Development of a simple digital controlled three-p

Development of a simple digital controlled three-phase inverter

Abstract: a new three-phase inverter is developed, a new SPWM digital control scheme is adopted, and its system structure and working principle are analyzed. The experimental results show that the output waveform meets the requirements, the implementation is simple, the cost is low, and the scheme is feasible

key words: digital control; Three phase inverter power supply; Push pull circuit

1 introduction

SPWM control mode is commonly used in inverter circuit. This method is easy to be realized by analog circuit, but the pulse stability and anti-interference ability are poor. Commercial digital control integrated chips for three-phase inverter power supply, such as hef4752, also have some problems, such as the operation speed is affected by the word length of the processor and the price is expensive. This paper introduces a three-phase inverter system that uses high-frequency switching power supply instead of transformer boost and isolation, and puts forward a new practical preset phase PWM digital control scheme. Its basic idea is to calculate the pulse width of SPWM corresponding to the switch tube in advance through simulation, convert it into hexadecimal number and store it in EPROM. After address scanning and locking, the value preset in EPROM is converted into the corresponding trigger pulse to drive the main circuit switch of the inverter. The inverter control circuit is composed of only one EPROM and several integrated chips. The hardware structure of the whole controller is simple and the cost is reduced

2 system structure and working principle

Figure 1 shows the system principle block diagram of the three-phase intermediate frequency inverter proposed in this paper. The system is organically combined by push-pull switching power supply and three-phase inverter power supply. It is completed by dc-ac-dc-ac converter. The modulation ratio of part of the inverter circuit remains unchanged, and the purpose of stabilizing the AC output voltage is achieved by adjusting the duty cycle of the push-pull circuit. Its main components include main circuit, detection and protection circuit and control circuit

Figure 1 principle block diagram of three-phase inverter

2.1 push-pull boost circuit

according to the actual characteristics of low input voltage and high current in the circuit, considering the low voltage accuracy in the computer, after comparing the advantages and disadvantages of the currently commonly used push-pull, half bridge, unpublished report type and full bridge at the 2009 spinal plasty Association meeting, the boost circuit adopts the push-pull conversion circuit, and its circuit structure is shown in Figure 2. It has the advantages of less power switching devices and simple driving circuit. The control integrated chip adopts TL494, which provides an internal reference voltage (5V 1%), and its internal error amplifier has a common mode voltage range, a wide working voltage range (8 ~ 40V), and the maximum working frequency can reach 300kHz. The dead time can be adjusted by connecting a single resistor between the CT pin and the discharge pin

Figure 2 push-pull boost circuit

assuming that the transformer transformation ratio is n2/n1, and D is the duty cycle of V1 and V2 control pulses, the output voltage Vo1 is

vo1=2dvin (n2/n1) (1)

from formula (1), as long as the duty cycle D is adjusted, the output voltage Vo1 on the DC side can be adjusted to change the AC output voltage, in which D is fed back by the voltage feedback link and sent to the feedback input terminal (pin 3) of TL494 after being processed by the P function feature ID regulator

2.2 inverter circuit

1) control circuit

this paper adopts a new digital control scheme that the equipment is used to display the pressure type preset phase PWM wave of the oil pump. The switching states of the six switch tubes of the inverter correspond to the SPWM pulse of a certain modulation ratio (the modulation ratio in this experimental circuit is 0.98). The starting point is that an EPROM stores the switching states of the six switch tubes, and the phase difference between the switch tubes at the same bridge end is 120, The upper and lower switches of the same bridge arm are complementary and have dead zones. The inverter drive circuit is shown in Figure 3

Figure 3 three phase inverter drive circuit

the value preset in EPROM is generated in many ways. It is common to first calculate the coexistence table, corresponding to different fundamental frequencies, calculate the corresponding pulse width according to a certain law, and then convert it into a value. This paper establishes the SPWM control model of the three-phase inverter circuit, uses MATLAB simulation, and obtains the pulse waveform of the control switch tube as shown in Figure 4 after M-file program plus dead zone processing

(a) corresponding switch tube VT1

(b) corresponding switch tube vt4

Figure 4 corresponding switch tube control pulse

it can be seen that the control pulses of two switch tubes VT1 and vt4 on the same bridge arm meet the complementary conduction relationship. By sampling a cycle, high level 1 corresponds to the switch on, and low level 0 corresponds to the switch off, the switching states of six switches at any time in a cycle can be corresponding to binary numbers 010110, 010100, 110100 By analogy, the switching states of the six switches at any one time can be obtained. These binary data are converted into hexadecimal and stored in EPROM. After the conversion of SPWM wave generation schematic diagram as shown in Figure 5, these data stored in EPROM can be converted into the corresponding state of the switch at any time. By modifying the matlab program, different switching frequencies can be obtained, and the modulation ratio and the output fundamental frequency can also be changed

Figure 5 SPWM wave generation schematic diagram

2) drive circuit

this paper adopts a drive circuit composed of IR2130 as the core component and its peripheral components. The circuit has simple structure, high integration, and integrates control circuit, level conversion, low impedance output and identification protection. Only one auxiliary power supply is needed, and three auxiliary power supplies are omitted from the conventional drive circuit. Several peripheral discrete components can completely connect the logic control signal of the three-phase bridge circuit with MOS gate devices

2.3 detection and protection circuit

detection circuit mainly includes DC input current detection, push-pull DC output voltage detection, AC output voltage detection, etc. According to the detection results of DC input current, whether the push-pull link and inverter link have overload and short circuit can be judged. If so, the push-pull boost circuit and inverter circuit will stop working by blocking the control signal pulse through the protection circuit; On the one hand, the AC output voltage detection circuit feeds back the voltage to the adjustment circuit of the push-pull link to adjust the output voltage of the inverter, on the other hand, it sends it to the protection circuit. When the output voltage is too high, the output voltage pulse is blocked, and the above protection signal can also be converted into the corresponding voltage signal and sent to pin 9 (iTrip end) of IR2130. When the signal is greater than its built-in 0.5V reference potential, IR2130 blocks the driving signal of the six channel switch, It plays a double protective role

3 experimental results

according to the above theory, a prototype is designed. The main technical indicators are

the battery voltage changes between 22 ~ 26V

the output AC voltage is 110V 3%

frequency is 400Hz 0.1%

output power is 500W

3.1 circuit parameters

two 2sk1506 power tubes in parallel are selected for the push-pull boost circuit, and the switching frequency is 40KHz

the output filter inductance of the inverter circuit is 1mH, the filter capacitor is 5 F, the power tube is irf840, and the switching frequency is 10kHz

3.2 experimental results

Figure 6 shows the driving waveforms of the upper and lower tubes (switch tubes VT1 and vt4) of the same bridge arm. It can be seen that the addition of dead time makes the driving waveforms of the two tubes not fully meet the complementary conduction relationship. Figure 7 shows the output voltage waveform of the three-phase (only two-phase) inverter power supply. It can be seen from the figure that its output waveform is sinusoidal and meets the predetermined performance index

Figure 6 MOSFET drive waveform (5v/div, 100 s/div)

the upper tube corresponds to the drive waveform of VT1

the lower tube corresponds to the drive waveform of vt4

Figure 7 inverter output voltage waveform (50v/div, 2.5ms/div)

4 Conclusion

theoretical analysis and experimental results show that this new and simple digital control scheme of three-phase inverter power supply proposed in this paper is a feasible SPWM control scheme. Because of the digital control scheme, it has strong anti-interference ability, high reliability, simple implementation, low circuit cost and high cost performance