120 VAC To 3.3 V Flyback Converter

This project designs a compact flyback converter that produces a regulated 3.3 V output from a 120 VAC input. It uses the UCC28881 flyback controller and a custom transformer to deliver a low‑voltage output with galvanic isolation. The converter operates over the 60–400 Hz mains frequency range and provides enough current for low‑power electronics.

Converter Topology

The flyback converter topology divides into distinct stages. This section summarises the purpose of each stage and the components involved.

• Input stage – The AC input passes through a fusible resistor, MOV and bridge rectifier to provide surge protection and full‑wave rectification. A TVS diode and bulk capacitors suppress transients and reduce ripple.
• Primary switcher – The UCC28881 controller drives a MOSFET to transfer energy into the custom transformer. A snubber network across the MOSFET damps ringing and improves efficiency.
• Output and feedback – On the secondary side, a fast diode rectifies the flyback pulse. Output filtering capacitors smooth the 3.3 V rail. An optocoupler and LMV431 shunt regulator provide isolated feedback to regulate the output voltage.
• Transformer design – The transformer’s turns ratio is chosen based on the input voltage, desired duty cycle and output voltage. The article provides equations for selecting the inductance and peak current and explains that a custom transformer may be required when size constraints prevent using standard parts.

Using The Converter

Follow these steps to safely connect mains power to the converter and obtain a stable 3.3 V output for your circuits.

• Connecting the input – Attach the AC mains to the input pads. Observe proper creepage distances and follow safety guidelines when working with mains voltages. The converter operates from 60 Hz to 400 Hz and will deliver a regulated 3.3 V output.
• Output connection – Connect your low‑power load to the 3.3 V and ground terminals. Ensure that the load current does not exceed the converter’s rated output.
• Feedback adjustment – If you modify the transformer or change the output voltage, adjust the resistor divider on the LMV431 and the optocoupler LED current to maintain regulation.

Design Considerations

Attention to isolation, transformer selection and output filtering is critical in high‑voltage power supplies. This section highlights the most important design choices.

• Isolation and safety – Maintain clearance and creepage distances between the high‑voltage primary and low‑voltage secondary. The board uses a four‑layer stack with an isolation boundary printed on the silkscreen. Large through‑hole components are placed on the primary side, while SMD components populate the secondary side to keep the isolation barrier clear.
• Transformer selection – Choose a core and turns ratio that meet the output voltage and power requirements. Calculate the magnetising inductance and peak current using the formulas provided in the article. Ensure the core can handle the operating frequency and flux density.
• Snubber network – Design the RC snubber across the MOSFET to control voltage spikes during turn‑off. Adjust the resistor and capacitor values experimentally to minimize ringing.
• Output filtering – Use low‑ESR capacitors to filter the 3.3 V output. The article notes that ripple can be reduced further by selecting a larger inductor or adjusting the switching frequency.

By following these design guidelines, engineers can adapt this flyback converter to produce other output voltages or support higher power levels. Pay careful attention to safety and isolation when working with mains power, and verify performance with laboratory measurements before deployment.

Flyback converters are sensitive to component tolerances; always derate capacitors and diodes for the expected voltage and temperature. Use Y‑capacitors across the isolation barrier to provide a return path for common‑mode currents while maintaining safety isolation. If higher output power is needed, scale the transformer’s core size accordingly and verify that the MOSFET and diode ratings are sufficient. Finally, perform load regulation and cross‑regulation tests to ensure that the 3.3 V output remains within specification over the full input voltage range and temperature.

Additional Resources

Read the full guide article on Altium Resources for a deep dive on this project. The video below shows the design approach and preparation for manufacturing of the Rev A and Rev B releases of this project.