Beyond the datasheet – PC efficiency for the real world

Beyond the datasheet – PC efficiency for the real world

Get proof of the datasheet by requesting efficiency test results from different loads and voltages

BY ERIC GLATFELTER, JUAN CARLOS PASTRANA, and LAURENT JENCK
ON Semiconductor, Phoenix, AZ
http://www.onsemi.com

Under commercial, environmental, and legislative pressures to deliver “greener” desktop PCs and servers, engineers are faced with the challenge of implementing PC power supplies that comply with efficiency standards such as 80 Plus, Energy Star, and the Climate Savers Computing Initiative (CSCI). Recognizing this, a growing number of semiconductor companies are now actively developing chipsets and reference designs that would seem, at first glance, to help designers achieve such compliance.

Unfortunately, not all solutions are equal, and the challenge for the engineer is how best to compare the various designs. Of particular concern is that demonstrating compliance with a given efficiency standard does not automatically mean that a power supply design will deliver the target efficiencies the designer is looking for once the PC is deployed in the field.

Datasheets, standards vs. real world

To illustrate the challenge, let us consider the example of an engineer tasked with designing a desktop computer using an ATX power supply compliant with the 80 Plus Silver performance specification. Funded by electric utilities in North America, the 80 Plus program is designed to encourage the use of more efficient power supplies in desktop PCs and servers. To this end, the 80 Plus Silver specification requires that multi-output power supplies in computers and servers have an efficiency of at least 85% at 20% and 100% of rated output power, and at least 88% at 50% of rated output power.

Table 1. Efficiency targets for multi-output desktop power supplies

The first problem is that, being a U.S.-centric specification, achieving the relevant certification requires only that the efficiency ratings be verified with nominal input voltages of 115 Vac. Many desktop computers, however, are designed to be sold and deployed around the world and, therefore, must accommodate “universal” line voltages of anywhere between 100 Vac (for example, in Japan) to 240 Vac (most of Europe). A PC manufacturer that is truly committed to delivering green PCs and that wants to turn this commitment into a commercial advantage with consumers will have to demonstrate claimed efficiency levels across the complete voltage range.

This issue of widely varying input voltage conditions presents a number of nontrivial challenges, such as the impact of conduction losses at the input stage. These losses can significantly impact overall efficiency at the low end of the input voltage range, making it difficult to maintain the efficiency ratings for compliance.

The engineer must also consider the issue of actual power versus rated power. An ATX power supply may be rated for 255-W operation, but, in real-world operation, will only operate near full load when running highly processor-intensive applications. In reality, most desktop PCs rarely operate at this full load, and it is estimated that over two-thirds generally operate under light-load conditions. Under light-load conditions, the losses associated with housekeeping operations represent a larger percentage of overall losses with a corresponding negative impact on operational efficiency.

Cable length matters

Another factor that the engineer should look into when considering a manufacturer’s claims is the length of cable that was used during the compliance testing and certification process. The certification requirements do not specify cable length, leading some manufacturers to claim efficiency levels that have been measured directly at power supply output (or with an unrealistically short cable).

In reality, the form factor and design of a working desktop PC means that the cable between the power supply and the point of power delivery typically measures around 16 in. (41 cm) and, therefore, is a contributor to overall losses. Because of this, laboratory simulation of real-world efficiency requires testing using similar cable lengths.

Matching real-world operation to efficiency claims

With these issues in mind, ON Semiconductor set out to develop an open, high-efficiency ATX power supply reference design that would allow manufacturers of desktop PCs and servers to deliver real-world efficiencies, across a variety of operating conditions, in line with the 80 Plus Silver, Energy Star 5.0, and Climate Savers Computing Initiative Step 3 efficiency standards. At the same time, the design would have to meet the IEC 61000-3-2 requirements relating to power factor correction (PFC), again across a range of real-world operating conditions.

Figure 1 shows a schematic of the resulting GreenPoint ATX reference design, which addresses all of the functional blocks of an ATX power supply including power factor correction, switch mode power supply control and regulation, post regulation, and standby power requirements.

Fig. 1. 255W ATX power supply reference design.

The new design brings together a number of semiconductor technologies, each of which is optimized to provide the best possible performance/power consumption ratio.

On the ac supply input or primary side, the NCP1654 continuous-conduction-mode (CCM) power-factor-correction controller reduces the component count required for the inclusion of PFC functionality and provides a robust, cost-effective front-end that is used over the 90 to 265Vac input voltage range. The PFC stage delivers a constant output voltage of 385 V to the second stage, the resonant half-bridge LLC converter. This topology optimizes efficiency and minimizes the EMI signature. The resonant half-bridge LLC converter is built around the NCP1396. Thanks to its proprietary high-voltage technology, this controller includes high- and low-side drivers for half-bridge applications, accepting bulk voltages up to 600 V.

On the secondary side, this architecture uses a synchronous rectification scheme built around ON Semiconductor’s NCP4302 controller in order to generate the 12V outputs. The controller is designed to simplify the implementation of synchronous rectification in switch-mode power supplies, improving overall system efficiency by between 2% and 4%. In addition to synchronous rectification control, the device incorporates an accurate shunt regulator with TL431 functionality, reducing board space and system cost.

Power losses are further reduced through the deployment of MBR20L45 dual Schottky rectifiers, which operate with a very low forward voltage drop.

Finally, two identical dc/dc controllers are used to down-convert the 12 V into +5, +3.3, and 12 V. The dc/dc controller is the NCP1587, a low-voltage synchronous buck controller in an SOIC-8 package. Each dc/dc controller drives two NTD4809N (30 V, 58 A, single n-channel power MOSFET) in a synchronous rectification scheme. This MOSFET has a low RDS(on) to minimize conduction losses, has a low capacitance for reduced driver losses, and has an optimized gate charge to keep switching losses to a minimum.

In addition, a compact flyback converter, built around the highly integrated switcher, NCP1027, delivers 15W standby power to another isolated 5V rail. The device includes a 700V MOSFET, and incorporates a proprietary skip-cycle that enhances efficiency at light load conditions.

Efficiency tests

Figure 2 illustrates the results of the efficiency tests conducted on the new 255W ATX power supply reference design across a variety of loading conditions at input voltages of 100 Vac, 115 Vac, 230 Vac, and 240 Vac. To simulate a real-world environment all of the test measurements were obtained at the end of a 16in. (41-cm) cable.

Fig. 2. Efficiency measurements at different loads and voltages.

As the diagram shows, at 50% load the reference design was able to achieve maximum efficiencies of 90% in the case of 230 and 240Vac inputs and above 88% when working with voltages of 100 and 115 Vac. In addition, efficiencies at 20% and 100% of load are above 85% for all input voltages. These results ensure that a computer OEM who chooses the reference design can correctly claim to be using an ATX power supply that exceeds the requirements of the 80 Plus Silver, Energy Star 5.0, and Climate Savers Computing Initiative Step 3 efficiency standards for desktop PC power supplies. 80 Plus Silver performance has been certified by an independent testing laboratory. The certification can be downloaded at http://www.80plus.org/manu/psu/psu_reports/ON%20SEMICONDUCTOR_ATX%20255W%2085+_ECOS%201283_Report.pdf ■