How To Design a 12V Switching Power Supply Using the UC3843BVDG

Introduction Power supply circuits are an essential part of almost all electronic devices. They convert one form of electrical power to another, ensuring that the right amount of voltage and current is delivered to the components within a device. In this project, we will design a 12V switching power supply using the UC3843BVDG, which is a high-performance pulse-width modulation (PWM) controller IC. This type of power supply is efficient, reliable, and perfect for powering low-voltage devices such as microcontrollers, sensors, and small motors.

The UC3843 is commonly used in power supplies due to its ease of use, high-speed switching capabilities, and built-in features like under-voltage lockout, overcurrent protection, and soft-start functionality. In this project, we will design a simple yet effective flyback converter to convert an AC voltage (e.g., 120V or 230V) to a stable 12V DC output using the UC3843BVDG PWM controller.

Project Overview This project involves creating a 12V switching power supply using the UC3843BVDG. The goal is to design and build a compact, efficient, and reliable power supply that can convert high-voltage AC power to a regulated 12V DC output suitable for powering low-voltage circuits and devices.

Components Required

● UC3843BVDG PWM Controller IC

● Transformer: 12V primary-to-secondary winding ratio suitable for the input and output voltage.

● Diodes: 1N5408 or similar for rectification.

● Capacitors: Electrolytic and ceramic capacitors (e.g., 220µF, 100µF, 0.1µF) for filtering and smoothing the output.

● Inductor: A flyback transformer with appropriate turns ratio.

● Resistors: For setting reference voltage and feedback loop.

● MOSFET: IRF540N or similar for the switch.

● Rectifier Diode: 1N5408 or 1N4007.

● Output Capacitor: To filter the rectified DC output.

● Heat Sink: For the MOSFET and rectifier diode to prevent overheating.

Step by step guide

Step 1: Understanding the UC3843BVDG The UC3843BVDG is a PWM controller designed for controlling the switching of power transistors in power converters. Its main features include:

● High-speed PWM control with a duty cycle up to 100%.

● Built-in reference voltage (5V) for feedback regulation.

● Under-voltage lockout (UVLO) to ensure the power supply only operates when the voltage is above a minimum threshold.

● Soft start to limit inrush current during power-up.

● Overcurrent protection to prevent damage to the circuit components.

In this project, the UC3843 will be used to control the MOSFET that drives the transformer. It will regulate the duty cycle of the switch to maintain a stable 12V DC output at the secondary side of the transformer.

Step 2: Design the Power Supply Circuit The power supply consists of several key stages: AC input rectification, DC filtering, flyback conversion, and output regulation.

AC Input Rectification: Start with a bridge rectifier circuit that will convert the AC input (e.g., 120V or 230V AC) into pulsating DC. This is done using four diodes arranged in a bridge configuration. After rectification, use a large electrolytic capacitor (e.g., 220µF) to smooth the rectified DC voltage.

Flyback Converter: The flyback converter is used to step down the high-voltage DC to 12V DC. The UC3843BVDG will regulate the operation of the flyback converter by controlling the duty cycle of the MOSFET switch. The flyback transformer has a primary side connected to the MOSFET and secondary side connected to the rectifier diodes and output filter capacitors.

Feedback and Regulation: To regulate the output voltage, a feedback loop is implemented. A voltage divider is used to scale down the 12V output to a lower voltage, which is fed to the non-inverting input of the error amplifier within the UC3843. The error amplifier compares this feedback voltage with the reference voltage (5V) and adjusts the duty cycle of the MOSFET to maintain the desired output voltage.

The UC3843 uses pulse-width modulation to control the MOSFET. When the output voltage is lower than the setpoint (12V), the error amplifier increases the duty cycle of the MOSFET, allowing more energy to be transferred to the output. If the output voltage exceeds the setpoint, the duty cycle is reduced.

Output Filtering: After the flyback transformer, the DC voltage is rectified using diodes (such as the 1N5408) and smoothed using a capacitor (e.g., 100µF or 220µF) to remove any ripple. This will give you a stable DC output voltage.

Step 3: Building the Circuit Once the design is complete, it’s time to assemble the components. Here's a step-by-step guide to the assembly process:

1. Power Input Stage:

● Connect the AC input terminals to the bridge rectifier (ensure the AC voltage is within the operating range).

● After the rectifier, place a large filtering capacitor (e.g., 220µF) to smooth out the ripple.

2. PWM Control with UC3843BVDG:

● Connect the UC3843BVDG to the MOSFET gate. This will control the switching of the MOSFET.

● The feedback from the output voltage is fed back to the error amplifier through a voltage divider network.

● Connect the reference pin of the UC3843 to ground.

● Add the required resistors and capacitors to stabilize the operation and ensure proper feedback loop functionality.

● The shutdown pin (SD) is connected to a pull-up resistor to disable any unwanted shutdown behavior.

3. Flyback Transformer:

● The flyback transformer should have a suitable turns ratio to convert the DC voltage from the primary side to 12V DC on the secondary side. The transformer will provide the necessary isolation between the high-voltage primary and low-voltage secondary.

● Connect the primary winding to the MOSFET and the secondary winding to the rectifier diodes and output filter capacitor.

4. Output Stage:

● The rectifier diodes (1N5408) will convert the AC voltage from the transformer secondary to DC.

● The output capacitor (e.g., 220µF) will smooth the rectified DC output.

● The final output should provide a stable 12V DC, which can be used to power your devices.

5. Safety:

● Since this is a high-voltage design, ensure that proper insulation and safety precautions are taken during assembly, especially when working with AC power.

● Use a fuse on the input side to prevent damage in case of a fault.

● Ensure that the components are rated for the voltage and current they will handle.

Step 4: Testing and Troubleshooting Once the circuit is assembled, carefully inspect all connections. Check for any short circuits or potential issues that could cause the circuit to malfunction. Connect the power supply to a test load (e.g., a resistor or small DC device) and measure the output voltage.

● If the output voltage is too high or low, adjust the feedback resistors to calibrate the voltage regulation.

● If the circuit fails to start or there is instability, check the feedback loop and ensure that the UC3843 is receiving proper input signals.

Use an oscilloscope to check the switching waveform at the MOSFET gate. It should show a pulse-width modulated signal, with varying pulse widths depending on the feedback from the output voltage.

Conclusion

In this project, we have successfully designed and built a 12V switching power supply using the UC3843BVDG PWM controller. The UC3843BVDG's ability to regulate the switching transistor based on feedback from the output voltage ensures that the circuit provides a stable, efficient power source for various low-voltage devices. This project is a great way to learn about switching power supplies and PWM control, and the resulting power supply can be used for a wide range of DIY electronics projects.