Extending the Life of IoT Devices with Energy–Harvesting Solutions

Extending the Life of IoT Devices with Energy–Harvesting Solutions

Many of the devices destined for the Internet of Things (IoT) aim to fulfill the promise of a more efficient and connected world. Many devices are expected to operate for several years with no updates, no recharging, and no human intervention. There are three ways to address the energy needs of power-hungry IoT devices:

1. Optimize the device for power consumption 
2. Increase the size of the device battery 
3. Continuously charge the device battery to extend longevity

Transforming electromagnetic (EM) radiation into electrical power is a promising solution for extending IoT battery life. Harvesting microwave band radiation (2.45 GHz Wi-Fi signals) provide a new source of energy to address the power issue.

In Europe, Geneva-based STMicroelectronics and French university Ecole Supérieure d’Electronique de l’Ouest (ESEO) collaborated on the development of “energy harvesting” circuits capable of transforming RF energy into DC power. The core of the design includes an antenna and a rectifier working together to capture the low-level energy. The antenna intercepts some of the power to produce an electric current and the rectifier converts the current from AC to DC. However, the low-impedence signals from the antenna are insufficient to drive the high-impedence requirements of the rectifier diode. STMicroelectronics and ESEO needed a design that could create sufficient voltage levels to charge the onboard batteries.

The Challenge: Matching Impedances and Transforming Voltages

The energy-harvesting circuit, antenna, and rectifier design included a Schottky diode, a load resistor, an input-matching circuit, and an output-bypass capacitor. Antennas typically have impedance ratings that are tens of ohms, while the diodes providing rectification have an impedance of several kilo-ohms. A large impedance transformation is required, and the design of the input matching circuit must also deliver on the forward voltage drop requirements of the diode, typically 0.2 to 0.4 volts. All of this must be accomplished in a circuit where the antenna typically gathers less than a microwatt of power, and in a system where impedance of the diode varies with the amplitude of the input voltage.

The Solution: Using Co-simulation to Optimize a Rectifying Circuit

Electronic design automation (EDA) tools help designers solve these challenges by fully optimizing the rectifying circuit. STMicroelectronics and ESEO chose Keysight Advanced Design System (ADS) and its co-simulation capabilities to accurately design and simulate the rectifying circuit for the energy harvesting solution. Co-simulation enables designers to create models of physical parts and add them to a circuit simulation. In this case, the designers used the co-simulation features of ADS to optimize the antenna/rectifier design. The ADS Momentum 3D planar field solver was used to perform passive modeling and analysis of the rectifying circuit. The designers also simulated the circuit in the frequency-domain with the ADS Harmonic Balance circuit solver. Unlike AC analysis in a generic Simulation Program with Integrated Circuit Emphasis (SPICE) tool, Harmonic Balance accounts for the nonlinear characteristics of the diode. For the energy harvester, the developers included diode package models to understand and minimize parasitic effects in the circuit. This enabled STMicroelectronics and ESEO to achieve good correlation between simulated and measured results.