Reducing phantom loads with mini power supplies

While the initial enthusiasm for renewable energies might have receded somewhat, energy efficiency is still a major concern for manufacturers and consumers. This article outlines the contribution of specially developed mini power supplies in reducing standby power consumption.

Days after the Fukushima disaster, Germany decided to exit nuclear power. The frightening pictures from Japan were just too confronting and the alternatives looked promising. People envisaged gigantic offshore wind parks along the coast and solar power plants in the Sahara Desert. These solutions would solve the problem of nuclear waste disposal and the risk of a maximum credible accident. However, in reality, things are obviously more complex. While the sun shines 365 days a year over the Sahara Desert, it does not shine at night. To provide electricity during night-time, we need huge storage capacities or other power sources. Other problems arise in relation to distribution of the power.

Electricity is traditionally generated close to the point of use and offshore wind parks and solar plants in far-flung countries require huge power pipelines. If such lines are to be installed near residential areas, the planning process is likely to take years. It is already obvious that the move towards renewable energy will take much longer than was initially envisaged. More immediate measures are necessary to prevent blackouts as are already happening in the USA and India. Energy-saving measures offer the best alternative.

According to the EU Commission, the power that can be saved by reducing the standby and off-mode consumption of the millions of electronic devices that are found in every office and household is estimated to correspond to the annual power consumption of Denmark. Suitable technical measures to reduce office and household consumption can reduce energy total consumption by 75% by 2020, according to the EU Commission.

Standby and off-mode losses are underestimated

An average consumer with limited technical knowledge probably believes that switching a device to standby mode is basically the same as switching it off. The consumer does not expect standby consumption to show up on the electricity bill. But the average consumer is wrong. In the old days when the garage door was controlled with a key and the off switch was actually a mains switch - off meant off. Today, most home and office electronics come with a ‘soft off’ button, which puts them into standby or off mode. This means that a power supply designed for normal operation remains powered in no-load mode, consuming power with extremely low efficiency.

Power supplies remain connected to the grid around the clock and consume considerable amounts of power even when not in use, depending on their circuitry. This can easily be verified, particularly with older, linear-controlled power supplies as they become warm.

The same applies to stereo systems, which, due to the low interference, are generally equipped with linear-controlled power supplies. In older devices, the standby electronics are powered directly from the mains power supply, so that they consume between 10 and 20 W even in standby mode, which is about half the power consumed in operating mode at normal room volume. Assuming that the stereo system is on standby mode for 20 hours a day, 70% of its overall consumption occurs during the standby time. Comfort thus comes at a high price.

Remote-controlled blinds and garage doors show even less favourable figures. They are normally operated for a few minutes a day and are idle and in standby mode for 99% of the time. The worst offenders are, however, old fax machines. If only a single fax is received per week, which is probably quite realistic for private fax machines, the device is in standby mode for 99.99% of the time, consuming up to 20 W just waiting for a message.

Apart from the high power consumption in standby mode, it’s the unreasonable ratio between actual operating time and standby time that’s concerning. In many homes, up to 50 electronic devices with standby function or external adapter are plugged in day and night. Most of these are only used for around 10% of the time. In a well-equipped household of four, the standby and off-mode consumption can easily amount to 200 W, costing the family around $537 a year.

The situation in offices is not much better. Legions of adapters power PC peripherals, communication devices and modems around the clock, even when there is nobody in the office. For older devices, details regarding power consumption in standby or off-mode are rarely included in the data sheets.

Automated systems are found everywhere - in homes, offices and factories, from sensor-controlled blinds to remote-controlled garage doors. For a few minutes of action, these systems remain powered around the clock. This also applies to remote access equipment for data loggers, weather stations, pump systems and similar equipment with SMS or GPRS modems. Low standby consumption has already become an important USP for manufacturers, as the marginally higher production costs are no longer an issue.

Stricter limits expand the market for mini power supplies

The European Union’s framework directive on energy-using products, known as the EuP Directive, came into force in 2010 and put a stop to the low-cost practice of feeding the standby electronics from a mains power supply. Manufacturers had to resort to technically more demanding solutions where the standby circuit is supplied through a separate mini power supply and the main power supply is fully disconnected from the grid by means of a relay.

From January 2013, the second phase of the EcoDesign Directive came into force, reducing the existing limit values by another 50%. According to these regulations, complex standby circuits with display, such as those of hard disk recorders, must not consume more than 1 W. For simpler systems and plug-in adapters, the maximum standby consumption is limited to 0.5 W. As standby consumption and energy-saving ratings are now a common feature in promotional material, mini power supplies with low no-load power have become extremely popular.

Low-power modules with no-load consumption of 0.03 W

Switched-mode power supplies are designed in such a way that their optimum efficiency is close to the rated power. The efficiency drops in line with the load, until it is near zero when the device is in no-load mode. This drop, however, is not linear, and many data sheets contain only the peak values. This is often misleading, as has been shown in recent comparison tests of a number of products from different manufacturers, carried out by Recom. In these tests, the efficiency, especially in the lower and medium load range, was compared. This efficiency is relevant as power supplies are normally not exactly matched to the rated power and must cater for a certain safety reserve. Despite comparable peak values around 80%, the Recom mini power supplies were considerably more efficient in the medium and lower load ranges. At 50% load, the efficiency was still near the peak value. At a load of 15%, the measured efficiency of the mini power supplies was still between 60 and 70%.

Manufacturers are advised to choose their power supplies with great care and to perform their own tests. As regards efficiency, the need to operate the power supply at near-full load level is less important today than before.

Apart from energy efficiency, electromagnetic compatibility is a major concern, as nobody wishes to get entangled in controversies about electro-smog and electromagnetic interference. In addition to emitted interference, low line-bound interference is an issue here.

Power supplies of the Recom RAC series conform to EN 55022, class B, and EN 55024 that apply to household applications. This is achieved without additional external components but by means of harmonised electronic scanning of the mains frequency. Interference is thus eliminated at the source, so that small filter components can be used, especially when compared with external solutions. This has a positive effect on the power consumption, contributing to the low no-load rating. RAC mini power supplies are available for a wide input voltage range from 90 to 305 VAC (except 1 W version) and can thus be used in conjunction with microcontrollers, sensors and similar equipment in both Europe and the US with its 227 VAC mains system. The modules come with a softstart function limiting the inrush current to 10 A/<0.5 ms (RAC01/02/03), or 30 A/< 0.2ms (RAC06), respectively. All modules are short-circuit-proof and protected against overload. The outlet voltage can be set to any value between 3.3 and 24 VDC. The 3 W module is also available with a 3.8 VDC output for chargers and GSM modems. For smart home applications, the company offers a mini power supply shaped like a flat disk. The prototypes of new power supplies undergo HALT tests, so that any problems are detected early during the development phase. The products of the RAC series are UL/EN and CE certified, and designed for continuous operation over many years.