Power Electronics

DC-DC Boost Converter Harvests Photovoltaic Energy

Energy harvesting with a photovoltaic cell requires an efficient dc-dc converter that operates with very low voltage inputs and whose output can charge an external battery. The newly introduced LTC3105 meets these requirements.

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ENERGY HARVESTING CAN EMPLOY SURPLUS OR AMBIENT energy to trickle charge a battery that supplies system power. This eliminates the need to for an ac power line-based recharging circuit, which may be impractical in a remote application. Typically, these applications require very low average power, but can require periodic pulses of higher load current. The Linear Technology LTC3105 is intended for these types of energy harvesting applications.

The LTC3105 is a high efficiency dc-dc boost converter (Fig. 1) that has a 250mVstart-up capability. A resistor divider connected between the VOUT and FB pins adjusts the converter output from 1.5V to 5.25V. An integrated maximum power point controller (MPPC) optimizes performance from photovoltaic (PV) cell inputs. The LT3105's 3mm × 3mm DFN package (or MSOP-12) and very small external components offer a compact solution for energy harvesting.

Internal 400 mA synchronous switches enhance the LTC3105's efficiency and its Burst Mode® operation results in 22µA quiescent current. Burst Mode, with proprietary self-adjusting peak current, optimizes converter efficiency and output voltage ripple over all operating conditions.

While the main output is charging the external battery, an auxiliary LDO delivers up to 6 mA for sensors and microcontrollers. You can adjust the LDO with a voltage divider consisting of a resistor from the LDO pin to FBLDO then another resistor to GND. As an alternative, you can connect the FBLDO pin to GND to deliver a nominal 2.2V output for the LDO. An optional 22pF feed-forward capacitor from VOUT to FB (paralleling the 1020 kΩ resistor in Fig. 1) reduces output ripple and improves load transient response.

This IC includes over-temperature protection during momentary overload conditions. Junction temperature can exceed 125°C when over-temperature protection is active. However, continuous operation above the specified maximum operating junction temperature may impair reliability.

The LTC3105 provides an output disconnect that prevents large inrush currents during start-up. This aids PV cells that can become overloaded if inrush current is not limited during power converter start-up. In addition, output disconnect isolates VOUT from VIN while in shutdown, resulting in 4µA quiescent current.

SHUTDOWN CONTROL

The pin is an active low input that places the IC into low current shutdown mode. This pin incorporates an internal 2MΩ pull-up resistor, which enables the converter when not controlled by an external circuit. Thresholds for the pin change dynamically with the maximum of VIN and VAUX. At low system voltages (VIN and VAUX), the shutdown pin thresholds are as low as 50mV from the voltage rails. In shutdown, the IC enables the internal switch connecting AUX and VOUT.

During start-up, the AUX output initially is charged with the internal synchronous rectifier disabled. After releasing, the enabled LTC3105 begins switching after a short delay. Once VAUX reaches approximately1.4V, 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. Fig. 2 shows a typical start-up sequence.

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When the converter is in the shutdown mode (due to undervoltage lockout or the enabled pin) the LDO goes into the reverse-blocking mode with reverse current limited to under 1µA. After ending the shutdown event, the LDO remains in reverse-blocking mode until VAUX rises above the LDO voltage. 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 redirects to the AUX output, instead of being used to charge the VOUT output. Once VOUT rises above VAUX, the IC enables an internal switch to connect these 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 (typical), its synchronous rectifiers are disabled and the converter operates in critical conduction mode. In this mode, the IC enables an N-channel MOSFET between SW and GND and remains on until the inductor current reaches the peak current limit. Then, it is disabled and the inductor current discharges completely before the cycle is repeated. When the output voltage is greater than the input voltage and greater than 1.2V, the IC enables the synchronous rectifier.

Upon reaching its current limit, an N-channel MOSFET turns off and the IC enables a P-channel MOSFET between SW and the driven output. This switch remains on until 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.

MPPC OPERATION

An integrated maximum power point controller (MPPC) allows direct operation from high impedance PV cells. Programmed by an external resistor (40.2k in Fig. 1), MMPC prevents the PV cell input voltage from collapsing below its programmed threshold. Peak current limits are automatically adjusted with proprietary techniques to maintain operation at levels that maximize power extraction from the PV cell.

The maximum power point control circuit allows the user to set the optimal input voltage operating point for the PV cell. 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, inductor current increases 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. For more information, see the sidebar “Maximum Power Point Control For A PV Cell.”

In PV cell applications, a diode can be used to set the MPPC threshold so that it tracks the cell voltage over temperature. The diode should be thermally coupled to the PV cell to ensure proper tracking. If the selected diode forward voltage is too low, a resistor in series with the diode can adjust the dc set point to better match the maximum power point of the PV cell. If the diode is located far from the converter inputs, a capacitor may be required to filter noise that may couple onto the MPPC pin. This method can be extended to stacked cell sources through use of multiple series connected diodes.

The LTC3105 maximizes efficiency at light load while providing increased power capability at heavy load by adjusting the peak and valley of the inductor current as a function of the load. Lowering peak inductor current to 100mA at light load optimizes efficiency by reducing conduction losses. As the load increases, the peak inductor current automatically increases to a maximum of 400mA. At intermediate loads, the peak inductor current can vary between 100mA and 400mA. This function is overridden by the MPPC and only occurs when the PV cell delivers more power than the load requires.

A power good output indicates that VOUT is in regulation. PGOOD is an open-drain output, and is disabled in shutdown and undervoltage lockout. PGOOD indicates that power is good at the beginning of the first sleep event after the output voltage has risen above 90% of its regulation value. PGOOD remains asserted until VOUT drops below 90% of its regulation value at which point PGOOD goes low.

PV cell outputs may be absent for long periods of time. To minimize output capacitor discharge in such cases, the LTC3105 incorporates undervoltage lockout (UVLO) that forces the converter into the shutdown mode if the input voltage falls below 90mV (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 4µA (typical). To minimize discharging of the output, the IC limits reverse current through the LDO to 1µA in shutdown.

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COMPONENT SELECTION

Low DCR (dc resistance) power inductors with values between 4.7µH and 30µH and sufficient saturation current rating are suitable for use with the LTC3105 (10 µH in Fig. 1). In applications where the input voltage is very low, a larger value inductor can provide higher efficiency and a lower start-up voltage. In applications where the input voltage is relatively high (VIN > 0.8V), smaller inductors may be used to provide a smaller overall footprint. If the inductor dc resistance is too high, it will reduce efficiency and increase the minimum operating voltage.

Input capacitor selection is important in low voltage, high source resistance PV cell systems. For general applications, a 10µF ceramic capacitor is recommended between VIN and GND. For high impedance PV cells, the input capacitor should be large enough to allow the converter to complete start-up mode using the energy stored in the input capacitor. When using bulk input capacitors that have high ESR, a small valued parallel ceramic capacitor should be placed between VIN and GND as close to the converter pins as possible.

A 1µF ceramic capacitor should be connected between AUX and GND. Larger capacitors should be avoided to minimize start-uptime. A low ESR output capacitor, 10µF or larger, should be connected between VOUT and GND. The main output charges energy storage devices, such as the li-ion battery in Fig. 1. When using output bulk storage devices with high ESR, a low value ceramic capacitor should be placed in parallel and located as close as possible to the converter pins.

When properly implemented, the LTC3105 can help designers develop circuits that efficiently harness photovoltaic energy in varying climatic conditions (see the Table “Energy Available At Various Light Conditions Relative to Full Sun”).

MAXIMUM POWER POINT CONTROL FOR A PV CELL

PV cells have a single operating point where the values of its current and voltage provide a maximum power output. As shown in Fig. 3, a PV cell has an exponential relationship between current and voltage, and the maximum power point (MPP) occurs at the knee of the curve, where the resistance is equal to the negative of the differential resistance (V/I = -dV/dI). The voltage at which PV cell produces maximum power depends on solar radiation, ambient temperature and PV cell temperature. Typical PV cells produce power with maximum power voltage of around 17 V when measured at a cell temperature of 25°C, it can drop to around 15 V on a very hot day and it can also rise to 18 V on a very cold day.

Maximum power point controllers utilize an algorithm to search for the MPP where a dc-dc converter extracts the maximum power available from a PV cell. The MPPC corrects variations in the PV cellís current-voltage characteristics.

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