Streamlined solar cell-based battery charging
25 March 2010
Solar power is a “hot” topic. Applications requiring large panels or arrays of panels have occurred in residential and commercial buildings. Now however, as Steve Knoth reports, technology is emerging for solar panels to have a role in power generation in off grid applications.
The goal of a solar powered charging system is not only to provide direct power to the system in daylight, but also to charge the storage element (typically a battery) during peak sunlight hours so that the battery may provide power to the system during night-time or partial sunlight periods, when the panel output power approaches zero. The majority of these applications traditionally use sealed lead acid (SLA) batteries as their primary storage element, but Lithium chemistries are becoming more common as application sizes decrease. Solar charging of these batteries is becoming more mainstream in portable and non-portable environments. Among the many emerging applications for single solar panels are ruggedised military laptops, industrial inventory and point-of-sale (POS) management devices, remote sensing units, portable automotive diagnostic equipment, marine solar buoys, road sign illumination, emergency roadside phones, pedestrian crosswalk indicator lighting, and even solar powered trash compactors.
Furthermore, there has been an emergence of Li-Iron Phosphate (LiFePO4) cells in many applications, offering improved safety and a lower float voltage (3.6V) than cobalt-based Lithium-Ion Polymer (typically 4.1V or 4.2V). This chemistry offers many of the other advantages of cobalt-based Li-Ion/Polymer cells as well, including a low self discharge rate and relatively low weight. Additionally, LiFePO4 offers, by comparison, longer cycle and overall life, higher peak-power rating, improved safety via higher resistance to thermal runaway, and lower environmental impact. Disadvantages of LiFePO4 when compared to cobalt-based Li-Ion/Polymer cells include lower energy density (capacity) and susceptibility to fail prematurely if the new cells are "deep cycled" too early.
Extracting peak power from a solar panel is typically either costly (as with solar regulator modules), or difficult to implement, typically requiring complex circuits using microcontrollers and numerous discrete components. These fully-contained, Maximum Peak Power Tracking (MPPT) modules have traditionally targeted large panel power applications, e.g. residential or commercial buildings, but the market application landscape is changing as more uses for solar power are discovered.
For a given amount of light energy, a solar panel has a certain output voltage at which peak output power is produced. Bypass diodes inside the panel can create complex power vs. current characteristics that are not easily optimised when partial shading exists on the panel. Nevertheless, virtually all of the 12V system solar panels currently on the market that are specified with maximum output power less than 25-30W are constructed from a simple series cell arrangement with no bypass diodes. This type of arrangement yields peak output power within a narrow band of panel output voltages, regardless of lighting conditions. Peak powers may be produced from panel voltages of 12.5V-18.5V, depending on the characteristics of the panel.
Li-Iron Phosphate cell batteries may not be charged with a standard Li-Ion/Polymer battery charger – given this cell type’s lower float voltage of 3.6V characteristic, possible irreparable damage to the cell may result if not properly charged. Accurate float voltage charging will prolong the life of the cell. Charge pre-conditioning (trickle charge) also helps to avoid damaging the cell, especially when deeply discharged.
There is currently a lack of solar-powered monolithic (onboard power device) battery charger IC solutions with onboard termination that operate at high voltages (>20V). There are some workarounds to accomplish this - especially to get solar capability - however, they comprise large, complex solutions that require many external components and take up valuable PCB real estate.
A Simple Solution
Any solution to satisfy the design constraints discussed above would have to be compact, high-voltage and monolithic, one that could handle the variation of solar power input voltages and multiple battery chemistries with onboard charge termination. Such a device would act as a catalyst to increase the installation of energy harvesting applications around the globe.
The LT3652 IC builds upon the strengths of Linear Technology’s popular LT3650 family. It is an innovative, solar power tracking, monolithic buck battery charger IC for modern battery chemistries. The device features an innovative input voltage regulation loop, which controls charge current to hold the input voltage at a programmed level. When the LT3652 is powered by a single solar panel, the input regulation loop forces the panel to operate at peak output power. This unique input voltage regulation loop circuitry delivers virtually the same output power generation as more complex and expensive MPPT techniques.
The LT3652 accepts a wide range of inputs from 4.95V to 32V with a 40V absolute maximum rating for added system margin. It charges a variety of battery pack configurations, including 1 to 3 Li-Ion / Polymer cells in series, 1 to 4 LiFePO4 cells in series, 12V sealed lead acid (SLA) batteries, as well as batteries up to 14.4V. Refer to Figure 1 for details.
The LT3652’s charge current is programmable up to 2A. This stand-alone battery charger requires no external microcontroller, and features user-selectable termination including C/10 or an onboard timer. The device’s high fixed switching frequency (1MHz) enables small solution sizes. Float voltage feedback accuracy is specified at 0.5%, charge current accuracy is 5% and C/10 detection accuracy is ±2.5%. Once charging is terminated, the LT3652 automatically enters a low current standby mode which reduces the input supply current to 85uA. In shutdown, the input bias current is reduced to 15uA. The LT3652 maximises battery life during all non-charging periods by draining <1uA from the battery. For autonomous charge control, an auto-recharge feature starts a new charging cycle if the battery voltage falls 2.5% below the programmed float voltage. Additional safety-related features include low battery preconditioning, a thermistor input for temperature-qualified charging, bad battery detection and binary coded status output pins. The LT3652 is available in a low-profile (0.75mm) 12-pin 3mm x 3mm DFN package, and is guaranteed to operate from –40C to 125C junction temperature.
The LT3652’s input voltage regulation control loop method compares very favorably to costly MPPT techniques, delivering virtually the same performance. The input voltage regulation loop:
• Extracts the maximum available power from the solar panel.
• Reduces charge current if the panel output voltage falls below a programmed level.
• Maintains the panel at the output voltage corresponding to the peak output power point for the particular solar panel being used.
• Programs the specific desired peak-power voltage via a resistor divider.
Figure 2 shows the maximum charger current as a function of input voltage for Figure 2’s application circuit, showing how the device lowers output current as the panel voltage falls.
The voltage monitor pin enables programming a minimum operational voltage. Connecting a resistor divider from VIN to the VIN_REG pin enables programming of minimum input supply voltage, typically used to program the peak power voltage for a solar panel. Maximum charge current is reduced when the VIN_REG pin is below the regulation threshold of 2.7V.
If the input supply cannot provide enough power to satisfy the requirements of the LT3652 charger, the supply voltage will collapse. A minimum operating supply voltage can thus be programmed by monitoring the supply through a resistor divider, such that the desired minimum voltage corresponds to 2.7V at the VIN_REG pin. The LT3652 servos the maximum output charge current to maintain the voltage on VIN_REG at or above 2.7V. Programming of the desired minimum voltage is accomplished by connecting a resistor divider as shown in Figure 3. The ratio of RIN1/RIN2 for a desired minimum voltage (VIN(MIN)) is:
RIN1/RIN2 = (VIN(MIN)/2.7) – 1
If the voltage regulation feature is not used, the VIN_REG pin may be connected to VIN.
A typical solar panel is comprised of a number of series-connected cells, each cell being a forward-biased p-n junction. As such, the open-circuit voltage (VOC) of a solar cell has a temperature coefficient that is similar to a common p-n diode, or about –2mV/°C. The peak power point voltage (VMP) for a crystalline solar panel can be approximated as a fixed voltage below VOC, so the temperature coefficient for the peak power point is similar to that of VOC. Panel manufacturers typically specify the 25°C values for VOC, VMP, and the temperature coefficient for VOC, making determination of the temperature coefficient for VMP of a typical panel, straight forward. The LT3652 employs a feedback network to program the VIN input regulation voltage. Manipulation of the network makes for efficient implementation of various temperature compensation schemes for an MPPT application.
Conclusion
Solar power has transformed from “trendy” to practical. Initial applications requiring large panels such as in residential & commercial buildings are being joined by smaller, off-grid, single panel applications less than 4 square feet in area, putting out <25-30W of power, with lower-power panels even smaller. High-end consumer, automotive, industrial, roadside, marine and military segments have developed portable and non-portable applications requiring solar power from these types of single panels. The LT3652 serves an unmet need in the battery charger space. It is a monolithic IC that offers a simple, innovative input voltage control loop for extracting peak power from solar panels, and features multi-chemistry charging capability including Li-Iron Phosphate, Li-Ion/Polymer and SLA cells. Charging efficiency is similar to alternative expensive and costly MPPT techniques. The device also features fast 2A charge capability, onboard charge termination, high voltage operation and comprises a compact, simple solution.
Steve Knoth is Senior Product Marketing Engineer, Power Products Group Linear Technology Corporation
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