EDN Access

July 3, 1997

Spice model simulates spark-gap arrestor

Christophe Basso, Sinard, France

Spark-gap arrestors, or surge arrestors, are highly nonlinear devices whose function is to block transient surges on dc or ac power-supply lines. Such transients can arise from lightning strikes, motor starts, and other causes. A spark gap uses two electrodes that oppose each other across a short distance in an atmosphere of inert gas, such as neon or argon. If the voltage on the arrestor is below its striking voltage (avalanche potential), the current in the arrestor is close to zero. Once the potential reaches the striking voltage, the voltage across the arrestor suddenly collapses to a level called the glow voltage. If the current increases further, the arrestor voltage decreases to a level called the arc voltage, where it stays until the surge passes. At this point, the arrestor stays conductive until its current falls below a sustaining value, much in the manner of a thyristor. Because the gas needs a certain amount of time for ionization, the ignition voltage depends on the dv/dt applied to the arrestor. The Spice model presented here takes into account the dv/dt effect.

You can model such a component with Spice in several ways (References 1 and 2). For simplicity and easy implementation, the method shown uses the macromodeling technique. The model assembles Spice primitives to describe a complex electrical function. Figure 1 shows the general spark-gap model. In the off state, the voltage-controlled switch is open, and only a leakage current flows in the spark gap. The switch stays off until VSG increases to the striking voltage, VSTRIKE. At this point, the switch immediately turns on and the network comprising the back-to-back zener diodes and the series resistance connects across the arrestor's terminals. The voltage then collapses to the arc value, and the current starts to rise.

When the surge passes, the arrestor current decays until the voltage reaches the sustaining value, and the switch opens. This simple model does not take into account the glow transition. The netlist in Listing 1 uses IntuSoft's (San Pedro, CA) IsSpice 4. (Click here to download the file from DI-SIG, #2053.) IsSpice 4 uses standard Spice 3 elements combined with one of IntuSoft's Spice extensions: an if-then-else behavioral element (BARC). The A1 element in the Spice routine depicts, point by point, the way the ignition voltage increases in
the presence of higher voltage slopes. You can easily obtain this ignition-voltage information by observing the VIGNITION/
dv/dt curves in the arrestor's data sheet. A classic differentiator calculates the slope of the input signal.

To adapt the model to a particular spark-gap device, you need only enter the parameters in the data sheet. The default values shown correspond to a Siemens (Iselin, NJ) A81-C90X surge arrestor. The first Spice test uses the self-relaxing configuration in Figure 2. Because the surge phenomena are fast, you need to view the raw, noninterpolated Spice data and not the interpolated (.PRINT) data. Using IntuSoft's IntuScope graphical-investigation tool, you can easily explore both types of data. Figure 3 shows the Spice results. Oscilloscope measurements on an actual circuit show close correlation with the Spice simulation.

A second test uses the spark-gap device as a real surge arrestor. The power mains supply a device protected by an arrestor. A 1-µsec transient, added to the supply voltage, sets off the arrestor. Figure 4 shows the Spice-simulated results. You simulate the dv/dt effects by driving the arrestor with different slopes. As the slope increases, so does the ignition voltage. Figure 5 shows the resulting curves. This model runs fast and converges with no difficulties. You could easily derive a warm- or cold-cathode fluorescent-lamp model from this method. IntuSoft also supplies a Spice-model library for various surge-arrestor devices. (DI #2053)

References

  1. "An electrical surge arrestor (ESA) model for electro-magnetic pulse analysis," Rockwell International Electronics Operations, IEEE Transactions on Nuclear Science, Vol NS-24, December 1977.

  2. Narui, M, "A Spice model for simulating arc-discharge load," 1991 IEEE Industry Applications Society Annual Meeting, Volume II.
Figure 1 Figure 2
 
Back-to-back zener diodes and a voltage-controlled switch form the heart of a circuit that simulates a spark-gap surge arrestor. A self-relaxing configuration replaces the Siemens A81-C90X arrestor with the model in Figure 1 for the Spice simulation.
Figure 3 Figure 4
The Spice simulation of the self-relaxing configuration in Figure 2 shows the beginning and end of the spark-gap arcing region. Spice results show that a surge in the line voltage provokes clamping at the arcing voltage; the normal line voltage returns after the current falls below the arc-sustaining value.
Figure 5
The ignition voltage of a spark-gap surge arrestor is a direct function of the dv/dt slope of the surge signal.
Listing 1--Spice simulation of a spark-gap surge arrestor

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