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PRODUCTS & TECHNOLOGY

May 2011

Energy Harvesting Power Conversion ICs Point to Easier, More Efficient Designs

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With the necessary expertise in short supply around the globe, it has been difficult to design effective energy harvesting systems. The primary hurdle has been the power management aspects associated with remote wireless sensing. Now, with the introduction of three dedicated ICs from Linear Technology, that is about to change. These devices can extract energy from almost any source of light, heat or mechanical vibration. Furthermore, with their comprehensive feature set and ease of design, they greatly simplify the hard-to-do power conversion design aspects of an energy harvesting chain. 

The LTC3109 is a highly integrated DC/DC converter and power manager. It can harvest and manage surplus energy from extremely low input voltage sources such as thermoelectric generators (TEG), thermopiles and even small solar cells. Its auto-polarity topology allows it to operate from input sources as low as 30 mV, regardless of polarity. 

The circuit in Figure 1 uses two compact step-up transformers to boost the input voltage source to the LTC3109, which then provides a power management solution for wireless sensing and data acquisition. It can harvest small temperature differences and generate system power instead of using traditional battery power. 

Figure 1
Figure 1: LTC3109 Typical application schematic

The LTC3109 converts the low voltage source and manages the energy between multiple outputs. The AC voltage produced on the secondary winding of each transformer is boosted and rectified using an external charge pump capacitor and the rectifiers internal to the LTC3109. This rectifier circuit feeds current into the VAUX pin, providing charge to the external VAUX capacitor and then the other outputs.

The internal 2.2V low-dropout regulator (LDO) can support a low power processor or other low power ICs. The LDO is powered by the higher value of either VAUX or VOUT. This enables it to become active as soon as VAUX has charged to 2.3V, while the VOUT storage capacitor is still charging. In the event of a step load on the LDO output, current can come from the main VOUT capacitor if VAUX drops below VOUT. The LDO is capable of providing 3 mA of output current.

The VSTORE capacitor may be a very large value (thousands of microfarads or even Farads), to provide holdup at times when the input power may be lost. Once Power-up has been completed, the Main, Backup and switched outputs are all available. If the input power fails, operation can still continue by operating off the VSTORE capacitor.

The LTC3588-1 is a complete energy harvesting solution optimized for low energy sources, including piezoelectric transducers. Piezoelectric devices produce energy by either compression or by deflection of the device. These piezoelectric elements can produce hundreds of uW/cm2 depending on their size and construction (Figure 2).

Figure 2
Figure 2: LTC3588-1 Typical application schematic

It should be noted that the piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of an electric potential when stress is applied) also exhibit the reverse piezoelectric effect (the production of stress and/or strain i.e., deflection when a voltage is applied).

The LTC3588-1 operates from an input voltage range of 2.7V to 20V, making it suitable for a wide array of piezoelectric transducers, as well other high output impedance energy sources. Its high efficiency buck DC/DC converter delivers up to 100 mA of continuous output current or even higher pulsed loads. Its output can be programmed to one of four (1.8V, 2.5V, 3.3V or 3.6V) fixed voltages to power a wireless transmitter or sensor. Quiescent current is 950 nA with the output in regulation (at no load), maximizing overall efficiency. 

The LTC3588-1 is designed to interface directly with a piezoelectric or alternative high impedance AC power source, rectify a voltage waveform and store harvested energy in an external storage capacitor while dissipating any excess power via an internal shunt regulator. An ultra-low quiescent current (450 nA) undervoltage lockout (ULVO) mode with a 1-1.4V hysteresis window enables charge to accumulate on the storage capacitor until the buck converter can efficiently transfer a portion of the stored charge to the output. 

The third device, the LTC3105, is an ultra-low-voltage step-up converter and LDO specifically designed to simplify the task of harvesting and managing energy from low voltage, high impedance alternative power sources such as photovoltaic cells, TEGs and fuel cells. Its synchronous step-up design starts up from input voltages as low as 250 mV, making it suitable for harvesting energy from even the smallest photovoltaic cells in less than ideal lighting conditions. Its wide input voltage range of 0.2V to 5V makes it attractive for a wide array of applications. An integrated maximum power point controller (MPPC) enables the LTC3105 to extract the maximum available power that the source is capable of providing. Without MPPC, the power converter can only produce a fraction of the source’s theoretical maximum capability. Peak current limits are automatically adjusted to maximize power conversion efficiency while Burst Mode operation reduces quiescent current to only 22 uA, minimizing the drain from the energy storage element. The ultra-low-Iq LDO is capable of directly powering popular low-power microcontrollers or sensor circuitry.

The circuit shown in Figure 3 uses the LTC3105 to charge a single-cell Li-Ion battery from a single photovoltaic cell. This circuit enables the battery to continually charge when the solar source is available and in turn the battery can power the application from the stored energy when the solar power is no longer available.

Figure 3
Figure 3: Single Photovoltaic Cell Li-Ion Trickle Charger

The LTC3105 provides the capability to start with voltages as low as 250mV. During start up the AUX output initially is charged with the synchronous rectifiers disabled. Once VAUX has reached approximately 1.4V, the converter leaves start-up mode and enters normal operation. Maximum power point control is not enabled during start up; however, the currents are internally limited to sufficiently low levels to allow start up from weak input sources. While the converter is in start-up mode, the internal switch between AUX and VOUT remains disabled and the LDO is disabled.

When either VIN or VAUX is greater than 1.4V, the converter will enter normal operation. The converter continues charging the AUX output until the LDO output enters regulation. Once the LDO output is in regulation, the converter begins charging the VOUT pin. VAUX is maintained at a level sufficient to ensure the LDO remains in regulation. If VAUX becomes higher than required to maintain LDO regulation, charge is transferred from the AUX output to the VOUT output. If VAUX falls too low, current is redirected to the AUX output instead of being used to charge the VOUT output. Once VOUT rises above VAUX, an internal switch is enabled to connect the two outputs together.

If VIN is greater than the voltage on the driven output (VOUT or VAUX), or the driven output is less than 1.2V, the synchronous rectifiers are disabled and operate in critical conduction mode enabling regulation even when VIN>VOUT

When the output voltage is greater than the input voltage and greater than 1.2V, the synchronous rectifier is enabled. In this mode, the N-channel MOSFET between SW and GND is enabled until the inductor current reaches the peak current limit. Once current limit is reached, the N-channel MOSFET turns off and the P-channel MOSFET between SW and the driven output is enabled. This switch remains on until the inductor current drops below the valley current limit and the cycle is repeated. When VOUT reaches the regulation point, the N- and P-channel MOSFETs connected to the SW pin are disabled and the converter enters sleep.

In order to power microcontrollers and external sensors an integrated LDO provides a regulated 6 mA rail. The LDO is powered from the AUX output allowing the LDO to attain regulation while the main output is still charging. The LDO output voltage can be either a fixed 2.2V or adjusted via resistor divider. 

The integrated maximum power point control circuit allows the user to set the optimal input voltage operating point for a given power source. The MPPC circuit dynamically regulates the average inductor current to prevent the input voltage from dropping below the MPPC threshold. When VIN is greater than the MPPC voltage, the inductor current is increased until VIN is pulled down to the MPPC set point. If VIN is less than the MPPC voltage, the inductor current is reduced until VIN rises to the MPPC set point.

In applications such as photovoltaic conversion, the input power source may be absent for long periods of time. To prevent discharge of the outputs in such cases, the LTC3105 incorporates an undervoltage lockout (UVLO) ,which forces the converter into shutdown mode if the input voltage falls below 90 mV (typical). In shutdown, the switch connecting AUX and VOUT is enabled and the LDO is placed into reverse-blocking mode, and the current into VOUT is reduced to 4 μA typical. Reverse current through the LDO is limited to 1 μA in shutdown to minimize discharging of the output. 

LiPPERT Embedded Computers
Mannheim, Germany. 
+49 621 4 32 14-0. 
[www.lippertembedded.com]