TISP61089 Gated Protector Series
APPLICATIONS INFORMATION
Gated Protectors
This section covers three topics. First, it is explained why gated protectors are needed. Second, the voltage limiting action of the protector is
described. Third, an example application circuit is described.
Purpose of Gated Protectors
Fixed voltage thyristor overvoltage protectors have been used since the early 1980s to protect monolithic SLICs (Subscriber Line Interface
Circuits) against overvoltages on the telephone line caused by lightning, a.c. power contact and induction. As the SLIC was usually powered
from a fixed voltage negative supply rail, the limiting voltage of the protector could also be a fixed value. The TISP1072F3 is a typical example
of a fixed voltage SLIC protector.
SLICs have become more sophisticated. To minimize power consumption, some designs automatically adjust the supply voltage, V
BAT
, to a
value that is just sufficient to drive the required line current. For short lines the supply voltage would be set low, but for long lines, a higher
supply voltage would be generated to drive sufficient line current. The optimum protection for this type of SLIC would be given by a protection
voltage which tracks the SLIC supply voltage. This can be achieved by connecting the protection thyristor gate to the SLIC supply, Figure 3.
This gated (programmable) protection arrangement minimizes the voltage stress on the SLIC, no matter what value of supply voltage.
TIP
WIRE
600
GENERATOR
SOURCE
RESISTANCE
600
RING
WIRE
AC
GENERATOR
0 - 600 V rms
C1
220 nF
I
G
I
SLIC
I
BAT
V
BAT
C2
D1
AI6XAGB
'61089
RSa
40
Th4
SLIC
RSb
40
Th5
SWITCHING M ODE
POWER SUPPLY
Tx
Figure 3. ‘61089 Buffered Gate Protector
Operation of Gated Protectors
Figures 4 and 5 show how the ’61089 device limits negative and positive overvoltages. Positive overvoltages (Figure 5) are clipped by the
antiparallel diodes in the ’61089 protector and the resulting current is diverted to ground. Negative overvoltages (Figure 4) are initially clipped
close to the SLIC negative supply rail value (V
BAT
). If sufficient current is available from the overvoltage, then the protector (Th5) will crowbar
into a low voltage on-state condition. As the overvoltage subsides the high holding current of the crowbar prevents d.c. latchup. The
protection voltage will be the sum of the gate supply (V
BAT
) and the peak gate-cathode voltage (V
GK(BO)
). The protection voltage will be
increased if there is a long connection between the gate decoupling capacitor, C1, and the gate terminal. During the initial rise of a fast
impulse, the gate current (I
G
) is the same as the cathode current (I
K
). Rates of 70 A/µs can cause inductive voltages of 0.7 V in 2.5 cm of
printed wiring track. To minimize this inductive voltage increase of protection voltage, the length of the capacitor to gate terminal tracking
should be minimized. Inductive voltages in the protector cathode wiring will also increase the protection voltage. These voltages can be
minimized by routing the SLIC connection through the protector as shown in Figure 3.
Application Circuit
Figure 6 shows a typical ’61089 part SLIC card protection circuit. The incoming line conductors, Ring (R) and Tip (T), connect to the relay
matrix via the series overcurrent protection. Fusible resistors, fuses and positive temperature coefficient (PTC) thermistors can be used for
overcurrent protection. Resistors will reduce the prospective current from the surge generator for both the ’61089 device and the ring/test
NOVEMBER 1995 - REVISED JANUARY 2007
Specifications are subject to change without notice.
Customers should verify actual device performance in their specific applications.
BOURNS [ BOURNS ELECTRONIC SOLUTIONS ]
