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REVIEW: Energy Star power supply reference design uses standard parts

ON Semiconductor Delivers GreenPoint ATX Reference Design that Exceeds New ENERGY STAR Efficiency Standards for Desktop Computers

Latest open reference design for 300 watt ATX power supplies reduces losses by 50 percent over existing solutions

INTEL DEVELOPER FORUM, BEIJING—ON Semiconductor unveils the industry's first open ATX reference design that meets the new ENERGY STAR performance requirements for ATX power supplies used in desktop PCs scheduled to take effect this July. The new reference design delivers efficiency that exceeds the ENERGY STAR requirements under all specified load conditions, and provides a blueprint for power supply manufacturers to bring highly efficient power supplies to market with readily available, cost-effective technology.

“ON Semiconductor has clearly demonstrated that it is both technologically possible and financially feasible to deploy highly efficient ATX power supplies to the market today,” said Andrew Fanara, director of ENERGY STAR Product Development at the U.S Environmental Protection Agency. “The availability of this open reference design prior to the effective date of the new ENERGY STAR requirements for ATX power supplies has very positive implications for energy savings in the desktop PC market.”

ON Semiconductor's reference design achieves full-load efficiency of 86.5 percent at high-line and 82.5 percent efficiency at 20 percent of load and low-line. This reduces power losses by approximately 50 percent, compared to the typical power supplies on the market today that average 70 percent power efficiency. Additionally, ON Semiconductor's ATX reference design meets IEC61000-3-2 power factor requirements.

“ON Semiconductor's 300 W GreenPoint ATX reference design demonstrates our unwavering commitment to deliver innovative, turn-key power solutions for extremely efficient power supplies,” said Andy Williams, senior vice president and general manager of ON Semiconductor's Automotive and Power Regulation Group. “We worked closely with worldwide regulatory organizations and our customers to develop this cost-effective ENERGY STAR compliant power supply solution. ON Semiconductor will continue to introduce additional GreenPoint power solutions addressing the global need for power efficiency in both active and standby modes.

The ATX Reference Design

The reference design addresses all functional blocks of the ATX power supply, including power factor correction, switch-mode power supply control and regulation, post regulation and standby power. The design utilizes several of ON Semiconductor's newest power management devices and draws from its leading discrete component portfolio to achieve superior efficiency performance.

ON Semiconductor has a complete documentation package for the ATX reference design, including description, schematics, bill-of-materials, Gerber files and evaluation guidelines. The documentation package is available at www.onsemi.com/ATX, as are links to relevant industry information and other supporting information.

Key products in the reference design include:

NCP1653: This compact continuous conduction mode (CCM) power factor correction (PFC) controller makes the choice of incorporating a PFC front-end into an ATX design easy, by reducing part count and providing a robust, cost-effective front-end without range switches. Offered in Pb-free 8-lead SOIC and PDIP packages, the NCP1653 is priced at $0.80 USD in 2,500-unit quantities.

NCP1562: This active-clamp pulse width modulation (PWM) voltage-mode controller has a +/-5% duty cycle limit with a soft-stop to prevent the typical oscillations in the forward active-clamp topology. The device has a dual-mode over-current protection circuit that handles momentary and continuous over-current conditions differently. Offered in Pb-free 16-lead TSSOP and SOIC packages, the NCP1562 is priced at $1.60 USD per unit in 2,500-unit quantities.

NCP1027: This high-voltage switcher includes a power MOSFET together with a startup current source, all directly connected to the bulk capacitor. To prevent lethal runaway in low input voltage conditions, an adjustable brown-out circuitry blocks the activity until sufficient input level is reached. Offered in a Pb-free 8-pin PDIP package, the NCP1027 is priced at $0.97 USD per unit in 1,000-unit quantities.

NCP4330: This synchronous post-regulator replaces the mag-amp to provide higher efficiency in the output section of the ATX power supply. Offered in a Pb-free 8-pin SOIC package, the NCP4330 is priced at $0.60 USD per unit in 2,500-unit quantities.

MSR860: This 8 amp (A), 600 V power rectifier features SWITCHMODE soft recovery. Offered in a Pb-free TO-220 package, the MSR860 is priced at $1.07 USD per unit in 50-unit quantities.

For more information, visit http://www.onsemi.com or contact Kelvin Tsiang at Kelvin.Tsiang@onsemi.com.

Today's PC power supplies go way beyond simple transformer/rectifier/filter designs. Safety, efficiency, and stability rule, making the design of power sources arcane and complex.

Enter Energy Star . It's a US government program (run by the U.S. Environmental Protection Agency and Department of Energy ). Its goal is to promote energy-efficient consumer products. The Energy Star logo now appears on many computer products and peripherals, and these days most consumer OEMs covet the logo.

The trick is implementing Energy Star. You could roll your own, but this reference design and chipset from ON Semiconductor should grease the skids.

What's more, ON's GreenPoint reference design confirms the feasibility of meeting Energy Star standards with existing technology. To date, ON Semiconductor has released a number of GreenPoint reference designs.

Industry-First?

As the press release (on the left) notes, ON Semiconductor's open ATX reference design can specifically help you meet Energy Star requirements for ATX power supplies as used in desktop PCs. Interestingly, ON Semiconductor claims it is the first chip company to provide a reference design for an ATX power supply (two years ago).

That first-generation certified reference design is now surpassed by this improved second-gen design, using newer ICs. The second-generation consists of a single board designed to fit into a standard ATX enclosure, along with a fan.

Clamp-Forward Topology

The system relies on an active clamp-forward topology based on the company's NCP1562 Active Clamp Controller IC. A NCP1653 CCM (continuous conduction mode) PFC (power factor correction) IC is also used. A standby controller, dubbed the NCP1027 , is optimized for the ATX power supply. It includes an on-chip high-voltage MOSFET .

On the secondary side, the architecture uses a post-regulator approach for generating a 3.3-V output. This is an alternative to the conventional mag-amp (magnetic amplifier).

It's interesting that ON Semiconductor believes its post-regulator will give you the highest efficiency, but if you wish to use a different topology, a design can be developed that uses all the other pieces of this architecture—without the post-regulator.

Significantly, ON's design closely follows ATX12V v2.2 power supply guidelines and specs.

While this reference design is obviously positioned to get you to buy ON Semi's chips, it does break new ground in a number of areas, and promises to streamline the design and integration process.

It's Coming

Indeed, the efficiency targets ON Semi aims for encompass the 80 PLUS program, and the Energy Star Program Requirements for Computers, v4.0 . As the ON Semi press release mentions, this is going into effect on July 20th.

The target specs for ON Semiconductor's reference design include operation with input voltages from 90-V ac to 265-Vac, at 47-Hz through 63-Hz (so-called universal mains), with a minimum efficiency of 80% (for 20%, 50%, and 100% of rated output) as defined by the 80 PLUS requirements as well as the Energy Star specs.

The reference design will yield a supply that will dish up at least 305-W total maximum output power, and operate at an efficient power factor of 0.9 or greater (at 100% load). It will also meet the ATX12V safety spec, as well as IEC1000-3-2 requirements, and do it over the input line range and under full load conditions. It will also hold up for a minimum of 20-ms during line sags and droops.

The traditional workhorse for ATX supplies has been the single switch-forward converter, operating at about 100-kHz. This is a robust approach that delivers good full-load efficiency at low cost. But, as power levels increase and regulatory requirements and energy conservation agencies push for ever higher efficiency under all load conditions, the single switch forward topology is reaching its limit.

To capitalize on the benefits of the active clamp topology alternative, the NCP1562 captures the necessary control features in a 16-pin package. To boost efficiency and maintain regulation, instead of the conventional mag-amp post-regulator, the 3.3-V output is derived from a 5-V winding of a supply's main transformer.

The MOSFET drivers, timing, sync, and control functions to support this output are provided by the NCP4330 controller, giving you a 6-W improvement. Gate charge and ON-resistance are also optimized in MOSFETs.

One Single-Sided Board

The reference design is implemented on a single-sided board, using daughter cards for the NCP1653 PFC chip and NCP1562 clamp controllers. A board is also used for the NCP4330-based 3.3-V post regulator and supervisory controllers.

This supports the use of newer generation GreenPoint controllers without the need for a re-layout of the main board. A daughter card is used for the supply's EMC components.

Let's look a bit closer at the silicon. The NCP1653 CCM device is what handles PFC (power-factor correction) in a supply. The 8-lead NCP1653's tiny form-factor belies its power-packed function mix. The NCP1653's output stage packs no less than ±1.5-A current handling ability. That can directly drive a MOSFET.

The device can also operate in a follower boost or constant output voltage mode, at either a fixed 67-kHz or a 100-kHz switching frequency. The follower boost mode, with reduced voltage, spells a reduction in the size and cost of the associated inductor and power switch. The output voltage isn't set at a constant level, but instead depends on the RMS input voltage or load demand.

Fixed-frequency operation also eases EMI (electromagnetic compatibility) compliance. Moreover, the CCM operation reduces di/dt and any resulting interference.


Click to view simplified block diagram

As you peruse the NCP1653 CCM chip's block diagram (above) you'll see additional circuits that support features such as soft-start and under-voltage lockout. The device also provides under-voltage and over-voltage protection, as well as programmable over-current protection and over-power limiting. It also includes thermal shutdown circuitry.

A PWM Controller

Next up in the GreenPoint mix is ON's NCP1562 PWM voltage-mode controller. It uses two in-phase outputs, with an overlap delay to prevent simultaneous conduction to ensure soft-switching.

The IC's main output can drive a forward-converter primary MOSFET. Its secondary output can drive an active clamp circuit MOSFET, a synchronous rectifier (on the secondary side), or an asymmetric half-bridge circuit.

Like the NCP1653, the companion NCP1562 can slash component-count. That's due, in part, to its accuracy with respect to critical specs such as maximum duty-cycle limit, under-voltage detection, and over-current threshold.

Soft-Stop And Current Limiting

The NCP1562 also provides soft-stop and a cycle-skip current limiter with a time threshold. The soft-stop circuitry powers-down the converter in a controlled manner if a severe fault occurs. The cycle-skip feature starts a soft-stop sequence if a continuous over-current condition happens.

The NCP1562 also includes line feed-forward frequency synchronization (up to 1-MHz), cycle-by-cycle current limiting with leading-edge blanking, and independent under-voltage and over-voltage detectors. The device also provides adjustable output overlap delay, and gives you a programmable maximum duty-cycle setting.

700-V MOSFET On-Chip

Next up is the NCP1027 switcher chip with on-chip MOSFET. This isn't just any MOSFET; it's a 700-V power MOSFET with very low ON-resistance specs. Nifty.

The NCP1027 is for output power levels ranging from a few watts up to 15-W in universal flyback applications. Current-mode operation, together with adjustable ramp compensation, lets it work well in universal mains applications.

Preventing Runaway

The NCP1027 also packs a start-up current source, and these are directly connected to a system's bulk capacitor. To prevent runaway in low input-voltage conditions, an adjustable brown-out circuit can block activity until a sufficient input-level is reached.

The device also includes an over-power protection pin that lets you precisely compensate all internal delays in high input-voltage conditions. It also optimizes maximum output-current capability.

The NCP1027 also includes a timer to detect any overload or short-circuit; this can stop all operations in their tracks, ensuring safe auto-recovery, low duty-cycle burst operation. Auto-recovery over-voltage protection permanently monitors VCC levels, temporarily shutting down drive pulses in case of an unexpected feedback loop runaway.

Replacing The Mag-Amp

The next component in the reference design is ON's NCP4330 synchronous post-regulator chip. It replaces the mag-amp, in order to provide higher efficiency in the output section of an ATX supply.

Operable at frequencies as high as 400-kHz, the NCP4330 includes an on-chip dual MOSFET driver. The N-channel MOSFET can gives an application an advantage in terms of efficiency. Being directly fed by a secondary AC signal, the device slashes dissipation.

Other features include under-voltage lockout, overlap management for soft-switching, thermal shutdown protection, and PWM operation that's synchronized to the converter frequency.

The NCP4330 should fill the bill for forward, multiple output supplies using synchronous rectification. One output would be regulated in a configuration that modulates the forward converter duty cycle. The other outputs are regulated by a dual MOSFET driven by the NCP4330.

In operation, the high-side MOSFET turns on during one part of the forward converter on-time, while the low-side power switch is on for the rest of the period (in a free wheeling mode). The sequencing of the switching phases, includes overlaps that result in only one hard switching at high-side turn-on. The three other transitions are soft for optimum efficiency.

Synchronous rectification ensures CCM operation whatever the load is. The technique sends back some energy towards the input under light load conditions.

Lastly, ON's reference design features the company's MSR860 TO-220 -packaged soft-recovery 600-V 8-A power rectifier.

Click here for a NCP1027 chip datasheet (in Adobe Acrobat .PDF format).

Click here for a NCP4330 datasheet (in Adobe Acrobat .PDF format).

Click here for an MSR860 diode datasheet (in Adobe Acrobat .PDF format).

Click here for a NCP1562 datasheet (in Adobe Acrobat .PDF format).

Click here for the NCP1653 chip datasheet (in Adobe Acrobat .PDF format).

For further details contact ON Semiconductor, 5005 East McDowell Rd., Phoenix, Ariz. 85008. Phone: 602-244-6600.

ON Semiconductor , 602-244-6600. www.onsemi.com

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