Yoshiyasu Nakashima

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.

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.

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.

Highly efficient 470 W solar-to-hydrogen conversion system based on concentrator photovoltaic modules with dynamic control of operating point

Using an optimized system comprising concentrator photovoltaic (CPV) modules, electrolyzers, and DC/DC converters for dynamic control, we achieved a one-day solar-to-hydrogen (STH) efficiency of 18.78% — the highest value for a sub-kilowatt-scale photovoltaic (PV) system under outdoor operation. We clarified that incorporating high-efficiency CPV modules and DC/DC converters in PV-driven solar hydrogen production systems is essential for obtaining the highest STH efficiency. Furthermore, we noted that the overpotential of electrochemical (EC) cells is a bottleneck for the improvement in STH efficiency; therefore, future work should focus on reducing such an overpotential.

Development of algorithms for real-time estimation of smartphone surface temperature using embedded processor

In smartphone and tablet devices, surface temperature management is required to ensure safety for the prevention of burn injuries. Therefore, mobile devices must have a mechanism of functional restriction to control the surface temperature, but this in turn also leads to performance degradation. It is preferable to control the surface temperature under the safety threshold while minimally affecting performance. The surface temperature must therefore be known precisely and in real-time. In this study, algorithms were developed for smartphone surface temperature estimation within 1 °C of the root mean square (RMS) error using internal thermistor sensor data that can be executed on a smartphones embedded processor using a negligible amount of resources.

Derivation of transfer function of LLC current resonant converter using numerical calculation

Among various converters, the LLC electric current resonant converter attracts attention because of high power density, low noise and high efficiency. However, this converter cannot apply the analysis of averaging method, and impossible to derivate the transfer function, so designing is difficult. In this study, we devised a method to derive a frequency response characteristic with a transfer function type using based on the way of thinking of averaging method, and succeeded expressing transfer function of the bode diagram which agreed with actual survey. Moreover, we derived poles from the transfer function, and showed the relation between the shape of the bode plots and poles.

Failure prediction using low stability phenomenon of digitally controlled SMPS by electrolytic capacitor ESR degradation

In this paper, we report the low stability phenomenon of digitally controlled switching mode power supply (SMPS) caused by Equivalent Serial Resistance (ESR) degradation of electrolytic capacitor. The ESR of the output capacitor stack of SMPS is known as the dumping factor in a control loop that enhances SMPS stability. However, while studying electrolytic capacitor degradation in digitally controlled SMPS, we found the low stability phenomenon of SMPS with ESR degradation. Unlike an analog controlled SMPS, a time delay between an analog-digital (AD) sampling and a pulse width modulation (PWM) duty update creates ringing on a voltage waveform at the step load change response. The amplitude and frequency of this ringing becomes larger and higher with a higher ESR and is easily evaluated with a Micro-Processing Unit (MPU) used for digital control. We propose a new failure prediction method for a digitally controlled SMPS using this low stability phenomenon.