Ayumu Hatanaka

Electrostatic potential distribution characteristics of glass surfaces in vacuums

The voltage of vacuum-insulated high-voltage systems typified by X-ray tubes has recently increased. In particular, the surface discharge on the insulation material surface in a vacuum is a major problem. Recent studies show that the electrostatic charge of the insulation material surface is closely related to surface discharge. Therefore, we developed a measurement system that measures the electrostatic potential distribution of an insulation material surface with a scanning electrostatic voltmeter in a vacuum. We used this system to measure the electrostatic potential distribution of the surface of a borosilicate glass plate energized with a high voltage. A local increase in the electric field was observed. This phenomenon is caused by a positive electrostatic charge generated by a secondary emission of field emission electrons from an electrode. On the other hand, a local increase in the electric field was not observed on a glass plate coated with silica particles and a glass plate roughened by sandblast. We reasoned that this could be because the electrons were trapped by the roughness of the surface. In this paper, an outline of the measurement system, experimental results, and a discussion are reported.

Study on low-loss gate drive circuit for high efficiency server power supply using normally-off SiC-JFET

We investigated how to reduce the energy loss of server power supplies equipped with vertical-trench normally-off Silicon Carbide junction-gate field-effect transistors (SiC-JFETs). High-speed driving circuits consisting of a speed-up capacitor with separated source terminal and timing adjust circuits to ensure a dead time margin are proposed. Applying the developed normally-off SiC-JFETs and the proposed gate driver to PFC circuits and DC/AC circuits resulted in, an increase of server power supply efficiency to 95.10%.

High-Speed Drive Circuit with Separate Source Terminal for 600 V / 40 A Normally-off SiC-JFET

When using JFETs with a threshold voltage lower than 2 V in a power supply system or inverter system, a high-speed drive circuit capable of precisely controlling the gate current and a mounting method are important to reduce the switching loss. In this paper, a drive circuit of a normally-off SiC-JFET with a separate source terminal is proposed and the effects are evaluated. By dividing the common source inductance and applying the speed-up capacitor, the turn-on time and turn-on energy losses can be decreased by 40% and 60%, respectively. A speed-up capacitor larger than 100 nF greatly decreases the rising time (tr) and turn-on energy losses. By applying the developed normally-off SiC-JFETs and proposed gate driver to PFC circuits and DC/DC circuits, a highly efficient power supply will be achieved.

Gate-Drive Voltage Design for 600-V Vertical-Trench Normally-Off SiC JFETs toward 94% Efficiency Server Power Supply

A gate-drive voltage for a normally-off silicon-carbide vertical-trench junction-gate field-effect transistor (JFET) was designed for a server power supply with 94% efficiency. Since the on-state resistance of the JFET is strongly depends on the gate voltage and a large gate-leakage current between the gate electrode and source flows by applying an excessively high-gate voltage, we therefore must set an adequate turn-on gate-drive voltage to suppress the increase in power loss. The optimum gate-drive voltage design was estimated to be 2.1 V, resulting in a high efficiency of 94% even with a gate-drive voltage variation of ±0.3 V.

Characteristics of electrical charging on smooth and rough surfaces of glass plates under vacuum

To investigate the electric-charging characteristics of an insulator under vacuum, a system for measuring the potential distribution on the surface of an insulator under vacuum was developed and used to measure potential distributions on smooth and rough surfaces of several glass plates. The measured potential distributions after surface discharges indicate that compared to discharges on the rough surface, the discharges on the smooth surface tend to occur at lower positive potential. On the other hand, during micro discharge caused by partial discharge of the electrode, in the potential distributions of both the smooth and rough surfaces, a negative peak near the negative HV electrode and a positive peak near the GND electrode were observed. Moreover, the positive electric field on the rough surface is lower than that on the smooth surface during micro discharge. These potential distributions are supposed to be caused by scattering and attachment of electrons drifting on the surface. Sandblasting was applied for roughening the insulators, and it is considered to be suitable for mass production of the vacuum-insulated DC high-voltage systems because it is low cost and easy to perform.

A 160-kW high-efficiency photovoltaic inverter with paralleled SiC-MOSFET modules for large-scale solar power

To reduce the life cycle cost of solar power plants, high conversion efficiency for inverters is necessary. The advantages of SiC MOSFETs include not only lower conduction loss but also the ability of high-speed switching. Lower switching loss is derived from high-speed switching. Especially with SiC MOSFETs, the tail current and switching recovery loss can be drastically reduced compared with Si IGBTs because of unipolar device operation. However, high-speed switching with SiC MOSFETs creates technical problems such as erroneous ignition at higher dv/dt and di/dt and device failure due to surge voltage. Therefore, we focused on high-speed, stable switching techniques for SiC-MOSFET-based modules. To solve the problems, the following steps were taken. 1) A compact gate driver output stage including a Miller clamp circuit was developed and arranged on the gate and the source terminals of each module directly. 2) A low inductance busbar configuration between the P terminal and the N terminal was developed. 3) For the 4-paralleled SiC-MOSFET modules, the modules having similar Vds characteristics were selected. To confirm the contribution of these techniques to the improvement in efficiency, a 160-kW prototype photovoltaic inverter (2-level) with SiC-MOSFET modules for large-scale solar power plants was developed. A higher peak efficiency (greater than 99.1%) was obtained for an input voltage of 470 VDC, an output voltage of 300 VAC, and a load factor of 30-60%. Our findings show that inverters with a SiC-MOSFET-based module have higher efficiency than 2- and 3-level single-stage inverters with a Si-IGBT-based module.

A recovery-diode model for analyzing EMC of an on-board power supply

An analysis technique for electromagnetic compatibility with recovery-diode model is proposed. The accuracy of the recovery-diode model is evaluated by comparison with measurement. Electromagnetic noise caused by the reverse recovery of the diode was predicted by the model, and countermeasures were implemented on the basis of the models predictions.