Principle of module planning for ultrasonic cleaning machine

1 Rectification filtering and power conditioning module

The 220 V50 Hz AC power is rectified by rectifier bridge B1 and filtered by electrolytic capacitor C12 to generate a DC output voltage. Among them, the bidirectional thyristor TR1 is used for power conditioning, C11 is a safety capacitor, and R11 and C11 are primarily used to eliminate high-frequency interference. U1 is an optocoupler, and the type can be MOC3021, with pin 1 and pin 3 connected to the power adjustment module. Optocoupler U1 plays a role in blocking strong and weak electricity, enhancing the reliability and safety of the circuit.

During the operation of the ultrasonic power supply system, the rectification and filtering module and the inverter module generate heat. The two modules can be installed on an aluminum heat sink for air cooling and heat dissipation. In this way, the system can work more safely and reliably.

2 Inverter and Pulse Drive Modules

Due to the use of fewer power devices, lower cost, and relatively simple operation, the half bridge inverter circuit is chosen for the ultrasonic cleaning machine planned in this article.

In a half bridge inverter circuit, two fully controlled switching devices are IGBTs, namely Q1 and Q2, and diodes D11 and D12, forming a half bridge inverter. Complementary signals are applied to Q1 and Q2, and the two IGBTs O1 and Q2 are triggered alternately, that is, they are replaced to conduct. Together, the capacitors C1 and C2 connected to the input terminal on the DC side should be sufficiently large, and C1=C2, with a capacitance value of 2 μ F or more. Similarly, resistors R14 and R15 should also be large enough, and R14=R15, with a resistance value of 100 k Ω or higher. Fuses F11 and F12 are used to maintain switch tubes Q1 and Q2 to avoid excessive current.

Transformer T1 and resistors R16, R17, R18, and R19 form a pulse drive module that provides complementary trigger signals for Q1 and Q2. Because the driving voltage of IGBT should be less than 20 V, and the input voltage between T12 and T14 is about 12 V, the transformer T1 transformation ratio is planned to be 1:1:1. R18 and R19 are used for current limiting effect, and resistors of around 20 Ω can be selected. In this ultrasonic cleaning machine, there is a certain dead time between the upper and lower IGBT devices to prevent them from conducting together.

3 Transformers and Linear Stabilizers

The 220 V50 Hz AC voltage is reduced to 12 V by transformer T4, then rectified by rectifier bridge B4, filtered by C41, and linearly stabilized by U1 (L7812) to output 12 V DC voltage, which supplies power to the PWM generator and control module. Together, the DC 12V is further stabilized by U2 (L7805) to 5V, providing power for the operation of processor IAP15F2K61S2. LED1 is a light-emitting diode that serves as a power indicator. In order to reduce the voltage ripple coefficient, capacitors C43 and C44 were added for multiple filtering.4 PWM generation and control module, as well as drive module

In this ultrasonic cleaning machine, KA3525A is used as the PWM generator and control chip. As shown in Figure 5, the set scale of KA3525A vibration frequency is 20-40 kHz. By connecting a resistor Rd in series between pin 5 and pin 7 of the chip, the dead time can be regulated on a larger scale. The vibration frequency of KA3525A can be expressed as:

In the formula, CT and RT are the capacitance and resistance of the vibrator connected to pins 5 and 6, respectively; Rd is the discharge terminal resistor connected to pin 7. Here: Rd, CT, RT are R52 C5、（R51 Rp51）。 Among them, Rp51 is a precision adjustable resistor, which can adjust the PWM output frequency through R1 and R2. Pin 8 is connected to a capacitor C51 for soft start, reducing the startup impact of the power switch tube. The 11 and 14 pins output two complementary PWM waves, which are amplified by medium power transistors Q1, Q2, Q3, and Q4, and then driven by pulse driven transformer T1 to drive two IGBTs. The inverter module is controlled to complete the half bridge inverter (as shown in Figure 3). The high-frequency transformer T1 has the effect of blocking strong and weak electricity, enhancing the driving capability and reliability of the power supply.

5 power conditioning modules

The principle of power regulation is to detect the voltage on the power regulating resistor through an AD port of the IAP15F2K61S2 microcontroller, and then obtain the AD value through analog-to-digital conversion. Then, based on this value, the bidirectional thyristor TR1 is triggered with zero crossing delay, that is, the output power is controlled by manipulating the phase of the trigger pulse. During this process, Figure 6 shows the principle diagram of zero crossing triggering. 12V AC current is rectified by diodes D31 and D32, and limited by current and voltage of R31, R32, and R33, and then detected by transistor Q3 for zero crossing. When the grid voltage crosses zero, a negative pulse occurs at P3.3. In addition, the P3.3 port of the IAP15F2K61S2 microcontroller is an external middle break, and the zero crossing pulse is detected to obtain the zero crossing point of the power frequency voltage.

The matching of ultrasonic power supply and transducer is primarily based on tuning matching and impedance matching. In tuning matching, in order to reduce the reactive power loss caused by electrostatic impedance and maximize the output power of the piezoelectric transducer, it is necessary to match the transducer to approximate a pure resistance state and improve the output efficiency of the ultrasonic power supply. In addition, if the tuning matching is completed, that is, when the load is in a pure resistance state, in order to maximize the power output of the power supply, it is necessary to make the actual load and the optimal output impedance of the power supply equal. The completion method is to use a high-frequency transformer to change the impedance of the transducer to the optimal output impedance of the ultrasonic power supply, so that the piezoelectric transducer can output the maximum power.

Among them, Co is the static capacitance of the piezoelectric transducer, primarily generated by clamping, and it is a real electrical quantity; Ro is the dielectric loss resistance of a piezoelectric transducer, which is generally considered infinite and is generally ignored; Ld, Cd, and Rd are the dynamic inductance, dynamic capacitance, and dynamic resistance of the piezoelectric transducer, respectively. When Ld and Cd are in resonance, the series branch is purely resistive. Under the effect of series inductance tuning matching, the entire load of the ultrasonic power supply exhibits pure resistance. When the output voltage of the power supply is stable, the power obtained on the resistive load is only related to the resistance value of the load. Therefore, it is necessary to use a high-frequency transformer for impedance conversion, so that the ultrasonic power supply can output at maximum power.

Article source: Kaiping ultrasonic cleaning machine http://www.kpzfcsb.com/