A brief discussion on the power supply architecture of communication systems

Modern telecommunication systems require wider bandwidth, faster data rates, tighter confidentiality measures, newer performance, more users and a wide range of user features, which has prompted the design of power supplies that provide dc voltage and current for modern telecommunication systems to change from traditional forms to new technical forms. The new generation of power supply systems based on dc-dc converters must operate in a wide input voltage range, sometimes reaching 30~100V. At the same time, the power supply system provides several low-level dc voltages for ASICs, DSPs and microprocessors designed with deep submicron CMOS processes in high-performance communication systems.

In communications and network server applications, this means not only converting 48V input voltages to traditional 5V and 3.3V, but also to new lower voltages (ranging from less than 1V to 2.5V, with load currents of 10~35A). In addition, the power system must maintain tight tolerances and generate minimal noise to maintain signal integrity. These increased requirements occur in environments where space is constrained and thermal management is a major consideration.

To meet these requirements, power system architectures are evolving from early centralized power supplies that provide lower voltage and current conversion to today’s distributed approaches. Instead of a single power supply generating all necessary voltage levels, power is now distributed along 2nd and 3rd buses to dc-dc converters that step down to voltage levels appropriate for individual circuits or subsystems.

At each level, designers can design or purchase a dc-dc converter that provides the necessary voltage and current for several ICs, ASICs, warm signal devices, or complete printed circuit boards. Each dc-dc converter has a specific topology that depends on many factors of the circuit it powers and the system it operates in, such as efficiency, noise level, physical factors (height, weight, size), the number of output voltages required, power consumption, and heat dissipation. This article will discuss the specific trade-offs and the best topology to meet different system power design goals.

Distributed power supply

In a distributed power architecture (Figure 1), the front-end power supply converts ac power to dc and distributes the dc voltage (typically -48V in telecom systems) to the dc-dc intermediate bus converter (IBC) via the first-level bus. The purpose of the IBC is first to provide isolation and to step down the voltage distributed by the ac-dc front end to a lower voltage level. This should happen before sending it to the final non-isolated dc-dc (step-down) converter via the second-level distribution bus. The point-of-load (POL) converter provides the required voltage and current to the system.

Figure 2 shows how dc-dc isolated current modules and POL point-of-load converters are configured into a typical distributed power system, providing multiple output voltages and currents. The DC voltage (-36~-72V) from the front-end ac-dc power supply is fed into an isolated power module, which represents a bus converter. The module is a complete isolated ac-dc converter and comes in different forms (full brick, half brick, 1/4 brick) with standard footprint, pinout and heat dissipation capabilities. The POL converter can be a combination of a switching regulator (buck or boost regulator) and a linear regulator or just a linear regulator, depending on the requirements of the circuit being powered. Sensitive circuits require low noise linear regulators, while high efficiency switching regulators are the choice for power systems that must have minimal power consumption.

-48V Telecom Distribution Power System

Figure 3 shows a block diagram of a -48V distributed power system for telecom applications. This diagram illustrates the flow of power from the input ac line to the low voltage dc-dc POL converter, with the battery (48V) backing up the ac-dc converter in case of power failure. The -48V hot-swap controller (IC) provides intelligent control of the power connection when hot plugging and unplugging circuit boards. This includes inrush current control, short circuit protection, and other protection functions to protect the power system. The first dc-dc conversion stage is an isolated converter, which means that the input dc power ground is isolated from the output ac power ground, usually using converter isolation. The isolation is limited to prevent hazardous voltage levels from being present during a failure condition. However, the isolation circuit makes the converter more expensive and affects efficiency. The POL converters that provide power to the system unit circuits do not require isolation because they are protected by the isolated power module that provides the dc input power to them.

Hybrid Power System

A power system can be designed with a combination of centralized and distributed units, such as the hybrid power system shown in Figure 4. The centralized power supply generates 5.0V and 3.3V logic supplies (whose input is the ac line) and 12V dc levels distributed to the voltage regulator module (VRM). The VRM is used to provide high-current, low-voltage core and I/O voltages for high-performance processors. The VRM power converter is placed on the motherboard near the "load point" of the processor, which reduces the circuit board trace voltage drop, which in other cases is unacceptable for effective converter operation.

Basic DC-DC conversion topologies

All dc-dc converters can be divided into two types: linear regulators and switching regulators. The advantages of linear regulators are simplicity, lower output ripple voltage and noise, and simple line and load regulation. Switching regulators have higher efficiency, up to 95% (linear regulator efficiency is about 50% or less), and have a larger power density (power to volume ratio, measured in W/in3). Switching converters are more efficient for wide input-to-output level ratios than linear converters because switching converters utilize output filtering components. Figure 5 shows the block diagram of linear and switching regulators.

Non-isolated buck topology

The buck converter is the basic topology that forms the basis of most switching converter architectures. It is the most common topology and is used in distributed power systems because a high dc voltage (48V) must be converted to a lower voltage with low power consumption. The switch is a power transistor (usually a MOSFET) whose gate is driven by an IC that performs pulse width modulation (PWM) . It controls the duty cycle (the time the transistor is switched on and off), thereby controlling the output voltage. Figure 6 shows a non-isolated buck topology.

The buck converter characteristics are:

No isolation

·Only reduce voltage

Only single output

Very high efficiency

Low output ripple current

High input ripple current

Requires high-side gate drive

Large duty cycle range

Wide voltage regulation range

Low power topology

Single transistor topologies (Figure 7) such as buck, boost, forward, and flyback are dc-dc converters designed for relatively low power loads in distribution systems (up to 100 W). Buck and boost circuits are non-isolated, while forward and flyback converters provide transformer isolation.

High Power Topology

Push-pull, half-bridge, and full-bridge dc-dc converters are isolated switching topologies that provide higher power outputs than single-transistor types. The advantage of the 2-transistor topology is that it provides twice the power of the single-transistor type with the same size transformer. The transistors in the bridge topology only care about the smaller voltage load in the half or forward (or push-pull) configuration. Therefore, the transistor voltage rating is a fraction of the value required for other topologies. Half-bridge and full-bridge converters are often used in offline applications and operate with very high 400V dc input voltages (from the rectified and power factor corrected ac input line voltage).

Forward and flyback converters are also used in lower power (less than 100W) offline applications. Unlike forward and flyback converters, bridge converters can provide high efficiency in high power dc-dc applications up to 1500W. Push-pull converters are particularly effective at low input voltages and can generate multiple output voltages, some of which can be of opposite polarity.