Energy-efficient solar charging controller design example

As is well known, solar panels have an IV curve that represents the output performance of the solar panel, representing the current and voltage values, respectively. The voltage and current indicated by the intersection of the two lines is the power of the solar panel. Disadvantageously, the IV curve will vary with irradiance, temperature, and age. Irradiance is the density of a given surface radiation event and is generally expressed in watts per square centimeter or square meter. If the solar panel does not have mechanical sunlight tracking capability, the irradiance will vary by about ±23 degrees with the movement of the sun during the year. In addition, changes in the irradiance of the sun moving from the horizon to the horizon each day can cause the output power to change throughout the day. To this end, ON Semiconductor has developed a solar cell controller NCP1294 for maximum solar power point tracking (MPPT) of solar panels to charge the battery with maximum energy efficiency. This article will cover some of the key features and applications of this device.

Enhanced Voltage Mode PWM Controller

The NCP1294 is a fixed frequency voltage mode PWM feedforward 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, the NCP1294 is optimized for high-frequency primary-side control operation with pulse-by-pulse current-limit and two-way synchronization for maximum power Up to 140 W solar panels. The MPPT function of this device is able to locate the maximum power point and adjust it in real time according to the environmental conditions, so that the controller stays 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, the NCP1294 features soft-start, precise control of duty cycle limits, startup currents below 50 μA, overvoltage and undervoltage protection. In solar applications, the NCP1294 can be used as a flexible solution for module level power management (MLPM) solutions. The NCP1294-based reference design has a maximum power point tracking error of less than 5% and can charge four batteries in series or in parallel. Figure 1 is a block diagram of the NCP1294 120 W solar controller.

Figure 1: Block diagram of ON Semiconductor's NCP1294 120 W solar controller

As shown in Figure 1, the core of the system is the power section, 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, a buck-boost topology must be available to support the application. Designers can choose from a variety of topologies: SEPIC, non-inverting buck-boost. Flyback, single-switch forward, two-switch forward, half-bridge, full-bridge or other topologies.

Design work involves isolating the topology based on increased power requirements. Management of the state of charge of the battery is accomplished by an appropriate charging algorithm. The solar panel installation technician can select the output voltage and battery charging rate. Since the controller is to be connected to a solar panel, it must have maximum power point tracking to provide high value to the end customer. The controller has two enable circuits, one that detects the night time and the other that detects the state of charge of the battery so that the external circuit does not discharge the battery to the point of damage. Since the controller will be installed by field technicians and novices with varying degrees of experience, it is important that the inputs and outputs have reverse polarity protection. In addition, the controller and battery may be installed in an overheated or subcooled position and the controller must be compensated for battery charging temperature. 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 power-variable power source (ie, a solar panel), the solar controller must use MPPT. The 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 drawn by the solar panel will also change significantly during each cycle. Dynamic MPPT utilizes the voltage dip of the solar panel multiplied by the increased current per 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, creating a power ramp that establishes a power representative of the duty cycle from the intersection of the error signals. 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

Feedforward voltage mode control

In conventional voltage mode control, the ramp signal has a fixed rise and fall slope. The feedback signal is only from the output voltage. Therefore, the voltage mode control circuit has a poor voltage regulation effect and has an audio susceptibility. The 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 also provides 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 Solar Charge Controller Application Design Flow

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 with a control signal from the NCP1294, and both Q1 and Q2 are simultaneously charged to charge L1. The four-switch buck-boost topology is shown in Figure 3, where the inductor is used to control 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 an input voltage pulse that is LC filtered to produce a lower DC output voltage. The output voltage can be varied 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 operates in buck mode. If the output voltage cannot be reached due to the duty cycle limit, it switches to buck-boost mode, which is reached at this point. The change from 89% to the lower duty cycle is shown in Figure 4.

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

It should be noted that when the converter mode is switched from buck to buck-boost, the error signal will take some time to change the duty cycle. A transient change in mode will cause the buck-boost converter to attempt to switch at 89% duty cycle and attempt to switch to 47%; this will cause the converter to attempt to output a 130 V result in the trade over region. The NCP1294 provides a pulse through the pulse current limiter that prevents the converter energy from reaching dangerous levels and achieves a moderate transition under duty cycle conditions.

Compensation network

To create a stable power supply, the compensation network around the error amplifier must be used with the PWM generator and power stage. Since the power stage design criteria are set according to 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 inductor and capacitor of the power stage form a double pole and the loop must compensate for this.

System on and battery current consumption

The system being created is connected to two finite sources that will power the load at different times of the day, and will not supply power at the same time, except for a short period of time. The system is not complete, no batteries and solar panels are installed, so it is beneficial to the detection of battery load and the presence or absence of solar panel sources. For example, if a battery is not connected, it does not consume the energy of the solar panel when the battery voltage is supplied. If a solar panel is connected, the battery will be exhausted in order to find the solar panel to be connected. A simple solution for checking solar panel connections and battery connections is to use a low current consumption comparator.

The system charges the battery during the daytime, while the battery discharge illuminates the defined space during the night. Although the input energy is not guaranteed, the output energy can remain constant for a considerable period of time. If the size of a system is not suitable, the battery may be damaged by electrical discharge. To prevent damage to the battery, the LED circuit must be used to suppress operation and prevent the battery from running out.