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It is known that piezoelectric transformers have several inherent advantages compared with conventional electromagnetic transformers. However, the maximum power capacity of piezoelectric transformers is not as large as electromagnetic transformers in practice, especially in the case of high output current. The theoretical power density of piezoelectric transformers calculated by stress boundary can reach 330 W/cm(3), but no piezoelectric transformer has ever reached such a high power density in practice. The power density of piezoelectric transformers is limited to 33 W/cm(3) in practical applications. The underlying reason is that the maximum passing current of the piezoelectric material (mechanical current) is limited by the temperature rise caused by heat generation. To increase this current and the power capacity, we proposed to add a thermal pad to the piezoelectric transformer to dissipate heat. The experimental results showed that the proposed techniques can increase by 3 times the output current of the piezoelectric transformer. A theoretical-phenomenological model which explains the relationship between vibration velocity and generated heat is also established to verify the experimental results.

The objective of this study was to increase the output current and power in a piezoelectric transformer (PT)-based DC/DC converter by using a cooling system. It is known that the output current of a PT is limited by temperature build-up because of losses, especially when driving at high vibration velocity. Although connecting different inductive circuits at the PT secondary terminal can increase the output current, the root cause of the temperature build-up problem has not yet been solved. This paper presents a study of a PT with cooling system in a DC/DC converter with a commonly used full-bridge rectifier and current-doubler rectifier. The advantages and disadvantages of the proposed technique were investigated. A theoretical?phenomenological model was developed to explain the relationship between the losses and the temperature rise. It will be shown that the vibration velocity as well as heat generation increases the losses. In our design, the maximum output current capacity can increase by 100% when the temperature of operation of the PT is kept below 55?? C. The study comprises a theoretical part and experimental proof-of-concept demonstration of the proposed design method.

In this paper a novel strategy to increase the power of piezoelectric transformer (PT) by using a cooling system is presented. A thermal pad is directly attached to a PT to dissipate the heat and enhance the power capacity. In a piezoelectric ceramic the heat increase quickly when the vibration velocity become too large. Therefore there is a limit in vibration velocity magnitude. To explain the relationship between the vibration velocity, the output current and the output power we propose a theoretical-phenomenological model based on nonlinear equivalent circuit. It will be shown that the vibration velocity and temperature influence the characteristics of the piezoelectric transformer significantly. A large vibration velocity may lead that the internally thermo-physical feedback loop of the PT enters into an unstable state. In this paper, a PT with the cooling system was implemented in a DC/DC converter. A zero voltage switching (ZVS) half-bridge is used to drive the PT and a full-wave rectifier is used to obtain the DC load voltage. As a result, the maximum power of the PT based DC/DC converter can be increased from 4.41 W to 10.2 W at specific temperature. The study comprises of a theoretical part and experimental proof-of-concept demonstration of the proposed system.

The purpose of this paper was to find the optimal parameters of moving multiple needle corona discharges by looking at the transport velocity and the distance of the needles. The parameters were then used to symmetrically arrange the discharge needles for large-scale mass production. A corona discharge method is typically used to perform charge injection in the creation of electret films. A static single needle corona discharge method typically leads to a non-uniform space charge distribution in electret films. Since obtaining uniformly injected charge distribution and uniform efficiency of large-area electret films have become important research topics recently, methods such as moving multiple needle corona discharges have been developed to improve production efficiency and uniformity in the spatial charge distribution of electret films for industrial applications. An automated control system was developed using VB (Visual Basic) to create a virtual instrument to perform the corona discharges and to measure the surface potential. Several important processing parameters were identified to enable us to obtain uniform spatial charge distribution in newly created electret films.

Corona discharge plays an important role in industrial and commercial applications, especially for large-area corona discharge need. It is a common method for the electrets industry. Nevertheless, achieving uniform charge distribution of the large area electret films is difficult. In order to achieve uniform surface charges distribution in large plane, the multi-pin-grid-plate electrode system which was powered from two continuously-adjustable DC high negative voltage suppliers (one for the multi-pins, the other for the grid) was employed. In this paper, electrostatic potential was measured by a large-scale automatic-controlled XY table which could efficiently measure over 30 cm × 30 cm electret materials. First of all, one pin experiment in large plane was established to find out the optimal charge range and surface potential with the temperature and relative humidity fixed. The set of associated parameters for the single pin system was identified with respect to a set of different spacing between the discharging pin and the sample. Then, the spacing that can has strong influence on the electric field applied to the film became the only parameter monitored. The surface potential measured was around 30 cm × 30 cm, which was limited by the operating ranges of the automatic-controlled XY table used. The aim of this paper is to establish the optimal distance between the multi-pins. In addition, the grid device was added to ensure uniform charge distribution within the samples.