Control of voltage resonance type power supplies by an auxiliary switch

In variable-period output control of a voltage resonance-type converter, the minimum output power is obtained in the subclass-E mode and the maximum output power is obtained at such a duty ratio that the off-time is half the resonance period. In an example, the ration of the maximum output power to the minimum output power was 16:1. When the period was fixed, the ratio of the maximum output power to the minimum output power was only 1.36:1. If the switching time was shortened by a longer interval than the damper period, the zero-voltage resonance condition could not be satisfied and the control range was narrowed. To overcome this problem, an auxiliary switch was provided in the resonance loop. As a result, the zero-voltage switch condition was expanded to a range such as that obtained in the case of a variable period. In the minimum output condition, the subclass-E mode was obtained for an on-time shorter than that in variable-period control, and the control range was expanded to 133:1 in terms of the maximum-to-minimum output power ratio