How Does a Solar Charge Controller Work?

The first solar charge controller schematic below (Figure 1) illustrates how a solar charge controller is connected to power a direct current (DC) load, and the second one (Figure 2) pertains to an alternating current (AC) load.

Figure1: Off-grid Diagram with DC Load

When installing a solar charge controller, it is recommended that you connect and disconnect in the following order:

1. Battery to the controller first

2. PV array to the controller

3. Electrical load to the controller

When disconnecting, you reverse that order. The battery provides power to the controller so always make sure that solar and loads are disconnected before connecting or disconnecting the battery from the controller. Connections between the battery, load, PV array, and the controller should have disconnect switches to enhance safety and facilitate ease of installation and breakdown.

In the wire diagram schematic above with DC load, sunlight contacts the solar modules, which convert solar into DC electrical power that it delivers to a charge controller. The charge controller regulates the amperage and voltage that is delivered to the loads and any excess power is delivered to the battery system so the batteries maintain their state of charge without getting overcharged. During the evening when there is no sunlight, battery power is used to run the load.

You’ll notice that the battery is grounded at the negative battery terminal. This is because all our PWM and MPPT controllers have a common negative ground. Therefore, it is possible to establish a common negative ground for the entire system: the solar array, controller, battery, and load. This meets NEC code requirements for grounding. If you need an equipment ground for any metal parts on a controller enclosure, some of our controllers include an equipment ground terminal lug. Otherwise, for our controllers that don’t have this terminal lug, you can connect an equipment ground directly to the controller enclosure.

The next diagram (Figure 2) depicts the components and connections to power an AC load. This diagram with an AC load looks similar to the previous example with a DC load, except that in this example, we have added an inverter to the system. The purpose of the inverter is to convert the DC power from the battery to AC power that can be used to run an AC load like the TV you see in the schematic.

Figure 2: Off-grid Diagram with AC Load

It’s important to note that the inverter is connected and powered from the battery, not the controller’s load terminals like we did in the DC load example. That’s because the inverter can have a high energy surge upon startup, and this high current surge might be higher than the rated capacity of the charge controller, whereas the batteries will be able to meet the high energy surge requirement.

Products and reference designs of Solar charge controller

Input enable power stage

Power from the solar module needs to be able to disconnect under various conditions, including no load and full battery, and running the load from just the battery, or when there is insufficient voltage able to be drawn from the solar module. A MOSFET with dedicated driving circuitry provides a safe and reliable method of disconnecting this input. Input enable power stage includes Low-side drivers、N-channel MOSFETs, the corresponding products are UCC27517 、CSD18532Q5B

Input current & voltage sense

Accurate and high speed sensory data of the various power busses in the solar charge controller is required to effectively control the power electronics and ensure operational efficiency and safety. High bias voltage sensory techniques are routinely used due to the high voltages involved. Input current & voltage sense includes General-purpose op amps、Precision op amps (Vos<1mV)、Analog current-sense amplifiers、Comparators、Shunt voltage references、Series voltage references、Signal conditioners, the corresponding products are TLV9002 、SM73307 、INA281 、LM2903 、TL431 、REF3425 、FDC2212

MPPT power stage

The core function of the solar charge controller is the efficient transfer of power from a solar module to a battery or load. The MPPT stage of the controller must be able to track the maximum power point of the module while maintaining the output voltage appropriate for the load and battery charge system. A buck topology is typically sufficient to support this architecture. MPPT power stage includes Isolated gate drivers、Half-bridge drivers、Low-side drivers、Isolated DC/DC converters & modules、N-channel MOSFETs、Inverting buffers & drivers, the corresponding products are UCC5320 、LM5109A 、UCC27517 、LM5017 、CSD18531Q5A 、SN74LVC1G04

Charge current & voltage sense

Accurate and high speed sensory data of the various power busses in the solar charge controller is required to effectively control the power electronics and ensure operational efficiency and safety. High bias voltage sensory techniques are routinely used due to the high voltages involved. Charge current & voltage sense includes General-purpose op amps、Precision op amps (Vos<1mV)、Analog current-sense amplifiers、Comparators、Shunt voltage references、Series voltage references、Signal conditioners, the corresponding products are TLV9002 、SM73307 、INA138、LM393 、LM4040、REF3425 、FDC2212

Output power relay

The load output of the charge controller should only be enabled when there is sufficient battery voltage or MPPT output voltage to support it. A MOSFET with dedicated driving circuitry provides a safe and reliable method of disconnecting this input. Output power relay includes Low-side drivers、N-channel MOSFETs, the corresponding products are UCC27517 、CSD18512Q5B

Output current sense

Accurate and high speed sensory data of the various power busses in the solar charge controller is required to effectively control the power electronics and ensure operational efficiency and safety. High bias voltage sensory techniques are routinely used due to the high voltages involved. Output current sense includes Analog current-sense amplifiers、Analog current-sense amplifiers with integrated shunt resistor, the corresponding products are LMP8601 、INA253

Non-isolated DC/DC converter

The internal electronics of a solar inverter require a number of separate voltage rails. MCUs and other digital controllers will typically run at 3.3V, some digital processors operate at however voltages like 1.2V, analog circuitry will need accurate low noise reference voltages, and power FETs and relays can require anywhere from 5-15V to turn on. Non-isolated DC/DC converter includes Linear & low-dropout (LDO) regulators、Buck converters (integrated switch)、Boost converters (integrated switch)、Supervisor & reset ICs、Charge pumps (inductorless), the corresponding products are TPS7A25 、TPS7A52 、LMR33610 、TPS61020 、TPS3306 、LM27762

MPPT/charger digital controller

The main power stage in a solar charge controller is controlled by a DSP. Using a DSP based solution, power can be more efficiently captured from the solar module, and battery charge profiles can be accurately mapped. Power flow can also be monitored and controlled via the DSP depending on the current system operation state. MPPT/charger digital controller includes C2000 real-time microcontrollers、MSP430 microcontrollers, the corresponding products are TMS320F28035 、MSP430F5132

Battery input protection

As the battery is typically a user serviceable item in a solar charge controller, protection schemes should be in place to prevent accidental reverse polarity connections from damaging the internal components of the controller, or worse, causing a catastrophic battery failure. Battery input protection includes Low-side drivers、N-channel MOSFETs, the corresponding products are UCC27517、CSD18533Q5A

Wired interface

Several wired interfaces may be used for a variety of use cases. RS-485 to connect to external meters and management systems and USB for configuration are just the start. Each interface also needs to comply with IEC ratings, and may additionally require isolation for common mode rejection. Wired interface includes RS-232 transceivers、RS-485 & RS-422 transceivers, the corresponding products are TRS3243E 、SN65HVD3082E

User interface

While there is little interaction required by an owner of a charge controller, there are some important pieces of data that need to be communicated. Onboard LCDs and status lights can give quick system status outputs, and help in installation and setup of the charge controller. User interface includes LCD & OLED display power & drivers, the corresponding product is TPS65130

Environmental monitor

A temperature sensor is required to monitor the system power FETs as they are prone to self-heating while under load. This temperature sensor can be used as a simple thermal switch, or provide the information back to the DSP so the inverter can throttle back its power output rather than just shut off. Environmental monitor includes Analog temperature sensors、Humidity sensors, the corresponding products are LMT70A、HDC3022

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