Design of Ultrasonic Transmitting Circuit for Low Voltage Power Supply

Design of Ultrasonic Transmitting Circuit for Low Voltage Power Supply

The application fields of ultrasound are very broad, such as military sonar technology, industrial nondestructive testing, ranging, thickness measurement, biomedical diagnosis and surgery, ultrasound breeding in agriculture, ultrasound seedling cultivation, ultrasound induction and so on.

In the ultrasonic application field, the ultrasonic transmission circuit is a key component in the system. With the development of electronic technology and the continuous improvement of performance and accuracy requirements of measurement systems, detection instruments have developed toward high integration, high sensitivity, low power consumption, and modularity. Among them, the ultrasonic transmission circuit is a key technology that affects its performance. The main function of the ultrasonic transmission circuit is to generate different forms of ultrasonic waves to meet the actual needs. At present, there are many design methods for the ultrasonic transmission circuit, and the DC voltage of the power supply is generally high, so as to generate an ultrasonic pulse tens to hundreds of volts to stimulate the electrical signal. Using low DC voltage to generate high voltage excitation pulses can not only improve the detection sensitivity, increase the detection effective range, and improve the anti-interference ability of the detection signal, but also reduce the size and cost of the transmitting circuit and facilitate the miniaturization of the instrument.

This paper designs a simple, reliable and practical transmitting circuit based on the research of the existing ultrasonic testing transmitting circuit. The circuit is powered by a 5V low voltage power supply, and the RLC series resonance generates a high-voltage pulse signal, which meets the requirements of circuit portability and safety, improves the sensitivity and anti-interference ability of the detection device, and achieves good results.

2. The basic structure of the ultrasonic detection transmitting circuit

The basic structure of the detection circuit is shown in Figure 1, which is mainly composed of control signals, isolation links, drive circuits, RLC circuits and DC high voltage. The control signal realizes the function of pulsed ultrasonic emission control. The isolation circuit is used to prevent the transmission circuit from causing electromagnetic interference to other circuits and to prevent other circuits from being burned out. The pulse signal is generated by the high-speed shutdown of the power insulated gate field effect transistor. Driving the power insulated gate field effect transistor is equivalent to driving a network with a capacitive load. The electrical characteristics of the electronic switch have a great impact on the performance of the system when working at high frequencies. As a result, the losses caused by the charge and discharge of the insulated gate field effect transistor capacitance are very significant. In order to increase the pulse amplitude, the switching characteristics of the insulated gate field effect transistor need to be enhanced, and a reasonable drive circuit is required. The commonly used field effect transistor drive circuit has CMOS buffer There are three types of parallel drive, field effect tube to tube drive and bipolar transistor power drive [1]. The RLC circuit generates a high-frequency signal through resonance, and tunes the matching network to make the circuit work at the resonance frequency of the transducer. The DC high-voltage power supply is realized by a DC inverter or other power modules.

The working process of the general DC high-voltage pulse transmitting circuit is shown in FIG. 2. When the control level V is low and the switch Q is turned off, the capacitor C is charged. The high-voltage power supply charges the capacitor C through the drain resistor R1. Since the charging process is completed in a short time, R1 and C cannot take values. Large, and C withstands high pressure. When the control level V is a high level, it is turned on, and the capacitor C discharges through R2 and D2, generating a negative pulse voltage on the probe, and exciting to generate an ultrasonic signal [1-3].
3. Design of RLC series resonant ultrasonic transmitter circuit based on low voltage power supply

There are two options for the generation of high-voltage narrow-band pulses: the first is to use a capacitor pre-charged to high voltage to quickly discharge to the transducer; the other is to generate an instantaneous discharge from the energy storage inductor. Tests have shown that for the transducer in the device to emit ultrasonic waves, an instantaneous high-voltage pulse of 100 volts needs to be added to both ends. In the first solution, a DC high-voltage power supply of several hundred volts is required. The second scheme is to use the instantaneous discharge of the energy storage inductor to generate instantaneous high-voltage pulses, which only needs DC low-voltage power supply to meet the requirements. Based on this idea, we selected the instantaneous discharge of the energy storage inductor to generate an instantaneous high voltage, R1 was replaced with an inductor L, and the power supply was replaced with a 5V low voltage power supply. Its resonance circuit is shown in Figure 3.
This circuit uses the power switch tube Q as the switching element, and the inductor L stores energy to form a trigger pulse. It does not need to provide DC high voltage. It passes through the photocoupler as an isolator to reduce electromagnetic interference and prevent other circuits from being burned out. When the pulse input to Q is positive, Q turns on. Q is equivalent to a small resistance, which is connected in series with resistor R1 and inductor L, and forms a loop with the low-voltage power supply. The current in L rises rapidly for energy storage. When the pulse input to Q is negative, the gate of Q is set low, Q is quickly turned off, and L, C, and R2 form a resonant circuit to quickly discharge, forming a high-voltage pulse on resistor R2, which can reach a voltage of 100 volts, as shown in Figure 4 As shown, D1 and D2 function as one-way switches. The matching network is realized by an adjustable resistor and an inductor in parallel. The amplitude of the pulse is changed by adjusting the adjustable resistor in the matching network. The matching inductor is tuned to make the circuit work at the resonance frequency. After tuning and matching, the high-voltage narrow-band pulse measured on the probe is shown in Figure 5.

4. Equivalent model analysis

The conduction of the field effect tube and the shutdown correspond to the charging and discharging states of the transmitting circuit, so that the transmitting circuit repeatedly works alternately in these two modes.

Analysis of the principle of charge state: when the control signal is high level, when the switch is on, Q is regarded as an ideal switch, its on-resistance R0, low-voltage power supply V1 and L, R1, Q form a loop, L resistance is RL, and the current The current in the inductance L can be approximated by the following formula:
At this time, the energy storage in the inductor L is proportional to the square of the current at the time of switching.

Analysis of the principle of discharge state: when the control signal is low, when the switch is off, L, C, R2 form a series resonance circuit, RLC zero input response, the equivalent circuit is shown in Figure 6, the initial state is (2), and No jump occurs in the current, and equation (3) is obtained according to FIG. 6.
Formula (3) The differential equation for the discharge of an unknown RLC series circuit, the characteristic equation is
As can be seen from the current expression, the waveform will show a state of decaying oscillation during the entire process. According to formula (6), we can get: It is the extreme point of current. Based on the above analysis, it is necessary to make the circuit generate high-frequency spikes in the selection of circuit parameters Parameters, transistor turn-on and response time should be short, damping ratio As small as possible, attenuation factor To be large, the oscillation frequency To be high. When the frequency matches, the probe works in the best state [4-6].

5. Summary

The ultrasonic testing and transmitting circuit is powered by a 5V low voltage power supply. RLC series resonance, inductive energy storage generates high-frequency trigger pulse, the circuit does not require high-voltage DC power supply, you can get the ideal trigger signal. And the influence of circuit parameters on pulse signal is expounded. The designed circuit is simple, safe, and practical, which improves the sensitivity and anti-interference ability of the detection device.


[1] Feng Hongliang, Xiao Dingguo, Xu Chunguang, Zhou Shiyuan. Pulsed ultrasonic sensor excitation / reception circuit design [J]. Instrument Technology and Sensors, 2003 (121): 30-32.
[2] Song Jianping. Research on the transceiver circuit of ultrasonic flaw detector [J]. Electronic Engineer, 2004, 30 (6): 16-18

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