Research on Single-Phase Inverter Based on DSP Closed-Loop Control

Abstract: Based on DSP closed-loop control inverter, TMS320F2812 is used as the controller. The article uses PWM and SPWM generated by DSP programming to drive the high-frequency inverter bridge and the power frequency converter respectively with optocoupler isolation. At the same time, the switching loss in the inverter bridge is analyzed, and the conversion efficiency is improved by improving the algorithm. Keywords: inverter; pulse width modulation; closed-loop control; switching efficiency0 Introduction With the over-exploitation of non-renewable energy, the energy crisis is imminent, and solar power generation will become one of the main energy sources in production, life and other fields. Photovoltaic power generation, as one of the main ways to utilize solar energy, has begun to attract widespread attention. Some developed countries have taken the lead in photovoltaic power generation, and their installed capacity has reached millions of megawatts. As a country with a large population and energy demand, China still has a considerable gap in the utilization of solar energy compared with developed countries. Based on this, this paper studies the basic structure and control principle of the inverter as the core device of photovoltaic power generation. 1 Overall design of closed-loop inverter 1.1 Technical indicators The output power is 500W, the output waveform is AC sine wave, the output voltage is 220V, the positive and negative deviation is ≤5%; the frequency is 50Hz, and the positive and negative deviation shall not be >0.2Hz. 1.2 System schematic diagram

The characteristics of this inverter are: 1) There is no DC/DC boost structure at the input stage, which improves conversion efficiency and safety. 2) The control method is highly digital, which maximizes the use of DSP's high-speed processing capabilities and its integrated peripherals, reduces the physical size of the inverter, and reduces costs. 3) The control drive circuit is all freewheeling through the diode. The phase-shift control method of zero voltage conduction and zero current cutoff is adopted. 4) The secondary side adopts the center-tapped output method, which greatly improves the utilization efficiency of the high-frequency transformer.

2 Design and analysis of the main circuit of the inverter
In the main circuit of the inverter, the high-frequency drive circuit drives the high-frequency inverter bridge, and the industrial frequency drive circuit drives the industrial frequency converter. During this period, the high-frequency transformer is directly boosted, and then the standard sine wave is obtained through the LC AC filter.
In order to ensure the stability of the output voltage and prevent overload, this system is designed with overvoltage, overcurrent and other circuits, and the protection of the main circuit is realized through the above protection module. At the same time, to prevent magnetic saturation, a capacitor C0 is connected in series in the primary winding of the pulse transformer.
(1) At t=t0, K1 and K4 are turned on, and the DC voltage Ui is applied to the primary winding N1 of the high-frequency transformer T. The secondary winding N21 generates an induced voltage. The terminal with the same name is marked ".", which is positive. Its voltage amplitude is , assuming that the input current is ii. When the current of the primary winding N1 increases linearly, the current i2 in the secondary winding N21 and the filter inductor L1 also increases linearly. The increase in the current of the inductor L1 is:

(2) At t=TON, K1, K2, K3, and K4 are all turned off. At this time, the current i2 of the inductor L1 is the maximum. Within the time of TON~Ts/2, this is the dead time set for the DSP programming. The primary winding current ii of the high-frequency pulse transformer cannot change suddenly, so it is continued through D2 and D3, and the energy stored in the primary winding is fed back to the power supply. At the same time, the current i2 of the secondary winding N21 and the filter inductor L1 cannot change suddenly. According to the second theorem, the induced voltage of the secondary winding N21 maintains the original polarity, the voltage polarity of the filter inductor L1 is reversed, and the Tg of the power frequency converter is 0.02s. K1 and K2 are leading arms, and K3 and K4 are lagging arms. By controlling the conduction sequence of the leading arms K1 and K2 and the lagging arms K3 and K4, the lagging arm is turned on with a lag of θ, that is, the phase shift angle, and the inductive load RL current continues through D9. The negative half cycle is similar.

(3) According to (1) and (2), the switching frequencies fb and fg of the inverter bridge and the power frequency converter are respectively equal to fb=1/Tb and fg=1/Tg=50Hz. Due to the different switching frequencies of the inverter bridge and the power frequency converter, the proportional coefficient is K=fb/fg. The main loss occurs in the commutation process. According to formula (2):

Where: VDS is the drain-source voltage, ic is the current, τ(t) is the duty cycle, and λ is the turn-on time. The loss is related to the voltage across the switch tube, the current when it is turned off, and the duty cycle (that is, the switching frequency). The turn-on loss of the inverter bridge is:

The waveform of the complementary phase shift control of K1 and K2 measured by DSP programming is as follows:
Figure 2 shows the phase difference of the driving waveform of K1 and K2 at two instants. It can be seen that the phase shift control is a phase difference between the two bridge arms. The upper and lower tubes of each bridge arm are complementary turned on, and the output voltage is controlled by changing the duty cycle. In order to ensure zero voltage conduction, VDS=0 must be ensured. The measured waveform is as follows:

Figure 3 shows the process of K1 being turned on at zero voltage. Channel 1 is the driving voltage between GS, and channel 2 is the voltage between DS. It can be seen from the figure that when K1 is turned on, the voltage between DS is zero. That is, the soft switching characteristic of K1 is achieved, so at this time VDS=0, that is, Pon=0. At this time, the turn-on loss is zero. The situations of the other switch tubes are similar.

3 Design and analysis of inverter closed-loop control circuit
The four-phase shifted SPWM drive signals generated by programming the two full comparison units of DSP are used to drive the two complementary IGBT switches of each bridge arm respectively. The specific method is shown in Figure 4.

The specific programming method is: set the 11-12 bits of the timer control TxCON to 01, that is, select the continuous increase and decrease counting mode, and the switching frequency is 20kHz. Then when GP1 increases from 0 to point A, the count value matches the comparison value of FCMP1, so the output level of FCMP1 jumps (the drive of K4 changes from 0 to 1, and K3 changes from 1 to 0). When GP1 increases from point A to point B, the count value matches the comparison value of FCMP2, and the output level of FCMP2 jumps (the drive of K2 changes from 0 to 1, and K1 changes from 1 to 0). When the count value of GP1 increases to its set value, it starts to count down. When it decreases to points C and D, the process is similar, and the output levels of FCMP1 and FCMP2 jump respectively. At the same time, in order to achieve the purpose of closed-loop control, in the underflow interrupt and match interrupt program of GP1, it is obtained by scanning the SPWM data table pre-stored in RAM. The comparison value of the full comparison unit updates a new SPWM data within half an open cycle, and the dead time of the drive signal is set by a dedicated register. The dead time control wave plow at a certain moment is shown in Figure 6.

The SPWM logic drive signal is generated by the table lookup method. The SPWM data table is calculated by the direct method and stored in the DSP's FLASH in advance. The SPWM data table is transferred to the high-speed RAM when the program is initialized. The modulation ratio M of S-PWM is 0.5 to 0.98. According to the switching frequency of 20kHz, 32 SPWM data tables are made, each table stores 200 data, and the symmetrical rule equal area method is used, so only the equivalent pulse width of 1/4 cycle, that is, 200 small cells, is calculated. A complete sine wave can be obtained by bidirectionally scanning the data table.

The calculation formula of the data table is as follows:

Where tk is the width of the kth square wave pulse, M2 is the modulation ratio, ω is the power frequency angular frequency, and Tk is the time value of the Kth moment (K=0~199).
According to the DSP working clock 20MHz, the value of the period register of the timer is calculated to be 500. According to the following calibration formula, the value of the data table is calculated and directly stored in FLASH:

Datak is the relative trigger time value of the drive signal in the [Tk, Tk+1] interval.
The high-frequency inverter drive circuit adopts a phase shift control method. K1 and K2 form the leading arm, and K3 and K4 form the lagging arm, which are ahead of Tm respectively. The time when the switch tube is turned on is TK1, TK2, TK3, and TK4 respectively.
The power frequency converter circuit also adopts a phase shift control method. Q5 and Q6 form the leading arm, and Q7 and Q8 form the lagging arm, which are ahead of Tn respectively. The time when the switch tube is turned on is TK5, TK6, TK7, and TK8 respectively.
Since tk is a parameter controlled by the PID regulator feedback, it causes TK (K=1, 2, 3, 4, 5, 5, 6, 7, 8) to change accordingly, realizing real-time closed-loop control. The
sampled voltage and current are converted through the interface circuit and input into the A/D of the DSP, and the digital PID regulator is realized by the DSP, so that the inverter can calculate the corresponding control quantity △k according to the error signal in real time according to the change of the load. After programming the DSP, according to the size of △k, the SPWM data table corresponding to different modulation ratios is searched, thereby achieving the purpose of closed-loop control. Compare the given voltage and current with the feedback voltage and current, adjust the output SPWM pulse width to control the drive circuit. Note: Be sure to enable the REVSOCE bit in the EV extended control register in the DSP, use the periodic interrupt to start the ADC, and the system enters closed-loop control.
The algorithm of the digital PID regulator is as follows:
ek is the error value of the Kth step, uk is the control quantity of the Kth step, u0 is the initial control quantity, Ik is the integral term, and the three coefficients kp, kl, and kD are obtained by parameter setting.

At the same time, the overheating, overcurrent and other signals are detected by the sensor, and converted into corresponding square wave signals by the signal conditioning circuit, which are captured by the DSP event manager capture unit. The detected change of PDPINTx level generates INT1 interrupt, and all driving signals are terminated within 200ns. The specific control process is shown in Figure 7.

4 Experimental results
Through continuous experiments, satisfactory results were finally obtained. Figure 8 shows the results of the experimental output waveform. The corresponding sinusoidal waveform was obtained after the frequency conversion inversion and filtering circuit. The output voltage 223V is compared with the standard voltage 220V, and its deviation is +1.3% <5%; 1 The output waveform frequency is 50.08Hz, and its deviation is <0.2Hz, which meets the standard frequency requirements. After filtering, the THD is 1.8%.

5 Conclusion
By using the event manager of TMS320F2812 to implement closed-loop control of single-phase inverters, the dynamic performance is greatly improved. At the same time, the setting of DSP algorithm parameters is related to the normal operation of the entire system and is a prerequisite for effectively realizing overvoltage, overcurrent and other protections. In addition, choosing a good sensor is the key to realizing overvoltage and overcurrent protection.