Control Strategy for the Discharge Phase of an Ultra-High Voltage (>7kV) Bi-Directional Flyback Converter Driving Capacitive Actuators

Dielectric Elastomer Actuators (DEAs) require ultra high voltages in the order of several kilo volts to operate. To supply such voltages, a DC-DC flyback converter topology was selected. One of the main advantage of using such a structure is the possibility to modify it to work bidirectionally. Meaning that electrical energy can not only flow from the power supply to the actuator, but also from the actuator back to the power supply. This characteristic is of significant importance because of the need to recuperate the electrical energy stored in DEAs to improve the overall efficiency of the system instead of dissipating it as heat. Indeed, due to their capacitive nature, DEAs can store an important amount of energy when fully deformed which needs to be removed to bring the actuator back to its original shape. This paper focuses on the control aspect to remove the stored energy when using a bi-directional flyback converter capable to supply a voltage of up to 7 kV to the load from a 12V power supply. After a brief presentation of the working principle of the discharge procedure, the implemented control strategy is detailed in depth with the presentation of the hardware used and the control algorithm put in place. Experimental results then show that the control strategy works well with an output voltage of up to 7 kV but is nonetheless close to its limits because of increasingly small measuring time spans going down to several micro seconds.

Ultra-High-Voltage (7-kV) Bidirectional Flyback Converter Used to Drive Capacitive Actuators

Morgan Almanza, Alexis Boegli, Raphaël Johannes Charles Mottet, Yves Perriard, Lucas Pniak

Dielectric elastomer actuators (DEAs) have extremely advantageous characteristics such as lightness, compactness, flexibility, and large displacements. However, in order to operate, they can require voltages in the order of several thousands of volts. Thus, to unleash the full potential of DEAs, it becomes essential to be able to generate and manipulate such voltages with an electronics as compact and efficient as possible. While previous works showed that it was possible to implement a system capable of supplying voltages of up to 2.5 kV and recovering part of the energy stored in DEAs (due to their capacitive nature), no work managed to go over that threshold for two main reasons: first, due to the absence of switches capable of withstanding more than 4.5 kV, and, second, due to parasitic capacitances of the flyback's coupled inductor, which steal an increasingly large part of the energy destined for the load when the output voltage is higher. This article, therefore, proposes a global design, where the factors limiting the increase in the output voltage have been mastered. Through a careful design of the coupled inductor combined with the use of MOSFETs put in series, thanks to the pulsed transformer gate drive topology to handle the high voltages, this work goes beyond the current state of the art. Indeed, here, we present various strategies undertaken, which led to the manufacture of a bidirectional flyback converter capable of amplifying an input voltage of 12 V to more than 7 kV across a capacitive load and recuperating parts of the energy stored. A preliminary study of the efficiency shows an approximate 58% efficiency during the charge phase from 0 V to 7 kV and a 54% efficiency during the discharge phase.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021

High voltage switch for compact bidirectional dc-dc converter driving dielectric electroactive polymer actuators

Morgan Almanza, Yoan René Cyrille Civet, Raphaël Johannes Charles Mottet, Yves Perriard, Lucas Pniak

Driving DEAs requires time-varying high voltage power supplies, up to 18kV for 200um polymer layers. HV amplifiers are commonly used but bulky and expensive. Designing embedded actuators requires compact and efficient powersupplies. Furthermore, only a little part of the electrical energy delivered to the DEA is converted to mechanical energy. The major part is stored in the capacitance of the actuator and needs to be recollected to maximise the system's efficiency. Low cost integrated bidirectional DC-DC converters (ex. Flyback) have been proven reliable to drive DEAs at voltages up to 2,5kV. To design a reversible converter with an output voltage up to 18kV, a high voltage switch is needed. Yet, the maximum breakdown voltage for a MOSFET with current ratings inferior to 200mA is 4,5kV. Our simulation and experimental work demonstrate a reliable driving circuit for triggering two series connected MOSFETs. A wide voltage switching range is achieved (0 to 8kV) with a 200mA current rating. Using the Transformer Gate Driver topology, galvanic insulation and synchronisation of each gate driver is ensured. To gain an equal distribution of the bus voltage over the MOSFETs, mastering the parasitic capacitances is mandatory. A design method has been developed to compensate these parasites and ensure the reliability of the device. The architecture of the high voltage switch is N-scalable and a switching range from 0 to 18kV could be achieved when stacking five MOSFETs in series.

2019

Multi-level high-voltage power supply for dea application

Morgan Almanza, Yoan René Cyrille Civet, Lucas Depreux, Yves Perriard, Lucas Pniak

In order to reap the benefits of Dielectric Elastomer Actuator (DEA) high energy density, the electronics powering the actuator should be as small as possible. Knowing that only a small fraction of the energy delivered to DEAs is effectively converted into mechanical energy, the power supply has to be highly efficient and bidirectional in order to recover the unused energy, which requires more volume. Furthermore, DEA applications call for high voltage (up to 18kV for a 200um film), but as the voltage increases, so does the volume. In addition, there are no MOS transistors rated above 4.5kV, which limits our options. Since these goals are contradictory, we need to demonstrate the feasibility of DEA integration and find its limits. Therefore we present a prototype of Multi-Level Converter designed for DEA, which is a bidirectional structure that has proven to be especially effective in the power electronics field. A Multi-Level Converter overcomes the voltage limitation by equally distributing the desired voltage among levels; because the voltage of each level is sufficiently low, smaller available parts can now be used. Our prototype includes 20 levels of 1kV each. A numerical study of this topology has demonstrated that the efficiency is above 90%. Upcoming experimental works will discuss the advantages and the drawbacks of such Multi-Level Converters. Finally, the volume of the DEA will be compared with the volume of electronics.

2019