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Power MOSFET Design Considerations

Wed, 12/02/2009 - 5:50am

The power MOSFET transistor, along with the decades of continuing device design optimization, has enabled new circuit topologies and improvements in power supply efficiency.  The conversion from current-driven to voltage-driven power devices resulted in a rapid market adoption of these devices.  The successful commercialization of the planar gate power MOSFET in the 1980s began with high voltage devices, in the 500 to 600V BVDSS range.  The conduction losses of the power MOSFET of this era were primarily determined by the channel density, JFET resistance and epitaxial resistance (See Figure 1).  As increasingly sophisticated photolithography tools became available to the industry, increases in the transistor cell density provided conduction loss improvements.  The increased cell density capable with these photo tools also enabled successful power MOSFETs with a BVDSS range less than 100V, enabling new automotive, power supply and motor control applications.  The conduction losses of high voltage MOSFETs became the epitaxial design.  The use of devices in buck converters and the increasing breadth of power supply requirements in the 30V range fostered a demand for increased performance devices.  

Figure-1-web-fairchild

As the planar power MOSFET technology advanced through the early 1990s, the introduction of the new class of trench gate power MOSFETs set a new standard of performance for low voltage devices.  These trench MOSFETs implement a gate structure embedded into a trench region that is carefully etched into the device structure, essentially doubling the channel density up to 12 million cells per square inch with the first generation.  The conduction improved about 30% with this new technology due to the ability to increase the quantity of parallel-conducting channels as well as eliminating the JFET resistance component.

A device designers’ challenge is that technology advancements implementing cell density improvements can also increase capacitance and gate charge.  This is due to the increased area of gate-to-drain region overlap area and gate-to-source overlap area.  Therefore the device designers have also focused on the reducing the switching-loss parameters through novel structures.  Fairchild Semiconductor introduced a trench gate power MOSFET optimized specifically to support high efficiency buck converters in 1998. Seven development generation cycles since that original PowerTrench® the sophisticated optimization culminates in the latest buck converter components.

Power MOSFET Optimization for the Synchronous Rectification Topology
When the first microprocessor started operating at a different voltage than the standard 5 or 12V supply in the computer, the ubiquitous application for power MOSFETs emerged. The age-old buck converter, used to efficiently change one DC voltage to another lower voltage, became the application driver for development of low voltage switching power devices. Development focus moved away from AC-DC switching power supplies and motor driving into the ever more demanding need of the processor and its entourage of peripheral components with specifically conditioned power requirements.

As the power source for the processor, the buck converter immediately adopted a synchronous rectifier to improve efficiency by supplementing and eventually replacing the rectifying Schottky diode with a synchronously switched power MOSFET to reduce conduction losses. With the advent of mobile computing, higher converter efficiency requirements emerged to drive the development toward the highly evolved forms found in modern power MOSFETs.

At a high level, the buck converter MOSFET requirements are easy to define. The synchronous rectifier or SyncFET™ operates predominantly in the on-state condition and should have very low on-resistance to minimize power losses. The high-side switching MOSFET delivers packets of energy from the DC supply rail to the load through an LC filter that smoothes the packets into a continuous voltage. The dominant power loss in this component results from switching and to a lesser extent on-state conduction, requiring a fast switching device with good on-resistance. These two components alternate their on-state condition but cannot overlap as that would result in power loss directly from the supply to ground a condition called shoot-through. When the switching device turn-on occurs the voltage transition on the drain of the SyncFET™ induces a current and a voltage on the gate CGS with a magnitude depending on the magnitude of and ratio between CGD and CGS and the switching transition rate. If the gate voltage exceeds the threshold this device will turn-on again resulting in shoot-through. A sufficiently large CGS to CGD ratio helps avoid shoot-through induced from the drain transient.

Analyzing the evolution of the technology, having now defined the MOSFET requirement, reveals the primary device technology drivers. The basic trench gate structure of Figure 2a enables lower on-resistance by increasing the channel width to length ratio. To improve switching performance and increase the CGS to CGD ratio, development of a thick oxide in the trench bottom followed in succession as shown in Figure 2b. In Figure 2c, the ultimate refinement utilizes an additional electrode embedded in the trench below the gate allowing on-resistance reduction by allowing increased drift region charge while also reducing CGD for fast switching, and modifying the CGS to CGD ratio so as to minimize shoot-through.

Figure-2-web-fairchild

Fairchild Semiconductor’s latest developments of the shielded device described above take this structure to new levels of refinement. The specific resistance, or unit-area resistance, has been significantly improved compared to the previous generation while improving on the already superior switching characteristic. Past generation devices like Fairchild Semiconductor’s pioneering SyncFET also required the integration of a Schottky diode on the low-side synchronous rectifier to reduce dead-time conduction loss of the MOSFET body diode and control transient behavior resulting from the body diode reverse recovery. The latest device generation manages to suppress undesirable transient behavior like over-shoot of the drain voltage by means of expert engineering of the body diode forward injection, minimal drain-shield capacitance, and low shield resistance, in order to eliminate the need for the relatively expensive Schottky.

As shown in Figures 3a and 3b, the over-shoot voltage and ringing are greatly reduced even when compared to components employing an integrated Schottky. Damped ringing of the SyncFET drain voltage dramatically reduces EMI noise, a condition frequently encountered in this application. This exceptionally quiet switching behavior of this solution can completely eliminate the need for any external snubber circuit to eliminate oscillation.

Figure-3-web-fairchild

As a result of the evolving device technology, new products have emerged that raise the performance and the maximum output current rating of the converter by reducing power loss in the MOSFET switches. Today, three milliOhm components are commonly used for the SyncFET and enable output current above 30A from each stage of a multi-phase converter. This is an extraordinary accomplishment considering that components of past generations had package interconnect resistance close to that of the entirety of today’s PowerTrench products. Four-fold improvement in the silicon-specific resistance during the last ten years has been complemented by improvement in package-interconnect resistance by a factor of eight enabling more than a two-fold increase in converter output current capability. Future progress enabled by new products will see operating frequency increase resulting in smaller filter inductors and capacitors enabling compaction of the circuit board area use.

Multi-chip modules, housing controllers and or drivers as well as the power switches, are finding their way into an increasing number of consumer electronics like game consoles and portable computers. Among the benefits of these new components is the reduction of parasitic inductance elements on the circuit board and inherent in the discrete solutions that generate transients and rob power from the converter. Longer battery life, cooler operating temperature, reduced radiated noise or EMI, and size reduction are realized.

Many of the improvements in packaging and MOSFET device technology derive from increasing use of simulation to direct and guide technologists to innovative solutions. The latest silicon technology development described relied on both finite element simulations of the device and application simulations to develop understanding of the interactions between the silicon, package, gate drivers, and circuit board parasitic elements. Simulation also provided insight into process steps responsible for device parametric variations and solutions to minimize sources of variation.

Synopsis
The development of commercially successful advanced power devices for advanced power supplies must consider and adapt to evolving application requirements. This entails considerable optimization of all elements in the application including the power device silicon, packaging, circuit board layout, and operating frequency of the converter. Recognizing the challenge and applying new design principles to the development of power MOSFETs, Fairchild Semiconductor has established expertise in power supply design resulting in leadership performance in their PowerTrench products.

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