Patent & Publications

An Energy Starved Flyback (ESF) ultra low power converter for a capacitive load is presented in this paper. For high voltage low power dc-dc converter, Discontinuous Conduction Mode (DCM) flyback topology is an optimum choice for its least component count and complexity. In conventional DCM flyback converters, in each cycle, energy is stored in coupled inductor or transformer during the on-time and transferred to the output side during off-time. For a capacitive load, this energy has to be dissipated through a bleeder resistor in order to maintain a constant output voltage which calls for power loss. This steady state power loss can be decreased by reducing the duty cycle or by increasing the inductance value. The former leads to instability at very low duty cycle and the later leads to bigger component size. In ESF converter, fix turn-on time approach is used to limit the energy stored in the inductor which eliminates the need of large inductor size and reduces the power loss in converter. Fixed turn-on time with variable frequency operation is used to achieve a wide range of output voltage as well as to regulate it. To verify the practical feasibility a prototype converter to drive a piezoelectric proportional flow control valve is realized using ESF topology for space applications. Its merits over conventional flyback converter are demonstrated and evaluated based on the experimental results... Read More

A discharge supply is a high voltage DC-DC converter that processes most of the power required by a plasma thruster. It must exhibit high efficiency for maximum utilization of the electrical power available in the satellite and high reliability for long-duration operation in a harsh environment. For this purpose, the phase-shifted full-bridge zero voltage switching topology is an optimum choice for its ability to achieve soft switching operation with the least component count. However, it has some fundamental limitations such as circulating current in the freewheeling interval and severe parasitic ringing across the rectifier diodes. In this paper, a zero-voltage zero-current switching topology with an energy recovery snubber (ERS) is used for designing the discharge supply. It eliminates the circulating currents resulting in increased efficiency and alleviates the parasitic ringing, improving system reliability. The design of ERS circuit and its impact on the converter operation are investigated in detail. To verify the practical feasibility and to evaluate the experimental performance, two 2.5kW power converters are realized with a peak efficiency of 94.65% for stepping up the 70V to 350V. Both converters are used in the current sharing mode to provide the discharge power to 5kW Stationary Plasma Thruster (SPT). This SPT will cater to the requirement of an all-electric propulsion system where both high thrust (300 mN for earth-orbit-raising) and throttled down operation (north-south-station-keeping) are required. Practical design considerations along with experimental results obtained from the integrated tests with SPT are also presented. Read More

In an electric thruster, a very high specific impulse can be achieved by increasing the discharge voltage which results in extended mission life for a given amount of fuel. For this reason, the interest in electric propulsion for deep space science and exploration mission has been increased significantly in recent years. However, the maximum discharge voltage is limited by the breakdown voltage of rectifier diodes. Avalanche breakdown happens when the electric field in the depletion region becomes large enough to force the electrons to knock out the covalent bonds. Breakdown voltage is usually increased by the means of decreasing the doping but it increases the diode resistance and forward cut-in voltage. Maximum achievable breakdown voltage is limited due to thermal consideration for the given semiconductor material. As wide band-gap diodes are still not in a fully mature stage to be used in deep space application, silicon diodes are widely used which have relatively less breakdown voltage.

Because current methods for voltage scaling do not facilitate soft-switching operation, the most popular method for voltage-scaling is to series multiple soft-switching converters. This approach is not an optimum one as if any of the converters fails then all the converters in the chain are rendered useless. [ NASA's 100kW Nested Hall Thruster - X3, currently under development at the University of Michigan, is powered by seriesed ZVS converters. ]

The present invention offers a voltage scalable method while achieving zero-voltage and zero-current switching. Because there is no seriesing of the converters is used, this method provides higher redundancy which is desirable for space applications. The voltage can be scaled virtually to any value.