Linear averaged and sampled data models for large signal control of high power factor AC-DC converters

Abstract

The present paper develops large signal linear models for the voltage loop. Specifically, we develop continuous This paper shows that the large signal behavior of time averaged models at the time scales of the switching a popular family of high power factor ac to dc power period and the input period, and also derive their samconditioners can be analysed via linear models, by uspled data counterparts These models yield efficient siming squared output voltage as the state variable. The ulations, and enable the simple design of control schemes state equation for a general (constant power plus resisthat permit recovery from large perturbations away from tive) load is obtained by a simple dynamic power balance. the operating point. Section 2 describes the operation of Timne invariant or periodically varying controllers, acting the inner current loop shown in Fig. 1. Section 3 presents at the time scales of the line or switching periods respeccontinuous time averaged and sampled data models for tively, can then be designed from the resulting averaged the dynamics of the outer voltage control loop. The conor sampled data models. Simulations and experiments tinuous time averaged models are verified in Section 4 by corroborate the results.comparison with both the results of SPICE implementa-1. Introduction tions of the models and experimental results for an actual ac-dc converter. Section 5 discusses the use of a sampled Recently, there has been much work on designing condata model to design a digital controller for the outer trol schemes for high power factor ac to dc converters.loop, including PI control, and presents simulation reSchlecht [1]-[3] discusses various topologies and control sults for the behavior of the full closed loop system. schemes for such converters. Subsequent work has largely focused on the scheme shown in Fig.l, using a boost 2. Current Loop Operation converter whose input voltage vn(t) is the rectified ac waveform. The inner current loop specifies the switching The current loop is responsible for obtaining the high sequence for the transistor to regulate the input current power factor by drawing a resistive current from the ac iia(t) around a reference in,,d(t) that is proportional to line. Any current mode control scheme may be used. the input voltage. The outer voltage loop varies the proThe operation of one such scheme is illustrated by the portionality constant k from cycle to cycle, to regulate simulation in Fig. 2. At the beginning of every switchthe output voltage v,(t) about the desired level, Vd.ing period, every Ts seconds, a decision is made to have Several recent papers discuss different approaches to the transistor on or off, as required to force the inductor designing the inner and outer loops. Henze and Mocurrent towards the switching boundary, io,,d(t). This is han [41 use a hysteretic current control loop, and implea compromise between the usual constant frequency disment the voltage control loop digitally, using a simple cipline and hysteresis band control. It provides ...

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A digital controller for a unity power factor converter

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