Introduction

An AC-DC power supply connects to the mains, so it is a safety-critical part of a product, and its efficiency, EMC and thermal behaviour decide whether the product meets its specification and its standard. This application note explains how to design the input protection, the filter, the isolation and the thermal path of an AC-DC supply, using the Hawun HW-AC-MOD-7W-05 and HW-AC-ENC-30W-12 as references.

Input Protection and Filter

The mains input must be protected against overcurrent, overvoltage and transients, and the input filter must limit the conducted emissions the supply produces. The fuse and the protection components are chosen for the application and the standard, and the filter is chosen to meet the EMC requirement. Because the mains is a hostile input, the protection and the filter are not optional; they are part of the supply's function.

Conducted Emissions

A switching supply produces conducted emissions at its switching frequency and its harmonics, and the input filter attenuates them. The filter's design follows from the spectrum the supply produces and the limit the standard sets, and it should be validated by measurement rather than by assumption.

Isolation and Safety

Isolation, creepage and clearance are safety-critical in a mains supply, because they separate the user and the low-voltage circuits from the mains. The layout must respect the creepage and clearance the standard demands, and the isolation barrier must be continuous. A supply that meets its electrical specification but not its isolation requirement is not a safe supply, so the safety design must be planned from the start rather than checked at the end.

Class and Standard

The safety class of the product, whether it has a protective earth or relies on double insulation, determines the isolation the supply must provide. Confirm the class and the standard before choosing the supply, because they constrain the design.

Thermal Design

A switching supply dissipates heat, and the package must carry that heat away. Size the supply for the peak load with margin for the derating at high temperature, and provide the cooling the datasheet requires. A supply that is cool on the bench in open air can overheat inside a sealed enclosure, so the thermal design must be validated in the real mechanical assembly rather than on a bare board.

Derating

Most supplies derate their output at high temperature, so the usable power depends on the ambient and the airflow. Confirm the derating curve against the enclosure and the worst-case ambient, and leave margin for the temperature rise inside the product.

Bench Validation

Before production, measure the efficiency across the load range, measure the conducted emissions, and confirm the temperatures under sustained load. BeiLuo's power lab can perform these measurements and supply samples for validation, so the design is confirmed before the board is committed and the standard is met.

Conclusion

An AC-DC supply succeeds when the protection, the filter, the isolation and the thermal design are planned together. Protect the mains input, filter the emissions, respect the isolation requirement, and validate the thermal design in the real enclosure; do those things and the supply will meet its specification and its standard, not only on the bench. That is the standard we hold our own recommendations to.