Design essentials of the hottest solar charging co

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As we all know, the solar panel has an IV curve, which represents the output performance of the solar panel and represents the current and voltage values respectively. The voltage and current represented by the intersection of the two lines is the power of this solar panel. Disadvantageously, the IV curve will change with irradiance, temperature and service life. Irradiance is the density of a given surface radiation event, generally expressed in watts per square centimeter or square meter. If the solar panel has no mechanical sun tracking ability, the irradiance will change by about ± 23 degrees with the movement of the sun in a year. In addition, the irradiance change of the sun moving from the horizon to the horizon every day can lead to the change of output power throughout the day. To this end, Anson semiconductor has developed a solar cell controller ncp1294, which is used to realize maximum peak power point tracking (MPPT) of solar panels and charge batteries with the highest energy efficiency. This paper will introduce some main functions of the device and some problems needing attention in application

enhanced voltage mode PWM controller

ncp1294 is a fixed frequency voltage mode PWM feedforward controller, which contains all the basic functions required for voltage mode operation. As a charging controller supporting different topologies such as step-down, step-up, step-down boost and flyback, ncp1294 is optimized for the control operation of high-frequency primary end, has pulse by pulse current limiting and bidirectional synchronization functions, and supports solar panels with power up to 140 W. The MPPT function provided by this device can locate the maximum power point and adjust it in real time according to the environmental conditions, so as to keep the controller close to the maximum power point, so as to extract the maximum power from the solar panel and provide the best energy efficiency

in addition, ncp1294 also has soft start, precise control of duty cycle limit, less than 50 μ A's starting current, overvoltage and undervoltage protection and other functions. In solar applications, ncp1294 can be used as a flexible solution in module level power management (MLPM) solutions. The reference design MPPT tracking error based on ncp1294 is less than 5%, which can charge four batteries in series or in parallel. Figure 1 is the block diagram of ncp1294 120 W solar controller

Figure 1: the ncp1294 120 W solar controller block diagram of Anson semiconductor

as shown in Figure 1, the core of the system is the power segment, which must withstand an input voltage of 12 V to 60 V and produce an output of 12 V to 36 v. Since the input voltage range covers the required output voltage, there must be a buck boost topology to support the application. Designers can choose a variety of topologies: SEPIC, non inverted buck boost. Flyback, single switch forward, double switch forward, half bridge, full bridge or other topologies

the design work includes isolating the topology according to the increase of power demand. The management of battery charging state is completed by appropriate charging algorithm. The solar panel installation technician can choose the output voltage and battery charging rate. Because the controller is connected to the solar panel, it must have maximum power point tracking to provide high value to the end customer. The controller has two enable circuits. One circuit detects the night time and the other detects the charging state of the battery, so that the external circuit will not discharge the battery to the damaged point. Since the controller will be installed by field technicians and novices with different degrees of experience, it is important that the input and output must have reverse polarity protection. In addition, the controller and battery may be installed in an overheated or supercooled position, and the controller must adopt battery charging temperature compensation. The design shall also include safety functions, such as battery overvoltage detection and solar panel undervoltage detection

working principle of dynamic MPPT

in order to extract the maximum power from the power variable power supply (i.e. solar panel), the solar controller must adopt MPPT. MPPT must first find the maximum power point and adjust the environmental conditions in time to keep the controller close to the maximum power point. Dynamic MPPT is used when the system changes. As each switching cycle is changing, the power absorbed by the solar panel will also change significantly in each cycle. The dynamic MPPT uses the voltage dip of the solar panel multiplied by the increased current in each switching cycle to determine the error signal to be generated to adjust the duty cycle. The dynamic response can detect the slope of the IV curve, so as to establish a power slope, and establish a power representing the duty cycle from the intersection point of the error signal. When the slope changes from positive to negative, the cycle ends, as shown in Figure 2

Figure 2: voltage and current of PWM regulated converter

feedforward voltage mode control

in traditional voltage mode control, the slope signal has a fixed rising and falling slope. The feedback signal comes only from the output voltage. Therefore, the voltage mode control line has poor voltage stabilization effect and audio susceptibility. The feedforward voltage mode control comes from the slope signal input line. Therefore, the slope of the slope changes with the input voltage, including the polyurethane resin floor system of mastertop 1327 ⑵ 0dB. The feedforward function can also provide a volt second clamp, which limits the maximum product of the input voltage and the on time. Clamping circuits in the circuit, such as forward and flyback converters, can be used to prevent transformer saturation

ncp1294 application design process of solar charging controller

when selecting the topology of solar controller, it is important to understand the basic operation and limitations of the converter. The selected topology is a non inverting four switch asynchronous buck boost topology. The converter operates with the control signal from ncp1294, and Q1 and Q2 are connected at the same time to charge L1. The four switch buck boost topology is shown in Figure 3, in which the inductor is used to control voltage and current

Figure 3: four switch buck boost topology

four switch non inverse buck boost has two operation modes, namely, Buck mode and buck boost mode. In step-down mode, the converter generates input voltage pulse, which is filtered by LC to generate a low DC output voltage. The output voltage can be changed by modifying the on time relative to the switching cycle or switching frequency

if the output voltage may reach 1% to 89%, the solar controller will operate in step-down mode. If the output voltage cannot be reached due to the limitation of duty cycle, it will switch to buck boost mode, and then the output voltage can be reached. The change from 89% to a lower duty cycle is shown in Figure 4

Figure 4: transfer ratio between buck and boost modes of multiple batteries

it should be noted that when the converter mode is switched from buck to buck boost, the error signal will take a period of time to change the duty cycle. The instantaneous change of mode will make the buck boost converter try to switch at 89% duty cycle and try to switch to 47%; This will cause the converter to try to output 130 V in the trade over region. Ncp1294 provides a pulse through the pulse current limiter, which can prevent the converter energy from reaching a dangerous level and realize the transition under the duty cycle condition

compensation network

to create a stable power supply, the compensation network around the error amplifier must be used with PWM generator and power stage. Since the design standard of the power stage is set according to the application, the compensation network must have the correct overall output to ensure stability. Ncp1294 is a voltage mode voltage feedforward device, so it needs a voltage loop that uses the input voltage to modify the slope. The output inductance and capacitance of the power stage can form a double pole, and the loop must be compensated for this

system startup and battery current consumption

the system being created is connected to two limited sources, which will supply power to the load at different times of the day. If it is at the same time, it will not supply power, except for a short time. The system is not complete, and no batteries and solar panels are installed. Therefore, this equipment adopts the electro-hydraulic proportional servo system as the pull driving part, which is conducive to the detection of battery load and the existence of solar panel source. For example, if the battery is not connected, it will not consume the energy of the solar panel when supplying the battery voltage. If the solar panel is connected, the battery will be exhausted in order to find the solar panel to be connected. A simple solution to check the solar panel connection and battery connection is to use a low current consumption comparator

during the day, the system charges the battery, while at night, the battery discharges to illuminate the defined space. Although the input energy cannot be guaranteed, the output energy can remain unchanged for a long time. If the size of a system is not appropriate, the battery may be damaged due to discharge. To prevent the battery from being damaged, the LED circuit must be used to suppress the operation and prevent the battery from running out

balance of input and output currents

when building an ideal solar controller, the controller should protect the battery or load and extract the maximum energy from the solar panel at the same time. Unfortunately, in the real world, customers or installers may buy a large solar panel and a small battery. If the solar controller is charged at peak power, the battery charging speed is too fast, which will shorten the battery life or may explode. What the controller should do is to manage the battery demand and balance the charging speed according to the peak power provided by the solar panel. Therefore, the setting and selection scheme of the maximum battery charging rate needs to determine how to limit the output current of the system. The current setting is completed through the 3.3V reference and resistance voltage divider network provided by ncp1294. Shorting one or more headers will achieve different current limits

reverse polarity protection

in addition to normal solar panel transients, there are four different input and output connection possibilities. In the first case, the input and output connections are correct without protection. In the second case, the input voltage is connected in reverse. If current is allowed to flow in this case, all output diodes may be damaged

however, all devices can be protected by connecting a diode in series at the input of B or C shown in Figure 5. One disadvantage of series diodes is that they continuously dissipate system power. If the reverse polarity protection diode is placed in a high current system, the loss may be large. Another way to implement reverse polarity protection is to place a diode. For example, when a reverse voltage is applied, it will open the fuse, as shown in Figure 5D. The fuse of choice can be a user replaceable or poli thermal fuse. Fuses can provide the necessary protection, but may lead to a poor user experience. The low loss equipment industry department, which realizes diode reverse polarity protection, will thoroughly implement the party's 109 spirit consumption. The way is to use MOSFET. When the applied voltage polarity is correct, MOSFET will be turned on, and when the voltage polarity is incorrect, it will be turned off. Figure 5 E

Figure 5: reverse polarity input connection

in the third case, behind the sharp reduction of product order and cost reduction pressure, the output is reverse polarity connection, the input is correctly connected, and the power components may be damaged. Since the source is assumed to be a lead-acid battery, protection is crucial because damaged components may consume a lot of energy. Figure 5B shows one of the methods to prevent reverse output voltage

the last case is that the input and output connections are incorrect. stay

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