The Work Coil and the Matching Transformer Are One Design
In induction heating, a work coil of a few microhenries is driven at 20–200 kHz from a resonant inverter. The matching transformer converts the inverter’s impedance to the coil’s and provides isolation, but its behaviour is inseparable from the coil: change the coil geometry, and the transformer’s operating point moves. Designing one without the other is the most common reason a heating station fails to reach temperature or trips on overcurrent.
This note covers the matching transformer and the coil-design considerations a buyer needs to specify both correctly.
Impedance Matching in Numbers
| Parameter | Typical range | Couples to |
|---|---|---|
| Work frequency | 20–200 kHz | Core material, penetration depth in the workpiece |
| Coil inductance | 2–20 µH | Resonant capacitor and transformer ratio |
| Coil current | 20–500 A RMS | Transformer secondary current, coil cooling |
| Workpiece coupling | Varies with load position | Reflected impedance seen by the inverter |
| Duty cycle | Intermittent heating cycles | Thermal design of coil and transformer |
Reflected impedance changes as the workpiece heats: cold steel couples strongly, Curie point crossing reduces permeability, and above the Curie point the load becomes essentially resistive. A transformer designed only for the cold state sees a very different load within seconds.
Core Material and Frequency
Below roughly 30 kHz, laminated steel or ferrite with a large cross-section works; from 30 kHz to 100 kHz ferrite dominates; above 100 kHz, ferrite with fine-grained grades and thin ribbon or powder cores are used depending on power. The transformer often runs with a substantial reactive current, so the core loss must be evaluated at the actual flux swing, not at the delivered power level.
Water cooling of the coil is standard at high power; the transformer may share the same cooling loop or be air-cooled with a thermal path to the enclosure. A potted transformer with a water-cooled copper block attached is a common, effective construction.
Coil Design Factors That Affect the Transformer
- Turns count. Fewer turns lowers coil voltage and raises current; more turns raises voltage and reduces current. The choice sets the transformer’s secondary rating.
- Coupling distance. A larger gap between coil and workpiece reduces efficiency and changes reflected resistance, sometimes by a factor of two.
- Coil geometry. Single-turn or multi-turn, helical or pancake, each has a different inductance per unit length and a different cooling requirement.
- Magnetic concentrators. Ferrite or laminated concentrators around the coil raise coupling; they also change the effective inductance, which must be re-tuned.
Thermal and Reliability Notes
Intermittent duty is deceptive: a transformer that reaches 60 °C in a 10 s heating cycle may reach 110 °C after an hour of repeated cycles because the thermal time constant of the assembly is minutes, not seconds. Specify the duty pattern and the number of cycles per hour, and verify the hotspot over a full production shift rather than a single cycle.
Insulation must handle the coil voltage transients from switching plus the reflected high voltage during load loss; a hipot test at the working voltage alone is not sufficient. Specify a test at the worst-case transient level and include a partial discharge check for high-voltage designs.
Related reading
- High-Frequency Transformer Design
- Inverter Welding Machine Magnetics
- Thermal Design of Transformers and Inductors
- Core Loss vs Copper Loss Optimization
We supply matching transformers and inductors for induction heating and welding platforms, sized against the reflected load across the heating cycle rather than the cold state alone. Send your frequency, coil inductance, workpiece material, and duty pattern to sales@sirui-tech.com and the Magnetic Core team will co-size the coil and the transformer.