Hiroyuki Handa

ZVS-PWM full-bridge converter using active current clamping with synchronous rectifiers

A ZVS-PWM full-bridge power converter using active current clamping with synchronous rectifiers is presented. A high performance DC/DC converter can be constructed using the proposed topology because of ZVS-PWM operation and convenience of utilizing synchronous rectifiers. In experiments, a high efficiency of 88% including 2.5% of improvement is achieved and the low noise characteristic is verified in a 400 W DC/DC power converter with 10 V output for high power density board-mounted power modules.

High-speed switching and current-collapse-free operation by GaN gate injection transistors with thick GaN buffer on bulk GaN substrates

GaN-based normally-off Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction are fabricated on bulk GaN substrates. Thickness of insulating GaN buffer layer is increased up to 16 μm for the presented device from 5 μm for conventional GITs on Si. The thick buffer reduces the parasitic output capacitances, which enables fast turn-off switching. The thick buffer and the use of bulk GaN substrate help to improve the crystal quality of AlGaN/GaN so that the sheet resistance is reduced. Improved crystal quality together with reduced trap density successfully suppresses the current collapse up to 1 kV or higher of the applied drain voltage. The resultant RonQoss (Ron: on-state resistance, Qoss: output charge) as a figure-of-merit for high speed turn-off switching is reduced down to 940 mΩnC that is one third from that of GITs on Si. The resultant turn-off dVds/dt reaches as large as 285 V/ns that is twice higher than reported values by GITs on Si.

GaN-based Gate Injection Transistors for power switching applications

GaN-based Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction serve normally-off operations with low on-state resistances owing to the conductivity modulation by injection of holes. Established basic technologies on the GIT have shown promising features for switching applications. Further improvement of the performances would extend the applications and lead to the widespread use. In this paper, recent technologies on the GITs to improve the performances and extract the full potential are described. These include extension of the wafer diameter of Si up to 8 inch, InAlGaN quaternary alloy to reduce the series resistances, shortening the gate length to improve the device performances, integration of the gate driver and flip-chip assembly for faster switching.

30.5 A GaN 3×3 matrix converter chipset with Drive-by-Microwave technologies

In this paper, we describe a GaN 3x3 matrix converter chipset, which are composed of a GaN integrated bidirectional switching chip and a GaN integrated gate drive transmitter chip using 5.0GHz Drive-by-Microwave technology. The extremely compact three phase AC-AC matrix converter such as a 25x18mm2 is realized by these GaN/Si integrated chips and novel isolated dividing couplers, which duplicate the gate signal with different references for dual-gate bidirectional switches and reduce gate lines and gate drive components by half. The proposed GaN 3x3 matrix converter is significantly more compact than the conventional one that requires numerous power switches, flywheel diodes, photo-couplers, isolated power supplies and gate drivers.

Vertical GaN-based power devices on bulk GaN substrates for future power switching systems

We propose a normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltage of 2.5 V and stable switching operations. The employed p-type gate does not give any concern of the gate instability. Note that formation of carbon doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. We also propose a novel vertical GaN-based junction barrier Schottky (JBS) diode with trenched p-GaN region on a bulk GaN substrate. A specific differential on-resistance of the GaN JBS diode is 0.9 mΩ·cm2 while keeping high breakdown voltage of 1.6 kV. These results indicate that the demonstrated vertical GaN devices are very promising for future high power switching applications.

A new composite substrate with high thermal conductivity for power modules

Recently, it has become more important to take the thermal dispersion of circuit boards into account. We have developed a new composite substrate with high thermal conductivity (HTC-CS) which is suitable for power modules. The main points of development of the substrate are: (1) newly developed composite materials with high thermal conductivity; (2) use of the lead frame (L/F) as a conductive layer; (3) use of thermally conductive sheets (TCSs) and realization of a simple procedure. Alumina and epoxy resin were mixed to make a slurry and were made into sheets by the doctor blade method. The sheet (TCS) was flexible while the resin was not hardened. The TCS was laid on the L/F and heated under pressure. The TCS moved into the gaps in the L/F patterns and the surface became flat; simultaneously, the resin in the TCS hardened to produce a rigid substrate. The substrate thermal conductivity was above 5 W/mK. The substrate was applied to intelligent power modules (IPM). These IPMs showed good reliability. In addition, it is simple to insert a shield layer in the substrate using the TCS procedure, and the substrate has high noise stability.