Electronic Design

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.

DIGITAL POWER CONTROL
Digital power control makes it possible to control, monitor, and maintain the power-management subsystem. Such control can be implemented from Power-One's Z-One technology and also the PMBus standard specification developed by a consortium of power-supply and semiconductor companies.

The exact nature of future digital power control systems depends on patent litigation covering digital power management and control incorporated in Power-One's ZOne system architecture. A Z-One system permits the central control of distributed point of load power regulators from a single digital power manager.

One of the associated patents (6,949,916, issued Sept. 27, 2005), "System and method for controlling a point-of-load regulator," describes the use of a serial bus (either passively or actively) with a point-of-load (POL) regulator. Patent 6,936,999, issued on Aug. 30, 2005, "System and method for controlling output timing of power converters," employs a controller to transmit output-timing data to at least one POL regulator.

With characteristics similar to Z-One technology is PMBus (Power Management Bus), an open standard that defines a digital communications protocol for controlling power conversion devices. PMBus would allow power converters to be configured, monitored, and maintained according to a standard set of commands.

Using PMBus commands, a designer could set a power supply's operating parameters, monitor its operation, and perform corrective measures in response to faults or operational warnings. The ability to set a power supply's output voltage enables the same hardware to provide different output voltages by merely reprogramming. The ability to monitor and maintain a PMBus system enhances its reliability and availability.

If implemented, the PMBus protocol would allow multisourced power management products. And, OEMs would be able to control compliant power converters using a standard set of commands.

TABLE 6: POWER-MANAGEMENT GLOSSARY

Parameter

Description

Drift

The variation in dc output voltage as a function of time at constant line voltage, load, and ambient temperature

Efficiency

Ratio of output-to-input power (in percent), measured at a given load current with nominal line conditions (POUT/PIN)

Hold-up time

Time during which a power supply's output voltage remains within specification following the loss of input power

Inrush current

Peak instantaneous input current drawn by a power supply at turn-on

International standards

Specify a power supply's safety requirements and allowable electromagnetic-interference levels

Isolation

Electrical separation between the input and output of a power supply measured in volts; a non-isolated power supply has a dc path between the input and output of a supply, whereas an isolated supply employs a transformer to eliminate the dc path between input and output

Line regulation

Change in value of dc output voltage resulting from a change in ac input voltage; specified as the change in ±mV or ±%

Load regulation

Change in value of dc output voltage resulting from a change in load from open circuit to maximum rated output current; specified as the change in ±mV or ±%

Margining

For system testing, this temporarily changes the output by a specified percentage of nominal

Noise

A power supply's output may consist of short bursts of high-frequency pulses caused by charging and discharging parasitic capacitances; this noise contributes to the supply's total instantaneous output voltage and can be coupled into nearby circuits, particularly analog circuits

Overcurrent

This failure mode causes load current that is greater than specified; it is limited by the maximum current capability and internal impedance of the power supply; it can also damage the power supply

Overtemperature

A temperature that is above the power supply's specified limit; it must be prevented or it will cause power-supply failure; many supplies include overtemperature protection that turns off the supply if the temperature exceeds the specified limit

Overvoltage

A failure mode in which the output voltage goes above the specified dc value; this can impose excessive dc voltage that damages the load circuits; to minimize this risk, many power supplies incorporate overvoltage protection

PARD (periodic and random deviation)

The sum of all ripple and noise components measured over a specified bandwidth and presented in either peak-to-peak or rms values

Power factor

Ratio of true input power (EI cos ) to the apparent power (Erms Irms) in ac circuits; it is the value of cos given in percent or decimals

Power sequencing

Establishes the sequence of voltages applied and/or removed to avoid conditions like excessive power dissipation or latch-up

Redundancy (N+1)

Parallel-connected power supplies configured so that if one fails, the others will continue delivering enough current to supply the maximum load

Remote on/off

Allows the system to turn the supply on or off to select or activate system functions; after turning on, the supply's output voltage must ramp up within a specified time interval, but without excessive overshoot

Remote sense

A power supply's internal voltage sense inputs bypass its output and connect directly to the load; sense inputs correct for any voltage drops that can occur between the supply's output and its load

Ripple

An ac frequency component that rides on the supply's dc output as a result of switching and filtering; it can usually be decreased using additional output filter capacitors

Soft start

Lowers inrush current during power-supply turn-on

Tracking

Capability to control the output voltage of two power supplies so they track each other and do not exceed a specified value during power-up, power-down, and steady-state operation

Transient response

The power supply's response to rapid changes in its load current; response should be fast enough so that the supply's output voltage remains regulated

Undervoltage lockout (UVLO)

Turns on the power supply when its input voltage reaches a turn-on threshold and turns it off if the input voltage falls below the turn-off threshold

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