How capacitive sensing can reduce standby power in household appliances
Practical implementation methods using popular SMPS controllers
Secondary side display and LED lighting enable/disable
LCD displays and LED lighting are always active on household appliances when the device is plugged in, often unnecessarily so. When the user’s presence is not detected it is advantageous to disable these energy consumers. Figure 3 illustrates this concept, where a typical LCD display is enabled/disabled by the microcontroller, based on data received from the capacitive proximity sensor. A microcontroller is used in conjunction with an IQS232 single-channel capacitive touch and proximity 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.
The LED backlighting and find-in-the-dark lighting can be disabled: the power savings possible can range from a single 5 mW LED to several LEDs totaling 100 mW or more.
If an interrupt input pin of the microcontroller is connected to the direct output pin of the IQS232, the microcontroller can be halted during periods of user absence, 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 an optimistic efficiency of 70% (note that typically this is not at full load, when the supplies are most efficient), it is clear that the power consumption will far exceed the typical requirement of 300 mW stipulated by most regulatory agencies (Energy Star V2.0).
Even with a 100% efficient power supply, the need to intelligently disable functionality without detrimentally affecting the functionality of the appliance is critical.
Disable the SMPS controller
Many modern SMPS controllers have disable or power-saving logic input pins that, when activated, lower the standby power to virtually zero. This is often a better alternative to disabling the supply as a whole when either limited intelligence exists on the secondary side or when a cost-effective supply with poor efficiency is used. In this example, the TopSwitch constant voltage SMPS controller from Power Integrations is used together with the IQS232 single-channel capacitive touch and proximity sensor from Azoteq.
Click on image to enlarge.
Figure 4 - Illustrating standby power saving by disabling the SMPS controller IC.
Standby power saving includes:
The potential standby power saving of the SMPS alone (with no secondary side power consumption), which can range from 100 mW to over 1 W, depending on the SMPS controller used. With the TOP265EG used in this example, the standby power saving averages 180 mW at 110 V and 100 mW at 220 V.
The IQS232 being powered through a simple resistive network from the primary bus voltage. Considering the power consumption in one of its low-power states, the estimated power consumption becomes 3.1 mW at 110 V and 6.2 mW at 220 V.
Even if an inefficient SMPS is used - or in the case of constant current SMPS supplies where the standby current is necessarily high - extremely low standby power figures at low cost are possible by intelligently detecting the user’s presence with a capacitive proximity sensor.
Conclusion
Governments and regulatory agencies worldwide are targeting standby power consumption as a substantial and unnecessary power loss. A considerable amount of pressure is being placed on electronic design engineers of household appliances to meet the requirements of regulatory agencies and governments, while at the same time keeping production costs to a minimum.
A considerable number of options for improving standby power consumption become available when the appliance is enabled to sense when the user is within a certain proximity of the appliance, as has been illustrated. Additional benefits of capacitive sensing include visual indication of the subsequent expected user interaction when the device is being approached, as well as informative display options at appropriate times when the user is present.
In a follow-up Product How To article we will show how an SMPS controller IC has been integrated into Azotec's latest range of capacitive proximity and touch sensing devices in order to provide significant power and cost savings.
Gerrit de Villiers completed his Bachelors in Electronic Engineering at the University of Pretoria and continued his studies with a Master’s degree in Engineering, specializing in optical sensors for nuclear environments. He then managed several turnkey products for Keystone, with a focus on consumer products. In 2011 he joined Azoteq as an application engineer and now develops intelligent SMPS supplies incorporating capacitive touch 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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