AT THE COMPONENT LEVEL
Power MOSFETs are a key component for virtually all POL converters. They're usually the semiconductors within a POL converter that consume the most power, so their performance has a major impact on circuit efficiency.
There's also a need to fully optimize the overall power-management solution. This requires selecting the correct control and driver ICs to work with the MOSFETs. Inadequate controllers and drivers can cause additional power dissipation in the MOSFET as well as in the driver itself. For the past two years, MOSFET manufacturers have pursued various packaging techniques to improve MOSFET performance, such as ball grid array (BGA), "bottomless" packages, and the DirectFET.
The International Rectifier DirectFET combines HEXFET power-MOSFET silicon technology with packaging that provides low on-state resistance in a package featuring the footprint area of an SO-8 and only a 0.7-mm profile (Fig. 5). The DirectFET package is compatible with existing layout geometries used in power applications; pc-board assembly equipment; and vapor-phase, infrared, or convection soldering techniques. The DirectFET product family is also optimized to work with IR's latest-generation controllers and drivers, including X-Phase and the bus converter chip sets.
With DirectFET packaging, dual-sided cooling can maximize thermal management and improve power density by a factor of two. It also cuts board space and component count from 50% to 60%, and it reduces junction temperatures up to 508C. Reduced losses and improved thermal performance foster high efficiency and low temperatures, and therefore enhance system reliability, suiting this device for POL converters.
Initially, IR introduced DirectFETs with 20- to 30-V ratings. Recently, the company introduced 40- and 100-V parts. The higher-voltage MOSFETs employ the same packaging concept and allow the benefits of the DirectFET technology to be employed in a broader range of applications, including 48-V input isolated dc-dc converters.
One of the DirectFET's major accomplishments is reduced on-state resistance, which includes the silicon itself as well as the means for connecting to external connections. For example, an SO-8 power MOSFET usually has about 1.4 mW of resistance for wire bonds and lead resistance. With the DirectFET, the equivalent resistance is only about 0.1 to 0.15 mW. The result is a lower overall on-resistance.
Lower on-resistance and improved thermal resistance breed higher-efficiency MOSFETs. This lets designers operate the MOSFETs at a lower junction temperature for a given power level. Or, they can drive the MOSFET harder without exceeding its maximum rated junction temperature, increasing system-level power density.
Currently, there's no second source for the DirectFET. However, International Rectifier has established similar production facilities in the U.S. and Europe.
ICs SUPPORT POL SYSTEMS
Input voltage for POL systems may range from 5 to 12 V. In a system upgrade, a 5-V dc supply may be available, and a specific load could require less than 1 V. Linear Technology's LTC3418, scheduled for introduction in the first quarter of 2005, fits this type of application.
This 4-MHz, monolithic, synchronous step-down dc-dc converter uses a constant-frequency, current-mode architecture. Operating from a 2.25- to 5.5-V input range, it provides a regulated output voltage from 0.8 to 5 V while delivering up to 8-A output. Switching frequency is set by an external resistor or synchronized to an external clock. OPTI-LOOP compensation optimizes the transient response over a wide range of loads and output capacitors.
Linear Technology also will introduce ICs that fit bus converter applications, such as the LTC3706, a PolyPhase secondary-side controller for synchronous forward converters. When used with the company's LTC3705 gate driver and primary-side controller, the combination creates a complete isolated power supply that combines the power of PolyPhase operation with the speed of secondary-side control.
The LTC3706 simplifies the design of highly efficient, secondary-side forward converters. The LTC3705 and LTC3706 form a robust, self-starting converter that eliminates the need for the separate bias regulator commonly used in secondary-side control applications. In addition, a proprietary scheme multiplexes gate drive signals and dc bias power across the isolation barrier through a single, tiny pulse transformer.
Another interesting IC for POL-system applications is the LTC3736-1. This two-phase, dual synchronous step-down switching controller has tracking that drives external complementary power MOSFETs. Its constant-frequency, current-mode architecture with MOSFET drain-to-source voltage (VDS) sensing eliminates the need for current-sense resistors, lowering cost and boosting efficiency. Operating the two controllers out of phase minimizes power loss and noise caused by the input capacitor's equivalent series resistance.
The LTC3736-1's unusual spread-spectrum architecture randomly varies the switching frequency from 450 to 580 kHz, significantly reducing the peak radiated and conducted noise on both the input and output supplies. This makes it easier to comply with international electromagnetic-interference standards. Pulse-skipping operation improves efficiency at light loads and 100% duty cycle capability offers low-dropout operation.