Design of photovoltaic power generation inverter circuit based on IGBT

Abstract: In order to meet the requirements of high-voltage and large-capacity inverter systems, a photovoltaic power generation system using an insulated gate bipolar transistor IGBT to form an inverter circuit is designed. Through a simple understanding of the principle of solar photovoltaic power generation, the inverter circuit composed of field effect tube MOSFET and insulated gate bipolar transistor IGBT is compared, and improvement schemes are proposed for the important links in the inverter circuit composed of IGBT to optimize the circuit design. In the end, it not only meets the requirements of high-voltage and large-capacity systems, but also improves the working efficiency of the entire system, so that the entire system reaches the optimal state.
Keywords: solar energy; photovoltaic power generation; inverter circuit; insulated gate bipolar transistor

Most photovoltaic power generation systems at home and abroad use an inverter circuit composed of a power field effect tube MOSFET. However, as the voltage increases, the on-state resistance of MOSFET will also increase. In some high-voltage and large-capacity systems, MOSFET will have the disadvantage of increasing switching losses due to its excessive on-state resistance. In contrast, the insulated gate bipolar transistor IGBT has a large on-state current, a relatively high forward and reverse configuration voltage, and is controlled to be turned on or off by voltage. These characteristics make IGBT more advantageous in medium and high voltage capacity systems. Therefore, using IGBT to form the switching device of the key circuit of solar photovoltaic power generation helps to reduce unnecessary losses of the entire system and achieve the best working state.

1 Principle
1.1 System Structure
The essence of solar photovoltaic power generation is that under the irradiation of sunlight, the solar cell array (i.e. PV module array) converts solar energy into electrical energy, and the output DC power is converted into AC power that can be used by users after passing through the inverter. The schematic diagram is shown in Figure 1.

[page] The inverter is a key component in the solar photovoltaic power generation system because it is a necessary process to convert direct current into alternating current that can be used by users, and it is the only way for solar energy and users to connect. Therefore, to study the process of solar photovoltaic power generation, it is necessary to focus on the inverter circuit. As shown in Figure 2 (a), it is a relatively simple push-pull inverter circuit composed of power field effect tube MOSFET. The neutral tap of the transformer is connected to the positive pole of the power supply, one end of the MOSFET is connected to the negative pole of the power supply, and the power field effect tubes Q1 and Q2 work alternately to finally output AC power. However, the disadvantage of this circuit is that the ability to carry inductive loads is poor, and the efficiency of the transformer is also low, so there are some conditions for its application. The full-bridge inverter circuit composed of insulated gate bipolar transistors IGBT is shown in Figure 2 (b). The phase difference between Q1 and Q2 is 180°, and the value of its output AC voltage changes with the output of Q1 and Q2. Q3 and Q4 are turned on at the same time to form a freewheeling loop, so the waveform of the output voltage will not be affected by the inductive load, thus overcoming the shortcomings of the push-pull inverter circuit composed of MOSFET. Therefore, the full-bridge inverter circuit composed of IGBT is more widely used.

The specific implementation is as follows: Rg2 and VD1 are connected in series and then in parallel with Rg1. When the IGBT is turned on, the positive voltage is output from the 3rd pin of EXB841 inside the driving circuit, VD1 is turned on, and Rg1 and Rg2 work together. Because the total resistance after parallel connection is less than the branch resistance of each branch, the value of the total resistance Rg in series on the gate is smaller than the value of Rg1 and Rg2. In this way, the switching time and switching loss are reduced as the total resistance value decreases, thereby reducing the driving loss. When the IGBT is turned off, the 5th pin of EXB841 inside the driving circuit is turned on, and the 3rd pin is not turned on. The emitter of the IGBT provides a negative voltage, so that VD1 in series with Rg2 is turned off, Rg1 works, and Rg2 does not work. At this time, the value of the total resistance Rg in series on the gate is the resistance value of Rg1. In this way, when the IGBT is turned off, the device will not be mis-conducted due to the small resistance between the gates, thereby improving the working efficiency.
2.2 Soft Switching
Soft switching technology is used to address switching losses. Soft switching technology is relative to hard switching. The traditional switching method is called hard switching. The so-called soft switching technology is that the semiconductor switch is turned on or off for a very short time, so that the current flowing through the switch or the voltage applied to the switch is very small, almost zero, thereby reducing the switching loss. In essence, it is to reduce the volume and weight of the transformer and filter by increasing the switching frequency, thereby greatly improving the power density of the converter, reducing the audio noise of the switching power supply, and thus reducing the switching loss.

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Q1~Q4 are 4 IGBT power switch tubes, among which Q1 and Q3 are leading arms, Q2 and Q4 are lagging arms, Q1 and Q3 are one phase ahead of Q2 and Q4. When Q1 and Q4 are turned off and Q2 and Q3 are turned on, the voltage across UAB is equal to the voltage across V1, and capacitor C1 is charged by the power supply voltage V1. When Q3 is turned from on to off, capacitor C3 is charged, and inductor L1 releases energy, causing capacitor C1 to resonantly discharge until the voltage on capacitor C1 is zero, so that Q1 has the conditions for zero voltage conduction. Similarly, the zero voltage conduction principle of leading arm Q3 can be known. When Q1 and Q4 are turned on and Q2 and Q3 are turned off, the voltage across AB is equal to the voltage across V1, and capacitor C3 is in a charging state. When Q1 and Q4 are continuously turned on, inductor L2 resonates with capacitor C8, so capacitor C8 is charged. When Q1 switches from on to off, capacitor C1 is charged, causing C3 to start discharging, and the voltage across AB decreases, causing C8 to resonantly discharge. C8 continues to discharge, and finally causes diode D7 to continue to flow, and the drive pulse of Q4 continues to drop until it reaches zero, and finally completes the zero-current shutdown of Q4. Similarly, the zero-current shutdown principle of the lagging arm Q2 can be known.
Therefore, it can be said that the leading arms Q1 and Q3 complete zero-voltage turn-on and turn-off through parallel capacitors C1 and C3, respectively, thereby reducing switching losses; the lagging arms Q2 and Q4 discharge C8 in the auxiliary circuit, reducing the current flowing through the primary side of the transformer to zero, and then completing zero-current turn-on and turn-off.

3 Experimental results
Based on the above analysis, the experimental results are shown in Figure 5.

Generally, the circuit waveform close to the square wave part indicates that its output contains more harmonic components, which will cause unnecessary additional losses in the system. As shown in Figure 5, the improved circuit using IGBT has a waveform very close to a sine wave. The total harmonic distortion of an ideal sine wave is zero, but it is difficult to achieve such a level in real life. Therefore, the requirements are basically met. At the same time, since the PIC16F873 microcontroller has multiple PWM generators and has better output sine waves, the feasibility of the experiment is verified and the expected effect is achieved.

4 Conclusion
Through the comparison and analysis of the devices, the improvement and optimization of the circuit, the integrated circuit EXB841 itself contains an overcurrent protection circuit, which solves the requirements of the insulated gate bipolar transistor IGBT for the drive circuit part, and reduces the design of the external circuit, making the entire design process simple and convenient. The soft switching technology solves the problem of excessive current and voltage on the IGBT when it is turned on and off. Finally, the driving loss and switching loss of the entire system are greatly reduced, and the output waveform is a relatively stable sine wave, thereby improving the working efficiency of the entire system.