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Built-in_PSU_UL_PCB_Safety
Edit: ANCI 2026-07-20 109
Built-in power supplies — open-frame AC-DC modules and DC-DC converters installed inside end-equipment — achieve North American safety compliance through UL Recognition (UR) under UL 62368-1. As components, built-in PSUs are evaluated for their internal safety: dielectric isolation between primary and secondary circuits, creepage and clearance on the PCB, over-temperature protection behavior, and single-fault immunity. The UR report's Conditions of Acceptability serve as the bridge between the PSU manufacturer and the end-equipment integrator — defining exactly how the PSU must be installed (spacing, cooling, external fusing) for the UR certification to remain valid. This article details the UL safety verification framework for built-in PSUs and the key PCB-level design decisions that determine certification outcomes.
UL Recognition vs. UL Listing — The Component vs. End-Product Distinction
Built-in PSUs follow the UL Recognition (UR) path because they are components — not standalone end-products. The UR mark (backwards "RU") indicates the PSU has been evaluated for its internal safety characteristics but requires specific installation conditions to be met in the end-equipment:
- PCB spacing: The UR report specifies the minimum creepage and clearance distances that must be maintained between the PSU's primary-side high-voltage section and any conductive parts of the end-equipment (metal chassis, adjacent PCBs). If the end-equipment's metal enclosure is closer than the specified distance from the PSU's primary traces, an insulating barrier (Mylar sheet, plastic shroud) is required.
- External fuse: Built-in PSUs without an onboard fuse require an external fuse of specified rating (e.g., T3.15A/250V) to be installed in the end-equipment. The UR Conditions of Acceptability state the required fuse type, rating, and breaking capacity.
- Thermal conditions: The PSU's rated ambient temperature Ta and the allowable temperature rise in the end-equipment enclosure are documented. The end-equipment integrator must verify that the PSU's local ambient temperature inside the enclosure does not exceed the UR-declared Ta.
Key PCB-Level Design Decisions for UL Compliance
Creepage Budgeting on the PCB
At 250 Vrms / PD2: primary-to-secondary creepage ≥ 5.0 mm (basic) or ≥ 6.4 mm (reinforced). For compact PSUs (< 15W/in³), the 6.4 mm requirement for reinforced insulation (Class II, no protective earth) often forces a PCB layout decision: use a slot (≥ 1.0 mm width) to extend the creepage path along the slot wall, or widen the PCB to accommodate the full 6.4 mm surface distance. Slots reduce the effective creepage requirement by routing the path vertically — but the clearance (through-air) distance must still be met in the plane perpendicular to the PCB surface. Slot width < 1.0 mm is treated as non-existent by UL — the creepage path bridges across the slot.
Thermal Copper Area vs. Creepage Conflict
The switching MOSFET and output rectifier require large copper areas for heat dissipation — but expanding the copper area in the primary zone reduces the available space for maintaining creepage distances to the secondary zone. The PCB layout must balance thermal copper area against creepage: use a rectangular copper pour with a controlled separation gap on the primary-secondary boundary, rather than a continuous pour that encroaches on the isolation zone. built-in PSU UL PCB layout thermal vs. creepage optimization is an iterative process that benefits from early-stage simulation before prototyping.
Single-Fault Testing — Verifying Protection Under Component Failure
UL 62368-1 abnormal testing requires simulating single-component failures: short-circuit across the optocoupler's primary-secondary isolation, open-circuit the feedback loop, short the output rectifier, and verify that the PSU does not produce fire, electric shock, or burn hazards. For built-in PSUs, the protection must be self-contained — the PSU cannot rely on the end-equipment to detect or respond to an internal fault. OVP (output overvoltage) must be implemented as a primary-side latch-off, not a hiccup-mode restart, if the fault could push the output above SELV limits during the hiccup interval.
Frequently Asked Questions
Q1 Does a built-in PSU need its own UL file if the end-product already has UL Listing?
The end-product manufacturer can either reference the PSU's UR file (preferred, as it avoids re-testing the PSU), or submit the PSU as an unlisted component within the end-product's UL Listing application — in which case the PSU will be evaluated as part of the end-product's testing scope. Having the PSU pre-certified with a UR mark reduces the end-product UL Listing timeline.
Q2 Can a conformal coating reduce the creepage requirement on a built-in PSU PCB?
Yes — Type 1 conformal coating (≥0.03 mm thickness, ≥99% coverage verified by UV inspection) allows the creepage to be calculated at PD1 (Pollution Degree 1), reducing the required distance by approximately 40-50%. The coating must be explicitly documented in the UR report and is verified during UL's factory inspection through coating thickness sampling.
Q3 How does a DC-DC converter (48V input) differ from an AC-DC PSU in UL spacing?
A DC-DC converter with SELV input (≤60 VDC from a pre-certified AC-DC front-end) has significantly reduced spacing requirements — the working voltage is the DC-DC's input voltage (< 60 V), not the AC mains voltage. Creepage is calculated per IEC 62368-1 Table 14 with the lower working voltage, resulting in much smaller required distances. The key condition: the DC-DC's input must be guaranteed SELV by the upstream AC-DC stage.
Q4 What is required for UL re-certification after a PCB layout change?
A PCB layout change affecting primary-secondary spacing, isolation slots, or the location of safety-critical components requires a UL Class II Permissive Change filing. Submit the revised PCB Gerber files, updated spacing measurements (using UL's CAD-based spacing verification or manual measurement), and any revised thermal data if the layout change affects heat dissipation. UL reviews the submission and issues an updated UR report.
Q5 Can a single UR certification cover multiple output voltage variants?
Yes — a family of built-in PSUs with identical topology and PCB layout, differing only in the output voltage set-point (e.g., 5V, 12V, 24V variants), can be covered under a single UR file. The highest-output-voltage variant is tested as the representative model (worst-case for creepage between output and other circuits). Lower-voltage variants are documented as model variants with the same spacing and thermal characteristics.
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