JP5640830B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply device that performs output voltage control by a switching operation, and more particularly to a switching power supply device having an overload protection function (OLP).

  In a switching power supply device that controls output voltage by switching operation to turn on / off the switching element, an overload protection function is generally provided to stop the switching operation when an overload condition caused by a short-circuit fault or the like is detected. It has been. As such an overload protection function, there has been proposed a self-recovery system that automatically returns to normal operation when an overload condition is detected and then recovered from the overload condition to a normal condition (Patent Literature). 1 and 2).

  In the switching power supply of Patent Document 1, the control circuit that controls the switching operation determines an overload based on the voltage drop of the control circuit power supply voltage Vcc. Therefore, the output voltage is maintained for a period immediately after the instantaneous interruption with respect to the decrease in the input voltage at the momentary interruption such as an instantaneous power failure, but the voltage decreases and the power supply is continued when the time further elapses. In this state, the maximum on-pulse width is obtained, and the voltage of the control circuit power supply voltage Vcc decreases in proportion to the output voltage, which may be erroneously determined as an overload. Therefore, in the switching power supply device of Patent Document 1, the operation of the overload protection function is stopped by utilizing the fact that the on-time of the switching element becomes large at the momentary interruption. This reliably prevents the power supply from starting up and the start-up of the power supply being delayed due to a malfunction of overload protection.

  The switching power supply of Patent Document 2 has both a soft start function and an overload protection detection delay function, and the controller IC that controls the switching operation is a capacitor that is used in combination with the soft start function when there is no feedback signal from the secondary side. To charge. When the charging voltage of the capacitor reaches a predetermined threshold value, the controller IC determines that an overload condition occurs, stops the switching operation, and latches it.

JP 2009-1000049 A JP 2008-160917 A

  However, in the prior art, when the overload state continues for a predetermined time, the overload protection function is activated. However, after the overload state is determined at the momentary interruption, the control circuit power supply voltage Vcc is set to the operation stop voltage. If the input voltage is not recovered, the overload protection function may malfunction after a predetermined time has elapsed. That is, when the capacity of the capacitor used for the control circuit power supply for the controller IC is large, the overload state is detected after the time until the control circuit power supply voltage Vcc drops and the controller IC is turned off after the instantaneous interruption. In some cases, the time from when the overload protection function is activated becomes earlier. In this case, if the power supply has not recovered even after a predetermined time has elapsed since it was determined that there was an overload at the time of a momentary interruption, it is still determined as an overload condition, so the overload protection function malfunctioned. There was a problem of doing.

  An object of the present invention is to solve the above-described problems of the prior art in view of the above-described problems, and to provide a switching power supply device that can reliably prevent malfunction of an overload protection function at the time of a momentary interruption.

The switching power supply device of the present invention applies input power to the primary winding of the transformer, and controls the switching element connected to the primary winding of the transformer by on / off control by a control circuit. A switching power supply device that rectifies and smoothes a voltage induced in the secondary winding and supplies the load to a load, an error amplification circuit that outputs a feedback signal according to a difference between an output voltage and a reference voltage to the control circuit; an overcurrent detection circuit for current flowing through the switching element is detected as equal to or higher than the over-current threshold to be in the overcurrent state, from on to off the switching element and the overcurrent state is detected by the overcurrent detection circuit an overcurrent protection circuit for shifting the overload detection circuit for detecting an overload state by the feedback signal, before the該過load detection circuit If an overload condition is detected continuously a first predetermined time period, and the overload determination circuit determines whether the current flowing through the switching element is the overload determination threshold above which is set smaller than the over-current threshold When the overload determination circuit determines that the current flowing through the switching element is less than the overload determination threshold, the overload protection function for holding the switching element in an off state is not activated, and the overload protection function is not activated. when the current flowing through the load determining circuit to the switching element is determined to the be the overload determination threshold above, characterized by comprising the overload protection circuit for actuating the overload protection function.
In the switching power supply device of the present invention, the first predetermined time is set longer than an overload period when the control circuit is activated.
In the switching power supply device of the present invention, when the overload determination circuit determines that the current flowing through the switching element is less than the overload determination threshold, the power supply voltage for the control circuit is the operation stop voltage. And a restart circuit for stopping the on / off control of the switching element until.
Further, in the switching power supply device of the present invention, by applying input power to the primary winding of the transformer and controlling the on / off of the switching element connected to the primary winding of the transformer by a control circuit, A switching power supply apparatus that rectifies and smoothes a voltage induced in a secondary winding of the transformer and supplies the voltage to a load, and outputs a feedback signal corresponding to a difference between an output voltage and a reference voltage to the control circuit A circuit, an overcurrent detection circuit for detecting an overcurrent state when a current flowing through the switching element exceeds an overcurrent threshold, and turning on the switching element when the overcurrent state is detected by the overcurrent detection circuit. an overcurrent protection circuit for shifting to the off from, the overload detection circuit for detecting an overload state by the feedback signal,該過load detection If the by road is overloaded state is detected continuously a first predetermined time period, the overcurrent flowing through the switching element you determine whether the overload determination threshold above which is set smaller than the overcurrent threshold A load determination circuit; and a case where the overload determination circuit determines that the current flowing through the switching element is less than the overload determination threshold; and the overload determination circuit determines that the current flowing through the switching element is the overload determination An overload protection function for holding the switching element in an off state until the overload state is continuously detected for a second predetermined time after the overload detection circuit is determined to be greater than or equal to a threshold value. without actuating the, after the current flowing through the switching element by the overload determination circuit it is determined to the be the overload determination threshold or higher, before If the overload condition by the overload detection circuit is detected continuously a second predetermined time, characterized by comprising the overload protection circuit that actuates the overload protection function.
In the switching power supply device of the present invention, the first predetermined time and the second predetermined time are set longer than an overload period when the control circuit is activated.
Further, in the switching power supply device of the present invention, the time obtained by adding the first predetermined time and the second predetermined time is set to be longer than an estimated time assumed to be used in the overload state. It is characterized by.
In the switching power supply apparatus of the present invention, prior Symbol overload determination circuit, after the current flowing through the switching element is determined to the be less than the overload determination threshold, said overload condition by again said overload detection circuit When it is detected continuously for a first predetermined time, it is determined again whether or not the current flowing through the switching element is equal to or greater than an overload determination threshold set smaller than the overcurrent threshold.
In the switching power supply of the present invention, the overload determination threshold is set in a range of 20% to 80% of the overcurrent threshold.

  According to the present invention, when an overload state is continuously detected for a predetermined time, whether the current flowing through the switching element is equal to or greater than an overload determination threshold set smaller than the overcurrent threshold for detecting the overcurrent. By determining whether or not and determining whether or not to activate the overload protection, there is an effect that the malfunction of the overload protection function at the time of a momentary interruption can be surely prevented.

It is a circuit block diagram which shows the circuit structure of embodiment of the switching power supply device which concerns on this invention. It is a flowchart for demonstrating operation | movement of 1st Embodiment of the switching power supply device which concerns on this invention. 1 is a circuit configuration diagram showing a specific circuit configuration of a first embodiment of a switching power supply device according to the present invention. It is a wave form diagram which shows the signal waveform and operation | movement waveform of each part of FIG. It is a flowchart for demonstrating operation | movement of 2nd Embodiment of the switching power supply device which concerns on this invention. It is a circuit block diagram which shows the specific circuit structure of 2nd Embodiment of the switching power supply device which concerns on this invention. FIG. 7 is a circuit configuration diagram showing a circuit example of a first OLP timer and a second OLP timer shown in FIG. 6. It is a wave form diagram which shows the signal waveform and operation | movement waveform of each part of FIG. It is a wave form diagram which shows the signal waveform and operation | movement waveform of each part of FIG.

  Referring to FIG. 1, the switching power supply according to the present embodiment includes a rectifier circuit DB, smoothing capacitors C1, C2, and C3, a transformer T, a controller IC1, diodes D1 and D2, and an error amplifier (E / A). ) 2, a light emitting diode PCD and a light receiving transistor PCTR constituting a photocoupler, a resistor R 1, and a capacitor C 4.

  A commercial AC power supply AC is connected to the AC input terminals ACin1 and ACin2 of the rectifier circuit DB having a diode bridge configuration, and the AC voltage input from the commercial AC power supply AC is full-wave rectified and output from the rectifier circuit DB. A smoothing capacitor C1 is connected between the rectified output positive terminal and the rectified output negative terminal of the rectifier circuit DB. Further, the rectified output negative terminal of the rectifier circuit DB is connected to the ground terminal. As a result, a DC power source obtained by rectifying and smoothing the commercial AC power source AC with the rectifier circuit DB and the smoothing capacitor C1 is obtained.

  The controller IC1 includes a switching element such as a power MOSFET (Metal Oxide Field Effect Effect Transistor) and a control circuit for performing switching control of the switching element, and a D / ST (MOSFET drain / starting current input) terminal and , S / OCP (MOSFET source / overcurrent protection) terminal, Vcc (control circuit power supply voltage input) terminal, FB / OLP (feedback signal input / overload protection signal input) terminal, and GND terminal .

The transformer T that supplies power from the primary side (input side) to the secondary side (load side) is composed of a primary winding P1, an auxiliary winding P2, and a secondary winding S1, and includes a rectifier circuit DB. Is connected to one end of the primary winding P1 of the transformer T, the other end of the primary winding P1 of the transformer T is connected to the D / ST terminal of the controller IC1, and the controller IC1. The S / OCP terminal is connected to the ground terminal via the resistor R1. As a result, by turning on / off the switching element built in the controller IC1, the power supplied to the primary winding P1 of the transformer T is transmitted to the secondary winding S1 of the transformer T, and the secondary of the transformer T A pulsating flow is generated in the winding S1. The resistor R1 functions as a current detection resistor that detects the current flowing through the switching element included in the controller IC1 as the voltage signal _Vocp . The controller IC1 is preset with the voltage signal _Vocp corresponding to the current flowing through the switching element. An overcurrent protection (OCP) function is provided to limit the power supplied to the secondary side when the overcurrent threshold is exceeded.

  A smoothing capacitor C2 is connected between both terminals of the secondary winding S1 of the transformer T via a rectifier diode D1, and the voltage induced in the secondary winding S1 of the transformer T is the rectifier diode D1 and the smoothing capacitor C2. Is rectified and smoothed by the power supplied to the load (RL) 3 as DC power. The line connected to the positive terminal of the smoothing capacitor C2 is a power line, and the line connected to the negative terminal of the smoothing capacitor C2 is a GND line connected to the ground terminal.

A light-emitting diode PCD of a photocoupler and an error amplifier 2 are connected in series between the power supply line and the GND line. The error amplifier 2 is connected between the power supply line and the GND line, and controls the current flowing through the light emitting diode PCD of the photocoupler according to the difference between the output voltage and an internal reference voltage (not shown). The FB / OLP terminal of the controller IC1 is connected to the ground terminal via the light emitting diode PCD and the capacitor C4 connected in parallel. As a result, a feedback (FB) signal corresponding to the output voltage is transmitted from the secondary side light emitting diode PCD to the primary side light receiving transistor PCTR and is input to the FB / OLP terminal of the controller IC1 as a voltage signal _VFB . The FB signal is transmitted when the output voltage is equal to or higher than the internal reference voltage of the error amplifier 2, and is configured to increase as the difference between the output voltage and the internal reference voltage increases. The controller IC1 controls the duty ratio of the switching element based on the voltage signal _VFB input to the FB / OLP terminal, controls the amount of power supplied to the secondary side, and short-circuit faults based on the voltage signal _VFB. It has an overload protection (OLP) function that detects an overload state caused by the above and stops the switching operation to stop the supply of power to the secondary side. Note that the overload state is a state in which a load higher than the steady operation is applied to the switching operation of the switching element. Further, it is possible to detect an overload condition by detecting a decrease in the voltage supplied to the load (RL) 3 due to the application of a load exceeding the setting.

  A smoothing capacitor C3 is connected between both terminals of the auxiliary winding P2 of the transformer T via a rectifier diode D2, and a connection point between the rectifier diode D2 and the smoothing capacitor C3 is connected to the Vcc terminal of the controller IC1. Yes. Thereby, the voltage generated in the auxiliary winding P2 is rectified by the diode D2, smoothed by the smoothing capacitor C3, supplied to the Vcc terminal of the controller IC1, and used as a control circuit power supply for the controller IC1.

(First embodiment)
The overload protection operation in the controller IC 1 of the switching power supply device according to the first embodiment will be described with reference to FIG.
The controller IC1 determines whether or not the voltage signal _VFB input to the FB / OLP terminal is greater than or equal to a preset overload threshold (step A1), and when the voltage signal _VFB is less than the overload threshold Is determined to be during normal operation (step A2), and the process returns to step A1.

When the voltage signal V _FB is equal to or higher than the overload threshold, the controller IC1 starts counting of the built-in first OLP timer (step A3), and whether or not the voltage signal V _FB is equal to or higher than the overload threshold (step A3). A4), monitoring whether or not the counting of the first OLP timer is finished (step A5). If the voltage signal V _FB is not greater than or equal to the overload threshold value in step A4, that is, if the voltage signal V _FB becomes less than the overload threshold value during the counting of the first OLP timer, the controller IC1 is not in an overload state. (Step A6), the first OLP timer is reset to stop counting (Step A7), and the process returns to Step A1.

When the count of the first OLP timer ends in step A5, that is, when the state where the voltage signal _VFB is equal to or higher than the overload threshold continues for a predetermined time (the count time of the first OLP timer), the controller IC1 It is determined whether or not the voltage signal V _ocp corresponding to the current flowing through is greater than or _equal to a preset overload determination threshold (step A8).

If the voltage signal V _ocp is greater than or _equal to the overload determination threshold value in step A8, the controller IC1 determines that it is in an overload state (step A9) and stops the switching operation by setting a built-in latch circuit. (Step A10), the overload protection function for stopping the supply of power to the secondary side is activated, and this state is maintained (Step A11).

On the other hand, if the voltage signal V _ocp is less than the overload determination threshold value in step A8, the controller IC1 determines that the commercial AC power supply AC is turned off and not the overload state (step A12). ), A restart operation is executed using the restart circuit (step A13).

The overload determination threshold value to be compared with the voltage signal V _{ocp in} step A8 is a threshold value for determining whether or not the overload condition is really over based on the voltage signal V _ocp immediately after the count of the first OLP timer is finished. It is set in the range of 20% to 80% of the overcurrent threshold, which is smaller than the overcurrent threshold used in the (overcurrent protection) function.

When the overload determination threshold is larger than 80% of the overcurrent threshold, there is a possibility that the overload state may not be determined despite the overload state. That is, when the capacity of the smoothing capacitor C1 is small when the AC power is reduced and when the rated input is overloaded, the switching operation (oscillation waveform) has a maximum on-duty (eg, 80%) at the lower limit voltage of the ripple of the smoothing capacitor C1. %), And the voltage signal V _ocp may reach about 80% of the overcurrent threshold without reaching the overcurrent threshold. Therefore, when the overload determination threshold is larger than 80% of the overcurrent threshold, the comparison between the voltage signal _Vocp and the overload determination threshold in step A8 is performed at a timing near the lower limit voltage of the ripple of the smoothing capacitor C1. Therefore, it is assumed that it is not an overload state, and there is a risk of continuing the switching operation (oscillation operation). When the switching operation (oscillation operation) continues, heat generation of the switching element such as the MOSFET is large, and in the worst case, the switching element such as the MOSFET is damaged.

If the overload determination threshold is smaller than 20% of the overcurrent threshold, it cannot be determined that the power supply is stopped, and may be erroneously detected as an overload state. That is, when the secondary side load is almost unloaded and the power supply is stopped, almost no power (energy) is sent to the secondary side, so the input voltage gradually decreases. Further, when the input voltage decreases, there is an input voltage that reaches a maximum on-duty (for example, 80%) or more at a certain voltage. Therefore, when the load on the secondary side is almost no load, the input voltage does not immediately become 0 V, and the oscillation operation is continued in the state of the input voltage that operates at the maximum on-duty (for example, 80%) or more. Become. As a result, when the overload determination threshold is smaller than 20% of the overcurrent threshold, the voltage signal V _ocp reaches the overload determination threshold, it is not possible to determine that the power supply is stopped, and the overload protection function (OLP) is erroneous. There is a risk of operation.

Incidentally, the on-duty, the input voltage V _IN, when the number of turns and the flyback voltage determined by the two-winding S1 turns, etc. of the primary winding P1 of the transformer T, respectively and V _F,
On-duty = {V _F / (V _F + V _IN )} * 100
When the input voltage _VIN becomes OV, it becomes 100%, and the maximum on-duty is always reached at a certain input voltage.

Next, a specific circuit configuration of the controller IC 1 in the switching power supply device according to the first embodiment will be described with reference to FIG.
Referring to FIG. 3, the controller IC 1 includes an N-type power MOSFET Q 1 (hereinafter referred to as MOSFET Q 1), a drive circuit 11, a latch circuit 12, a restart circuit 13, an OSC (internal oscillator) 14, a constant current a source 15, and the 1OLP timer 16, and the variable voltage _{V R,} the reference voltage _{V ref,} the aND circuit AND1, AND2, AND3, an OR circuit OR1, a flipflop FF1, a comparator COMP1, COMP2, COMP 3, the resistance R2, R3, R4 and a diode D3 are provided.

  The D / ST terminal is connected to the drain terminal of the MOSFET Q1, the S / OCP terminal is connected to the source terminal of the MOSFET Q1, and the gate terminal of the MOSFET Q1 is a drive circuit for controlling ON / OFF of the MOSFET Q1. 11 is connected. The output of the AND circuit AND1 is input to the drive circuit 11, and the inverted output of the latch circuit 12, the inverted output of the restart circuit 13, and the inverted output of the OR circuit OR1 are input to the input terminal of the AND circuit AND1. Yes. As a result, the AND circuit AND1 outputs an H level signal when the output signals of the latch circuit 12, the restart circuit 13 and the OR circuit OR1 are all at the L level, and the drive circuit 11 turns on the MOSFET Q1. become.

  The output of the OSC (internal oscillator) 14 and the inverted output of the Q terminal of the flip-flop FF1 are input to the input terminal of the OR circuit OR1, and the output of the OSC (internal oscillator) 14 is input to the S terminal of the flip-flop FF1. The output of the comparator COMP1 and the output of the comparator COMP2 are input to the R terminal of the flip-flop FF1. Thus, during normal operation, the output signals of the latch circuit 12 and the restart circuit 13 are at the L level, so the output signal of the OSC (internal oscillator) 14 is at the L level, and the flip-flop FF1 is set and the Q terminal When the output signal is at the H level, the output signal of the OR circuit OR1 becomes the L level, and the MOSFET Q1 is turned on.

A connection point between the S / OCP terminal and the source terminal of the MOSFET Q1 is connected to the non-inverting terminal of the comparator COMP1 and the non-inverting terminal of the comparator COMP2. The inverting terminal of the comparator COMP2, in the normal operation variable voltage V _R which is set in the overcurrent threshold is connected. As a result, during normal operation, the overcurrent protection function (OCP) is executed when the voltage signal V _ocp corresponding to the drain current flowing through the MOSFET Q1 serving as the switching element is equal to or greater than the overcurrent threshold. That is, when the output signal of the comparator COMP2 becomes H level, the flip-flop FF1 is reset, the output signal of the OR circuit OR1 becomes H level, and the MOSFET Q1 is turned off.

A diode D3, a resistor R2, and a resistor R3 are connected in series between the FB / OLP terminal and the GND terminal, and a resistor R4 is connected between the FB / OLP terminal and the reference voltage Reg. The connection point between the resistor R2 and the resistor R3 is connected to the inverting terminal of the comparator COMP1. Thus, the comparator COMP1 compares the voltage signal V _ocp with the _divided voltage signal V _FB, and when the voltage signal V _ocp is equal to or higher than the _divided voltage signal V _FB , the output signal of the comparator COMP1 is By going to the H level, the flip-flop FF1 is reset, the MOSFET Q1 is turned off, and feedback control is performed to control the duty ratio of the MOSFET Q1 based on the FB signal.

The FB / OLP terminal is connected to the constant current source 15 and the non-inverting terminal of the comparator COMP3, and the inverting terminal of the comparator COMP3 is connected to the reference voltage _Vref set to the overload threshold. . When the FB signal is not transmitted from the light emitting diode PCD on the secondary side to the light receiving transistor PCTR on the primary side, the capacitor C4 connected between the FB / OLP terminal and the GND terminal is charged by the constant current source 15, When the voltage signal V _{FB at the} FB / OLP terminal rises and the voltage signal V _FB becomes equal to or higher than the overload threshold V _ref , the output signal of the comparator COMP3 becomes H level.

  The output terminal of the comparator COMP3 is connected to the input terminal of the first OLP timer 16, and when the output signal of the comparator COMP3 becomes H level, the first OLP timer 16 counts a predetermined time T0 set in advance, and then goes to H level one. Outputs a shot pulse signal. The first OLP timer 16 is a timer circuit composed of a counter, a CR time constant, and the like. The first OLP timer 16 continues counting while the output signal from the comparator COMP3 is at the H level and is counting for a predetermined time T0. Even when the output signal from the comparator COMP3 is switched from the H level to the L level, the count operation is stopped.

The output signal of the 1OLP timer 16 is input to the variable voltage V _R, at the time the timer count completion of the 1OLP timer 16 outputs a one-shot pulse signal, the variable voltage V _R is switched to the overload determination threshold. Therefore, when the one-shot pulse signal is input, the comparator COMP2 compares the voltage signal V _ocp with the overload determination threshold value. When the voltage signal V _ocp is less than the overload determination threshold value, the L level output signal is _{When ocp} is equal to or greater than the overload determination threshold, an H level output signal is output from the comparator COMP2.

The output of the first OLP timer 16 and the output of the comparator COMP2 are input to the input terminal of the AND circuit AND2, and the output signal of the AND circuit AND2 is input to the latch circuit 12. Thus, when the one-shot pulse signal is output from the first OLP timer 16, if the voltage signal V _ocp is equal to or higher than the overload determination threshold, the output signal of the AND circuit AND2 becomes H level, and the overload (OLP) state is reached. And the state is held by the latch circuit 12, and the overload protection function is activated. Accordingly, the counting of the first OLP timer 16 is ended and the overload protection function is activated. The predetermined time counted by the first OLP timer 16 is T0 ms, and the one-shot pulse signal output from the first OLP timer 16 is Assuming that the pulse width is sufficiently smaller than T0 ms, if the overload state continues, it takes about T0 ms from the detection of the overload state until the overload protection function is activated.

The AND circuit AND3 receives the output of the first OLP timer 16 and the inverted output of the comparator COMP2, and the output signal of the AND circuit AND3 is input to the restart circuit 13. As a result, when the one-shot pulse signal is output from the first OLP timer 16, if the voltage signal V _ocp is less than the overload determination threshold, the output signal of the AND circuit AND3 becomes H level, and it is determined that the power supply is stopped. Then, the restart circuit 13 is set and the restart operation is executed.

Next, in the first embodiment, when the commercial AC power supply AC is turned off and the first OLP timer 16 starts counting, and the voltage signal V _ocp is less than the overload determination threshold at the end of the counting of the first OLP timer 16 Will be described in detail with reference to FIG.
FIG. 4 shows signal waveforms and operation waveforms of each part of FIG. 3, where (a) is an on / off signal of the commercial AC power supply AC, (b) is a voltage signal at the Vcc terminal, and (c) is an FB / OLP terminal. Is a voltage signal V _FB , (d) is V _OCP input to the S / OCP terminal, (e) is an output signal of the comparator COMP3, (f) is an output signal of the first OLP timer 16, and (g) is voltage signal of the variable voltage _{V R,} represents (h) the output signal of the aND circuit AND2, (i) the output signal of the aND circuit AND3, (j) is the output signal of the restart circuit 13, respectively.

As shown in FIG. 4A, when the commercial AC power supply AC is turned off (cut off) at time t0 during normal operation, the drain current flowing through the MOSFET Q1, that is, the peak value of the voltage signal V _OCP corresponding to the current is As shown in FIG. 4 (d), after the commercial AC power supply AC is cut off, the accumulated charge of the smoothing capacitor C1 is sent to the secondary side, so that it rises once, but then the accumulated charge is discharged and rapidly decreases (period). See T1). Further, since the output voltage decreases, the FB signal from the secondary side to the primary side disappears, and no current flows through the light receiving transistor PCTR. Therefore, the charge is accumulated in the capacitor C3 by the constant current source 15, and FB / OLP The voltage signal _VFB input to the terminal rises as shown in FIG.

When the voltage signal _VFB rises and exceeds the overload threshold at time t1, as shown in FIG. 4E, the output signal of the comparator COMP3 becomes H level, the first OLP timer 16 starts and is preset. The counting of the predetermined time T0 is started. The first OLP timer 16 counts a predetermined time T0 set in advance in the period of time t1 to t2, and then, as shown in FIG. 4F, the H level one-shot pulse signal (for example, pulse width of 1 ms) at time t2. Is output.

By a one-shot pulse signal output from the 1OLP timer 16, as shown in FIG. 4 (g), the variable voltage _{V R} is switched from the overcurrent threshold overload determination threshold, the input to the S / OCP terminal of a comparator COMP2 it is compared with the voltage signal V _ocp overload determination threshold being is either a power-off state, whether it is overloaded is determined. Note that the overload determination threshold is appropriately set in a range of 20 to 80% of the overcurrent threshold, and in the example illustrated in FIG. 4, the overload determination threshold is set to 50% of the overcurrent threshold.

As shown in FIG. 4, when the voltage signal V _ocp is less than the overload determination threshold during the period of time t2 to t3, it is determined that the power supply is stopped, and an L level output signal is output from the comparator COMP2. Thus, as shown in FIG. 4 (i), the output signal of the AND circuit AND3 becomes H level, the restart circuit 13 is set, and the restart operation is executed.

When the restart circuit 13 is set at time t3, the restart circuit 13 outputs an H level output signal and the AND circuit AND1 outputs an L level as shown in FIG. 4 (j). The switching operation of the MOSFET Q1 is stopped, the FB / OLP terminal is grounded, and the voltage signal _VFB becomes 0 potential as shown in FIG.

When the voltage signal V _ocp exceeds the overload determination threshold value and is determined to be in an overload state, an H level output signal is output from the comparator COMP2, and the AND circuit shown in FIG. The output signal of AND2 becomes H level, and the latch circuit 12 is set. When the latch circuit 12 is set, the latch circuit 12 outputs an H level output signal, the output signal of the AND circuit AND1 becomes L level, the switching operation of the MOSFET Q1 is stopped, and the overload protection state is maintained. Rutotomoni supplies power supply voltage V _cc of the control IC by the activation circuit (not shown).

Next, as shown in FIG. 4B, when the power supply voltage Vcc of the control IC becomes the operation stop voltage at time t4, the internal circuit of the controller IC1 is reset once and becomes an initial state. When the controller IC1 is in an initial state and the commercial AC power supply AC is turned on (turned on) at time t5 as shown in FIG. 4A, the capacitor C3 is charged by a start circuit (not shown), and FIG. As shown in FIG. 4, when the power supply voltage Vcc of the control IC rises and the power supply voltage Vcc of the control IC reaches the operation start voltage at time t6, the controller IC1 is activated and the switching operation is started. At time t7, since the voltage of the capacitor C2 has not risen to the rated voltage, the voltage signal _VFB immediately after the start of oscillation becomes equal to or higher than the overload threshold, and the output signal of the comparator COMP3 as shown in FIG. Becomes H level, and the first OLP timer 16 starts. However, at time t8 before the predetermined time T0 counted by the first OLP timer 16 elapses, the output voltage rises to the set voltage due to the return of the commercial AC power supply AC. When the output voltage rises to the set voltage, the feedback voltage signal _VFB falls below the overload threshold, and as shown in FIG. 4E, the output signal of the comparator COMP3 becomes L level, and the restarted first OLP timer. 16 is also reset. In other words, the commercial AC power supply AC is turned on (turned on) from the initial state, and the predetermined time T0 counted by the first OLP timer 16 is set longer than the overload period at the time of activation of the controller IC1, and the feedback voltage signal at the time of activation. Even if V _FB exceeds the overload threshold, overload is not detected.

As described above, according to the first embodiment, when the overload state is detected continuously for a predetermined time T0, the overload determination threshold V _OCP flowing to MOSFETQ1 is set smaller than the over-current threshold It is determined whether or not this is the case, and if it is determined that the V _OCP flowing through the MOSFET Q1 is equal to or greater than the overload determination threshold, an overload protection function for holding the MOSFET Q1 in the off state is activated. Thereby, there exists an effect that the malfunction of the overload protection function at the time of a momentary interruption can be prevented reliably.

  Furthermore, according to the first embodiment, the predetermined time T0 for counting the continuation of the overload state is set longer than the overload period when the controller IC is activated. Thereby, it is possible to prevent malfunction of the overload protection function when the controller IC 1 is restarted.

Furthermore, according to the first embodiment, when it is determined that the V _OCP flowing through the MOSFET Q1 is less than the overload determination threshold, the MOSFET Q1 is turned on / off until the power supply voltage for the controller IC1 becomes the operation stop voltage. It is comprised so that control may be stopped. Thereby, it is possible to promptly restart the controller IC 1 at the moment of interruption.

(Second Embodiment)
The overload protection operation in the controller IC1 of the switching power supply device according to the second embodiment will be described with reference to FIG.
The controller IC1 determines whether or not the voltage signal _VFB is equal to or higher than the overload threshold by comparing the voltage signal _VFB input to the FB / OLP terminal with a preset overload threshold (step S1). A1) When the voltage signal V _FB is less than the overload threshold, it is determined that the operation is normal (step A2), and the operations of steps A1 and A2 are repeated.

When the voltage signal V _FB is equal to or higher than the overload threshold, the controller IC1 starts counting of the built-in first OLP timer (step A3), and whether or not the voltage signal V _FB is equal to or higher than the overload threshold (step A3). A4), monitoring whether or not the counting of the first OLP timer is finished (step A5). If the voltage signal V _FB is not greater than or equal to the overload threshold value in step A4, that is, if the voltage signal V _FB becomes less than the overload threshold value during the counting of the first OLP timer, the controller IC1 is not in an overload state. (Step A6), the first OLP timer is reset to stop counting (Step A7), and the process returns to Step A1.

In step A5, when the count of the first OLP timer ends, that is, when the voltage signal _VFB is equal to or higher than the overload threshold value continues for a preset time (the count time of the first OLP timer), the controller IC1 It is determined whether or not the voltage signal V _ocp corresponding to the current flowing through is equal to or higher than a preset overload determination threshold value (step B1), and when the voltage signal V _ocp is less than the overload determination threshold value in step B1 Returns to step A1. As a result, when the voltage signal V _FB is equal to or greater than the overload threshold and the voltage signal V _ocp is less than the overload determination threshold, the count of the first OLP timer is repeated.

If the voltage signal V _ocp is greater than or _equal to the overload determination threshold value in step B1, the controller IC1 starts counting the built-in second OLP timer (step B2), and whether the voltage signal _VFB is greater than or equal to the overload threshold value. Whether or not (step B3) and whether or not the counting of the second OLP timer ends (step B4) are monitored. If the voltage signal V _FB is not greater than or equal to the overload threshold value in step B3, that is, if the voltage signal V _FB falls below the overload threshold value during the counting of the second OLP timer, the controller IC1 is not in an overload state. (Step A6), the first and second OLP timers are reset to stop counting (step A7), and the process returns to step A1.

When the count of the second OLP timer ends in step B4, that is, when the state where the voltage signal _VFB is equal to or greater than the overload threshold continues for a preset time (the count time of the second OLP timer), the controller IC1 It is determined whether or not the voltage signal V _ocp corresponding to the current flowing through is equal to or higher than a preset overload determination threshold (step B5).

If the voltage signal V _ocp is less than the overload determination threshold value in step B5, the process returns to step A1. On the other hand, if the voltage signal V _ocp is greater than or _equal to the overload determination threshold value in step B5, it is determined that the load is overloaded (step A9), and the built-in latch circuit is set to stop the switching operation. (Step A10) An overload protection function for stopping the supply of power to the secondary side is activated and the state is maintained (Step A11).

The overload determination threshold value to be compared with the voltage signal V _ocp in steps B1 and B5 is used to determine whether or not the overload state is based on the voltage signal V _ocp immediately after the first and second OLP timers are counted. The threshold value is set in the range of 20% to 80% of the overcurrent threshold value, which is lower than the overcurrent threshold value used in the OCP (overcurrent protection) function.

  Next, a specific circuit configuration of the controller IC1 in the switching power supply device according to the second embodiment will be described with reference to FIG.

  The second embodiment is different from the first embodiment in that a controller IC 1 provided with a second OLP timer 17 is used instead of the restart circuit 13. Hereinafter, a circuit configuration different from that of the first embodiment will be described, and description of the same circuit configuration as that of the first embodiment will be omitted.

The output of the first OLP timer 16 and the output of the comparator COMP 2 are input to the input terminal of the AND circuit AND 2, and the output terminal of the AND circuit AND 2 is connected to the input terminal of the second OLP timer 17. When the output signal of the AND circuit AND2 is H level, that is, when the voltage signal _Vocp is equal to or higher than the overload determination threshold when the one-shot pulse signal is output from the first OLP timer 16, the second OLP timer 17 is set in advance. After counting the predetermined time T3, an H-level one-shot pulse signal is output. Note that the first OLP timer 16 is reset when the output signal from the comparator COMP3 is switched from the H level to the L level and stops the counting operation even during the counting of the predetermined time T0. Similarly to the first OLP timer 16, the second OLP timer 17 continues counting while the output signal from the comparator COMP3 is at the H level, and the output signal from the comparator COMP3 even during the counting of the predetermined time T3. Is reset at the time of switching from the H level to the L level, and the count operation is stopped.

  In the second embodiment, the first OLP timer 16 and the second OLP timer 17 are configured separately. However, the first OLP timer 16 and the second OLP timer 17 are shared, that is, configured by one timer circuit. be able to. FIG. 7 shows an example of a timer circuit in which the first OLP timer 16 and the second OLP timer 17 are shared by using a CR time constant. In the timer circuit shown in FIG. 7, when the switch 19 is turned on, the capacitor C5 is charged by the constant current source 18, and the voltage of the capacitor C5 is compared with the voltage divided by the resistors R5 to R7 by the comparators COMP4 and COMP5. Thus, the functions of the first OLP timer 16 and the second OLP timer 17 are realized. The output of the comparator COMP4 corresponds to the output to the first OLP timer 16, and the output of the comparator COMP5 corresponds to the output to the second OLP timer 17. Further, the count time of the first OLP timer 16 and the count time of the second OLP timer 17 may be counted using a counter circuit by a logic circuit.

The output of the second OLP timer 17 and the output of the comparator COMP2 are input to the AND circuit AND4, and the output signal of the AND circuit AND3 is input to the latch circuit 12. Thus, when the one-shot pulse signal is output from the second OLP timer 17, if the voltage signal Vocp is greater than or equal to the overload determination threshold, it is determined that the overload (OLP) state is present, and the state is determined by the latch circuit 12. The overload protection function is activated. Therefore, after the count of the first OLP timer 16 is finished, the count of the second OLP timer 17 is finished and the overload protection function is activated, and the predetermined times counted by the first OLP timer 16 and the second OLP timer 17 are respectively set. If the pulse widths of the one-shot pulse signals output from the first OLP timer 16 and the second OLP timer 17 are sufficiently smaller than T0 and T3 ms, the overload state is maintained. It takes about (T0 + T3) ms from the detection of the overload state until the overload protection function is activated.

The predetermined times T0 and T3 counted by the first OLP timer 16 and the second OLP timer 17 are set to be longer than the overload period at the start of the controller IC1, and are fed back at the start. Even if the voltage signal V _FB exceeds the overload threshold, no overload is detected. Further, when the load (RL) 3 is a motor load, a period in which a current several times normal when the motor is driven is several tens to several hundreds of milliseconds, and the load (RL) 3 may be used in an overload state during the period. is assumed. Therefore, in such a case, a time obtained by adding a predetermined time T0 counted by the first OLP timer 16 and a predetermined time T3 counted by the second OLP timer 17 is longer than a period in which a current several times the normal current flows when the motor is driven. It should be set to be long.

Next, in the second embodiment, when the commercial AC power supply AC is turned off and the first OLP timer 16 starts counting, and the power supply voltage Vcc of the control IC reaches the operation stop voltage during the counting of the first OLP timer 16 Will be described in detail with reference to FIG.
FIG. 8 shows signal waveforms and operation waveforms of each part of FIG. 6, (a) is an on / off signal of the commercial AC power supply AC, (b) is a voltage signal at the Vcc terminal, and (c) is an FB / OLP terminal. Is a voltage signal V _FB , (d) is V _OCP input to the S / OCP terminal, (e) is an output signal of the comparator COMP3, (f) is an output signal of the first OLP timer 16, and (g) is voltage signal of the variable voltage _{V R,} represents (h) the output signal of the aND circuit AND2, (i) the output signal of the 2OLP timer 17, (j) is the output signal of the aND circuit AND4, respectively.

As shown in FIG. 8A, when the commercial AC power supply AC is turned off (cut off) at time t10 during the normal operation, the drain current flowing through the MOSFET Q1, that is, the peak value of the voltage signal V _OCP corresponding to the drain current. As shown in FIG. 8 (d), after the commercial AC power supply AC is cut off, the accumulated charge of the smoothing capacitor C1 is sent to the secondary side, so that it rises once, but then the accumulated charge is discharged, and then rapidly decreases ( (See period T1). Further, since the output voltage decreases, there is no FB signal from the secondary side to the primary side, and no current flows through the light receiving transistor PCTR. Therefore, electric charge is accumulated in the capacitor C4 by the constant current source 15, and FB / OLP The voltage signal _VFB input to the terminal rises as shown in FIG.

When the voltage signal _VFB rises and exceeds the overload threshold at time t11, as shown in FIG. 8 (e), the output signal of the comparator COMP3 becomes H level, the first OLP timer 16 starts and is preset. The counting of the predetermined time T0 is started. The first OLP timer 16 counts a predetermined time T0 set in advance during the period of time t11 to t12, and then, as shown in FIG. 8 (f), the H level one-shot pulse signal (for example, pulse width of 1 ms) at time t12. Is output. By a one-shot pulse signal output from the 1OLP timer 16, as shown in FIG. 8 (g), the variable voltage _{V R} is switched from the overcurrent threshold overload determination threshold, the input to the S / OCP terminal of a comparator COMP2 The applied voltage signal V _ocp and the overload determination threshold are compared. Note that the overload determination threshold is appropriately set in the range of 20 to 80% of the overcurrent threshold, and in the example shown in FIG. 8, it is set to 50% of the overcurrent threshold.

As in the example illustrated in FIG. 8, when the voltage signal V _ocp is less than the overload determination threshold during the period from time t12 to t13 and the voltage signal _VFB is still greater than or equal to the overload threshold, the output signal of the comparator COMP3 Since the signal remains at the H level, the first OLP timer 16 restarts and starts counting for a predetermined time T0 set in advance.

As shown in FIG. 8B, when the power supply voltage Vcc of the control IC becomes the operation stop voltage at time t14 when time T2 (T2 <predetermined time T0) has elapsed from time t13, the internal circuit of the controller IC1 is once turned on. The controller IC1 is reset to an initial state. As a result, as shown in FIG. 8C, the voltage signal _VFB becomes 0 potential, and as shown in FIG. 8E, the output signal of the comparator COMP3 also becomes L level, and the restarted first OLP The timer 16 is also reset. During the period up to time t14, the smoothing capacitor C3 maintains the power supply voltage Vcc of the control IC above the operation stop voltage, and the voltage signal _VFB is also maintained above the overload threshold.

  Next, as shown in FIG. 8 (a), when the commercial AC power supply AC is turned on (turned on) at time t15, the capacitor C3 is charged by an unillustrated starter circuit, as shown in FIG. 8 (b). When the power supply voltage Vcc of the control IC rises and the power supply voltage Vcc of the control IC becomes the operation start voltage at time t16, the controller IC1 is activated and the switching operation is started.

Next, in the second embodiment, the operation when the commercial AC power supply AC is turned off and the first OLP timer 16 starts counting, and the commercial AC power supply AC is turned on (turned on) during the counting of the first OLP timer 16. Will be described in detail with reference to FIG.
FIG. 9 shows signal waveforms and operation waveforms of each part of FIG. 6, where (a) is an on / off signal of commercial AC power supply AC, (b) is a voltage signal at the Vcc terminal, and (c) is an FB / OLP terminal. Is a voltage signal V _FB , (d) is V _OCP input to the S / OCP terminal, (e) is an output signal of the comparator COMP3, (f) is an output signal of the first OLP timer 16, and (g) is voltage signal of the variable voltage _{V R,} represents (h) the output signal of the aND circuit AND2, (i) the output signal of the 2OLP timer 17, (j) is the output signal of the aND circuit AND4, respectively.

As shown in FIG. 9A, when the commercial AC power supply AC is turned off (cut off) at time t20 during the normal operation, the drain current flowing through the MOSFET Q1, that is, the peak value of the voltage signal V _OCP corresponding to the drain current. As shown in FIG. 9 (d), after the commercial AC power supply AC is cut off, the accumulated charge of the smoothing capacitor C1 is sent to the secondary side, so that it rises once, but then the accumulated charge is discharged, and then suddenly decreases ( (See period T1). Further, since the output voltage decreases, there is no FB signal from the secondary side to the primary side, and no current flows through the light receiving transistor PCTR. Therefore, electric charge is accumulated in the capacitor C4 by the constant current source 15, and FB / OLP The voltage signal _VFB input to the terminal rises as shown in FIG.

When the voltage signal _VFB rises and exceeds the overload threshold at time t21, as shown in FIG. 9E, the output signal of the comparator COMP3 becomes H level, the first OLP timer 16 starts and is preset. The counting of the predetermined time T0 is started. The first OLP timer 16 counts a predetermined time T0 set in advance during a period from time t21 to t22, and then, as shown in FIG. 9 (f), the H level one-shot pulse signal (for example, pulse width 1 ms) at time t22. Is output.

By a one-shot pulse signal output from the 1OLP timer 16, as shown in FIG. 9 (g), the variable voltage _{V R} is switched from the overcurrent threshold overload determination threshold, the input to the S / OCP terminal of a comparator COMP2 The applied voltage signal V _ocp and the overload determination threshold are compared.

As in the example illustrated in FIG. 9, when the voltage signal V _ocp is less than the overload determination threshold during the period from time t22 to t23 and the voltage signal _VFB is still greater than or equal to the overload threshold, the output signal of the comparator COMP3 Since the signal remains at the H level, the first OLP timer 16 restarts and starts counting for a predetermined time T0 set in advance.

As shown in FIG. 9A, when the commercial AC power supply AC is turned on (turned on) at time t24 when time T4 (T4 <predetermined time T0) has elapsed from time t23, the switching operation is resumed. The first OLP timer 16 counts a predetermined time T0 set in advance during a period from time t23 to t25, and then, as shown in FIG. 9 (f), a one-shot pulse signal (for example, pulse width 1 ms) at H level at time t25. Is output. By a one-shot pulse signal output from the 1OLP timer 16, as shown in FIG. 9 (g), the variable voltage _{V R} is switched from the overcurrent threshold overload determination threshold, the input to the S / OCP terminal of a comparator COMP2 The applied voltage signal V _ocp and the overload determination threshold are compared.

As in the example illustrated in FIG. 9, in the period from time t25 to t26, when the switching operation is resumed, the voltage signal V _ocp is _equal to or higher than the overload determination threshold, and the voltage signal _VFB is still equal to or higher than the overload threshold. Since the output signal of the comparator COMP3 remains at H level, the second OLP timer 17 starts and starts counting for a predetermined time T3 set in advance. However, at time t27 before the predetermined time T3 counted by the second OLP timer 17 elapses, the output voltage rises to the set voltage by the return of the commercial AC power supply AC, so the feedback voltage signal _VFB falls below the overload threshold. Then, as shown in FIG. 9 (e), the first OLP timer 16 and the second OLP timer 17 are reset, and the function of latch determination, that is, the function of determining an overload state is released.

As described above, according to the second embodiment, when the overload state is detected continuously for a predetermined time T0, the overload determination threshold V _OCP flowing to MOSFETQ1 is set smaller than the over-current threshold When it is determined whether or not the V _OCP flowing through the MOSFET Q1 is equal to or greater than the overload determination threshold, and when an overload state is continuously detected for a predetermined time T3, the V _OCP flowing through the MOSFET Q1 is excessive again. An overload protection function that determines whether or not the overload determination threshold value set to be smaller than the current threshold value is greater than or equal to the overload determination threshold value and that is greater than or _equal to the overload determination threshold value. Is configured to operate. As a result, even if the commercial AC power supply AC is restored immediately before the timing for determining the V _OCP flowing in the MOSFET Q1 after the instantaneous interruption, the determination of the V _OCP flowing in the MOSFET Q1 is performed twice at different timings. There is an effect that the malfunction of the overload protection function at the time can be more reliably prevented.

  Furthermore, according to the second embodiment, the predetermined time T0 and the predetermined time T3 for counting the continuation of the overload state are set longer than the overload period when the controller IC is activated. Thereby, it is possible to prevent malfunction of the overload protection function when the controller IC 1 is restarted.

  Furthermore, according to the second embodiment, the time obtained by adding the predetermined time T0 and the predetermined time T3 for counting the continuation of the overload state is longer than the estimated time assumed to be used in the overload state. Is set to Thereby, even if the assumed overload period is relatively long, malfunction of the overload protection function can be prevented.

Furthermore, according to the second embodiment, after it is determined that the V _OCP flowing through the MOSFET Q1 is less than the overload determination threshold, the overload is again flowed into the MOSFET Q1 when it is detected continuously for a predetermined time T0. It is configured to determine again whether or not V _OCP is _equal to or greater than an overload determination threshold set smaller than the overcurrent threshold. Thereby, at the time of a momentary interruption, the overload protection function can be reliably restarted without malfunction.

  As mentioned above, although this invention was demonstrated by specific embodiment, the said embodiment is an example and it cannot be overemphasized that it can change and implement in the range which does not deviate from the meaning of this invention.

1 Controller IC
2 Error amplifier (E / A)
3 Load (RL)
11 Drive circuit 12 Latch circuit 13 Restart circuit 14 OSC (Internal oscillator)
15, 18 Constant current source 16 First OLP timer 17 Second OLP timer 18 Constant current source 19 Switch AND1, AND2, AND3, AND4 AND circuit C1, C2, C3 Smoothing capacitor C4, C5 capacitor COMP1, COMP2, COMP3, COMP4, COMP5 Comparator D1, D2, D3 Diode DB Rectifier circuit FF1 Flip-flop PCD Light emitting diode PCTR Light receiving transistor Q1 Power MOSFET (MOSFET)
OR1 OR circuit R1, R2, R3, R4, R5, R6, R7 Resistance T Transformer P1 Primary winding P2 Auxiliary winding S1 Secondary winding V _R Variable voltage V _ref Reference voltage

Claims (8)

Input power is applied to the primary winding of the transformer, and the switching element connected to the primary winding of the transformer is turned on / off by a control circuit, thereby being induced in the secondary winding of the transformer. A switching power supply device that rectifies and smoothes a voltage and supplies the load to a load,
An error amplifying circuit for outputting a feedback signal according to a difference between an output voltage and a reference voltage to the control circuit;
An overcurrent detection circuit for current flowing through the switching element is detected as equal to or higher than the over-current threshold to be in the overcurrent state,
An overcurrent protection circuit that shifts the switching element from on to off when the overcurrent state is detected by the overcurrent detection circuit;
An overload detection circuit for detecting an overload state by the feedback signal ;
Whether or not the current flowing through the switching element is equal to or greater than an overload determination threshold set smaller than the overcurrent threshold when the overload state is continuously detected for a first predetermined time by the overload detection circuit. An overload judgment circuit for judging,
When it is determined by the overload determination circuit that the current flowing through the switching element is less than the overload determination threshold, the overload protection function for holding the switching element in an off state is not activated. when the current flowing through the determination circuit to the switching element is determined to the be the overload determination threshold above, the switching power supply apparatus characterized by comprising the overload protection circuit for actuating the overload protection function.   2. The switching power supply device according to claim 1, wherein the first predetermined time is set longer than an overload period when the control circuit is activated.   On / off control of the switching element until the power supply voltage for the control circuit becomes an operation stop voltage when the overload determination circuit determines that the current flowing through the switching element is less than the overload determination threshold. The switching power supply device according to claim 1, further comprising a restart circuit that stops the operation. Input power is applied to the primary winding of the transformer, and the switching element connected to the primary winding of the transformer is turned on / off by a control circuit, thereby being induced in the secondary winding of the transformer. A switching power supply device that rectifies and smoothes a voltage and supplies the load to a load,
An error amplifying circuit for outputting a feedback signal according to a difference between an output voltage and a reference voltage to the control circuit;
An overcurrent detection circuit for current flowing through the switching element is detected as equal to or higher than the over-current threshold to be in the overcurrent state,
An overcurrent protection circuit that shifts the switching element from on to off when the overcurrent state is detected by the overcurrent detection circuit;
An overload detection circuit for detecting an overload state by the feedback signal ;
Whether or not the current flowing through the switching element is equal to or greater than an overload determination threshold set smaller than the overcurrent threshold when the overload state is continuously detected for a first predetermined time by the overload detection circuit. overload determination circuit you determination,
When the current flowing through the switching element is determined to be less than the overload determination threshold by the overload determination circuit, and the current flowing through the switching element by the overload determination circuit is greater than or equal to the overload determination threshold After the determination, until the overload state is continuously detected for the second predetermined time by the overload detection circuit, the overload protection function for holding the switching element in the off state is not activated. When the overload detection circuit continuously detects the overload state for a second predetermined time after the overload determination circuit determines that the current flowing through the switching element is greater than or equal to the overload determination threshold. the switching power supply apparatus characterized by comprising the overload protection circuit that actuates the overload protection function.   5. The switching power supply device according to claim 4, wherein the first predetermined time and the second predetermined time are set longer than an overload period when the control circuit is activated. 5. The apparatus according to claim 4, wherein a time obtained by adding the first predetermined time and the second predetermined time is set to be longer than an estimated time assumed to be used in the overload state. 5. The switching power supply device according to 5. Mae SL overload determination circuit, wherein after the current flowing through the switching device is determined to the be less than the overload determination threshold, if said overload condition by again the overload detection circuit is detected continuously a first predetermined time The switching power supply according to claim 4, wherein it is determined again whether or not a current flowing through the switching element is equal to or greater than an overload determination threshold set smaller than the overcurrent threshold. .   The switching power supply device according to claim 1, wherein the overload determination threshold is set in a range of 20% to 80% of the overcurrent threshold.

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