Development of pulsed power sources using self-sustaining nonlinear transmission lines and high-voltage solid-state switches

Abstract

The demand for short pulses capable of delivering high peak power is increasing by leaps and bounds over the last few decades. These pulses are garnering attraction from a wide variety of end-users associated with the military, medical, industry, etc. The diverse nature of the potential application spaces asks for agility and customization capability of the pulses in terms of frequency, voltage level, and current level to fit varying degrees of requirements. Several pulsed power approaches can generate high-power pulses including direct switch-based pulsers, pulse forming lines (PFLs), Marx generators, nonlinear transmission lines (NLTLs), pulse transformers, etc. Among these pulsed power approaches, NLTL showcases frequency-agility in output pulses that distinguishes it from the rest of the competition. NLTLs either sharpen the input pulse or break down the input pulse into a finite number of pulses with decaying amplitudes, known as solitons. This research work combines the uniqueness of the NLTL in terms of frequency agility and the robustness of the laterally-diffused metal-oxide-semiconductor (LDMOS) devices to produce self-sustaining oscillations from a single input pulse. To date, this research work is one-of-a-kind in the field of NLTLs used for generating medium-power RF signals. Multiple NLTLs have been fabricated to identify the best available candidate showcasing excellent nonlinearity. The NLTL, coupled with a custom high voltage pulse generator, an LDMOS based amplifier, and RF matching networks constitute a closed-loop setup that produces solitons with a peak power close to 2.1 kW. The experimental results are verified by LTSpice simulations as well. Direct high-voltage (HV) switch-based pulser is another intriguing option to produce pulsed power in a compact footprint. This approach can be easily streamlined compared to other pulsed power approaches thanks to the reduced number of required components. Traditionally, spark gap switches have been predominantly utilized in direct HV switch-based pulser. High voltage withstanding capability and high current carrying capability of the spark gap switches solidified their position during the early stages of pulsed power system development. That being said, the brilliance of the spark gap switches is overshadowed by the long jitter, limited lifetime, and inability to operate at higher switching frequencies which are some of the critical modern-day pulsed power requirements. To that end, the quest for an alternative switching candidate has led the researchers to solid-state semiconductor switches (e.g., MOSFETs, IGBTs, etc.) that can be employed in pulsed power systems. The solid-state semiconductor switches are inherently faster compared to the spark gap switches, thereby enabling operation at higher switching frequency as opposed to the spark gap switches. The lifetime of these switches is also much longer compared to the spark gap switches, and they offer mostly jitter-less operation. That being said, the voltage withstanding ability of the commercial-off-the-shelf (COTS) MOSFETs is limited to 3.3 kV which is inadequate for many pulsed power applications. Connecting multiple COTS MOSFETs in series is an intriguing way to increase the volage withstanding ability of the entire switch stack. However, ensuring voltage balancing among the series-connected MOSFETs during the OFF state as well as the switching transition is a major design challenge for the researchers working in this field. The capacitive coupling method is one of the mechanisms that ensure voltage balancing among the series-connected switches while occupying a smaller area compared to other competing methods. However, literature pertaining to capacitive coupling-based series-connected switches illustrates experimental results using a HV switch with a maximum stage count of two. This research work outlines the design and development process of a capacitively coupled 6.8 kV rated four-stage SiC switch. Simulation results exhibit excellent voltage balancing traits which are reinforced by the experimental results. Modularity has been introduced in the HV switch design to facilitate the scalability of the switch in terms of voltage withstanding capability. This feature allows the user to tailor the switch in accordance with the voltage requirement of a potential pulsed power application by connecting multiple modules in series. A 10 kV rated modular SiC switch has been designed and fabricated based on the capacitive coupling method. At a high voltage level (>5.7 kV), custom gate drivers are required to provide adequate galvanic isolation. To drive the 10 kV rated switch, a 10 kV rated custom isolated gate driver has been designed and developed. The design procedure of this custom gate driver is detailed in this dissertation. The modular SiC switch coupled with the custom isolated gate driver has been tested up to a supply voltage of 6 kV exhibiting excellent voltage balancing, thereby validating the modularity concept of the HV switch.