It is widely accepted that underwater shockwaves produced by high electric discharge can be effectively used for non-thermal food processing with minimal impact on nutritional properties and at low costs because several benefits, such as short processing time and low energy consumption, can be achieved [
1,
2,
3,
4,
5]. The underwater shockwave can be generated by utilizing gap discharge [
6,
7], wire explosion [
8,
9], wire discharge of dual electrodes [
10], or two pairs of restoration electrodes [
11]. A typical non-thermal food processing system using underwater shockwaves consists of a high-voltage generator, big capacitor, high-voltage switch, and pressure vessel [
10,
12]. Generally, a high-voltage multiplier (HVM) that can produce a high DC voltage such as 3.5 kV from a low AC voltage is utilized as the high-voltage generator. The big capacitor acts as a discharging circuit to produce a spark between two electrodes, which are placed in water stored inside the pressure vessel. The high-voltage switch is employed to control the charging/discharging status by connecting/disconnecting the big capacitor to/from the HVM. The output voltage of the HVM is charged to the big capacitor when turning off the high-voltage switch. On the other hand, in the case of turning on the high-voltage switch, the electric charge stored in the big capacitor is discharged from two electrodes to generate the shockwave in the pressure vessel. The target food is then processed by the mechanism of spall fracture [
6] or spalling destruction [
13].
The HVM, as well as its discharge technique, is obviously an important component in underwater shockwave generation [
10,
11]. For example, an HVM that can produce underwater shockwaves at high speed can provide high production capacity for non-thermal food processing [
12]. The other basic requirements for good HVMs used in non-thermal food processing are small size, light weight, and high voltage efficiency [
14]. One of the most famous switched-capacitor approaches that can usually be employed to realize HVMs is through the use of the traditional Cockcroft–Walton multiplier (CWM), which is an arrangement of capacitors and diode switches in various stages and in different capacitance values [
15,
16,
17,
18]. The key attractive feature of the half-wave CWM is that low-voltage components can be used in circuit realization because voltages across each capacitor and diode switch do not exceed two times the maximum value of the AC input voltage [
19]. This causes the size of the CWM circuit configuration to be smaller than that of a voltage multiplier utilizing high-voltage components. However, there are two major limitations in the designed circuit with the traditional CWM. The first limitation is that increasing the number of multiplier stages increases output voltage drop under load conditions because the voltage drops in all previous connecting capacitors are added up at the output terminal [
20]. The second limitation is that the operational speed of the traditional CWM is slow because it is driven by a sinusoidal waveform applied by a commercial power supply [
21]. Various design techniques to overcome these limitations have been proposed [
22,
23,
24,
25,
26,
27,
28,
29]. For example, to provide high voltage gain, transformer-less voltage multipliers made by combining the CWM with a boost-converter have been presented in 2013 [
22] and in 2015 [
23]. In addition, voltage multipliers in a bipolar topology and hybrid symmetrical topology have been introduced in 2007 [
24] and 2014 [
25], respectively, to minimize the voltage drop as well as to enhance the transient response. The bipolar voltage multiplier in [
24] is realized by using two multipliers with opposite polarities—a positive multiplier and a negative multiplier. The output of the bipolar structure is the sum of the positive multiplier output and the negative multiplier output. These positive/negative multipliers used are driven in parallel by a commercial AC voltage source. The hybrid symmetrical voltage multiplier in [
25] is realized by a cascading diode-bridge rectifier and symmetrical voltage multiplier. However, a high-voltage transformer is required to implement these two voltage multipliers. Thus, the usefulness of the bipolar and hybrid symmetrical voltage multipliers is rather limited. Alternatively, inductor-less HVMs in bipolar structure have been suggested in 2014 [
26], in 2015 [
27,
28], and in 2017 [
29]. The status of diodes placed in these bipolar HVMs is digitally controlled by rectangular pulses to produce a high DC output voltage at high speed. The bipolar HVM in [
26] consists of two switched-capacitor-based DC-DC converters in parallel, while the bipolar HVM in [
27] consists of two positive CWMs and two negative CWMs in parallel. The bipolar HVM in [
28] consists of two positive/negative CWMs in series. In order to reduce the number of circuit components, an HVM designed by combining a bipolar multiplier and the AC-AC converter has been proposed in 2017 [
29]. Nevertheless, the HVMs in [
26,
27,
28,
29] have been introduced for generating the output voltage between 3.5 to 4.0 kV by supplying 100 V and 50 Hz or 100 V and 60 Hz input. If there exists an HVM which can be applied with other AC inputs, then there are advantages of non-thermal food processing by using underwater shockwaves to be gained.
The goal of this paper is to propose a topological modification of three interesting design techniques for realizing bipolar HVMs with 220 V and 50 Hz input to produce the DC output between 3.5 and 4.0 kV, which is suitable to generate underwater shockwaves for non-thermal food processing apparatus. Three proposed HVMs, called HVM-A, HVM-B, and HVM-C, were modified from the CWM-based circuit configurations in [
10,
12,
28,
29], respectively. The design of the circuit topology of each HVM for 220 V and 50 Hz input requires considerable modification of the sub-circuits used for performing multiplication or amplification because the input signal requirement is changed. The electrical characteristics of each modified scheme can be estimated and compared from the results of theoretical analysis by using a four-terminal equivalent model. Moreover, simulation results verifying their performances are also given.