An overview of, matching
Ultrasonic generator and transducer matching includes two aspects, one is by matching the generator to the transducer output rated power, this is because the generator requires an optimal load to rated power output from the transducer, impedance transform into the optimum load impedance conversion effect, i.e.;
Two is the highest output efficiency of the generator through the match, this is due to electrostatic resistance transducer reasons, causing the operating frequency of the output voltage and current are in phase, so that the output power is not expected maximum output, so that the generator output efficiency is reduced, thus the generator output and or string of the last opposite of, make generator for pure resistance load, namely the tuning function.Thus matched directly affects the power ultrasonic source and efficiency.
Two, impedance matching
In order to make the power amplifier output rated power max power supply voltage; in given conditions mainly depends on the load impedance.In general D class switching type power amplifier of the generator transformer primary equivalent load on power output expression for Rl':
In the formula, VAm is the equivalent load of fundamental wave amplitude;
VCC is the power supply voltage; Vces power amplifier saturation voltage, so the
In order to ensure that the system has a certain power margin (because the output transformer, final matching circuit and the transistor loss resistance have loss, PO'needs to take the last approximately 1.4 - 1.5 of the coefficient.Output power Po 1.5Po';
From the type that, in the power supply voltage input, output depends on the size of the equivalent load RL'.Most of the current power ultrasonic generator load for piezoelectric transducer, its impedance in tens to hundreds of Ohm ohm, in order to meet the requirements of the rated power, thus requiring the transducer impedance transform load RL.High impedance transform to the low impedance.The commonly used methods, the primary and secondary coils through the output transformer turns ratio transform.Transformer primary turns ratio of N / m, then the output power of PO primary resistance
For example: a generator output on the transducer in the power of 1000W, a DC VCC for 220V, VCES=10V, power should be have a certain margin, PO=1.5PO'=1500W.Is the primary of the transformer
6.5 ohm
If the transducer resonance equivalent resistance RL = 200 Ω, then the output transformer secondary / primary cycle ratio
The above address impedance transformation, is implemented through the output transformer.
The output transformer is ultrasonic generator impedance matching, the transmission power of the important component, its design and winding technology for generator work safety is very important.It will not only to the leakage inductance, exciter current mode circuit, the leakage inductance of output voltage spikes or formed the main reason.Therefore, in the design, should be selected with a high magnetic flux density B, high permeability, high resistivity ρ C and low coercive force Hc of a highly saturated materials as the core.General in the prevention of high frequency transformer transient saturation, in the design should pay attention to the following points:
The selection of B 1 flux density
The core material of magnetic induction increment △ B is required, the number of turns of the coil DC resistance less, R also smaller, thereby coil copper loss Pm also smaller.Δ B achieved high, transmission pulse front edge is steep.Therefore, in the design of transformer, selecting high magnetic flux density of the materials as the core, which reduces the loss of transformer, reduces the volume and weight are very favorable.In order to avoid a steady or transition occurs during saturation, general selection of magnetic flux density B ≤ Bs / 3 respectively, where Bs is the largest magnetic core and magnetic flux density.
2 to ensure that the primary inductance is big enough
General requirements for transformer primary impedances of type: WLl ≥ 15RL', where RL' is the secondary load to the primary side of the equivalent resistance value, WLl as the primary inductive reactance, if the primary inductance is too small, the excitation current will be greater, excitation current is too large, the loss of the transformer will increase, temperature increase, resulting in lower Bs, so that the transformer to increase the possibility of saturation.
3 to consider "skin effect"
At high frequencies, the current flows through a wire will produce a "skin effect".This corresponds to a reduction of the wire cross-sectional area, increase the resistance of the wire, thereby causing the wire pressure drop increases, causing the temperature of the transformer, the transformer saturation increases in risk, suggested the use of small diameter strand and winding method.
Three, tuning and matching
Since the piezoelectric transducer static capacitance Co, magnetostrictive transducer has an LO, the transducer in the resonant state, the voltage and current transducer VRL IRL exists between a phase angle φ, the output power is PO = VRLIRLcos φ.Due to the presence of φ, reach the maximum output power.Only when φ =0, output power maximum.So in order to make the transducer voltage and current phase of VRL IRL (φ =0), it must be in the transducer, and or string the last phase offset resistance.For piezoelectric transducer, i.e. on or string and the last inductance L0 can be, and magnetostrictive transducer and or string of the last capacitor C0.
Piezoelectric transducer impedance or admittance equivalent circuit as shown in figure 1.52.
The equivalent circuit diagram
Type R'(f), X' (f) as the series resistance and reactance; R (f), G (f), B (f) is a parallel resistor, conductance and susceptance.They are a function of frequency.Parallel resonance and series resonance inductance by determined:
Here we compare the differences of two kinds of tuning
Figure 1-53, 1-54 is a transducer two tuning curves, by calculation,
1 because the transducer series reactance ratio parallel reactance is small, so there is a string of L
2 parallel tuned without changing the transducer parallel conductance response, and the series tuned conductance response in Shuangfeng, admittance circle diagram into two overlapping circles.
The 3 Series tuning active resistance than a parallel tuned after active resistance small, namely a series tuned for a relatively low input resistance.
In 4, a parallel tuned string from the input phase zero crossing situation, as the broadband characteristics of parallel tuned than series tuning.
5 is currently in power ultrasonic series tuned more, in addition to the above series properties, and when the transducer load short-circuit phenomenon, because the series tuned inductor in series in the generator output circuit of power amplifier, does not make the load causing complete short circuit.In the actual match regulation circuit, sometimes slightly by perceptual load as well, the power amplifier circuit, some ultimate power emitter strung on a small inductance may also have benefits.The front also mentioned, as voltage switches for class D power amplifier, a capacitive load caused the high harmonics of the short circuit effect, to switch in danger.But we should also pay attention to the perceptual load can make pipe peak inverse voltage increase.
Four, about the matching inductance design
Matching inductance is usually the iron core coil inductance, the inductance can be calculated by the following equation.
Type w as the number of turns of the coil, Sc as core effective cross-sectional area (cm2); LC core average path length (cm μ E); core permeability,
In the formula, LG μ ~ core permeability, core non magnetic gap length (CM); because the lg/lc '' 1 / μ ~, so
So
Thus the inductance L and gap LG approximately inversely proportional, adjusting the LG gap can be adjusted by LO.
Design of inductance has the following steps;
(1) by ω SC core
Type V for the effective value of output voltage (V); f (Hz; B) the working frequency of core, magnetic induction intensity.
The general election of MXO 2000E type iron core is more, the number of turns is calculated as follows;
(2) calculate the core gap LG
(3) to determine the conductor
Considering the high order harmonics and high frequency ultrasound, Shun and high frequency current proximity and skin effect factors.When f>10kHz proximity effect caused by the AC resistance of the DC resistance of Rd R~ is about 2 - 10 times, the consumption of copper PR than DC copper Pro increases the same multiples.To increase the multiple of K, then the:
Pr=kPro
Therefore, in order to maintain the coil inductance of the normal temperature, current density must be in accordance with the conventional permitted value of 1 / k-1 to select.
On skin effect of high-frequency current, the depth of penetration of the B to represent:
Type, μ for wire permeability, R conductor conductivity.
In order to reduce the influence of the skin effect, the selected wire diameter D must be less than two times the depth of penetration of B, or by using a flat line or high frequency line.
In the power ultrasonic in the frequency of 15-40 kHz matching inductance wires can be adopting a plurality of strands of plastic wire, the general problem.
Matching inductance 8 hours of continuous work if the normal temperature rise, indicates that the design is successful.