Design of APFC automatic voltage regulation circuit

Due to various reasons, there are current harmonics in the power grid. Due to the existence of power grid impedance, the harmonic current flows through the power grid impedance, which will cause the voltage waveform at the load end to be distorted. At this time, the power factor of the system is less than 1, which will bring "pollution" to the power grid, and will also affect the output voltage of the ultrasonic generator and the normal operation of the system. Therefore, it is necessary to design a special circuit to suppress harmonics, and the PFC circuit is one of them. The voltage output by PFC is generally constant, but in some cases, the output voltage needs to be adjusted, and the output power is controlled by adjusting the output voltage. Therefore, the study of the control strategy of PFC output voltage has certain practical significance.

Single-phase PFC technology

PFC (Power Factor Correction) is to suppress the height of the current pulse to make the current waveform as close to a sine wave as possible. Single-phase PFC can be divided into passive power factor correction and active power factor correction according to the specific methods used.

Figure 1 Single-phase passive power factor correction circuit

Figure 2 Single-phase APFC circuit and main waveforms

APFC circuit voltage control scheme

The APFC circuit uses a constant voltage feedback network composed of a resistor divider to control the output constant voltage. Based on this principle, this paper cleverly uses the APFC voltage feedback network to perform voltage control, that is, to adjust the output voltage by changing the resistor R2 of the resistor divider network. The change of R2 causes the voltage feedback signal to change accordingly. The voltage comparator compares the voltage setting, and the error of the comparison output controls the PWM generator, thereby adjusting the pulse width of the drive signal so that the output voltage can be linearly adjusted within a certain range. In this way, the input voltage of the inverter can be adjusted according to actual requirements. When applied in power control situations, when the load increases, the output current decreases instantaneously, and the power decreases accordingly. At this time, R2 is adjusted to increase the voltage, and the output current also increases until the product of the voltage and current (that is, the output power) is consistent with the given reference, thereby achieving the adjustment of the output power.

However, if only R2 is adjusted, it will cause hidden dangers. For example, if R2 is short-circuited, R1 will be directly grounded, and the output voltage will continue to rise; if R2 is open-circuited, the voltage feedback will be pulled up to the bus voltage, which may cause no output. Therefore, the voltage feedback network to be designed must prevent the above situation. In the voltage feedback network designed in this article, R3 is connected in series with R2 to prevent R2 from short-circuiting, and R4 is connected in parallel with R2 and R3 to prevent the occurrence of short-circuit conditions. Figure 3 shows the preliminary design of the voltage regulation feedback network.

Figure 3 Voltage feedback network

In the preliminary design, R2 is an adjustable potentiometer, which needs to be adjusted manually. However, this method is not only inaccurate, but also cumbersome to operate and has a low degree of automation. Therefore, this paper adopts the strategy of using a single-chip microcomputer to control a digital potentiometer instead of a mechanical potentiometer R2 for automatic adjustment.

This paper uses a non-volatile digital potentiometer, which is an intelligent device that can be programmed to operate automatically under computer control. It is not continuously adjustable like a mechanical or analog potentiometer, but its step-by-step resistance change has the characteristics of high adjustment accuracy and stable resistance. The more steps of resistance resolution, the finer the resistance change, and the higher the adjustment sensitivity [2][3]. The digital potentiometer used in this paper is the 100-section non-volatile digital potentiometer X9312 produced by Xicor. The schematic diagram of X9312 is shown in Figure 4 [3].

Figure 4 X9312 schematic diagram

Due to various reasons, there are current harmonics in the power grid. Due to the existence of power grid impedance, the harmonic current flows through the power grid impedance, which will cause the voltage waveform at the load end to be distorted. At this time, the power factor of the system is less than 1, which will bring "pollution" to the power grid, and will also affect the output voltage of the ultrasonic generator and the normal operation of the system. Therefore, it is necessary to design a special circuit to suppress harmonics, and the PFC circuit is one of them. The voltage output by PFC is generally constant, but in some cases, the output voltage needs to be adjusted, and the output power is controlled by adjusting the output voltage. Therefore, the study of the control strategy of PFC output voltage has certain practical significance.

Single-phase PFC technology

PFC (Power Factor Correction) is to suppress the height of the current pulse to make the current waveform as close to a sine wave as possible. Single-phase PFC can be divided into passive power factor correction and active power factor correction according to the specific methods used.

Figure 1 Single-phase passive power factor correction circuit

Figure 2 Single-phase APFC circuit and main waveforms

APFC circuit voltage control scheme

The APFC circuit uses a constant voltage feedback network composed of a resistor divider to control the output constant voltage. Based on this principle, this paper cleverly uses the APFC voltage feedback network to perform voltage control, that is, to adjust the output voltage by changing the resistor R2 of the resistor divider network. The change of R2 causes the voltage feedback signal to change accordingly. The voltage comparator compares the voltage setting, and the error of the comparison output controls the PWM generator, thereby adjusting the pulse width of the drive signal so that the output voltage can be linearly adjusted within a certain range. In this way, the input voltage of the inverter can be adjusted according to actual requirements. When applied in power control situations, when the load increases, the output current decreases instantaneously, and the power decreases accordingly. At this time, R2 is adjusted to increase the voltage, and the output current also increases until the product of the voltage and current (that is, the output power) is consistent with the given reference, thereby achieving the adjustment of the output power.

However, if only R2 is adjusted, it will cause hidden dangers. For example, if R2 is short-circuited, R1 will be directly grounded, and the output voltage will continue to rise; if R2 is open-circuited, the voltage feedback will be pulled up to the bus voltage, which may cause no output. Therefore, the voltage feedback network to be designed must prevent the above situation. In the voltage feedback network designed in this article, R3 is connected in series with R2 to prevent R2 from short-circuiting, and R4 is connected in parallel with R2 and R3 to prevent the occurrence of short-circuit conditions. Figure 3 shows the preliminary design of the voltage regulation feedback network.

Figure 3 Voltage feedback network

In the preliminary design, R2 is an adjustable potentiometer, which needs to be adjusted manually. However, this method is not only inaccurate, but also cumbersome to operate and has a low degree of automation. Therefore, this paper adopts the strategy of using a single-chip microcomputer to control a digital potentiometer instead of a mechanical potentiometer R2 for automatic adjustment.

This paper uses a non-volatile digital potentiometer, which is an intelligent device that can be programmed to operate automatically under computer control. It is not continuously adjustable like a mechanical or analog potentiometer, but its step-by-step resistance change has the characteristics of high adjustment accuracy and stable resistance. The more steps of resistance resolution, the finer the resistance change, and the higher the adjustment sensitivity [2][3]. The digital potentiometer used in this paper is the 100-section non-volatile digital potentiometer X9312 produced by Xicor. The schematic diagram of X9312 is shown in Figure 4 [3].

Figure 4 X9312 schematic diagram

Due to various reasons, there are current harmonics in the power grid. Due to the existence of power grid impedance, the harmonic current flows through the power grid impedance, which will cause the voltage waveform at the load end to be distorted. At this time, the power factor of the system is less than 1, which will bring "pollution" to the power grid, and will also affect the output voltage of the ultrasonic generator and the normal operation of the system. Therefore, it is necessary to design a special circuit to suppress harmonics, and the PFC circuit is one of them. The voltage output by PFC is generally constant, but in some cases, the output voltage needs to be adjusted, and the output power is controlled by adjusting the output voltage. Therefore, the study of the control strategy of PFC output voltage has certain practical significance.

Single-phase PFC technology

PFC (Power Factor Correction) is to suppress the height of the current pulse to make the current waveform as close to a sine wave as possible. Single-phase PFC can be divided into passive power factor correction and active power factor correction according to the specific methods used.

Figure 1 Single-phase passive power factor correction circuit

Figure 2 Single-phase APFC circuit and main waveforms

APFC circuit voltage control scheme

The APFC circuit uses a constant voltage feedback network composed of a resistor divider to control the output constant voltage. Based on this principle, this paper cleverly uses the APFC voltage feedback network to perform voltage control, that is, to adjust the output voltage by changing the resistor R2 of the resistor divider network. The change of R2 causes the voltage feedback signal to change accordingly. The voltage comparator compares the voltage setting, and the error of the comparison output controls the PWM generator, thereby adjusting the pulse width of the drive signal so that the output voltage can be linearly adjusted within a certain range. In this way, the input voltage of the inverter can be adjusted according to actual requirements. When applied in power control situations, when the load increases, the output current decreases instantaneously, and the power decreases accordingly. At this time, R2 is adjusted to increase the voltage, and the output current also increases until the product of the voltage and current (that is, the output power) is consistent with the given reference, thereby achieving the adjustment of the output power.

However, if only R2 is adjusted, it will cause hidden dangers. For example, if R2 is short-circuited, R1 will be directly grounded, and the output voltage will continue to rise; if R2 is open-circuited, the voltage feedback will be pulled up to the bus voltage, which may cause no output. Therefore, the voltage feedback network to be designed must prevent the above situation. In the voltage feedback network designed in this article, R3 is connected in series with R2 to prevent R2 from short-circuiting, and R4 is connected in parallel with R2 and R3 to prevent the occurrence of short-circuit conditions. Figure 3 shows the preliminary design of the voltage regulation feedback network.

Figure 3 Voltage feedback network

In the preliminary design, R2 is an adjustable potentiometer, which needs to be adjusted manually. However, this method is not only inaccurate, but also cumbersome to operate and has a low degree of automation. Therefore, this paper adopts the strategy of using a single-chip microcomputer to control a digital potentiometer instead of a mechanical potentiometer R2 for automatic adjustment.

This paper uses a non-volatile digital potentiometer, which is an intelligent device that can be programmed to operate automatically under computer control. It is not continuously adjustable like a mechanical or analog potentiometer, but its step-by-step resistance change has the characteristics of high adjustment accuracy and stable resistance. The more steps of resistance resolution, the finer the resistance change, and the higher the adjustment sensitivity [2][3]. The digital potentiometer used in this paper is the 100-section non-volatile digital potentiometer X9312 produced by Xicor. The schematic diagram of X9312 is shown in Figure 4 [3].

Figure 4 X9312 schematic diagram

Figure 5 is the schematic diagram of voltage adaptive control using digital potentiometer designed in this paper. The detection signal in the figure is the voltage feedback signal, and its output is used as the control signal after A/D conversion. The microcontroller gives the adjustment signal and counting pulse according to the control relationship and characteristics, so that the digital potentiometer changes its resistance value and acts on the level control circuit (i.e. APFC circuit) to achieve the requirement of adjusting the output voltage.

Figure 5 Voltage control schematic diagram

The software flow controlled by the single chip microcomputer is shown in Figure 6. In the figure, A1 is, A2 is, A3 is, and A4 is the blocking signal. A1, A2, and A3 control the digital potentiometer X9312, and A4 is the blocking signal to prevent the output from being too large and the control from failing. The protection circuit controlled by A4 blocks the input when the output is out of control, protecting the safety of the entire circuit.

Figure 6 MCU software control flow chart

According to the voltage regulation principle mentioned above, the circuit for realizing APFC output voltage adaptive control by using a single chip microcomputer to control a digital potentiometer is finally determined as shown in FIG7 .

Figure 7 Voltage control circuit

in conclusion

The APFC circuit is used to adjust the output voltage, which not only improves the power factor of the entire circuit, but also controls the output voltage. On this basis, the single-chip microcomputer is used to control the digital potentiometer to adjust the voltage, realizing digital control with high precision, good safety and good adaptive effect. This control strategy can be applied to the inverter circuit bus voltage control and the output power control system.