[Engineering Essentials]
Applications Dictate Power-Management Subsystem Design
Control of dc power ismore critical than ever in today's complex designs, and that requires a meticulously crafted power-management setup.
Designing power-management subsystems, which supply and control dc power in electronic systems, is much more complicated now than it was five years ago. These days, designers must cope with ICs that operate below 1 V, may consume over 100 A, and employ gigahertz clock rates. In addition, such subsystems involve more than just power-supply design. They also include system-oriented functions that require application-specific ICs.
A system viewpoint is necessary to set the stage for the optimum power-management subsystem design (See figure 1). First, you've got to select the electronic system's power-distribution technique (Table 1).
MEETING THE APPLICATION REQUIREMENTS Once the power-distribution system is selected, the actual subsystem design starts—with the power supplies. This requires the designer to look at the application's power-supply specifications (Table 2).
Except for battery-based systems, the designer can either make or buy power supplies and supporting circuits. Making power supplies in-house has its own design tradeoffs, such as whether or not to design a switching or linear regulator.
SWITCHING REGULATORS A switching regulator converts a dc input to a switched voltage controlling a power semiconductor switch, whose output is rectified and filtered to produce a dc output. If the output voltage tends to change, voltage feedback maintains the proper regulated voltage.
Switching regulators can be self-contained on one chip or use multiple chips. The single-chip switching regulator integrates either a bipolar junction transistor (BJT) or MOSFET power switch. Multiple-chip switching regulators employ a controller, gate drivers, and MOSFETs. Typically, the switching frequency falls within the 60-kHz to 3-MHz range.
Switching frequency determines the physical size and value of filter inductors, capacitors, and transformers. Higher switching frequencies allow for external components with smaller physical size and lower component values. To optimize efficiency, transformer/inductor core material must operate efficiently at the switching frequency.
Switching voltage regulators may be isolated or non-isolated. Non-isolated regulators have a dc path from input to output. An isolated regulator employs a transformer that isolates the input and output voltage.
Layout is important for all switching power supplies, especially with high peak currents and high switching frequencies. Therefore, use wide and short traces for the main current path and power ground tracks. Associated capacitors and inductors should be located as close as possible to the regulator IC.
Table 3 compares the characteristics of the three basic switching-regulator controller IC techniques: pulse-width modulation (PWM) (See figure 2), hysteretic (See figure 3), and multiphase (See figure 4). These techniques control the associated power-semiconductor switch to turn ON and OFF to maintain voltage regulation.
Table 4 compares the characteristics of the low-dropout (LDO) regulator IC and the basic charge-pump (switched-capacitor) IC. These regulators don't turn a power-semiconductor switch ON and OFF (See figure 5).
APPLICATION-SPECIFIC POWER-MANAGEMENT ICS Because electronic systems have grown in complexity and sophistication, several application-specific IC types are used in power-management subsystems (Table 5). These ICs are also used to design general-purpose power supplies (See figure 6).
POWER-SEMICONDUCTOR SWITCHES Power semiconductors used in power-management systems include power MOSFETs and BJTs. Both can be discrete devices or integrated with other circuits in a single package. They employ either internal or external gate drivers to turn them ON and OFF.
Power switching results in some power loss as the switch applies and removes power to a load. These power switches exhibit some delay when turning ON and OFF, causing power loss when the switch goes through the linear region between ON and OFF. There's an I2R power loss when the switch turns ON because semiconductor switches have a finite on-resistance. Although drain current is low when they turn off, there can be a slight power loss because they have a finite off-resistance. That power loss ultimately affects a power supply's efficiency.
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