Analysis of Load Impedance Characteristics of High Voltage Inverter

0 Introduction
At present, high-voltage inverter power supplies are being used more and more in technical fields such as ozone generators, sewage treatment, flue gas desulfurization, high-power lasers, and plasma discharge. Traditional high-voltage inverter power supplies are generally obtained by direct boosting of industrial frequency or medium-frequency transformers or LC series resonance, which inevitably has the disadvantages of large size and low efficiency. In many occasions where high-voltage power supplies are currently required, the use of high-frequency technology and soft switching technology can effectively reduce the volume, weight and loss of high-frequency power supplies, so high-frequency and high-voltage inverter power supplies are the direction of future development.
Due to the different application fields of high-voltage inverter power supplies, the impedance of the load will also be different; in addition, the load impedance will change with the change of temperature when the power supply is working, so it is of great significance to study the influence of load changes on the output parameters of the inverter power supply, and it is also the key to designing an excellent power supply. Based on the inverter power supply in practical applications, this paper analyzes the characteristics of load impedance changes.

1 Actual circuit and its working principle
The front stage of the high-voltage inverter power supply generally adopts uncontrolled rectification and full-bridge inversion, and the back stage adopts a series resonant soft switching structure. The power supply of this experiment also has this structure. Since the actual circuit requires that the load has little effect on the system when it is short-circuited, the load cannot be directly connected in series in the back stage resonant circuit, but the load is partially connected. The block diagram of this experimental circuit is shown in Figure 1.

Its specific components are as follows: 1) Voltage regulator. Input 220V industrial frequency voltage, output 0~220V adjustable; 2) Rectification: full-bridge rectification; Filtering: 4 450V/470 μ F capacitors in parallel; 3) Inversion. The full-bridge inverter circuit is composed of a full-bridge switch consisting of a control circuit, a drive circuit, a protection circuit and a MOS tube; the control circuit is composed of TL494, and the output parameters can be changed by changing the pulse width; the switch tube uses Mitsubishi's MOSFET tube, and the limit parameter is 20A/1200V; 4) High-frequency transformer. A high-frequency step-up transformer is used, with 3 strands of 50 turns on the primary side and 2 strands of 80 turns on the secondary side; 5) Resonant capacitor. It is composed of 14 22nf, /1200V capacitors to form a resonant soft switching circuit, and the designed resonant frequency of the circuit is 100KHz; 6) The load is connected in parallel with four of the capacitors, using a partial access form. The load of this circuit is replaced by a variable resistor during the test.

Working principle: After the AC power is regulated by the voltage regulator, it is rectified into DC power by the rectifier and filter circuit. The full-bridge inverter circuit converts the DC voltage into high-frequency AC power, which is then added to the load through a high-frequency step-up transformer and a series resonant circuit to make the load work. The resonant frequency of the circuit is adjusted to adjust the output current and voltage.

2 Analysis and simulation of load impedance variation characteristics
The resonant load circuit in the simplified inverter power supply system is shown in Figure 2, where L is the leakage inductance of the transformer, C1 and C2 are resonant capacitors, and Rp is the load resistance. For ease of analysis, we can equate Figure 2 to a standard series resonance form, as shown in Figure 3.

After equivalent conversion, the corresponding relationship between the parameter values ​​in the circuit is as follows:

For a full-bridge series resonant inverter, the corresponding relationship between its output current and load resistance is shown in formula (3):

Where Ude is the DC bus voltage, R is the load DC resistance, N1 and N2 are the turns of the primary and secondary sides of the transformer respectively. B is the phase difference between the fundamental component of the inverter input voltage signal and the output current.
In the series resonant circuit, the phase difference β between the output fundamental voltage and current is determined by the circuit's operating frequency f, the circuit load impedance R, the resonant inductor L and the capacitor C, and the relationship is:

In practical applications, the operating frequency of the circuit is about 110KHz. Ude and L are both known quantities. According to the working principle of the circuit analyzed above, substituting equations (1), (2), and (4) into equation (3) yields the function of the output current 10 and the load impedance Rp. When the value of Rp is between 100Ω and 10kΩ, we use Matlab to simulate the change curve of Rp and IO as shown in Figure 4.

Knowing the current of the loop, the output voltage UO across the load Rp can be obtained

Since IO is a function of the load Rp, it can be seen from formula (5) that U0 is also a function of Rp. When Rp is between 100 Ω and 10 k Ω, we use Matlab simulation to obtain the change curve of Rp and U0 as shown in Figure 5.

It can be seen from the simulation curve that when the load changes within a certain range, the current and voltage basically change linearly, and then gradually become flat, and the current and voltage characteristics are no longer affected by changes in load impedance.

The experimental high-voltage inverter power supply proposed in this paper has been applied to the power supply of the ozone generator. The discharge voltage and current are about 1600V and 4A respectively. It works in a flat area, and the voltage and current remain basically unchanged within the range of load impedance. The inverter power supply works stably and has been commercialized.

3 Conclusions
Based on an inverter power supply in actual application, this paper theoretically analyzes and derives the influence of load change of high-voltage inverter power supply on output parameters; based on theoretical analysis, the analysis results are simulated and simulation waveforms are given; finally, based on the inverter power supply in actual application, the correctness of the given theoretical analysis and simulation is proved. Therefore, the analysis results have practical guiding value for the design and parameter selection of the inverter power supply resonant circuit.