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Old 26th November 2005, 08:30 PM   #1
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Default small smps with uc3844

Hello!

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.
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Old 26th November 2005, 10:21 PM   #2
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Default TL494

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 ?
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Old 28th November 2005, 05:19 AM   #3
mflorin is offline mflorin  Romania
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http://schmidt-walter.fbe.fh-darmsta..._e/smps_e.html
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Old 28th November 2005, 08:43 AM   #4
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Nice page But it only calculates one output winding, i need two as a amp needs bipolar power supply.
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Old 28th November 2005, 11:56 AM   #5
mflorin is offline mflorin  Romania
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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 !
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Old 28th November 2005, 02:43 PM   #6
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Something like this ?

Click the image to open in full size.
Click the image to open in full size.
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Old 28th November 2005, 03:38 PM   #7
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Use a 3843 for 12V boost operation. It has a lower startup/shutdown threshold.
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Old 28th November 2005, 03:42 PM   #8
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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.
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Old 28th November 2005, 04:39 PM   #9
mzzj is offline mzzj  Finland
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2SC2625 is mediocre when switching 20amps at 12v. Try some mosfets.
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Old 28th November 2005, 08:44 PM   #10
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Default 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:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

A little video:
smps and amp running
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