Overview
A power supply can regulate perfectly on the bench and still fail certification. The cause is often not the control loop. It is a narrow isolation gap under a transformer, an uncertified substitute in the BOM, a plastic part used below its recognized thickness, or a fault condition that leaves one component hot enough to ignite nearby material.
Product-level safety design starts before the schematic is frozen. The engineer must identify the end-product standard and market, classify energy sources, define safeguards, and carry those decisions through the PCB, transformer, enclosure, component approvals, production controls, and test records. This guide explains that workflow for low-power AC-DC switch-mode power supplies used in industrial controls, commercial equipment, and AV/ICT products.
Start with the end product and its applicable standard
There is no single set of spacing, hipot, leakage-current, or temperature limits that applies to every switch-mode power supply. IEC 62368-1 is the hazard-based product safety standard for audio/video and information and communication technology equipment. IEC 61010-1 is commonly relevant to measurement, control, and laboratory equipment. Other end products may fall under appliance, lighting, medical, or machinery requirements. China, the EU, North America, Korea, and other markets then apply national or regional adoptions with their own edition dates and deviations.
Legacy references such as IEC/UL 60950-1 remain useful for understanding older designs, but they should not be treated as the default basis for a new AV/ICT project. IEC 62368-1:2023 is the fourth international edition, while a target market may still certify to an earlier adopted edition. Confirm the exact standard and edition with the certification body before finalizing the design.
The safety plan should record at least these inputs:
product category, installation environment, intended user, and accessible parts;
rated input range, mains supply system, overvoltage category, and altitude;
pollution degree, material group or comparative tracking index, and expected coating or enclosure protection;
Class I, Class II, or Class III construction and the protective-earth strategy;
required insulation type across every boundary: functional, basic, supplementary, reinforced, or double insulation.
This information drives the dimensions and tests. A familiar value from a previous 230 V design is not a substitute for a standards calculation.
Build a safety map before placing components
For an isolated offline supply, divide the design into electrical regions and mark every boundary on both the schematic and PCB. The primary side includes the AC input, fuse, inrush limiter, EMI network, rectifier, bulk capacitor, primary switch, transformer primary, and primary-referenced controller circuits. The secondary side contains SELV or other classified output circuits, rectification, filtering, feedback references, and accessible connectors. Optocouplers, safety capacitors, and the transformer cross the isolation boundary and therefore need particular scrutiny.
Do not assume every low-voltage-looking node is secondary-safe. An auxiliary winding referenced to the primary remains on the hazardous side. A heat sink attached to a primary switch may also require insulation or spacing because capacitance, mounting hardware, or an insulation-pad failure can make it hazardous. Similarly, a metal enclosure only becomes a dependable safeguard when the protective-earth path, bonding hardware, conductor sizing, and production test are all controlled.
Calculate clearance and creepage from real design conditions
Clearance is the shortest distance through air between conductive parts. It primarily addresses overvoltage and air breakdown. Creepage is the shortest path along an insulating surface. It is influenced by working voltage, pollution degree, and the material's resistance to tracking. The two distances solve different problems, so a slot that lengthens a surface path may improve creepage without providing the same improvement in clearance.
IEC 60664-1 provides the insulation-coordination framework used by many product standards. It explicitly considers factors such as rated or working voltage, impulse withstand, pollution, material properties, and altitude. Its general values apply up to 2,000 m; higher-altitude installations require correction for reduced air density.
The following table is a design-input checklist, not a universal spacing table.
| Input | Why it changes the requirement | What to document |
|---|---|---|
| Working and rated voltage | Drives steady-state insulation stress and creepage | Worst-case RMS, DC, and repetitive peak values across the boundary |
| Overvoltage category and mains transient | Determines impulse withstand and clearance | Installation category, supply environment, and required impulse voltage |
| Pollution degree | Moisture or conductive contamination increases tracking risk | Enclosure, condensation risk, dust, process contamination, and service conditions |
| Material group or CTI | Better tracking resistance can reduce required creepage in some tables | Laminate and molded-material recognition at the actual thickness and color |
| Altitude | Lower air pressure reduces dielectric strength | Maximum rated installation altitude and correction applied above 2,000 m |
| Insulation class | Reinforced insulation normally needs stronger safeguards than basic insulation | Boundary purpose and the safeguard it provides |
Practical layout margin still matters after the calculation. Account for PCB and assembly tolerances, component lead forming, solder fillets, contamination, conformal-coating process limits, board warpage, and sharp conductive features. Keep copper pours, test pads, mounting screws, shields, and component bodies out of the isolation corridor. If a routed slot is used, specify its minimum width and length on fabrication drawings and verify that the board supplier can hold those dimensions.
Coating is not a casual shortcut. Only a qualified coating system, applied through a controlled and inspectable process, should be credited when the chosen standard permits it. A missed edge, trapped contamination, or void can erase the assumed benefit.
Treat safety-critical components as controlled parts
Safety components must satisfy both the electrical function and the certification conditions of acceptability. A recognized part can still be misapplied if its voltage, current, temperature, mounting, thickness, spacing, or surrounding circuit falls outside the recognition.
| Component or material | Design question | Production control |
|---|---|---|
| Fuse and fuse holder | Is the breaking capacity, time-current behavior, voltage rating, and location suitable for the available fault current? | Lock manufacturer, series, rating, approvals, and approved alternates in the BOM |
| X capacitor | Is the across-the-line class correct, and is discharge after unplugging addressed where required? | Verify approval marks, capacitance, voltage class, and lot traceability |
| Y capacitor | Is the line-to-earth or primary-to-secondary class correct, and does its capacitance keep touch current within the product limit? | Control class, capacitance tolerance, dielectric system, and supplier |
| Optocoupler | Does the package provide the required insulation, internal separation, working voltage, and aging performance? | Use the exact recognized package and maintain the PCB boundary around its pins |
| Transformer | Do winding construction, margins, tape, bobbin, wire system, and terminations match the insulation plan? | Maintain an approved construction specification and audit winding production |
| MOV or surge suppressor | Can it survive expected surges, and is abnormal overheating or end-of-life failure contained? | Control rating, approval, spacing, thermal protection if needed, and sourcing |
| PCB and plastics | Are flammability and temperature ratings valid at the used thickness and color? | Retain material recognition records and prevent unapproved resin substitutions |
X and Y capacitors are not interchangeable with ordinary film, ceramic, or electrolytic capacitors. A Y capacitor is intentionally connected where its failure could affect electric-shock protection, so class and capacitance are tied to the insulation system and touch-current budget. Increasing its value may improve common-mode EMI but can also raise touch current. EMI and safety therefore have to be solved together.
UL 94 classifications describe small-scale flammability behavior under defined tests. V-0 is not a blanket statement that an enclosure or finished product is fireproof. The material must be used within the thickness, color, temperature, and end-product conditions covered by its recognition.
Design the transformer and isolation parts as one system
The transformer often carries the reinforced-insulation burden in an isolated converter. Its safety depends on more than a dielectric-strength number. Bobbin geometry, margin tape, winding wire, layer insulation, lead routing, solder joints, core and clamp accessibility, and the separation at pins all matter.
Triple-insulated wire can simplify some constructions, but the exact wire system, terminations, and manufacturing process must be recognized for the intended use. Where margin tape is required, winding operators need controlled dimensions and inspection criteria. Leads must not cross the isolation margin or be pulled into a smaller spacing during assembly. Varnish is useful for mechanical stability but should not be assumed to create reinforced insulation unless the approved insulation system says so.
The optocoupler and Y capacitor complete the isolation system. Check their external PCB spacing as well as their internal rating. It is possible to specify a high-isolation optocoupler and then defeat it with copper under the package.
Make abnormal operation safe, not merely recoverable
Protection circuits should prevent hazardous energy, excessive temperature, or fire when expected faults occur. The exact fault set comes from the product standard, but common investigations include output short circuit, overload, blocked airflow, feedback-loop open or short, cooling-fan failure, reversed connectors, transformer winding faults, and component short or open conditions.
Useful protections include current limiting or hiccup operation, independent output overvoltage protection, thermal shutdown or derating, input undervoltage/overvoltage handling, and appropriately coordinated fusing. A latched shutdown is not automatically better than hiccup or foldback; the right behavior depends on the load, fault energy, restart safety, and end-product requirements. What matters is that no single fault leaves a hazardous output, sustained ignition source, ruptured part, or defeated insulation safeguard.
Component spacing can change under fault. A resistor that chars, an MOV that vents, or a loose wire that moves toward the secondary can create a new conductive path. Place and restrain such parts with the fault outcome in mind.
Verify temperature and fire containment under realistic conditions
Temperature limits cannot be reduced to one generic table. They depend on the component's rated temperature, insulation system, material recognition, measurement method, ambient, and abnormal test. An 85 degrees C electrolytic capacitor operating at 85 degrees C may be within its absolute rating but unsuitable for the intended life. Conversely, a magnetic component can have different allowable temperatures depending on insulation class and the standard's measurement rules.
Run thermal evaluation at the worst rated input, maximum load, maximum declared ambient, and in the intended enclosure and orientation. Include tolerance and airflow variation. For production designs, use the result to define derating rules and critical-component limits, not just a one-time prototype pass.
Fire-enclosure decisions should consider ignition sources, available power, spacing to combustible materials, openings, barriers, and the behavior of nearby wiring and adhesives. Glow-wire testing is required in some product categories and locations, but a universal "750 degrees C for 30 seconds" claim is not appropriate across all power supplies. Follow the end-product standard and the component's actual role.
Plan compliance tests before the certification sample
Dielectric-strength testing, insulation resistance, touch current, earth-bond testing, temperature, abnormal operation, and mechanical checks should be planned while the design can still change. Test voltage and duration depend on the applicable standard, insulation type, working voltage, test method, and whether the test is type testing or routine production testing. A fixed 3 kVAC for 60 seconds is common in some specifications, but it is not a universal rule.
Pre-compliance testing should use calibrated equipment and a controlled setup. Ramp the hipot voltage where the method requires it, discharge stored energy afterward, and define leakage or trip settings from the standard and test equipment. Disconnect or account for components that the approved procedure permits, such as surge suppressors that would otherwise conduct during the test. Never invent a test configuration simply to make a failure disappear.
For Class I equipment, the protective-earth path deserves separate attention: terminal hardware, conductor color, crimp quality, resistance, resistance to loosening, and whether earth remains connected when other wiring is serviced. Routine earth-bond and dielectric tests must match the certified construction and factory procedure.
Control the design through mass production
Certification covers a defined construction, not an idea. A safe prototype can become a noncompliant product when purchasing changes a fuse, transformer, optocoupler, PCB laminate, plastic resin, wire, adhesive, or enclosure supplier without review.
Create a critical-component list tied to the certified BOM and drawings. Approved alternates need documented equivalence and, where necessary, certification-body acceptance. Incoming inspection should verify identity and rating, while line controls should cover transformer construction, isolation slots, component orientation, wire routing, fasteners, solder contamination, and labels.
Routine tests may include dielectric strength, protective-earth continuity, and functional safety checks. Record the test limits, equipment ID, calibration status, operator or station, serial or lot number, result, and disposition of failures. A 100 percent routine test may be required for certain constructions, but the exact factory test program must come from the applicable standard and certification report.
Use a gate review before releasing the design
| Review gate | Evidence to release the design |
|---|---|
| Standard and market | Applicable standards, editions, national deviations, product classification, and certification route confirmed |
| Insulation coordination | Boundary map plus clearance, creepage, solid-insulation, altitude, pollution, and material assumptions documented |
| Critical components | Approval files, ratings, conditions of acceptability, approved alternates, and traceable BOM entries complete |
| Fault and thermal safety | Worst-case temperature and abnormal tests completed with safe outcomes and recorded margins |
| Mechanical and fire safety | Enclosure, accessibility, strain relief, earth bonding, flammability, and ignition containment reviewed |
| Production readiness | Controlled drawings, work instructions, routine tests, calibration, labeling, and change control released |
This gate is most valuable before tooling and volume purchasing. Late fixes such as slots, insulation sheets, taller barriers, or a new transformer can affect EMI, thermal behavior, assembly yield, enclosure fit, and cost at the same time.
Conclusion
Good safety design is traceable. Every isolation boundary has a reason, every critical component has an approval and operating condition, and every production test links back to a safeguard in the certified design. That discipline prevents the familiar cycle of certification failure, emergency PCB rework, and uncontrolled substitutions after launch.
SIPURUI works with OEM and industrial buyers who need switching power supplies matched to real installation, safety, and production requirements. For a new program, share the target markets, end-product category, input and output ratings, enclosure class, altitude, and ambient conditions early; those details are more useful than asking for a generic "CE" or "UL" power supply.