AN9768 Datasheet, PDF (6/8 Pages) Littelfuse – Transient Suppression Devices and Principles

Application Note 9768
The gas discharge device may experience follow-current. As
the AC voltage passes through zero at the end of every half
cycle the arc will extinguish, but if the electrodes are hot and
the gas is ionized, it may reignite on the next cycle.
Depending on the power source, this current may be
sufficient to cause damage to the electrodes. The follow
current can be reduced by placing a limiting resistor in series
with the device, or, selecting a GDT specifically designed for
this application with a high follow-current threshold.
The gas discharge device is useful for high current surges
and it is often advantageous to provide another suppression
device in a combination that allows the added suppressor to
protect against the high initial impulse. Several hybrid
combinations with a varistor or avalanche diode are
possible.
Comparison of Zener Diode and Littelfuse
Varistor Transient Suppressors
Peak Pulse Power
Transient suppressors have to be optimized to absorb large
amounts of power or energy in a short time duration:
nanoseconds, microseconds, or milliseconds in some
instances.
Electrical energy is transformed into heat and has to be
distributed instantaneously throughout the device. Transient
thermal impedance is much more important than steady
state thermal impedance, as it keeps peak junction
temperature to a minimum. In other words, heat should be
instantly and evenly distributed throughout the device.
The varistor meets these requirements: an extremely reliable
device with large overload capability. Zener diodes dissipate
electrical energy into heat in the depletion region of the die,
resulting in high peak temperature.
Figure 6 shows Peak Pulse Power vs Pulse width for the
V8ZA2 and the P6KE 6.8, the same devices compared for
leakage current.
200
100
60kW
50
10kW
20
10
5
2
600W _ZENER
7kW
P6KE
6.8
V8ZA2 _{10mm DEVICE}
3.5kW
1
0.5
0.2
0.1
100ns 200
1µs 2
10µs 20
PULSE TIME
100µs
1000µs
FIGURE 6. PEAK PULSE POWER vs PULSE TIME
At 1ms, the two devices are almost the same. At 2µs the
varistor is almost 10 times greater, 7kW for the P6KE 6.8
Zener vs 60kW for the varistor V8ZA2.
Clamping Voltage
Clamping voltage is an important feature of a transient
suppressor. Zener diode type devices have lower clamping
voltages than varistors. Because these protective devices
are connected in parallel with the device or system to be
protected, a lower clamping voltage can be advantageous in
certain applications.
VARISTOR
ZENER
CURRENT
FIGURE 7. CHARACTERISTICS OF ZENER AND VARISTOR
Speed of Response
Response times of less than 1ps are sometimes claimed for
zener diodes, but these claims are not supported by data in
practical applications. For the varistor, measurements were
made down to 500ps with a voltage rise time (dv/dt) of 1
million volts per microsecond. These measurements are
described in Application Note AN9767. Another
consideration is the lead effect. Detailed information on the
lead effect can be found further in this section and in
Application Note AN9773. In summary, both devices are fast
enough to respond to real world transient events.
Leakage Current
Leakage current can be an area of misconception when
comparing a varistor and zener diode, for example. Figure 8
shows a P6KE 6.8 and a V8ZA2, both recommended by their
manufacturers for protection of integrated circuits having 5V
supply voltages.
10-107