Power Train Gears up for Integration

By Guy Moxey, Fairchild Semiconductor

A tremendous amount of attention has been placed on controller enhancements for low-voltage, DC/DC synchronous buck designs. Using digital or mixed-signal control, it’s possible to achieve high switching frequency and a fast transient response. Overcoming this daunting design hurdle represents an important step forward. Yet one must remember that the complete system design will only be as good as its most vulnerable part. In addition, converter losses will still be generated through the power train’s discrete implementation.

The optimization of the driver and switching devices within the power train is important—not only at specific thermal design points, but more so across the entire load spectrum. Thanks to advancements in discrete MOSFET silicon and advanced packaging technologies, low-voltage, DC/DC switch-mode power supplies like POLs, bricks, and VRMs have been pushed to the present limit of power density, efficiency, and thermal performance. Yet this achievement could not be accomplished without tradeoffs. Increasing power density is obtained at the cost of raising the overall power losses and higher temperatures on the silicon junction, device case, and overall printed-circuit board (PCB). In the same manner, optimizing a DC/DC power supply for medium to peak currents comes at the expense of efficiency in the light loads (such as standby or sleep mode) and vice versa.

So what’s the alternative? Increasingly, the integration of the power train is being adopted as a way of increasing efficiency while maintaining high levels of performance. Multichip modules (MCMs) are gathering momentum in the market—primarily because discrete solutions don’t solve the need for higher power density or parasitic issues at higher switching frequencies. These MCMs—for example, DrMOS—have been the subject of evaluation for quite some time. It can be proven that the performance of these devices is at par with or better than existing discrete solutions. Typical advantages include the following: low thermal impedance from the use of leadless packaging; simulated and proven internal wirebonding designs that minimize external PCB routing to reduce inductive and resistive PCB parasitics; compatibility with controllers that enable various modes of operation, such as discontinuous conduction mode for light load efficiency; and the flexible “system-solution” integration of drivers, FETs, diodes, and LDOs.

Consumers have enjoyed significant price decreases in the cost of computers, flat-panel TVs, and DVD players. Nothing is free, however. The machine-development teams therefore have to design with ever-tightening budgets. System efficiency is the key and integrating— hence optimizing—offers state-of-the art results that are in the region of up to 5% higher than the equivalent discrete designs. These points are taken at industry-typical switching frequencies (300 kHz). If the system switching frequency is doubled or even tripled, the benefits of using an integrated solution will be even more pronounced. Higher switching frequencies, comparable efficiencies, and a significant reduction in the size and cost of the output filter capacitors and inductor all make a compelling but somewhat elusive case.

Simply stated, design changes are required and power-train integration can offer significant advantages. Yet embracing this change means deviating from the tried and tested years of discrete driver and FET implementation. The positive argument is that power-semiconductor vendors take the ownership to simulate, study, and optimize all of the individual elements before implementation and manufacturing. The available solution can thus yield only the best performance possible for the specific application. With new energy-efficiency specifications emerging in the market, semiconductor suppliers need to adopt approaches that reduce conduction and switching losses. New approaches also must enable applications to achieve higher and higher levels of efficiency.

Guy Moxey is senior marketing director of computing, communications, and consumer products at Fairchild Semiconductor. He holds a BEng and MSc in power electronics and has spent 17 years in electrical power engineering.