Generally all surge protective devices can be placed into two categories. These two categories are clamps and crowbars. Clamps include metal oxide varistors and silicone avalanche diodes. Silicone Avalanche Diodes are also known as transient voltage suppressors (TVSs). Crowbars include gas discharge tubes, spark gaps, carbon blocks and thyristors.
Metal oxide varistors(MOVs) are among the most common devices used to provide surge protection for electrical and electronic circuits.
MOVs are nonlinear, voltage dependent devices that function similarly to back-to-back zener diodes. The symmetrical, sharp breakdown characteristics shown in Figure 1 provide excellent transient suppression performance. When exposed to high voltage transients the varistor impedance changes by many orders of magnitude from a near open circuit to a highly conductive level. This clamps the transient voltage to a safe level.
The potentially destructive energy of the incoming transient is absorbed by the varistor.
MOVs are constructed by sintering zinc oxide (ZnO) with small amounts of bismuth, cobalt, manganese and other metal oxides. The resulting ceramic forms a matrix of conductive zinc oxide grains separated by grain boundaries. This provides P-N junction semiconductor characteristics at the intergranular boundaries.
These boundaries are responsible for blocking conduction at low voltages and are the source of the nonlinear electrical conduction at higher voltages. In an MOV, energy is absorbed uniformly throughout the body of the device with the resultant heating spread evenly throughout its volume. Electrical properties are mainly controlled by the physical dimensions of the varistor. The energy rating is determined by volume, voltage rating by thickness or current flow path length, and current capability by area measured normal to the direction of current flow.
The electrical characteristics of an MOV are related to the bulk of the device. Each intergranular boundary functions as a P-N junction. Since the nonlinear electrical behavior occurs at the intergranular boundaries of the ZnO grains, the varistor can be considered a “multi-junction” device composed of many series and parallel connections of grain boundaries. Grain size and distribution play a major role in electrical characteristics.
MOVs for low voltage circuits range in size from 5 to 100mm. The most common discrete sizes of MOVs are 5mm, 7mm, 10mm, 14mm and 20mm. These vary in area from approximately 2mm2 to 127mm2 with each increase in size doubling the area.
In one particular manufacturer’s line the power handling capability for these diodes are 7.1J, 14.3J, 28.5J, 57J, and 114J respectively for 5 thru 20mm devices. Increasingly, multilayer MOVs are being placed directly on PC boards for component protection. Various types of MOV packages are depicted in Figure 2 Clamping values for MOVs are generally measured with an 8x20?S pulse at 500 amps.
A typical device rated for 130 VAC can be expected to clamp this pulse at 340 volts. MOVs can be employed in many surge protection applications. During a surge event, they can handle quite large amounts of current considering their size. Figure 3 and Figure 4 depict the pulse lifetime ratings for standard and high energy capable MOVs. The standard MOV can be expected to withstand a single 7kA 8x20?S pulse without changing its maximum varistor voltage by more than ±10% at 1 mA. Similarly the high energy rated MOV can withstand a single 10kA pulse. Of possibly greater interest are the 1000 (103) pulse ratings. Here the standard MOV is expected to withstand 1000 500A 8x20?S pulses while the high energy MOV should withstand 1000 600A 8x20?S pulses. While a 10kA surge is unlikely on residential interior wiring, a large number of lower amperage surges may indeed be possible and in many locations extremely likely.
MOVs can be combined in parallel and series combinations to increase current and voltage capabilities. While a series combination may be useful these devices are more often utilized in parallel. When combining devices in parallel with sharp knees in their conduction, close performance characteristic matching is especially crucial. Failure to closely match the characteristics will lead to uneven load sharing. This typically results in a cascading failure as the first device to begin conduction carries an inordinate share of the surge current. Another contributor to poor load sharing is differing path lengths.
The velocity of light is approximately 30 cm/nanosecond. When dealing with devices that have reportedly 5nS turn-on times a 10% difference in the turn on time could be generated by a 15cm difference in path length. Lead inductance is another crucial element in the use of these devices since the voltage drop per cm is equal to 3.67 volts for a 500A 8x20uS pulse.
While MOVs function well at damping high amplitude short duration transients they do not perform well at handling even low values of overvoltages. One of the limiting factors is their ability to dissipate the heat resulting from their absorption of the temporary overvoltage (TOV). The same is true of their ability to absorb multiple surges in a short timeframe. Many MOV packages are basically excellent electrical and thermal insulators. This further limits the MOVs ability to dissipate heat. A few manufacturers have resolved this problem by enclosing the MOVs in a conductive aluminum housing thus facilitating significant heat dissipation.
Devices packaged in this manner may be expected to withstand a TOV for several seconds or possibly more without significant degradation. In any case, a long term TOV will destroy any MOV. Failure mode of an MOV stressed in this manner is generally quite dramatic resulting in a rapid and violent deconstruction if the devices are not fused or protected by a PTC resistor.
Written by Ed Roberts of EF Roberts & Assoc