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5080H3

FORWARD-CONDUCTING UNIDIRECTIONAL THYRISTOR OVERVOLTAGE PROTECTORS

Power Innovations 

TISP5070H3BJ, TISP5080H3BJ, TISP5110H3BJ, TISP5150H3BJ

FORWARD-CONDUCTING

UNIDIRECTIONAL THYRISTOR OVERVOLTAGE PROTECTORS

JANUARY 1998 - REVISED MARCH 1999

broad-band protection.

The star-connection of three TISP5xxxH3BJ protectors gives a protection circuit which has a low differential

capacitance to ground (Figure 13). This example, a -100 V ISDN line is protected. In Figure 13, the circuit

illustration A shows that protector Th1 will be forward biased as it is connected to the most negative potential.

The other two protectors, Th2 and Th3 will be reverse biased as protector Th1 will pull their common

connection to within 0.5 V of the negative voltage supply.

Th1

Th3

Th2

C0.5 V

600 pF

SIGNAL

C-99.5 V

29 pF

29 pF

C-99.5 V

26 pF

26 pF

1 pF

A) STAR-CONNECTED

U-INTERFACE

PROTECTOR

- 100 V

B) EQUIVALENT

TISP5150H3BJ

CAPACITANCES

- 100 V

C) DELTA EQUIVALENT

SHOWS 25 pF

LINE UNBALANCE

- 100 V

AI4XAB

Figure 13. ISDN LOW CAPACITANCE U-INTERFACE PROTECTION

Illustration B shows the equivalent capacitances of the two reverse biased protectors (Th2 and Th3) as 29 pF

each and the capacitance of the forward biased protector (Th1) as 600 pF. Illustration C shows the delta

equivalent of the star capacitances of illustration B. The protector circuit differential capacitance will be 26 - 1

= 25 pF. In this circuit, the differential capacitance value cannot exceed the capacitance value of the ground

protector (Th3).

A bridge circuit can be used for low capacitance differential. Whatever the potential of the ring and tip

conductors are in Figure 14, the array of steering diodes, D1 through to D6, ensure that terminal 1 of

protector Th1 is always positive with respect to terminal 2. The protection voltage will be the sum of the

protector Th1, V(BO), and the forward voltage of the appropriate series diodes. It is important to select the

correct diodes. Diodes D3 through to D6 divert the currents from the ring and tip lines. Diodes D1 and D2 will

carry the sum of the ring and tip currents and so conduct twice the current of the other four diodes. The

diodes need to be specified for forward recovery voltage, VFRM, under the expected impulse conditions.

(Some conventional a.c. rectifiers can produce as much as 70 V of forward recovery voltage, which would be

an extra 140 V added to the V(BO) of Th1). In principle the bridge circuit can be extended to protect more than

two conductors by adding extra legs to the bridge.

RING TIP

1

D1 D3 D5

2 Th1

D2 D4 D6

AI5XAC

Figure 14. LOW CAPACITANCE BRIDGE PROTECTION CIRCUIT

PRODUCT INFORMATION

9

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