Development of intelligent high-power ultrasonic cleaning power supply

1 Introduction

Due to the advantages of ultrasonic cleaning, such as good effect, high efficiency and low cost, ultrasonic cleaning machines are widely used in electronics, machinery, clocks, optics, medical treatment, chemical fiber, electroplating and other industries [1]. In ultrasonic cleaning equipment, ultrasonic power supply is one of its important components. Most of the existing ultrasonic power supplies use dedicated integrated control chips (such as SG3525, TL494) or single-chip microcomputers to generate PWM pulse signals. After power amplification, impedance and tuning matching, they drive the transducer to convert electrical signals into mechanical vibrations to generate ultrasonic waves. Both power supplies have their own limitations. The former control method has slow dynamic response, inconvenient parameter adjustment and serious temperature drift [2]. Although the single-chip microcomputer is used to directly generate PWM signals, it can obtain high-precision and high-stability control characteristics and realize flexible and diverse control functions. However, due to the limitation of its operating frequency, the frequency resolution of the output PWM signal is low and it is difficult to meet the frequency fine-tuning. In view of the many problems existing in the above ultrasonic power supply, this paper uses a single-chip microcomputer combined with an analog integrated circuit to form an intelligent control system based on PWM technology to develop a digital ultrasonic power supply with stable performance, simple control and adjustment and low cost.

2 Composition and principle block diagram of ultrasonic power supply

The ultrasonic power supply mainly consists of two parts: the main circuit and the control circuit. Its structural diagram is shown in Figure 1. The main circuit adopts an AC-DC-AC structure. The single-phase AC power is rectified and filtered to form DC power. The DC voltage is converted into an alternating voltage with a frequency consistent with the resonant frequency of the transducer through a full-bridge inverter. The inverter outputs an alternating current, which is then sent to the load transducer through a matching network. The control circuit mainly provides a switching pulse signal for the inverter main circuit to drive the inverter main circuit to work, and uses the feedback loop and the given circuit to achieve closed-loop control of the inverter.

Figure 1 Schematic diagram of ultrasonic power supply structure

Figure 3 Driving circuit schematic

Figure 4 Input and output timing diagram of IR21844

[page] Figure 5 shows the full-bridge power amplifier circuit of the system. Its working principle is as follows: the alternating current is rectified and filtered to become a smooth DC voltage V+. This voltage is applied to the inverter bridge composed of MOSFET power tubes Q3, Q4, Q5, and Q6. When PW1 is high and PW2 is low, HO1 and LO2 are high, and HO2 and LO1 are low (see Figure 3). At this time, Q3 and Q6 are turned on, Q4 and Q5 are turned off, and the voltage across the primary of the transformer T is U=V+, and the current flowing through the primary coil of the transformer is from top to bottom; when PW1 is low and PW2 is high, Q4 and Q5 are turned on, Q3 and Q6 are turned off, and the voltage across the primary of the transformer T is U= -V+, and the current direction of the primary coil of the transformer is from bottom to top. Repeating the above working process, a quasi-square wave signal with the same frequency as the main oscillation signal and a higher voltage amplitude can be obtained at the secondary of the output transformer. Since the power amplifier tube operates in the saturation region or cutoff region of the volt-ampere characteristic curve, the collector power consumption is reduced to a minimum, thereby improving the energy conversion efficiency of the amplifier to more than 90% [3].

Figure 5 Full-bridge power amplifier and matching circuit

3.3 Matching Circuit

In power ultrasonic equipment, the matching design between ultrasonic power supply and transducer is very important, which largely determines whether the ultrasonic equipment can work normally and efficiently. The matching between ultrasonic power supply and transducer includes two aspects: impedance matching and tuning matching. The matching circuit is shown in the dashed box in Figure 5.
Impedance matching refers to changing the resistance of the load to make it equal to the optimal load value of the ultrasonic power supply to ensure that the load obtains the maximum electrical power, while the purpose of tuning matching is to make the transducer as close to a pure resistance state as possible and reduce the reactive component. When a piezoelectric ultrasonic transducer works near its resonant frequency, it is generally capacitive due to the influence of static capacitance. If the signal source is directly connected to the transducer, a part of the reactive component will be generated, resulting in a relative reduction in the active power of the transducer [4]. Therefore, it is necessary to match the opposite inductive reactance at the output end of the ultrasonic power supply to make its load a pure resistor.

At present, the most commonly used matching circuit is the series inductor matching method shown in the dotted box in Figure 5. By reasonably selecting the value of the inductor, the transducer can achieve resonance when driven by an ultrasonic power supply.

[page] 4 Ultrasonic power supply software design

The software design mainly involves programming the microcontroller to achieve frequency setting and adjustment, LCD display and keyboard input control, while monitoring various feedback signals, adjusting the duty cycle to change the output power, and completing frequency sweep, timing, soft start functions, etc. The program flow chart is shown in Figure 6.

Figure 6 Software Flow