Power Supply Design

Question: Why am I finding it difficult to find open frame power supplies that can operate above 85 C?

Original Question: Why am I finding it difficult to find open frame power supplies that can operate above 85 C? The maximum temperature range I could find was 0 to 70 C. What is the industrial standard for temperature range? S. R. 05/24/01

Answer: Semiconductor junction temperatures usually determine the maximum operating range of circuits, although electrolytic capacitors, if used, can also be a determining factor.

The ad-hoc standard temperature ranges for semiconductors are what you find in vendor catalogs. Here you can find three ranges.

Commercial: 0 C to 70 C

Industrial: -40 C to 85 C

Military: -55 C to 125 C

The parts get more expensive and the availability scarcer as the temperature range increases.

Your 70 C maximum for an open frame power supply probably means they used industrial range semiconductors and designed their thermal system for a 15 C delta between junction and ambient temperature. If you want a higher temperature, you probably will have to go to designs that use military range parts and look for power supplies in military catalogs. These will be much more expensive than designs intended for commercial or industrial environments.