Design of multifunctional power supply control system based on DSP2407

In the process of vehicle equipment inspection, maintenance and repair, the existing external power supply is large in size and weight and difficult to move, which brings a lot of difficulties to the actual inspection and maintenance work. Therefore, the development of a multifunctional vehicle maintenance power supply with high power and light weight is an urgent requirement for equipment support. This paper introduces in detail the design of a multifunctional power supply control system based on DSP2407 chip. The system mainly integrates the power supply part of functional equipment such as welding, welding and cutting, charging, starting, and voltage stabilization power supply, and realizes real-time control of power supply output through data acquisition, pulse width modulation and digital PID adjustment technologies.

Power supply overall solution

The multifunctional power supply integrates five power supply functions: welding, welding cutting, charging, starting, and voltage stabilization. The power supply control system is mainly composed of the power main circuit, DSP control circuit and other auxiliary circuits. The system schematic diagram is shown in Figure 1.

Figure 1 Multifunctional power system block diagram

Hardware design of control system

1. Power main circuit

The power main circuit includes an input rectifier and filter circuit, an IGBT inverter circuit, a high-frequency transformer, and an output rectifier and filter circuit, as shown in Figure 2.

Figure 2 Schematic diagram of the main power circuit

The input rectifier filter circuit includes: fuse SR, three-phase conversion switch K, three-phase bridge rectifier D0, closing surge limiting circuit (main circuit soft start circuit) and filter circuit. The function of the main circuit soft start circuit is to prevent the following from happening at the moment of closing: (1) the power switch contact is blown; (2) the input fuse is blown; (3) interference with other adjacent circuits; (4) deterioration of the performance of capacitors and rectifiers.

The inverter circuit adopts a bridge inverter. The transistors VT1, VT3 and VT2, VT4 form the front and rear conducting bridge arms. Their alternating conduction converts DC into high-frequency square wave AC, which is then rectified and output as low-voltage DC through a high-frequency transformer. Diodes D2, D3, D5, and D6 are clamping diodes, which have the following functions: (1) When the switch is turned off, the voltage spike caused by the transformer leakage inductance is clamped to the input voltage E; (2) The voltage spike energy is fed back to the input capacitors C1 and C3 to improve the utilization of electric energy. The R-C-D circuit connected in parallel at both ends of the switch is a peak voltage absorption network, which works together with the above-mentioned clamping diode to suppress the influence of the peak voltage. The connection of the diode in the R-C-D network can shorten the charging time constant of the capacitor and reduce its discharge current, which is beneficial to the function reduction of the switch.

High frequency transformers play the role of power conversion and isolation in circuits.

The output rectifier uses a high-power ultra-fast recovery diode. Considering the actual capacity of the diode, two sets of tubes are connected in parallel for rectification. After filtering by the reactor, the resistor R7 is maintained to establish the welding voltage. When designing and installing the rectifier circuit, pay attention to the connection between the high-frequency transformer winding and the two sets of rectifier diodes to avoid short-circuiting the winding. The R-C absorption circuit connected in parallel at both ends of the rectifier diode absorbs the peak voltage generated on the diode when it is turned on and off.

2 DSP control loop

The DSP control loop, as shown in Figure 3, is a real-time monitoring and control system that mainly monitors the voltage/current at the power output end, the temperature of the IGBT inverter, and analyzes and calculates the collected information. The control circuit mainly includes the DSP minimum system, pulse width modulation circuit, drive circuit, data acquisition circuit and protection circuit. The output signal of the control loop controls the conduction of the main loop, that is, the collected power supply related parameters are transmitted to the DSP chip for analysis and calculation, and the corresponding control data is obtained, and then the power is amplified by the drive circuit and sent to the control end of the inverter, thereby controlling the output voltage and current.

Figure 3 DSP control loop schematic

① DSP minimum system

The DSP controller is the core of the control loop and uses the TMS320LF2407 chip from TI. TMS320LF2407 is a single-chip DSP controller designed by TI for motor and inverter control. It has the characteristics of high performance, low power consumption and high integration. Its 16-bit fixed-point C2xLP core with a processing speed of 3×107 instructions/s provides designers of analog systems with a digital solution that does not sacrifice system accuracy and performance. By integrating a high-performance DSP core and the on-chip peripherals of the microprocessor into a single chip, it becomes a low-cost alternative to traditional microcontroller units (MCUs) and expensive multi-chip designs. The minimum DSP system includes clock circuits, power circuits, reset circuits, JTAG simulation interfaces, off-chip program/data memory and level conversion circuits. The clock circuit includes a crystal oscillator composed of an external crystal and an internal circuit, with a frequency between 6 and 16 MHz. A 15 MHz crystal oscillator is selected here. The power supply voltage of TMS320LF2407 is +3.3 V, while the commonly used TTL circuit power supply voltage is +5 V. Therefore, the power supply circuit uses the TPS7333QD chip to provide a stable +3.3 V operating voltage and power-on reset signal. The reset circuit uses a manual reset method, that is, a pull-up resistor with a resistance of ≥10 kΩ is connected in series between the VCC (+3.3 V) and reset (RS) pins.

② Pulse width modulation circuit

The control system uses a pulse width modulation circuit composed of an integrated pulse width modulation chip SG3525. The error signal input to SG3525 is amplified by the error amplifier and compared with the sawtooth wave generated by its internal oscillator. The output pulse width signal is then divided into two non-overlapping two-phase signals by the phase splitter and output from output terminals 11 and 14 with a totem pole structure. The larger the control signal, the wider the output pulse width. The principle of the pulse width modulation circuit is shown in Figure 4. Since the driving module M57959L requires a low-level input signal, the two PWM signals output by SG3525 are inverted and output by the transistor working in a saturated state and then added to the 13th foot of M57959L.

Figure 4 SG3525 pulse width modulation circuit

③ Driving circuit

The function of the driving circuit is to amplify the power of the two PWM pulses output by SG3525 to drive the IGBT. M57959L is a thick film integrated circuit designed by Mitsubishi Corporation of Japan to drive IGBT. It has a closed short-circuit protection function. In essence, it is an isolated amplifier that uses optical coupling to achieve electrical isolation between input and output. The isolation voltage is as high as 2500V; it is equipped with a short-circuit/overload protection circuit. The M57959L driving circuit is shown in Figure 5. Resistor Rg is the IGBT gate current limiting resistor, diode D1 is an overload/short-circuit detection diode, Zener diode D2 is used to compensate for the reverse recovery time of D1 (used when the reverse recovery time of D1 is long), and Zener diodes D3 and D4 are used to protect the emitter junction of the IGBT.

Figure 5 M57959L drive circuit

④ Data acquisition circuit

The multifunctional power supply needs to realize three output characteristics, namely constant voltage, constant current and pulse, which requires constant value control. Therefore, a constant value feedback sampling circuit with high precision and fast sampling speed must be designed. Linear Hall sensors are widely used in inverter circuits due to their small size, simple peripheral circuit, wide bandwidth, good dynamic characteristics, long life, and electromagnetic isolation.

The data acquisition circuit uses the CHB-500S Hall current sensor for current sampling, which has a rated current of 500A, an output current of 100mA, a response time of less than 1μs, and a power supply of ±15V. The voltage sampling uses the CHV-25P Hall voltage sensor, which has a maximum range of 500V, an output current of 25mA, a linearity of 0.2%, and also uses a power supply of ±15V. The tiny current signal output by the Hall current sensor is first converted into a voltage signal through the I/V circuit, and then amplified and buffered by the operational amplifier and sent to the DSP chip for A/D conversion.

The data acquisition circuit is the forward channel of the DSP microprocessor and is more susceptible to interference. Therefore, adding an RC resistor-capacitor circuit to the analog input end can act as a low-pass filter and reduce the impact of noise.

⑤ Protection circuit

TMS320LF2407 provides PDPINTA input signal, which can ensure the safe and reliable operation of power conversion circuit and inverter circuit in the system. The protection circuit structure block diagram is shown in Figure 6. After the overvoltage/undervoltage detection signal, overcurrent detection signal and overheat detection signal are integrated by the OR gate, they are inverted and input to the PDPINTA pin. If there is a fault, the OR gate output is high level, the pin is correspondingly low level, the PWM output pins of the DSP are all in high impedance state, and the PWM output is blocked. The whole process can be implemented by hardware.

Figure 6 Protection circuit structure diagram

Software Design of Control System

The control system software mainly includes system initialization program, ADC sampling program, PWM wave generation program, PID control algorithm program, etc. The overall software design process is shown in Figure 7.

Figure 7 Main program flow chart

The system software design adopts the incremental PID control algorithm to calculate the control quantity according to the deviation value at the sampling time. The sampling circuit collects and filters the voltage and current signals and inputs them into the DSP. After comparing them with the input values, the digital PID operation is performed to adjust the output control quantity. The incremental PID control algorithm flow is shown in Figure 8.

Figure 8 Incremental PID control algorithm flow chart