Building a 12V DC to 5V DC Power Converter Using the UC3845AN

In many DIY electronics projects, the need to convert power from one voltage level to another is common. Whether it's powering sensors, microcontrollers, or other low-power devices, having a reliable power supply is essential. A popular solution for power conversion is using a DC-DC converter, which efficiently steps down voltage with minimal energy loss.

This article will walk you through a specific project where we design and build a 12V DC to 5V DC power converter using the UC3845AN Pulse Width Modulation (PWM) controller. The UC3845AN is an integrated circuit (IC) that’s perfect for creating efficient switching regulators, specifically buck converters, that convert a higher DC voltage to a lower DC voltage. This type of converter is ideal for powering devices like Arduino boards, sensors, or other low-voltage electronics from a 12V battery or a 12V power supply.

Overview of the Project

In this project, we will build a 12V to 5V DC-DC buck converter that efficiently converts the 12V input from a standard DC power source into a regulated 5V output. The UC3845AN will serve as the core of the project, controlling the pulse width modulation (PWM) to drive a MOSFET that switches the power to the inductor, allowing us to efficiently step down the voltage.

This project involves several key components, including the UC3845AN controller, an inductor, a diode, capacitors, and a MOSFET. The final result is a compact and efficient voltage regulator capable of powering 5V devices from a 12V supply. This converter can be used for various applications, such as powering sensors, microcontrollers, or any low-power DC devices.

Components Required

· UC3845AN PWM Controller IC: This is the central component of the project. It controls the PWM signal to regulate the output voltage.

· MOSFET (e.g., IRLZ44N): Used as the switching device that drives the inductor in the buck converter.

· Inductor (e.g., 100µH): A crucial component for storing energy and smoothing the output current.

· Diode (e.g., 1N5822): A Schottky diode used for its low forward voltage drop, improving the efficiency of the converter.

· Capacitors (e.g., 47µF and 0.1µF): Used for filtering and smoothing the output voltage.

· Resistors (for setting feedback voltage and PWM control).

· 12V DC Power Supply: The input power source for the converter.

· Heat Sink (optional): To prevent the MOSFET from overheating if high current is drawn from the converter.

· PCB or Breadboard for assembling the circuit.

· Jumper wires for connections.

Why Use UC3845AN?

The UC3845AN is a popular PWM controller that offers the following benefits:

1. Integrated Features: It includes all the essential features for a switching regulator, such as an internal oscillator, error amplifier, feedback control, and totem-pole output stage.

2. Current Mode Control: The UC3845AN uses current mode control for regulating the output voltage, ensuring fast response times and high efficiency, even under varying load conditions.

3. Adjustable Output Voltage: The output voltage is set by external components, making the UC3845AN versatile for different voltage levels.

4. High Efficiency: With an efficient buck converter design, it minimizes energy loss compared to linear regulators.

5. Easy to Use: The IC is simple to interface with external components, making it ideal for DIY projects.

Circuit Design and Operation

In this project, the core of the power conversion is based on a buck converter topology. The basic operation of a buck converter involves switching the input voltage on and off at a high frequency, filtering the output, and using feedback to control the duty cycle of the PWM signal, which regulates the output voltage.

Step 1: Input Power

The 12V DC input is fed into the circuit. This could come from a battery or a regulated 12V power supply. The input is then filtered with a capacitor (usually around 100µF) to remove any noise or ripple from the power supply.

Step 2: PWM Generation by UC3845AN

The UC3845AN generates the PWM signal that controls the switching of the MOSFET. The duty cycle of the PWM signal determines how long the MOSFET stays on during each switching cycle, which in turn controls the average voltage sent to the inductor and the output.

· The UC3845AN uses feedback from the output voltage to adjust the duty cycle to maintain a stable 5V output.

· The feedback loop is achieved by connecting a voltage divider to the feedback pin (pin 1) of the UC3845AN, which senses the output voltage.

Step 3: Switching the MOSFET

The MOSFET (e.g., IRLZ44N) is driven by the output from the UC3845AN. The MOSFET switches rapidly, allowing current to flow through the inductor. The inductor stores energy when the MOSFET is on and releases it when the MOSFET is off. The inductor’s energy storage and release help smooth out the switching pulses and step down the voltage.

· When the MOSFET is on, the current flows through the inductor, and energy is stored.

· When the MOSFET is off, the inductor releases the stored energy through the Schottky diode (e.g., 1N5822), directing the current to the load.

Step 4: Output Filtering

The capacitors at the output (typically 47µF for bulk filtering and 0.1µF for high-frequency noise suppression) smooth the voltage and reduce ripple, providing a stable 5V output.

Step 5: Feedback Control

The feedback loop is essential for regulating the output voltage. The UC3845AN continuously compares the voltage at the feedback pin with a reference voltage (set internally to 2.5V). The error amplifier adjusts the PWM duty cycle to maintain a steady 5V output despite variations in input voltage or load conditions.

Schematic Diagram

Here is a simplified schematic for the 12V to 5V buck converter using the UC3845AN:

Step-by-Step Connections

1. Power Supply: Connect the 12V DC input to the input capacitor C1 to filter any ripple.

2. UC3845AN PWM Controller:

o Pin 1 (Feedback): Connect this pin to the voltage divider (R1 and R2) from the output to set the feedback voltage to the required 5V.

o Pin 8 (Vcc): Connect this to the 12V input.

o Pin 5 (Output): Connect to the Gate of the MOSFET (Q1), which switches the inductor.

3. MOSFET (Q1): Connect the drain of the MOSFET to the inductor and the source to the ground. The gate is driven by the UC3845AN.

4. Inductor (L1): Connect one end of the inductor to the drain of the MOSFET and the other end to the output.

5. Schottky Diode (D1): Place the diode between the inductor and the output, with the anode connected to the inductor and the cathode connected to the output to prevent backflow of current.

6. Output Capacitor (C2): Connect the output capacitor across the output to smooth the voltage.

Assembly Instructions

1. Connect Components on a Breadboard: Start by connecting the input power, capacitors, and the UC3845AN controller to the breadboard. Ensure that the feedback voltage divider is properly connected to the output.

2. Connect the MOSFET: Insert the MOSFET in the circuit as described, ensuring the gate is connected to the UC3845AN’s output.

3. Attach the Inductor and Diode: Place the inductor in series with the output, followed by the Schottky diode to prevent reverse current.

4. Test the Output: Power up the circuit and use a multimeter to check the output voltage at the load terminals. It should read a stable 5V.

Troubleshooting

If the output voltage is not stable or the circuit is not functioning correctly, check the following:

1. Check the Capacitors: Ensure that the capacitors are connected correctly and have the proper values.

2. Verify the Feedback Voltage Divider: Make sure the resistor values for the feedback loop are correctly chosen to set the output voltage to 5V.

3. Check the MOSFET: Ensure the MOSFET is switching correctly and not overheating.

4. Verify the Connections: Double-check all wiring and component placements on the breadboard or PCB.

Conclusion

This 12V to 5V buck converter using the UC3845AN provides an efficient solution for stepping down voltage in your DIY electronics projects. By following the steps outlined above, you can easily build a reliable power supply to power various 5V devices from a 12V source. Whether you are designing a battery-powered project, a sensor-based system, or simply need a stable 5V supply, this project is a practical and rewarding way to learn about power conversion, PWM control, and efficient voltage regulation in electronic systems.