Contact information
Free service hotline
net04@gtggroup.com
TEL: 0769-85075888-6618
13925591357
Fax: 0769-85075898
Mail: net04@gtggroup.com
ADD: Huacan Industrial Park, No. 2 Keji 8th Road, Songshan Lake Park, Dongguan City, Guangdong Province
Switch-Mode-Power-Supply-UL-Certification-Safety-Requirements
Switch mode power supplies (SMPS) sold in the United States must meet UL safety certification requirements. This guide covers the UL certification process for SMPS, applicable standards, and key safety testing requirements.
UL Standards for SMPS
SMPS products fall under different UL standards depending on application:
UL 62368-1 Safety of audio/video, IT and communication technology equipment (replaces UL 60950-1)
UL 508 Industrial control equipment (for industrial SMPS)
UL 1310 Class 2 power units
UL 60950-1 Legacy standard (still accepted but being phased out)
Most commercial SMPS products use UL 62368-1 as the primary safety standard. UL certification is voluntary but effectively mandatory for market access in the US — retailers and buyers typically require it.
Key Safety Testing Requirements
UL 62368-1 safety testing for SMPS covers these critical areas:
- Component evaluation:UL has specific requirements for critical components (transformers, capacitors, MOSFETs, optocouplers). These must be UL-recognized or meet specific performance criteria
- Transformer construction:Bobbins must meet specific flame ratings; windings must meet separation requirements; hi-pot testing of the transformer as a component
- Temperature testing:At 120V AC and 240V AC (US uses 120V, but universal input products must be tested at both extremes). Thermocouple measurements on all critical components
- Abnormal operation:Single-fault conditions including feedback loop open, output short, fan failure (for forced-air cooled units)
- Spacings:Creepage and clearance per UL 62368-1 tables, with specific UL interpretations that may differ from IEC
Practical Insight:A 500W enclosed SMPS for industrial use failed UL temperature testing because the Y1 safety capacitor on the primary side reached 112°C — exceeding its rated 110°C maximum. The capacitor was located near the primary heat sink and absorbed radiated heat. Relocating the Y1 capacitor to the opposite side of the PCB and adding a thermal shield between the heat sink and capacitor reduced its temperature to 98°C. When designing for UL certification, component placement relative to heat sources is critical.
UL Certification Process
UL certification for SMPS follows these steps:
- ①Application:Submit product information to UL (or an NRTL using UL standards)
- ②Engineering review:UL reviews documentation and identifies critical components requiring evaluation
- ③Sample testing:Full safety testing per UL 62368-1 at a UL-designated laboratory
- ④Initial factory inspection (IFI):UL conducts a factory inspection before listing
- ⑤UL Listing:Product added to UL's online directory with the UL mark authorization
Timeline: 8-16 weeks for new listings. Follow-up services (FUS) inspections occur quarterly at the manufacturing facility. GTG provides SMPS UL certification pre-testing and consulting services.
CB Report Conversion to UL
IEC 62368-1 CB reports can partially support UL certification:
- Core IEC test data can be referenced, reducing redundant testing
- UL national differences (component requirements, spacings, wiring practices) must still be tested
- CB conversion typically saves 2-4 weeks compared to full UL testing
This article is AI-assisted and for reference only. It does not constitute any certification commitment or legal advice. Please refer to the latest official regulations.
Contact: 13925591357 | net04@gtggroup.com