How to choose the right power supply design for communication structure equipment

The power supply system used in communication infrastructure equipment is composed of many different components. The AC/DC power supply with power factor correction (PFC) has load current sharing and redundant checking function (N+1) in the front-end part, which can feed the high-efficiency DC/DC modules and load point converters that are closely clustered in the back-end part. We must adopt a very energy-efficient power supply system design to provide power for high-voltage analog circuits and highly stable low-voltage power supply for high-speed digital communication application-specific integrated circuits (ASICs) and field-programmable gate arrays (FPGAs) chips.

Due to the different requirements of power supplies for different systems, and the communication market is constantly changing, and the changes are quite large, communication equipment manufacturers have to further save production costs and have to adopt more energy-efficient and more reliable power supply solutions to maintain their competitive advantage in the market.

Due to the current challenging business environment, new voltage distribution bus standards have emerged, and the recently introduced +12 volt (V) intermediate bus architecture (IBA) is a good example. We can use a new generation of low-cost point-of-load (POL) modules by using low-cost unregulated (open loop) "brick" converters to convert the -48V bus to a standard +12V intermediate bus. These modules are available in single in-line packages (SIP) and surface mount devices (SMD) packages.

Small point-of-load modules that can be installed in a system can provide low-voltage power to different loads, and this is a very cost-effective solution.
However, these new generation of point-of-load modules are also facing emerging competitors, such as compartmentalized hybrid power supply systems, including systems that use cascaded current-fed or voltage-fed push-pull converters. Some semiconductor suppliers are even providing design support for engineers who design power supplies, allowing them to embed low-cost small compartmentalized power supplies directly into the motherboard or line card. National Semiconductor's new highly integrated 100-volt high-voltage power application-specific integrated circuits (ASICs) such as the LM5041 cascaded pulse-width modulation (PWM) controller and the LM5030 push-pull pulse-width modulation controller not only minimize the number of external components required, but also minimize the area of ​​the printed circuit board. The cascade converter can directly use the power provided by the -48-volt bus to generate multiple low-voltage outputs, with higher overall efficiency and lower cost than point-of-load converters that use +12-volt intermediate bus converters to provide power.

Power supply designers must choose between using off-the-shelf point-of-load modules and intermediate bus converter modules or using semiconductor manufacturers' embedded power supply reference designs to reduce costs and improve efficiency. As information equipment manufacturers develop new generations of low-cost devices, they are more serious than ever about the trade-offs between cost, design complexity, and risk. Even though it is clear that embedded power supply solutions can save a lot of cost and energy, some manufacturers will refuse to use such embedded power supply solutions because the use of power switches and internal magnetic transformers will make the design of personal computer circuit boards too complicated.

For power supply systems with simpler designs and only need to provide one supply voltage, the additional cost of adding a transformer is insignificant and does not increase the complexity of the design. However, power supply systems that need to output multiple different voltages are more complicated in design, especially when transformers with multiple secondary windings are required, which makes the design more complicated. Designs that need to provide multiple voltage outputs can also use more complex voltage regulation circuits, which can control the feedback loop with the ability to sense multiple voltage outputs.

Power supply designs for VoIP, DSL, and third-generation mobile phone base stations all have varying degrees of complexity. There are several factors that affect the performance of each of these three power supply designs, which we will discuss below. The VoIP phone DC

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DC converter uses a relatively uncomplicated high-power single-output transformer design (typical voltage is between 250W and 500W) to buffer the main-48V voltage distribution bus. The single-voltage output transformer design minimizes the cost and capacitance of bulky capacitors required to maintain a stable voltage on the distribution bus in order to narrow the traditional operating voltage range of 36 to 72V to between 43 and 57V. All downstream inverters or other loads on the distribution bus also have fault protection and safety isolation. If we use DC/DC converters that support multiple parallel outputs and load current sharing, we can provide fault tolerance (N+1) and heat dissipation functions, which helps reduce the temperature of the system during operation, making the system more durable and reliable.
Generally speaking, the power supply circuit layout design required for VoIP converters must have advantages such as excellent performance (high conversion efficiency, very low line current), ease of use, cost-effectiveness, and small and slim form factor. There are many different layout designs available on the market, each of which can meet these requirements to some extent. For example, the flyback converter is very popular because of its simple layout. Unlike buck converters (such as forward converters), flyback converters do not require transformer flux reset mechanisms or output inductors. Although flyback converters have these advantages, when used in certain applications (especially high output voltage systems such as VoIP applications), expensive capacitors are required to filter large ripple currents at the input and output ends, which is a disadvantage of flyback converters. However, we can reduce the ripple current by interleaving the two converters in the opposite phase, and alleviate the ripple current problem of flyback and forward converters. If all factors remain unchanged, the input and output ripple currents of the interleaved system are much lower than those of the system using a single converter.
For VoIP systems, push-pull converters (Figure 1) are a far more effective solution than flyback converters. The push-pull converter basically consists of two interleaved forward converters, but only one of them is

A self-resetting transformer and an output inductor. In this sense, the push-pull converter is only slightly more complex than a stand-alone forward converter, and its ripple current is greatly reduced due to the interleaving effect, which also allows the push-pull converter to use a smaller input inductor. Since the output inductor will reduce the output ripple current, the push-pull converter can use low-cost capacitors with lower ripple current ratings. Conventional flyback converters are only suitable for power conversion up to about 150W, but push-pull converters can operate normally at power levels up to kilowatts with satisfactory results. In addition
, Internet phone systems that require higher conversion efficiency can use more complex topologies to ensure that the system can still achieve extremely high efficiency when the input voltage is at both extremes. The design of a cascaded buck topology with a current-fed push-pull converter is a good example. (Note: The LM5041 dedicated pulse width modulation controller that is best suited for this layout is already available in large quantities.) This hybrid topology is best suited for high-power systems. In addition, this layout is also suitable for high-efficiency and high-performance systems. Since adopting this layout will improve efficiency and performance, it is worth it even if the cost is higher.

Figure 1 Push-pull converter for VoIP applications

Digital Subscriber Line (DSL)
Digital Subscriber Line (DSL) applications can use a converter that provides multiple voltage outputs from a -48V supply. This converter contains a more complex, lower power multi-output transformer (50-100W). This DSL power supply system can provide power for high-voltage analog line drivers and amplifiers (typically +/-12V) and multiple low-voltage supplies (+5V, +3.3V, +1.8V and +1.5V) for special application integrated circuits. The power supply system with multiple output DSL converters must use a high-performance layout design, such as supporting high conversion efficiency and excellent load and line regulation capabilities, and must be simple, low-cost, and small.
We can ensure that the performance of the DSL power supply system meets our requirements by selecting the right layout design and control circuit. If the layout used in the DSL power supply system is supported by a new generation of controller chips with new functions, it will help reduce the number of components required and save board space, allowing the system design to be further streamlined. Small power supplies are typically designed with a printed circuit board (planar) transformer, output inductor, and surface mount input and output capacitors.
Multiple output power supplies typically require a multiple output flyback converter. While this is the simplest layout, it does not provide good load regulation for all outputs except the controlled output. Flyback converter efficiency is also not ideal because the low voltage outputs dissipate the most power, but synchronous rectification of the low voltage outputs requires additional application specific integrated circuits, which are rarely available on the market, so flyback converter efficiency is not easy to improve. The power supply topology
shown in Figure 2 is suitable for DSL applications and is an ideal performance topology. The push-pull converter is responsible for converting the 48V voltage to +/-12V and isolating the power supply. The synchronous buck converter uses the power provided by the +12V supply rail to generate multiple low voltage outputs. This push-pull intermediate bus design can take advantage of cost-effective power management chips such as the LM5030 push-pull controller and the LM5642 dual-channel current mode synchronous buck controller. The LM5642 is a high-performance chip that requires only two field-effect transistors, an output inductor, an output capacitor, and several resistors and capacitors for each channel.

Figure 2 Push-pull converter and synchronous buck controller for multiple output systems

Third Generation (3G) Base Stations
3G base stations require two converters to provide a +27 V distribution bus voltage both under normal conditions and during power outages. One high voltage converter is powered directly from the AC mains and uses the power to power the entire system during normal operation. The other converter continues to operate from a -48 V backup battery after an AC mains outage. This -48 V backup battery is similar in design and complexity to the single output, high power VoIP converter mentioned above. The power factor corrected (PFC) AC/DC converter provides the bus supply voltage for the point-of-load converters in addition to the typical 2.7 V supply voltage for the RF power amplifiers of the 3G base station.
The power supply system layout shown in Figure 3 uses a single conversion stage DC/DC converter to interleave the main DC/AC converter and the backup battery converter, eliminating the need for a separate 400 V to 48 V DC/DC converter stage. This design helps save cost while improving the overall efficiency of the system.
The design generates a 27V DC bus supply voltage using a two-FET forward converter. The forward converter has two top FETs, each connected to the primary coil with the appropriate number of turns on the transformer. Whenever the AC supply voltage is within the correct range, the input voltage sensing logic activates the top Q2 FET connected to the 400V bus. If the AC supply is lost, the top Q3 FET automatically activates to power the converter from the backup battery. The distribution bus powered by the backup battery provides 27V to the main power transmitter and the 3.3V "brick" converter, which then delivers the power to the point-of-load converter.

Figure 3: Power supply circuit diagram of the third-generation base station RF power amplifier

Summary
There are many power supply systems designed for telecommunication structure equipment on the market. The three solutions introduced above can stimulate the thinking of power system design engineers and encourage them to further analyze different distribution structures and converter layouts. DSL, Internet phone and third-generation base stations each use unique solutions, showing that there are various power system structures available on the market. Manufacturers can make full use of these technologies to develop highly integrated systems. Each application solution can highlight its unique advantages in terms of input voltage range, number of outputs, power supply requirements, cost, performance and size to provide more choices for the market.
Semiconductor manufacturers are launching various highly integrated controllers to reduce the cost of power management modules and streamline the design of embedded converters. Due to market competition, there is constant pressure to reduce system costs, forcing manufacturers to develop innovative structures. This continuous pursuit of innovation has promoted the rapid development of power system layout design.