Hiroshi Tadano

Theoretical analysis of short-circuit capability of SiC power MOSFETs

The short-circuit capability of Si power devices, defined in terms of critical energy density, is the product of the heat capacity in the heat generation region and the increase in temperature. However, for SiC power devices, the heat generation region is significantly smaller than that for Si power devices, because the drift-region thickness is about 10 times less in SiC power devices. Therefore, the formulae used for Si devices are not directly applicable to SiC devices. In this study, analytical formulae are derived for the short-circuit capability of a SiC power device and its dependence on the ambient temperature and the thickness of the n− drift region, on the basis of the thermal diffusion equation. The calculated results are found to be in good agreement with those of direct measurements.

Experimental and simulation studies of neutron-induced single-event burnout in SiC power diodes

Neutron-induced single-event burnouts (SEBs) of silicon carbide (SiC) power diodes have been investigated by white neutron irradiation experiments and transient device simulations. It was confirmed that a rapid increase in lattice temperature leads to formation of crown-shaped aluminum and cracks inside the device owing to expansion stress when the maximum lattice temperature reaches the sublimation temperature. SEB device simulation indicated that the peak lattice temperature is located in the vicinity of the n?/n+ interface and anode contact, and that the positions correspond to a hammock-like electric field distribution caused by the space charge effect. Moreover, the locations of the simulated peak lattice temperature agree closely with the positions of the observed destruction traces. Furthermore, it was theoretically demonstrated that the period of temperature increase of a SiC power device is two orders of magnitude less than that of a Si power device, using a thermal diffusion equation.

Novel dead time controlled gate driver using the current sensor of SiC-MOSFET

In comparison with Si-IGBT, Silicon Carbide (SiC)-MOSFET is expected to reduce switching loss and conduction loss of low-current region, as well as to remove external Free Wheeling Diode (FWD). However, because SiC-MOSFET bodydiode has high forward voltage, the diode conduction loss increases in the period of the dead time and, as a result, its loss reduction effect by using SiC-MOSFET decreases. This work proposes a novel dead time controlled gate driver using the current sense transistor integrated with SiC-MOSFET. Proposed gate driver has a high responsiveness and a high robustness against the switching noise, the dead time can be shortened within 0.1μs without external components. In addition, it can consist with both the dead time control and the detection short-circuit current, which is the radical function in one current sensor. In the experimental validation result of 10kW boost converter with SiC-MOSFET, the efficiency with the proposed gate driver was 1% higher than the efficiency without it.

Observation and Analysis of Neutron-Induced Single-Event Burnout in Silicon Power Diodes

Annular microvoids formed by neutron-induced single-event burnout (SEB) in Si power diodes were observed by a slice-and-view technique. The axial symmetry of damage region reflects the spatially isotropic thermal diffusion that occurred. Analytical formulas for the local rise in temperature during SEB were derived from the thermal diffusion equation. The local temperature was found to increase in direct proportion to the deposited energy, which was expressed as the time integration of the product of the applied voltage and the SEB current. This current is the result of charges generated by recoil ions and subsequent current-induced avalanche. The diameter of the damage region was estimated using the analytical formulas and the energy associated with Joule heating, which was calculated by technology computer-aided design device simulations, and was found to be comparable in size to the observed annular voids. The SEB current density was also calculated based on the simulated SEB current and the size of the damage region.

Capacitor size reduction of MMC-based STATCOM for medium voltage power distribution network

This paper presents the capacitor size reduction of a MMC-based transformer-less STATCOM for medium voltage ac distribution system. The instantaneous power swinging flows into the single-phase configuration results in a high capacitance needed; however, by accepting the swinging voltage in the capacitor, the capacitance can be drastically reduced. This paper analyzes the dimensional size reduction of this method and thermal characteristics by using a meta-parameterization technique based on existing film capacitor technologies, and estimates that the dimensional size of the capacitor can be about 34% of the conventional design.

Control of DC-capacitor peak voltage in reduced capacitance single-phase STATCOM

This paper discusses the design and control of a single-phase STATCOM with reduced dc capacitance. It is first demonstrated how operation with reduced dc capacitance and large dc voltage oscillations can be possible without increasing the maximum dc voltage, as long as only operation for reactive power injection is required. Then, a method for dc peak voltage control is proposed, based on adaptive filters for dynamically detecting the average value and the amplitude of the dc voltage oscillations. It is demonstrated by simulations and experiments that the presented control strategy can be effectively utilized to regulate the peak voltage in the dc capacitor under various operating conditions.

A dead-time minimized inverter by using complementary topology and its experimental evaluation of harmonics reduction

This paper proposes a “complementary inverter”, which consists of p-channel and n-channel MOSFETs in a leg. This configuration can minimize dead time; therefore, harmonics generated by the dead time can be eliminated, without any compensation controls. Possible leg configurations and appropriate gate driver are discussed. Model for estimating dead time is shown and was confirmed by experiments with a half-bridge inverter. The minimum dead time of 130 ns was achieved in a fabricated prototype. The advantage of the minimum dead-time operation by the complementary topology was verified by applying to a 400 W grid-tied inverter with 100 kHz switching frequency and a reduced impedance of the grid connecting inductor to 1.5% of the nominal, and 2.3% current THD (total harmonic distortion) was achieved.

Loss analysis of Z-source inverter using SiC-MOSFET from the perspective of current path in the short-through mode

This paper discusses suitable short-through mode for a Z-source inverter using SiC-MOSFET to achieve high switching frequency operation. The Z-source inverter has short-through mode for boost operation and several control techniques about short-through mode have been discussed from points of device stress and boost ratio; however, loss analysis from perspective of short-through mode implementation has not been studied well, especially in the case of using SiC-MOSFET. Losses with two types of short-through mode and three switching frequencies are investigated experimentally. As a result, it was confirmed that the conduction loss and switching loss of the Z-source inverter can be reduce by employing a 3-leg short through mode.

Influence of CCM and DCM operation on converter efficiency and power density of a Single-Phase Grid-Tied Inverter

In this paper the Continuous Current Mode (CCM) and Discontinuous Current Mode (DCM) operations of a Single-Phase Grid-Tied Inverter are compared regarding efficiency and power density. Main design guidelines and component models are introduced for both operation modes. Optimal design is used to perform the comparative analysis. It was found that for range of switching frequencies from 10kHz to 100kHz, and nominal power of the inverter between 0.5kW and 2kW, the CCM operation was the most efficient design and had the highest power density. Also, it was pointed out that the main drawback for design based on DCM operation are the high peak inductor current and the required output filter capacitor. The higher peak current for DCM designs produced higher losses in all cases. Also, the output filter capacitor limited the reduction in volume for high switching frequency operation.

Z-source with rectangular wave modulation inverter for Hybrid/Electric vehicles

An improved circuit topology of Z-source inverter is presented that will meet the requirements and features of the Hybrid/Electric vehicle power control system. It is able to perform rectangular wave modulation for motor drive control depending on the drive condition of a motor which cannot be achieved from the conventional Z-source inverter.