| SSR's & Snubber Networks |
 |
“RC Snubber Network” is simply a technical name for a resistor (R) and capacitor (C) wired in series. They are typically placed in parallel with semiconductors or other switching devices. In this case, we refer to their use within solid state relays.
RC networks serve two primary functions in a solid state relay. The first is to increase the solid state relays ability to withstand dv/dt phenomona. That is, how well the relay reacts when there is a rapid change in line voltage over a short period of time. One instance of such an event would be the closing of a main breaker in series with the solid state relay at peak line voltage. In this scenario, the potential across the output of the solid state relay could go from 0Vrms to peak line voltage in a few micro-seconds. This rapid change in potential can cause the SCR's in the solid state relay to conduct load current for the remainder of the half-cycle. If the load is capacitive, the ensuing surge current may be significant enough to destroy the load, the relay, or both (dv/dt resulting in a di/dt failure).
The RC network helps prevent this from occurring by attenuating the rapid change in potential across the relay. It accomplishes this by absorbing, or sharing, some of the energy in the line that the relay would otherwise be forced to withstand alone. In short, it “softens” the blow to the SCR die and other components in the output circuit of the solid state relay.
The second function of a RC network is to protect the device from transients on the AC mains. Transients and dv/dt are similar in the fact that they both represent a rapid change in line voltage over a short period of time. However, they are quite different for the simple reason that transients represent a rapid change in voltage in excess of the maximum line voltage. dv/dt represents a rapid change of voltage up-to the rated voltage of the AC mains. Regardless, as with the dv/dt phenomona, the snubber network accomplishes this by absorbing some of the energy in the transient.
However, there are consequences to using RC snubber networks. One is that there is a relatively significant amount of leakage current flowing through the RC netowrk when the solid state relay is in the off-state. This can be in excess of 5mA, depending upon the manufacturer, and flows directly through the load being switched. In some instances, the leakage is significant enough to cause the load to partially turn on. The leakage current also generates a voltage drop that may reach hazardous level with some loads.
The second concern is the RC network itself. The values and ratings for the resistor and capacitor are limited by the available space in the solid state relay to house the components. This restricts their ability to absorb significant levels of energy or survive repetitive occurrences. If the capacitor in the network fails (usually shorted), then the power dissipated by the resistor will increase dramatically and it will overheat. In the best-case scenario, the network simply fails and no longer provides protection for the solid state relay. However, it is quite possible that the components will overheat to such an extent that the failure is catastrophic. Especially in encapsulated solid state relays.
Crouzet's Approach to Snubber Networks
The previous section discussed the steps taken by Crouzet to design transient protection into a solid state relay. Thus, the Crouzet SSR's do not require an RC network to protect it from electrically fast transients. However, as with any solid state relay, they are still susceptible to dv/dt.
Unlike other solid state relay manufacturers, Crouzet decided to use a small inductor in series with the optocoupler in place of an RC network. This provides the solid state relay with a level of protection comparable to an RC network small enough to fit inside the housing. However, the off-state leakage current is roughly forty times less than an equivalent solid state relay with an RC network (typically < 100µA at 120Vac).
Unfortunately, neither method guarantees immunity from dv/dt. All switching devices tied into the AC mains are susceptibe to dv/dt, regardless of the method chosen to attenuate the phenomona.
 Next Section
|
|
|
