High frequency switching power supply double closed loop feedback parallel system

1. Introduction

After high-frequency switching power supplies entered my country in the 1980s, they entered my country's postal and telecommunications, power departments and other fields in large quantities due to their advantages of small size, light weight, high efficiency and low noise. They have developed rapidly and have huge market potential, replacing many traditional small and medium-power thyristor rectifier power supplies. However, in traditional industrial and mining enterprises, such as electrolytic plating, electrochemical, electric spark, battery charging, water treatment, heat treatment, welding, smelting and many other fields, traditional thyristor rectifier power supplies are still widely used, which does not comply with the national environmental protection and energy-saving policies. The power of a single high-frequency switching power supply on the market is restricted by the device and other factors, and it is difficult to use it in high power (above 50KW). In order to increase the power, a simple method is to simply connect the outputs of many single high-frequency switching power supplies in parallel to form an expanded current output. However, this method has a limitation, that is, the system after parallel connection can only have a steady current output, and cannot adapt to the application of voltage-stabilized output. The design idea of ​​this article is to design an output voltage negative feedback system on the basis of the above simple parallel connection, and use the output of the voltage feedback system to control each high-frequency switching power supply to form a double closed-loop feedback, so as to achieve a voltage-stabilized output of the parallel system. Since the working principle of a single high-frequency switching power supply is well known, the following focuses on the working principle of the parallel system from the perspective of the principle of the automatic control system.

2. System control schematic diagram

The automatic control principle of the parallel system is shown in Figure 1.

In the automatic control motor DC speed regulation system, there is a speed and current double closed-loop feedback system, also known as the cascade system.

The outer loop is speed feedback, and the inner loop is current feedback. Any speed change caused by internal or external disturbances in the system or changes in grid current can be adjusted through the feedback system of the outer loop or inner loop to achieve a stable speed output. Based on this design idea, this paper designs a high-frequency switching power supply double closed-loop feedback parallel automatic control system as shown in Figure 1. Each high-frequency switching power supply in the figure can work independently, and a voltage or current negative feedback system is formed internally. The voltage feedback of the parallel system belongs to the outer loop, and the inner loop is formed inside the high-frequency switching power supply. The reason why this parallel system is simple is that the output ends are simply connected in parallel on the basis of a single independently working power supply. The given input end is uniformly added to each independent high-frequency switching power supply by the outer loop.

In the dotted box in Figure 1, 1#.2#.....N# are each high-frequency switching power supply, and its internal automatic control principle diagram is simplified to a first-order system proportional integral link, so the current or voltage stabilization accuracy of each high-frequency switching power supply is very high. In the figure, they work in a steady current state.

3. System working design principle

3.1 Single high-frequency switching power supply design and overall block diagram

Technical indicators of a single high-frequency switching power supply:

Input voltage: 380V, 50HZ

Output voltage: DC 18V

Output current: DC 800A

Current limit: 850A

Voltage limit: 18.5V

Protection: overcurrent protection, thermal protection, overvoltage protection, undervoltage protection

Conversion efficiency: >80%

The overall block diagram of a single high-frequency switching power supply is shown in Figure 2.

The whole circuit can be divided into two parts: the main conversion circuit and the control circuit. The AC 380V voltage is input through the power filter. The input DC rectifier and filter obtain a DC voltage of about 550V, which is supplied to the pulse width modulator, which consists of two groups of IGBT modules, a high-frequency transformer and an output rectifier and filter.

The PWM control circuit provides alternating pulses to control the on and off of the IGBT module through the drive circuit, converting the DC voltage into an alternating 20KHZ pulse voltage, which is isolated and converted into the required voltage by the high-frequency transformer, and then full-wave rectified by the output rectifier diode to obtain a DC voltage with an average amplitude of 18V.

The control circuit consists of a PWM control circuit, a drive circuit, a feedback sampling circuit, a current limiting and voltage limiting circuit, and an auxiliary power supply.

The PWM control circuit outputs two pulses with a phase difference of 180 degrees and a certain dead zone. The pulses are amplified by the drive circuit to control the on and off of the IGBT module in the main circuit. In order to obtain a stable output voltage or current, the output voltage or current is sampled and fed back, compared with the reference value, amplified, and the pulse width of the PWM circuit is controlled, and the duty cycle of the IGBT is adjusted to achieve voltage or current stabilization. At the same time, the power supply itself is protected by soft start, overcurrent and overvoltage protection, short circuit protection, and voltage and current limiting circuits.

A single high-frequency switching power supply constitutes a current negative feedback control system, referred to as the inner loop. The automatic control principle is shown in Figure 3.

The single closed-loop current negative feedback control system using the PI regulator in Figure 3 not only ensures dynamic stability, but also achieves zero static error, thus resolving the dynamic-static contradiction. Its regulation principle: when the current given value remains unchanged, when the load changes or the power supply internal reasons cause the power supply output current to change, the automatic control regulation process is:

Through the above adjustment process, a single high-frequency switching power supply can be guaranteed to output a stable current. In this way, by connecting the outputs of each high-frequency switching power supply working separately in parallel and working in a steady current state and accepting the same current given value, it can be ensured that each high-frequency switching power supply outputs the same current.

Thus, the parallel system can achieve the expansion of current output. In order to improve the overall reliability of the system, N+1 redundant design can be added according to the requirements of the system. With this simple combination, when a high-frequency switching power supply fails, it can be immediately powered off and shut down, the output connection can be disconnected, and the backup high-frequency switching power supply can be powered on and put into operation. Thus, the time for handling the fault can be reduced to a minimum.

3.2 System automatic control principle

The automatic control principle of the double closed-loop parallel system is shown in Figure 4.

In Figure 4, a proportional-integral regulator is added to the high-frequency switching power supply system to adjust the voltage of the parallel system.

The output voltage feedback of the parallel system is compared with the given value of the parallel system, and the difference is amplified by the signal as the current given value of the high-frequency switching power supply system. The high-frequency switching power supply system adjusts its own output voltage according to the constantly changing current given value, so as to ensure that its own output current changes according to the given value. Thus, the output voltage of the parallel system is also guaranteed to be stable. From the structure of closed-loop feedback, the current regulation loop is inside the high-frequency switching power supply system, which is the inner loop; the voltage regulation loop is outside, which becomes the outer loop. The two are connected in series, that is, the output of the voltage regulator is used as the input of the current regulator, and the output of the current regulator is used as the control of the output voltage of the parallel system, so that the two regulators can cooperate with each other and complement each other. This forms a voltage and current dual closed-loop feedback control system. In order to obtain good static and dynamic performance, the two regulators generally use PI regulators.

When the voltage output of the parallel system changes due to load disturbance, the system automatic control adjustment process is:

The above voltage regulation process can ensure that the parallel system has a stable output voltage when in a voltage-stabilized working state. If the system is to work in a steady-current state, it is only necessary to turn off the outer loop voltage feedback unit through the selection switch inside the system, and directly add the voltage given signal to each high-frequency switching power supply. Since each high-frequency switching power supply itself works in a steady-current state, it can ensure that each high-frequency switching power supply in the parallel system outputs the same current.

From the perspective of dynamic stability, during the design process, the design of a single high-frequency switching power supply is first adjusted so that it can stably output the rated current. Then, the units are connected in parallel, and a voltage feedback outer loop is added. The outer loop is then adjusted according to the system design requirements to keep the system output voltage stable. It should be noted that the inner loops should be turned on one by one according to their design indicators and adjusted in conjunction with the outer loops. After all the inner loops are adjusted, all the inner loops are turned on to adjust the system output voltage and current together with the outer loops.

4. Experiments and Conclusions

Applying the above principle, a combined parallel 72KW high-frequency switching power supply was made. The specific parameters are: AC380V±10%, regulated output 18VDC; current limiting current 4100ADC; regulated current output 4000ADC; voltage limiting voltage 18.5VDC. The parallel system is composed of five separate high-frequency switching power supplies in parallel, and each high-frequency switching power supply outputs the same 800A/18V. When the system is working in voltage stabilization, it can limit the current to 4100A and work stably even if the output is short-circuited; when working in steady current, the output end can be open-circuited to achieve voltage limiting and stable operation. If you want to improve the reliability of the parallel system, you can also add a backup. The power supply has been running on-site in the chrome plating process of the electroplating industry for nearly two years, basically meeting the design requirements, and users have responded well.