3.3V 3Amp SMPS Circuit

This article teaches how to make 3.3V 3Amp SMPS Circuit.

It can power many digital circuit or microcontroller.

Main good thing of SMPS circuit is it does not make much heat like normal regulator as the normal one get hot and waste energy.

What is a 3.3V 3Amp SMPS Circuit:

3.3V 3A SMPS main job is to keep output voltage same which is 3.3 volts and give max 3 amp current.

This type power supply are mostly used for sensor, microcontroller and other small electronic things that need low voltage and medium current.

Circuit Working:

Parts List:

When transistor turns ON 1M 1W base resistor make it little ON.

Then 8 turn wire make positive voltage and turn transistor fully ON.

Transistor turn OFF when 2.2nF capacitor discharges.

Then filter capacitor get charged from secondary side.

This happen again and again when 2.2nF capacitor charge and transistor turn ON again.

When right voltage come TL431 start working, it uses 3.3k and 10k resistors to control.

To control power to transformer LED inside optocoupler light up and then phototransistor lower the base current of transistor by cutting PWM signal.

Voltage stay very stable only lose 0.01V even on full load.

33 ohm resistor added at output to fix no load problem.

If voltage goes too high then Zener diode protect devices where SCR and other protection are used.

10 ohm resistor used to stop big current at start and 68nF capacitor used to reduce EMI noise.

If there are changes in circuit design then be careful with 2.2nF capacitor value it changes the working speed.

Also keep good gap between high voltage and low voltage parts on PCB.

Transformer use EE ferrite core with the size 0.5 cm².

First wind 40 turns for primary side using 0.2 to 0.3 mm wire.

Then put 8 layers of tape for insulation.

Next wind secondary side and use thick wire if only 4 turns needed.

Again add 8 layers of tape.

Now wind 8 turns for auxiliary winding and same wire as primary.

Put one more insulation layer which is not too thick.

After that wind last 40 turns of primary winding.

Add few more layers of tape.

To stop core from saturate leave small air gap using tape between core halves.

Seal core with glue at the end.

To change output voltage make these changes:

For secondary winding 1 turn = around 1V and add or remove turns for needed voltage.

Change 39 ohm resistor by 10 ohm for each 1V change.

Keep output voltage stable by adjusting 3.3k resistor and make divider give 2.5V to TL431 input.

Use Zener diode with voltage little more than needed output.

Diode must handle reverse voltage at least 8 times more than output voltage and then se fast diode if voltage is high.

Use capacitor with enough voltage rating.

Formulas:

1.Transformer Turn Ratio (Np/Ns):

This ratio help to set output voltage from input voltage.

Np / Ns = Vp / Vs

where:

• Np is the number of primary turns
• Ns is the number of secondary turns
• Vp is the input in primary voltage
• Vs is the output is secondary voltage

2. Duty Cycle (D):

This is how long transistor stay ON vs OFF.

D = Vout / Vin

where:

• Vout is the output voltage like 3.3V
• Vin is the input voltage like 220V or 120V

3. Output Resistor (R):

This resistor is used to test or to limit the current.

R = Vout / Iout

where:

• Vout is the 3.3V
• Iout is the output current like 3A

Safety Warning:

Switching power supply is not good for beginners.

Many parts connect to high voltage from mains which is very dangerous.

If circuit design is not right then high voltage can come to output.

Also big capacitors can keep dangerous voltage even after unplug.

Do this project only at ones own risk and the writer does not take any blame if someone get hurt or things get damaged.

References:

3.3V @ 3A