High voltage DC-DC converter for a pulsed plasma thruster of a nano-satellite
Date of Issue2017-10-10
School of Electrical and Electronic Engineering
Satellite Engineering Centre
In nano-satellites, the pulsed plasma thruster (PPT) is a suitable electric propulsion system for orbit maintenance or attitude control due to its advantages of low mass (hundreds of grams), low power consumption (tens of Watts) and medium impulse bit (tens of μNs). The operating concept of PPT requires an igniter circuit to trigger the plasma generation, and an energy storage capacitor to ionize the propellant into plasma. In this thesis, a flyback circuit is first proposed for a PPT igniter. When compared with other topologies, the proposed circuit utilizes parasitic capacitances instead of physical high voltage capacitor. Consequently, the reliability is improved by 13.4% since capacitors are fragile in power electronic systems. With a reduced number of components, the proposed circuit suits well to the size constraint of a nano-satellite. However, the flyback circuit uses a hard-switching technique. By its nature, it has the shortcomings of switching noise and switching loss. The second topic of this thesis is to investigate an alternative soft-switching technique. This leads to a new quasi-resonant converter proposed for a PPT igniter. It consists of a resonant tank, a step-up transformer and a Cockcroft-Walton voltage multiplier. Inherent from the parallel resonant tank and series-parallel resonant tank, the proposed resonant tank has a boost capability, and the output voltage can be regulated at light loads. Unlike other resonant tanks, the proposed resonant tank can be driven by a single power switch. Thus, the switching control can be realized without a dead-time. In the quasi-resonant operation, the series resonant inductor operates in the discontinuous-current-mode. Hence, the switch can be turned on and off with zero-current to realize zero-current-switching (ZCS). This reduces switching noise, switching loss and voltage spikes. Compared to the resonant topologies using a switch bridge, the proposed quasi-resonant converter can realize ZCS at a switching frequency higher than half of the resonant frequency. This leads to a better voltage boost capability. For example, the voltage gain of a parallel resonant converter is less than 1.33, while that of the proposed topology can be higher than 6.2. Due to a single switch implementation of and ZCS operation, the efficiency has been best improved by 12.05% as compared to an LLC resonant converter. The third topic of this thesis investigates a quasi-resonant flyback converter for charging the capacitor in a PPT. By the quasi-resonant operation, the energy in the leakage inductance of transformer can be recycled to reduce energy loss. Moreover, the voltage spikes are relieved by the resonant capacitor when the switch is turned off. Depending on the operating conditions, the switch in quasi-resonant flyback converter is proposed to operate with zero-voltage-switching (ZVS) or hybrid switching throughout the capacitor charging process. In the hybrid switching mode, valley-voltage-switching is adopted in the initial charging cycles before zero-voltage turn-on is obtained, and ZVS is used in the remaining charging process. Due to the quasi-resonant operation, both the switching control methods can reduce switching loss. However, hybrid switching is preferred in terms of efficiency because of shorter charging time and less conduction loss. Compared to a conventional flyback converter using a hard-switching technique, the capacitor charging efficiency has been best improved by 10.3%.