Rectifier circuits are widely used in DC motor speed control, DC voltage regulation, and other applications. The three-phase semi-controlled rectifier bridge circuit is a common type of rectifier circuit, known for its ease of control and low cost. This paper introduces a three-phase semi-controlled rectifier circuit based on a PIC690 microcontroller and a dedicated integrated trigger chip TC787. It combines the advantages of a dedicated integrated trigger chip and a digital trigger, achieving high performance and highly symmetrical trigger pulses. It fully utilizes the microcontroller's internal resources, integrating phase sequence self-adaptation, online system parameter adjustment, and various protection functions, and can be used for constant voltage control of the load. The main circuit adopts a three-phase semi-controlled bridge structure, with an LC filter structure on the DC side to improve the output voltage quality. System Overall Design This system uses a PIC690 microcontroller as the main control chip and thyristors as the main switching devices. The design aims to maintain a stable DC output voltage, low output voltage ripple, low THD of the AC output current, and reliable performance. The main circuits of the system include: a three-phase bridge semi-controlled rectifier circuit, a synchronous signal sampling circuit, a microcontroller control circuit, and a thyristor trigger circuit. First, the synchronization signal is obtained by the synchronization signal sampling circuit and sent to the integrated trigger chip TC787. After zero detection, a corresponding delay is applied to achieve phase shifting. The ADC in the microcontroller is responsible for acquiring the DC bus voltage. Based on the deviation between the set value and the actual value of the voltage, the given output is adjusted by PI calculation. The PIC microcontroller outputs the reference value of the voltage to the TC787, which then triggers 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. The DC side adopts an LC filter current structure. The main current schematic diagram is shown in Figure 2. The semi-controlled bridge is selected from the SEMIKRON 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 has a better filtering effect than a single capacitor. In addition, it can also improve the THD of the AC input current. The main harmonic content on the DC side is 6 times the power frequency and integer multiples of 6. When designing an LC low-pass filter, resonance caused by high harmonic content should be avoided. In this design, a 5mH inductor and a 480μF filter capacitor are selected. Figure 2 shows the main circuit structure. The three-phase voltage obtained from the power grid is shaped by the synchronization circuit and then sent to the integrated trigger chip TC787 (pins 18AT, 2BT, and 1CT). The TC787 integrates three zero-crossing and polarity detection units, three sawtooth wave forming units, three comparators, one pulse generator, one anti-interference lockout circuit, and one pulse distribution and drive circuit. It provides digital phase-shift control voltage and can automatically identify the phase sequence. Control Circuit Design The PIC16F690 is used as the control chip. The PIC16F690 microcontroller has a built-in 10-bit AD converter; wide operating voltage (2.0～5.5V); low power consumption; PWM output function; and an internal crystal oscillator. The built-in 10-bit AD converter is used to perform AD conversion on the acquired DC side voltage. To reduce hardware costs, a voltage divider resistor is used instead of a voltage sensor to acquire the DC-side voltage. The voltage across the voltage divider resistor is fed to the microcontroller via two inverting proportional circuits. The microcontroller's analog ground and signal ground are directly connected (or connected via a ferrite bead to reduce interference). The PIC16F690 microcontroller enables or disables the TC787 chip's output through an I/O port, as shown in Figure 3. When the PIC microcontroller's I/O port RC3 outputs a high level (+5V), the Lock port is low; when the microcontroller's I/O port RC3 outputs a low level, the Lock port is high (+15V). One I/O port is selected as the reference voltage input signal for the TC787. A PWM pulse method is used, and the duty cycle is adjusted to regulate the output voltage. The PWM wave passes through an RC low-pass filter to become an approximate DC signal, which is used as the reference voltage input Uref, with a range of 0–5V. Since the TC787 chip requires a input range of 0–15V, the PWM wave needs to be level-converted via an optocoupler, as shown in Figure 3. Figure 3 shows the hardware structure of the control circuit. The grid voltage is input to the TC787 through a synchronous transformer. Pin 6 of the TC787 outputs a double pulse when high or a single-width pulse when low. Pins 12, 11, and 10 are the trigger output terminals A, B, and C, respectively, which are output to the thyristors through a pulse transformer. Trigger Drive Circuit Design The high-performance thyristor three-phase phase-shift trigger integrated circuit TC787 is selected as the trigger chip. The TC787 can operate with a single or dual power supply and is mainly suitable for three-phase thyristor phase-shift triggering and three-phase power transistor pulse width modulation circuits to construct various AC speed control and converter devices. The internal structure of the TC787 is shown in Figure 4. Figure 4 shows the internal structure of the TC787 chip. In this design, the TC787 uses a 15V power supply. Pin 4 (Vr): Phase-shift control voltage input terminal. The level of the input voltage at this terminal directly determines the phase shift range of the TC787/TC788 output pulse. In application, it is connected to the output of the given circuit. Pin 5 (Pi): Output pulse inhibit terminal. This terminal is used to block the output of TC787/TC788 in fault conditions. It is active high and is connected to the output of the protection circuit in application. Synchronization voltage input terminals: Pin 1 (Vc), pin 2 (Vb), and pin 18 (Va) are the three-phase synchronization input voltage connection terminals. In application, they are connected to the filtered synchronization voltage respectively. The peak value of the synchronization voltage should not exceed the operating power supply voltage VDD of TC787/TC788. The trigger drive circuit mainly consists of the mains 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. This design method requires more auxiliary components. By adjusting the three potentiometers RP1 to RP3, a phase shift of 0 to 60° can be achieved, thereby adapting to the needs of different main transformer connections. In Figure 5, the midpoint of the synchronization transformer is directly connected to the (1/2) power supply voltage, which simplifies the components used. Pin 4 of the TC787 outputs the microcontroller's given voltage (0～+15V), and pin 6 is the trigger pulse blocking pin. Pins 10-12 are trigger pulse output pins, connected to the isolation amplifier circuits of phases C, B, and A, respectively. Figure 5 shows the structure diagram of the synchronization circuit and pulse generation circuit. Figure 6 shows the voltage detection circuit. To reduce hardware costs, a voltage divider resistor method was used in the DC bus voltage detection circuit design instead of a voltage sensor. This method is simple in structure and easy to debug. The circuit is shown in Figure 6. The voltage obtained through 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 inverting proportional amplifier circuits, and then converted into a digital value by the PIC microcontroller's AD conversion.