Working Principle of Photovoltaic Inverter

1. Working principle of full-controlled inverter: It is a commonly used single-phase output full-bridge inverter main circuit, and the AC components use IGBT tubes Q11, Q12, Q13, and Q14. The conduction or cutoff of the IGBT tube is controlled by PWM pulse width modulation.

When the inverter circuit is connected to the DC power supply, Q11 and Q14 are turned on first, and Q1 and Q13 are turned off. Then the current is output from the positive pole of the DC power supply, and returns to the negative pole of the power supply through Q11, L or inductor, and the primary coil of the transformer (Figure 1-2), to Q14. When Q11 and Q14 are turned off, Q12 and Q13 are turned on, and the current returns to the negative pole of the power supply from the positive pole of the power supply through Q13 and the primary coil 2-1 inductor of the transformer to Q12. At this time, positive and negative alternating square waves have been formed on the primary coil of the transformer. Using high-frequency PWM control, the two pairs of IGBT tubes are alternately repeated to generate AC voltage on the transformer. Due to the action of the LC AC filter, a sinusoidal AC voltage is formed at the output end.

When Q11 and Q14 are turned off, in order to release the stored energy, diodes D11 and D12 are connected in parallel at the IGBT to return the energy to the DC power supply.

2. Working principle of semi-controlled inverter: Semi-controlled inverter uses thyristor components. Th1 and Th2 are thyristors that work alternately. If Th1 is triggered and turned on first, the current flows through Th1 through the transformer. At the same time, due to the inductive effect of the transformer, the commutation capacitor C is charged to twice the power supply voltage. Th2 is triggered and turned on. Because the anode of Th2 is reverse biased, Th1 is cut off and returns to the blocking state. In this way, Th1 and Th2 commutate, and then capacitor C is charged with reverse polarity. In this way, the thyristors are triggered alternately, and the current flows alternately to the primary of the transformer, and AC power is obtained at the secondary of the transformer.

In the circuit, the inductor L can limit the discharge current of the commutation capacitor C, prolong the discharge time, and ensure that the circuit shutdown time is greater than the thyristor shutdown time without the need for a large-capacity capacitor. D1 and D2 are two feedback diodes that can release the energy in the inductor L and send the remaining commutation energy back to the power supply to complete the energy feedback function.