Topswitch is nice for small powers .
Depending on what you need the powersupply for (or just for study purposes) have a look at this site .
This is a standard pushpull dc dc converter .http://sound.westhost.com/project89.htm
It has a relative understandable explanation .
Also look in the ebook section here there i posted some good books concerning smps
Just search for Brown or Pressman .
Good luck Posted on: May 30, 2008, 06:50:50 06:50 - Automerged
Let me try to explain the principia a little .
Yes it works out of 50 or 60 Hz (depending on what country you live )
(although dc-dc converter is more specificly when you convert for instance 12 Volt to 24 Volt Dc in Dc out).
Normal offline switchmode powersupply's rectify 230 Volt to Dc (offcourse with buffer capacitor on the DC side)
With that dc voltage you power a PWM generator like the SG3525 ,UC2856 or IR2153 which generates a high frequency .
That high frequency you use to switch mosfets to power a ferrite transformer .
(I'll add some links with examples later )
On these high frequency's your core is smaller and you can use less windings on your trnasformer .
Also is the efficiency much higher then with series regulators ,after all you switch & transform instead of burn the leftovers away .http://radio-konst.narod.ru/moi_konstrukcii/polumostov_preobr/polumostov_preobr.html
Have a look at the schema from the link (this is about as simple as it can get).
220 Volt in > diodebridge making +320 Volt DC ,here they create a low voltage for the IR2153 and further they devide the DC in 2 with C6 & C7 .
The IR2153 will make PWM which switches the mosfets ,one at a time first the upper then the lower etc .
At frequency of lets say 66 KHZ its not unusual to have 35 or 40 windings (making 320 / 2 /40 windings = 4 volt-winding)
This schema does about 3 to 400 Watt's although its very easy upscalable to more .
The topic itself is rather wide and i would advice you to download Marty Brown and Abraham Pressman .
I uploaded them some time ago on this site .http://www.sonsivri.com/forum/index.php?topic=12417.0
When you read a few chapters (Start with Push pull or halfbridge when you understand the idea have a look at the examples and get your calculator)
Just follow the example ,make those calculations ,when you manage to understand those you can fill in your own requirements and try)
But first a few words of advice .
A: ALWAYS use fuses
B : When you builded something start testing with lower voltages if things stay cool go up .
C : If you don't have a scope at hand it will be a matter of luck
D : read ,read ,read & read more smps's are very intresting matter but also something that needs time ,understanding and study .
Little advice start with low voltages first ,make a 12 volt push pull or something to get a little aquainted .
Maybe if you listened to Rule A: you might not blow up to much mosfets