This article will educate all to build a simple 3.3V 3 Amp SMPS circuit which powers nearly any type of digital electronics circuit or microcontroller.
The primary benefit of SMPS circuits is that they do not dissipate heat unlike linear regulators which produce a lot of heat and waste energy.
What is a 3.3V 3Amp SMPS Circuit:
A 3.3V 3A Switched Mode Power Supply circuits main objective is to maintain a constant and regulated output voltage of 3.3 volts while generating a maximum current output of 3 amps.
These kinds of power sources are commonly used to power sensors, microcontrollers, electronic devices and other devices that need low voltage and medium current.
Circuit Working:
Parts List:
| Category | Component | Quantity |
|---|---|---|
| Resistors | 10Ω 1W MFR | 1 |
| 1M 1W MFR | 1 | |
| 680Ω 1W MFR | 1 | |
| 100Ω 1W MFR | 1 | |
| 39Ω 1/4W MFR | 1 | |
| 3.3k 1/4W MFR | 1 | |
| 33Ω 1W MFR | 1 | |
| 10k 1/4W MFR | 1 | |
| 1M 1/4W MFR | 1 | |
| Capacitors | Electrolytic 33μF / 350V | 1 |
| Electrolytic 1000μF / 10V | 1 | |
| PPC 68nF / 400V | 1 | |
| PPC 2.2nF | 1 | |
| PPC 220pF / 1kV | 1 | |
| Semiconductors | Diode 1N4007 | 1 |
| Schottky Diode BA159 | 1 | |
| Schottky Diode 1N5822 | 1 | |
| Zener Diode 3.9V / 1W | 1 | |
| Transistor MJE13005 | 1 | |
| Optocoupler 4N35 | 1 | |
| IC TL431 | 1 | |
| IC 4N35 | 1 | |
| Ferrite core EE 0.5 cm² | 1 |
When the transistor is powered ON the 1M 1W base resistor partly opens it creating a positive voltage in the 8 turn auxiliary winding that causes the transistor to open completely.
The transistor shuts OFF when the 2.2nF capacitor discharges allowing the filter electrolytic capacitor to be charged by the secondary.
This cycle repeats when the transistor reopens after the 2.2nF capacitor has been charged.
When the desired voltage becomes available the TL431 circuit which is regulated by a resistive divider of 3.3k and 10k activates.
In order to control the energy supplied to the transformer the optocouplers LED illuminates and a phototransistor limits the current to the transistor base by reducing the PWM duty cycle.
With a maximum 0.01V loss in the full load voltage this stabilizing solution is quite successful.
A 33 ohm load resistor is connected to the output in order to fix the problem of no load functioning.
If stabilization fails a Zener diode protects the powered equipment from overvoltage.
SCRs and other overvoltage protection techniques are used.
A 10 ohm resistor controls the excess current during start up and a 68nF capacitor lowers EMI interference.
When modifying the design keep in mind that changes in the value of the 2.2nF capacitor may affect the operating frequency.
There should be enough space between the mains and secondary components of the PCB.
Transformer Winding Details:
The transformer is wound on an EE ferrite core with an effective cross section of 0.5 cm².
Start with the first half of primary turns of about 40 turns using wire with a diameter of approximately 0.2 – 0.3 mm.
Apply at least 8 layers of insulating tape.
Wind the secondary coils with wire having thick insulation which can be completed with just 4 turns.
Add another 8 layers of insulating tape.
Wind the auxiliary winding with 8 turns using the same wire as the primary.
Add an insulating layer which does not need to be as thick.
At last wind the primary final 40 turns and add a few more layers of insulation.
To avoid core saturation leave an air space between the core halves by using insulating tape.
Use glue to seal the core.
Voltage Adjustment:
This design is modified for different output voltages:
Adjust the number of secondary turns approximately 1 turn corresponds to 1V.
Change resistor 39 ohm by about 10 ohm for each 1V.
Maintain the output voltage by changing resistor 3.3k so that the divider provides 2.5V at the input of TL431.
Select a Zener diode slightly greater than the desired output voltage.
Ensure the rectifier diode can handle a reverse voltage at least 8 times larger than the output voltage switching to a fast diode for higher voltages.
Use an electrolytic capacitor rated for the sufficient voltage.
Formulas:
The following formulas are necessary for the switching power supply design:
The ratio of transformer turns (Np/Ns):
The transformers turns ratio decides the output voltage in terms of the input voltage.
Np / Ns = Vp / Vs
where:
- Np is the number of primary turns
- Ns is the number of secondary turns
- Vp is the primary voltage
- Vs is the secondary voltage
Duty Cycle D:
This is the ratio of the switching element transistor in the circuits ON to OFF times.
The following is an approximate duty cycle estimate:
D = Vout / Vin
where:
- Vout is output voltage 3.3V
- Vin is input voltage of standard 220V to 120V
Output Resistor R:
This is mainly used for current limitation and initial testing.
It may be measured as follows:
R = Vout / Iout
where:
- Vout is output voltage 3.3V
- Iout is desired output current 3A
Safety Warning:
Switching power supplies are not recommended for beginners as most of their circuits are connected to potentially dangerous mains voltage.
Poor design can result in mains voltage reaching the output and capacitors may provide dangerous voltage even after disconnecting from the mains.
Be in this project at ones own risk and I do not assume responsibility for any injuries or property damage.
Leave a Reply