Laurent Jenck, ON Semiconductor
The design of the power supply is clearly important, with any improvements in conversion efficiency helping, to some degree, to limit power consumption.
In the past, in order for people to enjoy a television program all they needed was the TV set and an antenna. With the advent of cable and satellite broadcasting, this is no longer enough, and the use of a set top box (STB) has become an almost unavoidable requirement.
Now the majority of viewers in the United States rely on some form of STB to provide them with entertainment, instead of analogue broadcasts. The American Council for an Energy-Efficient Economy (ACEEE) estimates that, by 2010, over 107 million US households will be subscribing to digital television in this way.
However, the added functionality and breadth of channels to choose from comes at a high price in terms of additional electricity consumption. In the following article, we shall explore how better power system design can curb the huge energy expenditure witnessed from use of STBs in the United States, and how OEMs can utilize technological advances to achieve this.
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Many STBs consume the same amount of energy when they are not in use as they do when operational, as they need to stay connected even when not in full operation (so they can receive programming guide information and software upgrades).
This means that there is little actual difference in their electricity consumption when in ON or OFF modes (as just minor changes, such as the front display not being lit, are all that this really effects). If the ongoing impact of STBs on carbon emissions is to be minimized, then more innovative power design techniques need to be developed.
It seems hard to imagine that such a small piece of equipment as an STB can have such a huge effect on our environment. To get some idea of the actual magnitude of the problem, it is worth looking at the current and projected electricity consumption levels involved.
An STB with built-in digital video recorder (DVR) consumes about 350 kWh per year. The STB/DVR and TV combined can actually constitute a bigger proportion of a household’s electricity bill than its refrigerator does.
Given that close to 70 percent of TV viewers in the USA watch their programs through an STB, this equates to 23 billion kWh of electricity being consumed in USA alone, according to figures by the National Defense Resource Council (NDRC), putting a total of close to $2 billion on the nation’s utility bills (and producing 15 million tons of CO2 in the process). This is more than the entire electricity consumption of the state of Nebraska.
Furthermore, the figure could potentially double in the next few years (needing the energy equivalent of 7 new power plants), based on the fact that more high-end STBs with built-in DVRs will be in use.
As Figure 1 describes, there is a clear correlation between STB device functionality (with additions like DVR, and gaming facilities) and increased power consumption. Since manufacturers are looking to add more features to their STB offerings in order to differentiate themselves from the competition, the strain being put on our energy resources will be heightened.
Figure 1: Comparison of STB device functionality and power consumption levels.
Increased levels of legislation governing energy consumption in electronics goods clearly have a part to play, with initiatives such as ENERGY STAR, and the European Union’s Code of Conduct gaining a great deal of traction on either side of the Atlantic. As part of the ENERGY STAR partnership agreement, telecom service providers must ensure that 50 percent of all their STB purchases in the 2009/2010 timeframe meet ENERGY STAR specs.
Though compliance with ENERGY STAR is not mandatory, that is not as great an issue now as it has been in the past. For many people, high electricity consumption is no longer simply an environmental concern, but also a commercial one.
The increasing cost of fossil fuels and rising utility bills now mean lower energy consumption is actually becoming a way for OEMs (and the service providers who buy from them) to set their products apart from competition. Service providers will look to their manufacturer partners to provide them with STBs that are feature-rich, but at the same time frugal in terms of energy use.
So what are the changes that can be made to improve energy efficiency levels in STBs? The design of the power supply is clearly important, with any improvements in conversion efficiency helping, to some degree, to limit power consumption.
The power supply of a STB usually comprises multiple output voltages, the regulation of which can pose significant challenges. Improvements to STBs’ power supply output regulation can be made by stacking the windings.
Another method is to stack the outputs, combining a number of outputs into the one feedback path. Stacking windings and outputs proves highly effective especially in the multiple output power converters used in modern electronics product such as STBs.
However, far more essential, is the design of the system as a whole. If a more energy conscious approach is taken to STB development, then far more substantial changes might be realized. Within the STB there are a number of different components that do not require power unless the system is tuning into live programming or recording content. It is therefore possible for certain components to be powered down for much of the time.
For example, the STB could be designed to power down the hard drive or tuner when not in use to help save energy. If the electricity consumption of STBs could be reduced by 50 percent while not in full operation, then 10 billion kWh ($1 billion) would not be wasted, according to the NDRC.
Optimized reference designs can assist OEMs in quickly producing more power efficient STBs that do not have to scrimp on the features that the subscriber base craves. They can avoid complicated design and development processes and effectively ‘buy in’ the expertise needed.
As part of its series of GreenPoint solutions, ON Semiconductor has developed a 40W STB power supply reference design using free running quasi-resonance (QR) operation, the primary side of which is built around the NCP1308 current-mode controller. The design (as shown in Figure 2) utilizes a flyback converter to store energy in the transformer during the conduction time of the primary switch, then delivers that stored energy to the secondary circuits after the primary switch has been turned off.
Because the energy storage occurs in the transformer, there is no need for energy-storage inductors in the secondary circuits as in the usual buck-derived topologies. This results in far simpler secondary circuits, and makes this strategy particularly suitable for multiple output supplies.
The design has a complement of four outputs (+5V, −5V, 3.3V and 12V) and can handle peak power levels of up to 50W. The QR flyback converter involved in this design is operated in the critical conduction mode, allowing the primary MOSFET to switch back on only when the drain-to-source voltage is at a minimum during the flyback ring out process. This ensures only very low switching losses are seen in both the MOSFET and the output rectifiers of the STB.
Different options for the structure of the secondary circuit designs were investigated. The circuitry included the use of MOSFET synchronous rectifiers for the main flyback windings and synchronous MOSFET buck converters for low voltage post regulators in several different configurations which are described in the GreenPoint reference design documentation.
The configuration described in Figure 2 produced the highest conversion efficiency. In this configuration, the 5 volt output is the main channel with the closed PWM loop while the 3.3 V output is derived from the synchronous NCP1587 buck DC-DC converter.
The transformer windings in this configuration are definitely easy to implement due to the so called “magic ratio” of 3 to 7 turns for the 5 V and 12 V outputs respectively. In this case, the quasi-regulated 12 volt secondary winding is again “stacked” onto the 5 volt winding with an additional 4 turns for improved cross regulation. Efficiency is further increased by the use of synchronous rectification on the 12 V output.
Figure 2: Simplified block diagram of ON Semiconductor’s GreenPoint STB power supply reference design.
In summary, manufacturers can utilize reference designs which are already fully optimized for having minimal power overhead, and this will allow them to create more efficient STBs without complicated, costly, or long drawn out development periods.
By using this, and other methods, it should be possible to keep the huge combined energy levy that these products represent down to manageable levels, and thereby restrict their effect on the environment.
ON Semiconductor has conducted extensive tests to discover which secondary circuit configurations, when combined with the main converter, yield the greatest efficiency levels, as well as which will offer the lowest system procurement costs.
If it was possible to deploy STBs where the ready mode power consumption was reduced to 5W, and sleep mode power to 1W, then projections suggest that over a 4-year period there would be a cumulative saving in the region of 4.4 billion kWh. The technology is there to make this feasible, and by utilizing it large scale energy conservation could be brought about.
For more information visit www.onsemi.com