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Tekko 26th November 2005 08:30 PM

small smps with uc3844

Is it possible to make a little bipolar 35v smps capable of delivering about 2.5A, that runs on 12vdc with a UC3844, four paralelled 2SC2625 transistors and some other components required around the transformer ?

The transformer i'll wind myself, is the winding ratio the same for a flyback type smps as for a pushpull one ? How big does the core need to be to make a 100-150w smps ?

I have this idea to use it with the 1982 elektor amp.

Tekko 26th November 2005 10:21 PM

EDIT: UC3844 needed 16v to start so lets go with TL494 instead.

Is this possible ?
How thick wire do i need to use to get 35v 2.5A out ?

mflorin 28th November 2005 05:19 AM


Tekko 28th November 2005 08:43 AM

Nice page :) But it only calculates one output winding, i need two as a amp needs bipolar power supply.

mflorin 28th November 2005 11:56 AM

You can take into account the total output power and efficiency.
For example if you want +35V/2.5A and -35V/2.5A, then your output power is Pout = 175W.
In order to use the program, you can set, for example, the output voltage equal to 35V and for output current use

Iout = Pout/(Vout*efficiency)

With this settings, program will return a value for the primary current which is close to reality. The value for primary wire diameter could be used.

For secondary windings, you have to set Iout = 2.5A and the program will do the rest. The specified value for secondary wire diameter is a good choice.

Have fun !

Tekko 28th November 2005 02:43 PM

Something like this ?

Ouroboros 28th November 2005 03:38 PM

Use a 3843 for 12V boost operation. It has a lower startup/shutdown threshold.

Ouroboros 28th November 2005 03:42 PM

At the sort of output power you're asking for, you'll probably need to run your flyback supply in continuous-conduction mode, so make sure your feedback stabilisation is happy with operation in both discontinuous and continuous mode. (The app notes for a TL3843, UC3843 etc will explain this).

Don't use a TL494, horrible old things! TL3843s cost pennies.

mzzj 28th November 2005 04:39 PM

2SC2625 is mediocre when switching 20amps at 12v. Try some mosfets.

Tekko 28th November 2005 08:44 PM

First prototype running
Now i have built a first prototype around a UC3844, two IRFI640 and a transformer made with a emi filter toroid.

The thing gives out around 20.2 volts and can actually drive the elektor amp, tho the voltage is alittle too low but i have tried to turn the bass up with no prob, i took it quite loud and voltage only drivved to 19v, pretty impressive for a first prototype built around a emi filter toroid.

Heres some pics:

A little video:
smps and amp running

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