| SSR's & Transient Protection |
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In the previous section we discussed the function and significance of the optical isolator in a solid state relay. We also touched upon it's susceptibility to transients and conducted/radiated emissions on the AC mains. Given this information, it only makes sense that the coupler(s), and other internal components inside a solid state relay, should be protected to prevent damage from occurring during this type of phenomena.
In this section we will discuss a few of the options available for protecting a solid state relay from electrical transients, and a few advantages & disadvantages of each.
Metal Oxide Varistors (MOV)
Transients, a term used here for simplicity, are actually "Transient Voltages". More familiar terms for disturbances carried on power lines may be “surges” or “spikes”.
Transients that “ride” on AC power lines can significantly increase the peak line voltage. Conversely (depending upon the polarity of the transient), they can cause significant sags in the peak line voltage as well. Devices that cannot withstand the peak value of a transient may breakdown and begin conducting load current once it's maximum voltage is exceeded. Since the breakdown occurs at a significantly elevated voltage level, the ensuing surge current will be equally impressive. This may result in permanent damage to the device being subjected to the transient. Furthermore, if the device is encapsulated, the damage may result in a catastrophic failure.
Basically, transients are momentary changes in voltage or current on the power lines or in a load circuit. They are generally considered as 'random' occurrences, but may be repetitive or re occurring depending upon the transient's source. Although they may come in many forms, voltage transients normally last only about 50 microseconds, and current transients last typically 20 microseconds (per ANSI C62.41-1991, the standard for transients in facilities operating under 600 Volts). Voltage transients can reach into the thousands of volts depending the source and circuit configuration.
MOV's are a commonly used transient suppressor, which are typically configured in a circuit to shunt energy to ground/neutral, or directly across the device being protected. The impedance of the MOV is inversely proportional to the voltage applied to it's terminals. In other words, the resistance of the MOV decreases as the voltage across it's terminals increases. Thus, when a power surge or voltage spike on the AC line exceeds the varistor's nominal rating, the decrease in resistance rapidly creates an shunt path for current. This shunt attenuates the transient, effectively protecting the selected device(s).
However, since the MOV creates a low impedance path to ground/neutral or through the load, the surge current during suppression can be quite significant. As a result, the MOV(s) may be “weakened” after only one or two transients occur in the circuit. This is one of the concerns that must be taken into consideration when selecting a suitable transient suppressor.
Concerns with MOV's
An MOV contains a mass of zinc oxide grains, in a matrix of other metal oxides, sandwiched between two metal plates (electrodes). The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs.
When a small or moderate voltage is applied across the electrodes, the reverse leakage through the diode junctions will allow a small amount of current to flow through the device. As the voltage begins to exceed the nominal rating of the MOV, the diode junctions break down because of the avalanche effect and allows a large amount of current to flow. The result of this behavior is a highly nonlinear current-voltage characteristic, in which the MOV has a high impedance at low voltages and a low impedance at high voltages.
The MOV may become damaged, or even melt, if the energy contained in the transient pulse (often measured in joules) exceeds it's rating. If this occurs, then the MOV can no longer protect the device for which it was selected.
MOV Failures
The AC mains is susceptible to many different forms of electrical phenomena. These include, but are not limited to, lightning strikes, switching transients, voltages dips & sags, and temporary overvoltage conditions. These conditions may result in the degradation or failure of an MOV within many applications.
In high-current conditions, the zinc oxide junctions in the MOV begins to degrade. This results in a lower MCOV, or turn-on voltage. As the degradation continues, the MCOV continues to drop until it reaches a point that it is conducting continuously. Furthermore, given a fixed voltage, MOV's exhibit greater power dissipation at higher temperatures. Thermal runaway can result if the power dissipation in the MOV occurs more rapidly than the device can transfer heat into the surrounding ambient. This will cause the MOV's temperature to increase until it is ultimately destroyed.
Crouzet Approach to Transient Protection
Due to the possibility of such failures with MOV's, Crouzet decided not to incorporate them in the design of their solid state relays. Instead, it was decided that the integration of transient voltage suppression diodes, or transzorbs, was the best method for protecting a solid state relay.
A transient voltage suppression diode is a device used to protect sensitive electronics, such as Microprocessors and Opto couplers, from voltage spikes carried on connected wires. TVS type devices are ideal for protecting semiconductor devices because they avalanche and transition into conduction much faster than MOVs. This provides a significant increase in protection, as the transient is “clipped” faster than with a MOV.
Crouzet's design places the TVS(s) in parallel with the optocoupler(s) in the gate circuit of the solid state relay. As stated in the previous section, the optocouplers are the components in the AC load circuit of the solid state relay most susceptible to damage from voltage transients. When a voltage surge appears at the SSR's output terminals (effectively across the optocouplers and the output semiconductor), the TVS avalanches and conducts current around the couplers and into the output semiconductor's gate. This turns on the output of the solid state relay and shunts the transient energy back into the AC load circuit, thus protecting the solid state relay from damage. Moreover, since the output of the solid state relay is designed to carry full load current in the application, no degradation occurs when the transient is suppressed. As opposed to MOV's, which are forced to dissipate the transient energy, this makes the protection circuit fully repeatable.
In summary, there are three significant advantages to using transzorbs rather than MOV''s in a solid state relay design;
 Varistors (MOV's) typically have an order of magnitude more leakage current than TVS devices (mA verses µA).
TVS devices avalanche and switch into conduction up to 5 times faster than MOVs, affording significantly faster reaction to transients.
Metal Oxide Varistors degrade over time. The degradation is accelerated with increased temperature and repeated surge events.
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