High efficiency control methods for class-E resonant converter for step-down applications using Piezoelectric Transformers (PT)
An array of solutions to handle the losses in switching mode power supplies such as zero-voltages-witching (ZVS), or as zero-current-switching (ZCS), have been developed and applied. There is still a limitation to eliminate the losses in the switching devices to be near to zero. The loss characteristics of a switching IGBT, such as turn-off switching loss due to the tail current under either ZVS or ZCS condition is defined as an important factor that limits the efficiency of the converter, especially in small size power supplies used at high operation switching frequency. In this work the dissipative energy of a resonant converter using PT is not reduced only by ZVS condition, but also by non-switched intervals during the operation, called burst mode. The system is classified to represent two cases of output load, called full load and light load. In the light load situation, the converter can optimize the transferred energy without losing the constant output regulation behaviour, which is done by burst mode control. Different burst mode classification methods between full load and light load have been investigated for different control techniques. The frequency based burst mode classification was evaluated to be most advantageous over phase-angle or switch current time interval observation, perspectively independent of the resonant converter topology. An active burst mode technique has been derived for output voltage feed-back. The work has been proved for a Class-E topology, fulfilling the "Code of conduct on efficiency of external power supplies" in a 3 Watts off-line power supply application .