|
For power system designers, the insatiable power appetite of DSP chipsets and application-specific microprocessors create a significant challenge. Their lower output voltages, higher current densities and increasing switching frequencies combined with smaller printed-circuit-boards (PCBs) are common; and are well-known trends in applications such as desktop computing, server, telecom POL and other similar DC/DC converter applications.
As designers strive to increase system efficiency, reduce bill-of-materials (BOM) cost and simplify design, tried and true design approaches implementing discrete components are nearing saturation and therefore small, but significant, changes in interpretation and approach are required.
Ultra-high switching frequency and fast transient response are two key design considerations that can be achieved through digital or mixed-signal IC control implementation. Improvements in discrete MOSFET silicon and packaging technology have resulted in lower RDS(ON) and lower gate charge ( Qg) which have significantly lowered switching and conduction losses within the device itself, thus lowering the overall loss for the power stage and therefore increasing system efficiency. However, there is a finite level of achievement in place, i.e., a bottleneck, to which the traditional synchronous buck converter designs are constricted by.
In addition recent legislation introduced to control consumer product energy consumption, such as ENERGY STAR and 85 PLUS, means every power system design has to conform to the regulations. Naturally, this is counter to the increasing feature sets that the consumer requires, all of which demand further power from the system budget. Because of this, any method of reducing power loss/increasing system efficiency is of optimal importance. As consumers, we have all enjoyed the significant decreases in the price of electronic goods such as computers, flat panel TVs and DVD players. Since nothing is for free, product development teams have to design with ever tightening budgets.
However, all is not lost and one method to combat these effects can be addressed through integrating the buck converter power stage in a multi-chip module ( MCM). Optimization of the driver and switching devices within the power stage module is now guaranteed, not only at specific thermal design points, but more so across the entire load spectrum.
MCM integration offers optimization
Factors such as driver output impedance and dead time are the two most significant influencers on system efficiency. However, with a MCM approach the vendor will evaluate and pre-select the best performing driver that matches the associated MOSFETs for the system to give the best balance between both peak ( light load) and full load efficiency.
Traditionally, the designer will also then select the best MOSFET for the high side and low side paying particular reference to RDS(ON) and Qg. What the designer does not have any control over are the second and third order characteristics of the discrete device. In effect what you see is what you get. By integrating the MOSFET silicon into a MCM the vendor has the flexibility to design the FET silicon exactly for the MCM in several ways. Namely, the MOSFET die aspect ratio has been created for maximum current capability, multiple wire bonding has been added to reduce package on resistance and provide short inter-package connections to reduce inductance — all of which cannot be achieved through using and placing single discrete devices together.
Advanced electrical and thermal simulation is performed to map current flow through the silicon and lead frame for the MOSFET's die in order to accurately predict and optimize the behavior and performance. Package on-resistance is also carefully designed by achieving the optimal lead frame thickness and having the layout for multiple bond wiring. Further parasitic effects such as stray capacitance and trace inductance of PCB tracks are also virtually eliminated with MCM integration as all internal connections are made with the shortest path of bond wires. Again this optimization is not possible with discrete devices or a discrete PCB layout approach.
In terms of the PCB footprint, the integrated MCM approach with a 64mm2 of total area shows a significant reduction in board space verses the discrete approach of three DPAK MOSFETs and one SO8 driver IC for a typical 30-A VRM phase leg. Per phase a discrete approach will consume 228mm2 for the devices alone, plus the additional copper PCB interconnect tracks. When the VCORE power train is 30% of the motherboard's total area, then moving to integrated MCM solutions can be a significant advantage.
MCM power-train performance versus discrete implementation
Typical peak efficiencies for a multi-phase VRM VCORE solution with a discrete driver and MOSFET implementation are typically at 85% peak at current ratings of 10 A per phase, reducing down to 80% at full loads of 30 A. This 10-15% loss in system efficiency is directly proportional to power dissipation and consequently heat. Suffice to say all power system designers strive to minimize losses and heat.
With the optimization and consequent performance advances as discussed with the integrated MCM solution, performing standard efficient testing on VRM systems will yield on average a 4% increase in efficiency at peak load and in the region of 2% at full loads.
We can see that multi-chip modules are gathering momentum in the market primarily because discrete solutions do not solve the need for higher power density nor parasitic issues at higher switching frequencies.
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) and if the system switching frequency id doubled or even tripled, then the benefits of using an integrated solution are 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 case.
About the author:
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. His experience includes electrical machine/drives design, power semiconductors and power IC applications. He is the author of many technical articles and application notes.
|
