06-05-2011, 02:35 PM
Pulse Shape Improvement in Core-Type High-Voltage Pulse Transformers With Auxiliary Windings
High-voltage pulsed power technologies are rapidly emerging as a key to efficient and flexible use of electrical power for many industrial applications. One of the most important elements in high-voltage pulse-generating circuit technology is the transformer, generallyused to further increase the pulse output voltage level. However, its nonideal behavior has significant influence on the output pulse shape.The most attractive winding configuration for high-voltage, the core-type transformer with primary and secondary on different corelegs, is seldom used in pulsed applications, because of its weak magnetic coupling between windings, which would result in a slow-risingoutput voltage pulse. This paper shows that auxiliary windings, suitably positioned and connected, provide a dramatic improvement inthe pulse rise time in core-type high-voltage pulse transformers. The paper derives a mathematical model and uses it to describe theobserved behavior of the transformer with auxiliary windings. It discusses experimental results, obtained from a high-voltage test transformerassociated with a high-voltage pulse generating circuit, and the simulation results obtained from the numerical evaluation of thedeveloped differential equations implemented in Matlab and taking into account the measured transformer parameters.Index Terms—High-voltage techniques, modeling, pulse-shaping methods, pulse transformers.
I. INTRODUCTION
TODAY the range of high-voltage applications is no longerconfined to the high power industry. Several applications,such as X-rays for medical use, gas lasers for plasma technology,food sterilization, air pollution control, and plasmatreatment surface techniques, need high-voltage (several kilovolts)fast rising pulses. This requires efficient and flexiblepulsed power circuits with optimization of all components[1]–[5].In spite of the new developments in solid-state-based pulsedpower circuits, achieved with the technological growth inpower electronics topologies and semiconductor characteristicsimprovements, the transformer is still a crucial part in mostof these high-voltage circuits. High voltage is often obtainedfrom a pulse-generating circuit driving a high-voltage pulsetransformer, which increases the output pulse voltage to thevalue needed at the load, reducing the voltage stress in the pulsegenerating circuit components, especially on the semiconductors[6], [7].However, the design of the pulse transformer is one of themost critical tasks, due to the characteristics of high-voltagetransformers (winding turns ratio usually greater than 1:10,large insulation gap between windings and between windinglayers), which increase the values of the parasitic elements,inter-winding capacitances, and equivalent leakage inductance,normally associated, respectively, with the electrostatic energystored between windings and with the magnetic energy storedoutside the core. These elements, related with the nonideal behavior of the transformer, extend the pulse rise time andcause overshoot and oscillations [8], [9].To overcome problems caused by the parasitic elements, newtransformer design methods [10], [11] and resonant topologies[11]–[13], have been used. Even though, all of these techniquesmust coexist with the fact that the transformer must sustainthe total voltage between the primary and secondary windings,which is sometimes fairly expensive to accomplish.The most attractive, easy to assemble, and least expensivewinding configuration for high-voltage is the core-type transformer,with the primary and secondary wound on differentcore legs, as shown in Fig. 1, keeping the necessary insulationdistance. This primary and secondary windings configurationis normally presented in high-leakage transformers, as in[14]–[16]. However, the magnetic coupling between primaryand secondary windings is relatively weak and the equivalentleakage inductance opposes significantly to the rise of theoutput voltage pulse.Considering the leakage inductance proportional to the magneticenergy stored outside the core, the pulse rise time can be reduced,decreasing this energy. A way to modify the distributionof the magnetic energy outside the core is to use auxiliary windings[17].
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High-voltage pulsed power technologies are rapidly emerging as a key to efficient and flexible use of electrical power for many industrial applications. One of the most important elements in high-voltage pulse-generating circuit technology is the transformer, generallyused to further increase the pulse output voltage level. However, its nonideal behavior has significant influence on the output pulse shape.The most attractive winding configuration for high-voltage, the core-type transformer with primary and secondary on different corelegs, is seldom used in pulsed applications, because of its weak magnetic coupling between windings, which would result in a slow-risingoutput voltage pulse. This paper shows that auxiliary windings, suitably positioned and connected, provide a dramatic improvement inthe pulse rise time in core-type high-voltage pulse transformers. The paper derives a mathematical model and uses it to describe theobserved behavior of the transformer with auxiliary windings. It discusses experimental results, obtained from a high-voltage test transformerassociated with a high-voltage pulse generating circuit, and the simulation results obtained from the numerical evaluation of thedeveloped differential equations implemented in Matlab and taking into account the measured transformer parameters.Index Terms—High-voltage techniques, modeling, pulse-shaping methods, pulse transformers.
I. INTRODUCTION
TODAY the range of high-voltage applications is no longerconfined to the high power industry. Several applications,such as X-rays for medical use, gas lasers for plasma technology,food sterilization, air pollution control, and plasmatreatment surface techniques, need high-voltage (several kilovolts)fast rising pulses. This requires efficient and flexiblepulsed power circuits with optimization of all components[1]–[5].In spite of the new developments in solid-state-based pulsedpower circuits, achieved with the technological growth inpower electronics topologies and semiconductor characteristicsimprovements, the transformer is still a crucial part in mostof these high-voltage circuits. High voltage is often obtainedfrom a pulse-generating circuit driving a high-voltage pulsetransformer, which increases the output pulse voltage to thevalue needed at the load, reducing the voltage stress in the pulsegenerating circuit components, especially on the semiconductors[6], [7].However, the design of the pulse transformer is one of themost critical tasks, due to the characteristics of high-voltagetransformers (winding turns ratio usually greater than 1:10,large insulation gap between windings and between windinglayers), which increase the values of the parasitic elements,inter-winding capacitances, and equivalent leakage inductance,normally associated, respectively, with the electrostatic energystored between windings and with the magnetic energy storedoutside the core. These elements, related with the nonideal behavior of the transformer, extend the pulse rise time andcause overshoot and oscillations [8], [9].To overcome problems caused by the parasitic elements, newtransformer design methods [10], [11] and resonant topologies[11]–[13], have been used. Even though, all of these techniquesmust coexist with the fact that the transformer must sustainthe total voltage between the primary and secondary windings,which is sometimes fairly expensive to accomplish.The most attractive, easy to assemble, and least expensivewinding configuration for high-voltage is the core-type transformer,with the primary and secondary wound on differentcore legs, as shown in Fig. 1, keeping the necessary insulationdistance. This primary and secondary windings configurationis normally presented in high-leakage transformers, as in[14]–[16]. However, the magnetic coupling between primaryand secondary windings is relatively weak and the equivalentleakage inductance opposes significantly to the rise of theoutput voltage pulse.Considering the leakage inductance proportional to the magneticenergy stored outside the core, the pulse rise time can be reduced,decreasing this energy. A way to modify the distributionof the magnetic energy outside the core is to use auxiliary windings[17].
Download full report
http://s3.amazonawspublicationslistdata/...edondo.pdf