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IEC 61558-2-16: Safety of Transformers, Reactors and Similar Products for Switching Mode Power Supplies
Tip: IEC 61558-2-16:2009 (with Corrigendum 1:2010) is a particular requirement within the IEC 61558 series, specifically covering the safety of transformers and reactors intended for use in switching mode power supplies (SMPS) with rated voltages up to 1000 V AC and 1500 V DC.
1. Scope and Relationship to the IEC 61558 Series
IEC 61558-2-16:2009 is part of the comprehensive IEC 61558 series that covers safety requirements for power transformers, power supplies, reactors, and similar products. The series is structured with Part 1 (general requirements) and numerous Part 2 particular requirements for specific product types. Part 2-16 specifically addresses transformers and reactors for switching mode power supplies (SMPS), which operate at frequencies typically between 20 kHz and 1 MHz and require design considerations fundamentally different from mains-frequency (50/60 Hz) transformers.
The standard covers SMPS transformers with rated output up to 20 kVA (single-phase) and 40 kVA (three-phase), intended for incorporation into equipment or as stand-alone units. It applies to both step-down (flyback, forward converter) and step-up topologies, as well as isolation transformers used for galvanic isolation in SMPS circuits. The corrigendum published in 2010 primarily clarified the creepage distance requirements for high-frequency windings and corrected inconsistencies in the thermal test procedures for ferrite core transformers.
The fundamental difference between SMPS transformers and mains-frequency transformers is captured in the standard’s approach to insulation design. At switching frequencies above 20 kHz, the voltage distribution within a winding is not uniform — the inter-turn voltage stress is influenced by the steep wavefronts of the switching waveforms (dv/dt typically 10-50 V/ns) and the distributed capacitance of the winding structure. IEC 61558-2-16 addresses these effects through specific partial discharge test requirements and enhanced insulation coordination rules.
Critical Design Context: SMPS transformers operate under conditions that are uniquely demanding compared to conventional power transformers. The combination of high-frequency operation (leading to skin and proximity effects in windings), high dv/dt stress (requiring careful insulation design), ferrite core losses (which increase with frequency), and compact construction (limited cooling surface) creates a multi-dimensional design challenge that the IEC 61558-2-16 standard directly addresses.
2. Insulation and Dielectric Requirements
IEC 61558-2-16 defines insulation requirements that account for the high-frequency voltage stress characteristic of SMPS operation. The standard requires that insulation between windings (reinforced insulation for safety isolation applications) withstand a dielectric test of 4000 V rms for 60 seconds for basic insulation between primary and secondary, and 6000 V rms for reinforced insulation. However, unlike the 50/60 Hz dielectric tests specified in Part 1, Part 2-16 also requires a partial discharge (PD) test at 1.5 x rated voltage with a maximum allowed PD level of 10 pC.
| Insulation Type | Dielectric Test Voltage (50/60 Hz) | Partial Discharge Test Voltage | Max. PD Level | Minimum Creepage (mm) @ 250 V |
|---|---|---|---|---|
| Basic insulation (primary-secondary) | 4000 V rms, 60 s | 1500 V rms | 10 pC | 6.4 mm (CTI Group IIIa) |
| Reinforced insulation (primary-secondary) | 6000 V rms, 60 s | 2500 V rms | 5 pC | 8.0 mm (CTI Group IIIa) |
| Supplementary insulation (primary-core) | 2500 V rms, 60 s | 1000 V rms | 10 pC | 5.0 mm |
| Inter-winding (multiple outputs) | 1500 V rms, 60 s | 500 V rms | 10 pC | 3.2 mm |
| Layer insulation (within winding) | 500 V rms, 60 s | No PD test required | N/A | Depends on layer voltage |
The partial discharge test is particularly critical for SMPS transformers because the high-frequency voltage stress can initiate PD that would not occur at 50/60 Hz for the same peak voltage. The standard specifies that the PD test be performed at the switching frequency of the intended application (or a representative frequency within the SMPS operating range), not at 50/60 Hz. This recognizes that PD inception voltage (PDIV) decreases with increasing frequency due to the reduced time available for charge dissipation on the insulation surface.
Creepage and clearance distances in IEC 61558-2-16 follow the principles of IEC 60664-1 (Insulation coordination for equipment within low-voltage systems) with additional requirements specific to SMPS operation. The standard recognizes three categories of insulation materials based on comparative tracking index (CTI): Group I (CTI >= 600), Group II (400 <= CTI < 600), and Group IIIa (175 <= CTI < 400). The creepage distances in the table above are for Group IIIa materials, with reductions of 20% for Group II and 35% for Group I.
Design Warning: The triple-insulated wire (TIW) commonly used in SMPS transformers for reinforced insulation between primary and secondary must be carefully evaluated for high-frequency PD performance. Many TIW constructions rated for 6000 V at 50/60 Hz show PD inception at voltages as low as 1500 V at 100 kHz. The standard requires that any wire insulation system used for reinforced insulation in SMPS transformers be PD-tested at the actual operating frequency, not just at 50/60 Hz. Designers should specify TIW with a PDIV of at least 3 x peak operating voltage at the switching frequency.
3. Thermal Performance and Heating Tests
Thermal management is a critical aspect of SMPS transformer design addressed by IEC 61558-2-16. The standard defines heating test conditions that reflect the actual operating conditions of SMPS transformers, including the effects of core losses (hysteresis and eddy current), winding losses (DC resistance + AC skin/proximity effects), and the thermal resistance of the construction.
| Component | Maximum Temperature (Class B) | Maximum Temperature (Class F) | Maximum Temperature (Class H) | Measurement Method |
|---|---|---|---|---|
| Ferrite core | 120 deg C | 150 deg C | 180 deg C | Thermocouple on core surface |
| Copper winding (solderable) | 120 deg C | 155 deg C | 180 deg C | Resistance method (preferred) |
| Copper winding (non-solderable) | 130 deg C | 165 deg C | 190 deg C | Resistance method (preferred) |
| Bobbin/former | 115 deg C | 145 deg C | 175 deg C | Thermocouple |
| Magnet wire insulation | 130 deg C | 155 deg C | 180 deg C | Thermocouple on winding surface |
| Terminal pins/solder joints | 110 deg C | 140 deg C | 170 deg C | Thermocouple |
The heating test is performed at the worst-case operating conditions: maximum ambient temperature (typically 40 deg C for general-purpose, 70 deg C for industrial), maximum rated load, minimum input voltage (which maximizes primary current in most SMPS topologies), and at the operating frequency that produces maximum core loss (typically the minimum switching frequency). The thermal stabilization criterion requires that the temperature change be less than 2 deg C per hour.
A unique aspect of the SMPS transformer heating test is the requirement to measure winding temperature using the resistance method rather than thermocouples. The resistance method provides the average temperature of the entire winding, which is more representative than point measurements with thermocouples. The standard specifies the formula: T2 = (R2/R1) x (T1 + 234.5) – 234.5 for copper windings, where R1 and T1 are the cold resistance and temperature, and R2 is the hot resistance.
Engineering Insight: The dominant thermal design constraint in SMPS transformers is often the hot-spot temperature within the winding, not the core temperature. The ratio of hot-spot temperature to average winding temperature (the hot-spot factor) varies from 1.1 for well-coupled planar transformers to 1.4 for poorly coupled多层 wound transformers. IEC 61558-2-16 requires that the designer account for this hot-spot factor, with a maximum allowable hot-spot temperature 10-15 deg C above the measured average winding temperature. Using interleaved winding construction (primary-secondary-primary-secondary sandwich) can reduce the hot-spot factor to 1.15-1.2 by improving thermal coupling and reducing AC winding losses.
4. Mechanical and Construction Requirements
IEC 61558-2-16 specifies mechanical requirements tailored to the SMPS environment. Transformers must withstand vibration testing at 2 g acceleration from 10 Hz to 500 Hz (simulating the fan-induced vibration and mechanical shock typical in SMPS equipment). The solderability of termination pins is tested per IEC 60068-2-20, requiring 95% coverage with no dewetting after accelerated aging (steam aging for 4 hours at 93 deg C).
The standard also requires that the transformer construction prevent the migration of core material (ferrite dust) into the winding structure, which could cause inter-turn shorts. An encapsulation or impregnation process is required for all SMPS transformers operating above 50 kHz. The impregnation material (typically vacuum-pressure-impregnated varnish or epoxy) must have a dielectric strength of at least 20 kV/mm and a glass transition temperature (Tg) above the maximum operating temperature of the transformer.
For transformers using ferrite cores with air gaps (common in flyback and filter inductor applications), the standard specifies that the gap must be filled with a non-conductive spacer or bonded using gap-filling adhesive to prevent the production of ferrite dust that could migrate into the winding area. The gap tolerance must be maintained within +/- 0.05 mm for gapped transformers, as the inductance tolerance is directly proportional to the gap tolerance.
Design Recommendation: When designing the bobbin for an SMPS transformer according to IEC 61558-2-16, pay careful attention to the creepage distances at the bobbin surface, particularly at the transition points where the winding exits the bobbin and connects to the termination pins. These locations are the most common sites for creepage breakdown in SMPS transformers. The use of a crepe paper or Mylar tube as an additional insulating layer between the winding and the bobbin end-wall can increase the effective creepage path by 50-100% and is strongly recommended for designs requiring reinforced insulation.
5. FAQs
Q1: Does IEC 61558-2-16 cover transformers for all types of SMPS topologies?
Yes, the standard applies to transformers and reactors for all common SMPS topologies including flyback, forward converter, half-bridge, full-bridge, push-pull, and resonant converters (LLC, CLLC). The standard also covers inductors and reactors used in SMPS output filters, PFC boost inductors, and EMI common-mode chokes, provided they fall within the voltage and power ratings specified in the scope. For resonant converters operating above 1 MHz, additional requirements from IEC 61558-2-17 (particular requirements for high-frequency transformers) may apply.
Q2: How does the standard address the use of planar transformers in SMPS?
Planar transformers (using PCB windings and low-profile ferrite cores) are covered by IEC 61558-2-16 with additional considerations. For planar designs, the creepage distances are determined by the PCB layout rules per IEC 60664-1 rather than the bobbin-based creepage rules. The partial discharge requirements apply equally to planar transformers. The thermal test for planar transformers must account for the reduced heat dissipation capability due to the compact construction and the thermal conductivity of the PCB material (typically 0.3 W/mK for FR-4, compared to 0.2 W/mK for air).
Q3: What is the difference between operational insulation and basic insulation in SMPS transformers?
Operational insulation is the insulation between windings that are at different voltages but both accessible to the user — it provides no shock protection and is only suitable for functional isolation within the same circuit. Basic insulation provides fundamental shock protection and is used between primary (mains-connected) circuits and accessible conductive parts. In an SMPS transformer, the insulation between the primary winding and the secondary winding must be at least basic insulation, and for SELV (Safety Extra-Low Voltage) outputs supplying user-accessible circuits, it must be reinforced insulation or double insulation (basic + supplementary).
Q4: How does the standard address the lifetime and reliability of SMPS transformers?
While IEC 61558-2-16 primarily addresses safety rather than reliability, it includes provisions that indirectly affect transformer lifetime. The thermal class rating (B, F, H) defines the maximum operating temperature that the transformer can withstand while maintaining a minimum lifetime of 20,000 hours (approximately 2.3 years of continuous operation) under rated conditions. The Arrhenius equation applied to insulation aging predicts a halving of lifetime for every 8-10 deg C increase in operating temperature above the rated class temperature. For applications requiring longer lifetime (e.g., industrial SMPS with 10+ year design life), the designer should operate the transformer at temperatures 15-20 deg C below the insulation class limit.
