News
Detailed explanation of solar charge controller and its design points
Detailed explanation of solar charge controller and its design points
As we all know, a solar panel has an IV curve, which represents the output performance of the solar panel, representing the current and voltage values respectively. The voltage and current represented by the intersection of the two lines is the power of the solar panel. The downside is that the IV curve changes with irradiance, temperature, and age. Irradiance is the density of radiation events at a given surface, typically expressed in watts per square centimeter or square meter. If a solar panel does not have mechanical sunlight tracking capabilities, irradiance will vary by approximately ±23 degrees over the course of a year as the sun moves. In addition, daily changes in irradiance as the sun moves from horizon to horizon can cause output power to vary throughout the day. To this end, ON Semiconductor has developed a solar cell controller NCP1294 to achieve the maximum peak power point tracking (MPPT) of the solar panel and charge the battery with the highest energy efficiency. This article will introduce some of the main functions of this device and issues that need to be paid attention to when applying it.
Enhanced voltage mode PWM controller
RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller is a fixed frequency voltage mode PWM feed forward controller that contains all the basic functions required for voltage mode operation. As a charge controller that supports different topologies such as buck, boost, buck-boost and flyback, RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller is optimized for high-frequency primary-side control operations and has pulse-by-pulse current limiting And two-way synchronization function, supporting 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 environmental conditions, keeping the controller close to the maximum power point, thereby extracting the maximum power from the solar panel and providing the best energy efficiency.
In addition, RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller also has functions such as soft start, precise control of duty cycle limit, starting current below 50 μA, overvoltage and undervoltage protection. In solar applications, RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller can be used as a flexible solution for module level power management (MLPM) solutions. The reference design based on NCP1294 has a maximum power point tracking error of less than 5% and can charge four batteries in series or parallel.
The heart of the system is the power section, which must withstand input voltages from 12 V to 60 V and produce outputs from 12 V to 36 V. Since the input voltage range covers the required output voltage, a buck-boost topology is necessary to support the application. Designers can choose from several topologies: SEPIC, non-inverting buck-boost. Flyback, single-switch forward, dual-switch forward, half-bridge, full-bridge or other topologies.
Design work includes increasing the isolation topology based on power requirements. Management of battery charge status is accomplished by appropriate charging algorithms. The solar panel installation technician can select the output voltage and battery charging rate. Since the controller is going to be connected to the solar panels, it must have maximum power point tracking to provide high value to the end customer. The controller has two positive enable circuits. One circuit detects the night time and the other detects the charging status 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 varying levels of experience, it is important that the inputs and outputs are protected against reverse polarity. In addition, the controller and battery may be installed in a location that is too hot or too cold, and the controller must use battery charging temperature compensation. The design should also include safety features such as battery overvoltage detection and solar panel undervoltage detection.
In order to extract the maximum power from a variable power source (i.e. solar panel), the solar controller must use 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. Since this changes with each switching cycle, the power drawn by the solar panel will also change significantly with each cycle. Dynamic MPPT uses the voltage sag of the solar panel multiplied by the increasing current of each switching cycle to determine the error signal that will be generated to adjust the duty cycle. The dynamic response detects the slope of the IV curve, thereby establishing a power ramp that establishes a power representative of the duty cycle from the point where the error signal intersects. The cycle ends when the slope of the ramp changes from positive to negative.
Feedforward voltage mode control
In traditional voltage mode control, the ramp signal has a fixed rising and falling slope. The feedback signal comes only from the output voltage. As a result, voltage mode control lines are less regulated and have audio susceptibility. Feedforward voltage mode control originates from the ramp signal input line. Therefore, the slope of the ramp changes with the input voltage. The feedforward function also provides a volt-second clamp, which limits the maximum product of input voltage and on-time. Clamping circuits in circuits such as forward and flyback converters can be used to prevent transformer saturation. NCP1294 solar charge controller application design process
When choosing a solar controller topology, it is important to understand the basic operation of the converter and its limitations. The topology chosen is a non-inverting four-switch non-synchronous buck-boost topology. The converter operates using the control signal from the NCP1294, with Q1 and Q2 conducting simultaneously to charge L1. A four-switch buck-boost topology in which an inductor is used to control voltage and current.
The four-switch non-inverting buck-boost has two operating modes, buck mode and buck-boost mode. In buck mode, the converter generates input voltage pulses, which are LC filtered to produce a lower DC output voltage. The output voltage can be changed by modifying the on-time relative to the switching period or switching frequency.
If the output voltage may reach 1% to 89%, the solar controller is operating in buck mode. If this output voltage cannot be reached due to duty cycle limitations, it switches to buck-boost mode, where it is achieved. Change from 89% to lower duty cycle
It is important to note that when the converter mode switches from buck to buck-boost, the error signal will take some time to change the duty cycle. The momentary change in mode will cause the buck-boost converter to attempt to switch at 89% duty cycle and attempt to convert to 47%; this results in the converter trying to output 130 V in the trade over region. The NCP1294 provides a pulse via a pulse current limiter that prevents the converter from reaching dangerous levels of energy and provides a gentle transition under duty cycle conditions.
compensation network
To create a stable power supply, a compensation network around the error amplifier must be used in conjunction with the PWM generator and power stage. Since the standards for power stage design are set based on the application, the compensation network must have the correct overall output to ensure stability. The NCP1294 is a voltage-mode voltage feedforward device and therefore requires a voltage loop that uses the input voltage to modify the ramp. The output inductance and capacitance of the power stage can create a double pole for which the loop must compensate.
System startup and battery current consumption
The system being created connects two finite sources that will supply power to the load at different times of the day and will not supply power at the same time except for short periods of time. The system is not complete and does not have batteries and solar panels installed, thus facilitating the detection of the presence or absence of battery loads and solar panel sources. For example, if no battery is connected, it will not consume energy from the solar panel when providing battery voltage. If a solar panel is connected, the battery will be drained trying to find the solar panel to connect to. A simple solution to check solar panel connections and battery connections is to use a low current draw comparator.
During daytime hours the system charges the batteries, while at night the batteries discharge to illuminate the defined space. Although the input energy is not guaranteed, the output energy can remain unchanged for a long time. If a system is not properly sized, the battery may be damaged by discharge. To prevent battery damage, LED circuitry must be used to inhibit operation and prevent battery exhaustion.
Balance of input and output current
When building an ideal solar controller, the controller should protect the battery or load while extracting maximum energy from the solar panel. Unfortunately, in the real world a customer or installer might buy a large solar panel and a small battery. If the solar controller is charging at peak power, the battery is charging too quickly, shortening battery life or possibly exploding. What the controller is supposed to do is manage battery demand, balancing charging speeds based on the peak power provided by the solar panels. Therefore, setting and selecting the maximum battery charge rate requires determining how to limit the system's output current. Current setting is accomplished through the 3.3V reference and resistor divider network provided by the NCP1294. Shorting one or more headers will achieve different current limit values.
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 are connected correctly and no protection is required. In the second case, the input voltage is connected in reverse. If current is allowed to flow in this condition, all output diodes may be damaged.
However, a diode in series with the input of B or C shown can protect all devices. One disadvantage of a series diode is that it continuously dissipates system power. If reverse polarity protection diodes are placed in high current systems, the losses can be significant. Another way to implement reverse polarity protection is to place a diode such that it opens the fuse when reverse voltage is applied, as shown in Figure 5 D. The fuse of choice can be a user-replaceable or polythermal fuse. Fuses can provide necessary protection, but may result in a less than optimal user experience. A low loss way to achieve diode reverse polarity protection is to use a MOSFET that conducts when the applied voltage is of correct polarity and turns off when the voltage is incorrect.
In the third case, the output is connected with reverse polarity and the input is connected correctly, the power components may be damaged. Since the source is assumed to be a lead-acid battery, protection is critical as damaged components can consume large amounts of energy. Figure 5 B shows one method of preventing reverse output voltage.
The last situation is that the input and output connections are incorrect. In this case, if the designer implements the second and third protections, both the input and output will be protected. Designers should not overlook voltage suppressors, which are installed at the input of transient voltages, which may or may not have the correct polarity. Therefore, it is important to have a bidirectional transient suppressor that can withstand normal reverse polarity voltages without damage.
There are three stages of lead-acid battery charging: constant current charging or high current charging, absorption or constant voltage mode, and float charging. During high current charging, the current is kept constant, which is accomplished by pulsing the NCP1294 pulse current limit and current setting circuit. Unless the maximum power point is lower than this level, the current will remain at the charge rate set by the designer or user, at which time the charge will be up to the maximum power point adjustment rate.
OOV comparator
RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller is equipped with an OOV comparator that can monitor the output battery voltage to determine if the feedback mechanism is damaged, or the remote detection is affected by battery voltage that exceeds battery temperature compensation. The system shuts down when OOV is disconnected. Comparators can be used at system input or system output, but are recommended as a fail-safe mechanism for the output. When using a single-battery system, you can use an 18V trip point or set the trip point based on charge status. If the floating voltage state is used, 15 V needs to be set as the trigger voltage.
OUV function
RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller’s undervoltage lockout (OUV) feature monitors the converter’s input voltage to determine if the input voltage level could cause thermal issues. The OUV can independently monitor the input voltage to ensure that the input voltage is at the ideal level to provide maximum output power.
OTP function
Since the solar controller may be used in an inappropriate manner, it is recommended that the temperature of the buck main switch be monitored to determine if it exceeds the maximum temperature level. If the temperature of the main MOSFET has exceeded the appropriate level, over-temperature protection (OTP) can suppress the current to reduce system power consumption.
Thermal management
RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller is a low power consumption device. Once the IC power dissipation is determined, the designer can calculate the thermal impedance required to maintain the specified junction temperature at the worst-case ambient temperature. The thermal performance of a solar controller is greatly affected by the PCB layout. Extra care should be taken during the design process to ensure that the IC and power switch operate within the recommended environmental conditions. Any power supply design should undergo appropriate laboratory testing to ensure the power consumption required by the design under worst-case operating conditions. Variables considered during testing should include maximum ambient temperature, minimum airflow, maximum input voltage, maximum load, and component variation (i.e., worst-case RDSON of the MOSFET).
solar panel
RAGGIE Power 60A 80A 100A MPPT Solar Charge Controller Evaluation Board supports solar panels between 5 W and 120 W. Considered here are industry standard types of solar panels. The most common type of solar cell is crystalline silicon, which comes in two main types: monocrystalline and polycrystalline. Monocrystalline silicon is the most energy efficient, but is also more expensive to produce and is generally limited to commercial and residential applications. Amorphous solar panels consist of a thin film of fused silicon coated on stainless steel or similar materials. The crystal structure is very fragile and is usually sandwiched between two pieces of glass for protection. The efficiency of monocrystalline silicon is 18%, polycrystalline silicon is 15%, and amorphous silicon is 10%.
Using this feature-rich and flexible solution, engineers can develop suitable products according to the requirements of different solar panels, allowing end users to enjoy the convenience and better user experience brought by advanced semiconductor technology.