Richard Tymerski

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>>

Application of the time varying transfer function for exact small-signal analysis

An analysis method for determining the control-to-output linearized describing function of time-interval-modulated switched networks using time-varying system theory was previously proposed by the author. A simplification is now presented which eases the analysis considerably. Use of the simplified approach is demonstrated in the derivation of the control-to-output frequency response of pulse-width-modulated (PWM) DC-to-DC switching power converters operating in discontinuous conduction mode (DCM) and current programmed converters operating in continuous conduction mode (CCM) as well as in DCM. Experimental results are presented which verify the modeling approach. >

Volterra series modelling of power conversion systems

The nonlinear control-to-output response of pulse-width-modulated (PWM) conversion systems is modeled via the Volterra functional series. The determination of the Volterra kernels in the transform domain is performed on a simplified state space model of the converter. The general system response was determined for the particular case of a two-tone excitation, which allowed the determination of the magnitudes of various harmonic and intermodulation frequency components in the output spectrum. For the frequency components of interest it was sufficient to only consider the first three Volterra operators. It was seen that the frequency components in the output spectrum have contributions from an infinite number of Volterra operators. Generally, the contribution diminishes with the order of the operator, such that the dominant contributions is attributed to the lowest order operator. It was seen that the magnitude (with an appropriate scaling factor and phase) of the Laplace transform of the lowest order Volterra kernel which contributes to the frequency component of interest.<<ETX>>

ZVS-PWM-controlled parallel-resonant converter applied to a constant-current power supply

A parallel-resonant converter featuring zero-voltage-switching and control by pulse-width-modulation is proposed. Similar to a previously proposed series-resonant counterpart, it has a simple structure and can be controlled at a constant switching frequency using an active-clamp technique. The nearly constant current output characteristic of the parallel-resonant converter lends itself beneficially to precisely controlled constant current power supply applications. An experimental breadboard featured an output-current accuracy of /spl plusmn/0.6% at 1 A over a range of input voltages from 34 to 46 V and output voltages from 2 to 20 V. A maximum efficiency of 80% was observed.

A fast time domain simulator for power electronic systems

A new power electronics circuit simulator is described. The simulator features a number of novel mathematical routines which result in fast and reliable operation. The simulator runs under DOS and uses extended and virtual memory management so that large circuits and/or long simulation runs can be handled. Integrated with the simulator are schematic capture and plotting facilities. A number of example power electronic circuits are simulated to demonstrate the capabilities of the simulator.<<ETX>>

A generalized approach for /spl mu/ synthesis of robust switching regulators

A general method using /spl mu/ synthesis to design controllers applicable to PWM power converters is described. System uncertainties from component tolerances are included in the synthesis procedure, adding a quantifiable measure of robustness. Furthermore, these uncertainties have been implemented in a structured format which maximizes available system performance. A boost power converter example is presented demonstrating the controller synthesis procedure and the advantages of this method.

Extended ripple analysis of PWM DC-to-DC converters

By employing a Taylor series expansion of the state, simplified expressions are derived for the first- and second-order terms of the state ripple of any pulse-width-modulated (PWM) DC-to-DC power converter operating in continuous conduction mode. Under the assumption that the ripple is dominated by its lowest-order nonzero component, along with the ripple order relationships between state variables, these expressions may be used on any PWM power converter to derive design guidelines for optimum ripple performance. An example is given of a fourth-order boost-type power converter in which, after determining conditions by which the first two lowest-order components in the output voltage ripple are nulled, further conditions are derived that minimize the third-order component, resulting in enhanced ripple performance. The results are verified by simulation. >

PECS-an efficient solution for simulating switched networks with nonlinear elements

PECS (Power Electronics Circuit Simulator) enables the time-domain simulation of switched networks that may contain nonlinear elements. Focus has been placed not only on obtaining high speed, but also on achieving a very high degree of accuracy. A set of optimized computer algorithms that have been incorporated into PECS to achieve these objectives are explained. Examples run, which include a power-factor correction circuit containing a multiplier/divider element, have shown over an order of magnitude speed advantage over other leading approaches.

Analysis of abnormal phenomena caused by synchronous rectifiers in a paralleled converter system

Problems of synchronous rectification in a parallel-module DC-DC converter system are described. These include excessive power dissipation due to the bidirectional current flow capability of MOSFETs and abnormal oscillation due to the cross coupling of the MOSFETs used as synchronous rectifiers. The abnormal oscillation, which causes excessive voltage stress, is analyzed; the results of which are subsequently confirmed by experiment.