Design of multi-chip microcomputer DC power supply control board

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, and together with rectifier transformers, batteries, instruments and other components, it constitutes 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 the main circuit is shown in Figure 1. The main circuit of the multi-MCU DC power control system is a three-phase bridge-type fully controlled rectifier circuit. The primary-side control and 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:

Figure 1 Rectifier transformer and main circuit

(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.

1.2 Power control board hardware block diagram

The DC power supply control board is composed of four 51 series single chip microcomputers, namely GMS97C52, GMS97C51 and two AT89C2051.

The GMS97 series microcontroller is produced by LG Corporation of South Korea, and is compatible with the Tntel MCS 51 series microcontroller. It has 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.

Figure 2 Hardware block diagram of power control board

The functions of the four MCUs 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 determine phase sequence and detect line breaks.

(2) U20 (GMS97C52) is used for A/D acquisition measurement. The acquired measurement value is digitally filtered and compared with the set value. The control amount is modified according to the deviation value, so as to achieve the purpose of adjusting the current and voltage. At the same time, the input value of the human-machine connection, the modified voltage, and the set value of the current are stored in the E2 PROM, and the parameters and fault information are displayed.

(3) U19 (GMS97C51) is used for remote control or grouping. It receives remote control information and transmits it to U20 (GMS97C52), and also processes the alarm information input by U20.

(4) U23 (AT89C2051) receives the phase-shifted pulse, adjusts its width, and outputs dual narrow pulses to trigger the thyristor.

2 Hardware Design

2.1 Synchronous pulse forming circuit

In the three-phase bridge full-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 in the full-controlled bridge rectifier circuit and connected to the circuit terminals K2c, K6b, and K4a, 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 4-pin of U32 is the earliest moment when the thyristors of each phase can be triggered to conduct. That is, at this moment, the coordination interruption is achieved to achieve synchronization. In addition, because the phases of the three-phase power supply are 120 degrees 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.

2.2 Driving circuit

In the circuit of Figure 4, RV6, R V12, and C+ 12 act as shunts to improve the anti-interference ability of the trigger circuit. U 6 is the 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 status of the thyristor. CF6 and RL7 can improve the anti-interference ability of the thyristor and reduce the gate input impedance.

Figure 3 Schematic diagram of synchronous pulse forming circuit

Figure 4 Pulse control unit and trigger circuit

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 TLC2543, 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.

Figure 5 Current and voltage sampling circuit

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.

3 Software Design

Figure 6 Watchdog circuit

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.

Figure 7 Synchronous pulse waveform

3.2 Trigger pulse phase shift and pulse width control

The system changes the trigger angle by changing the count value of the timer/counter inside the microcontroller, thereby changing the size of the DC voltage and DC current.

The synchronization pulse generated by the U18 microcontroller is sent to the U20 microcontroller. The U20 microcontroller compares the measured value with the set value and adjusts the ratio so that the measured value is equal to the set value. At the same time, it 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 at the same time, it is timed to send a coding pulse every 60° until the last encoding pulse is sent. Then it 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 microcontroller. 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:

Based on the value of N and the timer/counter operating mode, the initial value of the timer/counter can be calculated.

3.3 Proportional control adjustment procedure

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 constantly follows the set value and finally reaches consistency. In addition, in this subroutine, after the system starts, within 5 seconds, the adjustment subroutine only allows the control amount to increase in steps of one unit, so as to 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:

In the formula: 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 DC power supply control board has the following outstanding features in its design:

(1) Adopting interrupt function: There are four 51 series MCUs, each with four interrupt sources. The transmission of signals such as synchronization, phase shift, phase sequence discrimination and phase loss all utilize the 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 1.5 s, and the voltage and current accuracy can reach 5 ? when the disturbance is small.

(3) It has the functions of automatic identification of power supply phase sequence, automatic detection of phase loss, fault alarm and status display.

(4) Multi-MCU control design makes the MCU function single, improving the adjustment speed and reliability of system performance.