New approach to reducing standby power
Figures published by the Energy Information Administration (EIA) show that US electricity usage is more than double what it was back in the mid 1970s, however it has started to plateau in recent years due to increasing awareness of energy efficiency and more stringent government regulations. One of the key areas where progress has been made is in trying to reduce the electricity that is consumed by the multitude of different power supplies that are found throughout the home for powering consumer electronic devices. In the past, the focus has been on improving active mode efficiency of the power supply. Now many power supply manufacturers are taking notice of the energy consumed in standby mode.
Standby power describes the power which is consumed by electronic devices and equipment while turned off but still plugged in. This constitutes a large portion of the energy consumption in the average home (usually about 10% of the annual total). For example, many people leave their notebook adapters connected underneath their desks when they take their computer to work or school. Although there is no load connected to the adapter, the adapter still consumes a few hundreds of mW while it is plugged in. By eliminating this wasted energy, a considerable reduction could be made to the nation’s domestic electricity usage. The Department of Energy (DoE) has recently proposed new standards of efficiency for power supplies (to go in effect July 2013) that target reducing this wasted energy in standby mode. The new standard requires that power supplies with a nameplate output power of >1 W to <49 W must have power consumption of <100 mW during no load.
For power supplies between 49 W and 250 W, the consumption during no load must be less than 210 mW. Previously, government regulations called for no load power consumption of <300 mW and <500 mW respectively. In addition to the new government regulations, many power supply OEMs are in fact trying to outpace such regulations, setting their sights on product development roadmaps that go way beyond this standard in order to get a competitive edge and increase their market share.
Power supplies designed for charging the batteries of consumer electronic devices are one of the major contributors to wasted electricity. In the past, these chargers employed very basic charging systems such as the one in Figure 1. This circuit simply provides a constant charge supply to the battery. The low cost transformer used here has a considerable influence on no load power - drawing current and making the system very inefficient. With this style of battery charger, the charging continues indefinitely until the product is unplugged, even once the battery is full. Though today’s battery chargers have become more sophisticated, many chargers are still unable to detect whether the battery they are connected to is fully charged or not and will continue supplying electrical charge to the battery when it is no longer needed. A substantial amount of energy can be saved if smarter charging systems are implemented, which automatically turn off when no power is needed.
A lot of progress has been made with adapters for notebook computers. An example of an advanced notebook adapter circuit is detailed in Figure 2. While most modern notebook power adapters draw somewhere between 300 mW and 500 mW during no load condition, the power supply in Figure 2 draws only 10 mW during no load condition.
This circuit consists of a primary side IC which controls power delivery and a secondary side controller to monitor the output and provide feedback to the main controller. The primary side controller is the NCP1246 fixed-frequency current-mode controller from ON Semiconductor. This controller has been optimized to draw minimal power during no/light load conditions. The secondary side controller is the NCP4354, which is capable of detecting no load conditions and placing the power supply into a low consumption sleep mode.
During no load conditions, the adaptor’s electricity consumption is greatly affected by the X2 capacitor discharge circuitry. Adapters utilize high voltage X2 capacitors connected to their AC input for electro-magnetic interference (EMI) filtering. To safeguard against the threat of electrical shock, this capacitor needs to be automatically discharged to a safe voltage level within just 1 sec of the adapter being unplugged. Normally this is done via a string of high impedance bleed resistors which are placed in parallel with the capacitor. However, this resistive drain creates a constant draw of approximately 25 mW when supplied by 230 VAC. During no load condition, this 25 mW drain is a major contributor to the overall standby power consumption.
With more advanced AC-DC controllers, such as the NCP1246, an internal AC line detector with active X2 discharge circuitry is included in order to address this issue. With this feature, it is possible for the power supply to detect when an AC signal is no longer present and then activate an internal switch to discharge the X2 capacitor. The need for bleed resistors is thereby avoided and the constant 25 mW power drain is removed.
Another technique used in the latest generation of AC-DC power supplies for reducing standby consumption is to allow the power supply to drop out of regulation during no load. These “Sleep Modes” or “Off Modes” greatly reduce the switching losses as well as the Icc consumption of the controller. The challenge lies in entering and exiting this mode at the appropriate time. The NCP4354 controller is able to detect no load conditions and signal the NCP1246 to enter into the low consumption Off mode. When in Off mode, the primary side controller is deactivated and energy is supplied by the output capacitors. The output voltage of the adapter begins to drop as there is no more switching on the primary side. The output voltage is allowed to decrease to an adjustable level before the NCP4354 signals a primary restart to charge the output capacitors back up. When the adapter is reconnected to the battery, the NCP4354 controller automatically restarts the primary side controller. Feedback control as well as On/Off signal can be provided with only one optocoupler. When used together in a circuit of this type, the NCP1246 and NCP4354 are capable of achieving <10 mW no load input power consumption when connected to the US mains system.
Advanced techniques for reducing standby power consumption like the one described here are helping OEMs to meet the requirements of legislative bodies and set a new bar for energy efficiency. Improvements in semiconductor technology combined with increased awareness of the contribution of standby power losses are now enabling a new breed of adapters to be brought onto the market. These new power supplies help prevent the expansion of US electricity usage and therefore reduce the amount of per capita carbon emissions.