How does a Single Phase String Inverter work?

Single phase string inverters are an essential component of many solar energy systems. They are responsible for converting the direct current (DC) generated by solar panels into alternating current (AC) that can be used to power homes and businesses. In this article, we will discuss the basics of single phase string inverters, how they work, and their advantages and disadvantages.

What is a Single Phase String Inverter?

A single phase string inverter is a type of inverter that is used in small to medium-sized solar systems. It is designed to convert the DC power generated by solar panels into AC power that can be used to power homes or businesses. The name “string inverter” comes from the fact that it is typically used to convert the power from a string of solar panels, rather than just a single panel.

How Does a Single Phase String Inverter Work?

Single phase string inverters work by using a process called pulse-width modulation (PWM). In PWM, the DC power from the solar panels is chopped up into a series of pulses. The inverter then adjusts the width of these pulses to control the amount of power that is delivered to the grid.

Single phase string inverters typically have two stages: the DC-DC converter and the DC-AC inverter. The DC-DC converter is responsible for converting the high-voltage DC power from the solar panels to a lower voltage DC power that can be used by the inverter. The DC-AC inverter then converts this DC power into AC power that can be used to power homes or businesses.

Advantages of Single Phase String Inverters

One of the main advantages of single phase string inverters is their affordability. They are typically less expensive than other types of inverters, such as microinverters or central inverters. This makes them an excellent choice for small to medium-sized solar systems.

Single phase string inverters are also relatively simple to install and maintain. They do not require any special wiring or equipment, and they can be easily replaced if they fail.

Disadvantages of Single Phase String Inverters

One of the main disadvantages of single phase string inverters is that they are not as efficient as other types of inverters. Because they convert the power from a string of solar panels, they are susceptible to shading and other issues that can cause the output of the entire string to decrease.

Single phase string inverters are also limited in the amount of power they can handle. They are typically only used in small to medium-sized solar systems, and they cannot be used in large commercial or utility-scale systems.

Products and reference designs of Single phase string inverter

Input current & voltage sense

Accurate and high speed sensory data of the various power busses in the solar inverter is required to effectively control the power electronics and ensure operational safety. Isolated sensory techniques are routinely used due to the high voltages involved. Input current & voltage sense includes Hall-effect current sensors、General-purpose op amps、Precision op amps (Vos<1mV)、Analog current-sense amplifiers、Isolated amplifiers、Isolated ADCs, the Corresponding products are TMCS1126、LM2902B、TLV2376、INA213、AMC1100、AMC1304M25

DC/DC power stage

The first power stage in an inverter is used to generate a stable internal DC voltage suitable for the operation of the inversion stage. FETs must be sized appropriately, and matching dedicated gate drivers are used to ensure proper driving and operational efficiency. DC/DC power stage includes Isolated gate drivers、Low-side drivers、N-channel MOSFETs、Gallium nitride (GaN) ICs、Half-bridge drivers、Analog temperature sensors、Digital temperature sensors, the Corresponding products are UCC23514、UCC27524、CSD19505KCS、LMG3522R030、TPS28225、LMT87、LMT01

Bus current & voltage sense

Accurate and high speed sensory data of the various power busses in the solar inverter is required to effectively control the power electronics and ensure operational safety. Isolated sensory techniques are routinely used due to the high voltages involved. Bus current & voltage sense includes Hall-effect current sensors、General-purpose op amps、Precision op amps (Vos<1mV)、Isolated amplifiers、Isolated ADCs, the Corresponding products are TMCS1123、LM2902B、OPA2192、AMC1200、AMC1304M25

DC/AC power stage

The regulated output of the DC/DC stage of the inverter feeds the actual inverter power stage. The switching characteristics and output filter of this stage lead to different implementations and selections of the gate driver. DC/AC power stage includes Gallium nitride (GaN) ICs、Isolated gate drivers、Half-bridge drivers、Analog temperature sensors、Digital temperature sensors, the Corresponding products are LMG3522R030、UCC23514、TPS28225、TMP235、LMT01

Output current & voltage sense

Accurate and high speed sensory data of the various power busses in the solar inverter is required to effectively control the power electronics and ensure operational safety. Isolated sensory techniques are routinely used due to the high voltages involved. Output current & voltage sense includes Hall-effect current sensors、General-purpose op amps、Precision op amps (Vos<1mV)、Isolated amplifiers、Isolated ADCs, the Corresponding products are TMCS1126、LM2902B、OPA2192、AMC1200、AMC1304M25

Relay control power stage

A mechanical relay is needed to connect or disconnect the outputs of the inverter to the grid. Relay drivers ensure fast turn-on/off while maintaining low hold current when active and support fault diagnostics. Relay control power stage includes Solenoid drivers, the Corresponding product is DRV110

DC/DC logic

Glue logic is used to connect the power DSP to the gate drivers in order to correctly set logic voltage levels, active high or low settings, and serve as a buffer for multiple GPIO outputs. DC/DC logic includes AND gates、Noninverting buffers & drivers、Inverting buffers & drivers, the Corresponding products are SN74LV21A、SN74LVC1G34、SN74AHCT244、SN74HC05

DC/AC logic

Glue logic is used to connect the power DSP to the gate drivers in order to correctly set logic voltage levels, active high or low settings, and serve as a buffer for multiple GPIO outputs. DC/AC logic includes AND gates、Noninverting buffers & drivers、Inverting buffers & drivers, the Corresponding products are SN74ACT08、SN74LVC1G34、SN74LVC2G14

Self-diagnostics/monitoring

String Inverter require to monitor parasitic resistance from the HV-String inputs to protective earth (PE). If a certain parasitic isolation is reached the system needs to shut down. Another diagnostics feature is Arc Fault Detecting caused by arcing between PV panels or strings of PV panels. If arcing is detected the inverter needs to shut down. Self-diagnostics/monitoring includes General-purpose op amps、Precision op amps (Vos<1mV)、Isolated amplifiers、Solid-state relays, the Corresponding products are LM2902B、TLV2376、AMC3330、TPSI2140-Q1

Analog front end

To ensure tight control loops on the DC/DC and DC/AC stages, the DSP requires high accuracy data from the various analog sensors across the system. Dedicated analog filters and amplifiers help reduce noise and properly scale signals regardless of the ADC used. Analog front end includes Precision ADCs、Shunt voltage references, the Corresponding products are ADS8363、TLVH431B

Digital processing

Both main power stages in a solar inverter are controlled by a DSP. Using a DSP-based solution, power can be more efficiently captured from the solar array and output harmonics can be much lower. Dedicated DSPs for the application will have sufficient MIPS or accelerators, as well as performance for a communication layer. Digital processing includes Supervisor & reset ICs、C2000 real-time microcontrollers、Direction-controlled voltage translators, the Corresponding products are TPS386000、TMS320F28P650DK、SN74LVC4245A

Signal isolation

As working voltages continue to rise, so does the need for safety isolation. Any port that is human accessible on a piece of industrial equipment needs to meet certain safety ratings. In addition, communication between several pieces of equipment will need to be functionally isolated to guarantee immunity against common mode transients. Signal isolation includes Digital isolators, the Corresponding products are ISO6721、ISO7720

Wired interface

Several wired interfaces may be used for a variety of use cases. RS-485 to connect to external meters and building management systems, USB for configuration, and Ethernet to enable IP based communication 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-485 & RS-422 transceivers、Ethernet PHYs、RS-232 transceivers、ESD protection diodes、TVS diodes、Isolated RS-485 transceivers、Isolated CAN transceivers、High-speed op amps (GBW ≥ 50 MHz)、Line drivers, the Corresponding products are THVD1400、DP83826E、TRS3232E、TPD1E01B04、TVS0500、ISO1410、ISO1050、OPA2607、THS6222

High side isolated DC/DC power supply

The array input voltage is nominally in the 400 V range, and can be from very low voltages when there is low luminosity, or up to 600 V in some HV inverters. To support this, a wide input, HV, isolated DCDC converter is required to generate the internal DC voltages for the system. High side isolated DC/DC power supply includes PWM controllers、Flyback controllers, the Corresponding products are UCC28C54、UCC28881

Low side isolated DC/DC power supply

In solar inverters the switching devices are isolated from the control domain. Isolated gate drivers are used for that. For this an isolated DC/DC supply is necessary, which converts typical <24 V to an isolated unipolar (i.e. 0 V ... 20 V) or bipolar supply (i.e. -5 V ...15 V). Low side isolated DC/DC power supply includes Isolated DC/DC converters & modules、Buck converters (integrated switch)、PWM controllers、Transformer drivers, the Corresponding products are LM25180、DCH010505S、LMQ66430、UCC28C54、SN6501

Output user interface

While there is little interaction required by an owner of a solar inverter, 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 inverter. Output user interface includes RGB LED drivers、LCD & OLED display power & drivers、LED display drivers, the Corresponding products are TLC59108、TPS65130、TLC5928

Isolated AC/DC power supply

For safety, redundancy, and simple operation reasons, solar inverters will typically be able to power themselves from either the solar array or the grid itself. Both of these supplies are used to generate an internal primary power rail, which other DC/DC converts can use. As such, both of the main converters needs to be capable to fully powering all of the electronics inside the system. Isolated AC/DC power supply includes Flyback controllers, the Corresponding products are UCC28742、UCC28700

Non-isolated DC/DC power supply

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 power supply includes Linear & low-dropout (LDO) regulators、Buck converters (integrated switch), the Corresponding products are TPS562203、TPS562206

Wireless interface

Most modern residential solar inverters include a communication option to enable remote monitoring and control of the owner's system. In order to simplify installation, Wi-Fi is a popular option. Modern wireless SoCs can enable quick development and deployment of compliant 802.11 stacks, and offer multiple paths to FCC certification. Wireless interface includes Wi-Fi products、Low-power 2.4-GHz products, the Corresponding products are WL1837MOD、CC2642R

Conclusion

Single phase string inverters are a simple and affordable way to convert the DC power generated by solar panels into AC power that can be used to power homes and businesses. They are an excellent choice for small to medium-sized solar systems, but they are not as efficient as other types of inverters and are limited in the amount of power they can handle. Before choosing a single phase string inverter, it is important to consider the size of your solar system and your energy needs.