Build this 85V–265V AC to 12V 5A SMPS Design Using UC3843 (With Calculations)

This thing is a full SMPS design using UC3843, which takes wide AC input from 85V AC to 265V AC, then gives out 12V DC at 5A, so now it is like a universal input supply, which can be used for 110V AC supplies and also for 220V AC input supplies...

Input Protection Stage

At input side, we first pass AC through 1.5A fuse which saves things if current goes high, then NTC is there, so when power comes ON then inrush current stays limited, otherwise it can hit hard.

A MOV is also placed across AC lines which catches spikes, so if surge comes then it absorbs that, yep protection layer.

AC to DC Conversion

Then AC goes into bridge rectifier like KBP206 or DB207 which converts AC to DC, so now depending on input, we get around 120V DC at low side or near 375V DC at high side.

This DC is filtered using 220uF / 400V capacitor and 100nF which removes noise, makes line stable enough.

Circuit Diagram

UC3843 Startup and Vcc Supply

Now UC3843 needs around 12V to 16V to start, so at beginning it gets supply through 330k resistor from high voltage line, slow feeding, and 47uF capacitor is there to hold it steady.

When circuit starts running then auxiliary winding takes over, through UF4007 diode, so now IC does not depend on that startup resistor anymore.

Also 18V zener is placed across pin 7, so if voltage rises too much then it clamps, keeps IC safe.

Oscillator and Frequency Setting

Frequency part comes from Rt and Ct at pin 4 and formula is:

f = 1.72 / (Rt * Ct)

So if Rt = 10k and Ct = 2.7nF, then frequency comes near 63.7 kHz, that is switching speed.

MOSFET Drive and Switching

Now pin 6 drives MOSFET gate through 15 ohm resistor and 10k pulls gate to ground, so when OFF then it really turns OFF.

Current Sensing and Protection

Current sense resistor sits at source, and peak current follows:

Ipeak = 1 / Rsense

So if Rsense = 0.27 ohm, then current goes near 3.7A, which keeps things under limit.

Power and Input Current Calculation

Power side, output is 12V * 5A = 60W, and if efficiency is 80 percent, then input becomes around 75W.

So at 120V DC input, current comes near 0.62A, and at 375V DC then it drops near 0.2A, since voltage is higher.

Transformer Design and Calculation

Now transformer, hmm this is main part, flyback type, which stores energy when MOSFET is ON and then releases when OFF.

Primary turns follow:

Np = (Vin(min) * Dmax) / (Bmax * Ae * f)

Putting values gives near 55 turns, so we keep 55 to 60 turns.

Secondary comes from relation, gives near 7 turns, so 7 to 8 turns is fine.

Auxiliary winding depends on ratio, so around 9 to 10 turns.

Air gap also needed, around 0.5mm to 1mm, otherwise core can saturate, then trouble.

Transformer Wire Thickness

Wire size, primary uses 0.25mm to 0.3mm or two strands, since current is around 1.4A RMS.

Secondary uses thicker, like 0.8mm to 1mm or multiple strands, since current is 5A.

Auxiliary just small wire, around 0.3mm.

Output Rectification and Filtering

At output, secondary goes into Schottky diode MBR20100 then filtered using 2200uF / 25V and 100nF, so output becomes smooth.

Feedback and Regulation

Feedback side uses TL431 and optocoupler PC817 which gives isolation and control.

Voltage setting follows:

Vout = 2.5 * (1 + Rupper / Rlower)

So with 8.2k and 2.2k, output becomes near 12V.

Optocoupler LED connects from +12V through 1k to TL431, so when voltage changes then feedback adjusts duty cycle.

Universal Input Working Principle (110V and also 220V mains AC inputs)

Now wide input working, if input is low then current rises slowly so ON time increases, causing UC3843 duty cycle to go high.

If input is high then current rises fast so ON time reduces, then UC3843 duty cycle goes low.

That relation is:

Flux ∝ (Vin * Ton) / Np

So core stays safe, no saturation across range.

Snubber and Protection

Snubber is placed across primary, using UF4007, 100k resistor, and 2.2nF / 1kV capacitor, which absorbs spikes from leakage.

Safety and Practical Notes

Now some care, insulation between primary and secondary must be proper, PCB track gap must be safe, because high voltage is there.

Snubber values may need tuning depending on transformer, since build changes behavior.

When you first time power ON, always use series bulb, so if something is wrong then it does not blow the circuit part instantly, the bulb illuminates brightly and absorbs the faulty situation and let's you know about it....

Conclusion

So overall this UC3843 SMPS design gives around 60W SMPS, works across wide AC input from 85V AC to 265V AC RMS, to produce a stable 12V DC output at high current of 5 amps, and if transformer is built properly then it runs without any issues...

So yeah, simple in idea, but building care and proper testing matters a lot, that is where things could go right or wrong.