Design of photovoltaic grid-connected inverter control based on SG3525

1 Introduction

In the 21st century, mankind will face major challenges in achieving sustainable economic and social development. Under the dual constraints of limited resources and environmental protection, energy issues will become more prominent, which are mainly reflected in: ① Energy shortage; ② Environmental pollution; ③ Greenhouse effect. Therefore, when solving energy problems and achieving sustainable development, mankind can only rely on scientific and technological progress to develop and utilize renewable clean energy on a large scale. Solar energy has the advantages of large reserves, ubiquitous existence, economic utilization, clean and environmental protection. Therefore, the utilization of solar energy has been widely valued by people and has become an ideal alternative energy source. The 200W solar photovoltaic grid-connected inverter described in this article directly converts the DC power generated by the solar panel into 220V/50Hz industrial frequency sinusoidal AC power and outputs it to the power grid.

2 System working principle and control scheme

2.1 Principle of photovoltaic grid-connected inverter circuit

The main circuit schematic diagram of the solar photovoltaic grid-connected inverter is shown in Figure 1. In this system, the rated voltage of the solar panel output is 62V DC, which is converted into 400V DC through the DC/DC converter, and then 220V/50Hz AC is obtained after DC/AC inversion. The system ensures that the 220V/50Hz sinusoidal current output by the grid-connected inverter is synchronized with the phase voltage of the power grid.

Figure 1 Circuit block diagram

2.2 System Control Scheme

Figure 2 is the main circuit topology of the photovoltaic grid-connected inverter. This system consists of a front-stage DC/DC converter and a rear-stage DC/AC inverter. The inverter circuit of the DC/DC converter can be selected in half-bridge, full-bridge, and push-pull types. Considering the low input voltage, if the half-bridge type is used, the switch tube current will increase, while the full-bridge type will make the control complex and the switch tube power consumption increase, so a push-pull circuit is used here. The DC/DC converter consists of a push-pull inverter circuit, a high-frequency transformer, a rectifier circuit, and a filter inductor. It converts the 62V DC voltage output by the solar panel into a 400V DC voltage.

Figure 2 Main circuit topology

The main circuit of the DC/AC inverter adopts a full-bridge structure, consisting of 4 MOS tubes (with anti-parallel parasitic diodes inside the tube), which converts 400V DC power into 220V/50Hz industrial frequency AC power.

2.2.1 DC/DC Converter Control Scheme

The control block diagram of the DC/DC converter is shown in Figure 3. The control circuit is based on the integrated circuit SG3525. The two 50kHz drive signals output by SG3525 are added to the gates of the push-pull circuit switch tubes Q1 and Q2 through the gate drive circuit. In order to maintain the stability of the output voltage of the DC/DC converter, the detected output voltage is compared with the command voltage. The error voltage controls the duty cycle of the SG3525 output drive signal after passing through the PI regulator. The control circuit also has the protection function of limiting output overcurrent and overvoltage. When it is detected that the output current of the DC/DC converter is too large, SG3525 will reduce the width of the gate pulse, reduce the output voltage, and thus reduce the output current. When the output voltage is too high, the DC/DC converter will stop working. Since the push-pull circuit is prone to transformer saturation due to DC bias magnetism, the difficulty in designing the push-pull circuit lies in how to prevent the magnetic saturation of the transformer. In this circuit, in addition to paying attention to the symmetry of the circuit, a magnetic saturation detection circuit is also designed. When the current flowing through the two branches of the push-pull circuit is unbalanced, the soft start function of SG3525 will be activated, the DC/DC converter will be restarted, and the transformer will be reset.

Figure 3 Control block diagram of DC/DC converter

The bias magnetization detection circuit is shown in Figure 4. Only the secondary side of the magnetic ring is drawn in the figure. The two primary coils are connected to the two windings of the primary side of the transformer of the main circuit, and the current flowing through the two coils should be in opposite directions. When the transformer is biased, the current in a certain direction is abnormally large. Through the detection of the current transformer, a voltage can be generated on the output resistor R1 of the transformer. If the voltage is large enough, the voltage-stabilizing diode D5 can be turned on, and a voltage drop can be generated on the potentiometer. The value of the potentiometer is adjusted to a suitable resistance value, so that the voltage drop on the potentiometer is greater than the threshold voltage of the transistor, so that the transistor is turned on, and the capacitor connected between pin 8 of the chip SG3525 and the ground is discharged, and then the constant current source in the SG3525 charges it, and the SG3525 is restarted, thereby resetting the magnetic core of the transformer.

Figure 4 Bias detection circuit

2.2.2 DC/AC inverter control scheme

The DC/AC inverter is the key and difficulty of photovoltaic grid connection, so the following will focus on this part. The DC/AC inverter control block diagram is shown in Figure 5. The core control chip uses TI's TMS320F240. Although the microcontroller can also realize the pulse width modulation of the grid-connected inverter, the DSP has a stronger real-time processing capability, so it can ensure that the system has a higher switching operating frequency. From Figure 5, we can clearly see the input and output signals of the system.

Figure 5 Control block diagram of DC/AC inverter

2.3 Output power optimization control scheme

In static conditions, when the grid-connected inverter is connected to the solar cell, the grid-connected inverter can be equivalent to the load resistance of the solar cell. When the light intensity λ and temperature T change, the terminal voltage output by the solar cell will change accordingly. In order to effectively utilize solar energy, the output of the solar cell should always be at an appropriate operating point. Therefore, the control scheme requires that when the voltage of the solar cell increases, its output power can be increased; otherwise, its output power should be reduced.

The control scheme of DSP is shown in Figure 6. After the reference voltage is compared with the actual voltage of the solar cell, the error is adjusted by PI. The current command (DC value) IREF is multiplied by the sine table value in the ROM to obtain the alternating output current command iref. After comparing it with the actual output current value, the error is passed through the proportional (P) link, and the obtained command is inverted. After adding it to the collected AC side voltage Us, the obtained waveform is compared with the triangular wave to generate 4-way PWM modulation signals (the frequency of the triangular wave is 20kHz).

Figure 6 DSP control scheme