design features 80V Buck-Boost Lead-Acid and Lithium Battery Charging Controller Actively Finds True Maximum Power Point in Solar Power Applications Tage Bjorklund In solar power systems, the bulk of the expense is in the panel and batteries. Any cost-effective solar power solution maximizes the capacity usage and lifetime of these components. For instance, a high quality charger increases battery run time, reducing capacity requirements, and extends battery lifetime, minimizing maintenance and replacement costs. Likewise, using a DC/DC controller that extracts the maximum available energy from the solar panel reduces the size and cost of the panels required. The LT8490 is a charge controller for lead acid and lithium batteries that can be powered by a solar panel or a DC voltage source. It includes true maximum power point tracking (MPPT) for solar panels and optimized built-in battery charging algorithms for various battery types—no firmware development required. 80V input and output ratings enable the LT8490 to be used with panels containing up to 96 cells in series. The power stage uses four external N-channel MOSFETs and a single inductor in a buck-boost configuration. Unlike most charge controllers, the buckboost configuration allows the charger to operate efficiently with panel voltages that are below, above or equal to the battery voltage. The minimum panel voltage is 6V. Batteries live longer and run longer when the charge algorithm is optimized for the battery type. Likewise, a high performing MPPT charger, which tracks the solar panel maximum power point during partial shade conditions, allows the use of a smaller and lower cost solar panel. Creating a discrete-component charger solution to perform all of these duties would be costly and time consuming, typically requiring a microcontroller, a high performance switching regulator and a lengthy firmware development cycle. GATEVCC´ GATEVCC´ SOLAR PANEL LOAD TG1 BOOST1 SW1 BG1 CSP CSN BG2 SW2 BOOST2 TG2 VBAT CSPOUT CSNOUT EXTVCC CSNIN CSPIN VIN LT8490 GATEVCC´ AVDD TEMPSENSE + – RECHARGABLE BATTERY THERMISTOR GATEVCC INTVCC STATUS AVDD FAULT GND AVDD Figure 1. Simplified solar powered battery charger schematic July 2014 : LT Journal of Analog Innovation | 21 Dual local maximums are the downfall of the conventional MPPT found in a number of controllers. In contrast, the LT8490 finds the true MPP, yielding twice the charge power—or even higher in other shade conditions. 250 FULL ILLUMINATION PPANEL (W) 200 150 LOCAL MPP 50 0 Since the power stage is external, it can be optimized for the application. Charge current limits (and input current limit when a DC voltage source is used) can be configured as needed. TRUE MPP 100 True Maximum Power Point Tracking PARTIAL SHADE 0 10 20 voltage of 80V; a range corresponding to 16 to 96 series-connected solar cells. 30 40 VPANEL (V) Figure 2. The power curve of a 60-cell 250W solar panel with entire panel illuminated and with a small shadow partly covering one cell (Figure 3) COMPLETE SINGLE-IC SOLAR POWERED BATTERY CHARGER SOLUTION The LT8490 is an MPPT battery charger controller with a long list of features including: •integrated MPPT algorithm (no firmware development required) greatly reduces time to market •integrated buck-boost controller allows VIN to be above, below or equal to VBAT •supports lead-acid and lithium-ion batteries •6V–80V VIN and 1.3V–80V VBAT The LT8490 can be powered by a solar panel or any DC voltage source. For a particular battery voltage, a wide range of solar panel types can be used, as the panel voltage can be lower or higher than the battery voltage. The LT8490 accepts panel inputs from 6V to a maximum (cold temperature) open circuit 22 | July 2014 : LT Journal of Analog Innovation When operating from a solar panel, the LT8490 maintains the panel voltage at the panel’s maximum power point. Even during partial shade conditions, when more than one local maximum power point appears (an effect of bypass diodes inside the solar panel), the LT8490 detects and tracks the true maximum. Figure 2 shows the P-V curves for a common 60-cell 250W panel under two different lighting conditions. The maximum power point (200W) occurs at 25V when the panel is fully illuminated. In partial shade (see Figure 3), the available power at a 25V panel voltage drops to 50W, with the new true maximum power point (128W) appearing at 16V. Note that the original 25V/200W power peak actually moves to a local maximum ~32V/63W. This dual local maximum effect is the downfall of traditional MPPT functions found in a number of controllers, for they follow the initial 25V/200W peak as it shifts to 32V/63W. In contrast, the LT8490 finds the true MPP at 16V/128W, yielding an additional 65W from the panel. It does this by measuring the entire power curve of the panel at regular intervals and locating the true maximum power Figure 3. The solar panel shaded in top right corner peak at which to operate. In this case, more than twice as much charge power is extracted, with even greater gains possible in other shade conditions. Charge Control Functions Charge algorithms can be configured according to the requirements of each application by adjusting the voltage on two configuration pins. Lead-acid batteries built with AGM, gel and wet cell technologies require slightly different charge voltages for best lifetime, and Li-ion and LiFePO4 cells have charge requirements that are different from lead-acid batteries. Some of the built-in and configurable charge control functions are: design features ½W 7mΩ + CIN3 2.2µF ×2 CIN2 2.2µF ×2 SOLAR PANEL VOC < 53V – 10Ω 8.06k 110k GATEVCC ´ 4Ω 3.24k 5.49k 2Ω CSN EXTVCC CSPOUT CSNOUT 3.01k 21k 100nF LT8490 4.7nF SYNC 8.45k 11.5k 4.7µF + FLOODED LEAD ACID 10k AT 25°C ß = 3380 NTC 100nF SWEN SWENO CLKDET CLKOUT CHARGECFG2 STATUS FAULT CHARGECFG1 AVDD 1.3k 13k AVDD 3.32k 68nF 10nF DS 470pF 14.27V STAGE 2 (ABSORPTION) CHARGE VOLTAGE (VS2) AT 25°C 13.87V STAGE 3 (FLOAT) CHARGE VOLTAGE (VS3) AT 25°C 10A CHARGING CURRENT LIMIT 2.5A TRICKLE CURRENT LIMIT 7.2A INPUT CURRENT LIMIT 53V MAXIMUM PANEL VOLTAGE (VMAX) NO TIMER LIMITS TEMPERATURE COMPENSATION ENABLED –20°C TO 50°C BATTERY TEMPERATURE RANGE 175kHz SWITCHING FREQUENCY EXAMPLE SOLAR PANEL: SHARP NT-175UC1 175W 200k 90.9k DF 549Ω 549Ω M1, M2: INFINEON BSC028N06NS M3, M4: INFINEON BSC042N03LSG L1: 15µH COILCRAFT SER2915H-153KL DB1, DB2: CENTRAL SEMI CMMR1U-02 CIN1: 33µF, 63V, SUNCON 63HVH33M CIN2, CIN3, CIN4: 2.2µF, 100V, AVX 12101C225KAT2A COUT1: 150µF, 35V NICHICON UPJ151MPD6TD COUT2, COUT3: 10µF, 35V, MURATA GRM32ER7YA106KA12 COUT4: 1µF, 25V AVX 12063C105KAT2A •charge voltage temperature compensation (typically for lead-acid batteries) using NTC sensor •reduction of charge voltage to a lower float voltage level when the battery is fully charged •over or under battery temperature stops charge current to protect the battery •charging time limits can be set when operating from a DC voltage source •dead battery detection stops the charging, to avoid a hazard CONCLUSION •constant current charging that changes to constant voltage charging as the battery voltage reaches its final value 1µF – ECON 53.6k •adjustable trickle charging of a deeply discharged battery reduces risk of damage 124k 23.2k VDD LDO33 SRVO_IIN SRVO_FBIN SRVO_FBOUT SRVO_IOUT IOR IMON_OUT VC LOAD 0.082µF 26.1k TEMPSENSE AVDD IOW 97.6k 8.2nF 274k FBOR FBOUT FBOW RT SS IIR IMON_IN 32.4k 470nF BOOST2 TG2 1.05k 249k COUT4 1µF 220nF GND BG2 SW2 COUT1 150µF 10Ω DB2 MODE 93.1k 1µF 5mΩ INTVCC SHDN VINR FBIR FBIN FBIW COUT2 10µF ×2 GATEVCC ´ 3.3nF TG1 BOOST1 SW1 BG1 CSP CSNIN CSPIN VIN GATEVCC 4.7µF ×2 35.7k 220nF 2Ω CIN4 2.2µF M3 10Ω 3.3nF VBAT COUT3 10µF ×2 10Ω DB1 1W 5mΩ M4 M2 GATEVCC ´ CIN1 33µF ×3 470nF 196k L1 15µH M1 Figure 4. Complete solar power system with lead-acid battery charging/control an inductor, allowing the charger to operate with VIN above, below or equal to the battery voltage. All necessary functions are included, with built-in battery charging algorithms and MPPT control, requiring no firmware development. n The LT8490 is a full-featured true MPPT charge controller that can operate from a solar panel or a DC voltage source with a voltage range from 6V to 80V, charging lead-acid or lithium batteries from 1.3V to 80V. The power stage is easily configured by selecting four MOSFETs and July 2014 : LT Journal of Analog Innovation | 23