Semi-planar power combiner structure for IGBT-based pulsed power modulators

A semi-planar, rotationally symmetric power combiner has been realized using a hard-wired parallel circuit of four power transfer stages which feed the power into a common radial transmission line. For testing purposes, the radial transmission line is terminated with an ohmic resistance. The load and the characteristic impedance of the radial transmission line are matched to avoid any reflection inside the circuit. The semi-planar power combiner is designed for the damped case by optimizing the transmission line geometry with the help of electrodynamics modeling using CST microwave suite. Each of the four stages contains three parallel capacitors of 200nF each, the geometrical circuit inductance, a switch, and are discharged into the common load. The load is realized by using a set of parallel resistors at a total resistance of 0.5Ω around the radial transmission line, respectively, resulting in a characteristic impedance of 2 ohms for each of the power stages. The switch is realized with an industrial type IGBT module; the collector-emitter breakdown voltage is 6.5kV, at a useful DC link voltage of 4.5 k V. Lifetime estimations show a permissible peak current of up to 2kA for a single IGBT module, at a pulse duration of 1 μs. Hence, a peak current of 8 kA can be achieved by paralleling four of these power transfer stages in the semi-planar power combiner structure. To achieve a short switch turn-on time with an IGBT the gate drive unit must provide a high rate of rise of the gate voltage, which requires a gate pulse with a high current and a short rise time. Hence, a dedicated gate drive unit has been designed for the IGBT module which provides a peak current of 60A at a rise time of 106ns. Using such a fast gate drive unit also has the advantage of achieving a short delay time of the power stages, which eases the paralleling in the semi-planar power combiner. First results show that the semi-planar power combiner is a suitable functional unit for - ulsed power applications. The circuit was characterized at a DC link voltage of 1.65kV, a peak current of 1.68kA and a pulse duration of 1μs. The collector-emitter voltage fall time of the IGBT has been determined to be 239ns (t_20/80), while the rise time of the collector current is 160ns (t_10/90). The total switching losses have been measured to be around 1.57J and the peak loss power is ca. 1.41MW. The associated junction temperature increase of the individual IGBT chips of the module will be increased by approx. 0.5K per single shot. This allows to use the IGBT at high pulse repetition rates (PRF) up to 2kHz, at a pulse duration of 1μs, without cooling. The topology of the semi-planar power combiner can be scaled by different numbers of parallel power transfer stages to achieve the desired current and impedance, respectively. The circuit presented is also intended to be used as a modular component of an inductive voltage adder to increase the available voltage and peak power levels, respectively.