In this project, we’ll design a high-efficiency power supply circuit using the STF13N80K5, a powerful N-channel MOSFET designed for use in switching applications. This project will demonstrate how to use the STF13N80K5 MOSFET in building an energy-efficient, robust, and compact power supply for a range of devices, including industrial systems or DIY electronics. The STF13N80K5, rated for 13A and with an 800V drain-to-source voltage (Vds), is a suitable choice for handling high-voltage switching with low conduction losses.
Objective
The goal of this project is to design a power supply that can efficiently convert AC mains voltage to a stable DC output using the STF13N80K5 MOSFET in a flyback converter topology. Flyback converters are widely used in power supplies due to their simplicity, cost-effectiveness, and isolation properties, making them ideal for applications where isolation between input and output is needed.
Project Overview
● Input Voltage: 110V to 240V AC (AC mains)
● Output Voltage: 12V DC (regulable)
● Power Rating: 50W to 100W (depending on your transformer rating and components)
● Topology: Flyback converter
● Key Components:
— STF13N80K5 MOSFET
— Power transformer (flyback type)
— Diode (e.g., 1N5408)
— Capacitors (electrolytic and ceramic)
— Resistors (for feedback loop and voltage regulation)
— PWM controller IC (e.g., UC3842)
— Auxiliary components: Inductors, heat sinks, and connectors
Step 1: Understanding the STF13N80K5 MOSFET
The STF13N80K5 MOSFET is part of the SuperFET™ 2 series and features:
● Drain-Source Voltage (Vds): 800V
● Continuous Drain Current (Id): 13A
● Rds(on): Low (providing low conduction losses)
● Gate Threshold Voltage (Vgs(th)): 2V to 4V
● Total Gate Charge (Qg): 95nC
This MOSFET is capable of handling significant voltage and current, making it suitable for high-power applications like flyback converters. The key feature that allows the MOSFET to operate efficiently in this design is its fast switching capability, which reduces switching losses and enhances overall performance.
Step 2: Choosing the Transformer
For this project, the flyback transformer needs to be selected based on the input and output specifications. A transformer with a primary winding capable of handling the AC mains voltage and a secondary winding that provides the desired output voltage is essential.
● Primary winding: Designed for 110V or 220V AC input (depending on your region)
● Secondary winding: 12V DC at up to 5A (for a 60W output power, adjust based on desired output power)
● Turns ratio: The turns ratio between the primary and secondary windings is crucial for determining the output voltage. A typical transformer for such applications will have a turns ratio designed to step down the high-voltage AC input to a lower voltage suitable for the flyback converter’s operation.
Step 3: Building the Circuit
The flyback converter works by storing energy in the transformer during the “on” phase (when the MOSFET is conducting) and releasing that energy to the output when the MOSFET is off. Here's a step-by-step process for assembling the circuit.
1. Input Section
Connect the AC input to the primary side of the transformer. Ensure that you use proper fuses for protection against power surges and that your circuit is equipped with adequate insulation for safety. You can incorporate a bridge rectifier or a simple diode-based rectification circuit at the input to handle fluctuations in the AC mains supply. However, in a full flyback design, rectification usually occurs on the secondary side.
2. Control Circuit
To regulate the switching of the STF13N80K5 MOSFET, we need a pulse-width modulation (PWM) controller. The UC3842 is a popular PWM controller for flyback converters and offers a great combination of simplicity, performance, and reliability. This IC will drive the gate of the MOSFET, determining when it turns on and off.
● The UC3842 generates a pulse width modulated signal based on the feedback from the output voltage. This feedback is usually taken through an optocoupler to provide electrical isolation between the high-voltage side (primary) and the low-voltage side (secondary).
3. MOSFET Drive Circuit
The STF13N80K5 MOSFET will be driven by the PWM signal from the UC3842. The gate drive circuit should include:
● A gate resistor (typically 10Ω to 100Ω) to limit the current into the gate and prevent oscillations.
● A diode (like the 1N4148) to prevent reverse voltage spikes that could damage the MOSFET during the off-phase.
● A gate driver IC could also be used for more robust switching, although the UC3842’s internal gate drive is typically sufficient for moderate power levels.
4. Output Rectification and Filtering
After the transformer steps down the voltage, the secondary side of the transformer will provide an AC signal. You need to rectify this AC signal into DC using a high-speed diode such as the 1N5408. This diode will handle the high-current switching transients generated by the MOSFET.
Once rectified, the output should be filtered to smooth out the DC voltage. Use electrolytic capacitors for bulk filtering (typically 470µF to 1000µF) and ceramic capacitors for high-frequency decoupling to ensure stable voltage output.
5. Feedback and Regulation
To regulate the output voltage and maintain a stable DC output, feedback from the output voltage needs to be sent to the PWM controller (UC3842). This is usually achieved using an optocoupler. The optocoupler provides electrical isolation while sending the feedback signal from the low-voltage side of the circuit back to the high-voltage side. The UC3842 will adjust the PWM duty cycle to maintain the output voltage at the desired level.
Step 4: Testing and Fine-Tuning
Once the circuit is assembled, it's time to test it. Begin by connecting a variable load to the output and gradually increasing the load current while monitoring the output voltage. Verify that the converter maintains a stable output voltage under varying load conditions.
● Check for Heat Dissipation: The STF13N80K5 MOSFET should not overheat during operation. If it does, consider adding a heat sink to the MOSFET.
● Monitor Efficiency: To ensure that the converter is operating efficiently, measure the input power and output power, calculating efficiency as the ratio of output power to input power.
Step 5: Troubleshooting
If the power supply does not work as expected, consider the following steps:
● Check the MOSFET Switching: If the MOSFET isn’t switching, ensure that the PWM signal from the UC3842 is reaching the gate of the MOSFET.
● Transformer Wiring: Verify that the primary and secondary windings of the transformer are correctly wired, and the turns ratio is appropriate for your voltage needs.
● Feedback Loop: Check that the feedback loop is functioning correctly and providing the necessary voltage regulation. If the output voltage is unstable, adjust the feedback network.
Final Thoughts
By using the STF13N80K5 MOSFET in a flyback converter design, this project highlights how to create an efficient, high-power supply suitable for various DIY and industrial applications. The STF13N80K5's high-voltage rating and low conduction losses make it a great choice for power conversion applications, ensuring that your circuit can handle high-voltage switching with minimal energy loss.
This project can be adapted for different output voltages and power levels by adjusting the transformer and feedback components. Whether you’re powering low-voltage electronics or providing high-current output for industrial devices, this flyback converter design is versatile and scalable.