Switching power supply device

Abstract

To provide a switching power supply device capable of changing over an output control from a constant voltage control to a constant power control when an output current exceeds a certain value with use of a small number of components.SOLUTION: A clamp resistor R6 with a predetermined resistance value is connected in parallel to a transistor Q1. A terminal voltage VFB of an FB terminal is clamped to a clamp voltage β by the clamp resistance 6. I.e., the terminal voltage VFB is clamped to the clamp voltage β at an earlier time tb than a time ta, and thereafter, maintained to a constant voltage β. The clamp resistance 6 is set to a predetermined resistance value at which an output current Iout that flows corresponding to the clamp voltage β becomes lower than an output current Iout at which a CS terminal voltage Vcs becomes an OCP threshold value voltage α. Therefore, after the time tb, an IC 10 keeps an ON-width of a control signal output to a gate of a MOSFET 5 constant.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a switching power supply device (DC stabilized power supply device) having a function of switching an output control method from constant voltage control to constant power control when an output current exceeds a certain value. [Background technology]

[0002] Conventionally, a switching power supply device with a constant voltage control output control method is disclosed in, for example, Patent Document 1. In this switching power supply device, the output voltage is detected by a voltage detection circuit, and the voltage of a feedback terminal in an integrated circuit (IC) for controlling a switching power supply changes according to the detected output voltage. The switching power supply control IC controls the on-time and off-time of a switching element according to the voltage detected at the feedback terminal, i.e., the magnitude of the output voltage of the switching power supply device, to turn on and off the current flowing through the primary winding of the transformer and the voltage applied thereto. By turning on and off the voltage applied to the primary winding, a voltage of a magnitude according to the on-time and off-time of the switching element is induced in the secondary winding of the transformer, and this voltage is rectified and smoothed to output a constant output voltage.

[0003] Conventionally, in order to use this type of switching power supply device that performs constant voltage control as a power supply device with the function of switching from constant voltage control to constant power control, a circuit that monitors the magnitude of the output current was required. This is because the switching power supply control IC has an overload protection function, and if the current output to the load exceeds a certain threshold, the switching power supply control IC stops operating.

[0004] Conventionally, a circuit for monitoring the magnitude of the output current is configured with a detection resistor for detecting the output current, a differential amplifier for amplifying the voltage detected by the detection resistor, a voltage follower for impedance conversion provided on the output side of the differential amplifier, and a power supply circuit for supplying the operating power to the differential amplifier and voltage follower, all of which are provided on the secondary side of a transformer. The detection resistor voltage amplified by the differential amplifier is provided via the voltage follower to a voltage detection circuit for detecting the output voltage of the switching power supply device, and a voltage proportional to the output current is added to the reference voltage of the reference voltage source (shunt regulator) for output voltage control. This causes the output voltage to drop, and constant power control is performed to keep the output power constant. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2011-234518 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the above-mentioned conventional switching power supply device having the function of switching from constant voltage control to constant power control, many of the above-mentioned components are required on the secondary side of the transformer as a circuit for monitoring the magnitude of the output current in order to perform constant power control.

[0007] An object of the present invention is to provide a switching power supply device that can switch output control from constant voltage control to constant power control when the output current exceeds a certain value, without using so many components, and with a small number of components. [Means for solving the problem]

[0008] To this end, the present invention provides A transformer in which the power stored in the primary winding is released through the secondary winding, and a voltage proportional to the voltage generated in the secondary winding is generated in the auxiliary winding. a rectifying and smoothing circuit that rectifies and smoothes the AC voltage generated in the secondary winding and outputs the rectified voltage to a load; an output voltage detection circuit for detecting an output voltage output to a load; a feedback circuit that generates a detection signal according to an output voltage detected by the output voltage detection circuit, and feeds back the output voltage to the primary side of the transformer by causing a current of a magnitude determined by the detection signal to flow through a transistor provided on the primary side of the transformer; a clamp resistor having a predetermined resistance value connected in parallel with the transistor; a switching element connected to the primary winding for controlling the power discharged by the secondary winding by switching on and off the power stored in the primary winding; a current detection element that generates a voltage corresponding to a current flowing through the switching element; an integrated circuit (IC) for controlling a switching power supply, the integrated circuit having a current detection terminal to which a voltage generated in a current detection element is input, a feedback terminal to which a parallel circuit of a transistor and a clamp resistor is connected and which flows a predetermined current through the parallel circuit, and a control terminal which outputs a control signal to a switching element for controlling the on / off of the switching element according to the terminal voltage of the feedback terminal, the integrated circuit controlling the switching element to be turned off when the voltage input to the feedback terminal or the current detection terminal reaches a predetermined threshold, and an output current which flows in response to the voltage at which the terminal voltage of the feedback terminal is clamped by the clamp resistor is set lower than an output current at which the voltage input to the feedback terminal or the current detection terminal reaches the predetermined threshold; A switching power supply device comprising the above was configured.

[0009] According to this configuration, when the output current increases and the detection signal generated by the feedback circuit becomes smaller, the current that the detection signal can pass through the transistor provided on the primary side of the transformer becomes smaller. When the detection signal becomes even smaller and reaches the limit of the current that can pass through the transistor, which is determined by the magnitude of the detection signal, the terminal voltage of the feedback terminal that outputs the current to the transistor increases. When the terminal voltage of the feedback terminal increases, the current flowing through the clamp resistor connected in parallel to the transistor increases. When the detection signal becomes even smaller and the terminal voltage of the feedback terminal increases further, and the IC reaches the limit of its ability to output current from the feedback terminal, a current corresponding to the terminal voltage of the feedback terminal at that time continues to flow through the clamp resistor. Then, the terminal voltage of the feedback terminal is maintained at the clamp voltage determined by the current flowing through the clamp resistor.

[0010] On the other hand, when the output power increases and the terminal voltage of the feedback terminal rises, the IC performs intermittent control on the switching element to lengthen the on-time of the switching element, increasing the power released by the secondary winding of the transformer, and performing constant voltage control to keep the output voltage at a constant voltage despite the increased output power. When the terminal voltage of the feedback terminal continues to rise and reaches the clamp voltage, the output current exceeds a certain value, and the terminal voltage of the feedback terminal is maintained at a constant clamp voltage, the IC performs intermittent control to keep the on-time of the switching element constant. As a result, the power stored in the primary winding and released by the secondary winding becomes constant, and the output control switches from constant voltage control to constant power control.

[0011] Conventionally, when the output current increases and the voltage input to the feedback terminal or current detection terminal reaches a certain threshold, the overload protection function of the IC is activated, the switching element is controlled to be turned off by the IC, and power is no longer discharged on the secondary side of the transformer. However, in this configuration, the current discharged on the secondary side of the transformer and output to the load in response to the above clamp voltage on the feedback terminal is set lower than the output current at which the voltage input to the feedback terminal or current detection terminal reaches the certain threshold.

[0012] Therefore, it is possible to switch output control from constant voltage control to constant power control without worrying that the current output to the load will exceed a certain threshold and the IC will stop operating.As a result, it is possible to switch output control from constant voltage control to constant power control when the output current exceeds a certain value with a small number of components, without providing an output current monitoring circuit that requires many components as in the past.

[0013] The present invention also provides a method for producing a method for manufacturing a semiconductor device comprising the steps of: an auxiliary winding voltage drop detection circuit for detecting a voltage generated in the auxiliary winding and outputting a voltage drop detection signal when the voltage generated in the auxiliary winding drops to a predetermined voltage; The IC has a latch terminal that stops the operation of the IC when a voltage drop detection signal is input from the auxiliary winding voltage drop detection circuit. It is characterized by:

[0014] After the output control is switched from constant voltage control to constant power control, if the load becomes large and the load current increases, the switching power supply device will increase the output current and decrease the output voltage because the power it supplies to the secondary side of the transformer is constant. If the load increases further, the output current will increase and the output voltage will continue to decrease, but because the power that can be supplied from the primary side to the secondary side of the transformer is constant, ideally the voltage value will be zero and the current value will be infinite, which may eventually cause a malfunction of the switching power supply device.

[0015] However, with this configuration, when the voltage proportional to the secondary winding voltage generated in the auxiliary winding drops to a predetermined voltage, a voltage drop detection signal is output from the auxiliary winding voltage drop detection circuit to the latch terminal of the IC. When the voltage drop detection signal is input to the latch terminal of the IC, the IC stops operation of the IC. Therefore, when the output current increases and the output voltage drops to a certain value, the IC stops operation. This eliminates the risk of malfunction of the switching power supply device due to an increase in output current and a drop in output voltage during constant power control. Effect of the Invention

[0016] According to the present invention, it is possible to provide a switching power supply device that can switch output control from constant voltage control to constant power control when the output current exceeds a certain value, using a small number of parts. [Brief description of the drawings]

[0017] [Figure 1] 1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention; [Diagram 2] Graphs (a), (b), (c), and (d) show the drain current of a MOSFET, the terminal voltage of the CS terminal, the voltage applied to the gate of the MOSFET, and the terminal voltage of the FB terminal in a conventional switching power supply device, and graphs (e), (f), (g), and (h) show the drain current of a MOSFET, the terminal voltage of the CS terminal, the voltage applied to the gate of the MOSFET, and the terminal voltage of the FB terminal in a switching power supply device according to one embodiment. [Diagram 3] Graphs (a) and (b) show the change in output voltage and output power relative to the output current in a switching power supply device when the function for switching from constant voltage control to constant power control is not operating, and graphs (c) and (d) show the change in output voltage and output power relative to the output current in a switching power supply device according to one embodiment when the function for switching from constant voltage control to constant power control is operating. [Figure 4]1 is a circuit diagram showing a modified example of an auxiliary winding voltage drop detection circuit constituting a switching power supply device according to an embodiment of the present invention; [Diagram 5] FIG. 1 is a circuit diagram showing a conventional switching power supply device. [Figure 6] 5 is a graph illustrating switching from constant voltage control to constant power control, and operation stop due to a voltage drop during constant power control, in a switching power supply device according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Next, an embodiment of a switching power supply device according to the present invention will be described.

[0019] FIG. 1 is a circuit diagram showing a switching power supply device 1 according to an embodiment of the present invention.

[0020] In the switching power supply 1, AC input input to input terminals 1a and 1b is rectified and smoothed by a primary side rectifying and smoothing circuit 3 via a noise filter 2, and the rectified and smoothed DC current is supplied to a primary side winding 4a of a transformer 4. The noise filter 2 is composed of capacitors C1 and C2 and a coil L1, and prevents noise generated in the switching power supply 1 from being released from the input terminals 1a and 1b. The primary side rectifying and smoothing circuit 3 is composed of a diode bridge RC1 and an electrolytic capacitor C3. In addition, a fuse F1 is provided in the live input line (LIVE) connected to the input terminal 1a.

[0021] The DC voltage supplied to the primary winding 4a is switched on and off by a MOSFET (field effect transistor) 5, which is a switching element, so that the power stored in the primary winding 4a of the transformer 4 is released by the secondary winding 4b of the transformer 4, and an AC voltage is induced in the secondary winding 4b. The AC voltage induced in the secondary winding 4b is rectified and smoothed by a secondary rectifying and smoothing circuit 6 and supplied to a load from output terminals 1c and 1d of the switching power supply device 1. The secondary rectifying and smoothing circuit 6 is composed of a diode D1 and an electrolytic capacitor C4. The output voltage of the switching power supply device 1, which is output from the output terminals 1c and 1d to the load, is detected by an output voltage detection circuit 7, which is composed of a series circuit of resistors R1 and R2. The output voltage divided by the resistors R1 and R2 of the output voltage detection circuit 7 is input to a reference voltage terminal REF of a shunt regulator 8.

[0022] The shunt regulator 8 is a constant voltage source that outputs a constant voltage, and includes a reference voltage source and an amplifier. The shunt regulator 8 has a resistor R3 and a light-emitting diode D2 of a photocoupler 9 connected in series. The shunt regulator 8 controls the diode current flowing through the light-emitting diode D2 to be small when the output voltage detected by the output voltage detection circuit 7 decreases, that is, when the load current increases. The shunt regulator 8 also controls the diode current flowing through the light-emitting diode D2 to be large when the output voltage detected by the output voltage detection circuit 7 increases, that is, when the load current decreases. The light-emitting diode D2 is connected in parallel with a resistor R4, and a series circuit of a resistor R5 and a capacitor C5 is connected between the connection point of the resistor R3 and the shunt regulator 8 and the connection point of the resistor R1 and the resistor R2.

[0023] The diode current flowing through the light-emitting diode D2 corresponds to a detection signal corresponding to the output voltage detected by the output voltage detection circuit 7. The light-emitting diode D2 and the transistor Q1 provided on the primary side of the transformer 4 form a photocoupler 9. The series circuit of the shunt regulator 8, resistor R3, and light-emitting diode D2, and the transistor Q1 generate a diode current corresponding to the output voltage detected by the output voltage detection circuit 7 as a detection signal, and the diode current flows through the transistor Q1, thereby forming a feedback circuit that feeds back the output voltage of the switching power supply device 1 to the primary side. A current of a magnitude determined by the diode current flowing through the light-emitting diode D2, that is, a current proportional to the diode current, flows through the transistor Q1.

[0024] The transistor Q1 is connected in parallel with the capacitor C7 and the clamp resistor R6. The capacitor C7 is used to remove noise, and its capacitance is set to be small. This parallel circuit is connected to the feedback terminal (referred to as the FB terminal) of the switching power supply control IC10. Inside IC10, a constant voltage source is connected to the FB terminal via a resistor, and IC10 outputs current from the FB terminal to the parallel circuit using this constant voltage source. Therefore, if the current that can flow through the transistor Q1 decreases, the terminal voltage VFB of the FB terminal increases, and if the current that can flow through the transistor Q1 increases, the terminal voltage VFB of the FB terminal decreases.

[0025] A voltage generated in the auxiliary winding 4c of the transformer 4 is applied between the power supply voltage terminal (referred to as the VCC terminal) and the ground terminal (referred to as the GND terminal) of IC10. An AC voltage proportional to the AC voltage generated in the secondary winding 4b is generated in the auxiliary winding 4c. The AC current and AC voltage generated in the auxiliary winding 4c are converted to DC by a rectifying and smoothing circuit consisting of a diode D3 and an electrolytic capacitor C6, and the DC current is supplied between the VCC terminal and the GND terminal of IC10.

[0026] When an AC voltage is applied between input terminals 1a and 1b, the applied AC voltage is half-wave rectified by diode D4 or D5 and applied to the VH terminal of IC10 via resistor R11. This voltage application starts IC10.

[0027] The terminal voltage VFB of the FB pin changes according to the current flowing between the collector and emitter of transistor Q1. In other words, when the output current (load current) increases and the diode current generated by the feedback circuit decreases, the current flowing through transistor Q1 decreases and the terminal voltage VFB of the FB pin increases. Conversely, when the output current (load current) decreases and the diode current generated by the feedback circuit increases, the current flowing through transistor Q1 increases and the terminal voltage VFB of the FB pin decreases.

[0028] The IC10 controls the frequency and duty ratio of the voltage output from the gate terminal (referred to as GATE terminal) of the IC10 to the gate of the MOSFET5 according to the terminal voltage VFB of the FB terminal. That is, when the terminal voltage VFB of the FB terminal increases, the frequency and duty ratio of the voltage output to the gate of the MOSFET5 are controlled to increase. Also, when the terminal voltage VFB of the FB terminal decreases, the frequency and duty ratio of the voltage output to the gate of the MOSFET5 are controlled to decrease. The frequency and duty ratio of the voltage applied to the gate of the MOSFET5 are controlled by the FB terminal voltage that changes in proportion to the output power of the switching power supply device 1. The MOSFET5 is connected to the primary winding 4a and constitutes a switching element that controls the current and voltage discharged from the secondary winding 4b, i.e., the power, by turning on and off the current flowing through the primary winding 4a and the voltage generated in the primary winding 4a, i.e., the power stored in the primary winding.

[0029] The GATE terminal of IC10 is connected to the gate of MOSFET 5 via resistor R7 and a parallel circuit of diode D7 and resistor R8. When the terminal voltage VFB of the FB terminal increases, IC10 normally outputs a control signal to the gate of MOSFET 5 so that the on-time of MOSFET 5 gradually becomes longer, as shown in the graph of Figure 2(c), thereby controlling the on-off of MOSFET 5. The horizontal axis of the graph represents time t, and the vertical axis represents the control signal voltage applied to the gate of MOSFET 5. The GATE terminal constitutes a control terminal that outputs a control signal to MOSFET 5 that controls the on-off of MOSFET 5 according to the terminal voltage VFB of the FB terminal.

[0030] With this control, a drain current Id as shown in the graph in Fig. 2(a) normally flows through the MOSFET 5. The horizontal axis of the graph represents time t, and the vertical axis represents the magnitude of the drain current Id flowing through the MOSFET 5. High-frequency ringing occurs when the drain current Id of the MOSFET 5 is interrupted, but this high-frequency ringing is absorbed by a series circuit of capacitor C8 and coil L2 that constitutes a snubber circuit and is connected in parallel to the secondary winding 4b.

[0031] As shown in the graph, the drain current Id increases in value and the current-carrying time increases as the duration during which the control signal voltage applied to the gate of the MOSFET 5 is high, i.e., the time during which the MOSFET 5 is on, increases. Since the drain current Id is a current flowing through the primary winding 4a, the intermittent drain current Id flowing through the primary winding 4a induces power in the secondary winding 4b, and the magnitude of the power increases as the value of the drain current Id and the current-carrying time increase.

[0032] Therefore, when the diode current decreases with an increase in the output current and the FB terminal voltage increases, the output voltage increases. Conversely, when the diode current increases with a decrease in the output current and the FB terminal voltage decreases, the output voltage decreases.

[0033] The magnitude of the output power is detected by detecting the drain current Id flowing through the primary winding 4a of the transformer 4. This detection is performed by inputting the voltage generated across a resistor R9 connected in series with the MOSFET 5 to a current detection terminal (referred to as the CS terminal) of IC10 via a resistor R10 and a capacitor C9. The resistor R9 constitutes a current detection element that generates a voltage corresponding to the current flowing through the MOSFET 5. By detecting the voltage input to this CS terminal, IC10 detects the current flowing through the primary winding 4a of the transformer 4, and therefore the current flowing through the secondary winding 4b of the transformer 4.

[0034] The graph in Fig. 2(b) shows the terminal voltage Vcs that normally appears at the CS terminal when the drain current Id flows through the MOSFET 5 as shown in Fig. 2(a). The horizontal axis of the graph is time t, and the vertical axis is the terminal voltage Vcs of the CS terminal. As shown in the graph, the terminal voltage Vcs of the CS terminal increases as the drain current Id increases.

[0035] IC10 has an overcurrent / overload protection (OCP) function. When the current flowing through the secondary winding 4b increases, the terminal voltage Vcs reaches the IC-specified OCP threshold voltage α, shown by a horizontal line in the graph of FIG. 2(b), at time ta, for example. When the terminal voltage Vcs reaches the OCP threshold voltage α, IC10 normally turns off the MOSFET 5 and stops operation. Also, the terminal voltage VFB of the FB terminal normally rises to the right as time passes, as shown in the graph of FIG. 2(d). The horizontal axis of the graph is time t, and the vertical axis is the terminal voltage VFB of the FB terminal. In other words, the terminal voltage VFB continues to rise even after time ta has passed.

[0036] FIG. 3(a) is a graph showing the relationship between the output voltage Vout and the output current Iout of the switching power supply device 1 at this time, and FIG. 3(b) is a graph showing the relationship between the output power Pout and the output current Iout of the switching power supply device 1.

[0037] 3(a), the horizontal axis is the output current Iout, and the vertical axis is the output voltage Vout. As shown in the graph, normally, the output voltage Vout is kept constant until the terminal voltage Vcs reaches the OCP threshold voltage α, but when the terminal voltage Vcs reaches the OCP threshold voltage α, IC10 turns off the MOSFET 5, and the output current Iout becomes zero.

[0038] The horizontal axis of the graph in Figure 3(b) is the output current Iout, and the vertical axis is the output power Pout. As shown in the graph, the output power Pout usually increases with an increase in the output current Iout until the terminal voltage Vcs reaches the OCP threshold voltage α. When the terminal voltage Vcs reaches the OCP threshold voltage α, IC10 controls the MOSFET 5 to turn off, and the output power Pout becomes 0.

[0039] However, in this embodiment, as described above, a clamp resistor R6 having a predetermined resistance value is connected in parallel to the transistor Q1. In this embodiment, the terminal voltage VFB of the FB terminal is clamped to a clamp voltage β by the clamp resistor 6 as shown in the graph of FIG. 2(h). The horizontal axis of the graph is time t, and the vertical axis is the terminal voltage VFB of the FB terminal. That is, the terminal voltage VFB is clamped to the clamp voltage β at a time tb that is earlier than the time ta, and is maintained at a constant clamp voltage β thereafter. The clamp resistor R6 is set to a predetermined resistance value at which the output current Iout flowing corresponding to the clamp voltage β is lower than the output current Iout at which the terminal voltage Vcs becomes the OCP threshold voltage α.

[0040] As a result, IC10 keeps the on-width of the control signal output to the gate of MOSFET 5 constant from time tb as shown in Figure 2(g). Therefore, the drain current Id flowing through MOSFET 5 becomes constant from time tb as shown in Figure 2(e), and the CS terminal voltage Vcs is also kept at a constant voltage from time tb as shown in Figure 2(f).

[0041] Therefore, in this embodiment, the relationship between the output voltage Vout and the output current Iout of the switching power supply 1 is as shown in the graph of Figure 3(c), and the relationship between the output power Pout and the output current Iout of the switching power supply 1 is as shown in the graph of Figure 3(d).

[0042] The horizontal axis of the graph in Figure 3(c) is the output current Iout, and the vertical axis is the output voltage Vout. As shown in the graph, the output voltage Vout is maintained at a constant voltage until the terminal voltage VFB of the FB terminal reaches the clamp voltage β. However, when the terminal voltage VFB of the FB terminal reaches the clamp voltage β, IC10 controls the on-time width of the MOSFET 5 to a constant value, so that the output control switches from constant voltage control to constant power control, and the output current Iout decreases.

[0043] The horizontal axis of the graph in Figure 3(d) is the output current Iout, and the vertical axis is the output power Pout. As shown in the graph, the output power Pout increases with an increase in the output current Iout until the terminal voltage VFB of the FB terminal reaches the clamp voltage β. However, when the terminal voltage VFB of the FB terminal reaches the clamp voltage β, IC10 controls the on-time width of the MOSFET 5 to a constant value, so that the output control switches from constant voltage control to constant power control, and the output power Pout becomes constant.

[0044] IC10 also has an external latch signal input terminal (referred to as LAT terminal) that stops the operation of IC 10 when a high or low level external signal voltage is input. This latch mode cutoff function utilizes the fact that a voltage proportional to the voltage generated in the secondary winding 4b is generated in the auxiliary winding 4c, and when the voltage of the auxiliary winding 4c (Vcc terminal voltage) falls below a certain value, an external signal is input to the LAT terminal, causing IC10 to stop its operation.

[0045] In this embodiment, the auxiliary winding voltage drop detection circuit 11 is connected to the LAT terminal of the IC 10. The auxiliary winding voltage drop detection circuit 11 detects the voltage generated in the auxiliary winding 4c, and outputs a voltage drop detection signal to the LAT terminal when the voltage generated in the auxiliary winding 4c drops to a predetermined voltage.

[0046] That is, the auxiliary winding voltage drop detection circuit 11 detects the auxiliary winding voltage by a voltage divider circuit made up of resistors R12 and R13. When the auxiliary winding voltage falls below a certain value as the output voltage Vout falls, the voltage across resistor R13, i.e., the base-emitter voltage of transistor Q2, falls below the voltage that turns on transistor Q2, and transistor Q2 turns off. When transistor Q2 turns off, the potential at the connection point between resistor R14 and transistor Q2 goes high, and a high-level voltage is input to the LAT terminal via diode D6 as a voltage drop detection signal. This causes IC10 to stop operating.

[0047] Some types of IC 10 stop operating when a low-level external signal voltage is input to the LAT terminal. For such IC 10, an auxiliary winding voltage drop detection circuit 11' as shown in Figure 4 is used. In Figure 4, the same reference numerals are used for the same or corresponding parts as in Figure 1, and their explanation will be omitted.

[0048] The auxiliary winding voltage drop detection circuit 11' detects the auxiliary winding voltage using a voltage divider circuit made up of resistors R16 and R17. When the auxiliary winding voltage falls below a certain value as the output voltage Vout falls, the voltage across resistor R17 (i.e., the base-emitter voltage of transistor Q3) falls below the voltage that turns on transistor Q3, and transistor Q3 turns off. When transistor Q3 turns off, the potential at the junction between resistor R18 and transistor Q3 becomes high, and the voltage across resistor R20 (i.e., the base-emitter voltage of transistor Q4) exceeds the voltage that turns on transistor Q4, and transistor Q4 turns on. As a result, the LAT terminal is connected to the ground potential via resistor R21, and a low-level voltage is input to the LAT terminal as a voltage drop detection signal. This causes IC10 to stop operating.

[0049] That is, according to the switching power supply device 1 of this embodiment, when the output current Iout increases and the diode current generated by the feedback circuit decreases, the collector current that can be passed through the transistor Q1 provided on the primary side of the transformer 4 decreases. When the diode current further decreases and reaches the limit of the collector current that can be passed through the transistor Q1, which is determined by the magnitude of the diode current, the terminal voltage VFB of the FB terminal that outputs the current to the transistor Q1 increases. When the terminal voltage of the FB terminal increases, the current flowing through the clamp resistor R6 connected in parallel to the transistor Q1 increases. When the diode current further decreases and the terminal voltage VFB of the FB terminal further increases, and IC10 reaches the limit of the ability to output a current from the FB terminal, a current corresponding to the terminal voltage VFB of the FB terminal at that time continues to flow through the clamp resistor R6. Then, the terminal voltage VFB of the FB terminal is maintained at the clamp voltage β determined by the current flowing through the clamp resistor R6.

[0050] On the other hand, when the output current Iout increases and the terminal voltage VFB of the FB terminal rises, IC10 performs intermittent control on the MOSFET 5 to lengthen the on-time of the MOSFET 5, and increases the power induced in the secondary winding 4b of the transformer 4 to perform constant voltage control to keep the output voltage Vout at a constant voltage. When the terminal voltage VFB of the FB terminal further rises and reaches the clamp voltage β, the output current Iout exceeds a certain value, and the terminal voltage VFB of the FB terminal is maintained at the constant clamp voltage β, IC10 performs intermittent control to keep the on-time of the MOSFET 5 constant. As a result, the power induced from the primary winding 4a to the secondary winding 4b becomes constant, and the output control is switched from constant voltage control to constant power control.

[0051] Conventionally, when the output current Iout increases and the voltage input to the CS terminal reaches a predetermined OCP threshold voltage α, the OCP function of IC10 works, the MOSFET 5 is controlled to be turned off by IC10, and power is no longer induced on the secondary side of the transformer 4. However, in this embodiment, the current Iout induced on the secondary side of the transformer 4 and output to the load in response to the above clamp voltage β at the FB terminal is set lower than the output current Iout at which the voltage input to the CS terminal reaches the predetermined OCP threshold voltage α.

[0052] Therefore, it is possible to switch the output control from constant voltage control to constant power control without worrying that the current output to the load will exceed a certain threshold and cause the IC 10 to stop operating. Therefore, it is possible to switch the output control from constant voltage control to constant power control with a small number of components when the output current Iout exceeds a certain value, without providing an output current monitoring circuit that requires many components as in the past.

[0053] In the past, depending on the type of IC10, when the output current Iout increases and the voltage input to the FB terminal reaches a predetermined OCP threshold voltage, the OCP function of the IC10 is activated, the MOSFET 5 is controlled to be turned off by the IC10, and power is no longer induced on the secondary side of the transformer 4. For such IC10, the current Iout induced on the secondary side of the transformer 4 and output to the load in response to the clamp voltage β of the FB terminal is set to be lower than the output current Iout at which the voltage input to the FB terminal reaches the predetermined OCP threshold voltage. This provides the same effects as the present embodiment.

[0054] A conventional switching power supply device 21 is shown in Fig. 5, and requires many more components than the switching power supply device 1 according to this embodiment. In Fig. 5, the same or corresponding parts as those in Fig. 1 are denoted by the same reference numerals, and their description will be omitted.

[0055] That is, in the conventional switching power supply device 21, as described above, the switching power supply control IC 10 has an OCP function, and when the current output to the load exceeds a certain threshold, the IC 10 stops operating. For this reason, the output current monitoring circuit 22 that monitors the magnitude of the output current Iout is required to perform constant power control.

[0056] The output current monitoring circuit 22 is configured by a detection resistor R31 for detecting the output current Iout, a differential amplifier 31 for amplifying the voltage detected by the detection resistor R31, a voltage follower 32 for performing impedance conversion at the output side of the differential amplifier 31, a power supply circuit 33 for supplying operating power to the differential amplifier 31 and the voltage follower 32, and the like, which are provided on the secondary side of the transformer 4. The detection resistor voltage amplified by the differential amplifier 31 is provided via the voltage follower 32 to an output voltage detection circuit 7 for detecting the output voltage of the switching power supply device 21, and a voltage proportional to the output current Iout is added to the reference voltage of the shunt regulator 8. This causes the output voltage Vout to drop, and constant power control is performed to keep the output power Pout constant.

[0057] Furthermore, in the conventional switching power supply device 21, a stop detection circuit 23 is provided on the secondary winding 4b of the transformer 4, which detects a trigger for stopping the operation of the IC10 when the output voltage Vout drops. In the stop detection circuit 23, the resistance values ​​of the resistors R36 to R39 are set so that the voltage at point A is higher than the voltage at point B. When the output voltage Vout drops due to constant power control and the voltage at point A drops, the transistor Q5 turns off. When the transistor Q5 turns off, the voltage at point B rises and the transistor Q6 turns on. This causes a current to flow through the light-emitting diode D23 that constitutes the photocoupler 34.

[0058] A stop circuit 24 is connected to the LAT terminal of IC10, which stops the operation of IC10 when the stop detection circuit 23 detects a trigger to stop the operation of IC10. The stop circuit 24 is provided with a transistor Q7 that pairs with a light-emitting diode D23 that constitutes a photocoupler 34. When a current flows through the light-emitting diode D23, the transistor Q7 turns on, a high-level voltage is generated across the resistor R42, and this high-level voltage is input to the LAT terminal. This causes IC10 to stop its operation. That is, when the output voltage drops to a certain value, the drop in voltage is detected by the stop detection circuit 23, and the operation of IC10 is stopped by the stop circuit 24.

[0059] In this way, the conventional switching power supply 21 performs constant power control on the secondary side of the transformer 4, and therefore requires more components than the switching power supply 1 of this embodiment. In contrast, the switching power supply 1 of this embodiment performs constant power control on the primary side of the transformer 4, and therefore the only component required is the clamp resistor R6, making it possible to reduce the number of components compared to constant power control on the secondary side. Note that the conventional switching power supply 21 does not have the clamp resistor R6 in the above embodiment at the FB terminal of IC10.

[0060] Furthermore, in the switching power supply 1, normally, after the output control is switched from constant voltage control to constant power control, when the load becomes large and the load current increases, the power supplied to the secondary side of the transformer 4 by the switching power supply 1 is constant, so the output current Iout increases and the output voltage Vout decreases, as shown in the graph of Fig. 6. The horizontal axis of the graph is the output current Iout of the switching power supply 1, and the vertical axis is the output voltage Vout. When the load increases further, the output current Iout increases and the output voltage Vout continues to decrease, but because the power that can be supplied from the primary side to the secondary side of the transformer 4 is constant, ideally the voltage value becomes zero and the current value becomes infinite, which may eventually cause a malfunction of the switching power supply 1.

[0061] However, according to the switching power supply device 1 of this embodiment, when the output voltage Vout drops to the latch stop output voltage γ and the voltage proportional to the secondary winding voltage generated in the auxiliary winding 4c drops to a predetermined voltage corresponding to the latch stop output voltage γ, a voltage drop detection signal is output from the auxiliary winding voltage drop detection circuit 11, 11' to the LAT terminal of IC10. When the voltage drop detection signal is input to the LAT terminal of IC10, the operation of IC10 is stopped by IC10 as described above. Therefore, when the output current Iout increases and the output voltage Vout drops to the latch stop output voltage γ, the operation of IC10 is stopped. This eliminates the risk of malfunction of the switching power supply device 1 due to an increase in the output current Iout and a decrease in the output voltage Vout during constant power control. [Explanation of symbols]

[0062] 1...Switching power supply device, 1a, 1b...AC input terminals, 1c, 1d...DC output terminals, 4...Transformer, 4a...Primary winding, 4b...Secondary winding, 4c...Auxiliary winding, 5...MOSFET (switching element), 6...Secondary rectification and smoothing circuit, 7...Output voltage detection circuit, 8...Shunt regulator, 9...Photocoupler, 10...Switching power supply control IC, 11...Auxiliary winding voltage drop detection circuit, D2...Light-emitting diode, Q1...Transistor, R6...Clamp resistor

Claims

1. a transformer in which power stored in a primary winding is discharged through a secondary winding and a voltage proportional to the voltage generated in the secondary winding is generated in an auxiliary winding; a rectifying and smoothing circuit that rectifies and smoothes the AC voltage generated in the secondary winding and outputs the rectified AC voltage to a load; an output voltage detection circuit for detecting an output voltage output to the load; a feedback circuit that generates a detection signal according to an output voltage detected by the output voltage detection circuit, and feeds back the output voltage to the primary side of the transformer by causing a current of a magnitude determined by the detection signal to flow through a transistor provided on a primary side of the transformer; a clamp resistor having a predetermined resistance value connected in parallel to the transistor; a switching element connected to the primary winding for controlling power discharged by the secondary winding by switching on and off the power stored in the primary winding; a current detection element that generates a voltage corresponding to a current flowing through the switching element; an integrated circuit for controlling a switching power supply, the integrated circuit having a current detection terminal to which a voltage generated in the current detection element is input, a feedback terminal to which a parallel circuit of the transistor and the clamp resistor is connected and which flows a predetermined current through the parallel circuit, and a control terminal which outputs a control signal to the switching element to control the on / off of the switching element in accordance with a terminal voltage of the feedback terminal, the integrated circuit turning off the switching element when a voltage input to the feedback terminal or the current detection terminal reaches a predetermined threshold, and an output current which flows in response to a voltage at which the terminal voltage of the feedback terminal is clamped by the clamp resistor is set lower than an output current at which the voltage input to the feedback terminal or the current detection terminal reaches the predetermined threshold; A switching power supply device comprising:

2. an auxiliary winding voltage drop detection circuit for detecting a voltage generated in the auxiliary winding and outputting a voltage drop detection signal when the voltage generated in the auxiliary winding drops to a predetermined voltage; The integrated circuit has a latch terminal that stops the operation of the integrated circuit when the voltage drop detection signal is input from the auxiliary winding voltage drop detection circuit.

2. The switching power supply device according to claim 1.

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