In this project, we will build a simple Switch Mode Power Supply (SMPS) circuit using the FSL128MRT, an integrated circuit designed for efficient and reliable power conversion. The FSL128MRT is a highly integrated power IC from Fairchild Semiconductor (now part of ON Semiconductor) that combines multiple features into a single package, making it ideal for DIY power supply projects. This project will demonstrate how to build a basic SMPS capable of converting a 12V input to a stable 5V output, commonly required for powering low-voltage devices like microcontrollers, sensors, and small electronic gadgets.

Components Needed

Before we dive into the circuit design, let’s list all the components required for this project:

1. FSL128MRT Integrated Circuit (1 piece)

2. Inductor (10µH)

3. Capacitors:

● Input capacitor: 470µF, 25V

● Output capacitor: 220µF, 10V

● Bypass capacitor: 0.1µF (ceramic)

4. Diode (Schottky diode, 1N5819 or similar)

5. Resistors:

● 1kΩ (1 piece)

● 10kΩ (1 piece)

● 100Ω (1 piece)

6. Input Power Source (12V DC adapter or battery)

7. Output Load (e.g., a 5V LED strip or microcontroller)

8. PCB or Breadboard for assembly

9. Soldering tools (if assembling on a PCB)

Overview of the FSL128MRT

The FSL128MRT is a high-efficiency, low-power, step-down (buck) converter that integrates a number of key components in a single package. It uses a pulse-width modulation (PWM) control technique to efficiently step down the input voltage to a lower output voltage. With a wide input voltage range and integrated features such as an internal MOSFET, the FSL128MRT is an excellent choice for creating compact and reliable power supplies for a variety of applications.

This IC is designed to handle both switching and feedback regulation, and it can work with an array of components to provide a stable output voltage. Its efficient conversion ensures minimal heat generation, making it ideal for use in portable applications and systems where space and heat management are critical.

Step 1: Understanding the Circuit Design

The circuit we are going to design is a simple buck converter that steps down the input 12V to a stable 5V. The FSL128MRT handles most of the heavy lifting in terms of switching and regulation, but a few additional components are needed to ensure proper performance and efficiency.

1. Inductor (L1): The inductor stores energy during the switching cycle and releases it to the load. A 10µH inductor is ideal for this application, as it allows for efficient conversion with minimal ripple in the output voltage.

2. Capacitors (C1, C2): Capacitors are essential for smoothing out voltage spikes and filtering the output. The 470µF capacitor at the input helps smooth the DC input voltage, while the 220µF capacitor at the output helps smooth the stepped-down voltage to maintain a steady 5V output.

3. Schottky Diode (D1): The diode ensures the proper direction of current flow and prevents reverse current, which could damage the circuit. Schottky diodes are preferred in SMPS circuits due to their low forward voltage drop, which reduces power loss and increases efficiency.

4. Feedback and Control Resistors (R1, R2): These resistors are used to set the output voltage by providing feedback to the FSL128MRT IC. The 10kΩ resistor sets the desired feedback level for the 5V output.

5. Bypass Capacitor (C3): The 0.1µF ceramic capacitor is placed between the feedback pin and ground to filter out high-frequency noise and ensure stable operation.

Step 2: Preparing the Components and Breadboard Setup

Before assembling the circuit, lay out the components on a breadboard (if not using a PCB) to visualize the connections and check for potential issues.

1. Placing the FSL128MRT IC: Insert the FSL128MRT onto the breadboard with proper orientation. Pay attention to the pinout, as incorrect placement could damage the IC. Refer to the datasheet for the exact pinout.

2. Connecting the Input Power: Connect the input power supply (12V DC) to the appropriate pins of the FSL128MRT. This will typically involve connecting the positive lead of the 12V supply to the VIN pin and the negative lead to ground.

3. Inductor Placement: Insert the inductor (L1) between the SW (switch) pin of the FSL128MRT and the anode of the Schottky diode (D1).

4. Capacitors: Place the capacitors (C1, C2, and C3) at the appropriate points on the breadboard, following the layout guidelines in the datasheet. C1 goes from VIN to ground, C2 from VOUT to ground, and C3 between the feedback pin and ground.

5. Resistor Placement: Insert the 10kΩ resistor between the feedback pin (FB) and VOUT. The 1kΩ resistor will be used to connect the ground pin of the IC to the ground rail.

Step 3: Assembling the Circuit

1. Input Power Supply:

● Connect the positive terminal of the 12V input supply to the VIN pin of the FSL128MRT.

● Connect the negative terminal of the power supply to the GND pin of the IC and to the ground rail of the breadboard.

2. Inductor and Diode:

● Insert the 10µH inductor between the SW pin and the anode of the 1N5819 Schottky diode.

● Connect the cathode of the diode to the output pin (VOUT) of the FSL128MRT.

3. Capacitors:

● Place the 470µF capacitor between VIN and ground to stabilize the input voltage.

● Place the 220µF capacitor between VOUT and ground to smooth out the output voltage.

● The 0.1µF ceramic capacitor should be connected between the FB pin and ground.

4. Resistors:

● Connect the 10kΩ resistor between VOUT and the FB pin. This resistor helps set the output voltage to the desired 5V.

5. Output Load:

● Connect the output load (such as a 5V LED strip or a microcontroller) to the VOUT pin and ground. The load will draw power from the SMPS, and the output voltage will remain stable at around 5V.

Step 4: Testing the Circuit

1. Power Up:

● Power up the circuit by connecting the 12V input supply.

● Ensure that the output voltage at the VOUT pin is 5V. You can use a multimeter to check the output voltage.

2. Load Testing:

● Connect a small load, such as a 5V LED or a microcontroller, to the output. Verify that the output voltage remains stable even as the load is powered on and off.

3. Check Heat Dissipation:

● Touch the FSL128MRT and other components gently to check for excessive heating. If the circuit is running too hot, you may need to adjust the components or check for shorts.

4. Efficiency and Stability:

● You should observe minimal ripple in the output voltage and efficient operation with very little heat generation due to the high efficiency of the buck converter.

Step 5: Enclosure and Finalizing the Project

Once you have successfully tested the circuit on a breadboard, you may want to finalize the project by soldering the components onto a printed circuit board (PCB). This provides a more durable, compact, and professional finish for the power supply.

1. PCB Design:

● If you have access to a PCB design software, you can design a custom PCB for this project. Alternatively, you can use a pre-designed buck converter board and modify it to fit your requirements.

2. Enclosure:

● Use a small plastic or metal enclosure to house the circuit and prevent short circuits or damage from external factors. Ensure that the ventilation is adequate to prevent the components from overheating.

3. Testing After Assembly:

● Once assembled, perform another round of testing to ensure the power supply operates as expected. Check for stability, voltage accuracy, and heat dissipation.

Conclusion

By using the FSL128MRT integrated circuit, we have successfully built a simple and efficient 12V to 5V step-down power supply. This project demonstrates the ease with which you can design a reliable power supply for low-voltage electronics. Whether you're powering a microcontroller, sensors, or LED strips, the FSL128MRT ensures that you have a stable and efficient power conversion solution.

This project can serve as the foundation for more advanced power supply designs, such as creating multi-output power supplies or experimenting with different voltage levels. With a bit of creativity, the possibilities for expanding this project are limitless.