According to the working frequency of the inverter conversion circuit, inverters are divided into power frequency inverters and medium and high frequency inverters. The power frequency inverter first inverts the DC power into a power frequency low voltage AC power, and then boosts it into a 220V/50Hz AC power for the load through a power frequency transformer. The advantage of the power frequency inverter is that the structure is simple, and various protection functions can be realized at a lower voltage. Because there is a power frequency transformer between the inverter power supply and the load, the inverter runs stably and reliably, has strong overload and impact resistance, and can suppress high-order harmonic components in the waveform. However, the power frequency transformer has the problem of being bulky and expensive, and its efficiency is low, generally not exceeding 90%. At the same time, because the iron loss of the power frequency transformer is basically unchanged during full-load and light-load operation, the no-load loss during light-load operation is relatively large, and the efficiency is also low. The high-frequency inverter first uses high-frequency DC-DC conversion technology to invert low-voltage direct current into high-frequency low-voltage alternating current, then boosts the voltage through a high-frequency transformer, and then rectifies it into a high-voltage direct current of about 360V through a high-frequency rectifier filter circuit. Finally, the power frequency AC power of 220V is obtained through the power frequency inverter circuit for the load. Because the high-frequency inverter uses small-sized and light-weight high-frequency magnetic materials, the power density of the circuit is greatly improved, the no-load loss of the inverter power supply is small, and the inverter efficiency is improved. Therefore, high-frequency inverters are the first choice for general electricity use occasions, especially for photovoltaic power generation systems with higher costs.
The basic circuit composition of the inverter is shown in Figure 1. It is composed of input circuit, output circuit, main inverter switch circuit (referred to as main inverter circuit), control circuit, auxiliary circuit, protection circuit, etc. The functions of each circuit are as follows.
(1) Input circuit
The main function of the input circuit is to provide the main inverter circuit with a DC working voltage that can ensure its normal operation.
(2) Main inverter circuit
The main inverter circuit is the core of the inverter, and its main function is to complete the inverter function by turning on and off the semiconductor switching device. Inverter circuits are divided into two categories: isolated and non-isolated.
(3) Output circuit
The output circuit is mainly to correct, compensate, and adjust the waveform, frequency, voltage, amplitude and phase of the alternating current output by the main inverter circuit, so that it can meet the needs of use.
(4) Control circuit
The control circuit mainly provides a series of control pulses for the main inverter circuit to control the on and off of the inverter switching device, and cooperates with the main inverter circuit to complete the inverter function.
(5) Auxiliary circuit
The auxiliary circuit mainly transforms the input voltage into a DC voltage suitable for the operation of the control circuit. The auxiliary circuit also contains a variety of detection circuits.
(6) Protection circuit
The protection circuit mainly includes input over-voltage, under-voltage protection, output over-voltage, under-voltage protection, overload protection, over-current and short-circuit protection, over-heat protection, etc.