

Stating 'a couple of volts' and not a percentage, masks the degree of regulation currently achieved.

RL

• posted

Brr.

If I have the space, I usually like to use another op amp for the I term and let it saturate to limit the swing, using resistor ratios to set the gains. That way you either have a well-behaved PI loop or a well-behaved P loop with an offset--none of the weirdness you can get with partially-turned-on diodes.

Cheers,

Phil Hobbs

• posted

I still like to do that when possible. Putting control loops on things that are already nearly linear tends to make their behaviour much more benign in unusual conditions or when there's a fault like your tach going south. Thermoelectric cooler control loops spring to mind.

Cheers,

Phil Hobbs

• posted

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As with many thing, if you do it wrong the results aren't very good. The advantage of the diodes over other simple methods of doing the antiwindup is that you don't cause large difference voltages to appear on the input of the op-amp.

You never want to do just an integrator and then let it clip at the rails unless the op-amp has nose to tail diodes on its input. Hitting the rail builds up a large difference voltage on that op-amp that can lead to all manner of strangeness and long recovery times.

• posted

You can do both if you want. The feed-forward gets it almost right.

• posted

You need much more gain.

Also, on the feed back path, you may want the shift the reference ratio as current is detected. This will force the output set point to elevate to get better regulation. Normally, a pot in the circuit is needed to calibrate that. Years ago, we used to put incandescent lamps in series as part of the bias network on the regulator circuit :) They have CLD's for that now..