In this post we will learn how to build a simple 3.3V 3 Amp SMPS circuit which can be used for powering all types of microcontroller and digital electronics circuits efficiently.
The main advantage of this SMPS circuit is that it does not dissipate heat unlike linear regulators which generates a lot of heat and waste energy.
What is a 3.3V 3Amp SMPS Circuit:
A 3.3V, 3A SMPS (Switched Mode Power Supply) circuit is designed to generate a stable and regulated output voltage of 3.3V with a maximum current output of 3 amperes.
Such a power supply is commonly used to provide power to electronic devices, microcontrollers, sensors or other components that require a low voltage and moderate current.
Circuit Description
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 | |
| ICs | IC TL431 | 1 |
| IC 4N35 | 1 | |
| Other | Ferrite core EE 0.5 cm² | 1 |
Upon power connection the 1M 1W base resistor partially opens the transistor inducing a positive voltage in the auxiliary winding (8 turns) causing the transistor to open fully.
When the 2.2nF capacitor discharges the transistor turns off, allowing the secondary to charge the filter electrolytic capacitor.
Once the 2.2nF capacitor is charged again the transistor re opens and this cycle repeats.
The TL431 circuit controlled by a resistive divider of 3.3k and 10k, activates when the desired voltage is reached.
The optocouplers LED starts shining, and a phototransistor limits the current to the transistor base, reducing the PWM duty cycle to regulate energy delivered to the transformer.
This stabilization method is highly effective with the full load voltage dropping by no more than 0.01V.
To address the issue of no load operation, a load resistor of 33 ohm is connected to the output.
A Zener diode safeguards the powered device from overvoltage in case of stabilization failure.
Other over voltage protection methods like using an SCR can also be employed.
A 68nF capacitor reduces EMI interference, and a 10 ohm resistor limits inrush current during power up.
Changes in the 2.2nF capacitor value can impact the operating frequency, so consider this when adjusting your design.
The printed circuit board layout should keep the primary (mains) and secondary sections sufficiently apart.
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, 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 accomplished 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.
Finally, wind the remaining 40 turns of the primary and apply a few more layers of insulation.
Place a layer of insulating tape between the core halves to create an air gap that prevents core saturation.
Seal the core with glue.
Voltage Adjustment
This design can be 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.
Stabilize 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:
Here are some relevant formulas and equations related to the design of the switching power supply:
Transformer Turns Ratio (Np/Ns):
The turns ratio of the transformer determines the output voltage in relation to the input voltage.
Np / Ns = Vp / Vs
where:
- Np = Number of primary turns
- Ns = Number of secondary turns
- Vp = Primary voltage
- Vs = 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 preliminary testing.
It may be computed 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 fatal mains voltage.
Poor design can result in mains voltage reaching the output, and capacitors may retain dangerous voltage even after disconnecting from the mains.
Engage in this project at your own risk, and I do not assume responsibility for any injuries or property damage.
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