Energy harvesting design explored

Everyone’s talking about energy harvesting – but what exactly is it? The term is often used as a paraphrase for the whole range of low-power applications, yet the “harvesters” themselves are only one part of a larger system, writes Ingo Seehagen, a field application engineer at Avnet Memec.
Energy harvester circuits are used in electronic systems that need to be self-sufficient in energy – in other words, those that are not powered by mains electricity or batteries. They can also be used to extend battery life.

In all these systems, the collection, transformation, storage and use of energy need to be coordinated with one another. They capture and transform energy from different sources, including thermal, kinetic and optical. One of the issues here is that the energy that results from these transformation processes is often not sufficient to power electronic systems in a reliable way. This is where integrated circuits come in.

These manage all power management functions like capture and storage of the charge, protection against undervoltage, and provision of energy for the system – more or less autonomously.  Once they have gone to the trouble of capturing energy, the device clearly shouldn’t waste it. This is why it’s a good idea to use elements that have the lowest possible energy consumption. What do these circuits need to do when they’re in use?

Their objective is to transform, store and make available the energy surrounding the system as effectively as possible so that it can be powered without needing any other energy sources.

Autonomously-powered systems are coming into their own with the widespread and growing availability of electronic components that have very low energy requirements, and the energy harvesting trend is another factor contributing to the rapid growth of these technologies. Taking the example of a wireless sensor, the point is not only to save money implementing it but also reduce its operating cost throughout its useful life.  

An energy harvesting evaluation platform developed by Avnet Memec in collaboration with Maxim Integrated Products and Energy Micro is intended to enable developers to learn about energy harvesting and start using these energy-independent systems. The first version is based on the existing evaluation platforms for Maxim Integrated Products’ MAX17710 energy harvester and for the EFM32TG840F32 energy-saving microcontroller from Energy Micro’s Tiny Gecko range.

The MAX17710 incorporates all the power management functions needed for energy harvesting applications. These include functions for charging and protecting the THINERGY Micro-Energy Cell (MEC) from Infinite Power Solutions (IPS), which is integrated in the evaluation kit.

The IC can manage poorly regulated sources such as energy-harvesting devices with output levels ranging from 1µW to 100mW. The device also includes a boost regulator circuit for charging the cell from a source as low as 0.75V. The evaluation board is powered by a solar cell and an internal regulator protects the cell from overcharging. Output voltages supplied to the target applications are regulated using an efficient adjustable low-dropout (LDO) linear regulator with selectable voltages of 3.3V, 2.3V, or 1.8V.

It also includes protection against undervoltage and output buffer management for the lithium cell. The IC’s lithium charger only requires 1nA standby power (IQBATT).

The MAX 17710 does not require any complex external wiring and is delivered in a 3mm x 3mm x 0.5mm 12-pin UTDFN package.  It charges the cell via an external energy source on the CHG pin.

Whenever the voltage on the CHG pin is greater than the battery voltage (BATT), the cell is charged with no intervention. If CHG exceeds the VCE threshold, the internal regulator limits the voltage to 4.125V to protect the cell from overcharging.  Also at this time, any undervoltage lockout (UVLO) is reset, allow¬ing the LDO to power the application load and the UVLO is reactivated as soon as the charge is removed.

Whenever the external harvest source drives the CHG pin above 5.3V, an internal shunt regulator protects the CHG pin. This shunt can sustain currents of up to 50mA. If there is any chance of the harvest source exceeding this power level, an external protection circuit should be added to prevent damage to the MAX17710. 

A simple internal boost regulator also enables the use of low-voltage harvest sources, such as solar cells or thermo-electronic generators (TEG). The boost regulator can work with power from as low as 1µW (in pulsed harvest mode) and up to 100mW (continuous conversion). As a result, the IC can provide over 20mA (80mW) if it has a suitable harvest source (0.8V) and a lithium cell of 4.1V. 

An important component of the evaluation board is the THINERGY Micro-Energy Cell (MEC) from Infinite Power Solutions (IPS). These MECs are a completely new type of battery – a solid-state battery that lasts for a system’s entire lifecycle, has many advantages compared to conventional small batteries and supercapacitors, and actually combines the benefits of both.

This makes MECs ideal for use in energy harvesting systems, as they can handle high current levels, have up to 50 times more energy density and 4000 times less leakage than a supercap, and can be recharged 100 times more often than conventional rechargeable batteries. In addition, they are extremely compact with a thickness of just 0.17 mm and are flexible too, making them easy to use. These features also allow them to be used in applications that cannot use ordinary batteries.

The MCU used in the energy harvesting evaluation platform is from Energy Micro’s Tiny Gecko range. Based on a highly efficient ARM Cortex-M3 core, Gecko microcontrollers are currently the best devices available on the market from the energy consumption point of view.

They achieve 150µA/MHz in active mode and 900nA in deep-sleep mode (EM2), in which the real-time clock, brownout detection, power-on reset and RAM and CPU retention remain active. 

The evaluation platform uses the Tiny Gecko Starter Kit (EFM32-G8XX-STK) which includes an EFM32TG840F32 microcontroller. The device’s extremely low power consumption derives from a combination of the following ten factors.
1. Very low active power consumption in active mode:
210 µA/MHz at 3V at 1MHz
150 µA/MHz at 3V at 25 MHz
150 µA/MHz at 3V at 32 MHz
2. Reduced processing time due to the performance of the Cortex-M3 at  1.25 DMIPS/MHz
3. Very fast wake-up time from the sleep modes – only 2µs
4. Ultra-low standby current at 20 nA shutoff at 3V / 900 nA deep sleep (POR, BOD, RTC, RAM and CPU retained)
5. Autonomous peripherals work with sleeping CPU; comprehensive DMA support
6. Peripheral Reflex System – direct, configurable connections between the peripherals
7. Well-architected energy modes
EM0 “Run Mode”   : 150µA/MHz
EM1 “Sleep Mode”  : 45µA/MHz
EM2 “Deep Sleep Mode” : 900 nA (RTC, BOD, POR, RAM & CPU)
EM3 “Stop Mode”  : 600 nA (BOD, RAM & CPU)
EM4 “Shutoff Mode”  : 20 nA  (Pin / GPIO reset)
8. Ultra energy efficient peripherals
Analogue digital converter 12-bit at 1 MSamples/s  : 350 µA
    6-bit at 1 kSamples/s  : 500 nA
Low-power UART    150 nA at 9600 baud/s
LCD Controller   4 x 40    : 550 nA
9. Low Energy Sensor Interface
Autonomous monitoring in EM2, up to 16 sensors simultaneously, configurable
10. Advanced Energy Monitoring with a debugging tool for the real-time analysis of power consumption, dependent on object code.
Simplicity Studio is used as the IDE in the kit. From the interface, users have access to all updates and news about Energy Micro, as well as to the latest documentation for the MCUs, software and kits. They also have a direct link to the integrated “energy aware tools”. 

These enable developers to identify and eliminate any energy-related weak points in their software during the development phase. Compilers from IAR, KEIL and ROWLEY and an Eclipse-based free toolchain can be integrated if required.

As with all the evaluation boards from Energy Micro, the EFM32-G8XX-STK includes a fully-functional USB J-Link debugger. That enables developers to debug software cost-effectively on Energy Micro boards and use the debug-out interface later for their Energy Micro based hardware.

The Energy Micro ecosystem also includes debug adapters, IDE and compilers as well as middleware and programmers.

Energy Micro will also soon be launching the first of its low power radios that will initially support three system implementations:

1. Transceiver + MCU
2. Network Processor + MCU
3. System-on-Chip (SoC).

These support frequencies from 167MHz to 2.5GHz, enabling them to be used anywhere in the world. Third-party and Energy Micro protocol stacks are also available and include Wireless M-Bus, Bluetooth LE, KNX, and io-Homecontrol as well as ZigBee and RF4CE based on 802.15.4(g).