Introduction

A COSEL power supply delivers its specified performance only when the design around it respects the derating, the thermal path and the input filtering. The supply itself is rugged, but the installation decides whether it runs cool and compliantly for years. This application note explains the practical rules for applying a COSEL AC-DC supply, a DC-DC converter and an EMI filter in a real power system for an industrial, a medical or a telecom design.

Load and Derating

A power supply is rated at a temperature and a mounting, and it delivers less at a higher ambient, so the derating curve is the real specification. Compute the load, add margin for the start-up and the lifetime, and read the available power at the worst-case ambient, not the nominal. A supply that runs at a lower fraction of its rating runs cooler and lasts longer, so a modest over-size is a good choice for a long-life product.

Thermal Path and Mounting

The thermal path from the supply to the air or the chassis matters as much as the rating. A convection supply needs free air around the body, so leave clearance above and below and avoid a sealed pocket, and a fan-cooled supply needs an airflow path with a filter to keep the dust out. Mount the unit so the air can move past it and keep the sensitive load away from the hot parts.

Active PFC and the Input

A larger COSEL supply includes active power-factor correction, which keeps the input current close to a sine wave and meets the IEC 61000-3-2 harmonic limit, so no external PFC stage is needed. Confirm the PFC against the standard and plan the front end with a suitable fuse and, where the line is noisy, a surge protector. The inrush at power-up is limited by the supply, but the breaker and the wiring must still suit it.

Holdup and Ride-Through

The holdup time is how long the supply keeps the output in regulation after the input is removed, and it lets the equipment ride through a brief dip or save its state. Check the holdup against the load and the mains, and add output capacitance or a small holdup capacitor only where the design needs more, because extra capacitance raises the inrush and the cost.

EMC and Safety

The input of the equipment must meet the conducted-emissions limit, so a COSEL EMI filter sits between the mains and the supply. Choose the current above the load and the line-to-ground capacitor code for the leakage the standard allows, and mount the filter close to the input connector with the case bonded to the chassis. For a medical application, confirm the leakage against the patient-contact limit, and for a telecom application, confirm the isolation and the surge requirement.

Grounding and Layout

Keep the input and output wiring separated, bond the case to the chassis with a short connection and keep the switching loop small, because the layout decides the emissions as much as the filter. Run the filtered and the unfiltered lines apart, and keep the analog or the sensor wiring away from the switching nodes.

DC-DC and the Board Rail

On the board, a COSEL DC-DC converter derives an isolated rail from the bus, so the board is protected and the ground loop is broken. Keep the input capacitor close, follow the recommended input filter and respect the isolation in the layout. Confirm the derating of the converter in the warm enclosure, because a board-mount part relies on the board and the air for its cooling.

Verification

Validate the design on the bench by measuring the efficiency and the derating of the supply at the worst-case load and ambient, by checking the ripple, the holdup and the inrush, and by measuring the conducted emissions and the leakage with the real load. Our FAE team can review your measurements and your layout and help you interpret them, so the power system performs in the product as it does on the datasheet.