Switching power supply and its control method

Abstract

The objective is to provide a switching power supply equipped with an AC / DC converter having an AC filter circuit and a PFC circuit, which prevents a decrease in power factor even under light load conditions. [Solution] The problem is solved by a switching power supply device comprising an AC / DC converter having a PFC circuit and a pulse signal generation unit that generates pulse signals for driving the switch elements of the PFC circuit so that the voltage of the AC / DC converter becomes a predetermined target value, wherein the AC / DC converter further comprises an AC filter and a current sensor that monitors the current of the AC power supply in order to generate the drive pulse signals for the switch elements, and the current sensor is disposed on the AC power supply side of the AC filter.

Description

Technical Field

[0001] The present invention relates to a switching power supply device, more specifically, to a switching power supply device including an AC-DC converter having an AC filter circuit and a PFC circuit, and a control method thereof.

Background Art

[0002] A switching power supply device having an AC filter circuit and a PFC circuit is known. For example, Patent Document 1 describes a switching power supply device including an AC-DC converter having an AC filter circuit and a PFC circuit.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the device described in Patent Document 1 and other conventional switching power supply devices, AC power is converted into DC power by full-bridge rectification or the like, and a PFC circuit is provided to approximate the AC current to a sine wave to improve the power factor. Specifically, the current in the AC-DC converter circuit is detected by a sensor, and based on the detected current, the switching element of the PFC circuit is controlled to reduce the harmonic components of the input current and approximate it to a sine wave, thereby improving the power factor. However, at light loads, there is a problem that the power factor improvement by the PFC circuit is not sufficient and the power factor decreases. The reason for this will be described using the circuit diagram of a conventional switching power supply device and the waveform diagram of the circuit. Figure 1 is a circuit diagram showing an example of a conventional switching power supply. However, some details of the circuit elements are omitted and shown as functional blocks. Figure 2 is a waveform diagram of the AC system voltage and AC system current under light load and heavy load conditions in a conventional switching power supply. However, in the AC system current waveform diagram, the waveforms of the load current Ia of the AC / DC converter and the reactive current flowing through the capacitor in the AC filter are shown superimposed as dotted lines. Of the two dotted waveforms, the one with the larger amplitude is the waveform of the load current Ia, and the one with the smaller amplitude is the waveform of the reactive current. As shown in Figure 1, in conventional switching power supplies, AC power supplied from an AC power source is connected to the load via a surge protection circuit, an AC filter circuit, and a PFC circuit. The PFC circuit performs feedback control using a dual loop of voltage control and current control, and various sensors are provided around the PFC circuit to monitor electrical quantities. Specifically, a voltage sensor is provided on the output side of the PFC circuit to monitor the load voltage, and a current sensor is provided on the input side of the PFC circuit to monitor the load current. The load current Ia is detected by the current sensor and used to control the switching elements of the PFC circuit. Incidentally, due to the presence of reactive current flowing through the capacitor in the AC filter, the load current Ia detected by the current sensor is the AC system current minus the reactive current. Here, the magnitude of the reactive current is determined by the AC system voltage and the impedance of the capacitor, and is not affected by the magnitude of the load connected to the AC / DC converter. As a result, as shown in the waveform diagram of Figure 2, at high load, the amplitude of the reactive current becomes negligibly small with respect to the amplitude of the AC system current or load current, so the load current Ia used for switching control of the PFC circuit is approximately the same as the AC system current. On the other hand, at light load, the amplitude of the reactive current becomes a significant amount with respect to the amplitude of the AC system current or load current, and the load current Ia no longer matches the AC system current. In other words, the phase difference between the AC system current and the load current Ia increases, and the power factor decreases.

[0005] The present invention has been made in view of these circumstances, and its objective is to provide a switching power supply device equipped with an AC-DC converter having an AC filter circuit and a PFC circuit, in which the power factor does not decrease even under light load conditions. [Means for solving the problem]

[0006] To achieve this objective, the technical means according to the present invention is a switching power supply device having at least the following configuration.

[0007] A switching power supply device comprising an AC-DC converter having a PFC circuit, and a pulse signal generation unit that generates pulse signals for driving the switch elements of the PFC circuit so that the voltage of the AC-DC converter becomes a predetermined target value, wherein the AC-DC converter further comprises an AC filter and a current sensor that monitors the current of the AC power supply in order to generate the drive pulse signals for the switch elements, and the current sensor is disposed on the AC power supply side of the AC filter.

[0008] Furthermore, in order to achieve the above objective, the technical means according to the present invention is a control method for a switching power supply device comprising at least the following configuration.

[0009] A method for controlling a switching power supply device comprising: an AC / DC converter having a PFC circuit; a pulse signal generation unit that generates pulse signals to drive the switching elements of the PFC circuit so that the voltage of the AC / DC converter becomes a predetermined target value; an AC filter; a current sensor disposed on the AC system power supply side of the AC filter for monitoring the current of the AC system power supply; an output voltage sensor for monitoring the output voltage; and an AC system voltage sensor for monitoring the voltage of the AC system power supply, the method comprising: inputting an output voltage command value to the input of the pulse signal generation unit; calculating the difference between the output voltage command value and the output voltage monitored by the output voltage sensor; outputting an RMS current value by a voltage compensator that brings the difference in output voltages closer to zero; calculating a current command value that brings the difference in output voltages closer to zero by multiplying the RMS current value by the AC system voltage monitored by the AC system voltage sensor; calculating the difference between the current command value and the current of the AC system power supply monitored by the current sensor; outputting a duty cycle by a current compensator that brings the difference in currents closer to zero; and switching the PFC circuit by a pulse width modulation circuit according to the duty cycle. [Effects of the Invention]

[0010] Having these characteristics, the present invention provides a switching power supply device that reduces power factor reduction even under light load conditions. [Brief explanation of the drawing]

[0011] [Figure 1] This is a circuit diagram showing an example of a conventional switching power supply. [Figure 2] These are waveform diagrams of the AC system voltage and AC system current under light and heavy load conditions in a conventional switching power supply. [Figure 3] This is a circuit diagram of a switching power supply device according to the first embodiment of the present invention. [Figure 4]This circuit diagram shows the control flow by the pulse signal generation unit for driving the switch element of the PFC circuit for a switching power supply device according to the first embodiment of the present invention. [Figure 5] This circuit diagram shows the control flow by the pulse signal generation unit for driving the switch element of the PFC circuit for a switching power supply device according to the second embodiment of the present invention. [Modes for carrying out the invention]

[0012] The following describes an example of an embodiment of the switching power supply device according to the present invention, based on the drawings. The following drawings are created for illustrative purposes, and in order to make them easier to understand, some components that are not necessary for the explanation may be intentionally omitted. In the following explanation, the same reference numerals in different figures indicate parts with the same function, and redundant explanations in each figure will be omitted as appropriate.

[0013] <First Embodiment> Figure 3 is a circuit diagram of a switching power supply device according to the first embodiment of the present invention. In the switching power supply device according to the first embodiment of the present invention, as shown in Figure 3, AC system power supplied from an AC power source is connected to the load via a lightning surge protection circuit, an AC filter circuit, and a PFC circuit. A current sensor is provided between the lightning surge protection circuit and the AC filter, and the AC system current Ib is detected by the current sensor and used to control the switching element of the PFC circuit. Here, the configuration of the PFC circuit is not limited and may be a full-bridge rectifier or a bridgeless rectifier. In addition, a common-mode choke coil (CMC) is provided in the AC filter circuit to remove common-mode noise.

[0014] The current sensor is located on the AC power supply side of the AC filter. Therefore, the current used for switching control of the PFC circuit matches the behavior of the AC system current, without being affected by the reactive current flowing through the capacitor in the AC filter. As a result, the power factor of the switching power supply does not decrease even under light load conditions. Verification showed that, compared to a conventional switching power supply where the current sensor is located after the AC filter, the switching power supply of the first embodiment, in which the current sensor is located on the AC power supply side of the AC filter, showed a 3% improvement in power factor.

[0015] Although the current sensor is located on the AC power supply side of the AC filter, it is situated after the lightning surge protection circuit, thus reducing the risk of damage to the current sensor due to lightning surges. While there is a risk of failure due to surge currents bypassing the lightning surge protection circuit because the current flowing through the sensor does not pass through the AC filter, the impact of surge currents bypassing the lightning surge protection circuit is limited, so this is not a significant practical problem.

[0016] Furthermore, while conventional switching power supplies required filter design to mitigate power factor drop, the switching power supply according to the first embodiment of the present invention allows for more flexible filter design than conventional methods. Therefore, the AC filter is not limited and can be passive or active. Additionally, this flexibility in design allows for monitoring of the current value after switching noise has been filtered out, depending on the filter design. Moreover, it offers the advantage of monitoring the current input to the AC filter circuit.

[0017] FIG. 4 is a circuit diagram showing a switching power supply device according to the first embodiment of the present invention, together with a control flow by a pulse signal generation unit for driving a switch element of a PFC circuit. Here, the "pulse signal generation unit" can be constructed by a digital controller such as a microcomputer, for example. As shown in FIG. 4, the control flow by the pulse signal generation unit is a feedback control flow peculiar to the PFC circuit by a double loop of a voltage control loop and a current control loop. As the voltage control loop, the pulse signal generation unit changes the output gain by performing pulse width modulation control (PWM control) according to a predetermined target voltage. As the current control loop, the current Ib detected by the current sensor is compared with a current command value, and the power factor is improved by controlling the switch element of the PFC circuit by the current compensator. In addition, in order to give a current command value, the AC system voltage is monitored in the previous stage thereof. Thus, there is no problem even if there are other circuits such as a monitoring circuit between the current sensor and the AC filter. More specifically, when explaining the control flow, the following processing will be executed in the pulse signal generation unit. A step of inputting an output voltage command value to the input of the pulse signal generation unit. A step of calculating a difference between the output voltage command value and the output voltage monitored by the output voltage sensor. A step of outputting, by a voltage compensator, an effective current value of the current so as to bring the difference of the output voltage closer to zero. A step of calculating a current command value so as to bring the difference of the output voltage closer to zero by multiplying the effective current value by the AC system voltage monitored by the AC system voltage sensor. A step of calculating a difference between the current command value and the current of the AC system power supply monitored by the current sensor. A step of outputting, by a current compensator, a duty ratio so as to bring the difference of the current closer to zero. A step of switching the PFC circuit by a pulse width modulation circuit according to the duty ratio. Here, the pulse width modulation circuit may be appropriately constructed by a microcomputer, an FPGA, an ASIC, or the like.

[0018] <Second Embodiment> FIG. 5 is a circuit diagram showing a switching power supply device according to a second embodiment of the present invention, together with a control flow by a pulse signal generation unit for driving a switch element of a PFC circuit. Here, the "pulse signal generation unit" can be constructed by a digital controller such as a microcomputer, for example. As shown in FIG. 5, the control flow by the pulse signal generation unit is a feedback control flow peculiar to the PFC circuit by a double loop of a voltage control loop and a current control loop. As shown in FIG. 5, in the switching power supply device according to the second embodiment of the present invention, AC system power supplied from an AC power supply is connected to a load through a lightning surge protection circuit, an AC filter circuit, and a PFC circuit. A current sensor is provided between the lightning surge protection circuit and the AC filter, and an AC system current Ib is detected by the current sensor and used for controlling a switch element of the PFC circuit. Here, the configuration of the PFC circuit is not limited, and it may be a full-bridge rectification or a bridgeless rectification. Also, a common mode choke coil (CMC) for removing common mode noise is provided in the AC filter circuit.

[0019] As shown in the flow of the current control loop in FIG. 5, the current Ib detected by the current sensor is compared with a current command value, and the power factor is improved by controlling the switch element of the PFC circuit by a current compensator. In order to give a current command value, the AC system voltage is monitored in the previous stage thereof. A monitoring circuit for monitoring this AC system voltage, specifically, a voltage sensor, is provided between the AC filter circuit and the PFC circuit. In the latter stage of the AC filter, only the current is affected by the reactive current flowing through the capacitor in the AC filter, and the influence on the voltage can be ignored, so the voltage sensor can be provided in the latter stage of the AC filter.

[0020] Although the switching power supply device according to the embodiments of the present invention has been described in detail above, the specific configuration is not limited to these embodiments, and any design changes, etc., that do not depart from the gist of the present invention are also included. For example, it is possible to provide another circuit in addition to the voltage monitoring circuit described as an example in the first and second embodiments, as long as the current sensor is located on the AC power supply side of the AC filter. Due to the PFC circuit's characteristic of performing feedback control using a dual loop of voltage control and current control, the conventional technical approach of placing monitoring circuits for input current monitoring and output voltage monitoring around the PFC circuit, i.e., before and after the PFC circuit, and instead placing a current sensor far away from the PFC circuit, even before the AC filter circuit, to improve the power factor not only at high loads but also at light loads, should be correctly recognized.

Claims

1. A switching power supply device comprising: an ADC converter having a PFC circuit; and a pulse signal generation unit that generates pulse signals for driving the switching elements of the PFC circuit so that the voltage of the ADC converter becomes a predetermined target value, The ADC converter further includes an AC filter and a current sensor that monitors the current of the AC power supply to generate a drive pulse signal for the switch element. The current sensor is located on the AC power supply side of the AC filter. A switching power supply device characterized by the following features.

2. To generate the drive pulse signal for the switch element, an AC system voltage sensor is provided to monitor the voltage of the AC system power supply, separately from the output voltage sensor that monitors the output voltage. The switching power supply device according to feature 1.

3. It also includes a lightning surge protection circuit for lightning surge protection, The current sensor is disposed between the AC filter and the lightning surge protection circuit. The switching power supply device according to feature 1.

4. A method for controlling a switching power supply device comprising: an ADC converter having a PFC circuit; a pulse signal generation unit that generates pulse signals to drive the switching elements of the PFC circuit so that the voltage of the ADC converter becomes a predetermined target value; an AC filter; a current sensor disposed on the AC system power supply side of the AC filter for monitoring the current of the AC system power supply; an output voltage sensor for monitoring the output voltage; and an AC system voltage sensor for monitoring the voltage of the AC system power supply, wherein The step of inputting an output voltage command value to the input of the pulse signal generation unit, A step of calculating the difference between the output voltage command value and the output voltage monitored by the output voltage sensor. A step of outputting an effective current value using a voltage compensator that brings the difference in the output voltages closer to zero. A step of calculating a current command value that brings the difference in the output voltage closer to zero by multiplying the effective current value by the AC system voltage monitored by the AC system voltage sensor. A step of calculating the difference between the current command value and the current of the AC power supply monitored by the current sensor. A step of outputting a duty cycle using a current compensator that brings the difference in the aforementioned currents closer to zero, The step includes switching the PFC circuit using a pulse width modulation circuit according to the duty cycle. A control method for a switching power supply, characterized by the following features.

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