Application of Intel80C196MC microprocessor in static inverter power supply

     Abstract: Briefly introduce the hardware and software design scheme of the three-phase static inverter power supply circuit developed using 80C196MC microcontroller. The test results show that this solution can meet the application requirements.

     Keywords: sinusoidal pulse width modulation, static inverter power supply

                The Application of Intel 80C196MC Microprocessor

                             in Static-inverter Power Supply

    Abstract: This paper presents a design precept of static-inverter power supply using Intel 80C196MC microprocessor and shows the circuit block and the programming idea. The experimental results show that this precept can meet the requirements of the application.

    Keywords: SPWM, Static-inverter, PowerSupply

    1 Introduction

    PWM (Pulse Width Modulation) technology is a control technology that uses the on and off of semiconductor switching devices to turn DC voltage into a voltage pulse train, and controls the voltage pulse width to achieve the purpose of voltage conversion and frequency conversion. SPWM (Sinusoidal Pulse Width Modulation) is a control loop that generates a set of rectangular pulse trains of equal amplitude but not equal width, which is used to approximate a sinusoidal voltage wave.

    There are three commonly used methods: one is completely generated by analog circuits; the second is generated by digital circuits; the third is generated by dedicated integrated chips. The analog method has complex circuits, too many hardware, poor anti-interference performance, and temperature drift, making it difficult to achieve optimal PWM control (the modulation waves of the optimal PWM are not sinusoidal), and the system reliability is low; the digital method follows different digital models Use a computer to calculate each switching point, store it in memory, and then generate SPWM waves through table lookup and necessary calculations. This method does not have a wide frequency modulation range. The output PWM wave is asymmetrical on the 1/4 axis and will produce even-order harmonics, especially in the low-frequency area. It also takes up a large amount of memory and is inconsistent with system accuracy. The technology of generating three-phase SPWM waves by dedicated integrated chips has been used in recent years. Widely used, the commonly used ones are HEF4752, SLE4520, MA818, MA828, MA838 and three-phase and single-phase PWM generator SA828 and SA838 series chips developed by MITET company. They are mostly connected to microprocessors to complete peripheral control functions, but the system structure is still relatively complex. The 16-bit microprocessor 80C196MC recently launched by INTEL Corporation integrates a 3-phase waveform generator WFG (WaveFormGenerator) on-chip. This peripheral device greatly simplifies the control software and external hardware for generating synchronous pulse width modulation waveforms, and can It constitutes the smallest single-chip microcomputer system and coordinates SPWM waveform generation and detection, protection, intelligent control, etc. of the entire system. Based on the above reasons, this article uses 80C196MC to form the control circuit of the static inverter power supply.

   2 Introduction to 80C196MC on-chip waveform generator WFG

    2.1WFG functional features

    The 80C196MC on-chip WFG has three synchronous PWM modules. Each module contains a phase comparison register, a deadtime generator and a pair of programmable outputs. WFG can generate 3 independent pairs of PWM waveforms, but they have a common carrier frequency, no signal time and operation mode. Once started, WFG only requires the CPU to intervene when changing the PWM duty cycle.

    The SPWM waveform generated by WFG is completed under the control of the following special registers.

    (1) Bidirectional counting register WG-COUNT: 16-bit bidirectional counter, which is a time base generator that generates output signals. Each status cycle WG-COUNT changes a count value. The user can write to the WG-RELOAD register, and its value is loaded into the counter periodically.

    (2) Reload register WG-RELOAD: This register actually contains a pair of 16-bit registers. When reading or writing this register, the WG-RELOAD register is accessed. The value written to WG-RELOAD is periodically (depending on the mode of operation) loaded into the second register. This latter register is called the counter comparison register, which is the time register that WG-COUNT actually compares with.

    (3) Phase comparison register WG-COMPx: There are three (X=1, 2, 3) readable and writable 16-bit phase comparison buffers. Each phase comparison buffer has an associated comparison register, and its value is compared with the WG-COUNT after each count.

    (4) Control register WG-CON: WG-CON is a 16-bit register. It can control the counting mode and generate three 10-bit deadtimes.

    (5) Output control buffer register WG-OUT: can be used to select the output signal mode of the output pin. The active states can be defined independently for each pin.

    2.2Basic working principle of WFG

    (1) WFG consists of time base generator, phase drive channel and control loop:

    ①The time base generator establishes the carrier cycle for SPWM. The period value depends on the value of WG-RELOAD;

    ②The phase driver channel determines the duty cycle of the SPWM waveform and is programmable output. Each phase driver contains a programmable signal-free time generator;

    ③The control circuit is used to determine the operating mode and other register configuration information.

    (2) The time base generator WG-COUNT has 4 working modes. When the strobe waveform generator is working, according to the selected working mode, the WG-COUNT as the time base generator continuously counts up or counts up/down. Each time it counts, the content of WG-COUNT is compared with the value of the count comparison register. Compare, when the two match, the corresponding operation will be generated according to the selected working mode.

    In the center-aligned PWM mode, the carrier cycle Tc=(4×WG-RELOAD)/Fxtal(μs)

    Regardless of the no-signal time, the output "valid" time Toutput=(4×WG-COMPx)/Fxtal(μs). The no-signal time is not considered,

　　Duty cycle = (WGCOMPx/WGRELOAD) × 100% where WGRELOAD——16-bit value;

    Fxtal——crystal oscillator frequency on XTAL1 pin, MHz;

    WG-COMPx - 16-bit value, equal to or less than WG-RELOAD.

    Edge-aligned PWM mode, carrier cycle Tc=(2×WG-RELOAD)/Fxtal(μs)

    Regardless of the no-signal time, the output "valid" time Toutput=(2×WG－COMPx)/Fxtal(μs)

    Ignoring the no-signal time, duty cycle = (WG-COMPx/WG-RELOAD) × 100%

    It can be seen from the above formula that changes in the WG-COMPx value change the duty cycle of the PWM wave. The SPWM waveform is generated by calculating the stored sine function data value and assigning it to WG-COMPx. Each interrupt assigns a value to WG-COMPx that changes with the sinusoidal rule, thus generating a series of pulses with different pulse widths. to approximate a sine wave.

    (3) WFG interruption

    There are two types of interrupts related to the waveform generator: WFG interrupt and EXTINT interrupt.

    The WFG interrupt is generated when WG-COUNT is reloaded. Different working modes have different reloading modes. Each PWM cycle, mode 0 generates a WFG interrupt when WG-COUNT=WG-RELOAD, mode 1 generates a WFG interrupt when WG-COUNT=WG-RELOAD and WG-COUNT=1. All generate interrupts.

    The EXTINT interrupt is generated by the protection circuit. The mode of generating interrupts can be set programmably to detect overcurrent signals in the entire system and protect power electronic switching devices.

    3 Inverter power supply hardware circuit

    The hardware structure of the static inverter power supply is shown in Figure 1. It mainly consists of the following parts.

    3.1 Main circuit

    It is in the form of AC/DC/AC inverter circuit. The input three-phase AC voltage is rectified and filtered and then supplied to the inverter. The main switching device uses the 2-unit IGBT module CM75DY-24H of Japan's Mitsubishi Company, plus a buffer circuit to form a three-phase inverter power supply for this system. The output uses an isolation step-down transformer.

    3.2 Control circuit

    The minimum system of 80C196MC microprocessor and a small number of peripheral chips constitute the control circuit of this system. The microcontroller generates three-phase 6-channel SPWM signals, and simultaneously completes functions such as frequency display, closed-loop voltage stabilization and current limiting control, detection and protection, and blocking of SPWM pulse signals.

    3.3 Drive circuit

     This inverter power supply drive circuit uses the special integrated circuit M57959L designed by Mitsubishi Corporation of Japan for driving IGBT, and is composed of a few peripheral components. The 80C196MC output SPWM signal can directly drive the IGBT tube through the driver module M57959L. When the M57959L detects the overcurrent signal on the IGBT tube, if the duration is greater than 2.5μs, it will send a fault signal, otherwise the protection circuit will not operate. The fault signal generates an EXTINT interrupt, blocks each SPWM signal, and turns off the IGBT at high speed. Its typical application circuit is shown in Figure 2.

    4 Software design

    Software program design is the core of the entire inverter power supply control. It determines the characteristics of the inverter power supply output, such as: voltage range and stability, harmonic content, perfection of protection functions, reliability, etc. The block diagram of the voltage-stabilized and current-limiting inverter power supply is shown in Figure 3 .

    In software design, please refer to the references for 80C196MC initialization commands, parameter calculation , generation of SPWM waveforms, dead time, etc.

    The output voltage value and current limit value are given by the potentiometer and converted into digital quantities through the 80C196MC on-chip A/D channel. The voltage given value is processed as a modulation depth coefficient to control the M57959L to output SPWM signal.

   5 Experimental results and conclusions

    A three-phase 8kVA static inverter power supply was produced according to the above hardware circuit. The main parameters are three-phase input 380V, 50Hz, three-phase output 220V, 400Hz. Take the carrier frequency as 6.4kHz and the dead time as 5μs. The SPWM signal when the modulation depth coefficient M=0.87 before and after isolation was collected using SignalView general signal analysis software, as shown in Figure 4 and Figure 5.

    The output voltage waveform after using L-type filtering (filter capacitor 15μF, inductor 0.37mH) is shown in Figure 6 (for convenience, step-down collection).

　　The waveform after spectrum analysis of the output waveform using this software is shown in Figure 7.

     The main technical indicators of the static inverter power supply measured with the YOKOGAWA Model WT2030 Digital Power Meter tester are: voltage stability (100% load change, 10% input change) 1%; total harmonic content 2.7%; overall machine efficiency above 85%.

    Experiments show that in the process of developing the inverter power supply, after using the 16-bit microcontroller 80C196MC minimum system, the entire control circuit is greatly simplified, the components are reduced, the structure is compact, the cost is reduced, the data processing is fast in the 16-bit mode, and the system is responsive. Improved reliability. The test achieved relatively ideal results. At the same time, the system can be used for single-phase inverter power supply by simply changing two data in the programming software. It is believed that the 80C196MC microprocessor has good practical value and promotion prospects in the field of frequency conversion intelligent control.