Research and design of small and medium power high frequency and high voltage power supply with capacitive load

0 Introduction
Since the ozone generator load is capacitive and the high-voltage coil of the dual-high transformer is inductive, they are connected together to form a resonant circuit. The resonant circuit has its own inherent resonant frequency. Only at the inherent resonant frequency can the ozone generator obtain the maximum energy. In addition, there is a distributed capacitance in the high-voltage coil. On the one hand, this capacitance is connected in parallel with the inherent capacitance of the load to increase the capacitance of the resonant circuit, and at the same time consumes part of the circuit energy. In addition, the electromagnetic field generated by the alternating current of the circuit is coupled to the primary of the dual-high transformer, affecting the working condition of the power switch tube. In order to meet the technical requirements of the ozone generator, some information and basic circuit principles of the switching power supply can be used to design a high-frequency high-voltage power supply. The use of excellent pulse width modulators adopts structural current sharing technology and is equipped with various perfect protection functions, so that the reliability problem of the high-frequency switching power supply has been satisfactorily solved. It meets the low ripple requirements, and is small in size, light in weight, energy-saving and environmentally friendly.

1 Technical indicators of ozone generator
The technical indicators of the surface ceramic discharge ozone generator are as follows: model FCY-3500, capacitance 120±20pF, withstand voltage>7kV, output rate 3500mg/h, working frequency 20~30kHz, power supply power 45W
. According to the load requirements, the technical indicators of the high-frequency high-voltage power supply are as follows: output power (3500mg/h) ≥60W, output high voltage Up-p ≥7.5kV, working frequency ≥20kHz, power supply voltage DC24V±2V.

2 Working principle of high-frequency and high-voltage power supply
In order to enable the power supply to drive the 45W surface ceramic discharge ozone sheet, the developed dual-high power supply is defined within the range of small and medium power, and a single-tube self-excitation circuit is selected. This circuit is simpler and easy to debug. However, the switching power tube must withstand a higher reverse breakdown voltage (BVce0) than other circuits. Its principle block diagram is shown in Figure 1.

Its working principle is as follows: the switching power circuit adopts a separately excited single-tube pulse power amplifier circuit, which can provide a sufficiently large high-frequency, high-voltage pulse current for the ozone generator, so that the ozone generator produces sufficient corona. The switching power device uses a VMOS tube. The double-high transformer regards it as an adapter between the energy source and the load; it not only transmits the energy of the power circuit to the load, but also its performance (such as distributed capacitance, leakage inductance, etc.) restricts the working condition of the power circuit. In order to solve the difficulty of making this circuit. The drive circuit uses a CMOS AND gate integrated circuit. By using the characteristics of the AND gate, we can easily control the working state of the VMOS tube. Regarding the control interface circuit, when the control level is low, the output level of the drive circuit is also zero, and the power circuit works in the cut-off state. In order to prevent false triggering, an anti-false triggering circuit should also be set in the control circuit interface to protect the circuit from working accurately and avoid damage to high-power devices due to instantaneous high-frequency interference. And an overcurrent self-protection resistor is used in the circuit to ensure the safe operation of the circuit.

3 Design and manufacturing process of high-frequency high-voltage transformer
1) Design of high-frequency high-voltage transformer
In order to make the double-high transformer meet the proposed technical indicators, the manufacturing parameters of the double-high transformer are obtained by calculating the primary winding, core cross-sectional area and winding wire cross-sectional area of ​​the transformer Transformer
primary turns:
Core cross-sectional area:
Where Se: core window area, Po: ozone generator power × 1.3, Sc: core cross-sectional area (cm2), U: DC power supply voltage (V), Bm: is the saturation flux density of the magnetic material, its value is generally 800~2 000, f: oscillation circuit output frequency (Hz) Winding wire
diameter:
Where: Id: effective value of current flowing through the transformer coil, j: current density (2~3A/mm2) Regarding the
winding wire diameter, it is only an approximate calculation here. Due to the skin effect of high-frequency current, the high-frequency current only flows on the surface of the wire. Therefore, the wire diameter calculated in the formula should be appropriately increased until the coil skeleton can be installed. Through theoretical calculation and experiment, the parameters of the double-high transformer are determined as follows: the specification is DC24V3500, the ferrite core adopts E128, the primary turns are 20 turns, the secondary turns are 1300 turns, and the skeleton structure has 6 slots.
2) Production of high-frequency high-voltage transformers
In order to reduce the distributed capacitance, the transformer skeleton coil winding is processed with 6 slots, and the transformer high-voltage coil is also wound by slot winding. The slot winding coil is about 200 turns.
In order to solve the interlayer insulation of the double-high transformer, small and medium-sized high-voltage transformers generally use vacuum immersion paint and epoxy resin encapsulation, but this process is difficult to process in small batches and is expensive. Therefore, the more mature method of immersing the high-voltage transformer in insulating oil is adopted to solve the insulation treatment of the transformer. The key to the insulation design lies in the container, which requires the container to be able to withstand the temperature of 180°C and good heat dissipation performance in constant movement and bumps. The transformer has a high-voltage output of nearly 10,000 volts, so the container is required to withstand high voltage electricity. In order to meet the requirements of transformer manufacturing, the shell of the transformer container is made of 0.2 thick stainless steel sheet. The upper cover of the shell is a printed board, and the buffer circuit, transformer lead, etc. are made on this printed board. A copper frame is reserved around it. The copper frame is connected to the shell.