What Is a High-Voltage Pulse Generator?

High-voltage pulse generators are widely used in insulation performance tests in meters, household appliances, low-voltage electrical appliances, electromechanical and other related industries.

1.High-voltage DC power supply

The role of the high-voltage DC power supply is to rectify the AC power input from the power grid and then invert it into high-voltage AC power, and then rectify it into high-voltage DC power. It is mainly composed of rectification, power factor correction, inverter, transformer boost and voltage doubler rectification.

2.High-voltage pulse waveform

The pulse waveforms produced by the researched and designed high-voltage pulse generator mainly include square wave, exponential decay wave and oscillating wave. These kinds of pulse waveforms have the best sterilization treatment effect with square wave and the worst oscillating wave, and the faster the front and back edges of the square wave pulse, the higher the use of pulse energy, the higher the sterilization efficiency. The exponential pulse is generally generated by a circuit composed of a resistor and a capacitor, and its structure is simple and cheap, and it has entered industrial applications. However, a voltage lower than 36.8% of the highest voltage has no sterilization effect, and will increase the temperature of food and waste energy. The square wave pulse has a good sterilization effect. Generally, it forms a pulse formation network (PFN) through a capacitor bank, an inductor, a resistor, and a discharge switch. However, compared to an exponential pulse generator, its production cost is high and the debugging is troublesome. The shock wave can not continuously apply the pulse effect to microorganisms in a high-intensity electric field, thus preventing large-scale irreversible damage to the microbial cell membrane. Therefore, it has the worst lethal effect on microorganisms in food. ^[1]

In the 1860s, high-voltage pulsed electric fields were proven to be destructive to microbial cells. Since Sale and others systematically studied the impact of pulsed electric fields on microbial inactivation, many domestic and foreign research institutions and companies have begun to focus on

Research and design of high-voltage pulse generator for key components in PEF technology. There are many methods to generate high-voltage pulse output. One of the common high-voltage pulse generators is generated by Marx circuit. Its schematic diagram is shown in the figure.

The high-voltage pulse generator mainly includes two parts: a charging circuit and a pulse forming circuit. In addition, the pulse transformer is a key component in the high-voltage and high-power pulse generator. Its high power conversion efficiency plays a decisive role in reducing the volume and weight of the pulse generator.

1.Charging circuit

At present, the more common high-voltage pulse generator charging circuits include resistance charging circuits and inductive charging circuits. The resistance charging circuit has a simple structure and mature technology, but its charging efficiency is low. It is generally suitable for small and medium power, narrow pulse width, or low operating ratios. Inductive charging circuits have high efficiency, so they are used in high power and high frequency applications. Used often. In addition, there are flyback charging circuits and step charging circuits. In actual applications, a suitable charging circuit needs to be selected according to specific requirements.

2.High-voltage pulse forming

High voltage pulse forming is the main part of high voltage pulse generator. For general exponential pulses, by controlling the conduction of the modulation switch, the energy storage capacitor can be discharged to the load through the modulation switch, so as to obtain an output pulse on the load. This method is simple and mature, but its sterilization efficiency is obviously lower than that of square wave pulse. At present, the generation of high-voltage square wave pulses generally uses a full-bridge inverter plus a pulse transformer to boost the voltage. The advantage of this pulse-shaping circuit is that it reduces the design difficulty of the primary circuit, but there are also major shortcomings, such as the oscillation of the primary will be transmitted to the secondary, which will worsen the output waveform and adjust the duty cycle When the frequency is low, the pulse transformer is large and difficult to design. With the development of high-voltage and high-current switches, high-voltage DC power supplies and high-voltage modulation switches can be used to control the switch on and off to obtain pulse output on the load.

The switch connects power MOSFETs or IGBTs in series and parallel through a simple circuit. By selecting low-inductive components and reasonable layout, the high-voltage pulse generator with adjustable pulse width and frequency range can be realized. It has a long life and is easy to maintain. The design of the control circuit for turning on and off the switching device is more complicated, and issues such as voltage sharing and current sharing synchronization need to be considered. In addition, there is also a linear pulse modulator, which uses an artificial wire (pulse forming network) as an energy storage element, and a hydrogen thyristor or thyristor SCR as a switch to achieve a full discharge pulse modulator. The artificial line is composed of a capacitor and an inductor. As the number of stages increases, the waveform of the output pulse tends to be a square wave. However, once the parameters of the artificial line are determined, the output pulse width is basically determined. Therefore, this method is not suitable for situations that require a wide range of adjustable output pulse widths. In practical applications, a suitable pulse shaping circuit is selected according to the requirements of the actual output pulse index.

3.Design of high voltage pulse transformer

In order to solve the problems of voltage level and impedance matching of modulation switching devices in high-voltage pulse generators, pulse transformers are generally used. The use of a pulse transformer will limit its maximum output pulse to the available volt-second characteristics of the pulse transformer core. In order to increase the output pulse width, a demagnetization circuit is generally added to reset the magnetic core. The high-voltage pulse generator boosted by a pulse transformer has a reduced primary circuit voltage level and reduced design difficulty. However, this structure requires that a large current must flow through the primary of the pulse transformer, and the primary current is larger when the pulse transformer has a larger boost. Therefore, in the design, trade-offs should be made according to the output voltage amplitude, power level, switching capacity of the pulse modulation switch and the requirements of the pulse parameters to determine the appropriate pulse transformer boost ratio. The leakage inductance of the pulse transformer and the distributed inductance of the loop will affect the leading and trailing edges of the output pulse. Therefore, when the leading and trailing edges of the output pulse are required or narrow pulses are required, efforts should be made to reduce the leakage inductance of the pulse transformer and properly arrange the discharge circuit. ^[2]