[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.
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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