Design Con 2015

How capacitive sensing can reduce standby power in household appliances

Gerrit de Villiers, Azoteq

September 07, 2012

Gerrit de Villiers, AzoteqSeptember 07, 2012

Consumer electrical appliances consume a significant amount of power when left in standby mode or even when switched off. Standby power consumption has become one of the largest individual electrical end uses of the residential sector, averaging 10% (60W per home) [1] of the average power usage of households. Strict regulations are being set in place to limit standby power consumption, which is said to have exceeded 20 GW in the residential sector of industrialized countries.

This article shows how to reduce standby power to well below 50 mW, while at the same time minimizing cost/complexity and extending the product’s expected life. Special attention is given to capacitive proximity sensing, which allows the designer to add a substantial amount of intelligence, such as waking up and enabling the electronic device only when required, and permitting a visual indication to the user of what is required to access the specific features of the device.

The culprits
The term ‘off’ has become an increasingly relative one as far as electronic appliances are concerned and can refer to the many lower power modes of modern electronic devices. The most popular low-power modes currently in use include standby, sleep, standby active, and soft off mode. For the purposes of this article, all non-active modes in which the device is not performing its primary function will be referred to as standby mode.

Almost any product with an external power supply, LED, or display that runs continuously, a remote control, a battery-charging functionality, or any type of monitoring functionality will draw power continuously. Table 1.1 provides a condensed overview of some of the most common household appliances, together with their typical standby power consumption.




Click on image to enlarge.

Table 1: Typical household appliance standby power consumption [2]

Addressing standby power consumption
There are in essence three broad strategies available to reduce the standby power in household electrical appliances:

Social education. Educating the public on what to look for when purchasing an electronic appliance, as well as encouraging users to unplug devices not in use.
Technological innovation. This involves the implementation of innovative technology to improve the efficiency of power supplies, thereby minimizing the power consumed in relation to the functions being used.
Intelligent device behavior and interaction. This deals with the intelligent activation of low-power modes through user monitoring.

Social education has its practical limits. Even technological innovation has limits in terms of cost and what is possible given the energy required to keep circuit blocks active.

The third strategy is where the author believes room exists for dramatic improvement in terms of intelligently choosing to disable certain functionality based on whether the user is present or not. The following section offers a few suggestions and examples of user detection and the successive enabling/disabling of electronic circuits.

Intelligent low-power modes with capacitive proximity sensing
Traditionally, low-power modes are activated after a certain fixed delay, the time interval being dependent on the device. However, in many appliances this can limit the usefulness of and features presentable to the user. For example, it is highly desirable to display the current status or the time of day on appliances such as ovens, microwaves and DVD players, but it is useful to do so only when the user is in proximity of the appliance.

Intelligent low-power modes with capacitive proximity sensing are a practical solution due to the extremely low power consumption (in the order of 3-10 uA) [3] and the low implementation cost of the sensors available today.

Figure 1 illustrates the capacitive proximity sensing principle in which a low-cost PCB copper pour acts as a sense pad to detect the presence of the user. Detection distances of up to 10 cm are easily obtainable, while up to 30 cm and more can be obtained with special attention to the electrode design.



Figure 1 - Capacitive proximity sensing with a sense pad on an inexpensive PCB.

Detecting the presence of a user enables the design engineer to easily add and implement various features, while at the same time requiring lower standby power, as illustrated in Figure 2.


Click on image to enlarge.

Figure 2 - Intelligent power saving by enabling various functionality only when needed.

The designer has several options available for the manner in which the proximity detection information is used to activate the low-power modes. Capacitive proximity sensing ICs are available with direct outputs as well as I2C-compatible data interfaces [4], which easily allow them to either directly disable parts of the electronic circuit or to be connected to a microcontroller if available. Another is that modern SMPS controllers have standby and disable input pins that can be connected to the capacitive proximity sensing device. Practical implementation methods are illustrated in the section that follows.

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