# Cancer Treatment: An Overview of Pulsed Electric Field Utilization and Generation
## 3 Solid-State Marx PGs
### Modular PGs  
1. Modular Multilevel Converter-Based PGs  
Modular Multilevel Converters (MMCs) have been utilized for many different applications due to their modularity and scalability as well as the variety of available submodules which present the basic building blocks of the converter. A single leg of MMC, shown in Figure 18, is composed of two identical arms; each arm comprises an inductor and a series of submodules in cascading connection. Two different submodule configurations, namely, half-bridge (HB) and full-bridge, are shown in Figure 19. The most utilized submodule is HB, which is due to the lower number of switches and simple control compared to other submodules [94]. By defining the modulating reference, the output of the converter can be a pulsed shape instead of a staircase modified sine wave [29]. The output of the generator varies based on the load connection. For a load connected between the leg midpoint and the input capacitors midpoint, as in Figure 18a, the shape is bipolar; otherwise, for the connection between the leg midpoint and leg lower voltage point the output is unipolar, as in Figure 18b. Balancing the capacitor submodules is one of the major challenges facing MMC; in most cases, voltage sensors are needed to sort and charge the capacitors in order to maintain balance [95]. In contrast to Marx PGs, MMC PGs require an input source voltage equal to or greater than the pulse peak value. In [96; 97], the capacitor charge balance is maintained  
Figure 16: Capacitor diode voltage multiplier module. Figure 17: Power electronics-based Marx generator. by utilizing a diode between each two adjacent HB submodules without the need for voltage or current sensors. In addition, by controlling the submodule switching sequence, the pulse plateau peak can be reached in one step (rectangular shape) or several steps (ramp shape) to meet the load requirements and control rise/fall time. In [98], the authors presented a balancing algorithm and utilized the phase-disposition PWM technique to keep the balance and mitigate the need for additional diodes in order to reduce the system cost and complexity. The peak delivered pulse voltage for the previous method is half the input voltage. In [99], a two-legged MMC-based PG comprising four arms each was presented with the arms in an H-bridge shape. This converter has the ability to generate both unipolar and bipolar pulses with full-input DC voltage. The charging state is achieved by inserting an arm and bypassing the second arm of the same leg while the other arms are in idle mode. For pulse delivery, an arm is inserted and the same arm of the second leg is bypassed while the others are in idle state. Adopting the same hardware, [100] presented a modified and enhanced control algorithm for the PG to generate various pulse shapes (i.e., the ramp wave mimics exponential decaying, multipulse, and rectangular shapes) based on the flexibility of the MMC-based converter to deliver a multilevel output waveform. Sensing hardware and a sorting algorithm are utilized to maintain the voltage of each capacitor at the targeted value. To attain a rectangular shape, the generator operates in sensorless mode. Another enhancement by the same group was published in [101], in which submodules of the lower arms were replaced with a series of connected switches while manipulating zero current and voltage transitions to mitigate induced electromagnetic interference (EMI) and reduce the stress on the switches. The inserted modifications reduced the number of switches by a quarter, and that of the capacitors and voltage sensors by half as compared to the original state. Even with these modifications, the converter retained the ability to generate various pulse shapes using the presented control algorithm. To overcome the  
Figure 18: Single leg MMC-based PG showing two different load connection states: (**a**) for a bipolar pulse and (**b**) for a unipolar pulse.