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Registered Member #575
Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263
Hello, lately I've been brainstorming and sketching a digital controlled linear PSU. And I am general interested in your opinions regarding a few design aspects.
First off all, below are the intended specifications:
Output voltage: 0-20V Output current: 0-1A Total output ripple: 0.5mVpp
As mentioned the PSU is digitally controlled by a micro controller that via SPI/I2C adjusts the output on two 16-bit DACs (one for voltage, one for current limit) The whole feedback part will be done analog (if there is nothing to gain on a adjustable slew rate/transient response?) I am thinking of using a well known jelly bean regulator as an tracking pre-regulator (LM317, LT1085 etc.) followed by a OPA548 as an output regulator. The OPA548 was chosen mainly cause I have a few of them, secondly because it is simply to use. But the OPA548 have a typical input offset at about 5mV, I find this somewhat too high so I was thinking of using an low-offset opamp to take care of the feedback, I believe this should compensate for the offset voltage.
Any inputs?
And in my experience, a pair of banana plugs can have a total of about 0.2-0-4 ohm in resistance, this raise the question if I should have some type of remote sensing? 1A over 0.2 ohm is 200mV loss in just wiring.
If you have digital control, you could always subtract off the DC offset digitally each time on power up (or set once). Find out what DAC value that make output goes to zero (or change from 0 for -ve offsets).
I would assume that there is an A/D some where reading back the output voltage and current etc. if you want to (sort of like digital integrator) correct for drifts. I like analog feedback for transient, not so fond of the typical microcontroller doing control loops and have transient response dependent on code quality/loop timing.
Also note that there might be DC offset/drifts etc associated by whatever opamp they put in a DAC with voltage output . I usually use (multiplying) DAC with current output as I can pick the opamp and have much better slew rates in my design.
>this raise the question if I should have some type of remote sensing? Probably if you are concerned even with a +/-2mV (typ) and +/- 10mV (max) in DC offset on your OPA548.
What the store bought one usually does is to have two resistors (say 10 ohms) to connect the rails to the senses so that the feedback loop would also work when you only use 2 terminals. Watch closely which grounds you are using to output sense, decouple and which are signal ground and power ground.
As for current, you can always do high side sensing, so not have to break up the grounds. My personal believing in not cutting up the ground too much if I can help it. There are chips to do high side sensing and they are no more complex than your low side sensing.
There is also the approach by using LM317 (or its high current version) to drive the output directly and use your feedback path to control its output. It can't sink current like your circuit does. You heatsink also have to handle dissipation when your power opamp decided to sink current from the load.
Right now your current limit feedback path allows it to both increase and decrease the output voltage via L1. I wonder if you should restrict it to only lowering the output voltage with a diode or something.
The other concern I have is the load. As a power supply, it would have to handle all kinds of funny loads - low ESR ceramic caps, large values caps, inductive load etc. and not getting into stability issues.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The OPA548 has no local feedback round it in your circuit, and there are a total of three opamps connected round as the feedback loop. The only thing that can be guarranteed for that circuit is that it will honk. A power supply is an amplifier with feedback. Unless you know your Bode plots as a minimum, then you will make a power-oscillator. There are too many components to hope'n'poke them into order either on a circuit simulator, or in real life, without understanding gain and phase margins.
When I design a power supply, I tend to make the output capacitive load create a dominant pole with the power output device as a current source. That way, unlimited extra C can be added to the output without compromising stability. There are other more exciting ways to make a supply stable, but single dominant pole always
There's a lot to be said for leveraging existing an existing power supply regulator, like an LM317, and using your DAC to program its ADJ pin open loop. The result will be a voltage offset from the ADJ pin by about 1.2v, which you can calibrate out by the magic of uC, with inherent overcurrent protection and over temperature shutdown, stability into capactive loads, and ability to troubleshoot by replacing the DAC with a potentiometer. I doubt that would meet your stringent accuracy requirements, but it would work, and work well.
Registered Member #575
Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263
Neet Studio, Dr. Slack: As this will be a single rail PSU I don't really see the need for high-side current sensing, but I'll have to give it some though. But I've seen one some of the proprietary high-side sensors and they all have about 1-3% offset/drift. I would guess an instrumentation amplifier would serve better at it will reject common voltage.
"Control A High-Power Load With A Low-Power Microcontroller" Schematic (in case you miss it in the article)
Take a look at how they use an external opamp to control a LM317 from an external voltage Vc in the schematic. (They basically use a PWM as a replacement for a low res. DAC)
There is no reason why the LM317 has to run open looped.
Linear Tech: AN105 - Current Sense Circuit Collection - Linear Technology Lots of examples
Drift would be the opamp offset and whatever tempco of the sense resistor, Rin and Rout. Circuit limited by operating voltage, but you should be able to power a opamp with the supply input.
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