Design Note – DN06054/D Reference Design for Solar Power MPPT Controller Device Application Input Voltage NCP1294 Solar Street Lighting 12-24 V Output Voltage A 13.2V Output Current A Topology 2.0A Table 1: NCP1294 Output Statistics Characteristic Output Voltage Output Current Oscillator Frequency 1. 2. 3. 4. 5. Min 9 0 Typ 12 Max 13.5 2.0 100 < 2mA Current Consumption from Battery Maximum Power Tracking < 5% Error Can Charge 4 Batteries in Series or Parallel Can be Configured for use with 10W to 30W Solar Panels Can be used in Parallel with Other Systems Figure 1: Solar Controller Evaluation Board Unit V A kHz Flyback System Description The system under consideration is an off grid solar streetlamp. The solar streetlamp consists of high power LEDS, a lead acid battery, a solar controller, and a solar panel as shown in Figure 2. LED Street Light Solar Panel Lead Acid Battery lar So oller ntr Co - + Solar Controller V 12 Figure 2: Basic off Grid Solar Streetlamp System Solar Panel Characteristics Solar panels collect energy from the sun and convert it to electrical energy. Unfortunately, the sun is not consistent throughout the day due to cloud cover and the angle of the sun relative to the position of the solar panel. Further, the intensity of the sun varies with the season, geographic location, and reflections from adjacent surfaces. Figure 3 shows the monthly average daily total solar resource information on grid cells of approximately 40 km by 40 km in size [1]. Figure 3: Monthly Average Daily Total Solar Resource in December (Right) and July (Left) [1] Since the sun is not consistent, the solar panel rated for 30W peak power may only supply 24W in midday sun and 6 W in the evening sun. Figure 4 displays the voltage, current, and power characteristics of a solar panel at various times during the day with a resistive load. The voltage from solar panels can be configured by the manufacturer to supply almost any voltage and current depending on the solar cell configuration, but generally range from 20V to 48V. Solar Panel Power in Afternoon Sun Voltage Power 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Solar Panel in Evening Sun 20 18 16 14 12 10 8 6 4 2 0 Voltage Power 0 0.2 0.4 Current(I) 0.6 0.8 1 1.2 1.4 Voltage(V)/ Power (W) 24 22 20 18 16 14 12 10 8 6 4 2 0 Voltage(V)/ Power (W) Voltage(V)/ Power (W) Solar Panel IN Miday Sun 20 18 16 14 12 10 8 6 4 2 0 Voltage Power 0 0.2 Current(I) 0.4 0.6 Current(I) Figure 4: Solar Panel Available Power at Midday (Left) Afternoon (Middle) and Evening (Right) Controller Considerations When designing the solar street lighting system, the worst-case scenarios must be considered if the light provided is for safety rather than convenience. Solar energy is measured in hours of the day at which the maximum rated performance from a solar panel can be obtained (peak_power_time). On average, Arizona has six peak hours of sun daily. Solar panels are rated for the peak power they produce, thus the required energy from solar panels can be calculated since the required load energy is known. Excess solar capacity needs to be considered for charge recovery from a cloudy day giving the designer a fixed number of days from which to recover from a series of cloudy days (Recharge-Time). IDLPanelsize = LoadEnergy 712Wh 672Wh Example1 Example2 ⎯⎯ ⎯ ⎯→ = 119W ⎯⎯ ⎯⎯→ = 112W Peak _ Power_ Time 6h 6h RECHARGE Panelsize BatteryCapacity Example1 2.kW 2.4kW Example 2 ⎯⎯ ⎯ ⎯→ = 84W ⎯⎯ ⎯⎯→ = 80W Re ch arg e _ time 30 30 Example1 ⎯ ⎯→ 119W + 84W = 203W Total Panelsize = RECHARGE Panelsize + IDLPanelsize ⎯⎯ Example 2 ⎯⎯ ⎯⎯→ 112W + 80W = 192W The designer need only extract the peak power, deliver it to the battery, and then the system will be complete. Unfortunately, the solar panel characteristics make it difficult to find the peak power point as it moves with many variables. If the solar panel is simply connected to the battery and removed once it is charged, 20% - 30% of the peak power is lost depending on the state of charge of the battery. Commonly solar panels are the most expensive part of the system averaging $4/W/USD. Solar panels are typically manufactured in 5W increments, thus each increment in power costs $20/USD. The price of a linear charge controller may look attractive from the piece part standpoint, but may end with a less cost effective system. The panel size required for a linear charging system is calculated below. Total Panelsize 192W 203W LINEAR Panelsize = ⎯Example ⎯⎯ ⎯1→ = 290W ⎯Example ⎯⎯ ⎯2→ = 274W LIN EFF − PPLoss 0.95 − 0.25 0.95 − 0.25 The discrepancy between the peak power and the linear regulator power is 87W and 82W. If an algorithm for peak power tracking is implemented, there will be an error from the true peak power point and the algorithm operating point. The peak power error (PPerror) operation point results in unused energy from the solar panel, so the smaller the percentage of error, the greater the power extracted. The solar controller efficiency operates at 88% for the 12V system and 91% for the 24V system. The required solar panel size with peak power tracking is calculated below. MPTPanelsize = IDLPanelsize 203W 192 Example1 Example 2 ⎯⎯ ⎯ ⎯→ = 254W ⎯⎯ ⎯⎯→ = 223W η − PPERROR 85% − 5% 91% − 5% For Example 1, the 254W requirement should be rounded to 255W and can be divided into three 85W solar panels. In Example 2, the 223W requirement should be rounded to 225W and can be divided into three 75W solar panels. The system level diagrams are shown in Figure 7. 85W Solar Panel 85W Solar Panel Solar Controller 75W Solar Panel Solar Controller 75W Solar Panel LED Controller Solar Controller Solar Controller 12V 230Ah 85W Solar Panel ` Solar Controller LED Controller 24V 100Ah 75W Solar Panel ` Solar Controller Figure 7: System Diagram Circuit Description The input voltage for the solar controller enters from the solar panel through VIN and GND. The input voltage is filtered by C14. Input under voltage is sensed by R19, R25, and C2, which prevents the controller from operating when the solar panel cannot provide the minimum current. Input over voltage is detected by R27 and R26, which prevents the controller from turning on when the panel voltage is too high or an improper voltage source is connected. The frequency of operation is set by R22 and C1, and can be adjusted up to 1MHz. An external reference is provided that is capable of sourcing a minimum of 2mA when decoupled with C4. The NCP1294 allows the limiting of duty cycle, as well as providing voltage feed forward through R23 and C3. A linear regulator consisting of R20, Z1, and Q5 provide the startup current for the controller. Once the circuit is switching, the current for the controller is provided through R18, D2, C7, and clamped by Z2. The NCP1294 is equipped with power voltage line VC pin and power ground pin PG. The NCP1294 also has a logic voltage line VCC, which is filtered by R15 and C8 with a logic ground LGND to ensure optimal performance. Soft start of the controller can be programmed by adjusting C5. The switching of Q1 is accomplished with the gate pin and rise and fall times can be adjusted with R14. Snubbing of switching noise is provided on the primary side by C16, R11, D4, R17, C18, and R28. The current flowing through Q1 is sensed across R12 and R33 through the filtering of R13 and C6 at the ISENSE pin. Energy is delivered to the secondary side via D1 and D3 which are filtered by C15, C20, and C21. Snubbing is accomplished on the secondary side by C17, R6, R28, and C19. Isolated feedback is provided through U3, but can be modified to provide non-isolated feedback by connecting NIFB and removing ISFB, ISCOMP, ISI, R3, and R33. Phase and gain measurements during the prototyping stage can be accomplished with R9. Type 3 feedback is provided using U2, C11, R7, C12, R3, C10, R4, R5, C9, and R8. A standard ground bypass safety capacitor is provided in C13, but should be shorted if no- isolated operation is required. Peak Power Tracking The NCP1294 allows the user to adjust the current limit via R21 and R24, which are compared to the pulse by pulse current limit measured at Pin 2. The solar panel as discussed earlier has I V characteristics shown in Figure 8 when the current is ramped up. During each on period of a DCM Flyback, the current in Q1 is ramped up from zero current to the maximum allowed internal current limit of the part. If the maximum power of the solar panel is exceeded, the voltage falls to zero. Ideally, the current limit of the part would be set such that it limited at the maximum power point, utilizing the entire capacity of the provided solar panels. Figure 8: I V Characteristics of a Solar Panel in a Switching Application The circuit in Figure 9 takes advantage of the IV characteristics of the solar panel to find the maximum power point on a cycle-by-cycle basis. During each cycle the current ramps up in the transformer when Q1 is on. Once the peak power has been exceeded, the voltage of the panel drops and is measured through R30, R31, and is compared to 3.3V – 0.7V = 2.6V at the base of Q1. If the voltage falls below the 2.6V level, Q1 turns on and a voltage is dropped across R16 turning on Q2 and discharging the current limit set node. The designer can set two current limits adjustable by R24 and R16 if the on resistance is assumed to be << than the parallel combination of R16 and R24. If no lower limit is required, then R16 can be shorted to ground. ILIMIT 1 = ILIMIT 2 = Vref * R 24 Example 3.3V * 2.2kΩ ⎯⎯ ⎯ ⎯→ = 595mV R 24 + R 21 2.2kΩ + 10kΩ Vref * R 24 // R10 Example 3.3V * 687.5Ω ⎯⎯ ⎯ ⎯→ = 212mV R 24 // R10 + R 21 687.5Ω + 10kΩ Figure 9: Peak Power Tracking Circuit A 30W solar controller was designed to determine the maximum error in finding the peak power point using the methodology described above. The solar controller was then connected to a 30W solar panel. The solar panel I V curve data was gathered in a short duration using a resistive load. The solar panel was then connected to the solar controller, the solar controller was loaded with a battery type constant voltage load, and the solar panel I V characteristics were plotted in Figures 10 and 11. The resulting error in finding the maximum power point was less than 5%. Current and Voltage With Resistive Load and CS51221 Input Voltage (V) Input Power (W) 25 20 PNL IV 15 PNL PWR PNL CNTRL IV PNL CNTRL PWR 10 5 0 0 0.5 1 1.5 Input Current (A) Figure 10: I V Characteristics of Solar Panel and Solar Controller Current and Voltage With Resistive Load and CS51221 17.5 Input Power (W) 17 5% Error 16.5 16 PNL PWR PNL CNTRL PWR 15.5 15 14.5 14 0.8 0.9 1 1.1 Input Current (A) Figure 11: I V Characteristics of Solar Panel and Solar Controller Error The control scheme should react and find the maximum power point in both the static and dynamic cases. The street light may have been installed when trees were small and over time they may shade the solar panel or have moving shadows that pass over the solar panel on windy day. Figure 12 shows the case where a tree is casting a shadow on the solar panel, a gust of wind moves the leaves to expose the solar panel to full sun, covers it completely, then provides the panel with more sun than the original level, all taking place in less than 6 seconds. Shadow Removed and Current Limit Adjusted Panel in Slight Shadow Solar Panel in Partial Shadow Full Sun Panel in Full Shadow Panel Fails to Provide Minimum Power Figure 12: Shadow Reaction of Solar Controller Fast Moving Shadow and Full Sun Layout TOP BOTTOM Figure 13: Layout of Solar Controller Schematic Figure 14: Schematic of Solar Controller Table 2: Solar Controller Bill of Materials Designator Quantity Description Value Tolerance Footprint Manufacturer Manufacturer Part Number D2-4 3 Schottky 100V 0.79 Vf NA Axial 5.2X2.7 mm ON Semiconductor MBR1100RL U4 1 Small Signal N MOSFET 60V 380mA NA SOT-23 ON Semiconductor 2N7002KT1G C10 1 Ceramic Chip Capacitor 1.2nF 10% 603 TDK C1608C0G1H122J C3 1 Ceramic Chip Capacitor 1nF 10% 1206 Vishay VJ1206A102JXACW1BC C6 1 Ceramic Chip Capacitor 1nF 10% 603 Vishay VJ0603Y102KXACW1BC C1 1 Ceramic Chip Capacitor 2.2nF 10% 805 Vishay VJ0805Y222KXACW1BC C11 1 Ceramic Chip Capacitor 3.9nF 10% 603 TDK C1608C0G1E392J C22 1 Ceramic Chip Capacitor 390pF 10% 603 Vishay VJ0603A391JXACW1BC C12 1 Ceramic Chip Capacitor 470pF 10% 603 TDK C1608X7R1H471K ECJ-1VB1C563K C5 1 Ceramic Chip Capacitor 56nF 10% 603 Panasonic C9 1 Ceramic Chip Capacitor NI 10% 603 AVX C2 C4 C8 3 Ceramic Chip Capacitor 0.1uF ±10% 603 Vishay VJ0603Y104KXJCW1BC C21 1 Ceramic Chip Capacitor 1uF 10% 1206 Vishay VJ1206Y105KXJTW1BC C16 1 Ceramic Chip Capacitor 220pf 10% 1206 Vishay VJ1206Y221JBBAT4X C17-19 3 Ceramic Chip Capacitor 560pF 10% 1206 Vishay VJ1206Y561KBBAT4X C7 1 Ceramic Chip Capacitor 4.7uF ±20% 1210 TDK Corporation C3225X7R1E475M C14-15 2 AE Capacitor 35V 220 uF 20% Through Hole 8mmX11.5mm United Chemi-Con ESMG350ELL221MHB5D C20 1 AE cap 16V 220uF ±20% 6.3mm x 11mm United Chemi-Con ESMG160ELL221MF11D C13 1 1.5nF +/- 20% Panasonic ECKANA152ME U1 1 Ceramic Disk Capacitor Enhanced Voltage Mode PWM Controller 3V Reference NA SOIC 16 ON Semiconductor NCP1294 D1 1 Schottky Power Rectifier NA NA Axial Leaded On Semiconductor MBR3100RLG Q5 1 NPN Transistor 40V 200mA NA SOT-23 ON Semiconductor MMBT3904LT1G Q2 1 PNP Transistor PNP NA SOT-23 ON Semiconductor MMBT3906LT1G Z2 1 Zener Diode 14V ±5% SOD-123 ON Semiconductor MMSZ5244BT1G Z1 1 Zener Diode 6.2V ±5% SOD-123 ON Semiconductor MMSZ5234BT1G Q1 1 Power N-MOSFET 60V 30A NA DPAK 4 ON Semiconductor NTD5414NT4G Designator Quantity Description Value Tolerance Footprint Manufacturer Manufacturer Part Number U3 1 OTOCOUPLER 6V 80mA NA 4 PIN DIP CEL PS2503-1-L R33-34 2 Resistor 0R0 NA 603 Vishay / Dale CRCW06030000Z0EA R8 1 Resistor 1.2k ±1.0% 603 Vishay / Dale CRCW06031K20FKEA R1 1 Resistor 10.7k ±1.0% 603 Vishay / Dale CRCW060310K7FKEA R20-21 2 Resistor 10k ±1.0% 603 Vishay / Dale CRCW060310K0FKEA R3 1 Resistor 110k ±1.0% 603 Vishay / Dale CRCW0603110KFKEA R30 1 Resistor 12.4k ±1.0% 603 Vishay / Dale CRCW060312K4FKEA CRCW0603150RFKEA R5 1 Resistor 150R0 ±1.0% 603 Vishay / Dale R10 1 Resistor 1K ±1.0% 603 Vishay / Dale CRCW06031K00FKEA R2 1 Resistor 2.21k ±1.0% 603 Vishay / Dale CRCW06032K21FKEA R31 1 Resistor 2.67k ±1.0% 603 Vishay / Dale CRCW06032K67FKEA R9 1 Resistor 20R0 ±1.0% 603 Vishay / Dale CRCW060320R0FKEA R7 1 Resistor 20k ±1.0% 603 Vishay / Dale CRCW060320K0FKEA R16 1 Resistor 30.0k ±1.0% 603 Vishay / Dale CRCW060330K0FKEA R26 1 Resistor 4.02k ±1.0% 603 Vishay / Dale CRCW06034K02FKEA R23 1 Resistor 41.2k ±1.0% 603 Vishay / Dale CRCW060341K2FKEA R27 1 Resistor 49.9k ±1.0% 603 Vishay / Dale CRCW060349K9FKEA R4 R13 R22 3 Resistor 5.36k ±1.0% 603 Vishay / Dale CRCW06035K36FKEA R19 1 Resistor 54.9k ±1.0% 603 Vishay / Dale CRCW060354K9FKEA R12 R32 2 Resistor 0R1 ±1.0% 1206 Vishay / Dale WSL1206R1000FEA R15 R17 R29 3 Resistor 10R0 ±1.0% 1206 Vishay / Dale CRCW120610R0FKEA R6 R28 2 Resistor 15R0 ±1.0% 1206 Vishay / Dale CRCW120615R0FKEA R14 1 Resistor 1R0 ±1.0% 1206 Vishay / Dale CRCW12061R00FNEA R24 1 Resistor 2.20k ±1.0% 1206 Vishay / Dale CRCW06032K20FKEA R18 1 Resistor 36R0 ±5.0% 1206 Vishay / Dale CRCW120633R0JNEA R25 1 Resistor 4.99K ±1.0% 603 Vishay / Dale CRCW06034K99FKEA R11 1 51R0 ±5.0% 1206 Vishay / Dale CRCW120651R0JNEA U2 1 Resistor Programmable Precision Reference 2.5V ±0.4% TO-92 On Semiconductor TL431CLPG T1 1 Transformer 28.6 uH 10% TH 10 PIN 24X21X32.5 ICE TO08029 [1] NREL National Renewable Energy Laboratory webpage. 2008 <http://www.nrel.gov/gis/solar.html> 1 © 2009 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Tim Kaske and Bryan McCoy, e-mail: [email protected] ; [email protected] May 2009, Rev.1 www.onsemi.com 13