Transient Suppressor Design with Varistor Composite Materials

Abstract

The following paper describes the electrical behavior of varistor composite materials. These are powder-filled resin materials. Each powder particle is electrically conductive with a thin dielectric outer layer. This layer exhibits threshold conductivity like a Zener diode giving the bulk material a conductivity like a "soft" Zener with a parallel capacitance which is voltage-dependent. This paper shows analytical expressions to fit empirical data. These expressions are then used to describe the material and system response to a general exponential transient pulse. One of the results is that the material essentially absorbs fast pulses capacitively, and then dissipates the charge resistively over longer times. This capacitance-resistance characteristic time is essentially constant for all voltages above the conduction knee. The derivations are carried through, and graphs plotted for, a specific example case of the "standard" DOD-HDBK-263 15-kV electrostatic discharge (ESD) pulse with a 0.15-μs decay time and a material capacitance-resistance time of 0.4 μs. The analytical and graphical techniques are shown for solving the system response for a general exponential transient.