Minimization of the dc capacitor is an essential step towards developing and manufacturing compact low-cost hybrid electric vehicle (HEV) converter/inverter systems for high temperature operation, long life and high reliability. Traditionally, the dc capacitance has been determined according to empirical equations and computer simulations, which provides little insights into how to minimize the dc capacitor. In order to achieve an optimum minimization of the dc capacitor, an accurate theory to calculate the dc capacitor voltage ripple and current ripples must be developed first, then pulse-width modulation (PWM) and control techniques can be further developed to minimize both dc voltage and current ripples. This thesis presents an accurate theory of calculating the dc link capacitor voltage ripples and current ripples for inverters and PWM rectifiers, which are most commonly exist in HEV converter/inverter systems. The results are analyzed and summarized into graphs according to the theory, which helps to find the right capacitance value for a given voltage ripple tolerance and the rms ripple current that the capacitor has to absorb. A 150 kW inverter prototype has been built to verify the theory. The comparison between the calculation result and experimental result shows that they are in close agreement. Therefore, these theoretical calculation results help to minimize the dc capacitors rather than merely relying on simulation methods.
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