Design of digital inverter power supply based on ADMC331

1 Introduction
　　With the development of information technology, inverter power supplies are increasingly widely used in the fields of communications, military, aviation, aerospace, etc. Traditional inverter power supplies are mostly analog control or analog and digital combined control systems, which have poor reliability, complex structure, high cost and are not conducive to product replacement. Modern inverter power supplies are developing in the direction of full digitalization , intelligence and networking. With the emergence of high-performance digital signal processors (DSPs), the realization of full digitalization of inverter power supplies has become possible. Based on a detailed analysis of ADMC331 , this article introduces the specific application of ADMC331 controller in full digital inverter power supplies.

　　2  Structural features of ADMC331

　　ADMC331 is a motor controller based on DSP technology launched by Analog Devices (ADI). It integrates a 26MIPS (million instructions per second) fixed-point digital signal processor core and a complete set of motor control peripheral circuits, which creates very favorable conditions for users to develop motor controllers quickly and efficiently. The functional block diagram of ADMC331 is shown in Figure 1. Its main features are as follows:

　　● Using 16-bit fixed-point ADSP-2171 as the core, it is fully compatible with the code of ADSP-2100 digital signal processing series; it has three independent full-function computing units, namely: a 16-bit arithmetic/logic unit (ALU), a 32-bit multiplication accumulator (MAC) and a 32-bit barrel shifter (SHIFTER); it also has two independent data address generators (DAGs) and a powerful program sequencer. Since the architecture of the device is a parallel structure, it can speed up the execution of the program.

●The single-cycle instruction execution time is 38.5ns (external 13MHz crystal oscillator), which can realize the following functions: generate the next program address, fetch the next instruction, execute one or two data move operations, update one or two data address pointers, execute a calculation operation, etc.

　　●Built-in 2k×24-bit program memory ROM, 2k×24-bit program memory RAM and 1k×16-bit data memory RAM.

　　●Has a three-phase 16-bit midpoint-based pulse width modulation (PWM) generator that can be flexibly programmed to generate high-precision PWM signals with minimal processor overhead.

　　●With 2 8-bit auxiliary pulse width modulation (AUXPWM) channels, the frequency is programmable.

　　●It has seven Σ-Δ type A/D conversion channels with a maximum resolution of 12 bits and a maximum sampling frequency of 32.5kHz.

　　●It has 24 programmable digital input and output (PIO) ports, which can be individually set as input or output and support state change interrupts.

　　● Provides two double-buffered synchronous serial ports (SPORT0, SPORT1) to accomplish serial communication and communication between multiple processors.

　　●16-bit watchdog timer with real-time interrupt.

　　●The internal program memory ROM solidifies some utility programs, which facilitates the system program design and reduces the program calculation time of the digital control system.

　　3 Application Examples

　　The structure diagram of the fully digital inverter power supply composed of ADMC331 is shown in Figure 2.

 3.1 Formation of PWM signal

　　A general fully digital inverter must generate 4 precise PWM timing signals to drive the IGBT inverter bridge. At the same time, the PWM signal driving each pair of power tubes must have a conduction delay to prevent the upper and lower bridge arm power switches from being turned on at the same time and destroying the system. The generation of PWM signals is completed by the PWM controller in the ADMC331, and the flexibility and programmability of the PWM generation unit in the ADMC331 can fully meet the requirements of different digital PWM methods. Figure 3 is a functional block diagram of the ADMC331 three-phase PWM controller.

　　The PWM unit of ADMC331 is based on an independent three-phase timer, which can be controlled by three determined period registers, each of which controls a pair of PWM outputs. The DSP program required for control can be compiled according to the requirements of the application and the required PWM scheme. In the figure, PWMPOL can select the polarity of the PWM output according to the logic and structure of the gate drive circuit. Each PWM signal output can be enabled or disabled by an independent enable register PWMSEG. PWMTRIP is a hardware protection circuit. When this pin is low, the entire system is unconditionally reset to protect the entire circuit. In addition, there are clock output signals CLKOUT and PWM synchronization signals PWMSYNC.

3.2 Analog-to-digital (A/D) conversion interface circuit

　　The A/D converter in ADMC331 consists of 7 analog input channels and comparator circuits. Data conversion is achieved by judging the intersection of the analog input and the high-stability sawtooth wave on the chip. Considering that the level range that the A/D converter can accept is 0~+5V, the voltage detected by the voltage transformer and the current detected by the current sensor (converted into voltage) must be converted into this range according to a certain ratio. The output voltage and current detection interface circuits are shown in Figures 4 and 5 respectively. The input impedance of this part of the circuit should be relatively large to minimize the impact on the device signal, and the output impedance should match the input impedance of the A/D converter.

3.3 Protection measures

　　In order to ensure the normal and safe operation of the IGBT module and prevent abnormal phenomena from causing device damage and system crash, perfect protection measures must be taken. The fully digital inverter power protection circuit uses a GAL chip to block the system overcurrent, overvoltage, undervoltage, abnormality and other fault signals transmitted from the main circuit. When any of the above faults occurs, the signal generated by the protection circuit can directly block the output of the inverter bridge drive signal through hardware, which not only ensures the rapidity and reliability of the system's response to the fault, but also the protection signal can be sent to ADMC331 through the digital PIO port for software query, and thus determine whether the system has a fault and the type of fault, so that the system can provide corresponding fault handling procedures and alarm displays.

　　In addition, ADMC331 is also connected to an E2PROM that stores program execution instruction machine code. These instructions can be automatically imported through the serial port program after power-on, and the MAX232 chip is used as the level conversion between TTL and RS232 to realize communication between ADMC331 and the outside world.

4 Conclusion

　　With the increasing demand for green household appliances, the development of environmentally friendly and pollution-free power supplies has become a top priority. The green and environmentally friendly inverter power supply designed with high-performance embedded DSP motor controller ADMC331 can achieve true full digital control, which not only improves the integration of the control circuit, but more importantly, improves the reliability and plasticity of the control circuit and even the entire system.