Control circuit, switching power supply, and control method for 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

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

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

[0002] Conventionally, a switching power supply including a critical current control (PFC) circuit is known, which performs control to turn on a switching element when an 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 varies the increase or decrease of 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 the 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 when the inductor current becomes zero. Specifically, as shown in FIG. 10 , when the switching element Q is turned 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 rising auxiliary winding voltage VL then begins to fall at time t2, and by detecting the timing (time t3) when it falls below the predetermined threshold Vth, the timing when the inductor current becomes zero is detected, and the switching element is turned on at time t4.

[0005] JP 2017-118767 A

[0006] 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 come into use 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 an 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.

[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.

[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 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, 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, it is possible to detect the timing at which 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 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, with the second control circuit and switching power supply of the present invention, it is possible to detect the timing at which the inductor current becomes zero because the voltage based on the voltage applied to the switching element exceeds the second zero current detection threshold even in the case of (1) above, 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 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 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, it is possible to detect the timing at which the inductor current becomes zero because the voltage based on the voltage applied to the switching element exceeds the second zero current detection threshold even in the case of (1) above. Furthermore, it is possible to detect the timing at which the inductor current becomes zero because 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 even in the case of (2) above.

[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 fully increased, 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 at which 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 at which 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 at which the inductor current becomes zero can be detected.

[0019] 1 is a circuit diagram showing a switching power supply 1 according to a first embodiment. FIG. 2 is a block diagram shown for explaining a control circuit 30 according to the first embodiment. FIG. 3 is a timing chart when 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 when the second divided voltage VDZC exceeds only the second zero current detection threshold Vth2. FIG. 5 is a diagram shown for explaining a zero current detection threshold in the first embodiment. FIG. 6 is a diagram shown for explaining an overcurrent detection unit in the first embodiment. FIG. 7 is a flowchart showing a control method for a DC conversion circuit according to the first embodiment. FIG. 8 is a circuit diagram showing a switching power supply 2 according to a second embodiment. FIG. 9 is a circuit diagram showing a conventional switching power supply 9. FIG. 10 is a timing chart of a conventional switching power supply. FIG. 11 is a diagram showing problems of the switching power supply described in Patent Document 1.

[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 a switching power supply 1 according to embodiment 1 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 the 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 (terminal Vo and terminal 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 the 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) in 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) in the 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 a 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. 5A , 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 normal values ​​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 zero current detection thresholds. Any appropriate method can be used to calculate the zero current detection thresholds. 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 Figure 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 related to 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-crossing detection terminal ZC and outputs overvoltage detection information to the switching element drive control unit 34. If an overvoltage is detected, the switching element drive control unit 34 controls the switching element Q so as to reduce the output voltage.

[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) When the second divided voltage VDZC rises to a value greater than the first zero current detection threshold Vth1 (see FIG. 3), at time t1, when the switching element Q is turned off, the zero current detection voltage VDZC (second divided voltage) and the drain voltage VDS rise sharply 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). At time t1, the drain current Id falls sharply to zero. The diode current ID rises sharply and then 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 FIG. 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 FIG. 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. However, although the zero current detection voltage VDZC is larger than the second zero current detection threshold Vth2, it rises to a voltage smaller than the first zero current detection threshold Vth1.

[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 Converter Circuit According to Embodiment 1 Next, a control method of the DC converter circuit according to embodiment 1 will be described. Fig. 7 is a flowchart showing the control method of the DC converter circuit according to embodiment 1. The control method of the DC converter circuit according to embodiment 1 is a control method of the 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 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, 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 Control Method of the DC Conversion Circuit According to the First Embodiment According to the control circuit 30 and switching power supply 1 according to the first embodiment, the control circuit 30 includes a zero current detection threshold calculation unit 31 that calculates the zero current detection thresholds Vth1 and Vth2 based on the 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 fully increased, such as immediately after startup, the timing at which the inductor current becomes zero can be detected (see FIG. 5A).

[0043] 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 (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 increased, 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 it is therefore possible to detect the timing at which the inductor current becomes zero.

[0044] However, with the control circuit 30 and the switching power supply 1 according to the first embodiment, it is possible to detect the timing at which the inductor current becomes zero because the voltage based on the voltage applied to the switching element Q exceeds the second zero current detection threshold Vth2 even in the case of (1) above. Furthermore, it is possible to detect the timing at which the inductor current becomes zero because the voltage based on the voltage applied to the switching element Q after the inductor current becomes zero becomes lower than the first zero current detection threshold Vth1 even in the case of (2) above.

[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 is required when 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 to this, according to the control circuit 30 and switching power supply 1 of the first embodiment, the delay time from when a voltage exceeding (falling below) the first zero current detection threshold Vth1 is detected until the switching element Q is turned on is longer than the delay time from when a voltage exceeding (falling below) the second zero current detection threshold Vth2 is detected until the switching element Q is turned on. Therefore, it is possible to adjust the time from the falling edge time t3 until the next turning on of the switching element Q 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 the need to add 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 correction 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 fully increased, such as immediately after startup, the timing at which the inductor current becomes zero can be detected.

[0050] 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 30 a 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 30 a 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 it is therefore possible to detect the timing at which the inductor current becomes zero.

[0053] The switching power supply 2 and control circuit 30a according to the second embodiment have the same configuration as the switching power supply 1 and 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 control circuit 30 according to the first embodiment.

[0054] Although the present invention has been described above 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. The zero current detection threshold 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. 5B).

[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. However, 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.

[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. In a control circuit for controlling a DC conversion circuit having an inductor and a switching element that changes the increase and decrease of the current flowing through the inductor, 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 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; a switching element drive control unit that controls the 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, wherein the zero-current detection threshold 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. A control circuit characterized by this.

2. In a control circuit for controlling a DC conversion circuit having an inductor and a switching element that changes the increase and decrease of the current flowing through the inductor, 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 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 the voltage applied to the switching element; a switching element drive control unit that controls the 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. A control circuit characterized by this.

3. 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. The control circuit according to claim 1 or 2, characterized by this.

4. The delay time from detecting a voltage exceeding the first zero-current detection threshold to turning on the switching element is made longer than the delay time from detecting a voltage exceeding the second zero-current detection threshold to turning on the switching element. The control circuit according to claim 1 or 2, characterized by this.

5. 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 the valley of the resonance voltage. The control circuit according to claim 1 or 2, characterized in that.

6. The control circuit according to claim 1 or 2, 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 switching power supply comprising: a DC conversion circuit having an inductor and a switching element that changes the increase and decrease of the current flowing through the inductor; A control circuit for controlling the DC conversion circuit, The control circuit is the control circuit according to claim 1 or 2. The switching power supply is characterized in that.

8. A method for controlling a DC conversion circuit having an inductor and a switching element that changes the increase and decrease of the current flowing through the inductor, A zero-current detection threshold calculation step of calculating a 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 comparing a voltage based on the voltage applied to the switching element with the zero-current detection threshold to detect the timing when the current flowing through the inductor becomes zero; A switching element drive control step of controlling the turning on of the switching element based on the timing when the current flowing through the inductor detected in the zero-current detection step becomes zero. The method for controlling a DC conversion circuit is characterized by including.

9. A method for controlling a DC conversion circuit having an inductor and a switching element that changes the increase and decrease of the current flowing through the inductor, A zero-current detection step of 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 to detect the timing when the current flowing through the inductor becomes zero; A control method for a DC conversion circuit, comprising a switching element drive control step of controlling the turning-on 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.