SMPS Adjustable DC Power supply 0/10 - 300 volt/0.3-0.5 Amp

[pikstart]

Hi.

I 'd like to upgrade my design of such a DC voltage power supply that currently, to reduce power losses on the pass element, uses SCR controlled rectifier. The SCR is fired when the difference between the output and smoothing cap is less than about 10 volts.

This method has two main disadvantages.
1. Needs a rather bulky transformer to isolate main power.
2. Produces nasty pulses (of about 0.1 to 5 Hz) on the rectifier that are also audible on the transformer itself as ticks that user notice it.
3. Those rapid pulses will eventually reduce the life of smoothing capacitor.

I 'd prefer total SMPS solution with a smaller ferrite transformer, but the wide adjustable output is something that could not find how to do it.

Any ideas welcome.
Thank you
Ioannis

 

[rjenkinsgb]

I forgot to draw the resistor from gate to ground, of 270K.

A typical power MOSFET has a gate capacitance of several nanofarads and needs a high current, low impedance drive for both turn on and turn off, if it's to be used in any kind of fast switching system.

It's not so important for occasionally switching a load on or off, but for anything running at higher frequencies you need to use a reasonable gate drive system.

You need a floating supply and a buffer amp after the opto (or dedicated opto / gate driver device) to get something like reasonable efficiency.

eg.

https://docs.rs-online.com/f83e/0900766b80779700.pdf

Or even better, this one produces its own supply from the high voltage input:

https://www.ixysic.com/home/pdfs.nsf/www/CPC1596.pdf/$file/CPC1596.pdf

The data shows the load on the positive feed to the FET but as the IC + FET function as a floating switch, it should equally work for your 300V pre-regulator switch.

 

[pikstart]

It seemed to me that, since the input voltage is a recified 100Hz it would not matter, but proves it does.

Looking again the circuit I see another flaw in the design. That PNP current limiting transisor is limiting the speed that Mosfet is turning ON as it limits the current in safe levels, but this has the consequence the Mosfet work as a resistor for the time needed to turn ON.

I don't think this can work as desired.

Ioannis