Design of multi-chip microcomputer DC power supply control board

0 Introduction
The multi-MCU DC power supply control board includes A/D acquisition and conversion, measurement, display, synchronization, automatic phase sequence determination, phase shift triggering, overcurrent/overvoltage protection, phase loss detection and other parts, together with rectifier transformers, batteries, instruments and other components to form a complete set of devices. The device has charging, current stabilization, voltage stabilization and other working modes, and can be used as a DC power supply for control, operation or lighting in power plants, substations, hospitals, factories and other departments. The hardware circuit of the multi-MCU power supply control system is simple and clear, the digital trigger pulse is high in precision, the system adjustment speed is fast, and the performance index and reliability are high.

1 System structure
1.1 Rectifier transformer and main circuit
The circuit of the rectifier transformer and main circuit is shown in Figure 1. The main circuit of the multi-chip microcomputer DC power supply control system is a three-phase bridge-type fully controlled rectifier circuit. The primary side control protection devices of the rectifier transformer include relays, control switches, fuses, power indicator lights, etc. The primary side is connected to a 380 V AC power supply. The secondary side of the transformer serves as the power supply for the three-phase bridge-type fully controlled rectifier circuit. There are six thyristors in the main circuit. The trigger pulse circuit of the thyristor in this circuit must meet the following conditions:
(1) The trigger pulse must be synchronized with the main circuit power supply and have a certain phase shift range;
(2) The trigger pulse should have a certain width to ensure that the triggered thyristor is reliably turned on;
(3) The leading edge of the trigger pulse should be as steep as possible and have sufficient power.

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1.2 Hardware block diagram of power control board
The DC power control board consists of four 51 series microcontrollers, namely GMS97C52, GMS97C51 and two AT89C2051. The GMS97 series microcontrollers are produced by LG Corporation of South Korea, and are compatible with the Tnt-elMCS-51 series microcontrollers. They have the characteristics of low power consumption, low price, OTP (One Time Programmable), etc. The hardware block diagram of the power control board is shown in Figure 2.

The functions of the four microcontrollers are briefly described as follows:
(1) U18 (AT89C2051) is used to receive AC voltage, form rectangular synchronous pulses, and send phase-shift pulses to U20 for forwarding; it is also used to distinguish phase sequence and detect disconnection.
(2) U20 (GMS97C52) is used for A/D acquisition and measurement. The acquired measurement values ​​are digitally filtered and compared with the set values. The control quantity is modified according to the deviation value, thereby achieving the purpose of adjusting the current and voltage. At the same time, the input value of the human-machine connection, the modified voltage and current set values ​​are stored in the E2PROM, and the parameters and fault information are displayed.
(3) U19 (GMS97C51) is for remote control or group use, receiving remote control information and transmitting it to U20 (GMS97C52), and processing the alarm information input by U20.
(4) U23 (AT89C2051) receives phase-shift pulses, adjusts their width, and outputs double narrow pulses to trigger the thyristor.

2 Hardware Design
2.1 Synchronous Pulse Formation Circuit
In the three-phase bridge fully controlled rectifier circuit, the control angle is the corresponding line voltage zero crossing point. The abc three-phase phase voltage is obtained from the K terminals of the 4#, 6#, and 2# thyristors of the fully controlled bridge rectifier circuit and connected to the circuit terminals K2-c, K6-b, and K4-a, as shown in Figure 3. This circuit connects the two-phase voltage to the photocoupler U32. The negative jump moment of the output signal of the 4th foot of U32 is the earliest moment that the thyristors of each phase can be triggered to conduct. That is, at this moment, the interruption is coordinated to achieve synchronization. In addition, because the phases of the three-phase power supply are 120° apart, it can be analyzed that no matter what the order of the power supply line connection is, the phase sequence relationship has only two types: positive sequence and negative sequence. It can be further identified in the software and the correct six-way trigger pulse signal can be issued accordingly.

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2.2 Driving circuit
In the circuit of Figure 4, RV6, RV12, and C+12 act as shunts, which can improve the anti-interference ability of the trigger circuit. U6 is an output-stage power amplifier transistor, which amplifies the power of the trigger pulse from the microcontroller. T8 is a pulse transformer, L7 indicates the working condition of the thyristor, and CF6 and RL7 can improve the anti-interference ability of the thyristor and reduce the gate input impedance. 2.3
Current and voltage sampling circuit In the circuit of Figure 5, the analog/digital converter uses TI's 12-bit successive approximation chip TLC254 3, which has functions such as sampling and holding and serial three-state output, and is widely used in instruments. The current sampling circuit uses the true effective value conversion chip AD736, which simplifies the software design. The current and voltage signals output by the main circuit are sent to the microcontroller U20 after A/D conversion. The microcontroller then modifies the control amount according to the deviation value and realizes functions such as overvoltage and overcurrent protection and fault judgment.

2.4 Watchdog circuit
The control board is equipped with an up monitoring circuit with a watchdog timer, which uses the MAX813L chip, as shown in Figure 6. When the monitoring circuit is working, if it is not detected within 1.6 s, it will continue to send a reset signal until the program returns to normal. [page]

3 Software Design
3.1 Implementation of Synchronous Coordination
First, an interrupt signal is provided to the microcontroller according to the three-phase synchronous voltage signal. After the microcontroller responds to the interrupt, a trigger pulse is generated according to the requirements of the three-phase full-controlled bridge rectifier circuit for the trigger pulse. Whenever the phase switching point of phase a and phase b is reached, as shown in Figure 3 (a), the phase voltage comparison between phase a and phase b will generate a falling edge signal to the external interrupt 1 pin (INT1) of the U18 microcontroller, and a synchronous pulse is generated through the interrupt service event. The synchronous pulse is sent to U20 (AT89C52), and the waveform is shown in Figure 7.

3.2 Trigger pulse phase shift and pulse width control
This system changes the trigger angle by changing the count value of the timer/counter in the single-chip microcomputer, thereby changing the size of the DC voltage and DC current.
The synchronization pulse generated by the U18 single-chip microcomputer is sent to the U20 single-chip microcomputer. The U20 single-chip microcomputer compares the measured value with the set value, adjusts the ratio, makes the measured value equal to the set value, and generates a phase shift pulse to the external interrupt 1 (INT1) of U23. After responding to external interrupt 1, the P1 port starts to output the first encoding pulse and time it at the same time, corresponding to one encoding pulse every 60°, until the last encoding pulse is sent, and then returns to wait for the next interrupt request of external interrupt 1 (INT1), that is, the upper machine sends a phase shift synchronization pulse to the U23 single-chip microcomputer. Dual pulse encoding is used in pulse encoding to achieve dual pulse triggering. In this process, T0 is used as a counter, and a trigger pulse is sent when the count reaches N times. The calculation method of N is as follows:
Given: power supply frequency f = 50 Hz, crystal frequency F = 8 MHz, timing is 60°, then:

According to the N value and the working mode of the timer/counter, the initial value of the timer/counter can be calculated.
3.3 Proportional control adjustment program
In order to make the output voltage/current value match the set value, in the software control, the control amount is proportional to the deviation between the set value and the measured value, so that the actual output value continues to follow the set value and finally reaches consistency. In addition, in this subroutine, after the system is started, within 5 seconds, the adjustment subroutine only allows the control amount to increase in steps of one unit, which can prevent
the voltage from rising too fast, thereby achieving soft start.
The control quantity is proportional to the deviation between the measured value and the set value. Theoretically, the PID algorithm is used, and the discrete difference equation expression for regulation is as follows: Wherein

: en, en-1, en-2 are the deviation values ​​of the nth, n-1th and n-2nd voltages respectively; KP, TI, TD are the proportional coefficient, integral coefficient and differential coefficient respectively, and T is the adoption period.

4 Conclusion
The design of this DC power supply control board has the following outstanding features:
(1) Interrupt function: four 51 series single-chip microcomputers, each with four interrupt sources, such as synchronization, phase shift, phase sequence discrimination and phase loss signal transmission all use interrupt function, which significantly improves the system response speed and reliability.
(2) The resolution of A/D is 12 bits, the minimum counting interval of the timer is 1.5μs, and the voltage and current accuracy can reach 5‰ when there is a small disturbance.
(3) It has the functions of automatic identification of power phase sequence, automatic detection of phase loss, fault alarm and status display.
(4) The multi-chip microcomputer control design makes the single chip function single, improves the adjustment speed and reliability of system performance.