Half load does not give a power supply permission to live under a blanket or inside a sealed box. Its rating assumes particular cooling conditions. Ambient temperature, mounting position, airflow, and nearby heat sources can reduce the usable output even when the electrical load seems modest.
Rated output comes with thermal conditions
A maximum wattage on the label is not a promise for every installation. The documentation may specify an ambient temperature range, orientation, airflow, or enclosure arrangement. Derating means reducing allowable output when conditions become less favorable. Read the relevant curve and installation notes together: a temperature limit without its associated output limit is incomplete information. TDK-Lambda provides examples showing how a supply rated for full output at one ambient temperature must operate at reduced power as that ambient rises.
Half the output is not half the temperature rise
Losses do not necessarily scale directly with output power. Some consumption remains at light load, while other losses vary with current and operating mode. Temperature rise also depends on how effectively heat leaves the parts. In an illustrative comparison, 5W of internal loss with a poor thermal path can produce a higher temperature rise than 8W with good cooling. Output percentage alone cannot determine component temperature. The enclosure surface temperature is also only an indirect clue about hotter internal locations.
The air around the supply may be hotter than the room
A cabinet beside warm equipment can contain air well above room temperature. Closely packed adapters, blocked vents, and recirculating exhaust can make that difference worse. Convection depends on a path for warm air to leave and cooler air to replace it. A mounting position that interrupts that path can change performance even without adding another heat source. Follow the approved installation arrangement instead of assuming that an empty-looking corner provides adequate ventilation for every supply design.
Sealed products still need their specified surroundings
Some supplies are designed for sealed or conduction-cooled installations, while others depend on air movement through openings. Those are different designs with different instructions. A sealed wall adapter still transfers heat through its case and should not be insulated by bedding or piled under objects. Conversely, drilling vents in a sealed product can compromise its enclosure and protection. Use the intended mounting surface and clearances; changing the enclosure is an engineering modification, not a general solution to a hot adapter.
Use operating symptoms to support a proper diagnosis
A shutdown that appears only after sustained operation can be consistent with thermal protection, but load faults and other failures can look similar. Record the load, ambient conditions, mounting, and time to shutdown without opening the unit or defeating protection. Stop using equipment with damage, scorching, or other abnormal signs and obtain suitable service or replacement. For an engineered installation, temperature testing and derating review belong with qualified personnel using the manufacturer method and the worst expected conditions.
What to check before you act
- Read the derating curve and installation notes together.
- Consider local enclosure temperature, not just room temperature.
- Keep required vents and clearances unobstructed.
- Do not modify a sealed case to add improvised cooling.
Common questions
Is half load always safe in a hot cabinet?
No. The allowable output depends on the specified ambient, cooling, and mounting conditions.
Does thermal shutdown prove the supply is healthy?
No. Protection can limit damage, but repeated shutdown still needs the cause addressed.
The practical takeaway
A supply has to dispose of its losses at every load. Preserve its intended cooling path and apply the documented derating limits before deciding that unused wattage provides enough margin.
References and further reading
- TDK-Lambda: Understanding power-supply derating
- TDK-Lambda: Airflow direction and thermal performance
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
