Introduction: Fanless power supplies derate above 50°C because heat leaves through natural convection, and a warmer cabinet reduces the temperature difference that drives that heat out.
An automation engineer comparing power supply ratings often sees two numbers that look contradictory: a wide operating temperature range and a lower full-load range. A unit may be rated from -30°C to 70°C, yet full output is only available up to 50°C. The missing link is heat. A fanless supply still produces waste heat during conversion, and without a fan it depends on the surrounding air to carry that heat away. Once the cabinet air gets warmer, the cooling path becomes less effective, so the supply has to give up some output capability to stay within its thermal limits.
Heat is a byproduct of conversion loss before it becomes an ambient-temperature problem
A switching power supply converts incoming power into a regulated output, but the conversion is never perfect. Texas Instruments’ application report on switch-mode topologies explains the basic point: switching devices, magnetics, and control circuits all lose some energy during conversion. That loss shows up as heat inside the enclosure. The higher the load, the more power moves through the supply, and the more heat the loss produces. This is the first link in the thermal chain, and it happens before anyone thinks about cabinet temperature. The heat then has to travel from the components to the surrounding air. In an open-frame, fanless design, that path usually runs from the components to the metal housing and the vented openings, then into the cabinet air. Schneider Electric’s engineering note on open-frame power supply layout and component reliability makes the same practical point: local heat and placement affect component margin. A supply may be electrically fine and still run warm because the heat is trapped near sensitive parts. The thermal problem is not just the total heat produced; it is how effectively that heat reaches cooler air. This is why a fanless supply is best understood as a chain rather than a single rating. Conversion loss creates heat. Heat raises component temperature. The housing and vents pass heat to the cabinet air. The cabinet air carries heat away from the supply. If any link in that chain becomes weak, the supply runs hotter. A warm cabinet weakens the last link, which is why the ambient temperature around the supply matters so much.
Why natural convection slows down as cabinet temperature rises
Natural convection is simple and silent. Air near a warm surface picks up heat, becomes less dense, and rises. Cooler air moves in to replace it. That continuous movement carries heat away from the supply. The process needs no fan, no filter, and no motor, but it depends on a temperature difference between the warm surface and the surrounding air. A larger difference keeps the air moving; a smaller difference weakens the flow. This is the core reason a passive-cooled supply behaves differently in a comfortable room and in a warm control cabinet.
1. The Temperature Difference Between Components and Cabinet Air Drives the Heat Flow
The rate of heat transfer depends heavily on the gap between the hot component and the cooler air around it. If the cabinet air is 25°C, a component at 70°C has a useful temperature difference to work with. If the cabinet air is 50°C, the same component has much less room before it reaches its thermal limit. The supply can still release heat, but it needs a hotter surface to push the same amount of heat into the air. That is the quiet penalty of a warm cabinet: the cooling system still works, but it works with less margin. A fanless supply cannot solve this by moving more air. Its vents and open-frame layout are designed to support a natural upward flow, and that flow depends on cool air being available near the inlet and warm air being able to leave. In a crowded cabinet, air near the supply may already be warmed by other devices. The result is a local pocket of warm air around the supply. The component temperatures rise, even if the cabinet’s average temperature looks acceptable on a panel meter.
2. A Warmer Cabinet Lowers the Useful Cooling Margin Before the Supply Reaches Its Limit
At moderate cabinet temperatures, a well-designed fanless supply can run at full load because its thermal margin covers the internal rise. As the cabinet approaches 50°C, that margin becomes thinner. The components are closer to their design limits, and the natural convection path has less driving force. The supply still has a stated operating range beyond this point, but the full-load condition is no longer the right assumption. The thermal design needs a way to reduce internal heat, and the most direct way is to reduce output current. That is what derating does. It lowers the output capability so the supply produces less internal loss. Less loss means less heat, and the natural convection path can keep component temperatures within their intended range. IEC 60204-1 frames the industrial cabinet as an electrical environment where heat, placement, and protection all matter together. The supply is not floating in open air; it is part of that environment. Once the cabinet air gets warmer, the supply’s available output has to be matched to the actual thermal conditions.
What the 50°C to 70°C derating band means for output capability
The Schneider ABL2REM24045K gives a clear example of how to read this split. Its stated operating temperature range is -30°C to 70°C. It runs at full load from -30°C to 50°C, and it requires derating from 50°C to 70°C. Those two statements belong together. The operating range tells you where the supply can function; the full-load range tells you where the rated output can be used without thermal reduction. The 50°C to 70°C band is not a second full-load range. It is the part of the operating range where output must follow a derating curve. The reason is straightforward. Above 50°C, the cabinet air removes heat more slowly. If the supply kept delivering the same current, internal losses would stay at the same level while the cooling path became less effective. Component temperatures would climb. Derating reduces the load, which reduces the loss and keeps the internal thermal rise in check. The exact percentage of output available at 60°C or 70°C belongs to the model’s derating curve. The practical reading is simple: use the full-load figure for a cabinet that stays at or below 50°C, and use the derated figure when the cabinet runs hotter. A spec sheet from an electronic parts supplier or electrical components supplier can look simple: a temperature range, a current rating, and a line about derating. The line about derating is the one that connects the electrical rating to the real cabinet. For a fanless unit, ventilation around the housing and free space above the vents also matter because they support the natural convection path. The product listing for the Schneider ABL2REM24045K is useful because it states the operating range and the derating requirement in direct terms, giving engineers a clear starting point for load calculations in warm cabinets.
Conclusion
A fanless power supply derates above 50°C because its cooling depends on natural convection, and natural convection depends on a useful temperature difference between the supply and the cabinet air. Conversion loss creates heat; the housing and vents pass that heat to the air; the cabinet air carries it away. As the cabinet warms, that final step becomes less effective, so the supply reduces output to reduce internal heat. The -30°C to 70°C operating range and the -30°C to 50°C full-load range describe two different things. Keeping that distinction clear is the key to understanding why a fanless industrial power supply derates from 50°C to 70°C.
FAQ
Q: Why do fanless power supplies need derating above 50°C?
A: Above 50°C, the cabinet air is closer to the temperature of the supply’s internal parts, so natural convection removes heat more slowly. Derating lowers the output current and the internal loss, which keeps component temperatures within their intended thermal range.
Q: What happens to a passive-cooled supply when cabinet temperature rises?
A: The supply’s internal temperatures rise because the surrounding air carries heat away less effectively. Its cooling margin shrinks, and the natural convection path cannot simply move more air. The supply then needs a lower output load to stay within its thermal design.
Q: Does a -30°C to 70°C rating mean full load at 70°C?
A: The full-load portion is -30°C to 50°C. From 50°C to 70°C, the supply operates with derating, so the available output is lower than the full-load rating. The 70°C figure is the upper end of the operating range, while the derating curve defines the usable output in that band.
Sources / References
Switch Mode Power Supply (SMPS) Topologies - Texas Instruments Application Report
Risk Assessment of Tin Whisker Impact on Electronic Components | Schneider Electric
Related Examples
Schneider Open-type Switching Power Supply ABL2REM24045K Features
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