Toshiyuki Zaitsu

Ultra high efficiency of 95% for DC/DC converter - considering theoretical limitation of efficiency

The authors have considered the theoretical limitations in the efficiency of DC/DC power converters used in telecommunication applications. The theoretical limitation of efficiency in a single-stage topology and a two-stage topology is compared. As a result, they have selected a two-stage converter consisting of a buck+half bridge topology. This topology is suitable for high output current and low output voltage DC/DC converters. The prototype using a buck+half bridge topology has been made (3.3 V 60 A). The efficiency of the overall converter was 92% at full load, and an ultra high efficiency of 95% at half load was successfully achieved.

Zero-voltage-switching techniques and their application to high-frequency converter with piezoelectric transformer

This paper classifies the zero-voltage-switching, clamped-voltage (ZVS-CV) topologies for high-frequency switching dc-dc converters. These topologies are suitable for high-frequency operation because they can suppress the switching loss and voltage stress to the switching elements. In order to demonstrate this feature, we applied a ZVS-CV topology to the high-frequency dc-dc converter with a piezoelectric transformer which operates in megahertz frequency region. In our experiments, the efficiency of 77.8% at the switching frequency of 1.3 MHz was obtained.<<ETX>>

Steady-state characteristics of the push-pull piezoelectric inverter

Steady-state characteristics of the push-pull inverter with a piezoelectric transformer are analyzed. The piezoelectric transformer operating in the 3rd-order longitudinal vibration mode is used in place of a conventional magnetic transformer to produce a high output voltage to light up a cold cathode fluorescent lamp. The circuit operation, the load characteristics, the efficiency and the ZVS conditions are analyzed using equivalent circuits, analytical results are confirmed by experiments. An example of the output current control is also shown.

Stability improvement of distributed power system by using full-regulated bus converter

The bus architecture, consisting of bus converter and POLs (point of load), has become very popular since POL can respond to some demands of multi power supply voltage, low driving voltage and series sequence/tracking of LSI with more flexibility than isolated DC-DC solution. However, the customer suffers the system instability issue caused by impedance overlap between the bus converter and POL. Bus converter has three types i) un-regulated, ii) semi-regulated, iii) full-regulated, and each type has different output characteristic. This makes matter more complex to understand and difficult to resolve the instability phenomena. This paper presents, the availability of full-regulated bus converter for system stability as compared with un-regulated and semi-regulated case, and is examined analytically and experimentally. As a result, it has been found that in the case of full-regulated bus converter, the bandwidth is extended in the presence of feedback loop due to damping of output impedance peak, and achieved higher stability.

Stability Comparison of Three Control Schemes for Bus Converter in Distributed Power System

Recently, the distributed power system consists of bus converter and POL is usually used for IT infrastructure equipment. The system may become unstable depending on the bus converter design even if each converter has stable operation. Then, the bus voltage is oscillated. Recently, stability problems in distributed power system become series. It is required to overcome these problems as soon as possible. This paper investigates, the improvement of the system stability by the control method. As a result, the full-regulated bus converter is the best as a control method of the bus converter because the full-regulated bus converter can reduce the bus voltage oscillation and correspond to the input voltage variation flexible by the regulation.

Stability Design Consideration for On-Board Distributed Power System Consisting of Full-Regulated Bus Converter and POLs

The power supply system which requires the low-voltage / high-current output has been changing from conventional centralized power system to distributed power system. The distributed power system consists of bus converter and POL. The most important factor is the system stability in bus architecture design. The overlap between the output impedance of bus converter and input impedance of POL causes system instability, and it has been an actual problem. Increasing the bus capacitor, system stability can be reduced easily. However, due to the limited space on the system board, increasing of bus capacitors is impractical. The urgent solution of the issue is desired strongly. This paper presents the stability design for on-board distributed power system consisting of full-regulated bus converter and POLs. The output impedance of the bus converter and the input impedance of the POL are analyzed, and it is conformed by experimentally for stability criterion. As a result, the standard of the discrimination of stability on a frequency response of input and output impedance is clarified. Furthermore, the design process for system stability is proposed.

Frequency domain analysis of fixed on-time with bottom detection control for buck converter

The fixed on-time with bottom detection control having ripple injection has become very attractive due to practical usage and realization of ceramic capacitor solution for output while keeping ultra fast transient response. This paper presents quantitative analysis by introducing averaging method in comparator operation for the transfer function. As a result, the frequency response characteristic was quantitatively revealed. Good agreements between theoretical results and actual measurements were obtained.

Novel PWM-controlled resonant converter

A novel PWM-controlled resonant converter is proposed. The combination of a resonant converter and an active-clamp circuit makes it possible to control the output voltage of the resonant converter by PWM method. The good controllability and other characteristics of this new resonant converter are analyzed, and are confirmed experimentally.

Adaptive driving of synchronous rectifier for LLC converter without signal sensing

This paper presents the novel driving technique of synchronous rectifier (SR) for the LLC converter. The unique characteristic of “Vo*D=constant” of LLC converter is utilized to predict optimal duty ratio D, where D is the conduction period of secondary diode. This technique does not need the sensing of SR current neither SR voltage. Additionally, the auto tuning technique is implemented to adjust the duty ratio precisely as well by digital control using TIs DSP of Piccolo TMS320F28027. The technique is confirmed by experimentally..

PFM and PWM Hybrid controlled LLC converter

This paper proposes the new LLC converter with unique control method. Generally, LLC resonant converter, which is controlled by PFM, has the drawback of (1) difficulty of PWM controllability, (2) difficulty of constant current limitation, and (3) narrow input voltage range limitation. These difficulties come from the resonant operation. In order to overcome those drawbacks, a hybrid control of PFM and PWM is employed for LLC. The resonant frequency is set to lower frequency to generate triangle current waveform, and the output voltage can be controlled by PWM and PFM through adjustment of resonant inductor energy. The experimental results for 48Vin, 16Vo, 80W board achieves 90% efficiency and validated this method is useful practically.