Research on high-power DC/DC power supply parallel technology based on IGBT devices

Based on IGBT devices, this paper designs a high-power DC power supply using DC/DC power supply parallel technology. This power supply can be used as a high-power vertical displacement fast control DC power supply in the EAST tokamak device. The power supply device was simulated and tested, and relatively satisfactory dynamic and static performance was obtained.

With the development of power electronics technology, many occasions require high-power and high-current DC power supplies. The DC fast control power supply used in the magnetic confinement nuclear fusion device of EAST is a high-power DC power supply, and its technical requirements are: voltage response time 1ms peak voltage 50V; maximum current 20kA, and can achieve 4 quadrants of operation. In response to this requirement, it is inevitable to adopt power supply parallel technology, that is, power tube parallel or power supply device parallel. For a 20kA DC power supply, if power tube IGBT parallel is used, at least 15 power tubes are required in parallel for each bridge arm, which not only brings great difficulties to the drive, but also, in general, the voltage capacity of the power tube with a larger current capacity is also larger. When the actual voltage is only 50V, this is a great waste for the voltage capacity of the power tube. Therefore, a technical idea of ​​parallel connection of DC/DC power supply devices with domino structure is proposed.

The basic requirements for the power supply parallel system are:
1) Keep the output voltage stable under grid disturbance or load disturbance;
2) The modulation frequency of each module is consistent. If it is inconsistent, a low-frequency pulsating signal is generated to increase the ripple component of the output current and voltage;
3) Control the current of each module to evenly share the load current.

1 Topological structure of high-power DC power supply

There are two topological structures for parallel connection of DC/DC power supply. One is to use the input DC bus structure. The system structure block diagram is shown in Figure 1a, which mainly includes a rectifier transformer and an uncontrolled diode rectifier circuit, an N-way DC/DC converter, a pump-up voltage suppression circuit, etc.; the other is to use independent AC-DC/DC power supplies in parallel. The system structure diagram is shown in Figure 1b.

Using the topology shown in Figure 1a, the system requires a large-capacity non-adjustable DC power supply, which can generally be obtained by using a rectifier transformer to step down, diode rectification and capacitor filtering. Although this structure can ensure that each branch in parallel has a common DC voltage input, avoiding the imbalance caused by different DC side input voltages in the parallel branches, the capacity of the DC power supply is large, the current is up to 20KA, the load on the DC bus is too heavy, and the requirements for the front-stage AC-DC equipment are high, which is not easy to achieve. In addition, the shared bus at the input end is not conducive to the parallel connection of independent power modules in a complete sense. Therefore, the topology of AC-DC/DC DC power supply parallel connection as shown in Figure 1b is adopted.

The topology shown in Figure 1b can ensure the independence of each AC-DC/DC power supply module, that is, the parallel connection of DC power supply devices can be realized, and the number of modules can be freely increased or decreased according to the actual voltage, current and power requirements! There is a lot of room in practical applications and it has certain research value. But this topology also has its disadvantages! That is, if the output voltage of the transformer is slightly different, the output voltage of each rectifier module will be different, resulting in serious imbalance in the output current of each rectifier module.

However, this imbalance can be suppressed by taking the following corresponding measures: First, when using independent AC-DC/DC power supplies in parallel, the AC-DC/DC output DC voltage of each module should be as close to equal as possible; second, for the imbalance of the output current of each rectifier module caused by different output voltages of the transformer, current equalization measures can be set in the DC/DC link. The DC/DC module adopts a limited unipolar pulse width modulation method (PDW), which balances the load of each circuit by adjusting the duty cycle of each DC/DC module. When the given current of the power module is switched between positive and negative, operation in different quadrants can be achieved to meet the requirements of the system's 4-quadrant operation.

2 Control scheme of high-power DC power supply

In the application of tokamak fast-controlled power supply, the power supply output current is required to track the given current curve in real time. Therefore, the power supply system is a current follower system, and the rapidity of the system will be a more important performance indicator. The choice of control method will affect the static and dynamic performance indicators of the entire system.
In order to better improve the accuracy of the system's steady-state and dynamic performance indicators and achieve current tracking, a two-level current control is adopted (Figure 2), that is, the total current loop and the module current loop cooperate with each other, which can not only improve the performance indicators, but also achieve current balance in each module.

The main function of the outer loop is to achieve real-time tracking of current, using a composite control method of feedback plus feedforward. The feedforward control in the composite control does not affect the stability of the original system. However, it can greatly improve the steady-state accuracy and dynamic performance of the system without increasing the open-loop gain. In order to achieve the control effect. And do not make the structure of the feedforward channel complicated. The feedforward control uses the first-order derivative of the input signal and adds it to the input end of the signal. The main functions of the current loop of the inner loop module are as follows.

1) Transform the transfer function of the control object.
2) Limit the maximum current output, while achieving the current sharing of each power module.

3 Data transmission topology

The current sharing of the EAST plasma vertical displacement fast control power supply is an important issue in the parallel connection of the device. The CPU data transmission of the monitoring computer and the power module adopts the master-slave mode (Figure 3), that is, the CPU load of each power module realizes its own current control and sends the status information of the power module to the monitoring computer. The role of the monitoring computer is to achieve unified management of each power module, including sending start and stop instructions to each power module. Send current given signal, collect DC output total current, total voltage, AC input voltage and AC voltage and current of each power module, DC output current, temperature, fuse blown, access control and other physical quantities. At the same time, communicate with the upper level EAST master control computer and each power module of the system to complete the automatic reporting and downloading of various data information. Automatic removal and input of modules and other tasks. The monitoring computer transmits the same given current to each power module! Under the regulation and control of the power module current loop, the same load current is output through the software programming of the single-chip microcomputer! Get a better current sharing effect.

4 Conclusion

For power systems with large capacity and high requirements for quadrant operation and current tracking, such as the Tokamak fast-controlled power supply, multiple independent small and medium-capacity power modules can be connected in parallel to meet the total power capacity requirements. A key issue facing the parallel connection of multiple power supplies is the current sharing between the component modules. By using the intelligent power module and the theory of automatic control systems, the various component modules of the power supply can be made into a closed-loop system with current tracking capabilities! The current sharing between the modules is achieved by control laws rather than hardware. The system formed in this way will also be able to meet the fast current tracking requirements of the fast-controlled power supply. This design scheme can be realized. The key lies in the following conditions:

1) The embedded application of single-chip microcomputer in power module and parallel system realizes the intelligence of the device and greatly improves the consistency of module modulation frequency. It is beneficial to reduce the low-frequency ripple of output voltage and current! It overcomes the disadvantage that traditional methods are difficult to achieve the consistency of modulation frequency of each module.

2) The DC/DC link adopts PWM technology and has fast response capability;

3) The current tracking technology based on control theory can realize current sharing among modules with different ideas of hardware current sharing. Through the control of the monitoring computer, the same current setting is transmitted to the CPU of each module, realizing the static current sharing of the power module.

In situations where high power output is required, this system has good application prospects and is in line with the development trend of digital control of power supply systems.