Surge arrestors offer ultra capacitance for xDSL applications

12 July 2006 Circuit & System Protection

STMicroelectronics has introduced a range of TRISILs designed to protect ADSL/VDSL modems and similar high bit-rate communications equipment on the CPE (customer premises equipment) side.

Fully compatible with digital transmission standards such as ADSL2 and ADSL2+, the SMP80MC family of micro-capacitance transient surge arrestors is said to be the first TRISIL series for which the junction capacitance does not depend on the device breakdown voltage - the first family of devices to combine an 80 A repetitive peak pulse current rating with a low capacitance of only 12 pF, typical.

Telecoms systems are particularly vulnerable to lightning-induced surges and two levels of protection are normally required: a primary suppressor on each line to absorb the major part of the overvoltage transient and secondary protection devices mounted on individual PCBs to eliminate the residual overvoltage spikes. The primary suppressors must have high surge current capabilities and be able to withstand very high voltages.

According to ST, its TRISIL technology offers superior performance, including a long service life without ageing, tight voltage tolerance, fast response times and failsafe operation, compared to non-silicon alternatives, such as gas discharge tubes. Moreover, the failure mode in the event of an extreme lightning surge results in a short circuit that protects both equipment and personnel.

Compliant with major international standards, the expanded family provides fast, bidirectional crowbar protection. By optimising the trade-off between repetitive peak pulse current and capacitance for broadband transmission, ST has engineered devices having a surge capability of 80 A (10/1000 μs) and a capacitance of 12 pF at 50 V.

Key parameters of the devices include voltage range from 120 V to 270 V, a maximum leakage current of only 2 μA, a maximum breakover voltage up to 345 V and a minimum holding current of 150 mA. The devices are housed in surface-mount SMB packages (JEDEC DO-214AA).

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