Hidemine Obara

Development of high power density flying capacitor multi-level converters with balanced capacitor voltage

Among the various multi-level topologies, flying capacitor converters are promising from the view point of the high power density because the size of the capacitors, which is a major concern, is expected to be reduced. Although the voltages of the capacitors theoretically can stay in the balanced condition, they tend to be unbalanced under some practical conditions. So far, it has been clarified quantitatively that the capacitor voltage becomes unbalance due to inequality in switching delay of power devices. In this paper, some countermeasures to reduce the unbalance in the capacitor voltages are investigated to realize simple multi-level converters. The effectiveness of the initial charging resistors of the capacitors on the voltage balance is clarified analytically. Based on the investigations, a prototype is constructed to demonstrate attainable power density of the converters with balanced capacitor voltages. Some experimental investigations confirm that high efficiency and high power density can be realized.

Flexible power flow control for next-generation multi-terminal DC power network

To realize massive integration of distributed renewable energy resources in future power system, the development of efficient measures for their integration is required. Using DC grid is more efficient and more compatible for the integration of renewable energy resources and energy storage devices. In the next-generation DC power network, many kinds of nodes which consist of generators, loads, energy storage devices, and so on and links (distribution lines) are connected. Thus, the grid structure is complex, and flexible power flow controls are essential. In a DC distribution network, the only controllable parameter is voltage of nodes, thus it is difficult to control the power flow of each link independently. In this paper, we study a power flow control on the links named link voltage control based on a bidirectional buck-boost converter implemented on the link. In addition to the voltage difference between the nodes, the link voltage controller generates additional voltage difference on the link intentionally. Thus, it is possible to control power flow of a specific link independently without affecting power flow of other links. In this way, flexible controls of power flows in a multi-terminal DC power network become possible. The effectiveness of the power flow control is demonstrated by experiments. The high controllability of the link voltage controller will contribute to the realization of future DC power grid.

A concept of multi-level converter building modules to realize higher number of output levels

Multi-level converters can essentially reduce the harmonics and EMI. To realize extremely high quality output and low EMI, the number of output levels should be high. However, as the number of levels increases, the number of components also increases and implementation will be difficult significantly. Thus, flexible building platform of the multi-level converters suitable for the higher number of the output levels should be developed. In this paper, a concept of Multi-Level converter Building Modules (MLBM) is proposed to realize the multi-level converters with a higher number of output levels systematically. A prototype of the MLBM is constructed as a 5-level flying capacitor converter with extendable configuration. It is verified experimentally that single-phase and three-phase converters with 5-17 output levels are realized by various combinations of the six MLBMs. From these results, possibility of implementation of multi-level converters with further number of output levels with proposed MLBM is shown.

Theoretical analysis of self-balancing function of capacitor voltages in flying capacitor multi-level converters

Among the various multi-level topologies, flying capacitor converters will be promising multi-level converters from the view point of the high power density because the size of the capacitors is expected to be minimized in small converters with higher switching frequency. Although the voltages of the flying capacitors theoretically can stay in the balanced condition, they tend to unbalance under some particular conditions in the practical converters. So far, the mechanism of the voltage balance of the flying capacitors associated with the circuit operation has not been clarified fully. In this paper, the self-balancing function of the capacitor voltage is analyzed quantitatively. Based on the analysis, the operating condition under which the capacitor voltages are balanced is clarified. The analytical results are useful to realize the simple flying capacitor converters without additional forced balancing control.

A study on minimum required capacitance in flying capacitor multilevel converters for grid-connected applications

Multilevel converters are expected as a fundamental solution to harmonics and EMI problems caused by power converters. In general, the multilevel converters generate the output voltage using voltages of internal capacitors or floating voltage sources. From the practical point of view, implementation of the source of multilevel voltage without increase in the volume of the converter is strongly desired. In this context, flying capacitor multilevel converters are the most promising candidates for the practical multilevel converters. In this paper, the minimum required capacitance of the flying capacitors is investigated for the case of grid-connected inverters in which the reduction of the harmonics and EMI is strongly required. The minimum required capacitance is determined considering the allowable range of voltage variation in the flying capacitors to ensure proper operation both in the steady state and transient conditions. For the determination of the minimum capacitance, theoretical expression of the voltage ripple in the flying capacitors is derived. The validity of the theoretical expression is confirmed experimentally. In addition, the transient operation under severe disturbances such as a sudden change in power flow and a short-time power line fault is confirmed experimentally.

Active gate control in half-bridge inverters using programmable gate driver ICs to improve both surge voltage and switching loss

The requirements for peripheral circuits of power converters are becoming restrictive due to the enhancement of Si power devices and the practical use of SiC and GaN devices. In the design of recent converters with high-speed switching, we must consider the stray inductances and capacitances in the device package and the gate drive circuit in addition to those in the main circuit of the power converter. In these situations, the gate driving technique is a key technology to enhance the high-speed switching ability of power devices, as there are design limitations to reduce the stray inductances and capacitances. So far, several active gate control methods have been proposed. However, most conventional active gate drivers are configured using analog circuits such as transistors and diodes. Thus, it is difficult to reconfigure their control parameters to fit the stray inductances and capacitances after the implementation of power converter and gate circuits. As a solution to these problems, we have proposed a programmable gate driver IC, which is a digitally controlled circuit. This gate driver IC can control the gate current at 63 separate levels, operated by programmable full-digital 12-bit and the clock signals. In this study, an active gate current control based on the load current in a half-bridge inverter with two programmable gate driver ICs is demonstrated. It is verified that the proposed active gate control can effectively improve the trade-off relationship between the surge voltage and switching loss of the PWM half-bridge inverter circuit.

A realization of high-performance motion control systems by applying multi-level converters

In the field of motion control systems, voltage ripple, harmonics, and electromagnetic interferences (EMI) in an output of converters negatively affect the control performance. However, so far, there has been practically few works that explicitly consider the power converters in motion control systems. Currently, general 2-level converters are widely used. However, it is concerned that the 2-level converter becomes one of the obstacles to realize high-performance control because the output voltage has high harmonics and EMI. Those possibly degrade the position or force control performance. As the solution to this problem, linear amplifiers may be useful because they outputs continuous voltage without pulsed waveform. However, low efficiency of the linear amplifiers becomes critical issue as the power converter. In this paper, performance improvement of the motion control systems by applying multilevel converters is investigated as a solution to realize both high control performance and high efficiency at a higher level. From the results of some experiments, it is seen that current waveform is distorted in the case of using the 2-level converter due to the output voltage ripple of the converter. In addition, the operation of disturbance observer (DOB) causes current ripple with lower frequency. On the other hand, it is confirmed that 9-level converter reduces the ripples of the current effectively. Moreover, EMI which is major concern in high-performance control system such as extremely small motor applications for high-precision control, can be improved by using the multi-level converter even without any filters.

Power electronics 2.0: IoT-connected and Al-controlled power electronics operating optimally for each user

The emerging trend of internet of things (IoT) and artificial intelligence (AI) technologies will bring about a major change in power electronics and create a new generation of the power electronics (Power Electronics 2.0). To enable the IoT- and Al-assisted Power Electronics 2.0, the integration of the sensors, the programmable hardware, and VLSIs for the controller into the power devices/modules is very important. In this paper, a 6-bit programmable gate driver IC with automatic optimization of gate driving waveform for IGBT is presented as the first step toward Power Electronics 2.0. In the proposed gate driver, the 6-bit gate control signals with four 160-ns time steps are globally optimized using a simulated annealing algorithm, reducing the collector current overshoot by 37% and the switching loss by 47% at the double pulse test of 300V, 50A IGBT. The gate driver is also applied to a half-bridge inverter, where the gate driving waveform is changed depending on the load current.

Design for nonlinear current reference deadbeat control for boost converter

This paper describes a design for fast and robust nonlinear deadbeat control for boost DC-DC converters. Nonlinear current reference deadbeat control is derived based on nonlinear state equation of the converter. Load disturbance compensation is implemented thus forming a new robust nonlinear controller. Both experiments and simulations of boost converter with input 12 V, output 20 V, load 4 Ω and 100 kHz sampling frequency, confirmed the voltage command tracking capability 266 µs settling time, and also disturbance rejection 1.20 ms recovery time. The method is applicable to boost converters of various applications.

Experimental verification of “hardware fail-safe chopper” for electrical wheelchair

The role of electrical wheelchair has been more important in recent aging society. However, currently used electrical wheelchair is not always better solution considering human safety. For example, when the electrical wheelchair contacts a large step, it is dangerous to roll over the step with an ordinary speed. In addition, a human will be exposed to risk when a controller fails during step over. To solve these problems, we propose hardware fail-safe chopper to realize safe and secure electrical wheelchairs. In the proposed system, a wheelchair stops in front of a large step temporarily and the energy to climb the step is charged from battery to supercapacitor. Then, a large torque can be generated in the wheel with motor to achieve step-climbing by discharging the stored energy of the supercapacitor in short time. Therefore, the proposed chopper circuit operates as a buck converter on a flat ground and as a boost converter which is supplied the energy from the supercapacitor in the case of the step-climbing. An absolutely fail-safe operation can be provided by the hardware-based restriction of the motor torque. A validity of the proposed system is confirmed by the simulation and experiments.