Design of three-phase half-controlled rectifier circuit based on single-chip microcomputer

Rectification circuits are widely used in DC motor speed regulation, DC voltage regulation and other occasions. The three-phase half-controlled rectifier bridge circuit structure is a common rectifier circuit, which is easy to control and has low cost. This article introduces a three-phase half-controlled rectifier circuit based on PIC690 microcontroller and dedicated integrated trigger chip TC787. It combines the advantages of dedicated integrated trigger chip and digital trigger to obtain high performance and highly symmetrical trigger pulses. It makes full use of the internal resources of the microcontroller, integrates phase sequence adaptation, system parameter online adjustment and various protection functions, and can be used for constant voltage control of the load. The main circuit adopts a three-phase half-controlled bridge structure, and the DC side adopts an LC filter structure to improve the output voltage quality.

System overall design

This system uses the PIC690 microcontroller as the main control chip and uses thyristors as the main switching devices. The design goal is to keep the output DC voltage stable, the output voltage ripple small, the AC output current THD low, and the performance reliable.

The main circuits of the system include: three-phase bridge half-controlled rectifier circuit, synchronous signal sampling circuit, microcontroller control circuit, and thyristor trigger circuit. First, the synchronous signal is obtained by the synchronous signal sampling circuit and sent to the integrated trigger chip TC787. After zero detection, the corresponding delay is performed to achieve phase shift. The ADC in the microcontroller is responsible for collecting the DC bus voltage, and the given output is adjusted through PI operation according to the deviation between the set value and the actual value of the voltage. The PIC microcontroller outputs the reference value of the voltage to TC787, and TC787 realizes the phase shift triggering of the thyristor to achieve rectification and voltage regulation. The overall block diagram of the hardware circuit is shown in Figure 1.

Figure 1 Overall block diagram of system hardware

Main circuit design

The main circuit adopts a three-phase bridge semi-controlled rectifier circuit, and the DC measurement adopts an LC filter current structure. The main current schematic diagram is shown in Figure 2. The semi-controlled bridge selects SEMIKRON's SKDH146/120-L100 module, which has a rated current of 140A and a rated voltage of 1200V. The DC side adopts an LC filter circuit structure, which is better than a single capacitor filter. In addition, the current THD on the AC input side can also be improved. The main harmonic content on the DC side is 6 times the power frequency and an integer multiple of 6. When designing LC low-pass filtering, it is necessary to avoid resonance caused by high-content harmonics. In this design, the inductor is 5mH and the filter capacitor is 480μF.

Figure 2 Main circuit structure

The three-phase voltage obtained from the power grid is shaped by the synchronization circuit and sent to the integrated trigger chip TC787 pin 18AT, pin 2 BT and pin 1CT. TC787 integrates 3 zero-crossing and polarity detection units, 3 sawtooth wave forming units, 3 comparators, 1 pulse generator, 1 anti-interference locking circuit and 1 pulse distribution and drive circuit. The digital given phase shift control voltage can automatically identify the phase sequence.

Control circuit design

PIC16F690 is used as the control chip. The PIC16F690 microcontroller has a built-in 10-bit AD; wide operating voltage (2.0~5.5V); low power consumption; PWM output function; built-in crystal oscillator. The chip's built-in 10-bit AD is used to perform AD conversion on the collected DC side voltage. In order to reduce hardware costs, a voltage divider resistor is directly used instead of a voltage sensor to collect the DC side voltage. The voltage on the voltage divider resistor passes through two reverse proportional circuits to the microcontroller. The analog ground and signal ground of the microcontroller are directly connected (it can also be connected through magnetic beads to reduce interference). The PIC16F690 microcontroller enables or disables the output of the chip TC787 through an IO port, as shown in Figure 3. When the PIC microcontroller's I/O port RC3 outputs a high level (+5V), the Lock port is a low level; when the microcontroller's I/O port RC3 outputs a low level, the Lock is a high level (+15V). An IO port is selected as the given signal of the TC787 reference voltage, and the PWM pulse method is used to adjust the duty cycle to adjust the output voltage. The PWM wave passes through an RC low-pass filter to become an approximate DC signal. This signal is used as the reference voltage given Uref, which has a range of 0 to 5V. Since the given input range required by the chip TC787 is 0-15V, the PWM wave must pass through an optocoupler for level conversion, as shown in Figure 3.

Figure 3 Control circuit hardware structure

The grid voltage is input to TC787 through a synchronous transformer. The 6th pin of TC787 outputs double pulses when high or single width pulses when low. Pins 12, 11, and 10 are the trigger output terminals of A, B, and C respectively, and are output to the thyristor through a pulse transformer.

Trigger drive circuit design

The trigger chip selects the high-performance thyristor three-phase phase-shift trigger integrated circuit TC787. TC787 can work with a single power supply or a dual power supply. It is mainly suitable for three-phase thyristor phase-shift trigger and three-phase power transistor pulse width modulation circuits to form a variety of AC speed regulation and current conversion devices. The internal structure of TC787 is shown in Figure 4.

Figure 4 TC787 chip internal structure

In this design, TC787 is powered by 15V, and pin 4 (Vr) is the phase shift control voltage input terminal. The input voltage of this terminal directly determines the phase shift range of the output pulse of TC787/TC788, and it is connected to the output of the given link in the application. Pin 5 (Pi): Output pulse prohibition terminal. This terminal is used to block the output of TC787/TC788 under fault conditions. The high level is valid. In the application, it is connected to the output of the protection circuit. Synchronous voltage input terminal: Pin 1 (Vc), Pin 2 (Vb) and Pin 18 (Va) are the three-phase synchronous input voltage connection terminals. In the application, the input filtered synchronous voltage is connected respectively, and the peak value of the synchronous voltage should not exceed the working power supply voltage VDD of TC787/TC788.

The trigger drive circuit is mainly composed of the grid voltage synchronization circuit, the TC787 integrated trigger circuit and the pulse amplification isolation drive circuit. Figure 5 shows the synchronization circuit and the peripheral circuit of TC787. The first half is the voltage synchronization circuit, and this design method requires more auxiliary components. By adjusting the three potentiometers RP1 to RP3 differently, a phase shift of 0 to 60° can be achieved, thus meeting the needs of different main transformer connections. In Figure 5, the midpoint of the synchronous transformer is directly connected to the (1/2) power supply voltage, which simplifies the components used. Pin 4 of TC787 outputs the given voltage (0 to +15V) of the microcontroller, and pin 6 is the trigger pulse blocking pin. Pins 10 to 12 are trigger pulse output pins, which are connected to the isolation circuits of phases C, B, and A respectively.

Figure 5 Synchronous circuit and pulse generation circuit structure diagram

Voltage detection circuit design

In order to reduce hardware costs, the voltage divider resistor method is used instead of the voltage sensor when designing the DC bus voltage detection circuit. This voltage divider resistor method has a simple structure and is easy to debug. The circuit is shown in Figure 6. The voltage obtained by the voltage divider resistor is 1/31 of the DC bus voltage. This voltage is input to the AD1 input port of the PIC microcontroller through two reverse proportional amplifier circuits, and then processed into a digital quantity through the AD conversion of the PIC microcontroller.

Figure 6 Voltage detection circuit