First, the matching overview

Ultrasonic transducer and generator match includes two aspects, one is matched by the generator output to the transducer rated electrical power, which is due to the generator needs an optimal load to output the rated power due to the conversion Impedance converter into the best load, that is, the role of impedance conversion; Second, through the match so that the generator output efficiency is highest, which is due to the static impedance of the transducer, resulting in the operating frequency of the output voltage and current have a certain phase So that the output power can not be expected maximum output, so that the generator output efficiency is reduced, so the output of the generator and the string or on the opposite of a resistance, the generator load is purely resistive, that is, tuning. This shows that the quality of the match directly affects the power generation and efficiency of ultrasound sources.

Second, impedance matching

In order to maximize the power amplifier output rated power; under the given conditions of supply voltage depends on the load impedance. After the supply voltage is given, the magnitude of the output power depends on the equivalent load RL '. At present, most of the power ultrasonic generator load for the piezoelectric transducer, the impedance of about tens of ohms to several hundred ohms, in order to achieve the required rated power, so the need for transducer load RL impedance transformation. Transform from high impedance to low impedance. Commonly used method, through the output transformer primary winding turns ratio transformation.

The above title impedance transformation is implemented by the output transformer.

The output transformer is an important part of the impedance matching and transmission power of the ultrasonic generator. Its design and winding technology are very important to the working safety of the generator. Not only does it affect the operation of the circuit by way of leakage inductance, excitation current, etc., its leakage inductance forms the main reason of the output voltage spike. Therefore, in the design, should be selected with high magnetic flux density B, high permeability μ, high resistivity ρc and low coercivity Hc high saturation material as the core. General to prevent the transient high-frequency transformer saturation, the design should pay attention to the following points:

1. Core material increases the magnetic induction ΔB larger, the smaller the number of turns required coil, the smaller the DC resistance R, so the coil copper loss Pm also smaller. When ΔB is high, the leading edge of the pulse is transmitted steeper. Therefore, in the design of transformers, select the high magnetic flux density of the material for the core, which reduce the transformer loss, reduce size and weight are very beneficial. In order to avoid saturation in the steady state or transition, the working flux density B≤Bs / 3 is usually chosen, where Bs is the maximum and magnetic flux density of the core.

2. To ensure that the primary inductance is large enough General requirements of the transformer primary impedance should meet the following relationship: WLl ≥ 15RL ', where RL' is calculated by the secondary load equivalent resistance of the primary side, WLl primary inductance inductive reactance, If the primary inductance is too small, encourage 6.5Ω magnetic current will be relatively large, the excitation current is too large, the transformer loss will increase, the temperature increases, thereby reducing the Bs, the transformer into the possibility of saturation.

3. To consider the effect of "skin effect" When working at high frequencies, the current flowing through the leads creates a "skin effect." This is equivalent to reducing the effective cross-sectional area of the wire, increasing the resistance of the wire, causing an increase in the voltage drop across the wire, resulting in an increase in the temperature of the transformer, increasing the risk that the transformer will saturate. It is advisable to use a multi- And around the method.

Third, tuning and matching

Since the piezoelectric transducer has a static capacitance Co and the magnetostrictive transducer has a static inductance LO, there is a phase angle φ between the voltage VRL on the transducer and the current IRL when the transducer is in the resonance state. The output Power PO = VRLIRLcosφ. Due to the presence of φ, the output power does not reach the maximum value. Only when φ = 0, the output power reaches the maximum value. Therefore, in order for the transducer voltage VRL to be in phase with the current IRL (φ = 0), an offset cancellation must be applied to the transducer either on or in the string. For a piezoelectric transducer, that is, an inductance L0 may be applied to the string or a string of capacitances C0 should be applied to the magnetostrictive transducer.