Switching control circuits and power supply systems

Abstract

This invention relates to a switching control circuit and a power supply system. [Solution] The switching control circuit for controlling a switching element includes an input detection voltage acquisition unit that acquires an input detection voltage corresponding to an AC input voltage, a current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element, a short-circuit determination unit that compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether or not the current detection resistor is short-circuited, and a drive unit that drives the switching element using a pulse width modulation method. When the short-circuit determination unit determines that the current detection resistor is short-circuited, the drive unit changes the duty cycle for controlling the switching element to a predetermined first duty cycle.

Description

Technical Field

[0001] The present invention relates to a switching control circuit and a power supply system.

Background Art

[0002] Patent Document 1 describes a "switch control device having a protection function related to a short circuit of a sense resistor or the like". [Prior Art Document] [Patent Document] [Patent Document 1] Japanese Patent Application Laid-Open No. 2020-061821 [Patent Document 2] Japanese Patent Application Laid-Open No. 2020-089033 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-177594 [Patent Document 4] Japanese Patent Application Laid-Open No. 2024-017055

Summary of the Invention

[0003] In a first aspect of the present invention, there is provided a switching control circuit for controlling a switching element provided in a power supply system that generates an output voltage from an AC input voltage, the switching control circuit including: an input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage; a current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects a current flowing through the switching element; a short circuit determination unit that compares the current detection voltage with a predetermined short circuit threshold voltage to determine whether the current detection resistor is short-circuited; and a drive unit that drives the switching element in a pulse width modulation method. When the short circuit determination unit determines that the current detection resistor is short-circuited, the drive unit changes a duty ratio for controlling the switching element to a predetermined first duty ratio.

[0004] In the switching control circuit, the first duty ratio may be smaller than a maximum duty ratio preset in the switching control circuit.

[0005] In any of the above switching control circuits, if the number of switching cycles of the switching element at the first duty cycle exceeds a predetermined reference number, the drive unit may change the duty cycle controlling the switching element to a second duty cycle smaller than the first duty cycle.

[0006] In any of the above switching control circuits, the second duty cycle may be a predetermined minimum duty cycle for driving the switching element or 0.

[0007] In a second embodiment of the present invention, a switching control circuit for controlling a switching element in a power supply system that generates an output voltage from an AC input voltage is provided, comprising: an input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage; a current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element; a short-circuit determination unit that compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether or not the current detection resistor is short-circuited; and a drive unit that drives the switching element in a pulse frequency modulation manner, wherein the drive unit, when the short-circuit determination unit determines that the current detection resistor is short-circuited, changes the ON width for turning on the switching element to a predetermined first ON width.

[0008] In the switching control circuit described above, the first ON width may be smaller than the maximum ON width preset in the switching control circuit.

[0009] In any of the above switching control circuits, if the number of switching cycles of the switching element in the first on-width exceeds a predetermined reference number, the drive unit may change the on-width for turning on the switching element to a second on-width that is shorter than the first on-width.

[0010] In any of the above switching control circuits, the second ON width may be a predetermined minimum ON width for driving the switching element or 0.

[0011] In any of the above switching control circuits, the drive unit may have a timer circuit that counts the number of switching cycles of the switching element.

[0012] In any of the above switching control circuits, the drive unit may reset the timer circuit count in accordance with the input detection voltage.

[0013] In any of the above switching control circuits, the drive unit may reset the timer circuit's count if the determination value based on the input detection voltage is lower than a predetermined count reference value.

[0014] In a third embodiment of the present invention, a switching control circuit for controlling a switching element in a power supply system that generates an output voltage from an AC input voltage is provided, comprising: an input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage; a current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element; a short-circuit determination unit that compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether the current detection resistor is short-circuited; and a drive unit that drives the switching element, wherein the drive unit has a timer circuit that counts the number of switching cycles of the switching element when the short-circuit determination unit determines that the current detection resistor is short-circuited, and when the timer circuit counts that the number of switching cycles is equal to or greater than a predetermined reference number, it outputs a signal to the drive unit to stop driving the switching element, and the drive unit resets the timer circuit count according to the input detection voltage.

[0015] In the switching control circuit described above, the drive unit may reset the timer circuit's count if the determination value based on the input detection voltage is lower than a predetermined count reference value.

[0016] A fourth embodiment of the present invention provides a power supply system comprising any of the above-described switching control circuits, the switching element, and the current sensing resistor.

[0017] A fifth embodiment of the present invention provides a switching control circuit for controlling a switching element in a power supply system that generates an output voltage from an AC input voltage, comprising: an input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage; a feedback voltage acquisition unit that acquires a feedback voltage corresponding to the output voltage; a current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element; a short-circuit determination unit that compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether the current detection resistor is short-circuited; and a drive unit that drives the switching element according to the feedback voltage acquired by the feedback voltage acquisition unit, wherein the drive unit changes the drive of the switching element from a drive according to the feedback voltage to a different drive when the short-circuit determination unit determines that the current detection resistor is short-circuited, and does not change the drive of the switching element to the different drive according to the AC input voltage detected by the input detection voltage acquisition unit.

[0018] In the switching control circuit described above, the drive unit may drive the switching element using a pulse width modulation method corresponding to the output feedback voltage. If the short-circuit determination unit determines that the current detection resistor is short-circuited, the drive unit may change the duty cycle controlling the switching element to a predetermined first duty cycle as a different drive.

[0019] In the above switching control circuit, the driving unit may drive the switching element in a pulse frequency modulation method according to the feedback voltage. When the driving unit is determined by the short-circuit determination unit that the current detection resistor is short-circuited, as the different driving, the on-width for turning on the switching element may be changed to a predetermined first on-width.

[0020] In any of the above switching control circuits, the driving unit may have a timer circuit that counts the number of switchings of the switching element. When the timer circuit counts that the number of switchings is equal to or more than a predetermined reference number, the timer circuit may output a signal to stop driving the switching element to the driving unit. The driving unit may reset the count of the timer circuit according to the input detection voltage.

[0021] Note that the above summary of the invention does not enumerate all the features of the present invention. Also, sub-combinations of these feature groups may also be inventions.

Brief Description of Drawings

[0022] [Figure 1] Shows an overview of the configuration of the switching control circuit 100. [Figure 2] Shows an overview of the configuration of a modified example of the switching control circuit 100. [Figure 3A] Shows an example of a power supply system 10 including the switching control circuit 100. [Figure 3B] Shows an example of the switching control circuit 100. [Figure 4] Shows an example of the input detection voltage and the determination value. [Figure 5] Shows an example of a power supply system 10 including the switching control circuit 100. [Figure 6] Shows an example of a power supply system 10 including the switching control circuit 100. [Figure 7] Shows an example of the auxiliary winding voltage Vzcd. [Figure 8] An example of input detection voltage and judgment value is shown. [Modes for carrying out the invention]

[0023] The present invention will be described below through embodiments of the invention, but these embodiments are not intended to limit the invention as defined in the claims. Furthermore, not all combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0024] Figure 1 shows an overview of the configuration of the switching control circuit 100. The switching control circuit 100 in this example is a switching control circuit that controls a switching element provided in a power supply system that generates an output voltage from an AC input voltage. The switching control circuit 100 includes an input detection voltage acquisition unit 110, a current detection voltage acquisition unit 120, a feedback voltage acquisition unit 125, a short-circuit determination unit 130, and a drive unit 140.

[0025] The input detection voltage acquisition unit 110 acquires an input detection voltage corresponding to the AC input voltage. The input detection voltage acquisition unit 110 may acquire the voltage obtained by full-wave rectifying the AC input voltage as the input detection voltage. The input detection voltage acquisition unit 110 may acquire the voltage obtained by the voltage drop due to resistance from the voltage obtained by full-wave rectifying the AC input voltage as the input detection voltage. The input detection voltage acquisition unit 110 may acquire the voltage obtained by full-wave rectifying and then dividing the AC input voltage as the input detection voltage. The input detection voltage acquisition unit 110 may acquire the voltage generated in the auxiliary winding when the voltage obtained by full-wave rectifying the AC input voltage is input to the main winding of the transformer as the input detection voltage. The input detection voltage acquisition unit 110 may acquire the voltage obtained by dividing the voltage generated in the auxiliary winding when the voltage obtained by full-wave rectifying the AC input voltage is input to the main winding of the transformer as the input detection voltage. However, the input detection voltage acquired by the input detection voltage acquisition unit 110 is not limited to these examples. The input detection voltage acquisition unit 110 may supply the input detection voltage to the drive unit 140.

[0026] The current detection voltage acquisition unit 120 acquires the current detection voltage generated at a current detection resistor that detects the current flowing through the switching element. The switching element may be an element in a power supply system that generates an output voltage from an AC input voltage. The current detection resistor may be a resistor connected in series with the switching element and used to detect the current flowing through the switching element. Details of the switching element and the current detection resistor will be described later. The current detection voltage acquisition unit 120 may supply the current detection voltage to the short-circuit determination unit 130 and the drive unit 140.

[0027] The feedback voltage acquisition unit 125 acquires a feedback voltage corresponding to the output voltage. The feedback voltage acquisition unit 125 may acquire a feedback voltage generated according to the output voltage. For example, the feedback voltage is generated using a photocoupler according to the output voltage. The feedback voltage acquisition unit 125 may acquire a feedback voltage obtained by dividing the output voltage at a predetermined voltage division ratio.

[0028] The short-circuit detection unit 130 compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether or not the current detection resistor is short-circuited. The short-circuit detection unit 130 may determine that the current detection resistor is short-circuited if the current detection voltage does not exceed the short-circuit threshold voltage within a predetermined time after the switching element is turned on. The short-circuit detection unit 130 may supply the determination result of whether or not the current detection resistor is short-circuited to the drive unit 140.

[0029] The drive unit 140 drives the switching element. The drive unit 140 may drive the switching element based on at least one of the input detection voltage, current detection voltage, feedback voltage, or determination result. The drive unit 140 may drive the switching element using pulse width modulation. The drive unit 140 may drive the switching element using pulse frequency modulation.

[0030] For example, when the drive unit 140 drives a switching element using pulse width modulation, the drive unit 140 drives the switching element by setting a duty cycle that controls the switching element. The drive unit 140 may set the duty cycle so that the output voltage of the power supply system remains constant. The drive unit 140 may set the duty cycle so that the output voltage of the power supply system remains constant based on a feedback signal corresponding to the output voltage and a current detection voltage.

[0031] When the drive unit 140 determines that the current detection resistor is short-circuited by the short-circuit detection unit 130, it changes the duty cycle controlling the switching element to a predetermined first duty cycle. The first duty cycle may be smaller than the maximum duty cycle preset in the switching control circuit 100. For example, the first duty cycle is 50% of the maximum duty cycle. The first duty cycle may be a duty cycle that does not cause the switching element to be destroyed by self-heating.

[0032] If the current sensing resistor is short-circuited, the current sensing voltage will not rise, making it impossible to compare it with the feedback signal corresponding to the output voltage. This results in the maximum pulse width controlling the switching element. In this case, the current flowing through the switching element becomes excessive, potentially causing it to overheat and be destroyed. Alternatively, the output voltage may rise, causing the switching element to shut down due to output overvoltage protection. To prevent the destruction of the switching element, it may be necessary to use a switching element with a rated current value greater than the capacity of the power supply system.

[0033] The switching control circuit 100 in this example includes a short-circuit detection unit 130 and a drive unit 140. When the drive unit 140 determines that the current detection resistor is short-circuited, it changes the duty cycle to a first duty cycle. This reduces stress on the switching element due to heat generation, thus preventing damage to the switching element even when using a switching element with a low rated current value.

[0034] Furthermore, in the switching control circuit 100 of this example, when the drive unit 140 determines that the current detection resistor is short-circuited, it changes the duty cycle to the first duty cycle without stopping the switching element. As a result, even if a short circuit in the current detection resistor is falsely detected due to an instantaneous voltage drop in the AC input voltage, the switching control circuit 100 of this example can prevent damage to the switching element while continuing to generate an output voltage. Therefore, the switching control circuit 100 of this example can improve efficiency compared to the case where the switching element is stopped in response to short-circuit detection.

[0035] The process of determining whether a short circuit has occurred in the current sensing resistor by the short-circuit determination unit 130 and changing the duty cycle by the drive unit 140 may be performed for each switching cycle of the switching element. For example, if a short circuit is determined to have occurred in one cycle and the duty cycle is changed to the first duty cycle, and it is determined that no short circuit has occurred in the next cycle, the drive unit 140 may terminate control by the first duty cycle and return to normal control. As another example, if a short circuit is determined to have occurred in one cycle and the duty cycle is changed to the first duty cycle, and it is determined that a short circuit has occurred again in the next cycle, the drive unit 140 may continue control by the first duty cycle.

[0036] If the number of switching cycles of the switching element at the first duty cycle exceeds a predetermined reference cycle, the drive unit 140 may change the duty cycle controlling the switching element to a second duty cycle smaller than the first duty cycle. For example, the second duty cycle is a predetermined minimum duty cycle for driving the switching element or 0. The predetermined reference cycle may be 8 or 16, but is not limited to these.

[0037] In the switching control circuit 100 of this example, the drive unit 140 changes the duty cycle to a second duty cycle smaller than the first duty cycle when the number of switching cycles of the switching element at the first duty cycle exceeds a predetermined reference number. When the number of switching cycles at the first duty cycle exceeds the reference number, there is a high probability that the current sensing resistor is actually short-circuited. Therefore, the switching control circuit 100 of this example can prevent the destruction of the switching element when a short circuit actually occurs by having the drive unit 140 change the duty cycle to a second duty cycle smaller than the first duty cycle. On the other hand, if the short-circuit detection is false, that is, if the control returns to normal before the number of switching cycles at the first duty cycle exceeds the reference number, the switching control circuit 100 of this example can improve efficiency compared to when the switching element is stopped in response to a short-circuit detection.

[0038] As another example, when the drive unit 140 drives a switching element using a pulse frequency modulation method, the drive unit 140 drives the switching element by setting an on-width for turning on the switching element. The drive unit 140 may set the on-width so that the output voltage of the power supply system remains constant. The drive unit 140 may set the on-width so that the output voltage of the power supply system remains constant based on a feedback signal corresponding to the output voltage and a current detection voltage.

[0039] When the drive unit 140 determines that the current detection resistor is short-circuited by the short-circuit detection unit 130, it changes the ON width for turning on the switching element to a predetermined first ON width. The first ON width may be smaller than the maximum ON width preset in the switching control circuit. For example, the first ON width is 50% of the maximum ON width. The first ON width may be an ON width that does not cause damage to the switching element due to self-heating.

[0040] The switching control circuit 100 in this example includes a short-circuit detection unit 130 and a drive unit 140. When the drive unit 140 determines that the current detection resistor is short-circuited, it changes the ON width to a first ON width. This reduces stress on the switching element due to heat generation, thus preventing damage to the switching element even when using a switching element with a small rated current value.

[0041] Furthermore, in the switching control circuit 100 of this example, when the drive unit 140 determines that the current detection resistor is short-circuited, it changes the ON width to the first ON width without stopping the switching element. As a result, even if a short circuit in the current detection resistor is falsely detected due to an instantaneous voltage drop in the AC input voltage, the switching control circuit 100 of this example can prevent damage to the switching element while continuing to generate an output voltage. Therefore, the switching control circuit 100 of this example can improve efficiency compared to the case where the switching element is stopped in response to short-circuit detection.

[0042] The process of determining whether a short circuit has occurred in the current sensing resistor by the short-circuit determination unit 130 and changing the ON width by the drive unit 140 may be performed for each switching cycle of the switching element. For example, if a short circuit is determined to have occurred in one cycle and the ON width is changed to the first ON width, and it is determined that no short circuit has occurred in the next cycle, the drive unit 140 may terminate control with the first ON width and return to normal control. As another example, if a short circuit is determined to have occurred in one cycle and the ON width is changed to the first ON width, and it is determined that a short circuit has occurred again in the next cycle, the drive unit 140 may continue control with the first ON width.

[0043] If the number of switching cycles of the switching element in the first on-width exceeds a predetermined reference cycle, the drive unit 140 may change the on-width for turning on the switching element to a second on-width that is shorter than the first on-width. For example, the second on-width is a predetermined minimum on-width for driving the switching element or 0. The predetermined reference cycle may be 8 or 16. However, the reference cycle is not limited to these.

[0044] In the switching control circuit 100 of this example, the drive unit 140 changes the ON width to a second ON width, which is smaller than the first ON width, when the number of switching cycles of the switching element in the first ON width exceeds a predetermined reference number. When the number of switching cycles in the first ON width exceeds the reference number, there is a high probability that the current sensing resistor is actually short-circuited. Therefore, the switching control circuit 100 of this example can prevent the destruction of the switching element when a short circuit actually occurs by having the drive unit 140 change the ON width to a second ON width, which is smaller than the first ON width. On the other hand, if the short-circuit detection is false, that is, if the control returns to normal before the number of switching cycles in the first ON width exceeds the reference number, the switching control circuit 100 of this example can improve efficiency compared to when the switching element is stopped in response to a short-circuit detection.

[0045] If the drive unit 140 determines that the current detection resistor is short-circuited by the short-circuit detection unit 130, it may change the drive of the switching element from a drive corresponding to the feedback voltage to a different drive. For example, if the current detection resistor is not short-circuited, the drive unit 140 drives the switching element using a pulse width modulation method or a pulse frequency modulation method based on the feedback voltage.

[0046] When the drive unit 140 drives the switching element using a pulse width modulation method corresponding to the feedback voltage, the duty cycle of the switching element is determined based on the feedback voltage. In other words, driving according to the feedback voltage may mean driving the switching element by determining its duty cycle based on the feedback voltage. If the short-circuit detection unit 130 determines that the current detection resistor is short-circuited, the drive unit 140 may change the duty cycle controlling the switching element to a predetermined first duty cycle as a different drive.

[0047] When the drive unit 140 drives the switching element using a pulse frequency modulation method corresponding to the feedback voltage, the ON width of the switching element is determined based on the feedback voltage. In other words, driving according to the feedback voltage may mean driving the switching element by determining its ON width based on the feedback voltage. If the short-circuit detection unit 130 determines that the current detection resistor is short-circuited, the drive unit 140 may change the ON width for turning on the switching element to a predetermined first ON width as a different drive method.

[0048] If the current sensing resistor is short-circuited, control based on the feedback signal will not be performed properly, resulting in excessive current flowing through the switching element. In this case, the switching element may be destroyed by self-heating. Alternatively, the output voltage may rise, causing the switching element to shut down due to output overvoltage protection. To prevent the destruction of the switching element, it may be necessary to use a switching element with a rated current value greater than the capacity of the power supply system.

[0049] The switching control circuit 100 in this example includes a short-circuit detection unit 130 and a drive unit 140. When the drive unit 140 determines that the current detection resistor is short-circuited, it changes the drive of the switching element from a drive corresponding to the feedback voltage to a different drive. This reduces stress on the switching element due to heat generation, and thus prevents damage to the switching element even when using a switching element with a small rated current value.

[0050] Furthermore, in the switching control circuit 100 of this example, if the drive unit 140 determines that the current detection resistor is short-circuited, it changes to a different drive without stopping the switching element. As a result, even if a short circuit in the current detection resistor is falsely detected due to an instantaneous voltage drop in the AC input voltage, the switching control circuit 100 of this example can prevent damage to the switching element while continuing to generate an output voltage. Therefore, the switching control circuit 100 of this example can improve efficiency compared to the case where the switching element is stopped in response to short-circuit detection.

[0051] The process of determining whether a short circuit has occurred in the current sensing resistor by the short-circuit detection unit 130 and changing the driving method by the drive unit 140 may be performed for each switching cycle of the switching element. For example, if a short circuit is determined to have occurred in one cycle and the drive is changed to a different method, and it is determined that no short circuit has occurred in the next cycle, the drive unit 140 may terminate the different drive and return to the normal drive. As another example, if a short circuit is determined to have occurred in one cycle and the drive is changed to a different method, and it is determined that a short circuit has occurred again in the next cycle, the drive unit 140 may continue the different drive.

[0052] The drive unit 140 does not change the drive of the switching element to a different drive in response to the AC input voltage detected by the input detection voltage acquisition unit 110. For example, if the phase angle of the AC input voltage is low, the AC input voltage may be low, and a short circuit in the current detection resistor may be falsely detected. Also, a short circuit in the current detection resistor may be falsely detected due to an instantaneous voltage drop in the AC input voltage, etc. In the switching control circuit 100 of this example, the drive unit 140 does not change the drive of the switching element to a different drive in response to the AC input voltage detected by the input detection voltage acquisition unit 110. As a result, even if a short circuit in the current detection resistor is falsely detected, the switching control circuit 100 of this example can prevent the destruction of the switching element while continuing to generate an output voltage, and can improve efficiency compared to the case where the switching element is stopped in response to a short circuit detection.

[0053] Figure 2 shows an overview of a modified configuration of the switching control circuit 100. The switching control circuit 100 in this example is a switching control circuit that controls a switching element provided in a power supply system that generates an output voltage from an AC input voltage. The switching control circuit 100 includes an input detection voltage acquisition unit 110, a current detection voltage acquisition unit 120, a feedback voltage acquisition unit 125, a short-circuit determination unit 130, and a drive unit 140. The drive unit 140 has a timer circuit 142. The input detection voltage acquisition unit 110, the current detection voltage acquisition unit 120, the feedback voltage acquisition unit 125, and the short-circuit determination unit 130 may be configured the same as those described in Figure 1.

[0054] When the short-circuit detection unit 130 determines that the current detection resistor is short-circuited, the timer circuit 142 counts the number of switching cycles of the switching element. When the timer circuit 142 counts that the number of switching cycles is equal to or greater than a predetermined reference number, it may output a signal to the drive unit 140 to stop driving the switching element 20. If the number of switching cycles counted by the timer circuit 142 exceeds a predetermined reference number, the drive unit 140 may stop driving the switching element or drive the switching element under minimum driving conditions. Driving the switching element under minimum driving conditions may include setting the duty cycle to the minimum duty cycle in the pulse width modulation scheme, and setting the ON width to the minimum ON width in the pulse frequency modulation scheme.

[0055] The drive unit 140 resets the timer circuit 142 count according to the input detection voltage. For example, the drive unit 140 resets the timer circuit 142 count if the judgment value based on the input detection voltage is lower than a predetermined count reference value.

[0056] For example, if the phase angle of the AC input voltage is low, the AC input voltage may be low, and a short circuit in the current sensing resistor may be falsely detected. Also, a short circuit in the current sensing resistor may be falsely detected due to an instantaneous voltage drop in the AC input voltage. In the switching control circuit 100 of this example, the drive unit 140 resets the count of the timer circuit 142 according to the input detection voltage. That is, the drive unit 140 either stops driving the switching element or resets the count to drive under minimum driving conditions. As a result, even if a short circuit in the current sensing resistor is falsely detected, the switching control circuit 100 of this example can prevent damage to the switching element while continuing to generate an output voltage, and can improve efficiency compared to stopping the switching element in response to a short circuit detection.

[0057] Figure 3A shows an example of a power supply system 10 equipped with a switching control circuit 100. The power supply system 10 comprises the switching control circuit 100, a switching element 20, and a current sensing resistor 30. The power supply system 10 in this example is a flyback power supply system. The switching control circuit 100 in this example drives the switching element 20 using a pulse width modulation method. However, the configuration of the power supply system and the driving method of the switching element 20 are not limited to this example.

[0058] The power supply system 10 in this example is a power supply system that generates an output voltage Vout from a voltage obtained by full-wave rectifying an AC input voltage Vac by a diode bridge 40. Before the AC input voltage Vac is rectified by the diode bridge 40, noise may be removed by a noise removal circuit 42. The voltage rectified by the diode bridge 40 may be smoothed by a capacitor C1 before being input to the transformer 50.

[0059] A snubber circuit consisting of diode D1, capacitor C2, and resistor R1 is provided on the primary side of transformer 50 to suppress surge voltage. The voltage generated on the secondary side of transformer 50 is rectified by diode D2 and smoothed by capacitor C3. The output voltage Vout is divided by resistors R2 and R3 and input to shunt regulator D3. Shunt regulator D3 generates current so that the input voltage is equal to the internal reference voltage source, and current flows through the path consisting of resistor R4 and the light-emitting diode 62 of photocoupler 60. As a result, the output voltage Vout is fed back to the switching control circuit 100 via photocoupler 60.

[0060] The switching control circuit 100 generates a drive voltage Vdr to drive the switching element 20. The switching control circuit 100 may be connected to the gate of the switching element 20 via diode D4, resistor R5, and resistor R6. This sets different gate resistance values ​​for when the switching element 20 is on and when it is off.

[0061] The switching control circuit 100 is provided on the primary side of the transformer 50 and controls the switching element 20 so that the output voltage Vout remains constant based on a feedback signal from the secondary side of the transformer 50. For example, a photocoupler 60 consisting of a light-emitting diode 62 provided on the secondary side and a phototransistor 64 provided on the primary side generates a feedback voltage Vfb. The switching control circuit 100 may control the switching element 20 by comparing the current detection voltage Vcs generated at the current detection resistor 30 with the feedback voltage Vfb. The voltage generated at the auxiliary winding 52 may be rectified and smoothed via diode D5, resistor R7 and capacitor C4 and supplied as power to the switching control circuit 100.

[0062] In this example, the input detection voltage acquisition unit 110 of the switching control circuit 100 acquires the voltage obtained by the voltage drop across the resistor 70 from the voltage obtained by full-wave rectification of the AC input voltage Vac by diodes D6 and D7 as the input detection voltage Vin. The current detection voltage acquisition unit 120 of the switching control circuit 100 acquires the current detection voltage Vcs generated at the current detection resistor 30 that detects the current flowing through the switching element 20.

[0063] Figure 3B shows an example of an integrated switching control circuit 100. The switching control circuit 100 is an example of the switching control circuit 100 provided in the power supply system 10 of Figure 3A. The switching control circuit 100 in this example includes an input detection voltage acquisition unit 110, a current detection voltage acquisition unit 120, a short-circuit determination unit 130, and a drive unit 140. The switching control circuit 100 may also include an overcurrent determination unit 190.

[0064] The input detection voltage acquisition unit 110 acquires an input detection voltage Vin corresponding to the AC input voltage Vac. The input detection voltage acquisition unit 110 may have an input detection circuit 112. The input detection circuit 112 may supply a signal corresponding to the input detection voltage Vin to the drive unit 140.

[0065] The current detection voltage acquisition unit 120 acquires the current detection voltage Vcs generated at the current detection resistor 30 that detects the current flowing through the switching element 20. The current detection voltage acquisition unit 120 may supply the current detection voltage Vcs to the short-circuit determination unit 130, the drive unit 140, and the overcurrent determination unit 190.

[0066] The feedback voltage acquisition unit 125 acquires a feedback voltage Vfb corresponding to the output voltage Vout. The feedback voltage acquisition unit 125 may supply the feedback voltage Vfb to the drive unit 140.

[0067] The short-circuit detection unit 130 may include a comparator 132 and a voltage generation unit 134. The current detection voltage Vcs is input to the non-inverting input terminal of the comparator 132, and the short-circuit threshold voltage Vsh is input to the inverting input terminal of the comparator 132. The comparator 132 outputs a high-level signal when the current detection voltage Vcs is greater than or equal to the short-circuit threshold voltage Vsh, and outputs a low-level signal when the current detection voltage Vcs is less than the short-circuit threshold voltage Vsh. That is, the comparator 132 outputs a low-level signal when it determines that the current detection resistor 30 is short-circuited. The comparator 132 may supply a signal to the drive unit 140 according to the comparison result. The voltage generation unit 134 generates a predetermined short-circuit threshold voltage Vsh.

[0068] The drive unit 140 may include a timer circuit 142 and a pulse limiting circuit 144. The drive unit 140 may also include an OR circuit 146, an oscillator 150, a comparator 160, a gain circuit 162, a slope circuit 164, a comparator 166, an OR circuit 168, a flip-flop circuit 170, an OR circuit 172, an AND circuit 174, and a driver 180.

[0069] The timer circuit 142 may count the number of switching cycles of the switching element 20. For example, the timer circuit 142 counts the number of switching cycles of the switching element 20 when the short-circuit detection unit 130 determines that the current detection resistor 30 is short-circuited. The timer circuit 142 may be reset in response to a high-level signal from the comparator 132. That is, the timer circuit 142 is reset when the current detection voltage Vcs is greater than or equal to the short-circuit threshold voltage Vsh and it is determined that the current detection resistor 30 is not short-circuited, and the timer circuit 142 may continue counting when the current detection voltage Vcs is less than the short-circuit threshold voltage Vsh and it is determined that the current detection resistor 30 is short-circuited.

[0070] The timer circuit 142 may be reset in response to a signal from the input detection circuit 112. For example, the timer circuit 142 is reset when the determination value based on the input detection voltage Vin is lower than a predetermined count reference value. The determination value based on the input detection voltage Vin may be the voltage value of the input detection voltage Vin itself, or it may be a value calculated from the input detection voltage Vin. For example, the determination value may be a value calculated to compensate for the voltage drop due to the resistor 70.

[0071] In the switching control circuit 100 of this example, the timer circuit 142 count is reset according to the input detection voltage Vin. As a result, even if a short circuit in the current detection resistor 30 is falsely detected due to an instantaneous voltage drop in the AC input voltage Vac, the switching control circuit 100 of this example can prevent the destruction of the switching element 20 while continuing to generate the output voltage Vout, thereby improving efficiency compared to the case where the switching element 20 is stopped in response to short circuit detection.

[0072] The pulse limiting circuit 144 may generate a signal to limit the duty cycle of the switching element 20 to a first duty cycle. For example, the pulse limiting circuit 144 generates a signal to limit the duty cycle of the switching element 20 to a first duty cycle based on the signal of the first duty cycle generated by the oscillator 150.

[0073] The pulse limiting circuit 144 may generate a signal to limit the duty cycle of the switching element 20 to a first duty cycle in response to a signal from the comparator 132. For example, the pulse limiting circuit 144 generates a signal to limit the duty cycle of the switching element 20 to a first duty cycle when the signal from the comparator 132 is low level, and does not generate a signal to limit the duty cycle of the switching element 20 to a first duty cycle when the signal from the comparator 132 is high level. In other words, the pulse limiting circuit 144 may generate a signal to limit the duty cycle of the switching element 20 to a first duty cycle when it is determined that the current sensing resistor 30 is short-circuited.

[0074] In the switching control circuit 100 of this example, the pulse limiting circuit 144 generates a signal to limit the duty cycle of the switching element 20 to the first duty cycle when it is determined that the current sensing resistor 30 is short-circuited. As a result, even if a short circuit in the current sensing resistor 30 is falsely detected due to an instantaneous voltage drop in the AC input voltage, the switching control circuit 100 of this example can prevent the destruction of the switching element 20 while continuing to generate an output voltage, thereby improving efficiency compared to the case where the switching element 20 is stopped in response to a short circuit detection.

[0075] The feedback voltage Vfb is compared with a reference voltage Vref by the comparator 160. The comparator 160 may output the comparison result to the AND circuit 174. This allows the switching operation to be stopped and the safety of the power supply system to be enhanced when the output voltage Vout, which corresponds to the feedback voltage Vfb being lower than the reference voltage Vref, is higher than a predetermined voltage (overvoltage detection processing). The signal of the feedback voltage Vfb amplified by the gain circuit 162 is input to the inverting input terminal of the comparator 166. The signal of the current detection voltage Vcs slope-compensated by the slope circuit 164 is input to the non-inverting input terminal of the comparator 166. This allows the feedback voltage Vfb and the current detection voltage Vcs to be compared.

[0076] The oscillator 150 outputs a one-shot pulse. The one-shot pulse output by the oscillator 150 is input to the set terminal S of the flip-flop circuit 170. This causes the switching element 20 to turn on. If there is no short circuit in the current sensing resistor 30, the current sensing voltage Vcs rises in response to the switching element 20 turning on. When the voltage of the current sensing voltage Vcs, after slope compensation by the slope circuit 164, becomes greater than the output signal of the gain circuit 162, the comparator 166 outputs a high-level signal, and this high-level signal is input to the OR circuit 168. As a result, a high-level signal is input to the reset terminal R of the flip-flop circuit 170, and via the OR circuit 172 and AND circuit 174, the duty cycle of the drive voltage Vdr that the driver 180 uses to drive the switching element 20 is set. In this way, the switching control circuit 100 in this example may control the switching element 20 using a pulse width modulation method.

[0077] If the current sensing resistor 30 is short-circuited, the current sensing voltage Vcs does not rise, and the comparator 166 continues to output a low-level signal. As a result, a high-level signal is not input to the reset terminal R of the flip-flop circuit 170, the pulse width controlling the switching element 20 becomes maximum, and there is a risk that the switching element 20 will be destroyed by self-heating. In the switching control circuit 100 of this example, when it is determined that the current sensing resistor 30 is short-circuited, the pulse limiting circuit 144 generates a signal to limit the duty cycle of the switching element 20 to the first duty cycle, and this signal is input to the OR circuit 168 via the OR circuit 146. As a result, the output of the OR circuit 168 becomes high level, and a high-level signal is input to the reset terminal R of the flip-flop circuit 170, so that the duty cycle of the drive voltage Vdr used by the driver 180 to drive the switching element 20 is set to the first duty cycle via the OR circuit 172 and the AND circuit 174. Therefore, since the stress on the switching element 20 due to heat generation can be reduced, even when using a switching element 20 with a small rated current value, the destruction of the switching element 20 can be prevented.

[0078] Furthermore, in the switching control circuit 100 of this example, when the drive unit 140 determines that the current detection resistor 30 is short-circuited, it changes the duty cycle of the switching element 20 to the first duty cycle without stopping it. As a result, even if a short circuit in the current detection resistor is falsely detected due to an instantaneous voltage drop in the AC input voltage, the switching control circuit 100 of this example can prevent the switching element 20 from being damaged while continuing to generate an output voltage. Therefore, the switching control circuit 100 of this example can improve efficiency compared to the case where the switching element 20 is stopped in response to short-circuit detection.

[0079] The flow of determining whether a short circuit has occurred in the current detection resistor 30 by the short-circuit determination unit 130, changing the duty cycle by the drive unit 140, and determining whether or not to reset the count of the timer circuit 142 by the drive unit 140 may be executed for each switching cycle of the switching element 20. For example, if a short circuit is determined to have occurred in one cycle and the duty cycle is changed to the first duty cycle, and if it is determined that no short circuit has occurred in the next cycle, the drive unit 140 may terminate control based on the first duty cycle and return to normal control based on the current detection voltage Vcs and feedback voltage Vfb. As another example, if a short circuit is determined to have occurred in one cycle and the duty cycle is changed to the first duty cycle, and it is determined that a short circuit has occurred again in the next cycle, the drive unit 140 may continue control based on the first duty cycle.

[0080] If the number of switching cycles of the switching element 20 at the first duty cycle exceeds a predetermined reference number, the drive unit 140 may change the duty cycle controlling the switching element 20 to a second duty cycle that is smaller than the first duty cycle. For example, when the timer circuit 142 counts the predetermined reference number of cycles, a high-level signal is continuously input to the reset terminal R of the flip-flop circuit 170. As a result, the duty cycle controlling the switching element 20 is changed to the second duty cycle.

[0081] In the switching control circuit 100 of this example, a signal corresponding to the set pulse input to the set terminal S of the flip-flop circuit 170 is also input to the driver 180 via the OR circuit 172 and the AND circuit 174. The second duty cycle may be the duty cycle of the set pulse. For example, the duty cycle of the set pulse is a predetermined minimum duty cycle for driving the switching element 20. If the switching element 20 is completely stopped, it may be difficult to recover the switching element 20. By continuing to drive the switching element 20 at the minimum duty cycle without completely stopping it, the recovery of the switching element 20 can be facilitated. However, the driver 180 may not receive a signal corresponding to the set pulse, and the switching element 20 may be completely stopped. In this case, the second duty cycle is 0.

[0082] In the switching control circuit 100 of this example, when it is determined that the current detection resistor 30 is short-circuited, the drive unit 140 sets the duty cycle of the switching element 20 to the first duty cycle, and the timer circuit 142 counts the number of switching cycles of the switching element 20. On the other hand, the drive unit 140 can reset the count of the timer circuit 142 according to the input detection voltage Vin. For example, even if the count of the timer circuit 142 is reset according to the input detection voltage Vin, the duty cycle of the switching element 20 may be set to the first duty cycle. That is, even if the count of the timer circuit 142 is reset to prevent false detection of a short circuit in the current detection resistor 30, a short circuit in the current detection resistor 30 may actually occur, so the duty cycle of the switching element 20 may be set to the first duty cycle.

[0083] The overcurrent detection unit 190 may compare the current detection voltage Vcs with a predetermined overcurrent threshold voltage Voc to determine whether or not an overcurrent is flowing through the switching element 20. When the current detection voltage Vcs becomes greater than the overcurrent threshold voltage Voc, a high-level signal is output from the comparator 192, and a high-level signal is continuously input to the set terminal R of the flip-flop circuit 170, so that the switching element 20 is driven at the minimum duty cycle or stopped. This prevents the switching element 20 from being damaged by overcurrent.

[0084] The short-circuit threshold voltage Vsh may be between 1 / 40 and 1 / 10 of the overcurrent threshold voltage Voc. For example, the short-circuit threshold voltage Vsh is 1 / 20 of the overcurrent threshold voltage Voc.

[0085] Figure 4 shows an example of input detection voltage and judgment value. The upper graph shows the time change of the input detection voltage Vin, and the lower graph shows the time change of the judgment value.

[0086] The determination value based on the input detection voltage Vin may be the voltage value of the input detection voltage Vin itself, or it may be a value calculated from the input detection voltage Vin. For example, the determination value may be a value calculated to compensate for the voltage drop due to resistor 70.

[0087] The drive unit 140 may reset the timer circuit 142's count if the determination value based on the input detection voltage Vin is lower than a predetermined count reference value. The timer circuit 142's count may be reset in the section where the determination value shown by the solid line is below the count reference value shown by the dashed line, and the timer circuit 142's count may continue in the section where the determination value shown by the solid line is above the count reference value shown by the dashed line. In the switching control circuit 100 of this example, since the drive unit 140 resets the timer circuit 142's count in accordance with the input detection voltage Vin, the switching control circuit 100 of this example can prevent the destruction of the switching element 20 while continuing to generate an output voltage even if a short circuit in the current detection resistor 30 is falsely detected, and can improve efficiency compared to the case where the switching element 20 is stopped in response to short circuit detection.

[0088] The count reference value may be determined to correspond to the range in which the AC input voltage Vac has a low phase angle. For example, the count reference value may be set to correspond to the judgment values ​​when the phase angle of the AC input voltage Vac is 45 degrees and 135 degrees. Alternatively, a minimum value for input voltage detection (a threshold for resetting) may be set. As a result, if the phase angle of the AC input voltage Vac is in the range of 45 degrees or less or 135 degrees or more, or if the input voltage is below the minimum value, the drive unit 140 may reset the count of the timer circuit 142.

[0089] Figure 5 shows an example of a power supply system 10 equipped with a switching control circuit 100. The power supply system 10 comprises a switching control circuit 100, a switching element 20, and a current sensing resistor 30. The power supply system 10 in this example is a PFC power supply system. The power supply system 10 may be a PFC power supply system that operates in continuous current mode or discontinuous current mode. The switching control circuit 100 in this example drives the switching element 20 using a pulse width modulation method. However, the configuration of the power supply system and the driving method of the switching element 20 are not limited to this example. In this example, differences from the embodiment described in relation to Figures 3A and 3B will be explained in particular, and other aspects may be the same as the embodiment described in relation to Figures 3A and 3B.

[0090] The power supply system 10 in this example is a power supply system that generates an output voltage Vout from a voltage obtained by full-wave rectification of an AC input voltage Vac by a diode bridge 40. The switching control circuit 100 generates a drive voltage Vdr to drive the switching element 20. The switching control circuit 100 controls the switching element 20 so that the output voltage Vout remains constant. For example, the output voltage Vout is divided by resistors R2 and R3 to generate a feedback voltage Vfb. The switching control circuit 100 may control the switching element 20 by comparing the feedback voltage Vfb converted by an error amplifier or the like with a triangular wave that increases with a constant slope after the switching element 20 is turned on.

[0091] Oscillator 150 outputs a one-shot pulse and a triangular wave. The one-shot pulse output by oscillator 150 is input to the set terminal S of the flip-flop circuit 170. This causes the switching element 20 to turn on. The triangular wave output by oscillator 150 is input to the inverting input terminal of comparator 166. The triangular wave output by oscillator 150 increases at a constant slope in accordance with the rising edge of the one-shot pulse output by oscillator 150. When the output signal of comparator 166 becomes larger than the triangular wave, comparator 166 outputs a high-level signal, and this high-level signal is input to OR circuit 168. As a result, a high-level signal is input to the reset terminal R of flip-flop circuit 170, which sets the duty cycle of the drive voltage Vdr that the driver 180 uses to drive the switching element 20 via OR circuit 172 and AND circuit 174. In this way, the switching control circuit 100 in this example may control the switching element 20 using a pulse width modulation method.

[0092] In this example, the input detection voltage acquisition unit 110 of the switching control circuit 100 acquires the input detection voltage Vin as the voltage obtained by full-wave rectifying the AC input voltage Vac and dividing it by resistors 72 and 74. The current detection voltage acquisition unit 120 of the switching control circuit 100 acquires the current detection voltage Vcs generated at the current detection resistor 30 that detects the current flowing through the switching element 20.

[0093] The operation related to the timer circuit 142 and the pulse limiting circuit 144 is the same as in the embodiment described in relation to Figures 3A and 3B. Therefore, the switching control circuit 100 of this example can reduce stress on the switching element 20 due to heat generation, and can prevent the destruction of the switching element 20 even when a switching element 20 with a small rated current value is used. Furthermore, even if a short circuit in the current sensing resistor 30 is falsely detected due to an instantaneous voltage drop in the AC input voltage Vac, the switching control circuit 100 of this example can prevent the destruction of the switching element 20 while generating the output voltage Vout, thereby improving efficiency compared to the case where the switching element 20 is stopped in response to short circuit detection.

[0094] Figure 6 shows an example of a power supply system 10 equipped with a switching control circuit 100. The power supply system 10 comprises a switching control circuit 100, a switching element 20, and a current sensing resistor 30. The power supply system 10 in this example is a PFC power supply system. The power supply system 10 may be a PFC power supply system operating in current critical mode. The switching control circuit 100 in this example drives the switching element 20 using a pulse frequency modulation scheme. However, the configuration of the power supply system and the driving method of the switching element 20 are not limited to this example. In this example, differences from the embodiments described in relation to Figures 3A and 3B and / or the embodiment in Figure 5 will be described in particular, and other aspects may be the same as the embodiments described in relation to Figures 3A and 3B and / or the embodiment in Figure 5.

[0095] The power supply system 10 in this example is a power supply system that generates an output voltage Vout from a voltage obtained by full-wave rectification of an AC input voltage Vac by a diode bridge 40. The switching control circuit 100 generates a drive voltage Vdr to drive the switching element 20. The switching control circuit 100 controls the switching element 20 so that the output voltage Vout remains constant. For example, the output voltage Vout is divided by resistors R2 and R3 to generate a feedback voltage Vfb. The switching control circuit 100 may control the switching element 20 by comparing the feedback voltage Vfb converted by an error amplifier or the like with a triangular wave that increases with a constant slope after the switching element 20 is turned on.

[0096] In this example, the input detection voltage acquisition unit 110 of the switching control circuit 100 acquires the input detection voltage Vin as the voltage obtained by dividing the voltage Vzcd generated in the auxiliary winding when the AC input voltage Vac is full-wave rectified and the resulting voltage Vrec is input to the main winding of the transformer. The current detection voltage acquisition unit 120 of the switching control circuit 100 acquires the current detection voltage Vcs generated in the current detection resistor 30 that detects the current flowing through the switching element 20.

[0097] The drive unit 140 in this example includes a zero-cross detection circuit 152, a one-shot circuit 154, and a lamp oscillator 156.

[0098] The zero-cross detection circuit 152 may detect whether the inductor current Il has become zero based on the input detection voltage Vin. The zero-cross detection circuit 152 may detect whether the inductor current Il has become a predetermined current value close to zero based on the input detection voltage Vin. When the zero-cross detection circuit 152 detects that the inductor current Il has become zero or a predetermined current value close to zero, it may output a high-level signal.

[0099] The one-shot circuit 154 may generate a high-level pulse signal for a predetermined period of time. The one-shot circuit 154 may detect a high-level signal from the zero-crossing detection circuit 152 and generate a high-level pulse signal.

[0100] The ramp oscillator 156 may generate a triangular wave that increases in magnitude with a constant slope. The ramp oscillator 156 may generate a triangular wave that increases in magnitude with a constant slope in response to a high-level pulse signal from the one-shot circuit 154. The ramp oscillator 156 may supply the generated triangular wave to the comparator 166.

[0101] As described above, the zero-cross detection circuit 152 outputs a high-level signal when it detects that the inductor current Il has become zero or a predetermined current value close to zero. This causes a high-level pulse to be input from the one-shot circuit 154 to the set terminal S of the flip-flop circuit 170, causing the switching element 20 to turn on. If a short circuit does not occur in the current detection resistor 30, the comparator 166 outputs a high-level signal by comparing the feedback voltage Vfb converted by the error amplifier or the like with the triangular wave generated by the ramp oscillator 156, and this high-level signal is input to the OR circuit 168. This causes a high-level signal to be input to the reset terminal R of the flip-flop circuit 170, setting the on-width of the drive voltage Vdr that the driver 180 uses to drive the switching element 20. In this way, the switching control circuit 100 in this example may control the switching element 20 using a pulse frequency modulation method.

[0102] The timer circuit 142 may count the number of switching cycles of the switching element 20. For example, the timer circuit 142 counts the number of switching cycles of the switching element 20 when the short-circuit detection unit 130 determines that the current detection resistor 30 is short-circuited. The timer circuit 142 may be reset in response to a high-level signal from the comparator 132. That is, the timer circuit 142 is reset when the current detection voltage Vcs is greater than or equal to the short-circuit threshold voltage Vsh and it is determined that the current detection resistor 30 is not short-circuited, and the timer circuit 142 may continue counting when the current detection voltage Vcs is less than the short-circuit threshold voltage Vsh and it is determined that the current detection resistor 30 is short-circuited.

[0103] The timer circuit 142 may be reset in response to a signal from the input detection circuit 112. For example, the timer circuit 142 is reset when the determination value based on the input detection voltage Vin is lower than a predetermined count reference value. The determination value based on the input detection voltage Vin may be the voltage value of the input detection voltage Vin itself, or it may be a value calculated from the input detection voltage Vin. For example, the determination value may be a value calculated to compensate for the voltage drop due to the resistor 70.

[0104] In the switching control circuit 100 of this example, the timer circuit 142 count is reset according to the input detection voltage Vin. As a result, even if a short circuit in the current detection resistor 30 is falsely detected due to an instantaneous voltage drop in the AC input voltage Vac, the switching control circuit 100 of this example can prevent the destruction of the switching element 20 while continuing to generate the output voltage Vout, thereby improving efficiency compared to the case where the switching element 20 is stopped in response to short circuit detection.

[0105] The pulse limiting circuit 144 may generate a signal to limit the ON width of the switching element 20 to a first ON width. The pulse limiting circuit 144 may generate a signal to limit the ON width of the switching element 20 to a first ON width in response to a signal from the comparator 132. For example, the pulse limiting circuit 144 generates a signal to limit the ON width of the switching element 20 to a first ON width when the signal from the comparator 132 is low level, and does not generate a signal to limit the ON width of the switching element 20 to a first ON width when the signal from the comparator 132 is high level. In other words, the pulse limiting circuit 144 may generate a signal to limit the ON width of the switching element 20 to a first ON width when it is determined that the current sensing resistor 30 is short-circuited.

[0106] In the switching control circuit 100 of this example, the pulse limiting circuit 144 generates a signal to limit the ON width of the switching element 20 to a first ON width when it is determined that the current sensing resistor 30 is short-circuited. As a result, even if a short circuit in the current sensing resistor 30 is falsely detected, the switching control circuit 100 of this example can prevent the switching element 20 from being destroyed while continuing to generate an output voltage, thereby improving efficiency compared to the case where the switching element 20 is stopped in response to a short circuit detection.

[0107] In the switching control circuit 100 of this example, when it is determined that the current sensing resistor 30 is short-circuited, the pulse limiting circuit 144 generates a signal to limit the ON width of the switching element 20 to a first ON width, and this signal is input to the OR circuit 168 via the OR circuit 146. As a result, the output of the OR circuit 168 becomes high level, and a high-level signal is input to the reset terminal R of the flip-flop circuit 170, so that the ON width of the drive voltage Vdr used by the driver 180 to drive the switching element 20 is set to the first ON width. Therefore, the stress on the switching element 20 due to heat generation can be reduced, and even when using a switching element 20 with a small rated current value, the damage to the switching element 20 can be prevented.

[0108] Furthermore, in the switching control circuit 100 of this example, when the drive unit 140 determines that the current detection resistor 30 is short-circuited, it changes the ON width of the switching element 20 to the first ON width without stopping it. As a result, even if a short circuit in the current detection resistor is falsely detected due to an instantaneous voltage drop in the AC input voltage, the switching control circuit 100 of this example can prevent damage to the switching element 20 while continuing to generate an output voltage. Therefore, the switching control circuit 100 of this example can improve efficiency compared to the case where the switching element 20 is stopped in response to short-circuit detection.

[0109] The flow of determining whether a short circuit has occurred in the current detection resistor 30 by the short-circuit determination unit 130, changing the ON width by the drive unit 140, and determining whether or not to reset the count of the timer circuit 142 by the drive unit 140 may be executed for each switching cycle of the switching element 20. For example, if a short circuit is determined to have occurred in one cycle and the ON width is changed to the first ON width, and if it is determined that no short circuit has occurred in the next cycle, the drive unit 140 may terminate control by the first ON width and return to normal control based on the feedback voltage Vfb and triangular wave. As another example, if a short circuit is determined to have occurred in one cycle and the ON width is changed to the first ON width, and it is determined that a short circuit has occurred again in the next cycle, the drive unit 140 may continue control by the first ON width.

[0110] If the number of switching cycles of the switching element in the first on-width exceeds a predetermined reference number, the drive unit 140 may change the on-width for turning on the switching element to a second on-width that is shorter than the first on-width. For example, when the timer circuit 142 counts a predetermined reference number of cycles, a high-level signal is continuously input to the reset terminal R of the flip-flop circuit 170. As a result, the on-width that controls the switching element 20 is changed to the second on-width. For example, the second on-width may be a predetermined minimum on-width for driving the switching element or 0.

[0111] In the switching control circuit 100 of this example, when it is determined that the current detection resistor 30 is short-circuited, the drive unit 140 sets the ON width of the switching element 20 to the first ON width, and the timer circuit 142 counts the number of switching cycles of the switching element 20. On the other hand, the drive unit 140 can reset the count of the timer circuit 142 according to the input detection voltage. For example, even if the count of the timer circuit 142 is reset according to the input detection voltage, the ON width of the switching element 20 may be set to the first ON width. That is, even if the count of the timer circuit 142 is reset to prevent false detection of a short circuit in the current detection resistor 30, a short circuit in the current detection resistor 30 may actually occur, so the ON width of the switching element 20 may be set to the first ON width.

[0112] Figure 7 shows an example of the auxiliary winding voltage Vzcd. In this figure, the time variation of the drive voltage Vdr is shown along with the time variation of the auxiliary winding voltage Vzcd.

[0113] When the drive voltage Vdr becomes high and the switching element 20 turns on, a rectified voltage Vrec is applied to the positive side of the main winding, and the negative side of the main winding becomes the ground voltage, ignoring the voltage drop across the switching element 20 and the current sensing resistor 30. In this case, since the auxiliary winding has the opposite polarity to the main winding, the auxiliary winding voltage Vzcd is Vzcd = -2 1 / 2 The formula is ×Vrec × (Ns / Np), where Np is the number of turns of the main winding and Ns is the number of turns of the auxiliary winding.

[0114] On the other hand, when the drive voltage Vdr becomes low and the switching element 20 turns off, Vrec is applied to the positive side of the main winding and Vout is applied to the negative side of the main winding. In this case, the voltage on the negative side of the main winding becomes higher than the voltage on the positive side, so the auxiliary winding voltage Vzcd is Vzcd = Vout - 2 1 / 2 ×Vrec×(Ns / Np).

[0115] Therefore, when the switching element 20 is ON, the auxiliary winding voltage Vzcd is Vzcd = -21 / 2 The voltage changes along the envelope E1, which is represented by ×Vrec×(Ns / Np), and when the switching element 20 is off, the auxiliary winding voltage Vzcd is Vzcd = Vout - 2 1 / 2 It changes along the envelope E2, which is represented by ×Vrec×(Ns / Np).

[0116] Figure 8 shows an example of input detection voltage and judgment value. The upper graph shows the time change of the input detection voltage Vin, and the lower graph shows the time change of the judgment value.

[0117] The determination value based on the input detection voltage Vin may be the voltage value of the input detection voltage Vin itself, or it may be a value calculated from the input detection voltage Vin. As explained in relation to Figure 7, the shapes of the envelopes E1 and E2 of the auxiliary winding voltage Vzcd both have shapes corresponding to the rectified voltage Vrec. Therefore, the rectified voltage Vrec can be calculated based on the equations representing the envelopes E1 and E2 in both the ON and OFF states of the switching element 20. For example, the determination value may be the rectified voltage Vrec calculated based on the input detection voltage Vin.

[0118] The drive unit 140 may reset the timer circuit 142's count if the determination value based on the input detection voltage Vin is lower than a predetermined count reference value. The timer circuit 142's count may be reset in the section where the determination value shown by the solid line is below the count reference value shown by the dashed line, and the timer circuit 142's count may continue in the section where the determination value shown by the solid line is above the count reference value shown by the dashed line. In the switching control circuit 100 of this example, since the drive unit 140 resets the timer circuit 142's count in accordance with the input detection voltage Vin, the switching control circuit 100 of this example can prevent the destruction of the switching element 20 while continuing to generate an output voltage even if a short circuit in the current detection resistor 30 is falsely detected, and can improve efficiency compared to the case where the switching element 20 is stopped in response to short circuit detection.

[0119] For example, the count reference value may be determined to correspond to the range in which the AC input voltage Vac has a low phase angle. For example, the count reference value may be set to correspond to the determination values ​​when the phase angle of the AC input voltage Vac is 45 degrees and 135 degrees. In this way, the drive unit 140 may reset the count of the timer circuit 142 when the phase angle of the AC input voltage Vac is in the range of 45 degrees or less or 135 degrees or more.

[0120] The determination value based on the input detection voltage Vin may be the voltage value of the input detection voltage Vin itself. For example, when the switching element 20 is turned on and the auxiliary winding voltage Vzcd changes along the envelope E1, the drive unit 140 may reset the count of the timer circuit 142 if the absolute value of the input detection voltage Vin is lower than the count reference value. When the switching element 20 is turned off and the auxiliary winding voltage Vzcd changes along the envelope E2, the drive unit 140 may reset the count of the timer circuit 142 if the absolute value of the value obtained by subtracting the voltage corresponding to the output voltage Vout from the input detection voltage Vin is lower than the count reference value.

[0121] As described above, the switching control circuit 100 is applicable to any power supply system 10. The switching control circuit 100 may be applied to any power supply system 10 that includes a switching element 20 and a current sensing resistor 30. Figures 3A, 3B, 5, and 6 show examples in which the switching control circuit 100 is applied to a flyback power supply system or a PFC power supply system, but the power supply system 10 to which the switching control circuit 100 is applied is not limited to these.

[0122] Although the present invention has been described above using embodiments, the technical scope of the present invention is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various modifications or improvements can be made to the above embodiments. It will be clear from the claims that such modified or improved forms may also be included in the technical scope of the present invention.

[0123] It should be noted that the execution order of operations, procedures, steps, and stages in the apparatus, systems, programs, and methods shown in the claims, specifications, and drawings is not explicitly stated as "before," "prior to," etc., and that these can be implemented in any order unless the output of a previous process is used in a later process. Even if the operation flow in the claims, specifications, and drawings is described using phrases such as "first," "next," etc. for convenience, it does not mean that it is essential to perform the operations in that order. [Explanation of symbols]

[0124] 10 Power supply system, 20 Switching element, 30 Current sensing resistor, 40 Diode bridge, 42 Noise reduction circuit, 50 Transformer, 52 Auxiliary winding, 60 Photocoupler, 62 Light-emitting diode, 64 Phototransistor, 70 Resistor, 72 Resistor, 74 Resistor, 100 Switching control circuit, 110 Input detection voltage acquisition unit, 112 Input detection circuit, 120 Current detection voltage acquisition unit, 125 Feedback voltage acquisition unit, 130 Short circuit detection unit, 132 Comparator, 134 Voltage generation unit, 140 Drive unit, 142 Timer circuit, 144 Pulse limiting circuit, 146 OR circuit, 150 Oscillator, 152 Zero-crossing detection circuit, 154 One-shot circuit, 156 Lamp oscillator, 160 Comparator, 162 Gain circuit, 164 Slope circuit, 166 Comparator, 168 OR circuit, 170 Flip-flop circuit, 172 OR circuit, 174 AND circuit, 180 Driver, 190 Overcurrent detection unit, 192 Comparator

Claims

1. A switching control circuit for controlling a switching element in a power supply system that generates an output voltage from an AC input voltage, An input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage, A current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element, A short-circuit determination unit compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether or not the current detection resistor is short-circuited, A drive unit that drives the switching element using a pulse width modulation method, Equipped with, When the drive unit determines that the current detection resistor is short-circuited by the short-circuit determination unit, it changes the duty cycle controlling the switching element to a predetermined first duty cycle. Switching control circuit.

2. The first duty cycle is smaller than the maximum duty cycle preset in the switching control circuit. The switching control circuit according to claim 1.

3. If the number of switching cycles of the switching element at the first duty cycle exceeds a predetermined reference number, the drive unit changes the duty cycle controlling the switching element to a second duty cycle that is smaller than the first duty cycle. The switching control circuit according to claim 1.

4. The second duty cycle is a predetermined minimum duty cycle or zero for driving the switching element. The switching control circuit according to claim 3.

5. A switching control circuit for controlling a switching element in a power supply system that generates an output voltage from an AC input voltage, An input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage, A current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element, A short-circuit determination unit compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether or not the current detection resistor is short-circuited, A drive unit that drives the switching element using a pulse frequency modulation method, Equipped with, When the drive unit determines that the current detection resistor is short-circuited by the short-circuit determination unit, it changes the ON width for turning on the switching element to a predetermined first ON width. Switching control circuit.

6. The first ON width is smaller than the maximum ON width preset in the switching control circuit. The switching control circuit according to claim 5.

7. If the number of switching cycles of the switching element in the first ON width exceeds a predetermined reference number, the drive unit changes the ON width for turning on the switching element to a second ON width that is shorter than the first ON width. The switching control circuit according to claim 6.

8. The second ON width is a predetermined minimum ON width or 0 for driving the switching element. The switching control circuit according to claim 7.

9. The drive unit has a timer circuit that counts the number of switching cycles of the switching element. A switching control circuit according to any one of claims 1 to 8.

10. The drive unit resets the timer circuit count according to the input detection voltage. The switching control circuit according to claim 9.

11. The drive unit resets the timer circuit's count if the determination value based on the input detection voltage is lower than a predetermined count reference value. The switching control circuit according to claim 10.

12. A switching control circuit for controlling a switching element in a power supply system that generates an output voltage from an AC input voltage, An input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage, A current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element, A short-circuit determination unit compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether or not the current detection resistor is short-circuited, A drive unit for driving the switching element, Equipped with, The drive unit has a timer circuit that counts the number of switching cycles of the switching element when the short-circuit determination unit determines that the current detection resistor is short-circuited. When the timer circuit counts that the number of switching cycles is equal to or greater than a predetermined reference number, it outputs a signal to the drive unit to stop driving the switching element. The drive unit resets the timer circuit count according to the input detection voltage. Switching control circuit.

13. The drive unit resets the timer circuit's count if the determination value based on the input detection voltage is lower than a predetermined count reference value. The switching control circuit according to claim 12.

14. A switching control circuit according to any one of claims 1 to 8, 12, or 13, The aforementioned switching cable, The current sensing resistor and, Equipped with Power supply system.

15. A switching control circuit for controlling a switching element in a power supply system that generates an output voltage from an AC input voltage, An input detection voltage acquisition unit that acquires an input detection voltage corresponding to the AC input voltage, A feedback voltage acquisition unit that acquires a feedback voltage corresponding to the output voltage, A current detection voltage acquisition unit that acquires a current detection voltage generated in a current detection resistor that detects the current flowing through the switching element, A short-circuit determination unit compares the current detection voltage with a predetermined short-circuit threshold voltage to determine whether or not the current detection resistor is short-circuited, A drive unit drives the switching element according to the feedback voltage acquired by the feedback voltage acquisition unit, Equipped with, The aforementioned drive unit is When the short-circuit detection unit determines that the current detection resistor is short-circuited, the drive of the switching element is changed from a drive corresponding to the feedback voltage to a different drive. The input detection voltage acquisition unit does not change the drive of the switching element to the different drive according to the AC input voltage detected by the input detection voltage acquisition unit. Switching control circuit.

16. The drive unit drives the switching element using a pulse width modulation method corresponding to the feedback voltage. When the drive unit determines that the current detection resistor is short-circuited by the short-circuit determination unit, it changes the duty cycle controlling the switching element to a predetermined first duty cycle as a different drive. The switching control circuit according to claim 15.

17. The drive unit drives the switching element using a pulse frequency modulation method corresponding to the feedback voltage. When the drive unit determines that the current detection resistor is short-circuited by the short-circuit determination unit, it changes the ON width for turning on the switching element to a predetermined first ON width as a different drive. The switching control circuit according to claim 15.

18. The drive unit has a timer circuit that counts the number of switching cycles of the switching element when the short-circuit determination unit determines that the current detection resistor is short-circuited. When the timer circuit counts that the number of switching cycles is equal to or greater than a predetermined reference number, it outputs a signal to the drive unit to stop driving the switching element. The drive unit resets the timer circuit count according to the input detection voltage. A switching control circuit according to any one of claims 15 to 17.

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