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How to safeguard cellular base stations from five electrical hazards

Circuit-protection components such as fuses and TVS diodes protect power and data circuits from damage. Here’s where and how to insert them into your circuits.

how to safeguard cellular base stations from five electrical hazardsThe next generation of cellular communications, the 5G network, will help the IoT reach its full potential. IoT includes many devices and physical objects such as home appliances, vehicles, and “smart” cities.

IoT connectivity depends on the advancements of the 5G network compared to the current 4G LTE networks. The download speed of up to 1 Gbps for 5G, up to ten times faster than 4G, and lower latency, allows for swift file transfers. 5G networks also offer greater capacity to handle more traffic and greater consistency, which can unlock economic and societal opportunities.

With those opportunities comes the need for increased investment in supporting infrastructure. This need includes small-cell and macro cell base stations, with the small cells located on rooftops or light poles to transmit mmWave signals starting at 24 GHz. Millimeter waves can travel only a short distance and can be blocked or disrupted by walls, windows, and even weather. Macro cells generally operate via an antenna mounted on a more traditional tower, which is subject to hazards that can affect the reliability and lifetime of the equipment. Engineers must protect circuits and systems from:

  • Transient voltage surges resulting from large inductive load switching caused by motors,
  • Current overloads,
  • Electrostatic discharge (ESD), and
  • Short circuits.

As a design engineer, you can add protection to this 5G infrastructure by creating circuits to protect against electrical hazards.

Begin with a detailed description of a macro base station and recommendations for protecting the base station circuitry. Two crucial focus areas are the tower-mounted amplifier and the advanced antenna systems.

Protecting the macro base station
The base station connects to individual mobile phones and other wireless tools such as tablets, smartwatches, and IoT devices through a core network. The base station is a fixed transceiver that acts as the primary transmission and reception communication hub for wireless devices. The base station modulates baseband information and transmits it to mobile devices. Base stations also receive mobile device transmissions, modulate them, and send them to the wireline infrastructure.

Macro base stations reside on towers ranging in height from 50 ft. to 200 ft. These are highly visible structures and strategically located to maximize coverage within a defined geographic area. The base station connects to all wireless devices attempting communication within that geographic or coverage area.

A 5G base station will include advanced, active antenna systems populated by numerous antennas in multiple input-multiple output (MI MO) configurations. These antennas provide:

  • Faster data transmission rates,
  • Higher transmission and reception capacity, and
  • More efficient delivery of RF power.
cellular base station

Figure 1. Macro base station including with an Advanced Antenna Array needs circuit protection in several places.

Figure 1 details the elements that comprise a base station and lists the recommended protection, control, and sensing components—they supply a dual purpose: to protect and to improve the base station circuit efficiency.

circuit protection

Figure 2. Macro base stations include protection from lightning, over voltages, and other conditions.

Figure 2 shows a base station circuit block diagram.

Protection components inside the surge-protection device
The AC power line interfaces with the surge protection device (SPD). As a result, the SPD is also subject to all the transients that can impact the AC power line. In this situation, the best option is a surge-suppression fuse on the input of the surge-protection circuit. These fuses can withstand lightning surges up to 200 kA based on transient surges defined in UL 1449 Surge Protective Devices, UL 1449 and IEC 61000-4-5. Under short-circuit conditions, this fuse also provides current-limiting protection.

As further protection to absorb a lightning strike or other large transients provoked by load changes on the power line, consider using a series combination of a metal oxide varistor (MOV) and gas discharge tube (GDT). Place the combined MOV-GDT as close to the entrance of the AC line on the circuit as possible to minimize transient propagation into the circuit. Also, connect the MOV between Line and Neutral and connect the GDT from Neutral to Ground. As an alternative to an MOV, in certain circumstances, a high-power transient voltage suppressor (TVS) diode can suffice. The maximum surge-handling capacity of the TVS diode is adequate for the AC power line feed and it has faster response times and can clamp transients at lower voltages.

base station subsystems

Figure 3. Fuses, TVS diodes, and MOVs protect an Advanced Antenna System.

Advanced Antenna System protection
An Advanced Antenna System (AAS) receives and transmits information, audio, and data communications from and to mobile wireless devices within a defined geographic range. See the circuit block diagram for an AAS in Figure 3. The digital packets from the Baseband Unit (BBU) are converted to analog data, then upconverted for RF transmission. Any RF signals received are down-converted and digitized for transmission to the BBU.

Protecting the tower-mounted amplifier
Anything exposed to the outdoors, such as tower-mounted amplifiers, is prone to lightning strikes and ESD. A series fuse and a parallel TVS diode can work to protect against current overloads and absorb lightning or ESD transient strikes. A surface-mount TVS diode package can help overcome space constraints while safely absorbing current overloads as high as 10 kA.

Power input circuit
The power-input circuit provides DC power for the other AAS circuits. A best practice for this DC circuit would be a fast-acting fuse for overcurrent protection. Use surface-mount, fast-acting versions to save space. Consider an MOV and GDT in a series as an additional option to protect the front end of the Power Input circuit from transients that have passed through the SPD and the power supply and backup-battery circuit.

Additionally, a TVS diode at the Power Input circuit’s back end can also supply protection. TVS diodes have a lower clamping voltage than MOVs, which enables the use of lower-voltage-rated and lower-cost components in the downstream circuits.

Circuit-protection diode array

Figure 4. (a) Power-over-Ethernet (PoE) protection using two-line protection thyristor. (b) I-V curve of a protection thyristor.

Ethernet, RS-232, and RS-485 communication circuits
You can establish transient protection with crowbar protection components to protect the integrity of communication ports. Figure 4 shows a protection thyristor, which protects two data lines from ESD strikes when a Power-over-Ethernet (PoE) communication link is in play. Another option is a TVS diode array along with a gas discharge tube.

Figure 5 shows an example of a two-line TVS diode array that employs a Zener diode for clamping a transient, rather than a protection thyristor, which crowbars the transient. Consider using low-capacitance versions of these components to minimize the impact on the quality of the data transmissions. When the protocol is PoE, include a fuse to protect the Ethernet circuit from any overload from crossed lines connecting to the circuit.

Circuit protection for Ethernet

Figure 5. Two-line TVS diode array with a parallel Zener diode can protect Ethernet circuits.

For RS-232 and RS-485 interfaces, consider using a protection thyristor and a gas discharge tube (GDT) combination for transient protection. A resettable polymeric positive temperature coefficient fuse provides increased design flexibility when seeking crossed-line and current overload protection.

Baseband processor unit, network controller, and RF front-end power amplifier
Use a TVS diode to provide transient and ESD protection for these three circuits. Versions of TVS diodes can absorb up to 30 kV from an ESD strike. Also, versions are available in uni-directional or bi-directional formats in surface mount packaging.

Antenna Array
A polymer ESD suppression device, designed to absorb fast-rising ESD transients up to magnitudes of 30 kV, can help protect components such as the Antenna Array, which is directly exposed to atmospheric conditions. These devices have extremely low capacitance to minimize loading on the antenna that can distort transmitted and received signals. Versions of polymer ESD suppressors can have capacitances on the order of 0.09 pF and lower values.

BBU circuit protectin locations

Figure 6. Baseband Units need electrical protection at the power circuits, processors, and I/O lines.

Baseband Unit
The BBU links the AAS and the wireline infrastructure, encoding transmissions and decoding received signals while processing data from calls and transmissions. The same components recommended for the AAS power-input uUnit will supply the same protection for this circuit. A straightforward method is to copy the protection scheme used for the AAS Ethernet circuit on the BBU’s Ethernet circuit. Figure 6 illustrates the BBU and its dedicated power supply.

HDMI interface data lines should also have ESD protection. Consider using a 4-line TVS diode array, as shown in Figure 6, to absorb ESD strikes up to 20 kV. Look for TVS diode arrays that have low leakage currents (below 50 nA) and low capacitance (under 0.5 pF) to minimize disturbance for high-speed HDMI transmissions.

4-line TVS diode array

Figure 7. A 4-line TVS diode array provides I/O line protection.

Consider voltage transient protection for the DSP, the critical block in the BBU. As with other circuits in the AAS, consider a TVS diode (Figure 7) to provide unidirectional or bidirectional ESD protection up to 30 kV.
Power supply and backup battery system
When AC line power is down or disabled, attain AC line power and DC battery backup from the power supply and backup battery System. Figure 8 shows the circuit blocks of these systems.

The input protection, rectifier, and filter circuit (block 1 in Figure 8) convert the AC input to DC. Due to its interface with the AC line, it needs the full suite of overcurrent and overvoltage transient protection. For current overload and short circuit protection, consider:

  • Use a fast-acting fuse to prevent damage to the power semiconductors in the power supply.
  • Be sure that the fuse selected has a current rating to avoid nuisance failures due to the power supply’s inrush current,
  • Confirm the fuse’s voltage rating exceeds the voltage on the AC line.
Power supply BBU circuit protection

Figure 8. Power supply and backup battery systems are among the parts of a BBU that need electrical protection.

As with the other power-supply circuits, incorporate an MOV-GDT combination across the input line to absorb AC line-induced voltage transients and protect the circuit. Try a TVS diode in the circuit to increase immunity to transient surges and improve long-term reliability. Finally, consider adding a magnetic sensor into the circuit to ensure that the power supply is turned off when the electronics enclosure door is opened.

The high-frequency converter and clamp circuit (block 2) convert the rectified AC line voltage into a pulsed waveform in the kilohertz frequency range. Use a TVS diode to absorb any transients that passed through the input circuitry and protect the downstream circuitry. Try a MOSFET with low RDS(ON) and high dV/dt to maximize the switching power supply’s efficiency and reduce on-state power consumption and switching power losses.

The output rectification and filter circuit (block 3) converts the pulsed voltage back to DC. Use Schottky diode rectifiers with ultra-low forward voltage to reduce losses in the circuit. Improve supply efficiency with a low forward voltage drop, Schottky diode.

Utilize a fast-acting fuse in the output DC protection circuit (block 4) to protect the power supply from any overload failures in the loads, including the Advanced Antenna System and the Baseband Unit.

A backup battery (block 5) is one of the best ways to support the base station when AC power is interrupted. Support the base station by:

  • Providing a fast-acting fuse on the battery circuit for overload protection.
  • Monitoring battery temperature rise to ensure battery safety.
  • Placing surface mount thermistors on the battery pack modules.
  • Protecting the battery pack modules from overcharging.

A three-terminal device could replace a fast-acting fuse to detect an overvoltage condition and disconnect the modules from the charging voltage.

A mini-breaker, which can be a switch paralleled with a polymer-positive temperature coefficient, can back up the circuit’s battery management IC. This component provides the battery pack with overtemperature and overcurrent protection, preventing the battery pack from entering either an over-charge or under-discharge condition.

The battery management system in the backup battery circuit has voltage-sense lines connecting to each battery pack’s individual cells. These sense lines are susceptible to ESD and other voltage transients. Use TVS diode arrays consisting of a package with two TVS diodes connected anode-to-anode on the sense lines for bipolar transient protection.

When the backup battery circuit uses the I2C communication protocol to transmit the status of the battery pack from the fuel gauge IC to the battery management IC, consider utilizing a polymer positive temperature coefficient component for limiting current on the I2C lines during a high voltage transient. This series component protects the overvoltage components that safely absorb transients on the I2C data lines.

The importance of designing for maximum uptime
Communication infrastructure must have extremely high reliability so uptime can exceed 99.9%. Using the recommended component technologies will help provide the high reliability needed for wireless communication infrastructure by protecting the circuitry from the five sources of electrical hazards. The low cost of the components outweighs the high cost of a base station failure and the disruption to communication.

Protect your reputation as a reliable 5G equipment supplier and gain a competitive advantage by protecting the base station from overload current and voltage transient hazards.

Prasad Tawade LittelfusPrasad Tawade is a strategic marketing manager for the Electronics Business Unit of Littelfuse. His current responsibilities include managing go to market strategy for new products, developing monthly application spotlights and distribution marketing initiatives. Prasad joined Littelfuse in December 2018 during the acquisition of IXYS, where he was the EMEA Distribution Sales Manager. Before IXYS, he worked as a product manager at Pericom Semiconductor and Cypress Semiconductor. Prasad holds a BSEE from Pune University and a Master of Engineering Management degree from Duke University.

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