Slobodan Cuk

A general unified approach to modelling switching-converter power stages

A method for modelling switching-converter power stages is developed, whose starting-point is the unified state-space representation of the switched notworks and whose end result is either a complete state-space description or its equivalent small-signal low-frequency linear circuit model. A new canonical circuit model is proposed, whose fixed topology contains all the essential input-output and control properties of any d.c.-to-d.c. switching converter, regardless of its detailed configuration, and by which different converters can be characterized in the form of a table conveniently stored in a computer data bank to provide a useful tool for computer-aided design and optimization. The new canonical circuit model predicts that, in general, switching action introduces both zeros and poles into the duty ratio to output transfer function in addition to those from the effective filter network.

One-cycle control of switching converters

A pulsed nonlinear control technique, one-cycle control, is introduced. This technique takes advantage of the pulsed and nonlinear nature of switching converters and achieves instantaneous control of the average value of the chopped voltage or current. This technique provides fast dynamic response and good input-perturbation rejection. It is suitable for the control of pulsewidth-modulated (PWM) converters and quasi-resonant converters. The one-cycle control theory is developed based on the analysis of the basic buck converter with conventional feedback control and current-mode control. Experiments were conducted to verify the feasibility of one-cycle control of the buck converter. The dynamic behavior of one-cycle controlled switching converters is studied. The Cuk converter is used as an example for the analysis and experiments. The one-cycle control theory is generalized to control all types of switching converters: constant frequency, constant turn-on time, constant turn-off time, and variable switches.<<ETX>>

Switching converters with wide DC conversion range

Compared to basic converter topologies (buck, boost, buck-boost, Cuk, etc.), pulse-width modulation (PWM) converters with quadratic DC conversion ratios, M(D)=D/sup 2/, M(D)=D/sup 2//(1-D) or M(D)=D/sup 2//(1-D)/sup 2/, offer a significantly wider conversion range. For a given minimum ON-time and, consequently, for a given minimum duty ratio D/sub min/, D/sup 2/ in the numerator of M(D) yields a much lower limit on the minimum attainable conversion ratio. By applying a systematic synthesis procedure, six novel single-transistor converter configurations with quadratic DC conversion ratios are found. The simpler, single-transistor realization is the most important advantage over the straightforward cascade of two basic converters. As far as conversion efficiency is concerned, it is clear that a single-stage converter is usually a better choice than a two-stage converter. The quadratic converters proposed are intended for applications where conventional single-stage converters are inadequate-for high-frequency applications where the specified range of input voltages and the specified range of output voltages call for an extremely large range of conversion ratios. >

A general unified approach to modelling switching DC-tO-DC converters in discontinuous conduction mode

A method for modelling switching converters in the discontinuous conucction mode is developed, whose starting point is the unified state-space representation, and whose end results is a complete linear circuit model which correctly represents all essential features, namly, the input, output, and transfer properties (static dc as well as dynamic ac small signal). While the method is generally applicable to any switching converter operating in the discontinuous conduction mode, it is extensively illustrated for the three common power stages (buck, boost, and buck-boost). The results for these converters are then easily tabulated owing to the fixed equivalent circuit topology of their canonical circuit model. The outlined method lends itself easily to investigation of the discontinuous conduction mode in more complex structures (cascade connection of buck and boost converters, for example), in which more thean one inductor current may become discontinuous. As opposed to other modelling techniques, the new method considers the discontinuous conduction mode as a special case of the continuous conduction mode.

Dynamics of one-cycle controlled Cuk converters

One-cycle control is a nonlinear control method. The flow-graph modeling technique is employed to study the large-signal and small-signal dynamic behavior of one-cycle controlled switching converters. Systematic design method for one-cycle control systems is provided with the Cuk converter as an example. Physical insight is given which explains how one-cycle control achieves instant control without infinite loop gain. Experimental results demonstrate that a Cuk converter with one-cycle control reflects the power source perturbation in one-cycle and the average of the diode voltage follows the control reference in one cycle. >

A unified analysis of PWM converters in discontinuous modes

Three discontinuous operating modes of PWM (pulsewidth modulated) converters are considered: the discontinuous inductor current mode (DICM), the discontinuous capacitor voltage mode (DCVM), and a previously unidentified mode called the discontinuous quasi-resonant mode (DQRM). DC and small-signal AC analyses are applicable to all basic PWM converter topologies. Any particular topology is taken into account via its DC conversion ratio in the continuous conduction mode. The small-signal model is of the same order as the state-space averaged model for the continuous mode, and it offers improved predictions of the low-frequency dynamics of PWM converters in the discontinuous modes. It is shown that converters in discontinuous modes exhibit lossless damping similar to the effect of the current-mode programming. >

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.

Single stage, high power factor, lamp ballast

Unlike conventional ballasts requiring two cascaded stages, a new lamp ballast achieves near unity input power factor and high frequency sinusoidal lamp current in a single power conversion stage. A new discontinuous inductor current mode (DICM) of input inductor makes this possible by separating the input current shaping from high frequency output lamp ballasting function. The high efficiency is further enhanced by soft switching improvement, which is provided naturally through lagging current of the output resonant matching network. Design equations and experimental results verify all the advantages of the new lamp ballast.<<ETX>>

Large-signal modelling and analysis of switching regulators

A large-signal switching regulator model is derived, and prominent features of the transient response are determined. In particular, analytical expressions are found for the equilibrium points of the system which yield insight into the large-signal response, and computer-generated transient waveforms are obtained. As an example, a boost regulator is investigated, and is found to be stable for small signals but unstable for large transients.

Constant-frequency control of quasi-resonant converters

An additional independent control needed to eliminate the undesirable variable switching frequency of quasi-resonant (QR) converters is obtained by replacing the output rectifier by an active switch. The concept is applicable to all classes of converters. Compared to QR converters with conventional switch realization, constant-frequency quasi-resonant (CF-QR) converters exhibit the same type of switching transitions and similar switch voltage and current stresses. Advantages of CF-QR converters are not restricted to the constant-frequency control. In all classes, operation at zero load is possible, so that the available load range is unlimited. The range of attainable, conversion ratios is significantly extended in the classes of zero-voltage quasi-square-wave (CF-ZV-QSW) and zero-voltage multiresonant (CF-ZV-MR) topologies. A practical design example of a 25 W CF-ZV-MR buck converter is constructed and evaluated. The converter operates at 2 MHz from zero load to full load, with a full-load efficiency of 83%. Simple duty ratio control is used to maintain the output voltage constant for all loads. The circuit is inherently immune to the short-circuit condition at the output. Disadvantages of CF-QR converters are the increased gate-drive losses and increased complexity of the power stage and the control circuitry. >