Miniaturization in synchronous buck DC/DC applications employing DrMOS

Today’s high efficiency step-down DC/DC converters implement synchronous rectification to achieve the high efficiency requirements of today’s applications. This requires the driver and power-train to be optimized for specific operating points.

Advancements in packaging, silicon, and integration technology have enabled switch mode power supplies to push the limit of power density, efficiency, and thermal performance. Driver plus FET multi-chip modules allow for considerable space savings over the discrete approach and show performance advantages that are pivotal for power supply applications.

CHALLENGES

The ongoing trend for “green” systems not only means employing environmentally friendly components, it also challenges the electronics industry to embrace power conservation. Agencies such as EnergyStar and 80+ have released specifications for a variety of consumer electronics. Longer battery life in portable applications is also a demanded feature by today’s consumers. Therefore, extended battery life, reduced form-factors, and new governmental requirements are driving the need for careful component selection of power supplies, in particular for on-board synchronous buck converters. This translates into drastic improvements needed in the power-density, efficiency, and thermal performance of new platforms.

It is well known that designing the ideal buck converter is a game of tradeoffs. Increasing power density has usually meant increasing overall power losses and increasing temperature on the junction, the case, and the PCB. In the same manner, optimizing a DC/DC power supply for medium to peak currents, almost always means giving up efficiency in the light loads, and visa versa.

DRIVER PLUS FET MULTI-CHIP MODULE TECHNOLOGY

Typical multi-phase DC/DC buck converters employ one control FET (high-side) and one or two synchronous FETs (low-side) along with gate drivers. We refer to this as the “discrete solution.” Over the years, existing discrete designs have shown significant power efficiency improvements. Manufacturing advancements in packaging have increased the adoption of thermally-enhanced leadless MOSFET packages allowing DC/DC engineers to push the current capability of their power supplies further.

However, discrete solutions do not solve the need for higher power density nor parasitic issues at higher switching frequencies. As a result, multi-chip modules that integrate Drivers and MOSFETs, widely referred to as DrMOS, have begun to gain momentum. These devices are available in small form-factors and the performance is at par with, or better than, discrete solutions.

Performance of this Driver plus FET multi-chip modules (DrMOS) is achieved through:

• Low thermal impedance from the use of leadless packaging

• Internal wire bonding designs that minimize external PCB routing, thus reducing inductive and resistive PCB parasitics

• An improved trench silicon MOSFET process that significantly reduces conduction, switching, and gate charge losses

• Compatibility to controllers that enable various modes of operation, in particular, discontinuous conduction mode for light load efficiency improvements. New DrMOS devices have a low-drive disable, which turns off the low side FET

• Most importantly, integration of drivers, MOSFETs, diodes and LDOs.

EFFICIENCY

Many present day applications, such as computing devices, spend a majority of their operating life in a variety of states. Therefore it is essential that DrMOS be optimized for heavy loads as well as allow for light load efficiency management.

Figure 2 shows a comparison between a discrete solution and a DrMOS solution from Fairchild Semiconductor. This particular application is for a two-phase notebook CPU core supply. Upon a deep-sleep signal from the processor, the controller operates with a single phase.



When in single-phase operation, the power supply makes use of automatic discontinuous conduction mode (DCM) to enhance light load efficiency. In this mode, the external PWM controller turns off the synchronous FET as the inductor current ripple falls below ground, thus allowing the body diode to block reverse conduction. Switching frequency is decreased as the load current decreases. This controller scheme is becoming a popular trend in core power supplies for computing.

Some DrMOS products utilize a low-drive disable pin to accommodate discontinuous conduction mode operation. In this particular evaluation, the MOSFETs and drivers in the DrMOS have also been optimized for peak notebook power levels. During a two phase operation, the power supply is operating purely in PWM mode.

Overall efficiency of the DrMOS solution performs at par with or better than the discrete solution across all load currents depending on its targeted application.

POWER DENSITY

Power density is improved by way of integration and by increasing the switching frequency. For example, by increasing the switching frequency to 500KHz, the power supply evaluated in Figure 3, was able to employ a 7 x 10mm² (L x W) inductor.

This is a significant size reduction over the widely used 11 x 11mm2 (0.3uH – 0.5uH) inductors in notebooks at 300KHz. This yields an inductor area reduction of more than 30 percent. Lower inductor value also means lower DCR losses. Higher switching frequency also aides in the reduction of capacitor count.

THERMAL PERFORMANCE

As power supplies become denser, thermal limitations become all too evident. While it is more difficult to achieve better thermal performance from an integrated solution, most DrMOS products are comparable to discrete solutions. The comparison shown in Figure 3 shows a DrMOS solution versus two discrete thermally-enhanced S08 MOSFETs.

At an output current of 18A per phase, the temperature on the DrMOS is only 7ºC higher. This is due partly to the advanced packaging technology that is utilized. The thermal resistance from silicon to PCB is greatly reduced by attaching the drain of the FETs directly to the leadless frame with newly developed die-attach material. Heat easily flows to the PCB, thus keeping the device cool. In addition, new leadframe alloys and molding compound materials lend themselves to improved heat dissipation.

Thermal performance can be further improved by applying more copper on the board as well as by using vias to dissipate heat. Layout techniques become very critical in order to maintain thermal requirements when implementing DrMOS solutions.

SUMMARY

DrMOS shows a valuable performance advantage over discrete solutions. Miniaturization is also an obvious advantage. DrMOS reduces form factor and component count without sacrificing performance.

Many computing and networking systems are presently benefiting from this new technology. Suppliers such as Fairchild Semiconductor have released DrMOS portfolios and will continue to expand their offering of products to meet the design needs of many applications.



Click here for the illustrations:


Figure 1, Figure 2, Figure 3