CIRED2005 Session No 1 ANALYSIS AND SIMULATION OF PULSE TRANSFORMER CONSIDERING LEAKAGE INDUCTANCE AND CAPACITANCE ABOLFAZL VAHEDI, HOSSEIN HEYDARI, and FARAMARZ FAGHIHI Electrical Engineering Department, High Voltage & Magnetic Materials Research Center Iran University of Science and Technology Narmak, Tehran, 16844 IRAN avahedi@iust.ac.ir heydari@iust.ac.ir faramarz_faghihi@ee.iust.ac.ir INTRODUCTION Pulse transformers capable of transmitting substantially rectangular voltage pulses, with durations of less than one millisecond, were developed for radar application, NLC klystron pulse modulator, driving a microwave amplifier, x- rays for medical and industrial use, gas lasers for plasma technology and plasma immersion ion implantation [1-7]. Several application of them, need medium or high voltage pulses (1 up to 700kV) that medium or high voltage pulse transformer increases the output pulse voltage to the value required for the load. The usually large number of turns in the secondary winding (the transformer ratio is frequently 1:10), together with the insulation gap between windings and between winding layers increase the value of the equivalent parasitic elements (leakage inductance and inter-winding capacitance). These elements extend the pulse rise time and cause overshoot and oscillations. Hence, the design of the pulse transformer is critical, not only because all materials must sustain the medium or high voltage across them, but also because the output pulse shape depends heavily on several transformer parasitic parameters that are difficult to master [8]. Pulse transformer modeling is done by two methods: firstly, the well-known lumped parameter theory of transformer is used [3, 8]. Most pulse transformer models treat each winding as a single circuit element. This limits analysis of these types of models to the bulk properties of the model. Details of winding interaction with stray capacitances can only be lumped into a single circuit model element and these values are usually determined by measurement. This can limit a designer to a trial and error approach to the subtleties of pulse transformer design. Secondly, transformer considers as a distributed parameter circuit [9, 10]. However, nonlinear core are not included [11]. The windings are separated into multiple sections and all combinations of mutual inductances are calculated. The distributed capacitance between the core and primary, the primary and secondary, and the secondary to case are included. The individual inductance and coupling coefficients are calculated based on the magnetized inductances and the air mutual. These values are used in a circuit model developed in PSPICE. Especially, this method is used for simulation of air core pulse transformer [12-14]. On the other hand, because of the transformer parasitic elements involved, the transformer is the critical device in shaping the rising characteristics of the output pulse [3, 15- 19]. Other related works on pulse transformer are harmonics, thermal and mechanical force using finite element method (FEM), reducing size, and electromagnetic interference (EMI) and so on [4, 7, 20-22]. To contribute to a better understanding of pulse transformer operation considering leakage inductance and inter-winding capacitance, this paper proposes a mathematical model based on the flux linkage as state variable. Our main aim is to identify a critical values causing unsuitable rise time of the output, especially damaged output pulse completely. It is interesting to demonstrate conditions for leakage inductance of windings, pulse frequency, and leakage capacitance that are destroying output pulse that hasn’t been discussed clearly in the literature up to now. Finally results of simulation are able to show rated value of pulse transformer parameters. If parameters of pulse transformer are not allowable value obtaining measurement and calculation, we must choose methods for reduction of them, such as using auxiliary windings or active shielding and so on [8, 23]. Therefore, this model is then used to suggest approachable parasitic elements to optimize the design of a medium or high voltage pulse transformer. Besides, using two auxiliary winding for improvement of technical characteristics of output pulse is explained and new prove based on characteristic roots method is done. PULSE TRANSFORMER MODELING BASED ON FLUX LINKAGE AS STATE VARIABLE In this section, we will describe an arrangement by which the voltage and flux linkage equations of a two-winding transformer can be implemented in a computer simulation. There is of course, more than one way to implement a simulation of the transformer even when we are using the