The telecommunications sector has become an important element of modern society and instant global communication. Whether it's a phone call, a text message, or a web command, telecommunications equipment ensures reliable connections. The power supply running behind the scenes is an essential component that is rarely recognized.
This article focuses on the MAX15258 from Analog Devices, designed to accommodate up to two MOSFET drivers and four external MOSFETs in single-phase or two-phase boost/inverter-buckle-boost configurations. It is possible to combine two devices for three-phase or four-phase operation, achieving higher levels of output power and efficiency.
Meet the growing demand for electricity
Energy demand in the telecommunications sector has grown over time, driven by technological advances, increased network traffic and the expansion of telecommunications infrastructure. The transition from third generation (3G) to fourth generation (4G) and fifth generation (5G) networks has given rise to advanced and high-power equipment.
The implementation of 5G technology has had a significant impact on the power requirements of base stations and cell towers. Base stations, especially those located in urban areas, require higher power levels to support the increased number of antennas and radio units required for massive MIMO (multiple input, multiple output) configurations and beamforming.
Redundancy is another crucial factor. Power supplies must be designed with redundancy in mind and often include backup power sources such as batteries or generators to ensure uninterrupted operation in the event of power outages.
Compared to previous generations of wireless networks, the implementation of 5G mobile technology introduces several changes to the requirements of power devices. For 5G to fulfill its promise of offering reliable, high-speed, low-latency communications, some criteria need to be taken into account.
Power amplifier requirements
- It supports a broad spectrum of frequency bands, including sub-6 GHz and mmWave (millimeter wave) frequencies, which present unique challenges for signal propagation.
- They support wider signal bandwidths and higher power levels, and provide linear amplification to prevent distortion of high data rate signals.
- They work efficiently to minimize power consumption and heat generation, especially for battery-powered devices and remote small cells.
- They include a lightweight and compact form factor that can fit into small cabinets/enclosures, such as small cell sites and user equipment.
- They incorporate advanced materials and technologies, such as semiconductor devices made of gallium nitride (GaN) and silicon carbide (SiC) to provide higher power density, improve performance and increase operating frequencies.
Power conversion requirements
For historical, practical and technical reasons, telecommunications systems typically use a -48 V power supplyCC. In the event of a network failure or other emergency, telecommunications networks need reliable backup power supplies. Commonly used for backup power, lead-acid batteries can also operate at -48 VCC. Using the same voltage for primary and standby power makes the design and maintenance of standby systems easier. Additionally, lower voltages, such as -48 VCC, are safer for personnel working with telecommunications equipment, reducing the risk of electric shock and injury.
Power supplies for telecommunications equipment must meet specific operational requirements to ensure reliability and efficiency. Here are some important specifications:
- Input voltage range: The power supply must be designed to tolerate a wide input voltage range.
- Voltage Regulation: The power supply must provide a stable and regulated output voltage according to the requirements of the telecommunications equipment.
- High efficiency: Power supplies must be highly efficient to reduce power loss and energy consumption. Efficiencies of at least 90% are typical.
- Redundancy: To ensure uninterrupted operation, power supplies often include redundancy features such as N+1, where an additional power supply is used. If one fails, the other can take over.
- Hot-swappable: In mission-critical installations, power supplies must be hot-swappable, ensuring minimal downtime during replacement or maintenance.
- High reliability: The power supply must be equipped with protection mechanisms to prevent damage caused by adverse operating conditions such as overcurrent, overvoltage and short circuits.
The direct active clamp converter
The active clamp-on forward converter (ACFC) is a common DC/DC converter configuration in power supply systems, and is primarily used to convert -48VCC at positive tension levels. The ACFC is a voltage conversion circuit that integrates features of the forward converter and active clamp circuit to improve efficiency. This technology is prevalent in power supply systems for telecommunications devices and data centers.
The central element of the ACFC is a transformer (Figure 1). The primary winding of the transformer receives the input voltage, which causes the induction of a voltage in the secondary winding. The output voltage of the transformer is determined by its turns ratio.
The active clamp circuit, incorporating supplementary semiconductor switches and a capacitor, regulates and governs the energy contained within the leakage inductance of the transformer. When the primary switch is off, the energy stored in the leakage inductance is redirected to the clamp capacitor, thus preventing voltage spikes. This practice mitigates stress on the primary breaker and improves operating efficiency. The secondary winding voltage of the transformer is rectified by a diode, and the output voltage is smoothed by an output filter capacitor. Lastly, ACFC works with smooth switching, meaning that switching transitions are smoother and produce less noise. This reduces electromagnetic interference (EMI) and switching losses.
Figure 1: ACFC Topology. (Source: Analog Devices)
The ACFC circuit reduces voltage spikes and stress on components, resulting in improved efficiency, especially at high input to output voltage ratios. Additionally, it can handle a wide range of input voltages, making it suitable for telecom and data center applications with variable input voltages.
The disadvantages of the active clamp circuit are as follows:
- If not limited to a maximum value, an increase in duty cycle can cause transformer saturation or additional stress on the main switch, requiring precise sizing of the clamp capacitor.
- The ACFC is a single stage DC-DC converter. As the power level increases, the benefits of a multiphase design for power-intensive applications such as telecommunications will increase.
- A forward active clamp design cannot scale to higher power output and maintain similar performance.
Exceed the limits of the ACFC
The MAX15258 from Analog Devices is a high voltage multiphase boost controller with digital interface I2C designed for industrial and telecommunications applications. The device features a wide input voltage range of 8V to 76V for the boost configuration and -8V to -76V for the boost/buckle inverter configuration. The output voltage range, from 3.3 V to 60 V, covers the requirements of various applications, including telecommunications devices.
A typical application of this versatile IC is the power supply of a 5G macrocell or femtocell, as shown in Figure 2. The hot-swap function is ensured by a negative voltage hot-swap controller, such as ADI's ADM1073, powered by -48 VCC. The same voltage powers the MAX15258 buck/boost converter, which is capable of providing up to 800W of output power.
Figure 2: Block diagram of a power supply stage for 5G applications. (Source: Analog Devices)
The MAX15258 is designed to support up to two MOSFET drivers and four external MOSFETs in single-phase or two-phase boost/invert-buck-boost configurations. It also combines two devices for three-phase or four-phase operation. It has an internal high voltage FB level shifter to differentially detect the output voltage when configured as an inverter buck/boost converter. Via a dedicated reference input pin/jack or via a digital interface I2C, the output voltage can be adjusted dynamically.
An external resistor can be used to tune the internal oscillator or synchronize the regulator with an external clock to maintain a constant switching frequency. Switching frequencies from 120 kHz to 1 MHz are supported. The regulator is also protected against overcurrent, output overvoltage, input undervoltage and thermal shutdown.
The resistance on the OVP pin/pin designates the number of phases to the controller. This identification is used to determine how the controller responds to the multiphase clock signal from the primary phase. In a four-phase converter, the two phases of the MAX15258 driver or the target are interleaved by 180°, while the phase shift between the driver and the target is 90° (Figure 3).
Figure 3: Four-phase configuration: driver and target waveforms. (Source: Analog Devices)
In multiphase operations, the MAX15258 monitors the low-side MOSFET current to balance the active phase current. As feedback, current imbalance is applied to the current sensing circuit cycle by cycle to help regulate the charging current. This ensures an equitable distribution between the two phases. Unlike direct converter designs, designers do not need to take into account a possible 15% to 20% phase imbalance during the design calculation stages when using this IC.
In three-phase or four-phase operation, the average current per chip is transmitted between the driver and the target through specific differential connections. The current mode regulator and target devices regulate their respective currents so that all phases equally share the load current.
The four-phase interleaved inverting buck-boost power supply shown in Figure 4 is suitable for applications requiring large amounts of power. The CSIO+ and CSIO- signals connect the two controllers, and the SYNC pins are connected to ensure clock synchronization for the phase-coordinated phase interleaving scheme.
The MAX15258 is a low frequency boost converter. This reduces the main source of power loss in converters: switching losses. Since each converter operates in its low loss zone at low frequency, it delivers high output power at a high equivalent total frequency. This makes it the ideal device for converting -48 VCC.
Operating with a stable duty cycle, it obtains high power output with extremely high efficiency. Figure 5 shows the efficiency curves of an 15258 A MAX800 reference design based on a coupled inductor for various V combinations.IN and VOUT. As a result of reduced conduction losses, the graphs clearly show efficiency figures of over 98%.
Figure 4: Efficiency versus output load current of a MAX15258 CL 800 W reference design. (Source: Analog Devices)
Conclusion
Power supplies play an important role in the telecommunications sector. Due to their ability to achieve high efficiency and minimize power losses, active forward clamp converters (ACFCs) are preferred in telecommunications power supply designs. However, inherent limitations may hinder its effectiveness in specific circumstances. To overcome the limitations of active clamp-on converters, a new generation of power supply technologies has emerged that offer higher efficiency, higher power density and simplified control mechanisms. In the telecommunications sector, these innovative solutions pave the way towards more advanced and optimized power supplies.
Source: https://www.digikey.es/es/articles/achieving-high-efficiency-in-telecom-power-supplies
