Dc/dc converter efficiency revisited

Optimizing the converter FET switch improves energy efficiency

BY GUY MOXEY
Fairchild Semiconductor
Boston, MA
http://www.fairchildsemi.com/

The modest power semiconductor doesn’t often garner a lot of attention, but humble transistors can make a dramatic improvement in energy efficiency. The ongoing trend for “green” systems means employing environmentally friendly components, but more importantly, perhaps, it places a significant challenge upon the electronics industry to embrace power conservation and increase system-level efficiency. From this, power silicon products such as power MOSFETs are now very much in vogue, as these devices create and dissipate the vast majority of any low-voltage power conversion in-circuit losses.

There have been spectacular improvements in efficiency in computing and consumer electronic products. A strong emphasis has been placed on the ac/dc “silver box” conversion. However, with the emergence of legislation such as 80 PLUS, Climate Savers, and EnergyStar 5, designers are realizing that both the ac/dc and dc/dc power systems are in need of improvement.

With ac/dc system efficiencies averaging around 65% and dc/dc system efficiencies at 80%, it is understandable that more weight has been placed on ac/dc. Now is the time to reexamine dc/dc systems to find creative ways to improve efficiency.

Dc/dc systems for computing, communication, and consumer application systems convert, manage, and distribute power to supply such functions as graphic cards, processor chips, and memory, all of which face increasing demands for greater performance and functionality, which in turn make them more power hungry than ever. Research has been implemented to evaluate the available switching circuits and associated power transistor devices, embracing recent advances in MOSFET silicon and advanced thermal packaging technologies.

Careful component selection of power supplies, in particular for on-board synchronous buck converters, can translate into big improvements in the power-density, efficiency, and thermal performance of new platforms. For example, if half a million servers were fully compliant to 80 PLUS energy regulation, enough energy would be saved to power more than 377,000 households.

Circuits and losses

The stepdown or synchronous buck circuit is the workhorse of any low-voltage dc/dc power management system, and the main power loss component in any synchronous buck circuit will be the MOSFET switching and conduction losses.

A typical stepdown voltage regulator (VRM) that can be found in any desktop computer is shown in Fig. 1 , and will provide upward of 25 A at full load, with an output of 1.2 V taken from a 12-V input. For this, one MOSFET will be in the main path or high-side slot, and two MOSFETs in parallel will be in the flywheel or low-side socket. Converting down from 12-V input to 1.2-V output is at a 10% duty cycle so the high-side MOSFET will be tuned for low switching loss and the low-side MOSFET pair will be tuned for lowest RDS(on) to minimize conduction loss.

Fig. 1. A typical voltage regulator for a desk-top computer.

Typical peak efficiencies for a multiphase VRM VCORE solution with discrete driver and MOSFET implementation is 90% peak at current ratings of 10 A per phase, coming down to 85% at full loads of 30 A. For these designs today, the complete VRM system is normally at 100-W output and running at 85% efficiency — translating to a 15-W power waste.

Evolutionary improvements in MOSFET silicon

MOSFET vendors have optimized their silicon development primarily in two ways. Firstly, to improve the switching characteristics (switching speed) of their product advanced gate structure designs have been implemented which lowers the gate charge (Qg ) effect. Secondly, increasing cell density, which means that, for the same equivalent die size, the on state resistance is significantly reduced. RDS(on) and current are the two factors that dominate the MOSFET conduction loss given by the very basic formulas:

Ploss = I2 x RDS(on) Conduction loss

Ploss = ½V x I x (Tr+Tf)xF Switching loss

Figure 2 shows the improvements in cell density of ≤30-V n-channel MOSFETs from Fairchild. Each bar indicates a new process evolution. It can be seen that in the last decade cell densities have gone from 32 M cells/sq in. to today at 1 billion.

Fig. 2. The improvements in cell density of ≤30-V n-channel MOSFETs, with each bar showing a new process evolution.

MOSFET figure of merit

Within the industry, a generic performance measurement is always benchmarked as a figure of merit (FOM), and basically this is the sum of transistor on resistance (RDS(on) ) and gate charge (Qg )

FOM = RDS(on) x Qg

RDS(on) directly relates to conduction loss and Qg , directly relates to switching loss. The lower the FOM ,the better the performance.

Figure 3 shows the advancement in FOM for low voltage MOSFET process evolutions. For PowerTrench3, released in 2004 the best FOM was 240, for today’s PowerTrench5 silicon the best FOM is 126.

Unfortunately, reducing the FOM by 50% does not relate to a MOSFET loss reduction of 50%, as the relationship is not linear. However, by careful selection and optimization, today’s MOSFETs can still achieve significant reductions in system power loss.

Fig. 3. The advancement in figure-of-merit (FOM) for low-voltage MOSFETs.

System-level efficiency

So, the power MOSFET is the main culprit for power losses in dc/dc power circuits and by the adoption of advanced devices, this loss can be greatly reduced. How this relates to overall system efficiency?

Designers look for ways to improve system efficiency at light, medium, and heavy loads across the complete machine-operating spectrum. At full loads — for instance, a computer startup or during a heavy processing sequence — the power system is dominated by conduction losses. By simply selecting a low RDS(on) FET losses will be significantly minimized. It is interesting to note that most PCs spend a majority of their operating life in standby or sleep states, and therefore light load efficiency is really critical.

Fig. 4. Efficiency comparisons for a VR11.1 (Intel motherboard power supply spec) VCORE leg.

Figure 4 shows a real efficiency plot taken from a desktop voltage regulator module phase limb. The four curves are results using two differing MOSFET devices at 300 and 550 kHz. We can see efficiencies across the complete load current spectrum.

Focusing on the two upper curves (300 kHz), at full load (30 A) an efficiency improvement of 1.5% can be seen for the latest device. Following the same curves at light load (15 A) a difference of 0.69% can be realized. If integrated across the complete load spectrum, the average power loss reduction of 8% to 10% can be achieved by implementation of the latest MOSFET devices — compared to today’s typical solutions.

For a typical 100-W desktop computer a 10% saving in circuit losses yields 10 W. And in 2007 there were 170 million motherboards manufactured — which could have yielded a saving of 1,700 MW x the many hours they were operated.

Even at the higher switching frequency of 541 kHz we can see that system-level efficiencies are still above 80% at light load and >70% at full load. At higher frequency, the switching losses increase significantly. However, this can save significant costs on the downstream capacitors and inductor and make for a smaller package — both highly motivating for manufacturers.

The sweet spot for most dc/dc converters is 250 to 300 kHz, as it keeps acceptable switching losses verse conduction losses, and gives an acceptably low amount of output ripple to the load. Running below 250 kHz will yield a fraction more efficiency but the voltage output may be quite loose on tolerance and that can’t be used to power a Pentium chipset.

The same ideas can be applied to a notebook computer processor power supply, a game box, and although at much lower currents set-top boxes and other home consumer electronics. Every milliwatt of energy that is saved may appear to be taking baby steps. However, it can make a global difference to today’s environmental challenges. Many small changes in approach will yield significant results. ■

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