HIGH REP-RATE OPERATION OF PULSED POWER MODULATOR USING HIGH VOLTAGE STATIC INDUCTION THYRISTORS * Keiichiro Nishikawa, Akitoshi Okino, Masato Watanabe, Eiki Hotta, **Kwang-Cheol Ko and ***Naohiro Shimizu * Department of Energy Sciences, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ** Department of Electrical and Computer Engineering, Hanyang University, Seongdong-ku, Seoul 133-791 Korea *** Corporate Technical Center, NGK Insulators, Ltd., Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan Abstract A repetitive pulsed power modulator, which uses high voltage static induction thyristors as main switching devices, has been designed and constructed for application to discharge light source. The main components of the power modulator are a pulse forming network (PFN) and a semiconductor switch. The PFN consists of 100 ceramic capacitors (2000pF, 30kV) connected in parallel. The measured impedance and output pulse width of PFN are 0.75 ohm and 427ns, respectively. The semiconductor switch is made of 3 high voltage static induction thyristors connected in series, which is able to withstand 10kV. The significant feature of the static induction thyristor is that it has very low ON-state voltage. This feature is especially suitable for high rep-rate operation of pulsed power modulators, since energy loss by switch can be remarkably reduced. I. Introduction For pulsed power applications, discharge switches such as thyratrons have been widely used. However, these switches have limited lifetimes and these have to be replaced frequently. Therefore, they are not suitable for high-rep-rate operations. Semiconductor switches such as static induction thyristors have, as long as used within the rated values, semi-infinite lifetimes [1] and can be operated at high rep-rate. This feature is ideal for discharge light source application. Because the rated voltage of thyristor is limited to several kVs per device, it is sometimes necessary for them to be connected in parallel and in series in order to handle large current and high voltage. In this case, cares must be taken that current and voltage is equally divided to each device. For a pulsed power system, since the risetime and falltime of the pulse should be as short as possible, the thyristor should be triggered with large gate current. In addition, the design for reducing stray inductance of the system is also important to get an ideal pulse shape. In this paper, we present the experimental results on performance of the pulsed power modulator using static induction thyristors. II. Design of the System The main components of the pulsed power modulator are a pulse forming network (PFN) and a semiconductor switch made of static induction thyristors. The overview of the modulator is shown in Fig.1. Figure 1.Overview of the modulator A.PFN The PFN consists of 100 ceramic capacitors (2000pF, 30kV) connected in parallel. The designed characteristic impedance Z and the electrical length T 0 of the PFN are 1Ω and 200ns, respectively. The output pulse width corresponds to 2 T 0 . As shown in III.A, the total measured capacitance of the PFN C PFN is 0.265 µF, and the measured characteristic impedance and the electrical length of the PFN are 0.75Ω and 213nsec,respectively. B. Semiconductor switch The semiconductor switch is made of 3 high voltage static induction thyristors (SI-Thy) connected in series in order to withstand high switching voltage. The SI-Thy used is RT103N made by NGK Insulators, Ltd., and its specification is shown in Table 1. For protection against the reverse voltage applied to the SI Thy, fast recovery high-voltage diodes (K50UF made by Voltage Multipliers, Inc.) are connected in parallel with each thyristor. In addition, in order to distribute the applied voltage equally to each SI-Thy [2], shunt resistors of 10MΩ are connected in parallel with each SI-Thy. The thyristor of this type is triggered by electronic gate signal. To make the device conductive rapidly, the gate current should be sufficiently large and fast compared with that for the conventional applications. In this modulator, the fast gate current is supplied by using inductive storage.