Fuzzy Phase Control of Series Multicell Converters Guillaume Gateau, Pascal Maussion and Thierry Meyniard Laboratoire d'Electrotechnique et d'Electronique Industrielle, UPRESA C.N.R.S. 5004 BP 7122,2, rue Charles Camichel, 3 1071 Toulouse Cedex 7, France (gateau, maussion)@leei.enseeiht.fr Abstract In this papel; the authors propose a control strategy for multi cells converters based upon fuuy logic. The control variables are hereafter not the duty cycles but the phase shifts between the trigger signals of the switches. From a non exact but complex harmonic model of the system, a rule base has been built in order to provide fuuy model in- version. After assymptotic stability considerations through a Lyapunovfunction and on a 4-cell convertel; where ca- pacitor voltages natural balance is impossible, the fizy controller gives the system a rather dynamic and static be- havioul: 1. Introduction. Among the different topologies for high voltage power converters, the switches serial connection principle has re- cently given birth to the series multicell converters [2, 3, IO]. This particular type of converter has many advan- tages such as supply voltage sharing between the different switches and the multiplication of the switching frequency seen by the load. Moreover, with a suitable control strategy, the same topology could be used either for dcidc or dc/ac power conversion. For each of the n switches, the permanent off-state volt- age is n times smaller than the supply voltage e,, then they could be smaller and faster. The output waveform switching frequency is n times higher than that of one semiconduc- tor. Between two commutation cells, a floating capacitor is connected, which voltage have to be controlled in order to guaranty a converter secure operation, in case of load, supply voltage or setpoint variations. Generally, the duty cycle is used to control the load current and this will be done in our system. With a constant phase shift of % be- tween the trigger signals of the different switches, the volt- age balance is naturally assured by the harmonic currents, but rather slowly. This paper gives a solution for a closed loop control of the capacitor voltages by variable phase shifts to speed up the balance. A modelisation is made with fuzzy logic, try- ing to learn the behaviour of this non linear and complex system, under variable phase shifts. A knowledge base is built from an harmonic model [I, 61 of the system, more complete than an average value model [7]. Then, all the data are agregated into a fuzzy control rule base, leading through model inversion to good dynamic performances. 2. Harmonic model. Fig.( 1) shows the topology of a multicell converter. For the modelisation, the switches will be assumed ideal, i.e. no off-state current, no on-state voltage, neither delay or dead times. The floating capacitors are designed to limit their voltage variations during a switching period and the load bandwith is very small with respect to the switching frequency. L R I I Figure 1. Topology of multicell converter. For example, Fig.(2) shows the different waveforms dur- ing dciac operation of a :?+cell converter, with sinusoidal duty cycle variation and phase shifts equal to between the cells. The four levels could be seen, hardly improving tRe harmonic spectrum of the output voltage with respect to classical two levels inverter. The different steps lead.ing to the complete model could be found in [ 1,6]. An harrnonic model of this system could be expressed under the classical state space form 0-7803-3796-4/97/$10.0001997IEEE 1627