| In this multipart series we will cover surge and transient protection for all types of signal, control and power lines. My goal is to help the reader understand the basic principles and be able to assess the capabilities of the types of protection available on the market today. One of the most important skills you should learn from this series is the ability to ask the right questions and evaluate the answers received. In part 3 of this series we began looking at the types of devices employed in surge protection devices. In this segment we will continue to look at clamp type components.
Manufacturers of Metal Oxide Varistors (MOVs) list a number of performance characteristics. Table 1 lists a number of these characteristics.
Table 1 . MOV Electrical Characteristics
Varistor Voltage |
Voltage across the terminals of the device used to characterize the device. It is typically measured at a current through the device of 1mA. |
Maximum Operating Voltage |
The maximum sinusoidal RMS voltage or DC voltage that can be continuously applied without exceeding the specified environmental temperature range. (without inducing thermal runaway) |
Maximum Clamping Voltage |
The voltage across the device terminals with the standard 8x20µs impulse current. |
Withstand Surge Current |
The maximum 8x20µs impulse current that can be applied once or twice to the device without causing a varistor voltage change of more than ±10%. |
These values characterize the MOVs accurately enough for engineers to design surge protection devices to withstand certain threats. When SPD manufacturers put together packages that employ MOVs they come up with additional performance characteristics in hopes of differentiating themselves from other manufacturers. One of the favorite, a relatively meaningless one, is the number of joules that the device can absorb. Since a higher clamping voltage yields a higher energy, the device absorbing the greater energy may indeed offer less protection.
Silicon Avalanche Diodes are another useful clamp type surge suppression device. Various manufacturers refer to their particular SADs as transient voltage suppressors or transzorbs © . In all cases the function of these devices is essentially the same as that for zener diodes. They are discrete PN junction devices that must overcome the breakdown voltage of the junction in order to begin conduction. Because of the discrete nature of these devices the current handling capability is less per device than for an MOV. The time delay to initiate significant conduction is the junction propagation delay of a single PN junction and is thus more rapid than an MOV device which relies on multiple junctions. However, since these devices cannot handle as much power per device as an MOV they are generally utilized in arrays for power circuits. SADs are very effective when employed for protecting low voltage data and control circuits and are frequently employed on printed circuit boards and even on integrated circuit dies to provide protection against electrostatic discharges.
Some manufacturers of SPDs have attempted to differentiate their SAD based products from those utilizing MOVs in their construction of power line protection by claiming that the shorter propagation delay of the SAD allowed significantly improved performance. This is a spurious argument because the improved speed of the SAD is cancelled out by the need to employ it in an array which requires additional interconnecting wiring. In addition, the variation in speed is not enough to greatly increase the voltage across the SPD constructed with multiple MOVs or SADs.
The complexity of an array of devices will invariably lead to some conduction differences and thus not all paths will carry the same current. The most efficient path will thus carry the greatest current and receive the greatest stress on components. This is true of either a single or a multiple dimension array.
Next segment we will look at crowbar type devices and how they are utilized in SPDs.
Copyright © 2005 by E. F. Roberts and Assoc., www.efrobertsassoc.com
Ed Roberts, ed.roberts@efrobertsassoc.com
Lightning and Transient Protection, Grounding, Bonding and Shielding Education
2 0 0 5 G r o u n d i n g v e r s u s B o n d i n g S e m i n a r s |
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