Matching, Not Power, Is the Design Problem
An ultrasonic transducer presents a capacitive load (typically a few nanofarads) that must be driven at resonance by a power stage, usually a bridge at 20–100 kHz. The matching transformer’s job is to convert the bridge’s voltage and current to what the transducer needs and to isolate the DC path, while keeping the resonant tank stable as the transducer’s impedance shifts with temperature and load.
Designs that treat the transformer as a simple turns-ratio device usually oscillate, overheat, or fail to reach rated amplitude, because the resonance is a property of the transformer, the tuning inductor, and the transducer together.
Key Parameters
| Parameter | Typical range | Why it matters |
|---|---|---|
| Operating frequency | 20–100 kHz (40 kHz common for cleaning) | Sets core material and turns count |
| Power | 50 W to several kW per transducer | Drives core size and window |
| Transducer capacitance | 1–10 nF | Determines tuning inductance and Q |
| Voltage at transducer | 100–1,000 V RMS | Insulation class and creepage |
| Duty | Continuous or pulsed (welding, cutting) | Thermal design and potting |
Because the load is resonant, the current in the transformer can be several times the value implied by power divided by voltage: the circulating current in the tank dominates. Size the winding for that current, not for the delivered power, or the part will run hot at rated amplitude.
Core Choice and Gap
At 20–100 kHz with significant flux swing, MnZn ferrite is the standard choice; a small gap is often used to stabilize inductance against temperature and to prevent saturation from DC bias in half-bridge designs. Powder cores appear in low-cost low-power units but their loss rises with temperature, which is the wrong direction for a sealed handpiece.
The transformer usually carries a DC-blocking capacitor in series, so the core sees little DC bias; if the design omits that capacitor, saturation becomes a real risk and the gap must be sized for the worst-case asymmetry.
Thermal Behaviour
Ultrasonic transducers are often driven continuously for hours in cleaning baths, or in rapid pulses in welding. In both cases, the transformer sits near the transducer, frequently inside a handpiece or a sealed generator, at 50 %+ of its thermal budget. Potting with a thermally conductive compound, and specifying the hotspot at the actual duty cycle, is the only reliable approach. A bench test at 30 % duty will understate the temperature by 30–40 K.
Practical Design Checklist
- Define the transducer’s equivalent circuit at resonance, including capacitance and motional resistance.
- Choose the turns ratio for the voltage and current the transducer needs at rated amplitude, not for maximum power transfer.
- Select core size from the circulating current and the flux swing at resonance, with a gap if DC bias exists.
- Design the winding for the AC resistance at the operating frequency — Litz or foil, depending on current.
- Specify insulation for the transducer voltage plus margin, and hipot the primary-secondary barrier.
- Test at the real duty cycle and ambient, and record the resonant frequency shift over temperature.
Related reading
- High-Frequency Transformer Design
- Litz Wire Design
- Thermal Design of Transformers and Inductors
- Magnetic Core Selection Guide
We wind ultrasonic matching and drive transformers for cleaning, welding, and cutting equipment, and we report the resonant frequency shift and hotspot at your duty cycle rather than at a bench condition. Send the transducer capacitance, drive frequency, and amplitude target to sales@sirui-tech.com and the Magnetic Core team will size the transformer and the tuning inductor together.