Introduction

In the realm of modern electronics, power supplies are essential components for ensuring that your devices get the proper voltage and current for smooth operation. When designing a power supply for sensitive systems—especially where electrical isolation is necessary—optocouplers are invaluable components. One such optocoupler, the TLP222G, is often used for isolating low-voltage control circuits from high-voltage power circuits. It provides electrical isolation while still allowing signals to pass between circuits, ensuring safety and protecting sensitive components from power surges.

This project focuses on building an isolated switching power supply using the TLP222G optocoupler to separate the control side of the power supply from the high-voltage side. In this DIY project, we will create a flyback converter, which is a type of isolated switching power supply, using the TLP222G to transfer feedback signals from the output to the control circuit.

A flyback converter is an excellent choice for low-power applications, such as powering microcontrollers, sensors, or LED drivers, where isolation between the high-voltage side and the low-voltage side is necessary for safety and performance. In this project, we’ll focus on a practical example where the TLP222G is used for feedback control in the flyback converter, ensuring stable and regulated output voltage.

Components Needed

● TLP222G Optocoupler: For providing electrical isolation between the control and power sides of the converter.

● Flyback Transformer: For energy storage and voltage step-up or step-down.

● MOSFET (e.g., IRF540N): For switching the high-voltage side of the converter.

● PWM Controller IC (e.g., UC3842): To generate the switching frequency and control the operation of the MOSFET.

● Diodes (e.g., 1N4007): For rectifying the output voltage.

● Capacitors: To filter the output and smooth the DC voltage.

● Inductor: For energy storage and current regulation.

● Resistors: For controlling current in feedback loops and limiting input/output currents.

● Power Supply: For powering the control side of the circuit (e.g., 5V or 12V DC).

● Heat Sink: To prevent overheating of the MOSFET and other power components.

● PCB or Breadboard: For assembling the circuit.

● Connectors: For input and output connections.

Understanding the TLP222G Optocoupler

The TLP222G is a high-speed phototransistor optocoupler designed for providing electrical isolation in applications that require fast switching and efficient feedback. The optocoupler consists of an infrared LED and a phototransistor, allowing it to transmit signals from the low-voltage control circuit to the high-voltage power circuit while maintaining electrical isolation between the two.

Key features of the TLP222G include:

● High-Speed Operation: Capable of switching at speeds up to 2 MHz, making it suitable for fast feedback loops in power converters.

● High Isolation Voltage: Offers up to 5 kV of isolation, which ensures safety in high-voltage systems.

● Low Forward Voltage Drop: The phototransistor has a low forward voltage, allowing efficient signal transfer with minimal loss.

● Compact Package: Available in a 4-pin DIP package, which is easy to integrate into your designs.

In our power supply project, the TLP222G will be used to provide feedback from the output side of the flyback converter to the controller, allowing the system to adjust its output voltage and maintain regulation.

Step-by-Step Guide: Building the Isolated Switching Power Supply

Step 1: Designing the Flyback Converter

The flyback converter is a type of isolated power supply commonly used for low- to medium-power applications. It operates by storing energy in the transformer’s magnetic field when the MOSFET is on, and then releasing this energy to the load when the MOSFET is off.

1. Flyback Transformer Selection: Choose a flyback transformer that is appropriate for the input and output voltage you need. For this example, we’ll assume an input voltage of 12V DC and an output voltage of 5V DC at 1A. Select a transformer that can step down the voltage from 12V to 5V, with the necessary winding ratio and sufficient current rating.

2. MOSFET Selection: The MOSFET will switch the primary side of the transformer, storing energy during the "on" period. Choose a MOSFET with low Rds(on) to minimize switching losses. For example, the IRF540N is a suitable N-channel MOSFET for this application.

3. PWM Controller Selection: The UC3842 PWM controller IC will be used to drive the MOSFET. This IC generates the necessary pulse-width modulation signal to control the MOSFET, determining the power transferred to the transformer. The UC3842 also includes feedback features to regulate the output voltage.

Step 2: Adding the Feedback Loop with TLP222G

The TLP222G optocoupler plays a crucial role in providing feedback from the output side of the converter to the control side. This allows the controller to adjust the duty cycle of the MOSFET and maintain a stable output voltage.

1. Output Rectification and Filtering: After the flyback transformer, use a diode (e.g., 1N4007) to rectify the AC output and a filtering capacitor to smooth the output voltage. In this case, we want a stable 5V DC output. Choose an output capacitor (e.g., 100µF electrolytic capacitor) to filter the rectified DC voltage.

2. Voltage Sensing: The output voltage needs to be sensed to provide feedback. Use a voltage divider to scale the output voltage down to a level that can be safely applied to the TLP222G's input side.

● For example, if the output is 5V and you want to scale it down to 1.5V, use a pair of resistors (R1 and R2) to create the appropriate voltage divider.

● Connect the voltage divider to the input LED of the TLP222G.

3. Feedback to the Control Circuit: The TLP222G's phototransistor will transfer the feedback signal to the control side, allowing the PWM controller to adjust the duty cycle of the MOSFET to regulate the output voltage.

● The TLP222G will be connected to the feedback pin of the UC3842 PWM controller.

● The UC3842 uses this feedback signal to adjust the duty cycle of the MOSFET and maintain the desired output voltage.

Step 3: Assembling the Circuit

Once the components are selected, it’s time to assemble the circuit. The primary components—MOSFET, flyback transformer, PWM controller, and TLP222G—will be mounted on a PCB or a breadboard.

1. Power Supply for the Control Side: The control side of the circuit (PWM controller, optocoupler, etc.) requires a separate, low-voltage power supply. You can use a 5V DC or 12V DC supply, depending on the requirements of the control circuitry.

2. MOSFET and Flyback Transformer: The MOSFET will be placed in the primary side of the flyback converter, with the flyback transformer in between. The secondary side of the transformer will be connected to the output rectifier (diode and capacitor).

3. Optocoupler Connections: Connect the input LED of the TLP222G to the output voltage divider, and connect the phototransistor side of the optocoupler to the feedback pin of the PWM controller.

4. Resistors and Capacitors: Place the necessary resistors and capacitors to stabilize the feedback loop and ensure smooth operation. A 100nF capacitor is commonly used in parallel with the TLP222G for noise suppression.

5. Heat Management: Attach a heat sink to the MOSFET to ensure efficient heat dissipation. High-power components like MOSFETs can generate a significant amount of heat, so proper thermal management is crucial for stable operation.

Step 4: Testing the Circuit

Once the circuit is assembled, the next step is to test the power supply.

1. Verify Connections: Double-check all connections to ensure that the optocoupler is correctly wired and the feedback loop is properly set up.

2. Power On the System: Apply power to the input side and check the output voltage. Use a multimeter to verify that the output is stable at the desired value (e.g., 5V DC).

3. Adjust Feedback for Stability: If the output voltage is unstable or fluctuates, adjust the resistor values in the voltage divider or make adjustments to the feedback loop.

4. Load Testing: Test the power supply with a small load (e.g., a LED array or a low-power microcontroller circuit) to verify that the converter can handle the expected load without excessive ripple or voltage drop.

Step 5: Final Adjustments and Enclosure

Once the circuit is operating correctly, finalize the design by placing the components inside a protective enclosure. Ensure that the MOSFET and other high-power components are properly cooled, and that the circuit is safe for operation.

Conclusion

In this DIY project, we have designed and built an isolated switching power supply using the TLP222G optocoupler to provide feedback and maintain output voltage regulation. By incorporating a flyback converter design, we were able to step down a high input voltage (e.g., 12V DC) to a stable, regulated low output voltage (e.g., 5V DC) with electrical isolation for safety. The TLP222G played a critical role in the feedback loop, ensuring that the system remains stable and efficiently regulated.

This project demonstrates the power of optocouplers like the TLP222G in providing electrical isolation and feedback control in high-efficiency power supply designs. Whether you’re building a power supply for a microcontroller, sensor system, or battery charging circuit, this project provides a practical example of how to leverage optocouplers in isolated switching power supply designs.