Judicious use of the appropriate protective circuits and devices in cellular base stations can greatly improve reliability and ensure required compliance with YD5098-2005.

By Teddy To, China, Littelfuse Inc.

click to enlarge Figure 1. Cellular base stations require protection in multiple areas.

Despite the worldwide recession, the Chinese economy continues to expand, and in an effort to move it along faster the Chinese government announced in November of 2008 an economic stimulus package totaling a massive $586 billion. Part of this package ($8.8 billion) calls for greatly increasing the number of cellular towers as the best way to cover territory quickly and bring communications into new parts of the country. China Mobile, the country’s largest mobile carrier with 471 million subscribers as of February, plans to build 100,000 3G base stations by 2011. According to a recent report the company will build 85,000 TD-SCDMA cell towers in 238 Chinese cities by the end of 2009. Many of these new base stations will carry three different 3G standards at the same time: TD-SCDMA, W-CDMA and CDMA-2000.

All of these will need protection from over voltages and over currents caused by lightning and power line accidents and disturbances.

In order to protect the POTS, TDM (T1 and E1), Ethernet, and even DSL lines that are used in base stations, designers must know the most appropriate circuit protection methods for each of the different lines. For both baseband and broadband lines, the designer must understand how circuit protection devices affect capacitance and signal integrity. This article will describe the challenge facing the design engineer and offer recommendations on how to provide the needed circuit protection.

The Problem

click to enlarge Figure 2. The IEEE C62.45 8/20 µs combination waveform is used in evaluating protective devices.

The main sources of danger to cellular base stations are lightning and power faults. A nearby lightning strike can induce large voltages and currents in signal and power lines connected to the base station, as well as elevate the ground potential. A strike directly to a cell tower can produce very high voltages and cause huge currents to flow.

Base stations have multiple lines to protect (see Figure 1.) Antennas are susceptible to lightning and other atmospheric disturbances. The lines between different co-located services are a potential entry point for damaging voltages. The backhaul circuits between the base station and telecom network and between the base station and base station controller and, in some cases, even RS-232 and RS-485 interfaces for remote monitoring are also targets for surges. In addition to these are the AC and DC power input lines.

Antennas and their Cables

click to enlarge Figure 3. For lightning protection of antenna-connected equipment inside the base station, the usual practice is to use a GDT with a surge capability of at least 10 kA for the IEEE C62.45 8/20 µs combination waveform.

For protection of the equipment inside the base station, the normal practice is to use a GDT (Gas Discharge Tube, AKA a Gas Plasma Over voltage Arrester) with a surge capability of at least 10 kA for an IEEE C62.45 8/20 µs combination waveform (see Figure 2); this protects the coaxial cable connection at the input port of the RF board in the telecom equipment running from the antenna. Figure 3 shows a suggested arrangement.

Telecom Lines

The most common over voltage condition on telecom lines are longitudinal (common-mode), although metallic (differential mode) may also occur. Current surges can reach 3 kA 8/20 µs waveshape; there can also be single over voltages of 2500 V and over currents of 500 A. Protective devices are obviously necessary, but choosing specific devices to use is not such a simple task, because some protective devices are not compatible with the steady-state signals (Broadband or Baseband signals) on the lines.

Signals on telecom lines start with POTS (plain old telephone service), then frequently add DSL service, which can involve high frequencies: ADSL2 has a bandwidth of up to 1.1 MHz, ADSL2+ extends up to 2.2 MHz, and VDSL2 goes up to 30 MHz. At these frequencies the line’s impedance characteristics become important, but some protective devices exhibit voltage-variable capacitance.

Normal voltages on a telecom line include battery voltage up to -52 VDC plus a 20 Hz ringing voltage up to 90 Vrms, which gives a peak worst-case value of 180 V. This sets the lowest turn-on threshold value for the over voltage protection so that it does not interfere with normal operation of the telcom interface.

click to enlarge Figure 4. The best primary protection devices for telecom lines are generally two-chamber GDTs (3-lead devices), backed with fuses and/or fail-safe devices.

Telecom line protection is best done in two or three stages, with primary protection located at the entrance to the interface, secondary protection on the circuit boards themselves, and tertiary protection located on the secondary side of coupling transformers.

As a general rule the best primary protection devices for telecom lines are two-chamber GDTs (3-lead devices), backed with fuses and/or fail-safe devices (see Figure 4.) The GDT has low capacitance and can handle high currents, and in this application its physical size does not cause a problem.

For secondary protection, the most appropriate devices are a pair of solid-state thyristor based devices (SSTBD) to shunt over voltages on tip and ring to ground, backed up with a pair of fuses or PTCs (positive temperature coefficient thermistors) to handle the over currents during any surge events. The thyristor based devices’ small size allows them to be mounted directly to the circuit board. Figure 5 shows thyristor based protection in a biased balanced bridge circuit, which minimizes capacitance variations with voltage.

Backhaul Lines

click to enlarge Figure 5. For secondary protection of telecom lines the most appropriate devices are a pair of SSTBDs backed up with a pair of fuses or PTCs. This biased balanced bridge circuit minimizes capacitance variations in the SSTBD.

Depending on the installation area and the state of the available terrestrial facilities, backhaul lines may be leased line TDM (the older method) or IP (Ethernet 10/100/1000baseT), which may be deployed on bonded copper lines using such methods as 2Base-TL. There is also a hybrid approach, in which voice traffic is carried on TDM and data traffic via Ethernet. SSTBDs can provide good protection for TDM lines, as shown in Figure 6. While low-capacitance SSTBDs are available, in some high-speed Ethernet circuits ultrafast diodes (with capacitance of about 10 pF) connected in inverse parallel are needed to reduce the capacitance still further, as shown in Figure 7.

Power Input Lines

Chinese national standard YD 5098-2005, “Specifications of Engineering Design for Lightning Protection and Earthing Design for Telecommunication Bureaus (Stations),” available only in Chinese, covers mobile base station and telecom central office protection and specifies the different surge requirements in different parts of the power path, such as AC/DC and DC/DC.

The primary AC input to a base station can experience over voltages due to nearby lightning strike (through inductive coupling), through short circuits to higher-voltage transmission lines, and though switching transients. It has been found that the most appropriate protection for these lines is a combination of fuses and high-power MOVs installed in the AC power distribution box. The devices should be rated for 60 kA on the 8/20 waveform in urban and rural areas, and 80 kA in mountainous areas with higher chances of lightning strike events. In certain exposed environments the required surge capacity can reach 100 kA.

click to enlarge Figure 6. SSTBDs can provide good protection for TDM backhaul lines.

The secondary AC lines (for example, the input to a switching mode power supply), should be protected similarly, with fuses and high power MOVs, although the MOVs in this instance do not need as high a rating as those on the primary AC distribution box; a 40 kA 8/20 rating should be sufficient.

It is wise to choose an MOV that can not only handle the maximum anticipated current, but do it a reasonable number of times. MOVs have a wearout mechanism, and can absorb only so much energy over a lifetime (expressed as the Joule rating of the device); scrimping on the MOV can actually cost more than putting in a generous-size one, as it can require repeated trips for maintenance, and can also lead to unnecessary downtime.

The 48 V DC lines within the base station also require protection; here the devices of choice are fuses and high-power transient voltage suppressor (TVS) diodes, which in recent years have become available with ratings of 15 kA and more on the 8/20 combined waveform. TVS diodes feature fast switching times, low clamping voltages and small size, and do not suffer the aging effects that MOVs suffer.

RS-232 & RS-485 Lines

Figure 7. While low-capacitance SSTBDs are available, in some high-speed Ethernet circuits ultrafast diodes (with capacitance of about 10 pF) connected in inverse parallel are needed to reduce the capacitance still further.

Some base stations include an RS-232 or RS-485 interface for remote monitoring. The frequencies involved are much lower than those for DSL lines, and the voltages are also lower. Protection can be achieved using either SSTBDs or Silicon ESD diode arrays to guard against electrostatic discharge.

Conclusion

Cellular base stations are often installed in remote and lightning-prone areas, where access for repair may be difficult and expensive. Judicious use of the appropriate protective circuits and devices can greatly improve reliability and cut the need for field service calls, as well as ensure required compliance with YD5098-2005.

Teddy To is currently the Technical Marketing Manager and was previously the Telecom Segment Manager of Littelfuse Inc. He has an extensive background in power electronics and power management ICs. To has been working in the electronic industry for more than 15 years and more than ten years in circuit protection area, especially in semiconductor surge and ESD protectors, high power silicon avalanche diodes and polymer type over current protection devices.