This article will educate you to build a simple 3.3V 3 Amp SMPS circuit which powers nearly any type of digital electronics circuit or microcontroller.
This SMPS circuits main advantage is comparison to linear regulators which generate a lot of heat and waste energy, it does not dissipate heat.
What is a 3.3V 3Amp SMPS Circuit:
So the main idea of using a 3.3V 3A SMPS (Switched Mode Power Supply) circuit is to produce a maximum current output of 3 amperes with a constant regulated output voltage of 3.3V.
These power supplies could be used to supply current to sensors, microcontrollers, electronic devices and other devices that needs a 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 | |
| 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 are used.
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 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 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.
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.
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 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 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.
Be in this project at your own risk and I do not assume responsibility for any injuries or property damage.
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