Spectral characteristics of resonant link inverters

The limitations on the operating frequency of hard-switched and resonant-link inverters are studied. The maximum operating frequency of a hard-switched inverter is found to be well defined by the allowed power dissipation and the switching losses. For resonant link inverters the upper limit is not so well defined and depends on a trade-off between a number of different factors. For a system using high-power bipolar transistors is it seen that a hard switched inverter is limited to switching frequencies of below 2 kHz, while a comparable resonant link inverter is restricted to frequencies below 20 kHz. The frequency spectrum of the resonant link configuration has been compared with pulse width modulation (PWM). It was shown that the improvement in harmonic distortion associated with a delta-modulated resonant link is not as great as could be expected from the increase in operating frequency that may be obtained. This is a result of constraints placed on switching instants and the need to use integral pulse modulation. The simulations presented show that high-frequency resonant link systems generate more low-frequency components than those using low-frequency PWM