Multilevel converters for microwelding based on m-ary system

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MULTILEVEL CONVERTERS FOR MICROWELDING BASED ON M-ARY SYSTEM

V. Zhuikov, V. Spivak, T. Tereshchenko,

J. Yamnenko, V. Bezhenar, M.Tirsu

Abstract. Multilevel converter constructing and functioning principles are given. Two different approaches to increase efficiency are described. Both decisions are compared with typical applications, so advantages and drawbacks are specified.

Keywords: multilevel converter, microwelding.

Rezumat. Sunt prezentate principiile de construire єi de funcюionare ale convertizoarelor cu multe trepte. Douг abordгri de majorare a eficienюei de funcюionarea lor sunt descrise. Neajunsurile єi avantajele pentru diferite aplicaюii ale convertizoarelor sunt descrice.

Cuvinte-cheie: convertizor cu multe trepte, microsudare.

Аннотация. Представлены принципы проектирования и функционирования многоуровневых матричных конверторов. Рассмотрены два подхода повышения их эффективности.

Ключевые слова: многоступенчатый преобразователь, микросварка.

Introduction

multilevel converter microwelding current

Contact microwelding of the parts with the thickness from a few micrometers up to 0.5 millimetres is widely used in radioelectronic and instrument-making industry while producing circuit boards, integrated circuit and electronic components.

Special feature of the microwelding is high level of the heat emitted within the contact between the electrode and the part. This level is comparable with the heat level in welding contact due to relatively low own resistance of the parts and low welding effort. High quality of microwelding can be provided by forming of welding current pulses with assigned complicated waveform.

Method of welding current forming by assigned law on the base of m-ary counting system using multilevel converters and adding of output currents of each cell in common node is proposed in the paper. Such method allows remaining invariable structure of electrical circuit. Assigned waveform of welding current is defined by pulse-width modulator that allows simplifying the control system.

Proposed method of current forming for contact microwelding equipment allows significant decreasing of cells and switches quantity, increasing of energy efficiency with assigned high quality of welded joint.

Basic of m-ary System for n-stage Converters

The circuit of fig.1 shows the basic topology of one m-stage converter used for the implementation of multi-stage converters. It is based on the n-switches converter (in practice it could be less or more switches depending of principle scheme and total power), used for single phase converters or for dual converters. In general these converters are able to produce m levels of current in the load from 0 to Is with [Is(i+1)- Is(i)] current step and zero. Each current source produces Is(i) = mI0 that is the key feature of m-ary system conversion [1], so in this case current step is I0.

Developing of m-ary System Converters for Microwelding

Fig. 2 depicts base principle of organization m-ary converter with adding of output currents of each cell in common node. Output current of such converter has stepped waveform with equal level of each step. Durations of the steps are assigned by control system (CS) and define waveform of output current. When all steps durations are the same then graded approximation of linearly increased function will be formed at the output. For microwelding output current waveform depends of materials and usually non-linear function.

As each converter has different power supplies (PS) scaled in the power of m (for example m=3) it allow to obtain maximum output current levels with minimum switching cells (SC). All the switches operates in only switching mode in contrast to schemes with linear and combined regulation [2]. Using a high frequency modulation module (HFM) allows to generate PWM control signal equal to linear regulation method.

Fig. 1. n-stage current m-ary converter

a) b)

Fig. 2. Multilevel m-ary converters with single (a) and dual (b) power supply

a) b)

Fig. 3. Typical full-bridge (a) and dual current supply (b) switching cell in case of m=3

Fig. 4. Forming of the output waveforms for schemes with bridge switching cell without PWM (a) and dual current supply with PWM (b)

The main advantage of this technique is that switches don't operate at active mode so their power dissipation is much lower. Also the proposed approach overcome drawback of maximum amount of heat that can be emitted by switch during one cycle of welding. Use of transistors in switching mode only reduces their total power, cost and dimentions of device at all. Furthermore control system don't include DAC or any analog particles and could be performed as single microcontroller.

If output current waveform is not smooth enough an additional PWM of control signals could be used. In this case the structure of the power of the circuit remains unchanged, and given waveform of the output current is determined by the pulse-width modulator, which simplifies the control system versus linear regulator.

There are output currents (fig. 4) for both schemes (fig. 2) with different types of control system and switching cells (fig. 3). Fig. 4a depicts simple approach with standard current sources and bridge converter. Four transistors calculated at full power are required that is twice than the next one. Method (fig. 4b) is better efficient and required less switching elements. Also there is no current subtraction that increase power loses. The only one disadvantage is complicated power supplies.

Conclusions

Two structure schemes for multilevel transistor converter using m-ary system have been designed. Unified structure of the cells represented for both circuit solutions. Control system has been also improved and is completely digital now. As there are no transistors operated in active mode the dimensions and mass of the device could be significantly reduced.

References

1. J. Dixon, L. Moran, Multilevel inverter, based on multistage connection of three-level converters scaled in power of three, IEEE, 2002, vol. 2, pp. 456-461.

2. Y.E. Paerand, The former of special form current pulses for micro resistance welding, Compatibility and Power Electronics (CPE '09). - Badajoz, Spain. - 2009. - PP. 396-401.

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