Energy harvesting demands full-spectrum microcontroller efficiency
Energy harvesting provides the key to a new generation of devices that allow intelligent sensors to be deployed in a far wider range of situations than previously possible. Such sensors allow continuous condition monitoring in applications as disparate as industrial motors and body-worn sensors for long-term physical health measurement.
Although these systems could use battery power to avoid the need to connect the sensors to mains power, batteries will need to be replaced or recharged during their lifetime. Once placed around a large motor or turbine, for example, it can be difficult to gain access to replace them. The advantage of many of these applications however is that they can be harnessed for energy themselves.
The vibration of an industrial motors can be used, with an appropriate seismic mass and converter, to generate energy for the system that monitors it. Similarly, for body-worn sensors, vibration and thermal energy capture can trickle charge into a capacitor that can then be used to power a sensor. (fig 1)
Figure 1: Power density of Energy Harvesting Methods. (Source: The Journal of Technology Studies)
Although these systems provide mechanisms for capturing energy, they rarely generate the levels of power that designers are accustomed to working with on battery-fed systems. Therefore, it is vital to have a system engineered to consume the minimum power wherever possible.
A key target for reducing power in the logic circuitry is that of the operating voltage. In CMOS circuitry, there is a quadratic relationship between voltage and power consumption, as shown in the equation P = CV2f, where C is the circuit capacitance, f is the switching frequency and V is the voltage applied. Clearly, reducing the voltage offers the greatest potential for reducing power consumption. Near- and sub-threshold operation of transistors offers a unique approach to reducing the operating voltage in microcontrollers and other logic circuits to levels that are well below those required by standard logic.
The principle behind near- and sub-threshold operation is that the threshold voltage at which the device would normally be considered to be switched “on” need not be treated as a required target for logic and analogue circuits. Logic transistors have traditionally been designed to pass high levels of current when saturated in order to charge the capacitive paths that follow each gate. However, it is possible to charge these circuit paths without switching the transistor into full saturation and instead allow current to trickle through more slowly. This has the consequence of causing logic to switch state more slowly but in typical sensor applications, there is no need to switch at the highest possible speed.
However, as threshold voltages are driven lower, there is also an exponential increase in the transistor leakage currents (fig 2)
Figure 2. (Source: Ambiq)
As the voltage falls even further into the deep sub-threshold realm, the proportion of energy lost through leakage will dominate, which places a secondary limit, on top of performance considerations, as to how far the supply voltage can be lowered. (fig 3)
Figure 3. (Source: Ambiq)
A critical issue for designers of sub-threshold circuitry is process variation and its effects as the supply voltage approaches that of the threshold. Key to the effective design of sub-threshold circuitry are mechanisms to reduce the effects of this variation, such as adaptive circuitry that is specifically designed to overcome this variability. Development over many years at the University of Michigan and Ambiq Micro has led to many sub-threshold innovations such as this. The entire design flow must also be reengineered in order to effectively take advantage of this technology - from the standard cell libraries used to implement sub-threshold logic circuits all the way through to test strategies needed to test nanoamps and picoamps of current. Only with this level of investment is it possible to maximise the power-saving benefits of sub-threshold design.