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My bench PSU attempt

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I had a PSU designed schematic wise for some time but now that Dave did a blog about bench PSUs i decided to finally make a working implementation of it, i had the few boards i manged to shoehorn in some spare room of a PCB order for work.

This PSU had some special design features first is MCU controll so the current and voltage is set and read by a small voltage of about 0-2V then it goes down to 0V (Turned out it even goes a tad below 0V if it wants) and the last thing is something nearly no PSU does, it can sink current as well.I plan to use it as a lab supply and one whopsie that can let the magic smoke out is accidentally shorting say the 12V and 3.3V rail together. With almost all supplys the 3.3V will get pulled up to 12V while that 3.3V supply is happy thinking its running without a load.

So i built up the thing and it did regulate the output voltage, but then the more i tested it the more i found out how true it is when Dave said that this thing is difficult to make stable(See photo of what i mean).

So then as i was tinkering with it i kept soldering caps and resistors in various spots until it was stable, but the overcurent protection was still osculating when you show it something near a short(A few Ohm) Later the solution to it was doing it similar to Dave by a transistor to pull the voltage down since messing with the voltage regulators feedback loop was looking to be a bad idea for stability. Now the over current protection still osculates but only when you try to limit current very low. But i did find something interesting happen with it when the current limit hits very hard (Here it was the supply set to 10V 400mA and then an 1Ohm resistor put across it) It actually slams down in to 0V and then recovers, at first i tried to solve it but then after some thought i started to like it since the current limit kicks in super fast and it doesn't really take that long for it to come back on (Also if my MCU is very fast it can shut down the supply before it starts rising again)

Then when i got to trying the sinking current bit things didn't go well. I tested it by connecting various power resistors between 12V and its output and in almost all cases it caused it so start oscillating about 1V around its set voltage.Thats still much better than just letting its output go to 12V but id like it to regulate like that too.

So you guys have any tips on how to improve on this design?

Oh and btw that -0.7V supply comes from a capacitor charge pump that has its input wired directly on the transformers output and gets regulates trough a forward biased diode, this gets around the op amp output not reaching 0V problem

scope_22.png shows the output current in to a 1Ohm resistor(so 1V = 1A) when hitting the current limit hard
scope_21.png shows the output voltage(Yellow) and the output of the opamp(green) when giving it a load between its output and 12V

Look at TI's OPA548 or Linear's LT1970 (a really crazy IC) for the power stage.

And in general, give this HP app. note a look for some trusted and tried principles http://www.wfp76364.w1.com/HP-AN90B.pdf including occasional notes on stability.

That chip is quite a good idea, i didn't think op amps went to these sort of currents.

I asked LT for some samples of that little chip. I don't plan to use it in the final version because the package is a pain to heatsink but i can see what sort of performance these high current opamps give I imagine its near perfect apart from the output not reaching 0V problem but i can live with that if this thing really works as good as i imagine it to.

If the problem is that you think there is not enough current capacity to drive the pass transistor accordingly, try to use an n-channel mosfet, they only require some gate current when changing the voltage (the gate acts as a small capacitor).

I'm experimenting just with the voltage regulator by now, but it is surprisingly stable with capacitive loads, motors, etc.
It also features an extremely low dropout. It will require a charge pump to supply the op-amps driving the mosfet.

The power dissipation will be minimum since I will be using a tracking pre-regulator (i' thinking of a ready made adjustable laptop power supply) of a regular transformer with a circuit that only charges the filter capacitor up to 2V or 3V above the output.

I'm using an IRFZ44 mosfet now ( TO-220, 100W max power dissipation), and an OP07 precision op-amp, which is availaiable locally at 0.99€ each, but I might need to change it since it is only capable of 30V  on the power rails, and the outputs only go to 3V below the rails.
I'd like the PSU to go up to 30V at least, so, the op-amp would have to handle at least 41V on its rails (30V output + 5V Mosfet Vt + 3V positive rail drop + 3V negative rail drop).

Does any one know of readily available op-amps that can have 60V on their rails such as those rare OPA548?

I could have the op-amp supply floating, hanging on the output like many commercial power supplies, but that would require an additional transformer.


--- Quote from: BoredAtWork on December 26, 2011, 10:59:06 pm --- give this HP app. note a look for some trusted and tried principles http://www.wfp76364.w1.com/HP-AN90B.pdf including occasional notes on stability.
--- End quote ---
I have been searching for something like this.
I look forward to reading it in detail.


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