Ing. Pavel Skarolek, Ph.D.

Inductive power transfer systems are widely used in various applications, which increases the demand on improving their efficiency to avoid the energy waste. As a result, there is increasing research interest in the optimization methods to maximize their efficiency. While most of the existing optimization methods focus on fixed frequency systems this paper presents an optimization method for variable frequency systems. The method aims to maximize the average DC-DC efficiency across the whole load range by adjusting the coupling coil geometry and the input DC voltage. Special interest is paid to the implementation of variable frequency regulation in a microcontroller, which provides only discrete frequencies and thus may render the regulation unstable. The presented method is implemented in MATLAB using the Global Optimization Toolbox and the FEMM software package. The method is verified by a design of a Qi inspired wireless charger for a rocket. A constructed prototype provides constant output voltage of 30 V and up to 35 W of the output power. Its DC-DC efficiency has a maximum of 82.2 % and an average of 74.8 % for output power above 5 W.

In this paper, the comparison of efficiency was done by using Si and wide band gap (WBG) material for bi-directional DC/DC converter in buck mode. The simulations were done with LTSPICE software. The Hardware setup was made using silicon and WBG MOSFET’s for DC/DC converter with varying resistive load for the constant output step down voltage. Each converter is designed using all silicon, all SiC and all GaN MOSFET’s for both software and hardware designs.

The paper describes development of high-efficiency and high-power-density semiconductor converters intended for a two-stage charging station equipped with gallium nitride-based transistors. It consists of an input PWM rectifier that includes an active power factor correction (part PFC) and an output DC-DC voltage converter with an AC intermediate circuit, containing a single-phase bridge voltage resonant inverter with a series resonant LLC circuit on the primary side of the isolation transformer and a single-phase bridge synchronous rectifier on the secondary side of the transformer. The use of resonant topology significantly reduces the switching losses of transistors. The converter ensures the output voltage according to the requirements of the traction battery and the demanded galvanic isolating the feeding network from the being charged vehicle battery and other equipment. The maximum efficiency of the functional sample of the charging station is 97 % and the power density is 3.5 kW per liter.

The paper shows the designed converter output power changes with the temperature changes in case of various semiconductor switches. Mostly, wide band gap semiconductor-based switches like silicon carbide and GaN have a higher temperature capability than silicon for high power applications. 20-kW synchronous bi-directional DC/DC converters were designed for the silicon and silicon carbide switches using the LTSPICE simulations at different temperatures. The synchronous bi-directional DC/DC converters were designed for 75 kHz switching frequency having the duty cycle of 30 % to achieve the desired output power.

Power factor correction (PFC) bridgeless converter in Fig. 1 with GaN transistors achieves better efficiency and power density compared to other solutions in the field of PFC converters. Utilizing GaN transistors brings new challenges in the field of control algorithms. Mainly the high frequency operation demanding high resolution PWM generator timers and precise dead-time setting together with fast control loop calculations and feedback values measurements. This paper presents a software based solution to on-line deadtime adjustment which improves the efficiency of GaN based PFC converters.

The ever-increasing demands on the efficiency and power density of power electronics convert-ers lead to the replacement of traditional silicon-based components with new structures. One of the promising technologies represents devices based on Gallium-Nitride (GaN). Compared to silicon transistors, GaN semiconductor switches offer superior performance in high-frequency converters, since their fast switching process significantly decreases the switching losses. How-ever, when used in hard-switched converters such as voltage-source inverters (VSI) for motor control applications, GaN transistors increase the power dissipated due to the current conduc-tion. The loss increase is caused by the current-collapse phenomenon, which increases the dy-namic drain-source resistance of the device shortly after the turn-on. This disadvantage makes it hard for GaN converters to compete with other technologies in electric drives. Therefore, this paper offers a purely software-based solution to mitigate the negative consequences of the cur-rent-collapse phenomenon. The proposed method is based on the minimum pulse length opti-mization of the classical 7-segment space-vector modulation (SVM) and is verified within a field-oriented control (FOC) of a three-phase permanent magnet synchronous motor (PMSM) supplied by a two-level GaN VSI. The compensation in the control algorithm utilizes an offline measured look-up table dependent on the machine input power.

The current collapse phenomenon increases conduction losses in high electron mobility transistors. This paper presents a simple option to decrease these losses in 3 phase hard-switching inverter driving permanent magnet synchronous motor utilizing 5-segment SVPWM modulation. Converter DC-link input power decrease by 2 % was observed compared to classic 7-segment modulation. This decrease results in significant decrease of transistor operating temperature.

Current collapse in gallium nitride based transistors limit their use in high power and high frequency converters. In some cases, it makes the conduction losses to double. This paper investigates the measurement of dynamic on-state resistance for various cases such as blocking voltage and switching frequency. The optimum minimal length of pulses was determined in the case of three phase inverter to minimize the influence of current collapse.

This paper presents a comparison of three different transistor technologies Silicon Superjunction (Si SJ), Silicon Carbide (SiC) and Gallium Nitride (GaN) in respect to deadtime setting in a typical halfbridge converter. According to the measured results both new fast switching transistors SiC and GaN needs precise deadtime setting compared to the Si SJ devices. With wrong deadtime settings the converter efficiency drops more rapidly for GaN compared to SiC while the Si SJ device is the least affected by the deadtime length. The optimum deadtime in DC/DC converter can be found by tracking the maximum output voltage for given constant and compensated duty cycle and input voltage.

Gallium nitride (GaN) devices are becoming more popular in power semiconductor converters. Due to the absence of the freewheeling substrate diode, the reverse conduction region is used in GaN transistors to conduct the freewheeling current. However, the voltage drop across the device in the reverse conduction mode is relatively high, causing additional power losses. These losses can be optimized by adequately adjusting the dead-time issued by the microcontroller. The dead-time loss minimization strategies presented in the literature have the common disadvantage that either additional hardware or specific converter data are needed for their proper operation. Therefore, this paper’s motivation is to present a novel dead-time loss minimization method for GaN-based high-frequency switching converters for electric drives that does not impose additional requirements on the hardware design phase and converter data acquisition. The method is based on optimizing the current controllers’ output with a simple perturb-and-observe tracker. The experimental results show that the proposed approach can minimize the dead-time losses over the whole drive’s operating range at the cost of only a moderate increase in software complexity.

Electric vehicle equipped with power electronic system developed on the department of Electrical drives and Traction faculty of Electrical Engineering Czech Technical University in Prague is now used by students in lab classes as well as for qualification studies realizations.

Three methods of cooling gallium nitride (GaN) transistors have been tested and directly compared together. Under similar conditions, top side cooled GaN was compared to the bottom side cooled type; both were of the same electrical parameters and were mounted on FR4 (fibreglass resin) and aluminium printed circuit boards. At equal dissipated power the proposed bottom side cooled transistor on aluminium board reaches half the temperature compared to the top side cooled one and one fourth of the temperature compared to the bottom side cooled GaN on FR4 board. A High speed driver is placed close to the transistor also on the aluminium IMS (Insulated Metal Substrate) board to reduce parasitic inductance of the gate driving path from control board to the high power side on the aluminium board.

Modernization consists of a new compact drive unit for the Citröen Berlingo Electrique electric vehicle. The drive unit integrates a power converter for the traction motor, a DC/DC low voltage converter, an on board charger and an electronic control unit (ECU). The new air-cooled power converter is lightweight and more efficient due to the modern equipment used. The new ECU uses controller area network (CAN) to communicate with the power converters and also collects all the measured data that can be transferred in real-time during drive. This vehicle also currently provides a test and measurement platform for Master’s Degree students.

The presented method automatically adjusts the deadtime of gallium nitride (GaN) transistors in half-bridge to increase the efficiency. This removes the need of manual measuring and setting the deadtime of the finished converter. The developed algorithm was tested and compared with the fixed deadtime case. The obtained results show that the developed algorithm is achieving higher and more stable efficiency compared to selected fixed deadtime.

Redox flow batteries mounted on the vehicle chassis behaves better at low voltage and high current while the tractive system of electric vehicles is more efficient at high voltage and low current. The proposed DC/DC converter provides the necessary tractive system isolation from the flow battery and keeps the voltage ratio while working in two quadrants of the current flow direction. The Z-source converter topology enables the necessary wide range voltage control at high efficiency. The power density is increased using the Gallium Nitride (GaN) transistors. The tested prototype of the converter proves that the topology is suitable for this application.

This paper presents a method to obtain phase currents of the Permanent Magnet Synchronous Motor (PMSM) fed by three phase bridge converter equipped with single shunt resistor measuring the DC-link current in case the motor is driven by the Sliding Mode Control (SMC) algorithm running at high switching frequency. This method was successfully tested and the measured results are included. The main benefit of the presented design is the cost reduction of small to medium power drives utilizing the modern high efficient motor with simple converter not depending on motor parameters due to the Sliding mode control. This method brings accurate results at high frequency switching application used for motors with small phase inductance.

This paper presents a GaN transistor half-bridge prototype with robust pulse by pulse current limiting drivers designed to turn off safely the transistor for the rest of the PWM period when the drain current exceeds the set value. The half-bridge is intended as the key part of a DC/AC converter output stage with operating frequency up to 1 MHz. The current limiting circuit is designed to meet the requirements for safe operation of GaN transistors. The proposed current limiting driver is five times faster compared to common integrated drivers with included current limiting circuit.

This paper brings an idea of an application of a DC/DC converter for electric vehicle equipped with redox flow battery. The converter will increase and stabilize the voltage for the redox flow battery to be used as a replacement for today’s traction batteries. The main benefit is that the complex redox flow battery system can be mounted directly on the vehicle chassis providing power at safe low voltage which does not need to be electrically isolated from the chassis. The converter is based on Gallium Nitride (GaN) semiconductors for high power density and efficiency. The Bidirectional Quasi Z-Source converter topology had been chosen to provide the voltage regulation at the secondary side of the DC/DC converter. Schematic diagram of the proposed topology and simulation results are included. According to simulation results the proposed topology provides the necessary wide range regulation for the redox flow battery to be used as vehicle traction battery.

The goal was to design modern converter equipment for the electric vehicle bought by the department for research purposes. The new converter equipment consists of DC/DC converters for DC motor with separate excitation, on board electrical system converter and Electronic Control Unit. ARM microcontrollers had been used to control converters connected together with CAN bus. The equipment is built as a modular construc-tion to enable the possibility to be easy rebuilt for AC motor just by replacing the converter output stage and loading a new SW. Keywords: Electric vehicle;