J. Sebastian

The discontinuous conduction mode Sepic and Cuk power factor preregulators: analysis and design

Sepic and Cuk power converters working as power factor preregulators (PFP) in the discontinuous conduction mode (DCM) present the following desirable characteristics for a PFP: (1) the power converter works as a voltage follower (no current loop is needed); (2) the theoretical power factor is unity; and (3) the input current ripple is defined at the design stage. Besides, input-output galvanic isolation is easily obtained. This paper analyzes the operation of both power converters as DCM-PFP. Design equations are derived, as well as a small-signal model to aid the control loop design. Both simulation and experimental results are presented that are in agreement with the theoretical analysis and complement the work.

Design criteria for SEPIC and Cuk converters as power factor preregulators in discontinuous conduction mode

SEPIC and Cuk converters present a great advantage over boost and fly-back topologies in discontinuous conduction mode (DCM): an input current with low harmonic content can be obtained by correctly choosing the inductors L/sub 1/ and L/sub 2/ of the converter with a fixed operation frequency, as is demonstrated here. The authors also discuss the intermedium capacitor C/sub 1/ as well as some advantages and disadvantages of the application. Simulation and experimental results support the approach. It is concluded that SEPIC and Cuk converters in DCM seem to be good choices for use as power factor preregulators.<<ETX>>

Improving power factor correction in distributed power supply systems using PWM and ZCS-QR SEPIC topologies

The single-ended primary inductance converter (SEPIC) presents several advantages over boost and flyback topologies which make this converter convenient for use as a power factor preregulator (PFP). Due to this fact, both the pulsewidth modulation (PWM) and the zero-current switched quasi-resonant (ZCS-QR) SEPIC used as PFPs are studied. Stress in components and operation in continuous and discontinuous conduction mode are considered in the PWM case, while the use of full-wave and half-wave resonant switches is considered in the ZCS-QR case. The study of the PWM SEPIC used as a PFP reveals that both types of PFP control, multiplier approach and voltage-follower approach, can be used in PWM SEPIC, the first one when it operates in continuous conduction mode and the second one when it operates in discontinuous conduction mode. Regarding the ZCS-QR SEPIC used as PFP, both types of resonant switch (half-wave and full-wave) can be used.<<ETX>>

Design of a Soft-Switching Asymmetrical Half-Bridge Converter as Second Stage of an LED Driver for Street Lighting Application

High-brightness LEDs are considered remarkable lighting devices due to their high reliability, chromatic variety, and increasing efficiency. As a result, a high number of solutions for supplying LED strings are emerging. One-stage solutions are cost-effective, but their efficiency is low because they have to fulfill several purposes with only one converter: power factor correction (PFC), galvanic isolation (in some cases), and current regulation. Two-stage and three-stage solutions have higher efficiency because each stage is optimized for only one or two tasks and they are the preferred options when supplying several strings at the same time. In this paper, a two-stage solution is proposed. The first stage is the well-known PFC boost converter. The second stage, on which this paper is focused, is the asymmetrical half bridge (AHB). Its design has been optimized based on the needs and characteristics of LED-based street lighting applications. The proposed transformer design (with asymmetrical secondary windings) minimizes the conduction losses while the model of the converter during the dead times optimizes their duration, reducing switching losses in the MOSFETs and diodes. Experimental results obtained with a 40-W prototype show an efficiency as high as 94.5% for this second stage and validate the proposed design procedure and model.

Static and dynamic modeling of tapped-inductor dc-to-dc converters

Six different topologies of tapped-inductor dc-to-dc converters derived from buck, boost and buck-boost topologies are analyzed. Both dc and ac analysis in continuous and discontinuous operating modes are considered. The influence of the tapped inductor turn ratio (λ) is very important in the circuit behaviour. Particulary in the poles and zeros distribution of the transfer function. Some important conclusions about the dynamical characteristics of converters are derived from this study. Theoretical results are compared with those obtained from experiment carried out on some prototypes. Generalized conclusions are presented considering classical buck, boost and buck-boost (non tapped-inductor) as a particular case of these converters.

A Multiple-Input Digitally Controlled Buck Converter for Envelope Tracking Applications in Radiofrequency Power Amplifiers

Wireless communication transmitters have very low efficiencies due to the use of linear radiofrequency power amplifiers. Several techniques have been proposed over the years to improve the efficiency of these systems. One of the most promising is called the envelope tracking technique, which is based on using a fast switching mode power supply to provide a varying voltage to the power amplifier that tracks the envelope of the transmitted signal. The amplifier can, thus, operate continuously near its theoretical maximum efficiency, greatly improving the overall efficiency of the communication system. This paper proposes a multilevel digitally controlled power supply suitable for this application. It is shown to perform very well, achieving very high efficiency, high-output power capability and tracking bandwidths above 50 kHz. This paper also shows that the proposed system is able to produce a 15% overall increase in efficiency in a complete envelope tracking system.

An overall study of the half-bridge complementary-control DC-to-DC converter

The half-bridge complementary-control power converter has been recently proposed as a low-output voltage DC-to-DC power converter due to its excellent features (efficiency around 90% at as low an output voltage as 3.3 volts). A study of the statics and dynamics of this power converter is proposed in this paper. Thus, DC voltage conversion ratio both in continuous and discontinuous conduction mode, the boundary between both modes, the DC current level in the transformer and a small-signal average model have all been obtained. From the latter, transfer functions between duty cycle and output voltage and between input and output voltages have been also obtained, and some simplifications and design rules have been proposed to facilitate the design of the feedback loop.<<ETX>>

Single-switch three-phase power factor preregulator under variable switching frequency and discontinuous input current

A three-phase power factor preregulator with input inductor, diode bridge, and a single switch is analyzed. Operation with discontinuous input current and variable switching frequency is considered. Equations for average input current are presented, showing that quasi-sinusoidal waveforms are obtained. Also, the switching frequency equation shows an almost constant frequency operation. A design procedure is presented, where simulation and practical results show the good performance of the approach. Comparison with constant frequency operation also shows that a better harmonic distortion is obtained if the voltage in the switch (with switch off) is not far from the input voltage. Also, in this case, the high-order harmonic content is easily filtered.<<ETX>>

Different Purpose Design Strategies and Techniques to Improve the Performance of a Dual Active Bridge With Phase-Shift Control

This paper addresses the performance of the bidirectional Dual Active Bridge (DAB) converter. One of the advantages of the DAB is the possibility to achieve Zero Voltage Switching (ZVS) operation in all the switches of this converter. However, the ZVS operation range can be lost for light loads, especially if high voltage is required in at least one of the DAB ports and the phase-shift control is used to regulate the power processed by the converter. Theoretically simple averaged model is presented for the DAB converter. Using the study presented in this paper, the boundaries of ZVS operation can be easily evaluated. The proposed models and analysis of the ZVS boundaries allow the proposal and evaluation of two different design strategies with different purposes: on the one hand, increasing the ZVS operation range and, on the other, improving efficiency at full load. Moreover, some techniques are presented for increasing the ZVS operation range and improving the efficiency of the DAB at full load (both using phase-shift control) employing the aforementioned analysis to obtain certain design criteria and conclusions. Finally, the proposed models, design strategies and techniques to improve the performance of the DAB are experimentally tested using a 1kW prototype with input and output voltages of 48V and 400V, respectively.

An overall study of a Dual Active Bridge for bidirectional DC/DC conversion

The increase demand of an intermediate storage of electrical energy in battery systems, in particular due to the use of renewable energy, has resulted in the need of bidirectional DC/DC power converters with galvanic isolation. Uninterruptible Power Supplies (UPS), battery charging systems, photovoltaic equipment and auxiliary power supplies in traction applications are examples of some fields of application of this kind of converters. A Dual Active Bridge (DAB) bidirectional DC/DC converter is a topology with the advantages of decreased number of devices, soft-switching commutations, low cost, and high efficiency. The use of this topology is proposed for applications where the power density, cost, weight, and reliability are critical factors. In the present paper the steady-state analysis of the converter has been carried out, giving some guidelines for the design (considering soft switching limits and the amount of reactive current) and a small-signal model of the topology. Simulations and experimental results are also presented.