Posted : 10 Mar 2014
Operation of ac-dc power supplies from dc input
Embedded AC-DC power supplies are very often specified also with a DC input rating. Commonly the UL60950-1 safety approval relating to the product is valid for AC input only (since the internal fuse is usually only AC rated) and customers should take care of the necessary safety compliance requirements as part of their system integration. Normally DC rated fuses or fuses rated for both AC-DC and DC-DC operation are required for UL60950-1 approval with DC input.
Currently, certain TDK-Lambda AC–DC power supplies (Alpha ( CA), EFE, NV and Sirius ranges) have been approved by UL for DC operation with only an internal AC rated fuse accompanied by evidence of additional testing under fault conditions with AC and DC voltages, abnormal and normal conditions.
However UL have withdrawn this acceptance condition and therefore customers using these products with DC input within UL approved equipment after 10th July 2014 should contact their local TDK-Lambda Sales Office for advice regarding ongoing compliance.
Other approval bodies eg IEC, EN still accept the conditions stated above.
Please note that UL have advised that fitting an external DC rated fuse in addition to the internal AC rated fuse is not acceptable as the power supply will still be primarily protected by an AC rated fuse.