Detailed explanation of solar charge controller and its design points

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 change by about ±23 degrees as the sun moves throughout the year. In addition, the daily irradiance changes from the sun moving from horizon to horizon can cause the output power to change 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

The NCP1294 is a fixed frequency voltage mode PWM feedforward controller that includes 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 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, so that the controller remains close to the maximum power point, thereby extracting the maximum amount of power from the solar panel and providing the best energy efficiency.

In addition, 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, NCP1294 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. Figure 1 is a block diagram of the NCP1294 120 W solar controller.

Figure 1: Block diagram of the ON Semiconductor 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.

The design work includes isolation topology according to the increase of power demand. The management of battery charge state is done by appropriate charging algorithm. The output voltage and battery charge rate can be selected by the technician who installs the solar panel. 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 charge state of the battery so that the external circuit does not discharge the battery to the damage point. Since the controller will be installed by field technicians with different levels of experience and novices, 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 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.

Dynamic MPPT Working Principle

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 each switching cycle is changing, the power drawn by the solar panel will also change significantly from cycle to 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.

Figure 2: Voltage and current of a PWM regulated converter

Feed-Forward Voltage Mode Control

In conventional voltage mode control, the ramp signal has a fixed rising and falling slope. The feedback signal comes only from the output voltage. Therefore, voltage mode control circuits have poor regulation and are susceptible to audio frequencies. Feedforward voltage mode control derives the ramp signal from the input circuit. 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 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 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.

Figure 3: Four-switch buck-boost topology

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 period or switching frequency.

If the output voltage can reach 1% to 89%, the solar controller is operating in buck mode. If it cannot reach that output voltage due to duty cycle limitations, it switches to buck-boost mode, where it can reach that output voltage. The change from 89% to a lower duty cycle is shown in Figure 4.

Figure 4: Transfer ratio between buck and boost modes for multiple batteries

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 try 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 can prevent the converter energy from reaching dangerous levels, 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 the 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 that modifies the ramp with the input voltage is required. 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 a brief period. The system is not complete without the battery and solar panel installed, therefore, it is beneficial to detect the presence or absence of the battery load and the solar panel source. For example, if there is no battery connected, it will not consume the energy of the solar panel while providing the 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 the battery connection is to use a low current consumption comparator.

During the day the system charges the battery and at night the battery discharges to illuminate the defined space. Although the input energy is not guaranteed, the output energy can be kept constant for a considerable period of time. If a system is not properly sized, the battery may be damaged by discharge. To prevent battery damage, the LED circuit 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 the maximum energy from the solar panel. Unfortunately, in the real world, customers or installers may purchase a large solar panel and a small battery. If the solar controller is charged 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 according to 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 completed 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-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, placing a diode in series with the input of B or C as shown in Figure 5 will 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 losses can be significant. Another way to implement reverse polarity protection is to place a diode, for example, that will cause a fuse to open when a reverse voltage is applied, as shown in Figure 5 D. The fuse of choice can be a user replaceable or 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 diode reverse polarity protection is to use a MOSFET that turns on when the applied voltage is of the correct polarity and turns off when the voltage is of the incorrect polarity. This is shown in Figure 5 E.

Figure 5: Reverse polarity input connection

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 the damaged components may consume a large amount of energy. Figure 5 B shows one of the methods to prevent reverse output voltage.

The last case is when both the input and output are not connected correctly. 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 have 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 phases of lead-acid battery charging: constant current charging or high current charging, absorption or constant voltage mode, and floating 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. Unless the maximum power point is below this level, the current will be maintained at the charge rate set by the designer or user, at which time it will be charged 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 a battery voltage that exceeds 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 trigger voltage.

OUV Function

The NCP1294’s undervoltage lockout (OUV) function monitors the converter’s input voltage to determine if the input voltage level will cause thermal issues. OUV can independently monitor the input voltage to ensure that the input voltage is at an ideal level to provide maximum output power.

OTP Function

Since 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 exceeds 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 dissipation 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 undergo appropriate laboratory testing to ensure 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 are made 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%, multicrystalline silicon is 15%, and amorphous is 10%.

Figure 6: Solar controller circuit board

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.