Can anyone help with an Ultrasonic Transducer Transformer design. Air gap or no?

Question

I am looking at using a centre tap transformer with a 12mH secondary to drive a 50W 40kHz ultrasonic transducer (4100pF 10-20Ohm resonance impedance) for cleaning applications. The input voltage is 12V at the centre and 0V either side of the primary alternated to switch up the output for ~270VAC out of the secondary.

My question is what would be the best core/bobbin design for this? To air gap or not to air gap? If using no airgap and using a RC or LC what would be ideal starting values? I am assuming Inductor in series and Capacitor in parallel to the Transducer?

Thanks in advance.

Answer 1

The air gap is for energy storage, i.e. used mostly in flyback converters (where it's used as two inductor on the same core, not as a transformer).

For ultrasonic transducers you are actually looking at an impedance matching transformer (and you usually have to match it to the transducer for best efficiency, too!).

Consider that the piezo transducer is essentially a capacitor at resonance (and near resonance it has an RLC resonant equivalent model); the specs are hopefully declared by the manufacturer or you can measure them yourself. Also many transducer manufacturers also gives specifications for useful drive transformers (some directly sell them!)

40kHz is too much for an iron core (except special laminates, maybe) so you'll probably want to use a ferrite core. Size it for power handling (usually the core datasheets gives a starting idea of the power handling) and then determine the number of spires to avoid saturating the core itself. Usual transformer design really, at least for the first tries. Lots of documentation on the web too (search for 'ferrite transformer design' and similar things).

Ideally you'll want to have the impedance seen from the secondary matching to the transducer one (maximum power transfer law in AC: complex conjugate impedance) but it's almost impossible to compute that before building the transformer.

Answer 2

Normally, a forward (normal, ideal, power supply, impedance matching) transformer would be designed without an airgap to minimise energy storage in the core, and a flyback transformer would use an airgap for maximising the core energy storage.

However, for driving an ultrasonic transducer for cleaning applications, which presumably is single frequency, highest power possible, it may be beneficial to use a non-ideal transformer, to resonate out the transducer capacitance, making the driving amplifier's job somewhat easier.

The first thing to do would be to deliberately introduce some leakage inductance, by separating the primary and secondary windings just the way you wouldn't in a well-coupled transformer. This may only produce a limited amount of leakage, so the next thing is to use an airgap to enhance the effect.

Going from a theoretical leakage inductance to a physical design that will produce that is somewhat tricky, and you might be better off using an ideal (well coupled, no gap) transformer with an external inductance that would be easier to design.

Answer 3

I am looking at using a centre tap transformer with a 12mH secondary to drive a 50W 40kHz ultrasonic transducer (4100pF 10-20Ohm resonance impedance) for cleaning applications. The input voltage is 12V at the centre and 0V either side of the primary alternated to switch up the output for ~270VAC out of the secondary.

My question is what would be the best core/bobbin design for this? To air gap or not to air gap? If using no airgap and using a RC or LC what would be ideal starting values? I am assuming Inductor in series and Capacitor in parallel to the Transducer?

You seem to have chosen some components, without regard for the system design. The transformer output of 270 V is totally wrong, whichever way you drive it.

First, you have to decide whether your transducer is driven at resonance or not. I'm going to neglect whether figures are rms or peak, because as you'll see in a moment, it's irrelevant compared to the orders of magnitude involved.

If not driving at resonance, then the 270 V will send a current through the impedance of the transducer. Z = 1/sC, where C=4 nF and s = 2.pi.40k is about 1000 ohms. The current will therefore be about 270 mA, which will dissipate I²R in your transducer's real 14 ohms impedance, or about 1 watt, far short of the 50 watts you're aiming at. To get to 50 watts, you'll need about 2 kV.

If driving at resonance, let's assume you have added enough series inductance (about 4 mH) on the secondary side for resonance, and are driving that at 270 V. Now you see only the transducer's real impedance, and the power dissipation will be V²/R, or about 5 kW, rather more than the 50 the transducer can handle.

Let's go back to basics. You want about 50 watts dissipation in the transducer's real 14 ohms resistance. That requires about 28 volts at 2 amps, for 56 watts.

If your transformer has a centre tap primary driven in push-pull, that means you have a low impedance drive. You therefore need series resonance in the secondary with an added 4 mH inductor, and a well-coupled gap-less transformer, delivering 28 v RMS. A well-coupled transformer will have negligible leakage inductance, whatever the actual inductance of your secondary is.

Note that your primary square-wave drive is 12 v RMS and peak. Your secondary voltage will be square-wave, but your secondary (and primary) current will be (essentially) sinusoidal, due to resonance. You should use FETs rather than BJTs for the switching elements, so they can cope with any reverse current required of them.

That ratio, 14 ohms real part to the 1 kohm reactive part means you have a huge Q to deal with, around 70. The power you're going to deliver to the transducer will depend critically on the drive frequency. This means you either have to use a power oscillator to drive it at resonance automatically, or a tunable frequency source and some form of power or phase monitor to adjust it. You could deliberately drive off-resonance to reduce the power if you wanted.