Energy Efficient Solar Charge Controller Design Example

As we all know, solar panels have an IV curve, which represents the output performance of the solar panel, representing the current and voltage values ​​respectively. The intersection of the two lines represents the voltage and current, which is the power of the solar panel. Unfortunately, the IV curve changes with irradiance, temperature and age. Irradiance is the density of radiation events on a given surface, generally expressed in watts per square centimeter or square meter. If the solar panel does not have mechanical sunlight tracking capabilities, the irradiance will vary by about ±23 degrees as the sun moves throughout the year. In addition, the daily irradiance changes from horizon to horizon can cause the output power to vary throughout the day. For this reason, ON Semiconductor has developed a solar cell controller NCP1294 to implement maximum peak power point tracking (MPPT) of solar panels to charge the battery with the highest energy efficiency. This article will introduce some of the main functions of the device and the issues that need to be paid attention to when applying it.

Enhanced voltage mode PWM controller

NCP1294 is a fixed frequency voltage mode PWM feedforward controller that contains all the basic functions required for voltage mode operation. As a charging controller that supports different topologies such as buck, boost, buck-boost and flyback, the NCP1294 is optimized for high-frequency primary-side control operations, with pulse-by-pulse current limiting and bidirectional synchronization functions, supporting solar panels with a power of 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, so that the controller remains close to the maximum power point, thereby extracting the maximum amount of electricity from the solar panel and providing the best energy efficiency.

In addition, the NCP1294 also has functions such as soft start, precise control of duty cycle limit, startup current below 50 μA, overvoltage and undervoltage protection. In solar applications, the NCP1294 can be used as a flexible solution in module-level power management (MLPM) solutions. The reference design based on the NCP1294 has a maximum power point tracking error of less than 5%, and can charge four batteries in series or parallel. Figure 1 is a block diagram of the NCP1294 120 W solar controller.

As shown in Figure 1, the heart of the system is the power stage, 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 from a variety of topologies: SEPIC, non-inverting buck-boost. Flyback, single-switch forward, dual-switch forward, half-bridge, full-bridge, or other topologies.

Design work includes increasing isolation topologies based on power requirements. Management of the battery charge state is done by appropriate charging algorithms. The solar panel installer can choose the output voltage and battery charging rate. Since the controller is to be connected to the solar panel, 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 time of night and the other detects the battery's charge state so that the external circuit does not discharge the battery to the point of damage. Since the controller will be installed by field technicians with varying degrees of experience and novices, it is important that the input and output must have reverse polarity protection. Additionally, the controller and battery may be installed in an overheated or overcooled location, 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.

How Dynamic MPPT Works

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 environmental conditions in time to keep the controller close to the maximum power point. Dynamic MPPT is used when the system changes. Since each switching cycle is changing, the power drawn by the solar panel will also change significantly each cycle. Dynamic MPPT uses the voltage drop of the solar panel multiplied by the current increase in each switching cycle to determine the error signal to be generated to adjust the duty cycle. The dynamic response detects the slope of the IV curve to establish a power ramp and establishes a power representative of the duty cycle from the intersection of the error signal. The cycle ends when the slope of the ramp changes from positive to negative, as shown in Figure 2.

Feed-forward 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. Therefore, the voltage mode control line has poor regulation and is susceptible to audio. Feedforward voltage mode control is derived from the ramp signal input line. Therefore, the slope of the ramp varies with the input voltage. The feedforward function can also provide a volt-second clamp, which limits the maximum product of the input voltage and the on-time. The clamp circuit in the circuit, such as the forward and flyback converters, can be used to prevent transformer saturation.

NCP1294 Solar Charge Controller Application Design Flow

When selecting a solar charge controller topology, it is important to understand the basic operation of the converter and its limitations. The topology selected is a non-inverting four-switch non-synchronous buck-boost topology. The converter operates using the control signal from the NCP1294, and Q1 and Q2 are turned on simultaneously to charge L1. The four-switch buck-boost topology is shown in Figure 3, where the inductor is used to control the voltage and current.

The four-switch non-inverting buck-boost has two modes of operation, 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 cycle or switching frequency.

If the output voltage can reach 1% to 89%, the solar controller operates in buck mode. If the output voltage cannot be reached due to duty cycle limitations, it switches to buck-boost mode, at which time the output voltage can be reached. The change from 89% to a lower duty cycle is shown in Figure 4.

It is important to note that when the converter mode switches from buck to buck-boost, it will take some time for the error signal to change duty cycle. The instantaneous change in mode will cause the buck-boost converter to attempt to switch at 89% duty cycle and attempt to transition to 47%; this will result in the converter trying to output 130 V in the trade over region. The NCP1294 provides a pulse through the pulse current limiter, which prevents the converter from reaching dangerous levels of energy, allowing for a smooth transition in duty cycle conditions.

Compensation Network

To create a stable power supply, the compensation network around the error amplifier must be used in conjunction with the PWM generator and power stage. Since the criteria 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 feed-forward device, so a voltage loop is required that uses the input voltage to modify the ramp. The output inductor and capacitor of the power stage can form a double pole, and the loop must be compensated for this. System Turn-On and Battery Current Drain The system being created has two finite sources connected that will power the load at different times of the day and will not power at the same time except for brief periods. The system is not complete without the battery and solar panel installed, therefore, it is helpful to detect the presence or absence of the battery load and solar panel source. For example, if no battery is connected, it will not consume energy from the solar panel while providing battery voltage. If a solar panel is connected, the battery will be drained 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 drain comparator. During the daytime hours the system charges the battery, while at night the battery discharges to illuminate a defined space. While the input energy is not guaranteed, the output energy can remain constant for a considerable period of time. If a system is not sized appropriately, the battery may be damaged by discharge. To prevent battery damage, an LED circuit must be used to inhibit operation and prevent battery drain. Balancing of Input and Output Current When building an ideal solar controller, the controller should protect the battery or load while extracting the maximum energy from the solar panel. Unfortunately, in the real world a customer or installer may purchase a large solar panel and a small battery. If the solar controller is charging at peak power, the battery will charge too fast, shortening the battery life or possibly exploding. What the controller should do is manage the battery demand and balance the charging speed with the peak power provided by the solar panel. Therefore, the setting and selection of the maximum battery charging rate is required to determine how to limit the output current of the system. The current setting is done 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 case, all output diodes may be damaged. However, a diode in series with the input of B or C shown in Figure 5 can protect all devices. One disadvantage of a series diode is that it will continuously dissipate system power. If a 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, which will open a fuse when reverse voltage is applied, as shown in Figure 5 D. The fuse of choice can be a user replaceable or a Poly thermal fuse. Fuses can provide the necessary protection, but may result in a less than optimal user experience. A low-loss way to implement reverse polarity protection with a diode is to use a MOSFET that turns on when the voltage applied is of the correct polarity and turns off when the voltage is not of the correct polarity. See Figure 5.

In the third case, the output is connected with reverse polarity and the input is connected correctly, and the power components may be damaged. Since the source is assumed to be a lead-acid battery, protection is critical because a damaged component may consume a large amount of energy. Figure 5 shows one of the methods to protect against reverse output voltage.

The last case is when both the input and output are connected incorrectly. In this case, if the designer implements the second and third protections, both the input and output will be protected. Designers should not ignore voltage suppressors, which are installed at the input of transient voltages that may or may not be of the correct polarity. Therefore, it is important to have bidirectional transient suppressors that can withstand normal reverse polarity voltages without damage. Battery Charging There are three stages to charging a lead-acid battery: 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 the pulses of the NCP1294 pulse current limiting and current setting circuits. The current will remain at the charge rate set by the designer or user unless the maximum power point is below this level, at which time it will charge to the maximum power point regulation rate. OOV Comparator The NCP1294 is equipped with an OOV comparator that monitors the output battery voltage to determine if the feedback mechanism is damaged or if the remote sense is affected by the battery voltage exceeding the battery temperature compensation. The system shuts down when the OOV is disconnected. The comparator can be used at the system input or system output, but it is recommended to be used as a fail-safe mechanism for the output. When using a single-battery system, an 18V trip point can be used or the trip point can be set based on the charge state. If a floating voltage state is used, 15 V needs to be set as the trip voltage. OUV Function The NCP1294's undervoltage lockout function (OUV) function monitors the converter's input voltage to determine if the input voltage level will cause thermal problems. 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 Because the solar controller may be used in an inappropriate manner, it is recommended to monitor the temperature of the buck main switch to determine if it exceeds the maximum temperature level. If the temperature of the main MOSFET has exceeded the appropriate level, the over-temperature protection (OTP) can suppress the current to reduce system power consumption. Thermal Management The NCP1294 is a low power device. Once the IC power dissipation is determined, the designer can calculate the required thermal impedance to maintain the specified junction temperature at the worst-case ambient temperature. The thermal performance of the 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 switches operate under the recommended ambient conditions. Any power supply design should be properly tested in the laboratory to ensure that the required power dissipation is designed under the 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 MOSFET RDSON). Solar Panels The NCP1294 evaluation board supports solar panels between 5 W and 120 W. Industry-standard types of solar panels are considered here. The most common type of solar cell is crystalline silicon, which comes in two main types: monocrystalline and multicrystalline. Monocrystalline silicon has the highest energy efficiency, but is also more expensive to produce and is usually limited to commercial and residential applications. Amorphous solar panels consist of a thin film of molten silicon coated on stainless steel or similar materials. The crystalline structure is very fragile and is usually sandwiched between two sheets of glass for protection. The efficiency of monocrystalline silicon is 18%, polycrystalline silicon is 15%, and amorphous silicon is 10%.

With 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 usage experience brought by advanced semiconductor technology.