Like death and taxes, changing batteries in remote sensor nodes and wireless IoT devices is often inevitable. Huge effort has been expended on reducing power consumption for battery operated devices, but the day always comes when the battery goes dead. Taking care of this can be as simple as popping open a battery cover and swapping batteries, or as dreadful as driving or hiking to countless remote locations to do the same. Furthermore, batteries are expensive and wasteful. Energy harvesting promises to reduce or eliminate this problem by using among other things light, heat and motion to power electronics.
Huge gains in rechargeable battery life and capacity have enabled enormous progress for wireless IoT devices, but to eliminate the need for wired recharge or battery replacement other energy sources need to be harnessed. The most common sources for harvesting energy are solar, thermal and vibration. Because each of these sources are not consistent and deliver varying voltage and current profiles, the energy from them needs to be stored in a reservoir such as a capacitor or battery. To capture all the energy possible there is a need to up-convert low voltages to that the power is useful. Lithium Polymer (LiPo) batteries need to be charged up to 4.2V and require specialized charging profiles to avoid overcharging or reduced battery life.
IC’s that run off of 5V and charge LiPo batteries have been around for quite a while, but efficient energy harvesting demands the ability to operate with inputs well below 1V. Cypress offers several chips for converting power from solar, vibration and thermal sources. Each of these chips is designed for somewhat different applications.
The MB39C831 works for charging LiPo batteries with its built in charge controller. It will also charge up super capacitors which can replace batteries in many low power devices. It can drive circuit loads with lower voltage inputs, such as when 5V is needed from a 3.7V nominal LiPo battery, or even lower input voltages. The important feature of this chip for working off of solar is what is called Maximum Power Point Tracking (MPPT). This ensures the load that the MB39C831 presents to a solar panel optimizes power transfer. Solar panels provide their highest voltage under no load. If a high current load, such as charging a battery, is applied, the current will spike but the voltage coming from the panel will collapse. This can cause large fluctuations and inefficient power transfer. MPPT seeks the optimal I/V point for the connected panel.
The MB39C811 is suited for driving circuits rather than charging batteries. Despite the absense of charge control circuitry, it is still useful because it can handle power sources that deliver AC currents. It has a low-loss rectifier bridge to handle AC sources like piezoelectric generators. The other advantage this chip has for piezoelectric is a high input voltage rating – up to 24V, with over voltage protection that keep the unit operating and allows 100mA operation.
Both of these chips have extremely low quiescent current (1.5uA and 41uA). It would not do to have the voltage converter draw down what limited power should be available for the application circuit. The MB39C831 can start operating at voltages as low as 0.35V, which helps squeeze every bit of power out of a solar panel on a cloudy day. Remote sensor nodes must have the ability to tolerate periodic lack of sun if necessary.
Spansion (merged wtih Cypress) posted an interesting summary of the needs for energy harvesting on their web site. It also discusses several evaluation boards they offer to help with system prototyping. These sensor node boards come with the above PMIC’s and an ARM Cortex M3 with a FM3 MCU. There is an RF module that is included to provide communication. The board also comes with an LCD display, temperature, light sensor and I2C for attaching other peripherals.Share this post via: