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Single Phase Half Controlled Bridge Converter

Single Phase Half Controlled Bridge Converter

Introduction

Single phase fully controlled bridge converters are widely used in many industrial applications.  They can supply unidirectional current with both positive and negative voltage polarity.  Thus they can operate either as a controlled rectifier or an inverter.  However, many of the industrial application do not utilize the inverter mode operation capability of the fully controlled converter.  In such situations a fully controlled converter with four thyristors and their associated control and gate drive circuit is definitely a more complex and expensive proposition.  Single phase fully controlled converters have other disadvantages as well such as relatively poor output voltage (and current for lightly inductive load) form factor and input power factor. 

The inverter mode of operation of a single phase fully controlled converter is made possible by the forward voltage blocking capability of the thyristors which allows the output voltage to go negative.  The disadvantages of the single phase fully controlled converter are also related to the same capability.  In order to improve the output voltage and current form factor the negative excursion of the output voltage may be prevented by connecting a diode across the output as shown in Fig .4(a).  Here as the output voltage tries to go negative the diode across the load becomes forward bias and clamp the load voltage to zero.  Of course this circuit will not be able to operate in the inverter mode.  The complexity of the circuit is not reduced, however.  For that, two of the thyristors of a single phase fully controlled converter has to be replaced by two diodes as shown in Fig.4(b) and (c).  The resulting converters are called single phase half controlled converters.  As in the case of fully controlled converters, the devices T₁ and D₂ conducts in the positive input voltage half cycle after T₁ is turned on.  As the input voltage passes through negative going zero crossing D₄ comes into conduction commutating D₂ in Fig.4 (b) or T₁ in Fig.4(c).  The load voltage is thus clamped to zero until T₃ is fired in the negative half cycle.  As far as the input and output behavior of the circuit is concerned the circuits in Fig.4 (b) and (c) are identical although the device designs differs.   In Fig.4 (c) the diodes carry current for a considerably longer duration than the thyristors.  However, in Fig.4(b) both the thyristors and the diodes carry current for half the input cycle.  In this lesson the operating principle and characteristics of a single phase half controlled converter will be presented with reference to the circuit in Fig.4 (b).