I needed a 19V-20V source so I ordered some 24V LRS series SMPSs but they offer 10% adjustability, the voltage range just below that is 15V, the adjustability is also the same there. So none of them is going to provide me with the desired voltage range. I am wondering if there is an option to widen that range by changing the Pot or trimmer. Unfortunately, I have neither the product nor the full datasheet (no circuit diagram there only block diagram) to check the amplification from the voltage divider. I need a suggestion if changing the pot will affect the controller-IC / O.V.P or void my warranty. I don't want to use any buck-boost converter as that might introduce more ripples, and yes some diodes could be added to benefit from the forward voltage drop but that's a crude solution I hope there is something better.
Although you will probably void your warranty, if you measure all the R values around the Pot and the tap voltage Vref, you can reduce the voltage by adding a feedback resistor approx 4x the one from 24V Then the regulator will reduce the output to match Vref.
2.5 1.5 1.25 1V are common Vref voltages.
simulated here. Press the switch and adjust R to see results.
- it comes with a +/- 15% adjustment range trimpot according to the range in datasheet.
I have done this before on laptop chargers. It is a standard tuning method.
You can solder directly to the nodes Pot and Vout with short leads.
If you can modify it without a trace and remove it, if it fails for some other reason, you might get warranty service. (Assuming no foul modification or ESD damage). But technically CE, UL and OEM do not permit selling it modified. But DIY for your own use, you take the risk of failure to follow these simple instructions.
I would not recommend fiddling with the inner workings of budget AC-DC converters. There is not much to be gained and much to be lost.
Most laptop PSUs have ~19 V out of the box, so maybe their regulation is good enough for your purpose.
Otherwise, if you are concerned about ripple and noise: Maybe stick with the LRS and add an LDO after it. This depends on the current demand. Off-the-shelf parts exist for up to a few amps. If you need more you can design your own regulation with a beefy load transistor, controlled by opamp and Zener reference.