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Switching Power supply help

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As some of you know I have been fussing with a radio for quite some time now. The problem is that recently the power load on the power supply has gone up (due to new circuits going into the unit). The power supply is currently a liniar power supply and has been maxed on current (2A). Even though the transformer is giving 2A, that's not what the rest of the circuitry is consuming so I estimated that I'm loosing around 4W-7W of power somewhere.

The problem is that I have designed new cirucits to go into the unit. There is currently a lot of ripple from the over worked transformer so new stuff will just make it worse. I can't make the transformer bigger on acount of the batteries increasing in size (from AA cells to D cells). I want to retain some level of portability. This is why I decided to go to a switch mode power supply to make the transformer as small as possible.

The basic specifications of the new power supply are as follows:

-The input voltage will be 115V to 120V AC RMS (rectified to DC)
-The PWM IC will be a UC3842 (or similar)
-The Frequency will be about 30kHz to 40kHz
-The Primary output will be 20V @ 10A
-The 2 secondary outputs will be 6V @ 500mA each (connected in series)

The problem is that the datasheet for the UC3842 does not specify what the feedback voltage should be for the transformer. They had a 27W flyback transformer example in the datasheet but it didn't specify the feedback voltage. I would try to figuer this out myself but I have no clue what the calculations are for the transformer windings.

If anyone can help that would be appreciated.

And yes, I will be adding a circuit schematic for my power supply, calculations that an internet model came up with for the transformer, a datasheet for the chip that I will actually be using later.
 

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The error amplifier is internally referenced to +2.5 V. The feedback voltage out of the transformer is not critical - use a voltage divider so that at your desired voltage output, the input to the error amplifier is +2.5 V.
 
If the input to the error amp goes above 2.5 V, the output will lower its output voltage. If the input to the error amp goes below 2.5 V, the output will raise its output voltage. It does this by varying the duty cycle output that drives the primary of the transformer.
 
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