The working principle of the inverter is to convert DC power into AC power through the turn-on and turn-off of power semiconductor switching devices. There are three basic circuits of single-phase inverters: push-pull, half-bridge and full-bridge. Although the circuit structures are different, the working principles are similar. Semiconductor power devices with switching characteristics are used in circuits. The control circuit periodically sends switching pulse control signals to the power devices to control each power device to be turned on and off in turn, and then after the transformer is coupled to boost or step down, the AC power that meets the requirements is shaped and filtered to output.
(1) Push-pull inverter circuit
The principle of push-pull inverter circuit is shown in Figure 1. The circuit consists of two power switch tubes connected with a common negative electrode and a boost transformer with a center tap on the primary side. The center tap of the step-up transformer is connected to the positive pole of the DC power supply, and the two power switch tubes work alternately under the action of the control circuit to output square-wave or triangular-wave alternating current. Because the common negative pole of the power switch tube is connected, the drive and control circuit of the circuit can be relatively simple, and the transformer has a certain leakage inductance, which can limit the short-circuit current, thus improving the reliability of the circuit. The disadvantage of this circuit is that the transformer has low efficiency and poor ability to carry inductive loads, so it is not suitable for occasions where the DC voltage is too high.
(2) Half-bridge inverter circuit
The principle of the half-bridge inverter circuit is shown in Figure 2. The circuit consists of two power switch tubes, two energy storage capacitors and a coupling transformer. The circuit uses the midpoint of the two series capacitors as the reference point. When the power switch VT1 is turned on under the action of the control circuit, the energy on the capacitor C1 is released through the primary side of the transformer. When the power switch VT2 is turned on, the energy on the capacitor C2 is released through the primary side of the transformer. VT1 and VT2 are turned on in turn, and AC power is obtained on the secondary side of the transformer. The half-bridge inverter circuit has a simple structure. Due to the action of two series capacitors, there will be no magnetic bias or DC component, which is very suitable for the latter stage to drive the transformer load. When the circuit works at the power frequency (50Hz or 60Hz), a larger capacitance is required, which increases the cost of the circuit, so the circuit is more suitable for use in high-frequency inverter circuits.
(3) Full-bridge inverter circuit
The principle of the full-bridge inverter circuit is shown in Figure 3. The circuit consists of 4 power switch tubes and transformers. This circuit overcomes the shortcomings of the push-pull inverter circuit. The power switch tubes Q1, Q4 and Q2, Q3 are in reverse phase, and Q1, Q3 and Q2, Q4 are turned on in turn, so that the two ends of the load get AC power. For ease of understanding, the principle of the full-bridge inverter circuit is introduced with the equivalent circuit shown in Figure 3(b). In the figure, E is the input DC voltage, R is the pure resistive load of the inverter, and switches K1~K4 are equivalent to Q1~Q4 in Figure 3(a). When the switches K1, K3 are turned on, the current flows through K1, R, K3, and the voltage polarity on the load R is left positive and right negative; when the switches K1 and K3 are turned off and K2 and K4 are turned on, the current flows through K2, R and K4, and the voltage polarity on the load R is opposite. If the two groups of switches K1, K3 and K2, K4 are alternately switched at a certain frequency, the load R can obtain an alternating voltage of this frequency.
The above-mentioned circuits are the most basic circuits of the inverter. In practical applications, in addition to the use of this single-stage (DC-AC) conversion circuit in the main circuit of the low-power photovoltaic inverter, the main circuits of medium and high power inverters adopt the circuit structure of two-level (DC-DC-AC) or three-level (DC-AC-DC-AC). Generally speaking, the DC voltage output by the battery components or square arrays of medium and small power photovoltaic systems is not too high, and the rated withstand voltage value of the power switch tube is also relatively low. Therefore, the inverter voltage is also relatively low. To obtain 220V or 380V alternating current, whether it is a push-pull or full-bridge inverter circuit, its output must be processed by a frequency step-up transformer. Due to the large size, low efficiency and heavy weight of power frequency transformers, they can only be used in low-power applications.
With the development of power electronic technology, the new photovoltaic inverter circuit adopts high frequency switching technology and soft switching technology to realize multi-level inverter with high power density. The front-stage booster circuit of this inverter circuit adopts a push-pull inverter circuit structure, but the operating frequency is above 20kHz. The booster transformer uses high-frequency magnetic materials as the iron core, so it is small in size and light in weight. The low-voltage DC power is converted into high-frequency high-voltage AC power after high-frequency inverter, and then high-voltage DC power (generally above 300V) is obtained after passing through the high-frequency rectification filter circuit, and then 220V or 380V AC power is obtained through the power frequency inverter circuit. The inverter efficiency of the entire system can reach more than 90%. At present, most of the sine wave photovoltaic inverters use this 3-level circuit structure, as shown in Figure 4. Its specific working process: first of all, the direct current (such as 24V, 48V, 110V, 220V, etc.) output by the solar cell square array is inverted into an alternating current with a square wave waveform through a high-frequency inverter circuit. The inverter frequency is generally several kilohertz to tens of kilohertz. Then it is rectified and filtered by a high-frequency step-up transformer and converted into high-voltage direct current, and finally converted into the required 220V or 380V power frequency alternating current through the third-stage DC-AC inverter.