Vatche Vorperian

Quasi-square-wave converters: topologies and analysis

A class of converters with zero-voltage or zero-current switching characteristics is analyzed using a method originally developed for quasiresonant and PWM (pulsewidth-modulated) converters. The method relies on identifying simple three-terminal structures, called converter sections, that contain the switches and the resonant tank elements. The various zero-voltage-switched and zero-current-switched converters are obtained by permutation of these converter sections between source and sink. The method unifies the analysis of this class of converters in a single equivalent circuit model. The voltage and current waveforms in these converters are essentially squarelike except during the turn-on and turn-off switching intervals. >

Approximate small-signal analysis of the series and the parallel resonant converters

Approximate transfer functions of series and parallel resonant converters are given which are in good agreement with the results of exact analysis as well as the results of experiments. It is shown that the dominant behavior of these transfer functions is determined by the output low-pass filter modified by the internal impedance of the converter. The high-frequency behavior, on the other hand, is given by a second-order response whose frequency is at the difference between the resonant and the switching frequencies and whose Q is the original resonant Q modified by the internal impedance of the converter. >

A complete DC analysis of the series resonant converter

The dc-to-dc conversion ratio of the series resonant converter has been determined in the general discontinuous and continuous condection modes. This new analysis gives a complete description of the dc operation of the circuit for any load and for any switching frequency.

Multi-loop control for quasi-resonant converters

A multiloop control scheme for quasi-resonant converters (QRCs) is described. Like current-mode control for pulse width modulation (PWM) converters, this control offers excellent transient response and replaces the voltage-controlled oscillator (VCO) with a simple comparator. In this method, referred to as current-sense frequency modulation (CSFM), a signal proportional to the output-inductor current is compared with an error voltage signal to modulate the switching frequency. The control can be applied to either zero-voltage-switched (ZVS) or zero-current-switched (ZCS) QRCs. Computer simulation is method applied to a ZCS buck QRC. A circuit implementation is presented that allows multiloop control to be used on circuits switching up to 10 MHz. This circuit requires few components and produces clean control waveforms. Experimental results are presented for zero-current flyback and zero-voltage buck QRCs, operating at up to 7 MHz. Good small-signal characteristics have been obtained. >

Equivalent circuit models for resonant and PWM switches

The nonlinear switching mechanism in pulsewidth-modulated (PWM) and quasi-resonant converters is that of a three-terminal switching device which consists only of an active and a passive switch. An equivalent circuit model of this switching device describing the perturbations in the average terminal voltages and current is obtained. Through the use of this circuit model the analysis of pulsewidth modulated and quasiresonant converters becomes analogous to transistor circuit analysis where the transistor is replaced by its equivalent circuit model. The conversion ratio characteristics of various resonant converters and their relationship to a single function, called the quasi-resonant function, is easily obtained using the circuit model for the three-terminal switching device. The small-signal response of quasi-resonant converters to perturbations in the switching frequency and input voltage is determined by replacing the three-terminal switching device by its small-signal equivalent circuit model. >

Nonlinear modelling of the PWM switch

It is shown that nonlinearity due to the switching action in pulse width-modulated (PWM) DC-to-DC converters, DC-to-AC inverters or amplifiers, and input-current-shaping DC-to-DC converters can often conveniently be confined to a three-terminal structure referred to as the PWM switch. the PWM switch represents a static nonlinearity for which circuit models can easily be derived for frequencies harmonically related to the frequency of perturbation. Converter analysis can now be approached in an analogous way to ordinary transistor circuit analysis whereby the nonlinear three-terminal device is replaced by its circuit model. A first-order approximation of the model results in the small-signal model.<<ETX>>

Small signal analysis of resonant converters

It is known that the dc-to-dc conversion ratio of resonant converters can be controlled by changing the ratio of switching frequency to resonant frequency. In this work the small signal response of resonant converters to small signal perturbations in the switching frequency and input voltage is determined.

A simple scheme for unity power-factor rectification for high frequency AC buses

A simple scheme is proposed for offline unity power factor rectification for high-frequency AC buses (20 kHz). A bandpass filter of the series-resonant type, centered at the line frequency, is inserted between the line and the full-wave rectified load. The Q=Z/sub 0//R/sub L/ formed by the load and the characteristic impedance of the tank circuit determines the power factor, the boundary between continuous and discontinuous conduction modes, the peak stresses, and the transient response of the rectifier. It is shown that for Q>2/ pi the rectifier operates in continuous conduction mode and the output voltage is independent of the load. Also, it is shown that for Q>2 the line current is nearly sinusoidal with less than 5% third-harmonic distortion and the power factor is essentially unity. An increase in Q causes an increase in the peak voltages of the tank circuit and a slower transient response of the rectifier circuit. The DC, small-signal, and transient analyses of the rectifier circuit are carried out, and the results are in good agreement with simulation and experimental results. >

Synthesis of Medium Voltage dc-to-dc Converters From Low-Voltage, High-Frequency PWM Switching Converters

Low-voltage dc-to-dc power conversion is a very mature industry which uses high-frequency pulsewidth modulation (PWM) switching techniques. The passive and active components needed to build low-voltage dc-to-dc converters are highly available, affordable and constantly improving. In this paper, a very simple and systematic method of using a large number of low-voltage, high-frequency PWM converters to synthesize highly redundant, medium voltage (4-40 kV) dc-to-dc converters is presented. Theoretical and practical considerations are discussed in necessary detail and test results of an actual 10-kW, 10 kV-to-400 V, converter built from 48 low-voltage forward converters are presented. Real-time and average reduced circuit models are derived to predict the dynamical behavior of the converter and to design the feedback control loop.

The effect of the magnetizing inductance on the small-signal dynamics of the isolated Cuk converter

The magnetizing inductance of the isolated Cuk converter introduces an undesirable pair of closely spaced complex zeros and poles, or a glitch, in the control-to-output transfer function. Limited analysis and experimentation in the past have shown that sufficient increase in the magnetizing inductance or manipulation of the ratio of the capacitances of the energy transfer capacitors can reduce the glitch. In this work, the isolated Cuk converter with coupled input and output inductors has been studied and the dependence of the glitch on various circuit parameters has been determined analytically. A condition has been derived for the ratio of the capacitances of the two energy transfer capacitors which completely eliminates the glitch at a given operating point. With this condition satisfied, it is shown that the energy transfer capacitors can be easily damped by a simple RC network to eliminate the glitch from a wide range of operation about an operating point.