Yu Yonezawa

2.5-kW power supply unit with semi-bridgeless PFC designed for GaN-HEMT

Wide-gap semiconductors such as SiC and GaN have seen widespread increase in interest as promising materials for use in next-generation power devices. In this paper, we present a 2.5-kW power supply unit (PSU) designed for ICT systems with engineering samples of Fujitsu Semiconductor Ltd. (FSL) s GaN-HEMTs installed in the power factor collection (PFC) circuit. A new PFC circuit specially designed for GaN-HEMT is proposed: a semi-bridgeless design with a synchronous rectification return line. The bridgeless architecture is suitable for high-power and high-efficiency PSUs, and the synchronous rectification is designed to maximize the GaN-HEMT performance. As the GaN-HEMT does not have a body diode, the return current mainly flows through the return diodes in conventional semi-bridgeless PFCs. By applying synchronous rectifiers instead of return diodes, energy loss in the return line can be reduced to about 1/10. In our experiments, a maximum PSU efficiency of 94.3% was obtained with 230-V AC input and 12-V DC output.

Digital dead-time control for two phase double-ended forward converter

Two phase double-ended forward converter is suitable for more than 2kW high efficiency power supply by the feature of low surge voltage. One issue of the converter is dead-time loss of synchronous rectifier FETs. In this paper, we propose a precise digital dead-time control method for a 2.5 kW (12 V / 208 A) output power supply. Proposed method is confirmed by experiment and theoretical analysis that the method reduces the 6 W loss at half load.

Time-Coordinated Switching Relay for Arc Discharge Suppression

We propose a new electric contact device for arc discharge suppression. The functions of conventional electric contacts are categorized into energizing switch contacts and transient current switch contacts. A capacitor is connected in series to a transient current switch. Suppression of power consumption and arc discharge at breaking contacts are proposed, experimentally measured, and theoretically analyzed. The transient V-1 characteristics at breaking contacts are controlled by the transient current switch and the capacitor. The transient responses at contacts were numerically derived by SPICE, and the energizing switch contacts voltage could be controlled to less than the minimum arc voltage. Using 2 conventional relays, no arc ignition at breaking contacts was confirmed for 50 V/25 A.

Equivalent Circuit Analysis for Time-Coordinated Non-arcing Operation of Reed Switches

We proposed a method for suppressing arc ignition in mechanical contact devices using a transient current switch and a capacitor. We applied the method to conventional reed switches. For the electric circuit analysis, we clarified the momentary voltage-current characteristics at breaking operation of reed switches by FEM analysis. We could also estimate the capacitance of the contact electrodes at the metal bridge rupture by FEM analysis, and would derive the non-arcing condition using SPICE simulation. The suitable capacitor value in the transient current circuit for arc ignition suppression would be depend on the load impedance, the power supply, the time depending contact resistance R(t)s, the contact capacitance, and the minimum arc voltage and current.

Rapid control prototyping for server power supply with high-resolution PWM

In this paper, we report the first rapid control prototyping (RCP) system with a sub-nanosecond (ns) high-resolution Pulse Width Modulation (PWM) for server power supply circuits. Conventional RCP systems are designed mainly for engine and motor control. Based on the relevant response time and control accuracy requirements, their PWM update periods and resolutions are over 100 microseconds (μs) and 100 ns, respectively. The shortest update time reported thus far is 12.5 μs. However, for server power supply, the required PWM update period and resolution are less than 10 μs and sub-ns, respectively. Therefore, we developed a new interface board with high-speed and high-resolution PWM output, which is connected between an under-development power supply board and a commercially available RCP machine to increase the RCP systems speed and resolution. Moreover, we have developed new software that improves data transfer latency and maintains a constant PWM update period. The combination of the new interface board, high-performance RCP machine, and new software realizes the required short PWM update period and high-resolution PWM of 6.5 μs and 0.15 ns, respectively. This is the shortest PWM update period reported yet, and it is adequate for operating server power supply. We applied the new RCP system to a 500 W full bridge converter with 100 kHz feedback control. Frequency response and soft start functions are easily confirmed by the new RCP system.

Analysis of Mechanism of Improvement in Highly Accelerated Lifetime via Measurement of Vanadium Valence in Multilayer Ceramic Capacitors

The valence of vanadium was measured by electron paramagnetic resonance in order to explain the decrease in insulation resistance (IR) and the improvement in highly accelerated lifetime that resulted from the addition of vanadium. V4+ was detected in specimens with vanadium contents of 0.20 and 0.30 mol %, while no V4+ was detected in a specimen with a vanadium content of 0.06 mol %. It was also revealed that the content of the vanadium except for V4+ are the main factor responsible for the decrease in IR and the improvement in lifetime. The impedance of BaTiO3-based materials in multilayer ceramic capacitors with various vanadium contents was investigated in order to determine the mechanism of improving the highly accelerated lifetime using a four resistance and capacitor section electrical equivalent circuit. All four resistance components (R components) decreased with an increase in vanadium content. During the lifetime test, all four R components were degraded. In particular, the R component corresponding to the ceramic/internal electrode interface regions was more strongly degraded than the other three R components, and it was found that this component was the main factor responsible for the degradation of IR during the test. The resistance degradation of this component tended to occur slowly when the vanadium content increased, which resulted in the improvement in lifetime. The primary part of this degradation was implied to be controlled by diffusion.

Online Evaluation Method of Electrolytic Capacitor Degradation for Digitally Controlled SMPS Failure Prediction

In this paper, we propose a new capacitor degradation evaluation method aimed at failure prediction and suitable for digitally controlled switching mode power supply (SMPS) for servers. Electrolytic capacitors have one of the highest component failure rates in SMPS; therefore, we attempt to detect an equivalent series resistance degradation of the electrolytic capacitor directly from the data fetched to a digital controller of the SMPS. With a SPICE simulation and a rapid control prototyping evaluation, we confirm the degradation can be detected by the data at a transient response under a normal operation without additional circuits. Even only 10% of a load step change, which commonly occurs in SMPS for servers, causes detectable transient response degradation.

RCP evaluation of electrolytic capacitor degradation for SMPS failure prediction

This paper presents a new cost effective method for a failure prediction of a digitally controlled Switching Mode Power Supply (SMPS). Electrolytic capacitor is known as one of the highest failure rate components in SMPS, so we tried to detect an Equivalent Series Resistance (ESR) degradation of the electrolytic capacitor directly from the data fetched to a digital controller of SMPS. With a SPICE simulation and a Rapid Control Prototyping (RCP) evaluation, we confirm the degradation can be detected by the data at a transient response under a normal operation. Even only 10 percent of a load step change, which commonly occurs in SMPS for servers, causes detectable transient response degradation.

Gain controlled high efficiency power factor correction circuit

The purpose of this study is to improve the efficiency of a boost-type power factor correction (PFC) circuit with a digital controller. One of the biggest losses in PFC circuits is the switching loss. Switching loss can be reduced by low boost ratio operation, but these results in a greater voltage drop at transient response. The voltage drop can be suppressed by using a high gain setting for the voltage control loop, but this lowers the power factor, which creates a harmonic distortion of the AC power line. To improve this trade-off, we developed a gain control method that adjusts the gain to high only at the moment of transient response. This method improves both the efficiency and the power factor of PFC circuits. We implemented the proposed method using a digital controller and demonstrated a power factor improvement of 0.08 points and an efficiency improvement of 0.4 points for a 2.5 kW PFC circuit.

Measurement of Electric Contact Phenomenon Using LiNbO3 Piezoelectric Actuator.

An Au–Au contact gap was controlled by a LiNbO3 piezoelectric actuator. The control accuracy of the actuator was in the submicron range. Neither hysteresis nor creep was observed. Contact voltage, contact current, displacement of electrodes, and driving voltage of the actuator were continuously recorded for the opening contact. Measurement data for 1,500 contact operations (100 operations×15 steps of current value) were processed by a computer. Discharge and bridge phenomena were analyzed. For the electrical discharge phenomena, arc ignition probability, arc duration, and arc disappearance distance were derived. The length of the bridge and the bridge resistance were derived for the bridge phenomenon. In addition, the contact load was estimated from measurement data and numerical calculations. Our measurements show the possibility of data correlation among the factors affecting contact phenomena.