Ultrasonic generator and the matching design of transducer
1, matching an overview of
Ultrasonic generator and transducer matching consists of two aspects, one through the matching of the generator to the transducer rated output of electric power, this is because the generator needs one of the best rated power output of the load can be caused by the transducer device into the optimal load impedance transformation, namely the role of impedance transformation;
Second, by matching the output to make the most efficient generator, which is due to electrostatic transducer reasons for resistance, resulting in operating frequency on the output voltage and current have a certain phase, so that power output is not expected maximum output, so that reduce the generator output efficiency, and therefore generator output and the string on the opposite or anti-, so that generator load is purely resistive, ie tuning effect. This shows that match will have a direct impact on power generation and efficiency of the ultrasonic source.
Second, impedance matching
In order to make maximum power amplifier output power rating; in power supply voltage under the given conditions depends on the load impedance. Usually in the D class switching amplifier in its generator transformer primary equivalent load Rl 'on the output power of expression as follows:
Where, VAm for the equivalent load on the fundamental amplitude;
vcc for the supply voltage; vces for the power amplifier tube saturation voltage drop, so
In order to ensure the system has a certain power margin (due to the output transformer, the last stage matching circuits and transistors have the wear and tear wear and tear resistance, po 'need to multiplied by a coefficient equal to about 1.4-1.5. That the output power of po is 1. 5Po ';
From the style we can see that in a given supply voltage, the output power depends on the size of the equivalent load RL '. Most of the load power ultrasonic generator type piezoelectric transducer, whose impedance is about tens of ohms to several hundred ohms between, in order to meet the requirements of the rated power, so the need for the transducer to the load impedance RL transformation . Impedance transformation from high to low impedance. Commonly used approach by the beginning of the output transformer secondary coil of turns than a transformation. Transformer sub-primary turns ratio n / m, then the output power when the primary resistance of PO
For example: Request a generator output of the transducer on the power of 1000W, DC-based VCC to 220V, VCES = 10V, power should be left a certain margin, then PO = 1.5PO '= 1500W. Then the transformer primary of the
6.5Ω
If the transducer resonance equivalent resistance RL = 200Ω, then the output transformer secondary / primary ratio Circle
The above title impedance transformation is implemented through the output transformer.
Impedance matching output transformer is the ultrasonic generator, transmission power of the key components, its design and winding process on generator safety at work is very important. It is not only to leakage inductance, magnetizing current, etc. affect the circuit's operation, the leakage inductance is the main reason for the formation of the output voltage spikes. To this end, the design should be selected with a high flux density B, the high magnetic permeability μ, high resistivity ρc and low coercivity Hc of high saturation material for core. Generally transient in the prevention of high-frequency transformer saturation, in the design should pay attention to the following:
1. Selection of the work of flux density B
Magnetic core materials, the greater the increment ΔB required fewer coil turns, DC resistance R is also smaller, thus the coil copper loss Pm is also smaller. ΔB get high, the more steep front impulse transmission. Thus, in the design of transformer, select a high magnetic flux density of the material for core, which reduce the transformer losses, reduce size and weight are very beneficial. In order to avoid steady state or transition occurs during saturation, the general selected work flux density B ≤ Bs / 3 is appropriate, where the largest and Bs for the core flux density.
2. To ensure that the primary inductance is large enough
General requirements for the primary impedance transformer shall meet the following formula Relations: WLl ≥ 15RL ', in which RL' for the secondary to the primary side of the load are considered the equivalent resistance value, WLl for primary inductance inductive, if the primary inductance is too small, excitation current will be relatively large excitation current is too large, the transformer losses will increase, resulting temperature increase, thereby reducing the Bs, so that transformers into saturation more likely.
3. To consider the "skin effect" of
In the high-frequency work, the current flowing through the wire will have a "skin effect." This is equivalent to a decrease of the effective conductor cross-sectional area, an increase of wire resistance, causing increased pressure drop wires, resulting in transformer temperature increases, the result increases the risk of transformer into saturation, the proposed use of small diameter multi-strand wire and around methods.
3, tuning match
As the piezoelectric transducer with static capacity Co, magnetostrictive transducers are static sense of LO, the transducer resonant state, the voltage on the transducer VRL and current IRL exist between a phase angle φ, the output power PO = VRLIRLcosφ. Due to the existence of φ, the output power of up to less than the maximum. Only when φ = 0, the output power up to maximum. Therefore, in order to enable the transducer on the voltage VRL and current IRL-phase (φ = 0), you must transducer on and on or offset to a string of anti-. For the purposes of piezoelectric transducers, namely, and or you can string on a inductor L0, while the magnetostrictive transducer or string should be, and to a capacitor C0.
Piezoelectric transducer impedance or admittance equivalent circuit shown in Figure 1.52.
In the equivalent circuit in
Where R '(f), X' (f) for the series resistance and reactance; R (f), G (f), B (f) as a parallel resistance, conductance and susceptance. They are a function of frequency. Parallel and series tuning tuning inductance determined by the following formula:
Here we compare the differences between the two tuning
Figure 1-53,1-54 is a transducer two kinds of tuning curves calculated from the calculations show that
1 As the transducer in series reactance than the shunt resistance is small, so there are L string <L and,
Two parallel tuning does not change the transducer parallel conductance response, while the series tuned conductance response to bimodal, Admittance circle photo shows two overlapping circles.
3 series-tuned active resistance than the parallel tuned active resistance is small, the relatively low availability of series tuned input resistance.
4 from the series and parallel tuning the input phase angle of zero-crossing situation, as a superior broadband characteristics of parallel tuning tuning series.
5 currently in power are more tuned in series using ultrasound, in addition to the above-mentioned characteristics of series, there are also loads when the transducer has short-circuit phenomenon, due to series inductance tuning a string in the generator output circuit will not make the amplifier load led to a total short-circuit. In practice, the regulation of matching circuits, sometimes slightly inductive load transfer was good, beneficial to the power amplifier circuit, and some very power amplifier emitter on the end of string through a relatively small inductor may also be beneficial. Also mentioned earlier, as a Class D voltage switching power amplifier, capacitive load and short circuit on the role of high harmonics, will switch at risk. But we should also note that anti-inductive load, the tube will increase the peak voltage.
4, with regard to the design of matching inductor
Matching inductor is usually the iron core coil inductor, its inductance can be calculated.
Where ω as the coil turns, Sc for the core of effective cross-sectional area (cm2); lc average magnetic circuit iron core length (cm); μe core effective permeability,
Where, μ ~ core magnetic permeability, lg magnetic gap length of the core Central Africa (cm); because lg / lc>> 1 / μ ~, it is
So
This shows that inductance L and the gap is inversely proportional to lg approximation to regulate lg gap can be adjusted LO.
Inductor design has the following steps;
(1) The core ωsc election
Where the output voltage V rms (V); f is the operating frequency (Hz); B for the core magnetic induction intensity.
The general election MXO a 2000E-core are more turns are calculated as follows;
(2) The calculation of core gap lg
(3) to determine lead
Taking into account high-order harmonics and high frequency ultrasound, cis-taking into account high-frequency current skin effect and proximity effect factors. When f> 10kHz caused by the proximity effect the exchange of resistance R ~ about its DC resistance Rd of 2-10 times that of copper consumption pr than the DC is also the same increasing in multiples of copper consumption Pro. So increase the multiple of k, then:
Pr = kPro
Therefore, in order to maintain the normal inductor coil to heat up, the current density must follow the normal allowable value of the 1/k-1 to choose.
With regard to skin effect, commonly used high-frequency current to the penetration depth B said:
Where, μ for the conductor permeability, r the wire conductivity.
In order to reduce the impact of skin effect, the selected wire diameter D must be less than twice the penetration depth B, the introduction of the flat line or high-frequency line.
In the power of ultrasound in the frequency of 15-40 kHz matching inductor wire can be used multi-strand plastic lines, the general problem.
Matching inductor continuously work for 8 hours if the temperature to normal, it indicates that the design is successful.