Design of vehicle-mounted sinusoidal inverter power supply based on SG3525

With the development of society, cars are more and more closely related to people's lives, but the DC voltage used in cars is generally 12V, which cannot be directly used by portable electronic devices. For this reason, the development of car power supply (that is, converting DC 12V voltage into AC 220V/50Hz power supply) has attracted more and more attention.

Traditional on-board power supplies generally use inverters with power frequency transformers, which have defects such as large size and low efficiency. With the development of new power electronic devices and power electronic technologies, the use of high-frequency links to achieve inverter circuits without power frequency transformers can effectively solve the problems of traditional on-board power supplies and ensure that the output voltage of the on-board power supply is more stable and smoother.

1 Analysis of the structure and function of vehicle power supply circuit

The vehicle power supply system is shown in Figure 1. The 12V DC voltage is converted by high-frequency inversion and high-frequency rectification to obtain a DC voltage that meets the requirements: 350V. The control signal of this part is generated by the TL494 chip.

Figure 1 Vehicle power system structure

After passing through the full-bridge DC/AC inverter circuit, a 220V/50Hz AC voltage output is obtained. In order to ensure the reliable operation of the system and prevent the main circuit from interfering with the control circuit, the main and control circuits are completely isolated, that is, the drive signal is isolated by an optical coupler, the feedback signal is isolated by a transformer, and the auxiliary power supply is isolated by a transformer.

For the whole system, whether the inverter circuit can work normally determines whether the whole system can work normally. Therefore, the design focuses on the control and detection of the inverter.

1.1 SG3525 block diagram and pin functions

The system uses SG3525 to realize the output of SPWM control signal. The pins and internal block diagram of the chip are shown in Figure 2.

Figure 2 SG3525 pinout and internal block diagram

After the DC power supply Vs is connected from pin 15, it is divided into two paths, one is added to the NOR gate; the other is sent to the input end of the reference voltage regulator to generate a stable +5V reference voltage. The +5V is then sent to other components of the internal (or external) circuit as power supply.

The oscillator pin 5 must be connected to an external capacitor CT, and the pin 6 must be connected to an external resistor RT. The oscillator frequency f is determined by the external resistor RT and capacitor CT, f=1.18/RTCT. The inverter bridge switching frequency is set to 10kHz, CT=0.22m, RT=5kΩ.

One path is sent to the bistable trigger and two NOR gates in the form of clock pulses; the other path is sent to the non-inverting input of the comparator in the form of sawtooth waves, and the inverting input of the comparator is connected to the output of the error amplifier. The output of the error amplifier is compared with the sawtooth voltage in the comparator, and a square wave pulse with a width that changes with the output voltage of the error amplifier is output, and then this square wave pulse is sent to one input of the NOR gate. The other two inputs of the NOR gate are the bistable trigger and the oscillator sawtooth wave. The two outputs of the bistable trigger are complementary, and they output high and low levels alternately, sending the PWM pulse to the base of the transistors V1 and V2. The role of the sawtooth wave is to add dead time to ensure that V1 and V2 are not turned on at the same time. Finally, V1 and V2 output PWM waves with a phase difference of 180° respectively.

1.2 Generation of SPWM modulation signal

To get the output of sinusoidal voltage, the control signal of the inverter circuit should control the switch of the power tube in SPWM mode, and the obtained pulse square wave output can get the sinusoidal output voltage after filtering. To achieve the output of sinusoidal voltage through SG3525, the SPWM modulation signal must be obtained first. To get the SPWM modulation signal, there must be a steamed bun wave with an amplitude of 1 to 3.5V and a sinusoidal change. Add it to SG3525 pin 2 and compare it with the sawtooth wave to get the sinusoidal pulse width modulation wave.

The block diagram of the control circuit for implementing SPWM is shown in Figure 3(a), and the waveforms of each point in the actual circuit are shown in Figure 3(b).

(a) SPWM control circuit block diagram

(b) Waveforms of the main nodes of the SPWM circuit

Figure 3 Control circuit block diagram and waveforms at each point

As shown in Figure 3, the reference 50Hz square wave is generated by the 555 chip, which is used to control the error signal generated by comparing the output voltage effective value with the reference value, converting it into a 50Hz square wave, and then obtaining a sinusoidal control signal after low-frequency filtering. When the power supply output voltage changes, the amplitude of the sinusoidal signal will change, causing the output pulse width of the SG3525 to change accordingly, which forms a closed feedback loop that can effectively stabilize the output waveform.

1.3 Overcurrent protection

Overcurrent protection uses a current transformer as a current detection element, which has a fast enough response speed to shut down the IGBT within the overcurrent time allowed by the IGBT, thus playing a protective role.

As shown in Figure 1, the overcurrent protection signal is taken from CT2, and after voltage division and filtering, it is added to the non-inverting input of the voltage comparator, as shown in Figure 4. When the overcurrent detection signal at the non-inverting input is higher than the reference level at the inverting input, the comparator outputs a high level, causing D2 to change from the original reverse bias state to forward conduction, and raising the potential at the non-inverting end to a high level, so that the voltage comparator always outputs a high level stably. At the same time, the overcurrent signal is also sent to pin 10 of SG3525. When pin 10 of SG3525 is at a high level, the pulse width modulation pulses output on its pins 11 and 14 will immediately disappear and become zero.

Figure 4 Overcurrent protection circuit

1.4 Design of drive circuit

The design of the drive circuit must consider both the ability to quickly build up the drive voltage when the power tube needs to be turned on, and the ability to quickly discharge the charge on the gate capacitor of the power tube and lower the drive voltage when it needs to be turned off. The specific drive circuit is shown in Figure 5

Figure 5 Driving circuit

Its working principle is:

1) When the driving pulse current of the control circuit flows through the primary side of the optocoupler, the optocoupler is turned on, causing the base potential of Q1 to rise rapidly, resulting in D2 being turned on, and the gate voltage of the power tube rising, causing the power tube to be turned on;

2) When there is no driving pulse current from the control circuit flowing through the primary side of the optocoupler, the optocoupler does not conduct, causing the base potential of Q1 to be pulled down, while the voltage on the gate of the power tube is still high, causing Q1 to be turned on. The gate charge of the power tube is quickly discharged through Q1 and resistor R3, causing the power tube to be quickly and reliably turned off.

Of course, the protection of the power tube is equally important, so a buffer circuit should be added between the source and drain of the power tube to prevent the power tube from being damaged by excessive positive and reverse voltages.

Experimental Results

Based on the above analysis, the experimental prototype was tested. Its rated output power was 500W, and the filter parameters were L=3mH, C=2.2m. When the prototype was running with load, its output voltage waveform was measured as shown in Figure 6.

Figure 6 Output waveform of the inverter

3 Conclusion

The prototype output voltage waveform has good quality, strong output voltage stability, and the amplitude is basically not affected by load changes, with good results. Experiments show that the system solution proposed in this paper is feasible.