A design principle of three-phase combined inverter based on SA4828

introduction

Power supply equipment is a kind of electronic product with large quantity, wide application and strong versatility. Power supply is used in almost all departments such as modern communication, electronic instruments, computers, industrial automation, power engineering, national defense, etc. It is also widely used in other industries and daily life. With the development of science and technology, higher requirements are also put forward for power supply equipment. In order to meet the needs of many users, the specifications and varieties of power supply are increasing. Due to the diversity, novelty and complexity of the application objects of power supply, power supply equipment should not only be of high quality, high efficiency and high reliability, but also have output characteristics that adapt to its various loads. In most cases such as centralized power supply, the inverter power supply with three-phase output will face the problem of unbalanced three-phase load, and the serious problem is that the load is 100% unbalanced. Even if the power supply is installed and used, the three-phase load is distributed as much as possible to achieve balance, but it is impossible to ensure that the three-phase load is turned on and off at the same time during the power consumption process. In addition, the internal resistance of the inverter is much larger than that of the generator, so the various effects caused by it cannot be ignored.

In order to achieve consistent standards and facilitate analysis, the three-phase load is 100% unbalanced and defined as two situations. The first situation is that one phase is fully loaded and the other two phases are unloaded; the second situation is that two phases are fully loaded and the other phase is unloaded.

We roughly count the impact of load imbalance on output voltage, as shown in Table 1.

Table 1 Effect of load imbalance on output voltage

Three-phase load imbalance 0%～20% 21%～35% 36%～50% 51%～100%

Three-phase voltage unbalance <2% <5% <10% ≥10%

Generally, the output voltage imbalance index of three-phase inverter power supply is <2%. As can be seen from Table 1, when the load imbalance is >20%, the voltage imbalance has exceeded the standard.

When the three-phase load is 100% unbalanced, the output voltage of some phases of the inverter will increase, while that of others will decrease, making the inverter unable to work properly and even causing damage to the load. Therefore, it is urgent to solve the problem of the three-phase inverter power supply so that it can work properly when the load is 100% unbalanced. However, it is obviously unrealistic to simply require users to keep the three-phase load balanced during use. The only way is to find a practical solution in the inverter power supply itself.

At present, there are two ways to solve the problem of 100% load imbalance. One is to redistribute the three-phase windings of the output transformer and then connect them in series. Although this method is simple and partially solves the load imbalance, it does not completely solve the problem of 100% load imbalance, and the voltage stability is not high. The second is to divide the inverter part into three independent bridge inverters, and then combine them into a three-phase output inverter power supply according to the phase sequence of 120° and 240°. This system completely solves the problem of 100% load imbalance, and the structural relationship is very clear, ensuring the balance of the three-phase output voltage and high voltage stability. However, the circuit is more complex, the number of devices increases, and the cost increases. At the beginning of product design, the final technical indicators and performance of the product must be considered, and then the working mode and control method of the system are determined. Combining the above two solutions, we adopted the second solution in the process of developing new products in order to achieve better output voltage stability and balance. Optimize the circuit, reduce devices and reduce costs in system design. In particular, the use of MITEL's SA4828 and single-chip microcomputer to simplify the control circuit has achieved more obvious results.

Figure 1 System block diagram

Figure 2 SA4828 pin diagram

System Configuration

The main circuit is mainly composed of three parts: AC/DC rectification and filtering, three independent single-phase bridge inverters of DC/AC, and output filtering. The principle block diagram is shown in Figure 1.

Input three-phase 380V, 50Hz AC voltage, after EMI suppression, rectification and filtering, the DC high voltage is supplied to three single-phase half-bridge inverters. The three inverters output SPWM waveforms under the driving signals with phase differences of 120°C and 240°C in the control circuit respectively. Finally, through three independent transformers and filtering circuits, the required three-phase output voltage is combined. Among them, the inverter switch tube adopts IPM intelligent module. Since the peak voltage of the switch tube is very small, there is no need to add a special absorption circuit. The output transformer adopts integrated inductor technology to integrate voltage transformation and filtering, reduce noise, improve efficiency and increase reliability. The circuit is simple and clear. No matter how unbalanced the load is, the imbalance of the three-phase output voltage will not exceed the standard.

Control scheme

This system mainly uses the three-phase high-precision PWM wave generator SA4828 and a single-chip microcomputer to form the control circuit. SA4828 is a new generation of large-scale integrated circuits with better performance and more powerful functions launched by MITEL after the SA828 and SA838 PWM wave generator series. It is connected to the microprocessor and completes all peripheral detection, control and protection functions, making the system intelligent. 1 SA4828 Introduction

(1) Main features of SA4828

①Flexible control function SA4828 can select three different output waveforms, and can determine the amplitude of the three-phase output waveform through software, whether it is unified control or three-phase independent control. This function expands the user's scope of use, especially when it is necessary to solve the problem of 100% unbalanced inverter load, it is very important.

②Higher frequency accuracy The modulation wave frequency uses sixteen bits, which increases the frequency resolution and improves the accuracy of the inverter output frequency.

③ High reliability SA4828 has a new "watchdog" circuit for monitoring, so the program runs safely and reliably. SA4828 also uses harmonic suppression technology to reduce the loss of the switch tube.

(2) Pin Description

SA4828 is a standard 28-pin dual in-line package, as shown in Figure 2. The main pin classification is as follows:

① Driving signal and detection

RPHT, RPHB, YPHT, YPHB, BPHT, and BPHB are three-phase independently controllable TTL drive signals. The first letter represents the red, yellow, and blue phases, which are 120° and 240° apart respectively. The following letter "T" is the upper switch tube drive signal, and "B" is the lower switch tube drive signal.

Figure 3 SA4828 block diagram

Table 2 Register unit address and description

SETTRIP and RST are the shutdown signal input and reset terminals.

②Standard bus and control mode

AD7~AD0 is the address and data multiplexing bus.

WR, RD, and ALE are Intel control modes.

③Power supply and clock

VDD and VSS are +5V power supply and ground terminal respectively.

CLK is the clock input terminal.

If we compare it with SA8282, we can see that SA4828 can completely replace SA8282 in Intel mode as long as RS and MUX are connected to high level. However, the amplitude of the three-phase output waveform can be flexibly controlled separately.

(3) Working principle of SA4828

The SA4828 functional block diagram is shown in Figure 3.

The main working principle is similar to that of SA8282. The special principle of SA4828 is explained as follows:

① The selection of three different waveforms is mainly through the command transmitted to the initialization register and the control register to set the three-phase waveform ROM. They are sine, enhanced, and high-efficiency waveforms, so that they can be applied to various special occasions.

② When the "watchdog" circuit SA4828 receives a command from the microcontroller, if a problem occurs, the bus control will send a reset "watchdog" signal to delay the "watchdog" from shutting down the output drive signal.

③ Eight register units To improve frequency accuracy and independently control the amplitude of the three-phase waveform, SA4828 adds eight register units. See Table 2 for the address and description.

When transmitting the initialization command, the R4 and R5 registers are written as the delay control word of the "watchdog". When transmitting the control command, the R0 and R1 are written as the 16-bit frequency control word, and the R3, R4, and R5 are written as the three-phase output waveform amplitude control word. The above settings and adjustments are completed through the address/data bus and register unit, and stored in the initialization register and control register.

2. SA4828 is used for this system control

The control circuit of this system consisting of a single-chip minimum system, a small number of peripheral expansion chips, and SA4828 is shown in Figure 4.

The single-chip microcomputer first initializes the SA4828 and presets the output waveform, amplitude, frequency, etc. The digital-to-analog conversion circuit inputs the output three-phase voltage, current, and voltage and frequency given signals into the single-chip microcomputer, which performs open-loop and closed-loop control algorithm operations. After the single-chip microcomputer processes the data, the SA4828 adjusts the output voltage and frequency, and performs overcurrent delay protection, etc. Rapid shutdown is to immediately shut down the SA4828 output drive signal when it is found that the DC current detection signal (DCCT) and the IPM module have a direct or short circuit phenomenon.

3. Program flow

In program design, in addition to completing its control function, we should also strive for the rationality and simplification of the program, which determines the stability and reliability of the control system. Figure 5 is a flow chart of the control program.

In the main program, the parameter calculation and setting of SA4828 initialization command and control command are not described in this article.

Figure 4 Control circuit block diagram

Figure 5 Control program flow chart Test results and conclusion

The three-phase combined inverter power supply has been used and tested to achieve the following technical indicators:

Input voltage: 380V±15%(50Hz)

Output voltage: 115V (400Hz)

Output power: 6kVA

Voltage stability: <1%

Frequency stability: <0.1%

Three-phase voltage unbalance: <1%

Total harmonic content: <2%

Noise: <50dB

Protection function: Overload of 120% will delay shutting down the system for 6 minutes, and short circuit at the output will shut down the system immediately.

The test results show that the three-phase combined inverter fundamentally solves the problem caused by 100% unbalanced three-phase load. Because the power switch tube adopts IPM module and the output transformer uses integrated inductor technology, the circuit is simplified, the cost is reduced and the efficiency is improved. The control circuit is composed of a single-chip microcomputer and a three-phase high-precision PWM wave generator SA4828, making the system intelligent and the reliability improved. It has better solved the problems of complex circuits and high costs of previous three-phase combined inverter power supplies . It can be used as one of the better solutions in the application fields of medium and high power inverter power supplies.