High Voltage Metal Oxide Varistors for Surge Protection 2008-01-2850
Zinc oxide based metal oxide varistors (MOV) are widely used electrical surge protection components. Modern high power, high-density electronics post more requirements such as smaller footprints, higher current density and higher nonlinearity on MOVs. Such requirements can no longer be satisfied by commercially available MOVs due to their limited voltage capability, high leakage current and mechanical cracking related reliability issues, most of which are associated with the presence of non-uniformity, defects and coarse grain in their micro-structures. New formulations and processes have been developed to overcome such limitations. This work has identified compositions that can be sintered at relatively lower temperatures than typical commercial MOVs, but with largely improved I-V characteristics due to refined and uniform microstructure. These compositions show not only high breakdown voltages of up to 10 kV/mm with low leakage current, but also a large nonlinear alpha coefficient of great than 50 at high fields, a measure of the speed of the transition from insulating to conducting state and effectiveness of over-voltage protection. The demonstrated breakdown strength of greater than ten times higher than commercial MOVs, along with much higher nonlinearity, will enable MOV miniaturization, high voltage surge protection, and open up new areas of application.