Research on Corona Treatment Power Supply Based on PDM Control

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Research on Corona Treatment Power Supply Based on PDM Control [Copy link]

The series resonant inverter corona treatment power supply based on pulse density modulation (Pulse Density Modulation) control strategy is studied. The basic idea of PDM is to generate a square wave voltage or a zero-level voltage at the AC output end within a certain power modulation period, so that the power output can be controlled within a wide range even if the corona discharge load has a strong nonlinearity. Simulation and experiments were carried out on the corona treatment power supply, and the results verified the theoretical analysis.

Keywords: corona treatment power supply; power control; pulse density modulation

1 Overview

Compared with traditional packaging materials (such as paper, glass, and metal) [1], plastics have the advantages of being light, moisture-proof, corrosion-resistant, inexpensive, and easy to shape. The principle of corona treatment on the surface of plastic film is to apply a high-frequency, high-voltage power supply to the electrode (for the surface treatment of plastic film, the voltage is generally between 10kV and 13kV, and the frequency is around 10kHz to 30kHz) to discharge the electrode. The various energy particles (such as positive and negative ions, electrons, photons, etc.) generated after the gas is ionized are accelerated to impact the polymer surface between the electrodes under the action of a strong electric field, causing the chemical bonds connecting the surface molecules to break and dissolve, thereby increasing the surface roughness.

_{The physical structure and equivalent} circuit of the corona discharge load are shown in Figure 1. When the applied voltage across the corona load is lower than the gas discharge starting voltage _Vs , no discharge occurs in the discharge channel. At this time, the corona load can be equivalent to the gap capacitance Cg and _the insulating medium capacitance Cd of the discharge channel in series.

(a) Corona treatment discharge electrode structure

(b) Corona treatment discharge load equivalent circuit

Figure 1 Corona treatment discharge electrode structure and equivalent circuit

_{When the} applied voltage is higher than Vs , the discharge channel begins to discharge, _and the insulating dielectric capacitance Cd remains basically unchanged, but _the total equivalent capacitance Cz of the load gradually increases with the increase of the applied voltage. Its equivalent circuit is shown in Figure 1 (b). The resistance R is equivalent to the energy consumption during discharge [2].

The purpose of corona discharge treatment is to increase the adhesion of the plastic surface. In general, the corona discharge treatment process requires a specially designed power supply that can provide a voltage of 10 to 20 kV, 20 to 50 kHz, and maintain a stable discharge under atmospheric pressure [3]. In industrial applications, the power supply is also required to be able to treat different materials and materials of different thicknesses accordingly. This requires the power supply to have the ability to adjust power over a wide range. The pulse density modulation (PDM) control strategy can meet the above requirements.

2 Basic principles of PDM control

For the sake of simplicity, the step-up transformer and the corona discharge load are represented by a simple LCR resonant circuit, as shown in Figure 2. Figure 3 shows the switch operation mode of the voltage-type series resonant inverter PDM. The traditional voltage-type inverter works alternately between mode 1 and mode 2, thereby generating a square wave AC state. In addition to mode 1 and mode 2, the PDM inverter also has mode 3, which is to provide the gate drive signal to S3 _and S4 _, so that the anti-parallel diodes of one IGBT and another IGBT are turned on, providing a bidirectional flow path for the output current, so that the output end produces a zero voltage state. In this way, the PDM modulates the output voltage with a certain control sequence and synchronizes with the resonant current of the resonant load.

Figure 2 Equivalent main circuit

(a) Mode 1

(b) Mode 2

(c) Mode 3

Figure 3 Switching mode in PDM inverter

　

3 Implementation of PDM control system

FIG4 shows a control block diagram of a PDM inverter. The control circuit is divided into two parts: one is a phase-locked control circuit in PDM, and the other is a PDM feedback control circuit. The phase-locked circuit in PDM control includes a phase detector (PD), a low-frequency filter (LPF ₁ ), a voltage-controlled oscillator (VCO), an analog switch (AS), and a peak detector (PCD). Because it is impossible to accurately detect a current with a very small value, the traditional PLL circuit cannot work properly during mild surface treatment. The combination of AS and PCD can solve this problem. The output signal detected from the PCD turns the AS on or off at the input of LPF _1. When the peak value of the output current is greater than the preset level, the AS remains off, thus being a traditional phase-locked circuit. When the peak current is less than the preset current, the AS is turned on. In this case, the capacitor in LPF _{1 causes the VCO to operate at the same frequency as before the AS is turned on.}

Figure 4 PDM control system block diagram

The PDM feedback circuit includes a comparison circuit, a synchronization circuit and a low-pass filter circuit (LPF ₂ ). The control signal waveform is shown in Figure 5. The reference voltage V ^* of the average output voltage controls the width of the zero level. The actual average voltage _Vo is the actual AC voltage state M obtained by LPF _{2. The comparison between} V ^* and Vo generates an AC voltage state reference, which will select the mode in which the PDM inverter operates, either a square wave AC state or a zero voltage state in a sub-resonance sequence. The synchronization circuit including a D-type flip-flop will prevent the current state from changing during the resonant cycle. The output signal of the VCO and the AC state reference _M * ^are input into the D-type flip-flop as a clock signal and a data signal respectively. The synchronization circuit reads M ^* at each rising edge of the clock signal, that is, the output of the VCO , and maintains the logic signal M in the next sub-resonance cycle. The logic circuit generates alternating modes 1 and 2 in the AC square wave state M=1, and generates a third mode when M =0. The signals A, B and their inverted signals obtained through logic operation are input to the dead zone circuit, thereby generating a 2μs dead zone to avoid short circuit of the DC input.

Figure 5 PDM control system waveform

4 Simulation and Experimental Results

After PSPICE simulation of the PDM corona treatment power supply, a 20kHz/5kW corona discharge power supply device was designed and completed. Based on this device, the system was experimentally studied. Figures 6 and 7 are the simulation and experimental results, respectively. The two figures show the output voltage waveform of the series resonant inverter and the output current waveform on the load. In the experiment, a current transformer is used to detect the current signal. It can be seen from the results that in the first half of the power regulation cycle, the load current has basically reached a steady state, and the voltage and current are basically in phase. After that, the power switch enters the third working mode. At this time, the output is zero level and the output current is a decaying damped oscillation. Under the excitation of the output power supply in the next power regulation cycle, the current continues to increase, and this cycle repeats.

Figure 6 System simulation waveform

Figure 7 System experimental waveform

5 Conclusion

The PDM inverter works well in three switching modes and plays the role of phase locking, switch softening and power regulation. The successfully developed PDM corona treatment power supply works stably and reliably, successfully meeting the requirements of strong or weak treatment of different films.

　

About the Author

Lu Taotao (1979-), male, is a master's student at the Institute of Power Electronics, Zhejiang University. His main research direction is the industrial application of resonant power supplies.

Liu Yong (1975-), male, is a doctoral student in power electronics and power transmission at Zhejiang University. His research direction is the industrial application of high-frequency and high-voltage generators.

Zhang Zhongchao (1942-), male, doctoral supervisor, his main research direction is power electronics technology and its applications.