switching ps logic

[hacknet]

switching pwr supplies are juz mosfets ossilating at a high freq (>20khz).... then put into an air core trafo before being rectifed then smoothen wif caps rite? by rite i dun need to use any funny ics to do the switching rite?

[JBL]

In theory it could be simplified to that but it will probably don't work in real life. If you want to experiment I recommend that you simulate first with switchercad.

http://www.linear.com/software/

Also search the internet for information. Some interesting stuff can be found at:

International rectifier

Also at:

ESP

And in the power supply section of:

Epanorama

[hacknet]

esp's design requires me to get funny ic dat cost 30 bucks dat prob wont work becuz of my lousy workmanship...

im trying to build something like the computer psu

[JBL]

You should buy a computer psu instead. A computer psu is extremly complicated to build and require many ic. It could be possible to build a simple one with just a 555.

For what application?

[hacknet]

but a 555 has a very low max current...

btw... im looking at +- 35v at abt 10amps without a trafo...

the important part is the trafo because of iz weight...

[JBL]

What the input. AC or DC.

The 555 is for switching the mosfet. But you would not be able to do that without the transformer but they are easy to make.

[hacknet]

i intend to do so with a small trafo... when i mean small.. i mean like 10va... not some chunky trafo that wud power the amp..

im looking for somthing that draws itz power straight from the mains... AC... not from a trafo...

although switching components looks like it needs a small trafo

[Rarkov]

Hi,

I saw a schemeatic on the net for a 24V@40A (!!!) bench PSU that was switched mode...I would steer clear...All the inductors had to be self wound and moulded using epoxy etc etc...

It was no beginners project - that's for sure (I'm not suggesting you're a beginner...Just stating a point)

Good luck though,
Gaz

[Jeff R]

Originally posted by hacknet
switching pwr supplies are juz mosfets ossilating at a high freq (>20khz).... then put into an air core trafo before being rectifed then smoothen wif caps rite? by rite i dun need to use any funny ics to do the switching rite?

There is a lot more to it than that!

Few switch mode power supplies use air core indutors, for one, and you frequently need a custom designed and built inductor, unless you are using one of the special switchers manufactured by National or Linear Tech that are designed around standard inductors specially made for these devices. If you are trying to directly convert AC rather than first convert it to DC with a rectifier and step down transformer, these devices won't work.

You need to design what is called an "offline" power supply, which is short for "off of the line", where the AC mains are directly switched by a high voltage MOSFET. Do a search at companies like Texas Instruments (who bought out Unitrode, a major manufacturer of power supply controllers) and ON semiconductor for offlline power supply controllers for some design ideas.

Note that in order to control the output voltage of the supply, the period that the MOSFETs are conducting has to be controlled precisely. For lower input voltages (low line), the ON period is low, and it has be ON longer for higher line voltages. A simple oscillator driving a MOSFET will be unable to regulate the output voltage. So, you need to use some sort of pulse width modulator with feedback. Also, as the output load varies, the output voltage will vary in relation to losses in the switching transformer, MOSFETs and wiring. Voltage feedback is needed, therefore, to compensate for both line and load variations. I don't think a 555 could give very good performance.

You have a very ambitious project to get +/-35 volts at 10A directly of the mains without an AC transformer. I really think this is not a project for the average DIY'er, but if you want to go for it, good luck - just do your research and be VERY SAFE!!!

[hacknet]

thanks for the warning guys..

im trying to emulate it on the program state above and i cant seem to do it