The viability of an energy harvesting application often depends on components that can efficiently extract very low levels of power at low current and/or low voltage, and deliver these to a storage battery or capacitor. The premise is simple: scavengers of ambient energy rely only on what they are given and what is available, sometimes more, sometimes less.
This self-evident truth places great importance on products such as step-up low voltage boosters, which are self-powered modules that convert a low DC voltage input to a higher AC or DC voltage output suitable for low-power energy harvesting applications using photodiodes, thermoelectric or electromagnetic generators as the input source.
Consider, for example, the Advanced Linear Devices EH4205, designed primarily for driving loads such as the company’s ALD EH300/EH301 Series Energy Harvesting Modules. The AC outputs of the EH4205 are connected directly to the input terminals of the EH300/EH301 modules with a two-wire cable. EH300/EH301 modules can accept energy from many types of electrical energy sources and store this energy to power conventional 3.3 V and 5.0 V electrical circuits and systems. They can also be used for trickle-charge applications such as battery chargers or super-cap chargers, even in situations where the energy input is not well controlled or regulated.
To test the applicability of the modules to specific purposes, several evaluation kits are offered with booster modules paired with a harvesting storage module (Figure 1). One example is the EH4205/EH300KIT kit that includes the EH4205 booster along with the EH300 harvester.
Figure 1: Advanced Linear Devices Inc. EH4205/EH300KIT Evaluation Kit.
The EH4205/EH300KIT booster-harvester demonstration system is at the small end of both capacity and physical size. Reduced dimensions are naturally advantageous, but these do not limit performance, despite the reduced capacity of this harvester module compared with the rest of the company’s product lineup. The EH4205 booster is designed for energy harvesting at minimal voltage and power levels; the module draws input power levels starting at as low as 200 µW, which enables an on-board self-starting oscillator. The module is specified for extracting energy with input voltages down to 80 mV. The booster module provides DC voltage gain of 75, allowing output voltages of up to 6 V.
For energy harvesting, the EH4205 is ideal for low voltage sources such as single photovoltaic cells or thermoelectric generators. The module derives power only from the scavenged energy, so there is no drain from the energy storage component. Start-up occurs once the energy transducer supplies a voltage.
Figure 2: Advanced Linear Devices Inc. EH4205 Voltage Booster Module.
The EH4205 is designed around a transformer to provide the stepped up voltage. The booster circuit start-up is the key to the success of the part for ultra-low power performance. The booster module contains a self-starting oscillator with a natural frequency of 9 kHz. The system depends on the transformer coupled to the Advanced Linear Devices EPAD MOSFET array.
Briefly, the self-oscillating operation of the transformer EPAD-MOSFET system can be described in the following way. The energy source is coupled through a capacitor that filters and integrates the input driving the transformer primary winding. An EPAD MOSFET array is simultaneously turned on. The current in the primary winding induces a corresponding current in the secondary transformer winding. An RC network provides negative feedback to the MOSFET array, turning it off. The circuit then cycles the MOSFET back to the ON state, and the process repeats.
This self-starting oscillator boost circuit can capture very low levels since oscillation can be initiated for input power below 1 µW. The minimum input voltage to initiate oscillation is 120 mV while operation will continue if the input drops as low as 80 mV. This extremely low input voltage specification opens up many new applications for energy harvesting, but it does not limit boost operation to only very low voltages. The design of the boost module allows it to accept a very wide range of inputs as high as 3 V, limited only by the maximum output voltage of 15 V.
The AC output of the EH4205 is intended to drive its companion energy harvesting module. However, the circuit board allows users to add a full-wave rectifier to allow the boost circuit to use scavenged power to charge batteries or supercapacitors.
The preferred configuration for the booster module is to couple it to its energy harvesting sibling. The EH300 energy harvesting module is conveniently sized to fit inside the volume of a standard AA battery. Energy is stored in an on-board capacitor bank. Operation could not be simpler, as the harvesting module is shipped in a fully configured and calibrated condition requiring no user inputs or mode settings.
Figure 3: Advanced Linear Devices Inc. EH300 Harvester Module.
Considering the allowable input voltage range of the harvesting module, it is obvious that the EH300 module is an ideal companion to the boost module. The harvester module is always in an active state, functioning for any non-zero input voltage. The EH300 can accept either AC or DC voltages from 0 V to ± 500 V. Also, input currents from 0.2 to 400 mA are possible. The harvesting module conditions and outputs either 1.8 V or 3.6 V depending on user requirements. The useful energy output is 4.6 mJ while the output on time rating is 68 msat 25 mA. The EH300 energy harvesting module can store a full charge within 40 minutes of receiving an average current of as little as 1 µA.
The Advanced Linear Devices energy harvesting modules are always in the active mode. The EH300 is designed around a storage capacitor rather than a battery. As a result, the most appropriate applications are very low power and infrequent, low duty cycle operations such as condition-based monitoring. The capacitor energy storage makes the EH300 well-suited to systems requiring extreme lifespan operation.
Earlier we mentioned Advanced Linear Products’ EPAD technology. Literally, EPAD stands for electrically programmable analog devices. The EPAD technology allows electrical trimming of device parameters without resorting to conventional techniques like laser trimming during wafer fabrication. EPAD technology allows electrical programming of MOSFET threshold voltage and ON resistance for precise analog operation using CMOS devices.
The capability of the micropower boost harvesting solutions discussed opens up a new range of applications where conventional charging systems were incapable of extracting and storing the scavenged power. Especially where only intermittent operation is required, but over an extended system —lifetime, the performance of the EH4205 boost and EH300 harvesting tandem modules is difficult to beat. For more information use the links provided to product pages on the Hotenda website.