Inductorless bi-directional piezoelectric transformerbased converters: Design and control considerations

Piezoelectric transformers were introduced to the world in 1954 and turned into the best alternative for replacing the magnetic transformers. Recently, the development of research on piezoelectric-based switch-mode power supplies has gathered pace and led to extensive research development. However, this brings an open area for conducting further research which has been subject of this project.

The research on this type of power converters are progressive but still very new in the technology to become a successful commercial product. The unique characteristics of piezoelectric transformers i.e. low electromagnetic interference, compact, light, high power density and low cost allows for promising market in the near future. The piezoelectric transformer technology has the potential to be used in various applications e.g. motor driver for magnetic resonance imaging scans, the electronic ballast for fluorescent lamps, backlight for LCD displays in notebook computers. Piezoelectric ceramic devices vibrate at their mechanical resonance.

The operating principle of the piezoelectric transformers is based on electromechanical energy conversion. There is electromechanical coupling between the primary- and secondaryside of piezeo ceramic, where the primary acts as a piezoelectric actuator and the secondary acts as a piezoelectric transducer.

Therefore, piezoelectric transformers can be used as a replacement of resonant circuits in the power converters. This introduces piezoelectric transformers as applicable candidates for applications that have a high sensitivity to electromagnetic interference. The nonmagnetic bidirectional piezoelectric transformer-based switch-mode power supplies as the topic of this thesis is one of these applications. The dissertation presents the design, control and implementation of inductorless switch-mode power supplies employing piezoelectric transformers.

The main focus of this research is on the functionality of the piezoelectric transformer-based power converters and applying control techniques in order to exploit advantages of the piezoelectric transformers for the power converters. Therefore, the research is devoted to stepwise development of all parts of the iductorless piezoelectric transformer-based switch-mode power supply.

The developments have been mainly on the transformer design for internal resonant current sensing, increasing their capability in transferring energy and soft switching, following changes of the piezoelectric transformer in order to control operation of the piezoelectric transformer for the benefit of the power converters, digitizing control system, applying new control techniques compared to previously applied methods, implementing dynamic optimum dead time detector applicable for switch-mode power supplies, optimum phase detector, bi-directional wide bandwidth current sensor and a comprehensive analysis of piezoelectric transformer-based switch-mode power supplies for zero-voltage switching, where all finalized with improving the unidirectional topology with resistive and passive rectifiers as well as bi-directional topology with a capacitive load. The investigation of the piezoelectric transformers in terms of sensing the resonant current and increasing their capability of handling high power was carried out in collaboration with the project’s industry partner.

New samples of piezoelectric transformers were designed, fabricated and tested. Experiments showed promising results on sensing the resonant current, but could not be used for the control system in this research since having both sensing electrode and zero-voltage switching could not be obtained in one package.

Moreover, a progress of increasing power capability of piezoelectric transformer’s was a step forward to overcome the limitations in the technology of the piezoelectric transformers. Operation of unidirectional topology deals with finding solutions for advance control of piezoelectric transformers in terms of operating under various load, frequency and temperature changes.

In order to follow changes in the characteristic of the transformers and control their operation, a digitized self-oscillating loop designed and implemented. The main advantage of the digitized self-oscillating loop is that the time delay inside the loop is able to be changed with the resolution of 1 ns. This provides the possibility of sweeping the operating point on the characteristic over the frequency range by very fine frequency steps where considerable changes in the output voltage of the transformer are visible.

This is done by shifting the gate voltages of the switches versus the resonant current. Having access to the mechanical resonance of the piezoelectric material inside the piezoelectric transformer, known as "resonant current" in its equivalent electrical circuit, simplifies the control system of the entire converter.

Therefore, attempts were made to find solutions for sensing the resonant current over the whole or part of the switching time period. The first approach was to design a piezoelectric transformer with feedback to sense the mechanical resonance. The second approach was to design a current sensor for measuring the input current to the transformer which is equal to the resonant current during the on time of the switches.

The second approach requires a wide bandwidth bi-directional current sensor to be able to operate in the presence of the high common-mode voltage. Part of this research allocates to implementation of the current sensor. A new method for optimizing the dead time in every switching cycle is proposed. The dynamic optimum dead time detector starts to detect the time point where the switching voltage reaches the rails or passes through its local maxima.

This results in the minimum or the optimum dead time between the switching transitions and further expanding the duty cycle of the switches in order to provide more energy to the converter. The advantage will be reduction of the start-up time in the converter and the switching losses. The main achievement of this research is a new implementation of bi-directional piezoelectric transformer-based switch-mode power supply with two configurations. This implementation is applicable for the control system of switch-mode power supplies, specially resonant power converters.

Finally, the outcome of the research is implemented and tested in the final prototype with combining all the designed sub-circuits to investigate the bi-directional functionality of the power supply, which resulted in a successful outcome in the control techniques.