ON Semiconductor NCP1294 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 panels), a 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 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 [page]

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