Reference design powers desktop printers

Design is a blueprint that will help engineers quickly deliver power adapters to market that provide both low active-mode and low standby-mode power consumption.

ON Semiconductor has released its latest GreenPoint reference design - this one for 40W power supplies used to power desktop printers.

The design is a blueprint that will help engineers quickly deliver power adapters to market that provide both low active-mode and low standby-mode power consumption.

Providing active-mode energy efficiency above 83% when delivering an output power between 5 and 40W, this reference design achieves a no-load power consumption of less than 150mW.

Typical 40W printers in use today consume on average 450mW in similar conditions.

The frequency foldback technique implemented in the design, keeps the efficiency of the power supply very high under light load conditions.

(With an output power from 0.5 to 2W, the efficiency varies from 66 to 79%).

The combination of these characteristics makes this GreenPoint reference design compliant with the most stringent requirements from Energy Star to the Group for Energy Efficient Appliances (GEEA) via the US Executive Order "1W Standby" Order and the Japanese Eco Mark programme.

"The new 40W printer power supply reference design joins our 200, 60 and 90W GreenPoint power adapter designs, and strengthens our position as the leading silicon semiconductor solution provider for energy efficient power", said Andy Williams, Senior Vice President and General Manager of ON Semiconductor's Automotive and Power Regulation Group.

"The ability of this design to improve a printer's power supply efficiency by more than 12% in light load conditions and 2.5% under nominal load is further evidence that some of the most significant improvements in energy savings can be achieved through advanced silicon and system level design techniques that are extremely cost effective".

The new 40W reference design enables a peak power of 80W to be transiently delivered - a feature that makes it particularly suitable for printer applications.

An over power protection below 100W has been implemented for the entire input voltage range (88-265V AC) to comply with the Limited Power Source (LPS) test.

In addition, the design uses a smaller bulk capacitor than those typically used in 40W power adapter designs with similar performance.

This helps reduce both board space and the overall system cost.

The enabling device for the 40W reference design is the newly introduced NCP1351, a current mode controller targeting low-power, offline flyback switched mode power supplies (SMPS).

Based on a fixed peak current technique (quasi-fixed on-time), the controller reduces its switching frequency as the load becomes lighter.

As a result, a power supply using the NCP1351 naturally offers excellent no-load power consumption, while optimising the efficiency in other loading conditions.

When the frequency decreases, the peak current is gradually reduced down to approximately 30% of the maximum peak current to prevent transformer mechanical resonance.

The risk of acoustic noise is thus greatly diminished while keeping good standby power performance.

An externally adjustable timer permanently monitors the feedback activity and protects the supply in the presence of a short circuit or an overload.

Once the timer elapses, NCP1351A and NCP1351C stop switching and stay latched, while NCP1351B and NCP1351D attempt restart.

Both NCP1351C and NCP1351D feature a dual over current protection, a better fit for applications facing large output power transients.

The internal structure of the device features an optimised arrangement which allows one of the lowest available startup currents, a fundamental parameter when designing low standby power supplies.

The negative current sensing technique minimises the impact of the switching noise on the controller operation and allows the user to select the maximum peak voltage across the current sense resistor.

Power dissipation can then be optimised for the application.

The NCP1351 A, B, C and D are all offered in Pb-free SOIC-8 packages, and are priced at US $0.32 per unit in 2500-unit quantities.

A complete documentation package for the 40W power adapter is available - including description, block diagram, schematic, bill-of-materials, Gerber images, and evaluation results.

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