How capacitive sensing can reduce standby power in household appliances

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.



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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 .


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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.


Practical implementation methods using popular SMPS controllers

Secondary side display and LED lighting enable/disable
LCDdisplays and LED lighting are always active on household applianceswhen the device is plugged in, often unnecessarily so. When the user’spresence is not detected it is advantageous to disable these energyconsumers. Figure 3 illustrates this concept, where a typical LCDdisplay is enabled/disabled by the microcontroller, based on datareceived from the capacitive proximity sensor. A microcontroller is usedin conjunction with an IQS232 single-channel capacitive touch andproximity sensor from Azoteq.

Figure 3 – Secondary side display and LED lighting enable/disable with capacitive proximity sensing.

Standby power saving includes:

The LCD can be switched off completely: up to 500 mW power savings if a small LCD color display is used.
TheLED backlighting and find-in-the-dark lighting can be disabled: thepower savings possible can range from a single 5 mW LED to several LEDstotaling 100 mW or more.

If an interrupt input pinof the microcontroller is connected to the direct output pin of theIQS232, the microcontroller can be halted during periods of userabsence, adding another potential 50 mW power saving.

The power consumption of the IQS232 is negligible, averaging 4 uA in its lowest power states.

A total standby power saving of well over 600 mW is possible.Considering that the typical SMPS supply powers the devices with anoptimistic efficiency of 70% (note that typically this is not at fullload, when the supplies are most efficient), it is clear that the powerconsumption will far exceed the typical requirement of 300 mW stipulatedby most regulatory agencies (Energy Star V2.0).

Even with a 100%efficient power supply, the need to intelligently disable functionalitywithout detrimentally affecting the functionality of the appliance iscritical.

Disable the SMPS controller
Many modern SMPScontrollers have disable or power-saving logic input pins that, whenactivated, lower the standby power to virtually zero. This is often abetter alternative to disabling the supply as a whole when eitherlimited intelligence exists on the secondary side or when acost-effective supply with poor efficiency is used. In this example, theTopSwitch constant voltage SMPS controller from Power Integrations isused together with the IQS232 single-channel capacitive touch andproximity sensor from Azoteq.



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Figure 4 – Illustrating standby power saving by disabling the SMPS controller IC.

Standby power saving includes:

Thepotential standby power saving of the SMPS alone (with no secondaryside power consumption), which can range from 100 mW to over 1 W,depending on the SMPS controller used. With the TOP265EG used in thisexample, the standby power saving averages 180 mW at 110 V and 100 mW at220 V.

The IQS232 being powered through a simple resistivenetwork from the primary bus voltage. Considering the power consumptionin one of its low-power states, the estimated power consumption becomes3.1 mW at 110 V and 6.2 mW at 220 V.

Even if aninefficient SMPS is used – or in the case of constant current SMPSsupplies where the standby current is necessarily high – extremely lowstandby power figures at low cost are possible by intelligentlydetecting the user’s presence with a capacitive proximity sensor.

Conclusion
Governmentsand regulatory agencies worldwide are targeting standby powerconsumption as a substantial and unnecessary power loss. A considerableamount of pressure is being placed on electronic design engineers ofhousehold appliances to meet the requirements of regulatory agencies andgovernments, while at the same time keeping production costs to aminimum.

A considerable number of options for improving standbypower consumption become available when the appliance is enabled tosense when the user is within a certain proximity of the appliance, ashas been illustrated. Additional benefits of capacitive sensing includevisual indication of the subsequent expected user interaction when thedevice is being approached, as well as informative display options atappropriate times when the user is present.

In a follow-upProduct How To article we will show how an SMPS controller IC has beenintegrated into Azotec's latest range of capacitive proximity and touchsensing devices in order to provide significant power and cost savings.

Gerrit de Villiers completed his Bachelors in Electronic Engineering at the University ofPretoria and continued his studies with a Master’s degree inEngineering, specializing in optical sensors for nuclear environments.He then managed several turnkey products for Keystone, with a focus onconsumer products. In 2011 he joined Azoteq as an application engineerand now develops intelligent SMPS supplies incorporating capacitivetouch sensing for the consumer market.

References
[1] Meier, A., “Standby: where are we now?” Proceedings of the ECEEE 2005 Summer Study, Mandelieu, pp. 847-854
[2] A report for the Equipment Energy Efficiency (E3), Standby Power – Current status, October 2006, Report number 2006/10
[3] IQS232 Datasheet
[4] www.azoteq.com

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