Low-cost vehicle inverter

The power supply is the power part of electronic equipment and is a very versatile electronic product. It has been widely used in various industries and daily life. Its quality greatly affects the reliability of electronic equipment. Its conversion efficiency and load capacity are directly related to its application range. Square wave inverter is a low-cost and extremely simple conversion method. It is suitable for various rectifier loads, but it is not very adaptable to the transformer load and has a lot of noise.

Based on the basic principle of inverter power supply and the analysis and deduction of existing data, this paper proposes a method for making and debugging a square wave inverter.

1 Basic principles of the system

The input end of this inverter power supply is a battery (+12V, capacity 90A·h), and the output end is an industrial frequency square wave voltage (50Hz, 310V). Its structural block diagram is shown in Figure 1.

Figure 1 Block diagram of a square wave inverter

At present, there are many new technologies for DC/AC inversion, but considering the specific conditions of use, cost and reliability, this power supply still adopts the typical two-stage conversion, namely DC/DC conversion and DC/AC inversion. First, the DC/DC conversion inverts the DC 12V voltage into a high-frequency square wave, which is then boosted by a high-frequency step-up transformer, and then rectified and filtered to obtain a stable DC voltage of about 320V; then the DC/AC conversion inverts the stable DC voltage into a square wave voltage with an effective value slightly greater than 220V in the form of square wave inversion; and then the LC power frequency filtering obtains a 50Hz AC voltage with an effective value of 220V to drive the load.

2 DC/DC conversion

Since the primary voltage of the transformer is relatively low, in order to improve the utilization rate of the transformer and reduce the cost, the DC/DC conversion is shown in Figure 2, using a push-pull circuit, the center tap of the primary side is connected to the battery, and the two ends are controlled by switch tubes, working alternately, which can improve the conversion efficiency. The push-pull circuit uses fewer switching devices, and the size of the double-ended transformer is relatively small, which can increase the duty cycle and increase the output power.

Figure 2 DC/DC conversion structure diagram

The core area product formula of the double-ended square wave inverter transformer is:

AeAc=Po（1＋η）/（ηDKjfKeKcBm）（1）

Where: Ae (m2) is the cross-sectional area of ​​the core;

Ac (m2) is the window area of ​​the core;

Po is the output power of the transformer;

η is the conversion efficiency;

δ is the duty cycle;

K is the waveform factor;

j (A/m2) is the average current density of the conductor;

f is the inverter frequency;

Ke is the effective coefficient of the core section;

Kc is the window utilization coefficient of the core;

Bm is the maximum magnetic flux.

The switch tubes S1 and S2 on the primary side of the transformer each use IRF32055 in parallel. The reason for parallel connection is that when the inverter power supply is connected to the load, the current on the primary side of the transformer is relatively large. Parallel connection can be used for current diversion, which can effectively reduce the power consumption of the switch tube and avoid damage.

PWM control circuit chip SG3524 is a voltage-type switching power supply integrated controller with output current limiting, adjustable switching frequency, error amplification, pulse width modulation comparator and shutdown circuit. The peripheral circuit required to generate PWM square wave is very simple. When pin 11 and pin 14 are used in parallel, the duty cycle of the output pulse is 0-95%, and the pulse frequency is equal to 1/2 of the oscillator frequency. When pin 10 (shutdown end) is high, the output pulse can be blocked. If it is properly connected to the external circuit, the undervoltage and overcurrent protection functions can be realized. The duty cycle D of the output drive waveform of SG3524 is adjusted by using the operational amplifier inside SG3524 to make D>50%, and then after CD4011 is reversed, the drive waveform of the tube is obtained to be D<50%, so that the two sets of switch tubes can be driven with a common dead time.

3 DC/AC conversion

As shown in Figure 3, the DC/AC conversion adopts single-phase output and full-bridge inverter form. In order to reduce the size of the inverter power supply and reduce the cost, the output uses industrial frequency LC filtering. The bridge inverter circuit is composed of 4 IRF740s. The maximum withstand voltage of IRF740 is 400V, the current is 10A, and the power consumption is 125W. The half-bridge driver IR2110 is used to provide the drive signal, and its input waveform is provided by SG3524. Similarly, the output drive waveform of the SG3524 can be adjusted to D < 50%, ensuring that the inverter drive square wave has a common dead time.

Figure 3 DC/AC inverter circuit structure

IR2110 is a high-power MOSFET and IGBT dedicated driver integrated circuit produced by IR. It can achieve optimal drive for MOSFET and IGBT, and also has fast and complete protection functions, so it can improve the reliability of the control system and reduce the complexity of the circuit.

The internal structure and working principle block diagram of IR2110 are shown in Figure 4. In the figure, HIN and LIN are the drive pulse signal input terminals of the upper and lower power MOS in the same bridge arm of the inverter bridge. SD is the protection signal input terminal. When this pin is connected to a high level, the output signal of IR2110 is completely blocked, and its corresponding output terminal is always at a low level; when this pin is connected to a low level, the output signal of IR2110 changes with HIN and LIN. In the actual circuit, this terminal is connected to the output of the user's protection circuit. HO and LO are two drive signal output terminals that drive the MOSFET in the same bridge arm.

Figure 4 IR2110 internal structure and working principle block diagram

If the bootstrap capacitor of IR2110 is not selected properly, it is easy to cause chip damage or malfunction. The capacitor between VB and VS is the bootstrap capacitor. The bootstrap capacitor voltage must reach above 8.3V to work properly. Either use a small-capacity capacitor to increase the charging voltage, or directly provide a 10-20V isolated power supply between VB and VS. This circuit uses a 1μF bootstrap capacitor.

In order to reduce output harmonics, the DC/AC part of the inverter generally adopts bipolar modulation, that is, the pair of tubes of the inverter bridge are turned on and off at high frequency and complementarily.

4 Some problems in the design and debugging of protection circuits

The protection circuit is divided into undervoltage protection and overcurrent protection.

The undervoltage protection circuit is shown in Figure 5. It monitors the voltage of the battery. If the battery voltage is lower than the preset 10.8V, the protection circuit starts to work, causing the pin 10 shutdown terminal of the controller SG3524 to output a high level and stop the drive signal output.

Figure 5 Undervoltage protection circuit diagram

图5中运算放大器的正向输入端的电压由R1和R3分压得到，而反向输入端的电压由稳压管箝位在＋7.5V，当蓄电池的电压下降超过预定值后，运算放大器开始工作，输出跳转为负，LED灯亮，同时三级管V截止，向SG3524的SD端输出高电平，封锁IR2110的输出驱动信号。

The overcurrent protection circuit is shown in Figure 6. It monitors the output current and is preset to 1.5 A. The output current of the square wave inverter is sampled and enters the reverse input terminal of the operational amplifier. When the output current is greater than 1.5 A, the output terminal of the operational amplifier jumps to negative. After passing through the RS trigger composed of CD4011, the signal of the base of the triode V1 is low level, the triode is cut off, and a high level is output to the SD1 terminal of IR2011 to achieve the purpose of protection.

Figure 6 Overcurrent protection circuit diagram

One of the more important issues encountered during the debugging process is the selection of the bootstrap capacitor of IR2110. The upper tube driver of IR2110 is powered by an external bootstrap capacitor, which greatly reduces the number of driver power supply paths, but also has certain requirements for the selection of the bootstrap capacitor between VB and VC. After testing, a 1μF electrolytic capacitor was finally used, which can effectively meet the bootstrap voltage requirements.

5 Test results and output waveform

The DC/DC conversion output voltage is stabilized at 320V, and the switching frequency of the half-bridge driver IR2110 that controls the switch tube is 50Hz. The experimental circuit waveforms are shown in Figures 7 to 14.

Figure 7 IR2110 lower tube drive waveform

Figure 8 IR2110 upper tube drive waveform

Figure 9 SG3524 output drive waveform (DC/AC)

Figure 10 SG3524 driving waveform (DC/DC)

Figure 11 Output voltage waveform of external rectifier load

Figure 12 Output voltage waveform with external 300Ω resistor load

Figure 13 Output voltage waveform with external 500Ω resistor load

Figure 14 Output voltage waveform with external 600Ω resistor load

6 Conclusion

In the development direction of inverter power supply, light weight, small size and high efficiency are the goals it pursues. The inverter power supply circuit introduced in this article mainly adopts integrated chips, which makes the circuit structure simple, the performance stable and the cost low. Therefore, this circuit is a circuit with simple control, high reliability and good performance. Therefore, the entire inverter power supply has a high cost performance and market competitiveness.