PCSS Lifetime Testing for Pulsed Power Applications

Trigger systems are becoming increasingly important in pulsed power systems with large numbers of switches or large numbers of different switching times. Performance can be critical with demands for fast rise-times, sub-nanosecond jitter, and long lifetimes . In particular, component lifetimes affect maintenance costs and the available operational time of the system. High gain photoconductive semiconductor switches (PCSSs) deliver many of the desired properties including optical-isolation, 350 ps risetime, 100 ps r-m-s jitter, scalability to high power (220 kV and 6 kA demonstrated), and device lifetimes up to 10b shots with 20 A 5 ns wide pulses. However, higher current and longer pulse applications can drastically reduce device lifetime. For typical single shot pulsed power applications, lifetimes of several thousand shots are required and much longer-lived switches are required for repetitive pulsed power applications. The key parameters that impact PCSS lifetime are voltage, current, and pulse width. Voltage affects the lifetime of a lateral switch when the electric field near the surface of the switch approaches the surface breakdown limit, which is approximately 100 kV/cm for pulse charged switches under transformer oil or Fluorinert (a liquid dielectric). The current in high-gain GaAs PCSS always forms filaments, so this lifetime dependence can be considered in terms of the current per filament, and most of our lifetime testing is done with switches producing a single main filament. Since PCSS can have sub-nanosecond risetime and jitter, most of our interest in lifetime is for switches that produce 1-100 ns long pulses. These requirements lead us to testing high gain PCSS in high speed, 50 ohm transmission line, discharge circuits that can deliver up to 300 A. Control of the PCSS is achieved with a fiber-coupled laser directed between the PCSS metal contact pads resulting in randomly formed filaments (one primary). Devices demonstrating long lifetimes are ope- rated at up to a few kilohertz, whereas higher current and longer pulse tests are operated at lower repetition rates. In all cases, rep-rates are below the limit where bubbles form in the liquid dielectric. New switches are always tested at 20 A for direct comparison to the lifetime data that we have accumulated over the last 20 years. Fligher current tests are performed to predict switch lifetimes for specific applications that don´t require such long device lifetimes. This paper will discuss testing procedures, circuits, and dramatic changes in PCSS component lifetime due to contact methods (e.g. soldering versus ribbon bonding).