Current sharing among the multiphase cells is necessary so one does not hog too much current. Ideally, each multiphase cell should consume the same amount of current. To achieve equal current sharing, the output current for each cell must be monitored and controlled.
The multiphase approach also offers packaging advantages. Each converter delivers 1/n of the total output power, reducing the physical size and value of the magnetics employed in each phase. Also, the power semiconductors in each phase only need to handle 1/n of the total power. This spreads the internal power dissipation over multiple power devices, eliminating the concentrated heat sources and possibly the need for a heatsink. Even though this uses more components, its cost tradeoffs can be favorable. And, multiphase converters have a number of important technical advantages:
• Reduced rms current in the input filter capacitor allows use of smaller caps
• Distributed heat dissipation reduces the hotspot temperature, increasing reliability
• Higher total power capability
• Increased equivalent frequency without increased switching losses, which allows the use of smaller equivalent inductances that shorten load transient time
• Reduced ripple current in the output capacitor reduces the output ripple voltage and allows the use of smaller caps
Multiphase converters also have some disadvantages that should be considered when choosing the number of phases:
• The need for more switches and output inductors than in a single-phase design, which leads to a higher system cost than a single-phase solution, at least below a certain power level
• More complex control
• The possibility of uneven current sharing among the phases
• Added circuit layout complexity
As current requirements increase, so does the need for increasing the number of phases in the converter. ICs providing just two phases may not be adequate because of their limited output current range. An optimum design requires tradeoffs between the number of phases, current per phase, switching frequency, cost, size, and efficiency. Higher output current and lower voltage also require tighter output voltage regulation.
To evaluate multiphase design decisions, we have to review the approaches employed by available ICs. One approach is to use a pulse-width modulation (PWM) controller IC with integrated MOSFET drivers. Yet this technique presents several challenges.
First, the heating and noise generated by the on-chip gate drivers may affect controller performance. Second, for most of these chips, it is impractical to cascade them for additional phases. Next, accurate current sharing is difficult with this configuration. Finally, three phases appears to be the limit.
Another approach is to employ a separate controller and separate gate drivers. This method isolates the PWM controller from the heat and noise of the gate drivers. Also, because the current sense signal is routed to the controller, current sharing is more complex. There are additional controller-to-driver delays because of the separated ICs as well.
Still another approach is to use a dual-phase controller with integrated gate drivers and built-in synchronization and current sharing. This technique, though, only allows an even number of phases. Although it simplifies the design, it also may result in unused or redundant silicon, pins, and external components. And, the driver heat and noise generated on-chip can degrade controller performance.
STANDBY POWER
Standby power, also called vampire or phantom power, is the electricity consumed by electronic equipment when it’s switched off or in standby mode. The typical power loss per equipment is low, from 1 to 25 W. But when it’s multiplied by the billions of devices in homes and businesses, standby losses represent a significant fraction of total world electricity use.
Technical solutions exist in the form of a new generation of power transformers that use only 100 mW in standby mode and can reduce standby consumption by up to 90%. Another solution is the “smart” electronic switch, which cuts power when there is no load and restores it immediately when required.
The One Watt Initiative is an energy-saving proposal by the International Energy Agency to reduce standby-power use in all equipment to just 1 W. The International Energy Agency launched the One Watt Initiative in 1999 to ensure through international cooperation that all new equipment sold in the world only uses 1 W in standby mode by 2010.
In 2001, U.S. Executive Order 13221 stated that every government agency, “when it purchases commercially available, off-the-shelf products that use external standby power devices, or that contain an internal standby power function, shall purchase products that use no more than one watt in their standby power consuming mode.”
SIDE-EFFECT DESIGN BENEFITS
One side effect of improving energy efficiency is the extension of battery life in portable systems. In addition, space is usually limited in battery-based systems, so appropriate energy efficiency can result in simpler, less bulky heat-removal techniques. In contrast, poor energy efficiency can cause excessive heating that may require exotic and costly cooling techniques that drive up system costs and can potentially lengthen design cycles.
For all electronic systems, enhanced energy efficiency can improve a system’s reliability. Semiconductors must have controlled operating temperatures, which affects reliability as defined by their failure rate (useful system life in failures per 106 hours). The Arrhenius reliability model states that failure rate is a function of the temperature stress—the higher the stress, the higher the failure rate.
Typically, each 10°C rise in temperature increases the failure rate by 50%. Conversely, cutting the operating temperature by 10°C reduces the failure rate. Thus, failure rate and its inverse, mean time between failures (MTBF), can be improved by emphasizing energy efficiency.
Besides reliability and performance issues, semiconductor thermal management involves an economic and mechanical challenge that good energy efficiency can minimize. Cost is an important consideration that might be able to be reduced. Sizing considerations are equally important when increasingly higher-power semiconductors must be accommodated in next-generation system designs.