Watkins-Johnson Topology Integrated in a Full-Bridge Converter

Abstract

Electrical thrusters require high power and High Voltage (HV) of 300V to 2000V depending on the thruster technology. A converter suitable for this application should have a high power handling capability (full bridge converter) and a simple output stage (fly-back converter). Other desirable characteristics would be: simplicity, able to supply a wide range of loads, low stress on switches, high stability and galvanic isolation between input and the output power.The Watkins-Johnson (W-J) topology is used mainly in HV applications due to the fact that its magnetic elements are on the input side. However the power output of this converter is constrained by the "single switch topology". Further downsides of the basic W-J topology are the relatively high voltage stress on the switching elements and the fact that is not galvanically isolated .This paper will present a new topology of a Full Bridge Isolated Converter which is based on the W-J topology. Steady state behaviour in Continuous Current Mode (CCM) and in Discontinuous Current Mode (DCM) will be investigated. Dynamic performances of the converter will be investigated with the analysis of its transfer function. An equivalent circuit suitable for PSPICE simulations will be presented.The topology presented potentially offers the following advantages which are suited to providing power conditioning to sub-systems which require high voltage and high power: x Simple output stage suitable for HV.x Buck type transfer function, hence simple control scheme. x Low stress on components.x Single stage converter x Suitable for modular high power converter due to the simplicity of the output stage WATKINS-JOHNSON TOPOLOGYIn figures 1 and 2 the W-J topology and a practical realization of it is presented. . Watkins-Johnson topology practical realizationIts peculiarity is that energy is transferred from input to output during the ON time of M1, while during the OFF time the energy stored in the core of the mutual inductor L1 L2 is given back to the input source.It can be shown that in CCM the conversion ratio is of the basic topology is (į refers to duty cycle):Equation (1) is also valid for the practical realization of fig. 2 but only for CCM DQG į greater than 0.5.For the practical realization of fig. 2 the max static voltage stress on the switching elements is 2*Vin. Considering that the input voltage for high power satellites is already 100V and could rise higher in the, this level of voltage stress can seriously limit the choice of power components. ISOLATED VERSION OF THE WATKINS-JOHNSON TOPOLOGY AND ITS INTEGRATION IN A FULL BRIDGEThe isolated version of the topology under study can be implemented as a push pull or full bridge that follows a Watkins-Jonson stage, as depicted in figure 3 and 4.