COMPACT AND PORTABLE, REPETITIVE HIGH PEAK POWER GENERATOR FOR AN ULTRA-WIDEBAND SOURCE B. Cassany, B. Cadilhon, P. Modin, CEA, CESTA, F-33114 Le Barp, France L. Pécastaing, M. Rivaletto, A. Silvestre de Ferron Laboratoire de Génie Electrique, Pau University, F-64000 Pau, France M. Teboul Technix, F-94000 Créteil, France
Abstract The development of an autonomous, repetitive, pulsed power generator is presented. This work is a coordinate effort between CEA, Pau University and Technix to develop a tightly integrated unit, including a battery pack, an intermediate dc/dc converter, a high voltage dc/dc converter, the control system and a high PRF Marx generator. Pau University has designed the Marx generator. They have built a 170 mm diameter, 330 mm length Marx generator capable of delivering 200 kV pulses into 50 impedance with tens nanosecond rise times and a 100 Hz repetition rate, enabling it to drive a pulse forming line and peaking switches. The French Atomic Energy Commission has worked closely with the French company Technix in developing a rapid charging power supply to meet stringent package constraints and still permit high pulse repetition rates. This system has already demonstrated the ability to charge, from DC battery power, a 5 nF capacitance up to 50 kV in 5ms in a burst of one thousand pulses with 100 Hz repetition rate, delivering a peak power of 3.2 kW. The autonomy is more than 35000 shots or 35 bursts. This generator is equipped with a microcontroller which is remote at a distance up to 75 m with an optical fiber interface. Details of this repetitive peak power generator are presented in this paper. Results of preliminary tests are also included.
I. INTRODUCTION The field of susceptibility and hardening of electronic systems to transient threats has experienced a significant growth during the past few years. Driven by the development in the area of non-lethal electromagnetic weapons it has become necessary to extend the classical set of transient threats, by a fast transient threat with an
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extreme bandwidth. The investigation of the susceptibility to those UWB threats, characterized by a bandwidth of more than a quarter of the center frequency, rise times of less than 200 ps and pulse durations in the ns regime, is of special interest [1]. Wideband and ultra-wideband (UWB) technologies have achieved notable progress in recent years. The advent of many UWB sources capable of producing output powers in the GW range allows managing real investigations of the susceptibility of electronic systems as well as their protection and hardening against such UWB threats. However, in the meantime, the development of very compact and autonomous UWB sources remain for real interests in the way to have at one’s disposal, easy to use, transportable experimental devices [2], [3]. ARC technology and the Texas Tech University have demonstrated, in 2005, for the US Army Space and Missile Defense Command, the feasibility of a microprocessor-controlled 28 l / 35 kg pulsed power generator. It is capable of producing a 250 kV output voltage on a 50 load at 20 Hz repetition rate during 180 seconds from a 12 V battery module. In this way, the DGA (French ministry of Defense) is anxious to dispose of an ultra compact UWB source capable of figure-of-merit of hundreds of kilovolts at a highly repetition rate. The compact and autonomous, repetitive high peak power generator presented in this paper is the prime driver of an ultra-wideband source.
II. ULTRA COMPACT AND AUTONOMOUS 50 kV 100 Hz MODULATOR The high voltage modulator is made of four modules: the battery power source, an intermediate dc converter, a high voltage dc converter and a control system. The whole modulator is included in a metallic electromagnetically
tight suitcase. It represents a volume of 10.4 l (390 × 280 × 95 mm) for a weight of 9.2 kg. We show on figure 1 two photography of this HV suitcase opened and closed.
Figure 1: Photography of the compact and autonomous dc/dc converter. The battery module performance clearly dictates upper theoretical limits to both the maximum average power and overall energy that can be delivered by the high voltage modulator. It was found that the energy storage requirements for operation of some bursts of tens seconds specify a battery pack that could easily deliver a high instantaneous power. Li-polymer battery offers the best power density. We have chosen to associate six 7.2 V / 1350 mA/h battery units in a series/parallel arrangement to create a 16.8 V / 1.5 kW dc primary source. This arrangement represents 0.3 l and 0.7 kg. These batteries permit to achieve, in real experimental conditions, autonomy of more than 35000 shots. Evolution of the battery voltage in function of the number of shots is represented in figure 2.
100% represents 16.8 V battery voltages, 0 % is for 12.8 V voltage batteries. Deep discharges and work over a 60 °C temperature are strictly prohibited, as a consequence, battery voltage and temperature in the suitcase are monitoring by the control system. This primary power source is associated to an intermediate dc/dc converter to raise the battery voltage up to the voltage level required by the high voltage dc converter, typically 300 V to 380 V. This intermediate converter is designed as two halves standard H bridges operating in parallel at high frequency to drive each a transformer. The resulting outputs are then rectified and added to achieve a nominal 350 V / 700 Wdc output with power efficiency of 90 %. Figure 3 demonstrates results on a resistive load. Output power and output voltage are a function of the battery voltage. The electrical circuit structure of this converter is classical, but special efforts were made on the choice of semi-conductors and the arrangement of components to avoid heaters and cooling system and improve the size and volume of the overall converter. 1000 900
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Figure 2: Experimental test on battery autonomy.
The high voltage dc converter is an IGBT H-bridge which drives the primary side of two transformers from the dc output voltage of the intermediate converter. Secondary windings of the transformers are fed into a capacitordiode multiplier to achieve its 50 kV output. A minimum of 340 Vdc at the input is required to charge a 5 nF capacitor at 50 kV in less than 6ms. Figure 4 shows results of a 340 Vdc test on a 5 nF capacitor representing the equivalent capacitance of the Marx generator. This charger has demonstrated the ability to charge 5 nF of capacitance up to 50 kV at a repetition rate of 100 Hz, which is 1.3 kW of average power. Maximum output peak power of 3.2 kW is achieved with a 400 Vdc input voltage. The biggest mass and volume reductions from classical designs were achieved by considering an instantaneous peak power operation during bursts sequence of few
seconds where no current regulation is required. As a consequence, heater and cooling system were removed, IGBT were mounted on the metallic box containing the capacitor-diode multiplier. Finally, the HV converter represents a volume of 5.5 l (270 × 254 × 77 mm) and a weight of 5.4 kg.
air, and 560 pF / 50 kVdc ceramic capacitors. Spark gaps are made of two stainless steel spheres screwed in a Plexiglas tight cylinder. Inductive charging (two inductances of 20 µH per stage) is incorporated in order to achieve high pulse repetition rate. These eight stages are vertically piled up in a cylindrical PVC tight enclosure filled with oil. Package dimensions for this Marx generator are 175 mm in diameter and 340 mm in length equivalent to a volume of 8.2 l for a mass of 9.6 kg with oil. Figure 5 is a 3D drawing of the compact Marx generator.
Figure 4: Rapid charging of a 5 nF capacitor (equivalent capacitance of the Marx generator) in 5.9 ms at 50 kV voltage. Input voltage : 340 Vdc.
This HV modulator is equipped with a microcontroller which is remote at a distance up to 75 m with an optical fiber interface. This control system permits to define the burst parameters (number of shots and repetition rate), to check the battery status (voltage, temperature), to sequence the different stages of a shot (ON/OFF of the intermediate converter, H-bridge electronic command, HV inhibition ...) and to return voltage data.
Figure 5: 3D view of the compact Marx generator.
IV. EXPERIMENTAL INVESTIGATION III. MARX GENERATOR: DESIGN AND REALISATION Marx generators are voltage multiplication circuits where capacitors are charged in parallel and discharged in series. The key to low jitter operation and good output voltage reproducibility lies, for a great part, in the design of the first stage and the setting of spark gap distances. The gap lengths present a strong ascending order from the first spark gap to the last one. It means that, when the capacitors are charged to their maximum voltage, the spark gaps erect successively from the first (bottom) to the last (top). It is all the more necessary since the Marx is in non triggered operation. This adjustment avoids prepulses and bumps in the rise of the pulse. The length of the first gap is set to 2.5 mm. The Marx generator was designed by the Electrical Engineering Laboratory of Pau University from the experience of a former project [4]. It is made of eight stages employing spark gap switches, pressurized in pur
Marx generator is driven by the 50 kV autonomous modulator. Test were done on a high resistive load (1.5 k) which is in the same time a 50 resistive divider. The Marx spark gaps are pressurized under few atmospheres in pur air. For pressure values higher than that of the minimum of the Paschen’s law, the gas breakdown voltage in the spark gaps increases in direct ratio to the air pressure. We observe, in figure 6, the linear variation of the breakdown and output voltages as a function of the pressure in spark gaps. Tests were carried on up to 3.5 bars corresponding to a charging voltage closed to 40 kV which could be damaging for inducance in repetitive operations. The output voltage reach 280 kV for the highest charging voltage. At a pressure of 3 bars, output voltage is higher than 240 kV, for a charging voltage of around 30 kV.
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Figure 8: Output signals in fastframe acquisition mode for a burst of 50 shots at 50 Hz. 30 kV/div – 20 ns/div
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Figure 6: Voltage caracteristics of the Marx on a 1.5 k resistive load.
V. SUMMARY We present, on figure 7, the output signal on a 1.5 k resistive load for switches pressurized in air at 3.5 bars. The amplitude is 275 kV and rise time is 17 ns. Breakdown voltage of spark gap is 36 kV.
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An autonomous, repetitive and ultra portable (
