More on Fav Designs - switchmode power supplies

[D. Schouten]

> All you have to do is find the +- 5 volt supply windings on the
> transformer. Usually it's only a couple of turns. Rewind it for whatever
> voltage you want and change the filter caps' for ones with the new working
> voltage. The new windings can be snaked through the ferrite core without
> much trouble. The supply remains regulated because of the feedback winding
> being independant of the +- 5 and +- 12 volt windings.
> 
> Hope this helps some.
> 
> Doug

This is true for some types of SMPS's were, indeed, there is a 
separate winding for feedback purposes, and it usually supplies the 
power to the PWM controller too. 
But there are also a lot of SMPS's which are optically fed back from 
the highest power winding (in this case the +5V winding) to the PWM. 
In this case, increasing the secondary number of turns on the ferrite 
transformer will only lower the effective duty-cycle, so the output 
voltage remains the same. 

What you have to do, to really regulate this output at +15V in this 
case, is to modify the voltage divider on the secondary side of the 
optocoupler, and increase the number of turns a bit.
Be carefull to not only replace the output capacitors to higher 
voltage types (do not use regular capacitors by the way, these 
things will usually break down after a certain period), But look also 
to the maximum reverse voltage of the output diode(s). In most cheap 
power supplies they are very tight specified.

Not that I think that you guys are stupid or something, but I 
would like warn you for the possible shock hazards in off-line 
SMPS's. Primary capacitors can hold pretty large voltages and in 
some cases, small heatsinks are connected to the line voltage too.

But unlike the above story, I don't recommend it too modify existing 
SMPS's. Because these kind of supplies are in most cases designed to 
work properly only, with the components the designer choosed to 
implement. A simple example is the ESR value of the output capacitor. 
This value is, among others, used to design a stable feedback loop. 
If you change the used capacitor with a regular general purpose 
capacitor which has a much higher ESR value, the supply can become 
unstable and oscillate, or it's regulation can be very poor.
This is just one example, but I can think of a lot more.

Hope the above helps a bit.

Bye,

Daniel Schouten
SMPS designer at TBS Electronics