Design of Modular DC Power Supply Based on TMS320LF2407A

1 Introduction

Compared with the traditional centralized power supply system, the modular distributed power supply has the characteristics of high reliability, high efficiency, small size and easy use, but it needs to be equipped with reliable current sharing measures to ensure the uniform distribution of electrical and thermal stresses between modules. This paper proposes a modular high-power DC power supply. The single power module is based on DSP and adopts high-frequency switching power conversion technology to achieve voltage stabilization and current sharing of multiple modules in parallel operation.

2 Overall design of power supply

The basic module is a power supply with a voltage adjustment range of 22 V to 30 V DC and a power of 4 kW. The PWM control method is used to control the output by changing the PWM duty cycle. The main circuit of the basic module adopts a half-bridge topology; the control circuit is based on TMS320LF2407A to achieve voltage regulation, current limiting, and current balancing functions, and the protection circuit includes input voltage detection, output overcurrent detection, output overvoltage detection, and whole machine overheating detection.

3 Hardware Design of Basic Modules

The circuit principle of the power module is shown in Figure 1. Since the voltage stress of the primary switch tube in the half-bridge structure is the smallest, for offline switching power supplies, L1 in the circuit is the main filter inductor with a large inductance; L2 is the auxiliary inductor with a small inductance, which is mainly used to further reduce the output switching noise.

The rated power of a single power module is 4 kW, and the rated output voltage is 28 V. Considering the overload problem, the actual maximum output current is 150 A. According to the equal power of the primary and secondary sides of the transformer, the current flowing through the power module is estimated. Considering that the three-phase AC power supply voltage fluctuates within the range of ±15%, the voltage of the primary winding of the half-bridge converter transformer is 230 V, and the maximum current flowing through the primary winding of the transformer, that is, the power module, is 18.3 A. The reverse pressure on the power module is the three-phase rectifier voltage; the voltage calculated based on the maximum value of the three-phase AC voltage is 618 V. In order to ensure the reliable operation of the device, the rated current of the power module is more than twice the operating current. Therefore, the IGBT module CM100DY-24NF of Mitsubishi Corporation is selected, which has a rated current of 100 A, a withstand voltage of 1,200 V, and a two-unit half-bridge structure.

The control circuit is mainly composed of TMS320LF2407A subsystem, A/D input, voltage regulation, power-on protection and other circuits. Among them, the TMS320-LF2407A subsystem mainly includes clock, reset, JTAG simulation interface and power supply system.

4 Software Design of TMS320LF2407A

TMS320LF2407A DSP is the control core of the power module. It realizes voltage regulation, current sharing and other functions by changing the duty cycle of the output PWM. The main program is in the waiting interrupt state after completing system initialization, A/D initialization and timer start. Each functional module is realized through the interrupt service program.

4.1 Software Design of Voltage Stabilization Function

The sensor outputs a voltage signal, which is filtered and sampled, and then compared with a given voltage signal. The deviation is sent to the PID regulator to change the PWM duty cycle to keep the output voltage constant. The PID regulator uses an incremental PID control algorithm that discretizes and improves the analog PID control law. The control is simple and the parameters are easy to set. The control increment of the duty cycle can be obtained by measuring the deviation three times.

4.2 Software Design of Current Sharing Function

There are many ways to achieve the current sharing function for a power system with several power modules connected in parallel, such as output impedance method, master-slave current sharing method, automatic current sharing method, democratic current sharing method, etc. This power system module adopts the automatic current sharing method (also known as the external current sharing controller method). This method requires a communication line with a narrow bandwidth (called the current sharing bus) to connect each module. The voltage of the current sharing bus is proportional to the average value of the current of each module. This power system module uses TMS320LF2407A as a controller for voltage regulation and current sharing at the same time, and improves the average automatic current sharing method. The average current is calculated and the current sharing threshold is set through the sampling data of the circulating current error and the load current. Each power module decides whether to participate in the current sharing according to the specific values ​​of its load current and circulating current. In this way, current sharing can be achieved accurately, and the automatic current sharing method is also avoided. Failures and instability problems are easy to occur. Figure 2 shows the flow chart of the current sharing subroutine.

5 Conclusion

In addition to the above designs, in order to ensure the reliable and efficient operation of the power supply, this modular power supply system also designs surge current suppression circuits, generalized soft switches - absorption circuits, protection circuits, and anti-electromagnetic interference designs for the main circuit and control circuit. Finally, four series of DC power supplies were developed. The input voltage is three-phase three-wire AC 380 V. The output voltage is DC 28 V, and the rated capacities are 4 kW, 8 kW (current sharing error <2%), 16 kW (current sharing error <3%), and 24 kW (current sharing error <5%).

Now, with TMS320LF2407A DSP as the control core, the design of a high-power digital DC switching power supply with multi-module parallel operation has been realized. After experimental testing and actual use, the power supply works stably, and the voltage stabilization and current sharing effects are good, meeting the expected requirements.