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Offline switching power supply with microcontrolled output voltage

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KX36

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Hi all,

I am trying to design a switch mode power supply to run off mains supply of 230Vac with a variable output voltage up to about 25Vdc set by a separate microcontroller. I'm not an electrical engineer and this is the first time I've done anything like this, I'm learning as I go along, but I'm not in a rush and this is all theory right now for me.

Here's what I need from it:
Offline SMPS, running on 230Vac mains supply
It would have to be an isolated topology
It would be probably around medium power (up to 100W load)
The output voltage has to be able to be adjusted over a wide range, from near 0V up to around 25V, by the output of a microcontroller (analog or digital output).
I want to keep it as simple as possible while maintaining function and stability (don't we all).

I have been looking at some application notes and datasheets for PWM controllers and I think that an isolated flyback circuit could work but I don't know how to control it from a microcontroller.


**broken link removed**

Here's a simple draft of a schematic based on this application note: https://www.ti.com/lit/an/slua143/slua143.pdf
The opto-isolator circuit is from this application note: https://www.fairchildsemi.com/an/AN/AN-400A.pdf
As you can probably see, I just patched it together from these documents to get something to start with; it's not a finished circuit and I doubt it is complete. I'm certainly not comitted to using all or any of it.

What I want to know is what is a good way of achieving my aim of having a microcontroller control the output voltage of a circuit such as this. Can someone please give me a push in the right direction.

Cheers,
Matt
 

The easiest is to have the microcontroller vary the voltage feedback loop of the 3844 using a digital potentiometer.
Does the microcontroller also need to be isolated or can it run off the same supply as the 3844 (still requires regulator to microcontroller's Vdd)?
In any case the digital pot is not, and its SPI data lines could be driven via opto-couplers.
Its easy because the microcontroller simply sets the resistance value, and the voltage is controlled as normal with the analog
circuitry of the 3844.
The digital pot usually has a Vdd of 5V, and must ensure the voltage at the resistance pins don't exceed Vdd either,
so need to use appropriate voltage dividers ahead of it.

You could put the microcontroller IN the feedback loop, as a PID controller, reading an ADC input, and
outputing a PWM to Analog (low pass filter) as the feedback input into the 3844, but its not easy to get right, and there are all kinds
of compromises on response. The microcontroller can't be isolated in this case.
 

I don't really know where I could put a digital potentiometer without it exceeding Vdd though. I suppose I could use a potential divider so that; e.g. an output voltage of 0-25V scales down to 0-2.5V on the digital pot, which is then scaled back up with an op-amp to 0-25V and fed into the zener voltage reference adjustment pin. I'm a bit worried that the extra complexity in the feedback loop would cause phase shifts that make it difficult to stabilise.

The microcontroller dictating the output voltage is isolated (powered by a separate power supply with a common ground in fact) as it would be performing other functions as well, but there's nothing to stop me using another microcontroller if necessary.
 

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