Protecting Plain Old Telephone Customer Premise Equipment with a TISP

Category: Telephone, cellular phone and intercom
Manufacture: Power Innovations
Datasheet: Download this application note

Circuit Protection Technical Article Protecting Plain Old Telephone Customer Premise Equipment with a TISP Written by: Tim Ardley 2003 This engineering note has been written to provide the Engineer with a first principles theory when designing twisted-pair, Plain Old Telephone Service (POTS) equipment such as the telephone, fax or modem. Telecom chipsets are now commonly used in the front-end of these applications where protection against lighting disturbances is a key concern for increased reliability of the equipment and ensuring telecom standards conformance. Telecom standards for Customer premise equipment The countries standards or recommendations govern what protection is required for Customer Premise Equipment (CPE). CPE needs to conform to the specifications defined for the country where the USA equipment is required to meet TIA-968-A for lighting and UL 60950 (UL 1950) for a.c power line cross. Most other countries have adopted the ITU-T recommendations. The ITU-T K.21 document is pertinent to CPE equipment and uses the test circuits defined in ITU-T recommendation K.44. These recommendations have gone through a major iteration in year 2000 to include a higher (enhanced) test level for locations that have severe lighting currents and a primary and secondary protection impulse co-ordination test. More information is available in the article The new ITU-T Telecommunication Equipment Resistibility Recommendations by Mick J. Maytum and a copy is available from Bourns. USA Customer Premise Equipment Requirements The TIA-968-A is has two types of lighting surge test, A and B. Type A has been designed to test for any high stress failure mechanism of the CPE. The test allows equipment to fail, but only in a safe mode that is not harmful to the network. Any protection failure must be an open circuit condition making the CPE noticeably unusable after the surge. Type A metallic (transverse) testing applies two surges (one of each polarity) between any pair of line connectors on which lighting surges may occur. This test will be applied between Tip to Ring connections. For a 4wire connection that uses simplexed pairs for signaling, additional impulse test is required between Tip to Ring1 and Ring to Tip1. The impulse voltage and current waveform is a 10/560 s with an open circuit voltage of 800 V and current of 100 A. For longitudinal applications, voltage surges between the conductors and earth grounding connections and to all leads intended for connection to non-registered equipment. Longitudinal testing uses a different 10/160 s test waveform with a peak open-circuit voltage of 1500 V and short circuit current of 200 A.
TIA-968--A Specifications Longitudinal Metallic Longitudinal Metallic Wave Shape (t1/t2 s) 10/160 10/560 9/720, (5/320) 9/720, (5/320) Open circuit voltage 1500 V 800 V 1500 V 1000 V Short circuit current 200 A 100 A 37.5 A 25 A B Surge Type
Type B surges reflect normal lighting surge exposure and the CPE must not degrade or fail during these tests. The metallic test (one of each polarity between Tip to Ring) uses a 9/720 s, 1000 V open-circuit voltage with 5/320 s, 25 A short circuit current. Longitudinal tests use the same generator, but with an open-circuit voltage of 1500 V and short circuit current of 37.5 A.
Circuit Protection Technical Article
Selecting over-voltage protection
Figure 1. Telephone/FAX Protection The maximum working voltage (VDRM) of the thyristor needs to be considered to ensure there is no clipping during normal system operation. TIA-968-A in the USA requires a minimum VDRM of --269 V (VPEAK= -56.5 V + (-150 V x 2 ) to work with a battery-backed B type ringer voltage levels. Low ambient temperatures also need to be considered for reliable operation. The o thyristor s VD R M value decreases as the ambient temperature falls. To work down to 0 C o ambient, the thyristor s VD R M should be rated for at least 275 V at 25 C. If lower operating o ambient temperature, TAMIN is required, the 25 C ambient VDRM selection can be calculated by o using a temperature coefficient of --0.000846/ C.
VPEAK 1 - 0.000846 25o C - TAMIN
The protection voltage, V(B O ) of the thyristor is set by the maximum voltage rating of the DAA (Data Access Arrangement) chipset or hook switch. The hook switch is now commonly implemented using either a transistor or opto-coupler. A 400 V switch transistor will require a thyristor protection voltage to be limited to 350 V. 15 % of headroom between the maximum withstand voltage and V(B O ) of the thyristor to provide reliable operation under extended temperatures. Some DAA chipsets have a maximum switch transistor rating of 300 V and therefore a thyristor with a protection voltage V(B O ) of 290 V or lower should be considered. The TISP4290M3BJR has an impulse break-over voltage rating of 298 V. However, this has a minimum VD R M of 220 V and ring clipping could occur under the type B ringer conditions. The type B ringer levels are historical and in practice with a --60 V battery and 100 V rms ring, a VD R M of 200V will be suitable in most applications. If high temperature operation is required, a thyristor with a V(BO) of 265 V (VDRM = 200 V) should be considered to provide the additional 15 % of voltage headroom. Selecting over-current protection Most applications also require an over-current protector for high current a.c power line cross tests. If a fuse is deployed, then the thyristor needs to be rated for the impulse short circuit currents applied. In the case of the TIA-968-A, the thyristor needs to meet 100 A at 10/560 s for metallic (2-wire applications). Bourns TISP4350T3BJR would be a suitable thyristor to consider in this instance.

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