SMPS stands for switch mode power supply, as the name suggests this power supply concept utilizes high frequency switching pulses for converting mains AC voltages to a particular specified low voltage DC such as 12V or 24V.
I have discussed this in detail in one of my previous posts how to understand SMPS circuits
The SMPS concept today has almost completely replaced the traditional iron core transformers and have transformed these units into a much compact, light weight and efficient power adaptor alternatives.
However since SMPS units are commonly available as fixed voltage modules achieving a preferred voltage as per the users application needs becomes quite difficult.
For example for charging a 12V battery one may need an output voltage of around 14.5V, but this value being quite odd and non-standard we may find it extremely difficult to get an SMPS rated with these specs in the market.
Although variable SMPS circuits can be found in the market, these may be costlier than the ordinary fixed voltage variants, therefore finding a method of transforming an existing fixed voltage SMPS into a variable type looks more interesting and desirable.
By investigating the concept a little I was able to find a very simple method of implementing the same, let's learn how to conduct this modification.
You will find one popular 12V 1amp SMPS circuit in my blog which actually has an in built variable voltage feature.
We find that it employs a shunt regulator circuit stage for executing the variable voltage feature in the design.
Another interesting aspect is that this shunt regulator device implements the feature by regulating the input of the opto coupler of the circuit.
Now since a feedback opto coupler stage is invariably employed in all SMPS circuits, by introducing a shunt regulator one can easily transform a fixed SMPS into a variable counterpart.
In fact one can also make a variable SMPS circuit using the same principle as explained above.
You may want to learn more about what's a shunt regulator and how it works.
How to Make a Variable SMPS circuit
Referring to the following example circuit, we are able to find the exact location of the shunt regulator and its configuration details:
See the bottom right side of the diagram marked with red dotted lines, it shows the variable section of the circuit we are interested in. This section becomes responsible for the intended voltage regulation actions.
Here the resistor R6 can be replaced with a 22K pot for making the design variable.
Magnifying this section provides a better view of the involved details:
If you have a fixed voltage SMPS circuit, open it and just look out for the optocoupler in the design, it would be mostly located just around the central ferrite transformer, as may be seen in the following image:
Once you have found the opto-coupler, clean up by removing all the parts associated on the output side of of the opto, meaning across the pins which may be towards the output side of the SMPS PCB.
And connect or integrate these pins of the opto with the assembled circuit using the TL431, shown in the previous diagram.
You can assemble the TL431 section on a small piece of general purpose PCB and glue it on the main SMPS board.
If your SMPS circuit does not have an output filter coil, you can simply short the two positives of the TL431 circuit and join the termination to the cathode of the SMPS output diode.
However suppose your SMPS already includes the TL431 circuit with the opto coupler then simply find the position of the R6 resistor and replace it with a pot (see R6 location in the first diagram above).
Don't forget to add a 220 ohms or 470 ohm resistor in series with the POT otherwise while adjusting the pot to the upper most level could instantly damage the TL431 shunt device.
That's it, now you know exactly how to convert or make a variable voltage SMPS circuit using the above explained steps.
If you have any further doubts regarding the design or the explanation, feel free to express through your comments.