Control circuit and switching power supply

Abstract

A control circuit 100 according to the present invention comprises: a soft start control unit 130 that provides a soft start period when the starting voltage Vcc of a switching power supply 1 has exceeded a prescribed starting threshold voltage or when the COMP voltage has become equal to or less than a prescribed standby voltage and then has once again exceeded the standby voltage; an error voltage dischargeable period setting unit 140 that sets, on the basis of the soft start period, an error voltage dischargeable period during which the COMP voltage can be discharged; and an error voltage discharge unit 170 that defines, as a discharge period within the error voltage dischargeable period, the period from when the feedback voltage FB has exceeded a prescribed error voltage discharge threshold value until when the feedback voltage FB falls below the error voltage discharge threshold value or when the COMP voltage has dropped to an oscillation stop voltage, whichever occurs first, and discharges the COMP voltage within said discharge period. The control circuit 100 according to the present invention can suppress overshoot more than in the past and can stabilize the output voltage.

Description

Control circuit and switching power supply Cross Reference

[0001] This application claims priority based on Japanese Patent Application No. 2024-061210, filed on April 5, 2024, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a control circuit and a switching power supply.

[0003] Generally, in a PFC circuit, the output voltage control delays its response so as not to respond to the commercial power supply voltage, but there is a risk of the output voltage overshooting during startup, a sudden load change, etc. Therefore, a control circuit has been known that clamps the output voltage when it exceeds a predetermined threshold voltage to prevent overvoltage (see Patent Document 1).

[0004] As shown in FIG. 12 , the conventional control circuit 900 includes a startup voltage monitoring unit 910, an output current control unit 950, a gate control unit 960, a drive unit 962, a zero current detection unit 990, an on-width calculation unit 992, an error amplifier 972, and an overvoltage detection unit 980.

[0005] In the conventional control circuit 900, the overvoltage detection unit 980 monitors the feedback voltage FB corresponding to the output voltage Vo, and when the overvoltage exceeds a predetermined overvoltage threshold voltage (see the OVP voltage in FIG. 13), it outputs a signal to the gate control unit 960 to stop the on / off operation of the switch element, and clamps the output voltage Vo (see FIG. 13).

[0006] Japanese Patent Application Laid-Open No. 2004-297915

[0007] However, the conventional control circuit 900 cannot control the feedback voltage FB until it reaches the overvoltage threshold voltage (OVP voltage). For this reason, there has been a demand for a control circuit that can suppress overshoot and stabilize the output voltage.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a control circuit that can suppress overshoot more effectively than conventional circuits and stabilize the output voltage, and also to provide a switching power supply equipped with such a control circuit.

[0009] A control circuit of the present invention is a control circuit for controlling a switching element of a switching power supply having a switching element, the control circuit comprising: a soft-start control unit that provides a soft-start period that changes an overcurrent threshold that limits a maximum amount of current flowing through the switching element when a start-up voltage of the switching power supply exceeds a predetermined start-up threshold voltage, or when an error voltage generated based on a difference between a feedback voltage corresponding to an output voltage of the switching power supply and a reference voltage of an error amplifier falls below a predetermined standby voltage and then exceeds the standby voltage again; an error voltage dischargeable period setting unit that sets an error voltage dischargeable period during which the error voltage is able to be discharged, based on the soft-start period; and an error voltage discharge unit that sets a discharge period within the error voltage dischargeable period from when the feedback voltage exceeds the predetermined error voltage discharge threshold to when the feedback voltage falls below the error voltage discharge threshold or when the error voltage drops to an oscillation stop voltage, whichever occurs first, and discharges the error voltage within the discharge period.

[0010] A control circuit of the present invention is a control circuit for controlling a switch element of a switching power supply having the switch element, the control circuit comprising: a start-up voltage monitoring unit, an error voltage monitoring unit, a soft-start control unit, an error voltage dischargeable period setting unit, an output current control unit, a gate control unit, a drive unit, and an error voltage discharge unit, the start-up voltage monitoring unit detects a start-up voltage, and sends a start-up signal to the gate control unit and the soft-start control unit when the start-up voltage exceeds a predetermined start threshold voltage, the error voltage monitoring unit monitors an error voltage generated based on the difference between a feedback voltage corresponding to the output voltage of the switching power supply and a reference voltage of an error amplifier, and outputs a standby signal to the gate control unit when the error voltage falls below a predetermined standby voltage, and outputs a restart signal to the soft-start control unit and the gate control unit when the error voltage exceeds the standby voltage again after falling below the standby voltage, and the soft-start control unit sets a soft-start period for changing an overcurrent threshold that limits a maximum amount of current flowing through the switch element for a predetermined period based on the start-up signal or the restart signal, and the output current control unit and the error voltage dischargeable period setting unit, the error voltage dischargeable period setting unit sets an error voltage dischargeable period during which the error voltage can be discharged based on the soft start signal, and outputs this as error voltage dischargeable period information to the error voltage discharge unit, the output current control unit monitors the amount of current flowing through the switch element, and sets the overcurrent threshold for limiting the maximum amount of current flowing through the switch element based on the soft start signal during the soft start period, and when the maximum amount of current reaches the overcurrent threshold, outputs a stop signal to the gate control unit, the gate control unit starts outputting control signals for turning on and off the switch element to the drive unit based on the start signal or the restart signal, and stops outputting the control signals when the stop signal or the standby signal is input, the drive unit turns on and off the switch element based on the control signals output from the gate control unit, and the error voltage discharge unit, based on the error voltage dischargeable period information,The period from when the error voltage falls below the reference voltage of the error amplifier or when the error voltage falls to an oscillation stop voltage, whichever comes first, is defined as a discharge period, and the error voltage is discharged during this discharge period.

[0011] A switching power supply according to the present invention is characterized by comprising a switch element and a control circuit according to the present invention that controls the switch element.

[0012] According to the control circuit and switching power supply of the present invention, within the error voltage dischargeable period, the discharge period is defined as the period from when the feedback voltage exceeds a predetermined error voltage discharge threshold to when the feedback voltage falls below the error voltage discharge threshold or when the error voltage drops to the oscillation stop voltage, whichever occurs first, and the control circuit and switching power supply are provided with an error voltage discharge unit that discharges the error voltage within this discharge period, so that the output voltage can be suppressed before the feedback voltage reaches the overvoltage threshold voltage (a relatively early stage, namely, when the error voltage discharge threshold is exceeded), preventing the output voltage from significantly exceeding the rated output voltage (see FIG. 3 ). As a result, overshoot can be suppressed more effectively than in conventional circuits, and the output voltage can be stabilized.

[0013] 3 and 4 are circuit diagrams showing a switching power supply 1 according to an embodiment; FIG. 4 is a block diagram shown for explaining a control circuit 100 according to an embodiment; FIG. 5 is a timing chart showing the start-up of the switching power supply 1 according to an embodiment under heavy load; Ｄ6(a) is a timing chart of the COMP voltage and the output voltage Vo when the discharge current is large and the COMP discharge period is short, FIG. 6(b) is a timing chart of the COMP voltage and the output voltage Vo when the discharge current is appropriate and the COMP discharge period is appropriate, and FIG. 6(c) is a timing chart of the COMP voltage and the output voltage Vo when the discharge current is small and the COMP discharge period is long. FIG. 6(b) is a timing chart of the COMP voltage and the output voltage Vo when the discharge current is appropriate and the COMP discharge period is appropriate, and FIG. 6(c) is a timing chart of the COMP voltage and the output voltage Vo when the discharge current is small and the COMP discharge period is long. FIG. 6(c) is a timing chart of the switching power supply according to the second embodiment when it is started up. FIG. 6(d) is a timing chart of the switching power supply according to the second embodiment when it is restarted. FIG. 6(e) is a timing chart of the switching power supply according to the third embodiment when it is started up. FIG. 6(f) is a timing chart of the main switch current I shown to explain the soft start period. Ｄ 1 is a schematic graph of a voltage drop across a switching power supply in accordance with a first embodiment of the present invention; FIG. 2 is a diagram illustrating an overcurrent threshold during a soft start period according to a modified example; FIG. 3 is a block diagram illustrating a conventional control circuit 900; and FIG. 4 is a timing chart illustrating a start-up of a conventional switching power supply.

[0014] The control circuit and switching power supply of the present invention will be described below based on the embodiments shown in the drawings. Note that the embodiments described below do not limit the invention according to the claims. Furthermore, not all of the elements and combinations thereof described in the embodiments are necessarily essential to the solution of the present invention.

[0015] [Embodiment 1] 1. Configuration of a switching power supply 1 according to embodiment 1 A switching power supply 1 according to embodiment 1 includes a rectifier circuit 10, a converter 20, and an output capacitor C3, as shown in Fig. 1. The converter 20 performs current criticality operation.

[0016] The rectifier circuit 10 includes a bridge diode BD that performs full-wave rectification on the AC current of the commercial power supply to convert it into a pulsating current, and a smoothing capacitor C2.

[0017] The converter 20 includes an inductor 21, a switch element 22, a diode D1, and a control circuit 100. The inductor 21, the switch element 22, and the diode D1 form a boost chopper circuit. Note that the circuit that forms the converter 20 is not limited to a boost chopper circuit, and any appropriate circuit may be used.

[0018] In the first embodiment, the switch element 22 is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), but may be an IGBT or any other suitable switch element. The inductor 21 and the diode D1 may also be suitable inductors and diodes.

[0019] The control circuit 100 has seven terminals: a Vcc terminal, an OUT terminal, a GND terminal, a ZC terminal, an OCL terminal, a COMP terminal, and an FB terminal.

[0020] The Vcc terminal is connected to an external power supply Vcc and receives a voltage from the external power supply Vcc. The OUT terminal is connected to the gate electrode of the switch element 22 and outputs a signal to the gate electrode that controls the on / off of the switch element 22. The GND terminal is connected to a reference potential.

[0021] The ZC terminal is connected to the midpoint of resistors R1 and R2 connected in series to the drain electrode of the switch element 22, and receives a voltage obtained by dividing the drain voltage of the switch element 22 by the resistors R1 and R2. Note that an auxiliary winding may be provided in the inductor 21, and the ZC terminal may be connected to the auxiliary winding.

[0022] The OCL terminal is connected to a resistor R4 and a capacitor C7, and is connected to the source electrode of the switch element 22 and the midpoint of the resistor R3 via the resistor R4. The OCL terminal monitors whether an overcurrent is flowing through the switch element 22 by monitoring the voltage determined by the resistor R3.

[0023] The FB terminal is connected to a capacitor C5 and resistors R5 and R6. A voltage obtained by dividing the output voltage by resistors R5 and R6 is input to the FB terminal. The COMP terminal (error voltage terminal) is the output terminal of an error amplifier 172 (described later) that is connected to the capacitor C6.

[0024] 2. Configuration of the Control Circuit 100 According to the First Embodiment Next, a description will be given of the internal configuration of the control circuit 100. As shown in Fig. 2, the control circuit 100 includes a startup voltage monitor 110, a COMP voltage monitor 120 (error voltage monitor), a soft-start control unit 130, an error voltage dischargeable period setting unit 140, an output current control unit 150, a gate control unit 160, a drive unit 162, a COMP discharge unit 170, an error amplifier 172, a discharge switch 174, a discharge current source 176, an overvoltage detection unit 180, a zero current detection unit 190, and an ON width calculation unit 192.

[0025] The start-up voltage monitoring unit 110 detects the start-up voltage VCC (voltage from the external power supply Vcc) of the switching power supply 1, and when the start-up voltage VCC exceeds a predetermined start-up threshold voltage VCC(start), it outputs a start-up signal (see VCC(start) in Figure 3) to the gate control unit 160 and the soft-start control unit 130.

[0026] The COMP voltage monitoring unit 120 monitors an error voltage (COMP voltage) generated based on the difference between the feedback voltage FB and the reference voltage FB_ref of the error amplifier 172. Then, when the startup is completed and the switching power supply is performing normal switching operation, if the COMP voltage falls below a predetermined standby voltage Vcomp(stb) (see FIG. 5 ), the COMP voltage monitoring unit 120 outputs a standby signal to the gate control unit 160 to stop the switching operation, and if the COMP voltage falls below the standby voltage Vcomp(stb) and then exceeds the standby voltage VCOMP(stb) again, the COMP voltage monitoring unit 120 outputs a restart signal (see VCC(start) in FIG. 5 ) to the soft start control unit 130 and the gate control unit 160.

[0027] The soft-start control unit 130 provides a soft-start period in which it changes the overcurrent threshold that limits the maximum amount of current flowing through the switch element when the start-up voltage VCC of the switching power supply 1 exceeds a predetermined start-up threshold voltage VCC(start), or when the error voltage (hereinafter referred to as the COMP voltage) generated based on the difference between the feedback voltage FB corresponding to the output voltage Vo of the switching power supply 1 and the reference voltage FB_ref of the error amplifier 172 exceeds the standby voltage VCOMP(stb) again after falling below the predetermined standby voltage VCOMP(stb).

[0028] In other words, the soft start control unit 130 controls the output current I of the switch element 22 based on the start signal from the start voltage monitoring unit 110 or the restart signal from the COMP voltage monitoring unit 120. Ｄ The soft start signal is output to the output current control unit 150 and the error voltage dischargeable period setting unit 140 .

[0029] Specifically, a soft start timer (not shown) in the soft start control unit 130 starts simultaneously with the start of the soft start period (see the SS timer in FIG. 3), and a soft start signal is output to the output current control unit 150 and the error voltage dischargeable period setting unit 140. Then, the soft start signal is output to the output current control unit 150 at predetermined time intervals, and the output current control unit 150 gradually increases the output current I Ｄ (the main switch current I Ｄ Then, at the end of the soft start period, a soft start signal is output to the output current control unit 150 to control the output current I Ｄ The limit on the maximum current value (overcurrent threshold) is released, and a signal is output to the error voltage dischargeable period setting unit 140 to notify that the soft start period has ended (see the SS timer in FIG. 3).

[0030] In the first embodiment, the soft start period is set to the overcurrent threshold (main switch current I in FIG. 3) that limits the maximum value of the current flowing through the switch element 22. Ｄ The period is the time until the output voltage (see dashed line) reaches the normal value (until the same waveform is repeated).

[0031] The error voltage dischargeable period setting unit 140 sets an error voltage dischargeable period during which the COMP voltage can be discharged based on the soft start signal from the soft start control unit 130, and outputs the error voltage dischargeable period information to the COMP discharge unit 170. In the first embodiment, the soft start period and a predetermined period thereafter are set as the error voltage dischargeable period (see the COMP discharge detectable period in FIG. 3 ).

[0032] The "predetermined period thereafter" refers to the period from the end of the soft start period until the voltage stabilizes, i.e., until voltage fluctuations with a period longer than the AC period subside; specifically, this period is approximately 10 periods of the commercial power supply. Note that the PFC circuit delays its response so as not to respond to the commercial AC period, causing long-period voltage fluctuations. The "predetermined period thereafter" refers to the period until these long-period voltage fluctuations subside. In other words, the "predetermined period thereafter" is a period set by counting backward from the timing of starting the power supply, including the "time during which the power supply is desired to start up during the period during which the voltage stabilizes after the end of the soft start period," with a predetermined margin.

[0033] The output current control unit 150 monitors the amount of current flowing through the switch element 22 and calculates the output current (main switch current I Ｄ ) reaches a predetermined overcurrent threshold, a stop signal is output to the gate control unit 160 to turn off the switch element 22 (main switch current I Ｄ (See ). During the soft start period, the output current control unit 150 adjusts the output current so that it increases stepwise at predetermined time intervals based on the soft start signal. Note that during normal operation (after the soft start period), a predetermined overcurrent threshold is set.

[0034] The gate control unit 160 outputs a control signal to the drive unit 162 to turn on and off the switch element 22 based on a startup signal from the startup voltage monitoring unit 110 or a restart signal from the COMP voltage monitoring unit 120. The gate control unit 160 turns on and off the switch element at the timing when the zero current detection signal is input from the zero current detection unit 190, and outputs a control signal to adjust the on time (on width) of the switch element based on on width information from the on width calculation unit 192. Furthermore, when a stop signal from the output current control unit 150 or the overvoltage detection unit 180 or a standby signal from the COMP voltage monitoring unit 120 is received, the gate control unit 160 stops outputting the control signal and stops the switching operation of the switch element 22.

[0035] The drive unit 162 turns on and off the switch element 22 based on the control signal output from the gate control unit 160 .

[0036] The COMP discharge unit 170 sets the discharge period as the period from when the feedback voltage FB exceeds a predetermined error voltage discharge threshold to when the feedback voltage FB falls below the error voltage discharge threshold or when the error voltage (COMP voltage) drops to the oscillation stop voltage, whichever comes first, within the error voltage dischargeable period set by the error voltage dischargeable period setting unit 140, and discharges the error voltage (COMP voltage) within this discharge period.

[0037] Specifically, the COMP discharge unit 170 receives the error voltage dischargeable period information input from the error voltage dischargeable period setting unit 140 and determines whether it is the error voltage dischargeable period. If it is the error voltage dischargeable period, the COMP discharge unit 170 compares the feedback voltage FB with a predetermined error voltage discharge threshold, and when the feedback voltage FB exceeds the predetermined error voltage discharge threshold, starts the discharge period and outputs a signal to the discharge switch 174 to turn on the discharge switch 174.

[0038] Furthermore, when the feedback voltage FB falls below the error voltage discharge threshold or when the error voltage (COMP voltage) drops to the oscillation stop voltage, whichever occurs first, the discharge period is terminated and a signal to turn off the discharge switch 174 is output to the discharge switch 174.

[0039] In the first embodiment, the error voltage discharge threshold is the reference voltage FB_ref of the error amplifier 172. Furthermore, the discharge current of the COMP voltage during the discharge period is larger than the discharge current of the COMP voltage in normal operation and smaller than the discharge current of the error voltage when the feedback voltage FB exceeds the overvoltage threshold voltage.

[0040] When the feedback voltage FB exceeds the error voltage discharge threshold, the COMP discharge unit 170 increases the discharge current at a rate that slows down the rate of increase of the output voltage and prevents undershoot. Also, the discharge current of the COMP voltage (error voltage) during the discharge period is larger than the discharge current of the COMP voltage under normal conditions.

[0041] In this specification, "discharge of the COMP voltage (error voltage)" refers to discharging the charge stored in the capacitor C6 connected to the COMP terminal, and "discharge current of the COMP voltage (error voltage)" refers to the current flowing from the capacitor C6 via the COMP terminal toward the discharge current source in order to discharge the charge stored in the capacitor C6 connected to the COMP terminal.

[0042] The error amplifier 172 generates a COMP voltage based on the difference between the feedback voltage FB and the reference voltage FB_ref. The input terminal of the error amplifier 172 is connected to the FB terminal, and the output terminal is connected to the COMP terminal.

[0043] The discharge switch 174 has a drain electrode connected to the COMP terminal, a source electrode connected to the discharge current source 176, and a gate electrode connected to the COMP discharge unit 170. The discharge switch 174 performs a switching operation based on a signal from the COMP discharge unit 170.

[0044] A discharge current source 176 is connected to the source side of the discharge switch 174 .

[0045] The overvoltage detection unit 180 monitors the feedback voltage FB, and when the feedback voltage FB exceeds a predetermined overvoltage threshold voltage (OVP voltage), it outputs a signal to the gate control unit 160 to stop the on / off operation of the switch element 22. The overvoltage threshold (OVP voltage) at which the overvoltage detection unit 180 detects an overvoltage is greater than FB_ref, and the COMP voltage is discharged by the COMP discharge unit 170 during the soft start period, so that the overvoltage detection unit 180 does not normally detect an overvoltage.

[0046] The zero current detection unit 190 detects the timing at which the current flowing through the inductor 21 becomes zero based on the voltage obtained by dividing the drain voltage of the switch element 22 input from the ZC terminal by resistors R1 and R2, and outputs a zero current detection signal to the gate control unit 160.

[0047] The ON width calculation unit 192 determines the ON width of the control signal for the switch element 22 based on the COMP voltage, and outputs it to the gate control unit 160 as ON width information.

[0048] 3. Operation of the Control Circuit 100 and the Switching Power Supply 1 According to the First Embodiment (1) At Start-Up Next, the operation of the control circuit 100 and the switching power supply 1 according to the first embodiment at start-up will be described with reference to the timing charts of FIGS.

[0049] (1-1) Heavy Load Condition Fig. 3 is a timing chart showing the start-up of the switching power supply 1 according to the first embodiment under heavy load conditions. First, when the external power supply Vcc is turned on, the start-up voltage VCC starts to rise. Then, when the start-up voltage VCC exceeds a predetermined start-up threshold voltage VCC(start), the switching power supply 1 starts up and starts the on / off operation of the switch element 22 (main switch current I Ｄ reference).

[0050] At this time, a start signal (VCC(start)) is output to the soft start control unit 130, and the soft start timer of the soft start control unit 130 starts measuring, thereby starting the soft start period (see the SS timer in FIG. 3). Then, the soft start control unit 130 outputs a soft start signal to the error voltage dischargeable period setting unit 140, and the error voltage dischargeable period is set and started during the soft start period and a predetermined period thereafter (see the COMP discharge detectable period).

[0051] Furthermore, since the switching power supply 1 starts to start up, the output voltage Vo, the feedback voltage FB, and the COMP voltage start to rise.

[0052] During the soft start period, when a predetermined time has elapsed, the soft start control unit 130 outputs a soft start signal to the output current control unit 150, and the output current control unit 150 increases the overcurrent threshold stepwise every time it receives a soft start signal (the main switch current I Ｄ reference).

[0053] When the feedback voltage FB rises and exceeds the reference voltage FB_ref, which is the error voltage discharge threshold, the error voltage dischargeable period setting unit 140 outputs COMP voltage dischargeable information to the COMP discharge unit 170, and the COMP discharge unit 170 outputs an SS_COMP discharge signal (see SS_COMP discharge in FIG. 3 ) to turn on the discharge switch 174. This discharges the COMP voltage.

[0054] When the COMP voltage is discharged, the COMP voltage decreases. Accordingly, the ON width is shortened in the ON width calculation unit 192, the output voltage Vo of the switching power supply 1 decreases, and the feedback voltage FB also decreases accordingly.

[0055] Eventually, the soft start period ends and the overcurrent threshold voltage returns to the normal value, but during the error voltage dischargeable period, COMP discharge is maintained as long as the feedback voltage FB exceeds the reference voltage FB_ref.

[0056] When the feedback voltage FB drops to the reference voltage FB_ref within the error voltage dischargeable period, a signal to turn off the discharge switch is output from the COMP discharge unit 170 (SS_COMP discharge is turned off), and the discharge period ends. Then, the feedback voltage FB behaves so as to maintain the reference voltage FB_ref. Then, the error voltage dischargeable period ends a predetermined period after the soft start period ends.

[0057] In this way, the switching power supply 1 can suppress the output voltage Vo from a relatively early stage at startup. This prevents oscillation from stopping due to COMP discharge, output undershoot, and the main switch current I Ｄ (1-2) Light Load Condition FIG. 4 is a timing chart showing the startup of the switching power supply 1 according to the first embodiment under a light load condition. The operation under a light load condition is basically the same as that under a heavy load condition, but the timing for stopping discharge differs from that under a heavy load condition. That is, as shown by the waveforms of the FB and COMP voltages in FIG. 4, under a light load condition, even if oscillation stops, it takes time for the feedback voltage FB to drop to the reference voltage (FB_ref). Therefore, discharge is stopped when the COMP voltage drops to the oscillation stop voltage (the voltage at which discharge stops when oscillation stops). This prevents excessive discharge of the COMP voltage, which would otherwise cause a long wait until oscillation resumes.

[0058] (2) At Restart Next, at restart, the operation will be described with reference to Fig. 5. At restart, the soft start period and the error voltage dischargeable period are also set.

[0059] That is, the error voltage (COMP voltage) generated based on the difference between the feedback voltage FB and the reference voltage FB_ref of the error amplifier 172 is monitored, and when the error voltage (COMP voltage) falls below a predetermined standby voltage VCOMP(stb), a standby signal is output to the gate control unit 160, and when the COMP voltage falls below the standby voltage Vcomp(stb) and then exceeds the standby voltage Vcomp(stb) again, a restart signal (see VCC(start) in FIG. 5) is output to the soft start control unit 130 and the gate control unit 160. As a result, the switching operation of the switching power supply 1 is restarted (see the main switching power supply I in FIG. 5). Ｄ ), the soft start period and the error voltage dischargeable period start (see the SS timer and COMP discharge detectable period in FIG. 5).

[0060] The subsequent operation is the same as that at the time of startup. Note that although Fig. 5 is a timing chart corresponding to a heavy load, a timing chart corresponding to Fig. 4 also applies to a light load, although not shown.

[0061] In this way, the output voltage Vo is controlled by setting the soft start period and the error voltage dischargeable period not only at the time of start-up but also at the time of restart.

[0062] 3. Effects of the Control Circuit 100 and the Switching Power Supply 1 of the First Embodiment According to the control circuit 100 and the switching power supply 1 of the first embodiment, within the error voltage dischargeable period, the discharge period is defined as the period from when the feedback voltage FB exceeds a predetermined error voltage discharge threshold (FB_ref) to when the feedback voltage FB falls below the error voltage discharge threshold (FB_ref) or when the COMP voltage drops to the oscillation stop voltage, whichever occurs first. Since the control circuit 100 and the switching power supply 1 are provided with the COMP discharge unit 170 that discharges the COMP voltage within this discharge period, the output voltage Vo can be suppressed relatively early, i.e., when the error voltage discharge threshold (FB_ref) is exceeded, and the output voltage Vo can be prevented from significantly exceeding the rated output voltage (see FIG. 3 ). As a result, overshoot can be suppressed more effectively than in the past, and the output voltage can be stabilized.

[0063] According to the control circuit 100 and the switching power supply 1 of the first embodiment, the error voltage dischargeable period setting unit 140 sets the error voltage dischargeable period to a period that includes at least both the soft start period and a predetermined period thereafter. Therefore, even if the timing at which an overshoot is likely to occur varies depending on the input voltage, the load state, etc., it is possible to more reliably suppress the overshoot.

[0064] According to the control circuit 100 and the switching power supply 1 of the first embodiment, the error voltage discharge threshold is the reference voltage FB_ref of the error amplifier 172, so that the output voltage Vo can be reliably suppressed from a relatively early stage and the output voltage Vo can be prevented from significantly exceeding the rated output voltage. As a result, overshoot can be more reliably suppressed than in the past, and the output voltage can be stabilized.

[0065] According to the control circuit 100 and switching power supply 1 of the first embodiment, the discharge current of the COMP voltage during the discharge period is larger than the discharge current of the COMP voltage during normal operation and smaller than the discharge current of the COMP voltage when the feedback voltage FB exceeds the overvoltage threshold voltage, so that the COMP voltage can be prevented from reaching the overvoltage threshold voltage (OVP voltage) and the feedback voltage FB can be made to approach the reference voltage FB_ref. Specifically, the discharge current is about twice the maximum discharge current (the discharge current immediately before reaching OVP) of the amplifier and about 1 / 10 of the discharge current during OVP operation.

[0066] According to the control circuit 100 and switching power supply 1 of the first embodiment, the discharge switch 174 is turned on based on a signal from the COMP discharge unit 170 to connect to the discharge current source, thereby causing a discharge current to flow toward the discharge current source and discharging the COMP voltage, so that the COMP voltage can be discharged at an early stage after the feedback voltage FB exceeds the reference voltage FB_ref.

[0067] According to the control circuit 100 and switching power supply 1 of the first embodiment, the error amplifier 172 is a transconductance amplifier, so that the control circuit can be configured at relatively low cost and with a simple configuration.

[0068] The control circuit 100 and the COMP discharge unit 170 of the switching power supply 1 according to the first embodiment control the discharge switch 174 to flow a discharge current when the feedback voltage FB corresponding to the output voltage Vo exceeds the reference voltage FB_ref of the error amplifier 172, slowing the rate of increase of the output voltage Vo and preventing undershoot. This configuration prevents the occurrence of an undershoot due to a sudden decrease in the output voltage Vo, as shown in FIG. 6(a), or the occurrence of an undershoot due to a lack of significant change in the rate of increase of the output voltage Vo, which makes it impossible to sufficiently suppress the output voltage Vo and, in some cases, causes the output voltage Vo to exceed the OVP voltage, as shown in FIG. 6(c). Therefore, as shown in FIG. 6(b), undershoot does not occur and the output voltage Vo can be appropriately suppressed.

[0069] According to the control circuit 100 and the switching power supply 1 of the first embodiment, during the soft start period, the overcurrent threshold voltage is smaller than that in the steady state and increases at predetermined time intervals, so that a sudden increase in the output voltage Vo can be suppressed and a soft start can be achieved.

[0070] The control circuit 100 and switching power supply 1 according to the first embodiment include an overvoltage detection unit 180 that monitors the feedback voltage FB and outputs a signal to the gate control unit 160 to stop the on / off operation of the switch element 22 when the feedback voltage FB exceeds a predetermined overvoltage threshold voltage (OVP voltage). Therefore, when the output voltage Vo rises unexpectedly, the switching operation is stopped, and it is possible to prevent the switching power supply from being destroyed.

[0071] [Embodiment 2] A control circuit according to embodiment 2 basically has the same configuration as the control circuit 100 according to embodiment 1, but the error voltage dischargeable period is different from that of the control circuit 100 according to embodiment 1. In embodiment 2, the error voltage dischargeable period setting unit sets the soft start period itself as the error voltage dischargeable period, instead of the "soft start period and a predetermined period thereafter" (see FIGS. 7 and 8).

[0072] As described above, the control circuit according to the second embodiment has a different error voltage dischargeable period from that of the control circuit 100 according to the first embodiment. However, as in the case of the control circuit 100 according to the first embodiment, the control circuit according to the second embodiment defines a discharge period within the error voltage dischargeable period as the period from when the feedback voltage FB exceeds a predetermined error voltage discharge threshold (FB_ref) to when the feedback voltage FB falls below the error voltage discharge threshold (FB_ref) or when the COMP voltage drops to the oscillation stop voltage, whichever occurs first. Since the control circuit according to the second embodiment includes the COMP discharge unit 170 that discharges the COMP voltage within the discharge period, the output voltage Vo can be suppressed relatively early, i.e., when the error voltage discharge threshold (FB_ref) is exceeded, thereby preventing the output voltage Vo from significantly exceeding the rated output voltage (see FIGS. 7 and 8 ). As a result, overshoot can be suppressed more effectively than in the past, and the output voltage can be stabilized.

[0073] Furthermore, in the control circuit according to the second embodiment, the error voltage dischargeable period setting unit sets the soft start period itself as the error voltage dischargeable period, so that the output voltage Vo can be reliably suppressed within the soft start period, which is a period in which overshoot is likely to occur during startup and restart. Furthermore, if only the soft start period is set as the error voltage dischargeable period, the configuration of the error voltage dischargeable period setting unit may be included in the soft start control unit, resulting in a simpler configuration.

[0074] The control circuit of embodiment 2 has the same configuration as the control circuit 100 of embodiment 1 except for the error voltage dischargeable period, and therefore has the corresponding effects of the control circuit 100 of embodiment 1.

[0075] [Embodiment 3] A control circuit according to embodiment 3 basically has the same configuration as the control circuit according to embodiment 2, but the error voltage discharge threshold is different from that of the control circuit according to embodiment 2. That is, in embodiment 3, the error voltage discharge threshold is a predetermined voltage value that is greater than the reference voltage FB_ref of the error amplifier 172 and less than the overvoltage threshold voltage OVP (see FIG. 9).

[0076] Although not shown, the error voltage discharge threshold is a predetermined voltage value that is greater than the reference voltage FB_ref of the error amplifier 172 and less than the overvoltage threshold voltage OVP not only at startup but also at restart.

[0077] As described above, the control circuit according to the third embodiment has a different error voltage discharge threshold from that of the control circuit according to the second embodiment. However, like the control circuit according to the second embodiment, the control circuit defines a discharge period within the error voltage dischargeable period as the period from when the feedback voltage FB exceeds a predetermined error voltage discharge threshold to when the feedback voltage FB falls below the error voltage discharge threshold or when the COMP voltage drops to the oscillation stop voltage, whichever occurs first. Since the control circuit according to the third embodiment includes the COMP discharge unit 170 that discharges the COMP voltage within the discharge period, the output voltage Vo can be suppressed relatively early, i.e., when the error voltage discharge threshold is exceeded, and the output voltage Vo can be prevented from significantly exceeding the rated output voltage (see FIG. 9 ). As a result, overshoot can be suppressed more effectively than in the past, and the output voltage can be stabilized.

[0078] Furthermore, according to the control circuit of the third embodiment, the error voltage discharge threshold is a predetermined voltage value that is greater than the reference voltage FB_ref of the error amplifier 172 and less than the overvoltage threshold voltage OVP voltage, and therefore it is possible to control not only the reference voltage of the error amplifier but also the output voltage more precisely.

[0079] The control circuit of embodiment 3 has the same configuration as the control circuit of embodiment 2 except for the error voltage discharge threshold, and therefore has the corresponding effects of the control circuit of embodiment 2.

[0080] Although the present invention has been described based on the above embodiment, the present invention is not limited to the above embodiment and can be embodied in various forms without departing from the spirit of the present invention, and for example, the following modifications are also possible.

[0081] (1) The positions, connections, numbers, etc. described in the above embodiments (including each modified example; the same applies below) are examples and can be changed within the scope that does not impair the effects of the present invention.

[0082] (2) In the above-described first embodiment, the soft start period and a predetermined period thereafter are defined as the error voltage dischargeable period, and in the second embodiment, the soft start period is defined as the error voltage dischargeable period. However, the present invention is not limited to this. The error voltage dischargeable period may be a period including a predetermined period before the soft start period, or may be a period including predetermined periods before and after the soft start period.

[0083] (3) In each of the above embodiments, the overcurrent threshold (main switch current I in FIG. 3) is used to limit the maximum value of the current flowing through the switch element during the soft start period. Ｄ The soft start period is the period from when the overcurrent threshold, which limits the maximum amount of current flowing through the switch element, reaches its normal value (see dashed line in FIG. 10A), but the present invention is not limited to this. The soft start period may be the period from when the overcurrent threshold, which limits the maximum amount of current flowing through the switch element, reaches its normal value until a predetermined time has elapsed (see FIG. 10B).

[0084] (4) In the above embodiments, the overcurrent threshold voltage increases stepwise at predetermined time intervals during the soft start period. However, the present invention is not limited to this. The overcurrent threshold voltage may be set to be smaller than that during steady state and to increase continuously over time (see FIG. 11). This also allows for soft start.

[0085] (5) In the above embodiments, the error amplifier is a transconductance amplifier, but the present invention is not limited to this. An amplifier other than a transconductance amplifier may also be used.

[0086] (6) In the above embodiments, the control circuit includes a startup voltage monitor, an error voltage monitor, a soft-start controller, an error voltage dischargeable period setting unit, an output current controller, a gate controller, and a driver. However, the present invention is not limited to this. Even if one or more of these elements are not included, the present invention can be applied as long as the control circuit includes an error voltage discharge unit and discharges a COMP voltage generated based on the difference between the feedback voltage FB corresponding to the output voltage Vo and the reference voltage of the error amplifier during the period from when the feedback voltage FB exceeds the reference voltage of the error amplifier to when the feedback voltage FB falls below the reference voltage of the error amplifier.

[0087] (7) In the above embodiment, during the soft start period, the output current (main switch current I Ｄ However, the present invention is not limited to this. The ON width of the switch element may be narrowed and then widened over time.

[0088] 1... Switching power supply, 22... Switch element, 100... Control circuit, 110... Start-up voltage monitoring unit, 120... Error voltage monitoring unit (COMP voltage monitoring unit), 130... Soft start control unit, 140... Error voltage dischargeable period setting unit, 150... Output current control unit, 160... Gate control unit, 162... Drive unit, 170... COMP discharge unit (error voltage discharge unit), 172... Error amplifier, 174... Discharge switch, 176... Discharge current source, 180... Overvoltage detection unit, FB... Feedback voltage, FB_ref... Reference voltage

Claims

1. A control circuit for controlling a switching element of a switching power supply having a switching element, comprising: a soft-start control unit that provides a soft-start period for changing an overcurrent threshold that limits a maximum amount of current flowing through the switching element when a start-up voltage of the switching power supply exceeds a predetermined start-up threshold voltage, or when an error voltage generated based on the difference between a feedback voltage corresponding to the output voltage of the switching power supply and a reference voltage of an error amplifier falls below a predetermined standby voltage and then exceeds the standby voltage again; an error voltage dischargeable period setting unit that sets an error voltage dischargeable period during which the error voltage can be discharged, based on the soft-start period; and an error voltage discharge unit that discharges the error voltage within the error voltage dischargeable period, setting a discharge period from when the feedback voltage exceeds the predetermined error voltage discharge threshold to when the feedback voltage falls below the error voltage discharge threshold or when the error voltage drops to an oscillation stop voltage, whichever comes first, and discharges the error voltage within the discharge period.

2. The control circuit according to claim 1, wherein the error voltage dischargeable period setting unit sets the soft start period as the error voltage dischargeable period.

3. The control circuit according to claim 1, wherein the error voltage dischargeable period setting unit sets a period including at least both the soft start period and a predetermined period thereafter as the error voltage dischargeable period.

4. The control circuit according to any one of claims 1 to 3, wherein the error voltage discharge threshold is a reference voltage of the error amplifier.

5. The control circuit according to any one of claims 1 to 3, further comprising an overvoltage detection unit that monitors the feedback voltage and outputs a signal to stop the on / off operation of the switch element when the feedback voltage exceeds a predetermined overvoltage threshold voltage, wherein the error voltage discharge threshold is a predetermined voltage value that is greater than the reference voltage of the error amplifier and less than the overvoltage threshold voltage.

6. The control circuit according to claim 5, wherein the discharge current of the error voltage during the discharge period is greater than the discharge current of the error voltage under normal conditions and is smaller than the discharge current of the error voltage when the feedback voltage exceeds the overvoltage threshold voltage.

7. A control circuit according to any one of claims 1 to 3, further comprising: the error amplifier that generates the error voltage; an error voltage terminal connected to a capacitor that holds the error voltage; a discharge switch that is connected to the error voltage terminal and is turned on and off based on a signal from the error voltage discharge unit; and a discharge current source that is connected to the discharge switch, wherein the error voltage discharge unit turns on the discharge switch to connect the error voltage terminal and the discharge current source, thereby causing a discharge current to flow from the capacitor to the discharge current source and discharging the error voltage of the capacitor.

8. The control circuit according to claim 7, wherein the error amplifier is a transconductance amplifier.

9. A control circuit as described in any one of claims 1 to 3, characterized in that when the feedback voltage exceeds the error voltage discharge threshold, the error voltage discharge unit increases the amount of discharge current at a rate that slows down the rate of increase of the output voltage and prevents undershoot.

10. A control circuit according to any one of claims 1 to 3, further comprising an output current control section that monitors the amount of current flowing through the switch element, and when the amount of current reaches the overcurrent threshold during the soft start period, stops the switching operation of the switch element, and during the soft start period, the overcurrent threshold increases over time.

11. A control circuit according to any one of claims 1 to 3, further comprising an output current control section that monitors the amount of current flowing through the switch element, and when the amount of current reaches the overcurrent threshold during the soft start period, stops the switching operation of the switch element, and during the soft start period, the overcurrent threshold increases stepwise at predetermined time intervals.

12. A control circuit for controlling a switching element of a switching power supply having a switching element, the control circuit comprising: a start-up voltage monitoring unit, an error voltage monitoring unit, a soft-start control unit, an error voltage dischargeable period setting unit, an output current control unit, a gate control unit, a drive unit, and an error voltage discharge unit; the start-up voltage monitoring unit detects a start-up voltage, and sends a start-up signal to the gate control unit and the soft-start control unit when the start-up voltage exceeds a predetermined start-up threshold voltage; the error voltage monitoring unit monitors an error voltage generated based on the difference between a feedback voltage corresponding to the output voltage of the switching power supply and a reference voltage of an error amplifier, and outputs a standby signal to the gate control unit when the error voltage falls below a predetermined standby voltage, and outputs a restart signal to the soft-start control unit and the gate control unit when the error voltage exceeds the standby voltage again after falling below the standby voltage; the soft-start control unit sets a soft-start period for changing an overcurrent threshold that limits the maximum amount of current flowing through the switching element for a predetermined period based on the start-up signal or the restart signal, and outputs the soft-start signal to the output current control unit and the error voltage dischargeable period setting unit; the error voltage dischargeable period setting unit sets an error voltage dischargeable period during which the error voltage can be discharged based on the soft start signal, and outputs it to the error voltage discharge unit as error voltage dischargeable period information; the output current control unit monitors the amount of current flowing through the switch element, and sets the overcurrent threshold that limits the maximum amount of current flowing through the switch element based on the soft start signal during the soft start period, and outputs a stop signal to the gate control unit when the maximum amount of current reaches the overcurrent threshold; the gate control unit starts outputting a control signal to the drive unit that turns on and off the switch element based on the start signal or the restart signal, and stops outputting the control signal when the stop signal or the standby signal is input; and the drive unit turns on and off the switch element based on the control signal output from the gate control unit.the error voltage discharge unit determines, based on the error voltage dischargeable period information, within the error voltage dischargeable period as a discharge period from when the feedback voltage exceeds a reference voltage of the error amplifier to when it falls below the reference voltage of the error amplifier or when the error voltage drops to an oscillation stop voltage, whichever comes first, and discharges the error voltage within the discharge period.

13. The control circuit according to claim 12, further comprising an overvoltage detection unit that monitors the feedback voltage and outputs a signal to the gate control unit to stop the on / off operation of the switch element when the feedback voltage exceeds a predetermined overvoltage threshold voltage, wherein the discharge current of the error voltage during the discharge period is larger than the discharge current of the error voltage under normal conditions and smaller than the discharge current of the error voltage when the feedback voltage exceeds the overvoltage threshold voltage.

14. A switching power supply comprising: a switch element; and the control circuit according to claim 1 or 12 that controls the switch element.

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