Control circuit, switching power supply, and method for controlling DC conversion circuit

Abstract

A control circuit 30 of the present invention comprises: a zero current detection threshold calculation unit 31 which calculates zero current detection thresholds Vth1, Vth2 on the basis of a first divided voltage VFB obtained by dividing an output voltage of a DC conversion circuit 20; a zero current detection unit 32 which detects timing, at which a current flowing through an inductor L becomes zero, by comparing the zero current detection thresholds Vth1, Vth2 calculated by the zero current detection threshold calculation unit 31 and the voltage VFB based on a voltage applied to a switching element Q; and a switching element driving control unit 34 which controls turn-on of the switching element Q on the basis of the timing at which the current flowing through the inductor L, which is detected by the zero current detection unit 32, becomes zero.　According to the control circuit 30 of the present invention, the timing at which the inductor current becomes zero can be detected even in a case, such as immediately after startup, where the output voltage has not increased completely.

Description

[Technical Field]

[0001] The present invention relates to a control circuit, a switching power supply, and a method for controlling a DC conversion circuit. [Background technology]

[0002] Conventionally, there is known a switching power supply equipped with a critical current operation type DC converter circuit (PFC circuit), which performs control to turn on the switching element when the inductor current becomes zero.

[0003] 9 is a circuit diagram showing a conventional switching power supply 9. As shown in Fig. 9, the conventional switching power supply 9 includes a rectifier circuit 10, a DC conversion circuit 20, a control circuit (IC) 30, an input capacitor C1, and an output capacitor C2. The DC conversion circuit 20 includes an inductor L1 through which a current output from the rectifier circuit 10 flows, an auxiliary winding L2 of the inductor L1, a switching element Q that increases or decreases the current flowing through the inductor L1, and a diode D. One end of the auxiliary winding L2 is connected to a ZC terminal of the control circuit 30, and a gate electrode of the switching element Q is connected to a VG terminal of the control circuit 30.

[0004] FIG. 10 is a timing chart of a conventional switching power supply. Note that the symbol Δt indicates a delay time. In a conventional switching power supply 9, the voltage of the auxiliary winding L2 is detected and the detected auxiliary winding voltage VL is compared with a predetermined zero current detection threshold Vth to detect the timing at which the inductor current becomes zero. Specifically, as shown in FIG. 10, when the switching element Q turns off at time t1, the drain current Id drops sharply, and the auxiliary winding voltage VL, diode voltage ID, and drain voltage VDS rise sharply. The auxiliary winding voltage VL then begins to fall at time t2, and by detecting the timing at which it falls below a predetermined threshold Vth (time t3), the timing at which the inductor current becomes zero is detected, and the switching element is turned on at time t4. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-118767 Summary of the Invention [Problem to be solved by the invention]

[0006] FIG. 11 is a diagram showing a problem with the switching power supply described in Patent Document 1. In recent years, in order to make switching power supplies more adaptable to design changes, switching power supplies have begun to be used that include a control circuit that detects the timing at which the inductor current becomes zero by comparing a voltage based on the voltage applied to a switching element (for example, a voltage obtained by resistively dividing the drain voltage of the element) with a threshold value, instead of using an auxiliary winding of the inductor (see, for example, Patent Document 1).

[0007] However, in this case, since the voltage applied to the switching element fluctuates depending on the state of the input voltage and output voltage, if the output voltage has not yet risen to its full potential, such as immediately after startup, the voltage based on the voltage applied to the switching element may not exceed the threshold voltage (see the waveform on the left side of Figure 11(a)), making it difficult to detect the timing at which the inductor current becomes zero.

[0008] Therefore, the present invention has been made to solve the above-mentioned problems, and aims to provide a control circuit that can detect the timing when the inductor current becomes zero even when the output voltage has not yet risen to its full potential, such as immediately after startup. [Means for solving the problem]

[0009] The first control circuit of the present invention is a control circuit for controlling a DC conversion circuit having an inductor and a switching element that changes the increase or decrease of a current flowing through the inductor, characterized in that the control circuit comprises: a zero current detection threshold calculation unit that calculates a zero current detection threshold based on a first divided voltage obtained by dividing the output voltage of the DC conversion circuit; a zero current detection unit that detects the timing at which the current flowing through the inductor becomes zero by comparing the zero current detection threshold calculated by the zero current detection threshold calculation unit with a voltage based on the voltage applied to the switching element; and a switching element drive control unit that controls the on of the switching element based on the timing at which the current flowing through the inductor becomes zero, detected by the zero current detection unit.

[0010] A second control circuit of the present invention is a control circuit for controlling a DC conversion circuit having an inductor and a switching element that changes the increase or decrease of a current flowing through the inductor, characterized in that the control circuit comprises: a zero current detection threshold acquisition unit that acquires a first zero current detection threshold and a second zero current detection threshold that is set to a voltage lower than the first zero current detection threshold; a zero current detection unit that detects the timing at which the current flowing through the inductor becomes zero by comparing the first zero current detection threshold and the second zero current detection threshold acquired by the zero current detection threshold acquisition unit with a voltage based on a voltage applied to the switching element; and a switching element drive control unit that controls on of the switching element based on the timing at which the current flowing through the inductor becomes zero, detected by the zero current detection unit.

[0011] The switching power supply of the present invention is a switching power supply comprising an inductor, a DC conversion circuit having a switching element that changes the increase or decrease of a current flowing through the inductor, and a control circuit that controls the DC conversion circuit, wherein the control circuit is the first control circuit or the second control circuit of the present invention.

[0012] The first DC conversion circuit control method of the present invention is a control method for a DC conversion circuit that controls a DC conversion circuit having an inductor and a switching element that changes the increase or decrease of a current flowing through the inductor, and is characterized by including a zero current detection threshold calculation step of calculating a zero current detection threshold based on a first voltage obtained by dividing the output voltage of the DC conversion circuit, a zero current detection step of detecting a second voltage based on a voltage applied to the switching element and comparing it with the zero current detection threshold to detect the timing at which the current flowing through the inductor becomes zero, and a switching element drive control step of controlling the on of the switching element based on the timing at which the current flowing through the inductor becomes zero, detected in the zero current detection step.

[0013] The second control method for a DC conversion circuit of the present invention is a control method for a DC conversion circuit that controls a DC conversion circuit having an inductor and a switching element that changes the increase or decrease of a current flowing through the inductor, and is characterized by including a zero current detection step of detecting the timing at which the current flowing through the inductor becomes zero by comparing a voltage based on the voltage applied to the switching element with a first zero current detection threshold and a second zero current detection threshold set to a voltage lower than the first zero current detection threshold, and a switching element drive control step of controlling the on of the switching element based on the timing at which the current flowing through the inductor becomes zero, detected in the zero current detection step. [Effects of the Invention]

[0014] According to the first control circuit and switching power supply of the present invention, the control circuit includes a zero current detection threshold calculation unit that calculates a zero current detection threshold based on a first divided voltage obtained by dividing the output voltage of the DC converter circuit, and a zero current detection unit that detects the timing when the current flowing through the inductor becomes zero by comparing the zero current detection threshold calculated by the zero current detection threshold calculation unit with a voltage based on the voltage applied to the switching element, so that the zero current detection threshold can be set according to the output voltage. Therefore, even if the output voltage has not yet fully increased, such as immediately after startup, it is possible to detect the timing when the inductor current becomes zero.

[0015] According to the second control circuit and switching power supply of the present invention, the control circuit includes a zero current detection threshold acquisition unit that acquires a first zero current detection threshold and a second zero current detection threshold that is set to a voltage lower than the first zero current detection threshold, and a zero current detection unit that detects the timing at which the current flowing through the inductor becomes zero by comparing the first zero current detection threshold and the second zero current detection threshold acquired by the zero current detection threshold acquisition unit with a voltage based on the voltage applied to the switching element.Therefore, even when the output voltage has not yet fully risen, such as immediately after startup, the voltage based on the voltage applied to the switching element will exceed the second zero current detection threshold, making it possible to detect the timing at which the inductor current becomes zero.

[0016] However, there is a problem in that it is difficult to detect the timing when the inductor current becomes zero in the following cases: (1) when the input voltage is low, such as when the input voltage AC is at a valley, and the voltage based on the voltage applied to the switching element does not exceed the threshold; or (2) when the input voltage is high, the amplitude of the resonant voltage is small, and the voltage based on the voltage applied to the switching element does not fall below the threshold even after the inductor current becomes zero (see FIG. 11(b)). However, with the second control circuit and switching power supply of the present invention, even in the above case (1), the voltage based on the voltage applied to the switching element exceeds the second zero current detection threshold, making it possible to detect the timing when the inductor current becomes zero. Also, even in the above case (2), the voltage based on the voltage applied to the switching element after the inductor current becomes zero falls below the first zero current detection threshold, making it possible to detect the timing when the inductor current becomes zero.

[0017] The first DC converter circuit control method of the present invention includes a zero current detection threshold calculation step of calculating a zero current detection threshold based on a first voltage obtained by dividing the output voltage of the DC converter circuit, and a zero current detection step of detecting the timing when the current flowing through the inductor becomes zero by comparing a second voltage based on the voltage applied to the switching element with the zero current detection threshold, so that the zero current detection threshold can be set according to the output voltage. Therefore, even if the output voltage has not yet risen to its full potential, such as immediately after startup, the timing when the inductor current becomes zero can be detected.

[0018] According to the second DC converter circuit control method of the present invention, a zero current detection step is included in which a voltage based on a voltage applied to a switching element is compared with a first zero current detection threshold and a second zero current detection threshold set to a voltage lower than the first zero current detection threshold to detect the timing when the current flowing through the inductor becomes zero. Therefore, (1) even when the input voltage is low, such as when the input voltage AC is at a valley, and the voltage based on the voltage applied to the switching element does not exceed the threshold, the voltage based on the voltage applied to the switching element exceeds the second zero current detection threshold, so that the timing when the inductor current becomes zero can be detected. Also, (2) even when the input voltage is high and the amplitude of the resonant voltage is small, and the voltage based on the voltage applied to the switching element does not fall below the second zero current detection threshold even after the inductor current becomes zero (see FIG. 11(b)), the voltage based on the voltage applied to the switching element after the inductor current becomes zero falls below the first zero current detection threshold, so that the timing when the inductor current becomes zero can be detected. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a circuit diagram showing a switching power supply 1 according to a first embodiment. [Figure 2] FIG. 2 is a block diagram illustrating a control circuit 30 according to the first embodiment. [Figure 3] 10 is a timing chart showing a case where the second divided voltage VDZC exceeds the first zero current detection threshold Vth1 and the second zero current detection threshold Vth2. [Figure 4] 10 is a timing chart showing a case where the second divided voltage VDZC exceeds only the second zero current detection threshold Vth2. [Figure 5] FIG. 4 is a diagram illustrating a zero current detection threshold in the first embodiment. [Figure 6] FIG. 3 is a diagram illustrating an overcurrent detection unit according to the first embodiment. [Figure 7] 4 is a flowchart showing a control method for the DC converter circuit according to the first embodiment. [Figure 8] FIG. 10 is a circuit diagram showing a switching power supply 2 according to a second embodiment. [Figure 9] FIG. 1 is a circuit diagram showing a conventional switching power supply 9. [Figure 10] 1 is a timing chart of a conventional switching power supply. [Figure 11] 1 is a diagram illustrating a problem with the switching power supply described in Patent Document 1. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0020] The control circuit, switching power supply, and DC converter control method of the present invention will be described below based on the embodiments shown in the drawings. Note that the embodiments described below do not limit the invention according to the claims. Furthermore, not all of the elements and combinations thereof described in the embodiments are necessarily essential to the solution of the present invention.

[0021] [Embodiment 1] 1. Configuration of the switching power supply 1 according to the first embodiment Fig. 1 is a circuit diagram showing a switching power supply 1 according to embodiment 1. As shown in Fig. 1, the switching power supply 1 according to embodiment 1 includes a rectifier circuit 10, a DC conversion circuit 20, a control circuit 30, an input capacitor C1, an output capacitor C2, and voltage dividing resistors R3 and R4.

[0022] The rectifier circuit 10 converts AC power input from an input power source AC-IN into DC power. The input capacitor C1 is arranged between the rectifier circuit 10 and the DC conversion circuit 20. The output capacitor C2 is arranged between the DC conversion circuit 20 and the output terminals (terminals Vo and GND). The voltage-dividing resistors R3 and R4 are connected in series and divide the output voltage between the output terminals Vo and GND. The connection point of the voltage-dividing resistors R3 and R4 is connected to the output voltage detection terminal FB of the control circuit 30.

[0023] The DC conversion circuit 20 includes an inductor L1 through which the current output from the rectifier circuit 10 flows, a switching element Q that changes the increase or decrease of the current flowing through the inductor L1, a diode D, and voltage-dividing resistors R1 and R2. The DC conversion circuit 20 is a critical control type power factor correction circuit (PFC circuit) that suppresses harmonics to improve the power factor and converts the power output from the rectifier circuit 10 into DC power.

[0024] One end of inductor L1 is connected to the positive output terminal of rectifier circuit 10, and the other end is connected to the drain terminal of switching element Q and the anode electrode of diode D. Switching element Q has a drain electrode connected to inductor L1 and the anode electrode of diode D, a source electrode connected to ground potential, and a gate electrode connected to gate drive terminal VG of control circuit 30. Diode D has an anode electrode connected to inductor L1 and the drain electrode of switching element Q, and a cathode electrode connected to output terminal Vo. Voltage-dividing resistors R1 and R2 are connected in series and divide the voltage across switching element Q. The junction of voltage-dividing resistors R1 and R2 is connected to ZC terminal of control circuit 30.

[0025] 2. Configuration of the control circuit 30 according to the first embodiment FIG. 2 is a block diagram illustrating the control circuit 30 according to the first embodiment. FIG. 3 is a timing chart illustrating a case where the second divided voltage VDZC exceeds the first zero current detection threshold Vth1 and the second zero current detection threshold Vth2. FIG. 4 is a timing chart illustrating a case where the second divided voltage VDZC exceeds only the second zero current detection threshold Vth2. FIG. 5 is a diagram illustrating the zero current detection threshold according to the first embodiment. FIG. 5(a) is a schematic waveform diagram illustrating the relationship between the second divided voltage VDZC and the zero current detection threshold when the output voltage is small (left waveform), medium (center waveform), and normal (right waveform) according to the first embodiment. FIG. 5(b) is a schematic waveform diagram illustrating the relationship between the second divided voltage VDZC and the zero current detection threshold when the output voltage is small (left waveform), medium (center waveform), and normal (right waveform) according to a modified example.

[0026] As shown in FIG. 2, the control circuit 30 according to the first embodiment includes a zero current detection threshold calculation unit 31, a zero current detection unit 32, a delay circuit 33, a switching element drive control unit 34, an overvoltage detection unit 35, a gate drive terminal VG, a zero-cross detection terminal ZC, and an output voltage detection terminal FB.

[0027] The zero current detection threshold calculation unit 31 (sometimes referred to as a zero current detection threshold acquisition unit) calculates the zero current detection threshold based on the first divided voltage VFB obtained by dividing the output voltage of the DC conversion circuit 20. Therefore, as shown in the left and center diagrams of FIG. 5(a), when the output voltage (first divided voltage) has not yet fully increased, the zero current detection thresholds (first zero current detection threshold Vth1 and second zero current detection threshold Vth2) are calculated to be smaller than those in normal operation in accordance with the output voltage, and gradually increase in accordance with the output voltage. Note that, because the first zero current detection threshold Vth1 on the higher voltage side is more significantly affected by the output voltage, only the first zero current detection threshold Vth1 may be calculated based on the first divided voltage VFB obtained by dividing the output voltage of the DC conversion circuit 20.

[0028] 2, the zero current detection threshold calculation unit 31 is connected to the output voltage detection terminal FB and receives the first divided voltage VFB. The zero current detection threshold calculation unit 31 calculates a first zero current detection threshold Vth1 and a second zero current detection threshold Vth2, which is set to a voltage lower than the first zero current detection threshold, as the zero current detection thresholds. Any appropriate method can be used to calculate the zero current detection threshold. For example, the zero current detection threshold may be calculated by subtracting a predetermined value from the output voltage (first divided voltage), or the zero current detection threshold may be calculated from the ratio of the magnitudes of the output voltages (first divided voltages).

[0029] The zero current detection unit 32 detects the timing when the current flowing through the inductor L becomes zero by comparing the zero current detection thresholds (first zero current detection threshold Vth1 and second zero current detection threshold Vth2) calculated by the zero current detection threshold calculation unit 31 with a voltage based on the voltage applied to the switching element Q, specifically, a second divided voltage VDZC obtained by dividing the voltage applied to the switching element Q. When the second divided voltage VZDC rises to a voltage value that exceeds both the first zero current detection threshold Vth1 and the second zero current detection threshold Vth2 when the switching element Q is turned off, the zero current detection unit 32 detects the timing when the VDZC voltage becomes equal to or less than the first zero current detection threshold Vth1 as the timing when the inductor current becomes zero (see time t3 in FIG. 3 ). Furthermore, if the VZDC voltage only rises to a voltage value that is greater than the second zero current detection threshold Vth2 and smaller than the first zero current detection threshold Vth1, the timing at which the VZDC voltage becomes equal to or less than the second zero current detection threshold Vth2 is detected as the timing at which the inductor current becomes zero (see time t3 in FIG. 4).

[0030] The zero current detection unit 32 receives the zero current detection thresholds (first zero current detection threshold Vth1 and second zero current detection threshold Vth2) from the zero current detection threshold calculation unit 31, and receives the second divided voltage VDZC from the ZC terminal. It also outputs zero current detection information to the switching element drive control unit 34. It also outputs zero current detection information ZC1 based on the first zero current detection threshold Vth1 or zero current detection information ZC2 based on the second zero current detection threshold Vth2 to the delay circuit 33. The zero current detection information ZC1 and the zero current detection information ZC2 include information related to the thresholds and information related to the timing at which the inductor current becomes zero.

[0031] The delay circuit 33 calculates a delay time from detecting zero current to turning on the switching element Q based on the zero current detection information ZC1 based on the first zero current detection threshold Vth1 or the zero current detection information ZC2 based on the second zero current detection threshold Vth2 input from the zero current detection unit 32, and outputs information about the delay time to the switching element drive control unit 34. In the delay circuit 33, the delay time Δt1 from detecting zero current based on the first zero current detection threshold Vth1 to turning on the switching element Q is set to be longer than the delay time Δt2 from detecting zero current based on the second zero current detection threshold Vth2 to turning on the switching element Q. This is because the time from time t2 when the drain voltage VDS starts to fall to time t3 when zero current is detected is shorter in the case of zero current detection based on the first zero current detection threshold Vth1 than in the case of zero current detection based on the second zero current detection threshold Vth2. The delay time is preferably set so that the switching element Q turns on at the valley of the amplitude of the resonant voltage.

[0032] The switching element drive control unit 34 controls the turning on of the switching element Q based on the zero current detection information output from the zero current detection unit 32 and the information on the delay time output from the delay circuit 33. After the zero current detection information is input, the switching element drive control unit 34 turns on the switching element Q after a delay time Δt1 or Δt2 has elapsed.

[0033] The overvoltage detection unit 35 compares a second divided voltage VDZC obtained by dividing the voltage applied to the switching element Q with a predetermined overvoltage detection threshold Vth_OC to detect whether the output voltage of the DC conversion circuit has become an overvoltage (see FIG. 6). That is, the overvoltage detection unit 35 detects an overvoltage when the second divided voltage VDZC exceeds the predetermined overvoltage detection threshold Vth_OC. The overvoltage detection unit 35 receives the second divided voltage VDZC from the zero-cross detection terminal ZC and outputs overvoltage detection information to the switching element drive control unit 34. When an overvoltage is detected, the switching element drive control unit 34 controls the switching element Q so that the output voltage decreases.

[0034] 3. Operation of the control circuit 30 according to the first embodiment Next, the operation of the control circuit 30 according to the first embodiment will be described. (1) In the case where the second divided voltage VDZC rises to a value greater than the first zero current detection threshold Vth1 (see FIG. 3), when the switching element Q is turned off at time t1, the zero current detection voltage VDZC (second divided voltage) and the drain voltage VDS rise abruptly and maintain a predetermined voltage (a voltage greater than the first zero current detection threshold Vth1). At this time, the first divided voltage obtained by dividing the output voltage of the switching power supply 1 is input to the zero current detection threshold calculation unit 31, which calculates the first zero current detection threshold Vth1 and the second zero current detection threshold Vth2 (see FIG. 2). Also, at time t1, the drain current Id falls abruptly and becomes zero. Also, the diode current Id rises abruptly and gradually decreases after time t1.

[0035] Then, at time t2, when the diode current ID becomes zero, the zero current detection voltage VDZC and the drain voltage VDS begin to fall. At this time, the zero current detection unit 32 compares the second divided voltage VDZC with a zero current detection threshold (first zero current detection threshold Vth1). Then, at time t3, when the zero current detection unit 32 detects that the zero current detection voltage VDZC is lower than (exceeds) the first zero current detection threshold Vth1, the zero current detection unit 32 outputs zero current detection information ZC1 based on the first zero current detection threshold Vth1 to the delay circuit 33, as shown in FIG. 2, and the delay circuit 33 calculates a delay time. Information regarding the delay time calculated by the delay circuit 33 is output to the switching element drive control unit 34, and the switching element drive control unit 34 outputs a signal to the gate electrode of the switching element Q to turn on the switching element Q at time t4 (see Figure 3) that is delayed by Δt1 from time t3 when the zero current detection voltage VDZC falls below the first zero current detection threshold Vth1, thereby turning on the switching element Q.

[0036] Thereafter, the output voltage (first divided voltage) and the switching voltage of the switching element Q (second divided voltage) are detected and controlled for each cycle, thereby controlling the timing at which the switching element Q is turned on.

[0037] (2) In the case where the second divided voltage VDZC rises to a value smaller than the first zero current detection threshold Vth1 (see Figure 4), when the switching element Q is turned off at time t1, the zero current detection voltage VDZC and the drain voltage VDS rise sharply and maintain a predetermined voltage, similar to (1) above, but the zero current detection voltage VDZC rises to a voltage smaller than the first zero current detection threshold Vth1, although it is larger than the second zero current detection threshold Vth2.

[0038] At time t2, when the diode current ID becomes zero, the zero current detection voltage VDZC and the drain voltage VDS begin to fall. At this time, the zero current detection unit 32 compares the second divided voltage VDZC with a zero current detection threshold (second zero current detection threshold Vth2). At time t3, when the zero current detection unit 32 detects that the zero current detection voltage VDZC is lower than (exceeds) the second zero current detection threshold Vth2, the zero current detection unit 32 outputs zero current detection information ZC2 based on the second zero current detection threshold Vth2 to the delay circuit 33, and the delay circuit 33 calculates a delay time. Information regarding the delay time calculated by the delay circuit 33 is output to the switching element drive control unit 34, and the switching element drive control unit 34 outputs a signal to turn on the switching element Q to the gate electrode of the switching element Q via the terminal VG at time t4, which is delayed by Δt2 from time t3 when the zero current detection voltage VDZC falls below the second zero current detection threshold Vth2, thereby turning on the switching element Q.

[0039] The timing t3 of zero current detection based on the first zero current detection threshold Vth1 is earlier than the timing t3 of zero current detection based on the second zero current detection threshold Vth2 (see FIGS. 3 and 4). Accordingly, the delay time Δt1 after zero current detection based on the first zero current detection threshold Vth1 is longer than the delay time Δt2 after zero current detection based on the second zero current detection threshold Vth2. This makes the time from time t2 to time t4 approximately equal, thereby suppressing fluctuations in the switching frequency.

[0040] 4. Control method of the DC conversion circuit according to embodiment 1 Next, a description will be given of a control method for the DC converter circuit according to embodiment 1. Fig. 7 is a flowchart showing the control method for the DC converter circuit according to embodiment 1. The control method for the DC converter circuit according to embodiment 1 is a control method for a DC converter circuit that controls a DC converter circuit having an inductor L and a switching element Q that changes the increase or decrease of the current flowing through the inductor L (see Fig. 7).

[0041] The control method for the DC converter circuit according to the first embodiment includes a zero current detection threshold calculation step of calculating zero current detection thresholds Vth1 and Vth2 based on a first divided voltage VFB obtained by dividing the output voltage of the DC converter circuit 20, a zero current detection step of comparing a voltage (second divided voltage VDZC) based on a voltage applied to the switching element Q with the zero current detection thresholds Vth1 and Vth2 to detect the timing at which the current flowing through the inductor L becomes zero, a delay time determination step of determining a delay time, and a switching element drive control step of controlling the on-state of the switching element Q based on the timing at which the current flowing through the inductor L becomes zero, which is detected in the zero current detection step. In the zero current detection step, a first zero current detection threshold Vth1 and a second zero current detection threshold Vth2 set to a voltage lower than the first zero current detection threshold are used as the zero current detection thresholds.

[0042] 5. Effects of the control circuit 30, switching power supply 1, and DC conversion circuit control method according to the first embodiment According to the control circuit 30 and switching power supply 1 of the first embodiment, the control circuit 30 includes a zero current detection threshold calculation unit 31 that calculates zero current detection thresholds Vth1 and Vth2 based on a first divided voltage VFB obtained by dividing the output voltage of the DC conversion circuit 20, and a zero current detection unit 32 that detects the timing at which the current flowing through the inductor L1 becomes zero by comparing the zero current detection thresholds Vth1 and Vth2 calculated by the zero current detection threshold calculation unit 31 with a voltage VDZC based on the voltage applied to the switching element Q. Therefore, the zero current detection thresholds Vth1 and Vth2 can be set according to the output voltage. Therefore, even if the output voltage has not yet risen to its full potential, such as immediately after startup, the timing at which the inductor current becomes zero can be detected (see FIG. 5(a)).

[0043] According to the control circuit 30 and switching power supply 1 of embodiment 1, the control circuit 30 includes a zero current detection threshold calculation unit 31 (zero current detection threshold acquisition unit) that calculates a first zero current detection threshold Vth1 and a second zero current detection threshold Vth2 that is set to a voltage lower than the first zero current detection threshold, and a zero current detection unit 32 that detects the timing at which the current flowing through the inductor L becomes zero by comparing the first zero current detection threshold Vth1 and the second zero current detection threshold Vth2 calculated by the zero current detection threshold calculation unit 31 with a voltage based on the voltage applied to the switching element Q. Therefore, even when the output voltage has not yet fully risen, such as immediately after startup, the voltage based on the voltage applied to the switching element Q exceeds the second zero current detection threshold Vth2, and therefore it is possible to detect the timing at which the inductor current becomes zero.

[0044] However, there are problems in that it is difficult to detect the timing when the inductor current becomes zero in the following cases: (1) when the input voltage is low, such as when the input voltage AC reaches a valley, and the voltage based on the voltage applied to the switching element Q does not exceed the threshold (see the left waveform in FIG. 11(a)); or (2) when the input voltage is high, the amplitude of the resonant voltage is small, and the voltage based on the voltage applied to the switching element does not fall below the threshold even after the inductor current becomes zero (see FIG. 11(b)). However, with the control circuit 30 and switching power supply 1 according to the first embodiment, even in the case of (1), the voltage based on the voltage applied to the switching element Q exceeds the second zero current detection threshold Vth2, making it possible to detect the timing when the inductor current becomes zero. Furthermore, even in the case of (2), the voltage based on the voltage applied to the switching element Q after the inductor current becomes zero falls below the first zero current detection threshold Vth1, making it possible to detect the timing when the inductor current becomes zero.

[0045] According to the control circuit 30 and the switching power supply 1 of the first embodiment, the voltage based on the voltage applied to the switching element Q is the second divided voltage VDZC obtained by dividing the voltage across the switching element Q, so that an auxiliary winding is not required, and it can be applied without having to design a choke coil according to the circuit as in the case where an auxiliary winding is used, making it easier to respond to design changes.

[0046] In the control circuit 30 and switching power supply 1 according to the first embodiment, zero current detection is performed using the first zero current detection threshold Vth1 and the second zero current detection threshold Vth2, but because the first zero current detection threshold Vth1 is a higher voltage, the time from the start of the fall to time t4 at which zero current detection is performed is shorter when the first zero current detection threshold Vth1 is used. Therefore, if the delay time is the same when zero current detection is performed using the first zero current detection threshold Vth1 and when zero current detection is performed using the second zero current detection threshold Vth2, the switching frequency will change, and the parameters of the output power of the switching power supply will change. In contrast, according to the control circuit 30 and switching power supply 1 of embodiment 1, the delay time from detecting a voltage that exceeds (falls below) the first zero current detection threshold Vth1 to turning on the switching element Q is longer than the delay time from detecting a voltage that exceeds (falls below) the second zero current detection threshold Vth2 to turning on the switching element Q. Therefore, the time from the falling time t3 to the next turning on of the switching element Q can be adjusted depending on whether zero current detection is performed using the first zero current detection threshold Vth1 or the second zero current detection threshold Vth2.

[0047] The control circuit 30 and switching power supply 1 according to the first embodiment include an overvoltage detection unit 35 that detects that the output voltage of the DC conversion circuit 20 has become an overvoltage when the voltage based on the voltage applied to the switching element Q exceeds a predetermined overvoltage detection threshold Vth_OC. Therefore, even if a malfunction such as an increase in the voltage VFB due to a change in resistance value caused by electrolytic corrosion of the voltage dividing resistor of the feedback circuit occurs, making it difficult to detect an overvoltage from the output voltage, an overvoltage can be detected safely and with high accuracy without adding an additional overvoltage detection terminal.

[0048] According to the control circuit 30 and the switching power supply 1 of the first embodiment, the DC conversion circuit 20 is a critical control type power factor improvement circuit, and therefore, by detecting zero current and controlling the on-state of the switching element Q, it is possible to reduce switching loss.

[0049] The control method for the DC converter circuit according to the first embodiment includes a zero current detection threshold calculation step of calculating the zero current detection thresholds Vth1 and Vth2 based on the first divided voltage VFB obtained by dividing the output voltage of the DC converter circuit 20, and a zero current detection step of comparing the second divided voltage VDZC based on the voltage applied to the switching element Q with the zero current detection thresholds Vth1 and Vth2 to detect the timing at which the current flowing through the inductor L becomes zero, so that the zero current detection thresholds Vth1 and Vth2 can be set according to the output voltage. Therefore, even if the output voltage has not yet risen to its full potential, such as immediately after startup, the timing at which the inductor current becomes zero can be detected.

[0050] [Embodiment 2] 8 is a circuit diagram showing a switching power supply 2 according to embodiment 2. The switching power supply 2 and control circuit 30a according to embodiment 2 basically have the same configuration as the switching power supply 1 and control circuit 30 according to embodiment 1, but differs from the switching power supply 1 and control circuit 30 according to embodiment 1 in that the zero current is detected using an auxiliary winding voltage instead of using the second divided voltage VDZC (see FIG. 8).

[0051] In this case as well, the zero current detection thresholds Vth1 and Vth2 are calculated based on the output voltage.

[0052] As described above, the switching power supply 2 and control circuit 30a according to the second embodiment differ from the switching power supply 1 and control circuit 30 according to the first embodiment in that they use an auxiliary winding voltage instead of the switching voltage of a switching element. However, like the switching power supply 1 and control circuit 30 according to the first embodiment, the control circuit 30a includes a zero current detection threshold acquisition unit that acquires a first zero current detection threshold Vth1 and a second zero current detection threshold Vth2 that is set to a voltage lower than the first zero current detection threshold, and a zero current detection unit that detects the timing at which the current flowing through the inductor L1 becomes zero by comparing the auxiliary winding voltage with the first zero current detection threshold Vth1 and the second zero current detection threshold Vth2 acquired by the zero current detection threshold acquisition unit. Therefore, even if the output voltage has not yet fully increased, such as immediately after startup, the auxiliary winding voltage exceeds the second zero current detection threshold, and therefore it is possible to detect the timing at which the inductor current becomes zero.

[0053] The switching power supply 2 and the control circuit 30a according to the second embodiment have the same configuration as the switching power supply 1 and the control circuit 30 according to the first embodiment except that the auxiliary winding voltage is used instead of the second divided voltage VDZC, and therefore have the corresponding effects of the switching power supply 1 and the control circuit 30 according to the first embodiment.

[0054] Although the present invention has been described based on the above embodiment, the present invention is not limited to the above embodiment and can be embodied in various forms without departing from the spirit of the present invention, and for example, the following modifications are also possible.

[0055] (1) The positions, connections, numbers, etc. described in the above embodiments (including each modified example; the same applies below) are examples and can be changed within the scope that does not impair the effects of the present invention.

[0056] (2) In the above embodiments, the zero current detection threshold is configured with a first zero current detection threshold and a second zero current detection threshold set to a voltage lower than the first zero current detection threshold. However, the present invention is not limited to this. It may be configured with a single zero current detection threshold. Even in this case, the zero current detection threshold calculation unit calculates the zero current detection threshold based on the output voltage (see FIG. 5(b)).

[0057] (3) In the above embodiments, the zero current detection threshold is calculated based on the first divided voltage obtained by dividing the output voltage of the DC converter circuit, but the present invention is not limited to this. The first zero current detection threshold and the second zero current detection threshold may be determined in advance (for example, they may be obtained from a memory unit provided inside or outside the zero current detection threshold obtaining unit), or may be determined based on a parameter other than the first divided voltage.

[0058] (4) In the first embodiment, the zero current is detected using the first divided voltage obtained by dividing the output voltage, and in the second embodiment, the zero current is detected using the auxiliary winding voltage. However, the zero current may be detected by any other appropriate method.

[0059] (5) In the above embodiments, a step-up chopper circuit is used as the DC conversion circuit, but the present invention is not limited to this. A step-down chopper circuit or any other appropriate circuit may be used. [Explanation of symbols]

[0060] 1, 2, 9... Switching power supply, 10... Rectifier circuit, 20... DC conversion circuit, 30, 30a... Control circuit, 31... Zero current detection threshold calculation unit, 32... Zero current detection unit, 33... Delay circuit, 34... Switching element drive control unit, 35... Overvoltage detection unit, AC-IN... Input power supply, C1... Input capacitor, C2... Output capacitor, D... Diode, FB... Output voltage detection terminal, GND... Terminal, L, L1... Inductor, L2... Auxiliary winding, VG... Gate drive terminal, Q... Switching element, R1, R2, R3, R4... Voltage dividing resistor, Vo... Output terminal, Vth1... First zero current detection threshold, Vth2... Second zero current detection threshold, Vth_OC... Overvoltage detection threshold, ZC... Zero cross detection terminal, Δt, Δt1, Δt2... Delay time

Claims

1. A control circuit for controlling a DC converter circuit having an inductor and a switching element for varying the increase or decrease of a current flowing through the inductor, The control circuit a zero current detection threshold calculation unit that calculates a zero current detection threshold based on a first divided voltage obtained by dividing the output voltage of the DC conversion circuit; a zero current detection unit that detects a timing at which the current flowing through the inductor becomes zero by comparing the zero current detection threshold calculated by the zero current detection threshold calculation unit with a voltage based on a voltage applied to the switching element; a switching element drive control unit that controls the on-state of the switching element based on the timing at which the current flowing through the inductor, detected by the zero current detection unit, becomes zero; The zero current detection threshold comprises a first zero current detection threshold and a second zero current detection threshold set to a voltage lower than the first zero current detection threshold.

2. A control circuit for controlling a DC converter circuit having an inductor and a switching element for varying the increase or decrease of a current flowing through the inductor, The control circuit a zero current detection threshold acquiring unit that acquires a first zero current detection threshold and a second zero current detection threshold that is set to a voltage lower than the first zero current detection threshold; a zero current detection unit that detects a timing at which the current flowing through the inductor becomes zero by comparing the first zero current detection threshold and the second zero current detection threshold acquired by the zero current detection threshold acquisition unit with a voltage based on a voltage applied to the switching element; a switching element drive control unit that controls turning on of the switching element based on the timing at which the current flowing through the inductor, detected by the zero current detection unit, becomes zero.

3. 3. The control circuit according to claim 1, wherein the voltage based on the voltage applied to the switching element is a second divided voltage obtained by dividing the voltage across the switching element.

4. 3. The control circuit according to claim 1, wherein a delay time from detecting a voltage exceeding the first zero current detection threshold to turning on the switching element is longer than a delay time from detecting a voltage exceeding the second zero current detection threshold to turning on the switching element.

5. 3. The control circuit according to claim 1, wherein the delay time from detecting a voltage exceeding the first zero current detection threshold to turning on the switching element, and the delay time from detecting a voltage exceeding the second zero current detection threshold to turning on the switching element are determined so as to turn on the switching element at a valley of a resonant voltage.

6. 3. The control circuit according to claim 1, further comprising an overvoltage detection unit that detects that the output voltage of the DC conversion circuit has become an overvoltage when a voltage based on the voltage applied to the switching element exceeds a predetermined overvoltage detection threshold.

7. a DC converter circuit having an inductor and a switching element that changes the increase or decrease of a current flowing through the inductor; A switching power supply including a control circuit for controlling the DC conversion circuit, 3. A switching power supply, wherein the control circuit is the control circuit according to claim 1.

8. A method for controlling a DC converter circuit having an inductor and a switching element that changes the increase or decrease of a current flowing through the inductor, comprising: a zero current detection threshold calculation step of calculating a zero current detection threshold, which is composed of a first zero current detection threshold and a second zero current detection threshold set to a voltage lower than the first zero current detection threshold, based on a voltage obtained by dividing the output voltage of the DC conversion circuit; a zero current detection step of detecting a timing at which the current flowing through the inductor becomes zero by comparing a voltage based on a voltage applied to the switching element with the zero current detection threshold; a switching element drive control step of controlling the on-state of the switching element based on the timing at which the current flowing through the inductor, detected in the zero current detection step, becomes zero.

9. A method for controlling a DC converter circuit having an inductor and a switching element that changes the increase or decrease of a current flowing through the inductor, comprising: a zero current detection step of detecting a timing when the current flowing through the inductor becomes zero by comparing a voltage based on the voltage applied to the switching element with a first zero current detection threshold and a second zero current detection threshold set to a voltage lower than the first zero current detection threshold; a switching element drive control step of controlling the on-state of the switching element based on the timing at which the current flowing through the inductor, detected in the zero current detection step, becomes zero.

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