JP4919762B2 - Overcurrent protection circuit - Google Patents

Description

  The present invention monitors a current detection signal from a current detector that detects a load current, and when an overcurrent flows through the load, the pulse drive signal supplied to the switching element is changed to reduce the load voltage. It relates to a protection circuit.

  In general, a switching power supply device equipped with this type of overcurrent protection circuit, as disclosed in, for example, Patent Document 1, switches a switching element by a pulse drive signal provided from a control circuit to form a primary winding of a transformer. When the input voltage is applied intermittently, the voltage induced in the secondary winding of the transformer is rectified and smoothed, the output voltage is supplied to the load, and when the overcurrent state of the load is detected by the current detector, A command signal for narrowing the conduction width of the drive signal is supplied from the overcurrent protection circuit to the control circuit.

  FIG. 4 shows a more specific circuit configuration. In the figure, reference numerals 1 and 2 denote input terminals to which a DC voltage Vi is applied, and reference numeral 3 denotes an inverter having a so-called half-bridge configuration, which is connected between the input terminals 1 and 2 and charges by dividing the DC voltage Vi. Two switching elements 7 and 8 that are connected between a series circuit composed of two capacitors 5 and 6 and are similarly connected between the input terminals 1 and 2 and receive a pulse drive signal from a control IC 23 to be described later, respectively. And one end (dot side terminal) of the primary winding 9A of the transformer 9 is a connection point between the switching elements 7 and 8 and a transformer 9 for power transmission that insulates the primary side and the secondary side. And the other end (non-dot side terminal) of the primary winding 9 </ b> A is connected to a connection point of the capacitors 5 and 6.

  The secondary winding 9B of the transformer 9 is connected to the anodes of the diodes 11 and 12 as output rectifier circuits at one end and the other end, respectively, and the center tap of the secondary winding 9B and the diodes 11 and 12 are connected. A series circuit of a choke coil 13 and a capacitor 14 constituting an output smoothing circuit is connected between a connection point where the cathodes are connected.

  When the switching elements 7 and 8 are alternately turned on and off with a dead time when both are turned off, the charging voltage of the capacitor 5 is applied to the primary winding of the transformer 9 during the on-period of the switching element 7. A dot-side terminal is applied to 9A with a positive polarity, and during the ON period of the switching element 8, a charging voltage of another capacitor 6 is applied to the primary winding 9A of the transformer 9 with a non-dot-side terminal as a positive polarity. Therefore, on the secondary side of the transformer 9, the diodes 11 and 12 are alternately turned on and off in synchronization with the switching elements 7 and 8 being alternately turned on and off, and the rectified voltage is supplied to the choke coil 13. As a result of smoothing by the capacitor 14, a predetermined output voltage Vo is supplied to the load Z connected between the output terminals 15 and 16.

  On the other hand, as a feedback circuit for stabilizing the output voltage Vo, here, a change in the output voltage Vo is detected on the secondary side of the transformer 9, and the detection signal is electrically insulated by the photocoupler 21 so that the transformer 9 For control as pulse width modulation control means for variably controlling the conduction width of the pulse drive signal supplied to each of the switching elements 7 and 8 based on the output voltage detection circuit 22 transmitted to the primary side and the detection signal of the output voltage Vo An IC 23 is provided. As a result, when the output voltage Vo increases, the voltage level of the detection signal applied to the light emitting element 21A of the photocoupler 21 increases, and the current flowing through the light receiving element 21B of the photocoupler 21 and thus the control terminal COMP of the control IC 23 increases. Then, the control IC 23 narrows the conduction width of the pulse drive signal supplied from the output terminals OUT1 and OUT2 to the switching elements 7 and 8. Conversely, when the output voltage Vo decreases, the voltage level of the detection signal applied to the light emitting element 21A of the photocoupler 21 decreases, and the current flowing through the light receiving element 21B of the photocoupler 21 and thus the control terminal COMP of the control IC 23 decreases. Then, the control IC 23 widens the conduction width of the pulse drive signal supplied to the switching elements 7 and 8 from the output terminals OUT1 and OUT2. As a result, the output voltage Vo is stabilized by the conduction width control of the pulse drive signal.

  31 is a current transformer as a current detector for detecting an output current Io flowing through the load Z. Here, the primary winding 31A is inserted and connected in series with the primary winding 9A of the transformer 9, and the primary winding 31A The current detection signal corresponding to the winding ratio of the current flows through the secondary winding 31B of the current transformer 31 as a current. Further, 32 monitors the current detection signal obtained by the current transformer 31, and when an overcurrent flows through the load Z, the conduction width of the pulse drive signal supplied to the switching elements 7 and 8 is narrowed to reduce the output voltage Vo. This is an overcurrent protection circuit that outputs a command signal that lowers the voltage to the control IC 23. The current detector may be a resistor other than the current transformer 31 and may be connected to any position as long as it is a wiring path that can directly or indirectly detect the output current Io.

The overcurrent protection circuit 32 includes bridge-connected diodes 35A to 35D for full-wave rectification of the current flowing through the secondary winding 31B of the current transformer 31, a connection point between the diodes 35A and 35B, and a connection point between the diodes 35C and 35D. a resistor 34 connected between the, and the resistor 3 6 current detection signal rectified by the diode 35A~35D is Ru is supplied, to constitute a parallel circuit with resistor 37 to ground the other end, the rectified current detection A capacitor 38 for determining a time constant of a discharge section in which the voltage of the signal drops, a backflow prevention diode 39 for blocking current flow from a connection point of the resistors 36 and 37 to the diodes 35A to 35D as rectifier circuits, and an auxiliary Resistors 41 and 42 as reference voltage generating circuits for dividing a constant operating voltage Vcc generated by a power supply circuit (not shown), and resistors 36 and 37 The voltage level based on the current detection signal generated at the connection point is input to one input terminal (inverting input terminal), and the reference voltage level at the connection point of the resistors 41 and 42 is input to the other input terminal (non-inverting input terminal). ) And outputs the comparison result from the output terminal, the transistor 44 for increasing the current flowing through the control terminal COMP of the control IC 23 when the voltage level at the output terminal of the operational amplifier 43 increases, and the operational amplifier The resistor 45 supplies a base current corresponding to the voltage generated at the output terminal 43 to the transistor 44, and a diode 46 connected between the control terminal COMP of the control IC 23 and the collector of the transistor 44. .

  Next, the operation in the above circuit configuration will be described based on the waveforms of the respective parts shown in FIG. In FIG. 5, the voltage waveform of the pulse drive signal at point A supplied from the output terminal OUT1 of the control IC 23 to the one switching element 7 and the voltage waveform of the pulse drive signal supplied from the output terminal OUT2 of the control IC 23 to the other switching element 8 are shown. The voltage waveform of the pulse drive signal at point B, the voltage waveform of the rectified current detection signal at point E output from the diodes 35 </ b> A to 35 </ b> D, and the voltage waveform at point F applied to the non-inverting input terminal of the operational amplifier 43. They are shown in order from the top.

A pulse drive signal is alternately supplied from the output terminals OUT1 and OUT2 of the control IC 23 to each of the switching elements 7 and 8 with a certain dead time, thereby being induced in the secondary winding 9B of the transformer 9. The voltage is rectified by the diodes 11 and 12, and the rectified voltage is smoothed by the choke coil 13 and the capacitor 14, whereby a predetermined output voltage Vo is supplied to the load Z. Further, the current transformer 31 detects a current flowing through the primary winding 9A of the transformer 9 proportional to the output current Io of the load Z as a current detection signal, and passes this as a current through the secondary winding 31B. This current is rectified by the diode 35A~35D overcurrent protection circuit 32, and charging current of the rectified current detection signal from the resistor 36 through the diode 39 to the capacitor 38, the voltage level across the capacitor 38, The reference voltage level generated at the connection point of the other resistors 41 and 42 is compared by the operational amplifier 43.

Current through the primary winding 31A of the mosquito Rent transformer 31 is increased during the ON period of the switching elements 7 and 8, decreases during the dead time of the switching elements 7 and 8 are both turned off. Therefore, the voltage generated at the point E also rises during the on period of the switching elements 7 and 8 and falls during the dead time when both of the switching elements 7 and 8 are turned off, but the output terminals of the bridge-connected diodes 35A to 35D. Since a time constant circuit in which a resistor 37 and a capacitor 38 are connected in parallel is connected, the voltage waveform at point E shown in FIG. 5 is between the both ends of the resistor 34 during the ON period of the switching elements 7 and 8. (Refer to the solid line), while the dead time when both the switching elements 7 and 8 are turned off, it falls depending on the time constant determined by the resistor 37 and the capacitor 38. (See dashed line). In FIG. 5, the waveform at the point F is drawn in a gentle curve. However, since the voltage generated at the point E is actually smoothed , a ripple (pulsation) component is included.

  Here, during steady operation when the output current Io to the load Z is not in an overcurrent state, the voltage level at the non-inverting input terminal of the operational amplifier 43 is lower than the reference voltage level at the inverting input terminal of the operational amplifier 43, and the transistor 44 is turned off. Thus, the overcurrent protection circuit 32 is disconnected from the control terminal COMP of the control IC 23. Accordingly, the control IC 23 in this case receives the detection signal from the output voltage detection circuit 22 and variably controls the conduction width of the pulse drive signal to the switching elements 7 and 8 so that the output voltage Vo becomes constant. .

On the other hand, the output current Io to the load Z increases, the voltage level at the non-inverting input terminal of the operational amplifier 43 becomes higher than the reference voltage level at the inverting input terminal of the operational amplifier 43, and the collector of the transistor 44 is connected via the diode 46. When the current begins to flow from the emitter to the emitter, this becomes a command signal for starting overcurrent protection from the overcurrent protection circuit 32 toward the control IC 23, and the waveforms at points A and B at the start of the overcurrent protection operation in FIG. As shown in FIG. 2, the control IC 23 gradually narrows the conduction width of the pulse drive signal to the switching elements 7 and 8 and lowers the output voltage Vo. At this time, the voltage waveform of the rectified current detection signal at point E also gradually decreases in pulse width when the voltage rises, but the time constant at the time of voltage drop, that is, during discharge is constant, so the voltage waveform of point E is constant. The ripple component increases, and the potential at the point F applied to the non- inverting input terminal of the operational amplifier 43 decreases. However, in practice, the overcurrent protection circuit 32 operates so that the potential at the point F becomes equal to the reference voltage level determined by the resistors 41 and 42. In order to increase the potential at the point F, Since the peak of the voltage waveform increases, the output current Io exhibits a constant power drooping characteristic in which the output voltage Vo decreases while the output current Io further increases from the start of the overcurrent protection operation.
JP 2002-84744 A

  However, in the conventional overcurrent protection circuit 32 described above, when an excessive output current Io flows through the load Z, the output current Vo decreases with an increase in the output current Io with characteristics close to constant power droop. Stress increases. In order to avoid such a problem, it is sufficient to design a new overcurrent protection circuit having a constant current drooping characteristic at the time of overcurrent. In that case, the existing overcurrent protection circuit 32 as shown in FIG. It is necessary to make a major design change.

  The present invention has been made paying attention to each of the above problems, and provides an overcurrent protection circuit that can easily realize a constant current drooping characteristic at the time of overcurrent without modifying the existing circuit configuration. And its purpose.

In order to achieve the above object, the overcurrent protection circuit according to claim 1 of the present invention switches the switching element and supplies an output voltage to a load by a pulse drive signal given from the control means to the switching element. In order to stabilize the output voltage, the output voltage is detected by an output voltage detection circuit, and the control means variably controls the conduction width of the pulse drive signal based on a detection signal from the output voltage detection circuit. The switching power supply device includes a diode that rectifies a current detection signal from a current detector that detects an output current flowing through the load, and a current of the current detection signal rectified from the diode during an ON period of the switching element. A capacitor charged by flowing in, and connected between both ends of the capacitor, the switching A resistor that discharges the capacitor during the off period of the child, and when the voltage level across the capacitor becomes higher than a reference voltage level, the command signal that narrows the conduction width of the pulse drive signal In an overcurrent protection circuit that flows toward the control means and droops the output voltage, a voltage generation circuit that generates a voltage of a level corresponding to a conduction width of the pulse drive signal, and a voltage obtained by the voltage generation circuit A variable current source that generates, as an input, a current that changes according to the conduction width of the pulse drive signal as an input, and adjusts the amount of current that flows from the rectified current detection signal to the capacitor according to the current of the correction signal. When the conduction width of the pulse drive signal becomes narrow, the amount of current of the correction signal decreases and the amount of current flowing through the capacitor increases. So, the voltage level across the capacitor by more than the reference voltage level, the output current has a drooping characteristic correction circuit for suspending the output voltage at constant current.

According to the configuration of claim 1, when an overcurrent flows through the load, the drooping characteristic correction circuit generates a correction signal according to the change of the pulse drive signal using the pulse drive signal supplied to the switching element, By adjusting the amount of current flowing through the capacitor from the rectified current detection signal, the output voltage can be drooped while maintaining a constant current value without further increasing the output current to the load. This can be realized simply by adding a drooping characteristic correction circuit to the existing circuit configuration, and originally using the pulse drive signal to the switching element well. The constant current drooping characteristic can be easily obtained at the time of current.

Further , the voltage generation circuit generates a variable voltage level using the increase / decrease of the conduction width of the pulse drive signal, and based on this voltage, the amount of current flowing from the rectified current detection signal to the capacitor by the variable current source Therefore, the drooping characteristic correction circuit can be easily configured with a voltage generation circuit and a variable current source.

  Hereinafter, a preferred embodiment of a switching power supply device having an overcurrent protection circuit according to the present invention will be described in detail with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the part same as the location demonstrated by background art, and common description is abbreviate | omitted as much as possible in order to avoid duplication.

  FIG. 1 is a circuit diagram of a switching power supply apparatus according to the present invention. Here, the circuit configuration of FIG. 4 shown as the conventional example is commonly used as it is, and when the load Z enters an overcurrent state, A correction signal generated from a pulse drive signal applied from the control IC 23 to the switching elements 7 and 8 is generated so that the output current Io to the load Z is a constant current and the output voltage Vo drops. A drooping characteristic correction circuit 51 for correcting the amount of current of the current detection signal is provided. In order to prevent the drooping characteristics from fluctuating due to the ambient temperature, the resistances 41 and 42 as the reference voltage generation circuit and the thermistor correction circuits 51 are provided with thermistor resistors 52 and 53 as temperature compensating elements, respectively. It has been. In other words, the circuit configuration is the same as that of FIG. 4 except that the drooping characteristic correction circuit 51 and the thermistor resistor 52 are added.

The drooping characteristic correction circuit 51 receives a voltage generation circuit 56 that generates a voltage at a level corresponding to the conduction width of the pulse drive signal, and the voltage obtained by the voltage generation circuit 56 as an input, according to the conduction width of the pulse drive signal. the varying current Te generated as a correction signal, constituted by a current mirror circuit 57 as a variable current source to perform operations such as changing the impedance of the pre-Symbol F point by the current of the correction signal. More specifically, the voltage generation circuit 56 includes a series circuit of a diode 61 and a resistor 62 having one end connected to the output terminal OUT1 of the control IC 23, and another diode having one end connected to the output terminal OUT2 of the control IC 23. 63 and a resistor 64, a charge / discharge capacitor 65 connected between a connection point connecting the other ends of these series circuits and the ground line, and connected between both ends of the capacitor 65. A discharge resistor 66. The current mirror circuit 57 converts the voltage generated between both ends of the NPN type transistors 68 and 69 having bases connected to each other and both ends of the capacitor 65 into a current. A series circuit of resistors 71 and 72 supplied to the bases of the transistors 68 and 69, a thermistor resistor 53 for temperature compensation connected in parallel between both ends of the resistor 71, and emitters of both transistors 68 and 69 and a ground line. And resistors 73 and 74 for determining emitter potentials of the transistors 68 and 69, respectively.

  The inverter 3 in this embodiment may adopt any circuit system such as a forward type, a flyback type, and a full bridge type in addition to the illustrated half bridge type. Therefore, the secondary side of the transformer 9 can employ various circuit methods such as a single-ended configuration. Further, as the switching elements 7 and 8, a switching element with a control terminal such as a transistor may be used in addition to the MOS FET shown in FIG. Further, the overcurrent protection circuit 32 and the drooping characteristic correction circuit 51 are not limited to the circuit configuration shown in FIG.

  Next, the operation in the above circuit configuration will be described based on the waveform of each part shown in FIG. In FIG. 2, the voltage waveform of the pulse drive signal at point A supplied from the output terminal OUT1 of the control IC 23 to one switching element 7 and the voltage waveform of the pulse drive signal supplied from the output terminal OUT2 of the control IC 23 to the other switching element 8 are shown. The voltage waveform of the pulse drive signal at point B, the power supply voltage waveform of the current mirror circuit 57 at point C generated at one end of the capacitor 65, and the current waveform on the output side of the current mirror circuit 57 at point D flowing into the collector of the transistor 69 The voltage waveform in the rectified current detection signal at point E output from the diodes 35 </ b> A to 35 </ b> D and the voltage waveform at point F applied to the non-inverting input terminal of the operational amplifier 43 are shown in order from the top.

  The output voltage Vo is supplied to the load Z by the switching operation of the switching elements 7 and 8, and the conduction width of the pulse drive signal is variably controlled from the control IC 23 based on the detection signal obtained by the output voltage detection circuit 22 in the steady state. Since a series of operations for stabilizing the output voltage Vo is as described in the background art, description thereof is omitted here.

  When an on-pulse is output from the output terminal OUT1 of the control IC 23, the voltage generation circuit 56 of the drooping characteristic correction circuit 51 conducts the diode 61, causes a current to flow from the resistor 62 to the capacitor 65, and the output of the control IC 23. When an ON pulse is output from the terminal OUT2, the diode 63 is turned on, a current is passed through the capacitor 65 via the resistor 64, and the capacitor 65 is charged. Therefore, the voltage at the point C generated at one end of the capacitor 65 increases as the conduction width of the pulse drive signal supplied from the output terminals OUT1 and OUT2 to the switching elements 7 and 8 increases, and decreases as the conduction width decreases. Further, since the voltage at the point C becomes a power supply voltage input to the current mirror circuit 57, the current at the point D flowing from the collector of the transistor 69, which is the correction signal of the drooping characteristic correction circuit 51, to the base is the pulse drive signal. It increases as the conduction width increases, and decreases as the conduction width decreases.

On the other hand, the overcurrent protection circuit 32 rectifies the current flowing through the secondary winding 31B of the current transformer 31 by the diodes 35A to 35D, adjusts the amount of current of the rectified current detection signal with the correction signal, and supplies it to the capacitor 38. The operational amplifier 43 compares the voltage level of the signal across the capacitor 38 with the reference voltage level generated at the connection point of the other resistors 41 and 42.

The current flowing through the primary winding 31A of the current transformer 31 increases during the ON period of the switching elements 7 and 8, and decreases during the dead time when both the switching elements 7 and 8 are OFF. Therefore, the voltage generated at the point E also rises during the on period of the switching elements 7 and 8 and falls during the dead time when both of the switching elements 7 and 8 are turned off, but the output terminals of the bridge-connected diodes 35A to 35D. Since a time constant circuit in which a resistor 37 and a capacitor 38 are connected in parallel is connected, the voltage waveform at the point E shown in FIG. 2 depends on the current detection signal during the ON period of the switching elements 7 and 8. While rising (see the solid line), during the dead time when both the switching elements 7 and 8 are turned off, it falls depending on the time constant determined by the resistor 37 and the capacitor 38 (see the broken line). Further, in FIG. 2, the waveform at the point F is depicted in gentle curve shape, since in practice such that smoothed the voltage generated at point E, contains ripple components.

  Here, at the time of steady operation in which the output current Io to the load Z is not in an overcurrent state, the current flowing through the secondary winding 31B of the current transformer 31 is less than that at the time of overcurrent, and each output terminal OUT1, of the control IC 23 The conduction width of the pulse drive signal generated from OUT2 is wider than that during overcurrent. For this reason, the power supply voltage of the current mirror circuit 57 at point C is high, and the amount of correction signal current at point D is large, so that the non-inverting input terminal of the operational amplifier 43 passes through the diode 39 from the output terminals of the diodes 35A to 35D. The flowing current decreases, and the voltage level at the point F at the non-inverting input terminal of the operational amplifier 43 becomes lower than the reference voltage level applied to the inverting input terminal of the operational amplifier 43 determined by the resistors 41 and 42. Then, the transistor 44 is turned off, so that the overcurrent protection circuit 32 is disconnected from the control terminal COMP of the control IC 23, and the control IC 23 receives the detection signal from the output voltage detection circuit 22 and outputs it. The conduction width of the pulse drive signal to the switching elements 7 and 8 is variably controlled so that the voltage Vo is constant.

  On the other hand, when the output current Io to the load Z increases, the current flowing through the secondary winding 31B of the current transformer 31 increases, and the voltage generated at the point E also increases. Accordingly, when the voltage level at the non-inverting input terminal of the operational amplifier 43 becomes higher than the reference voltage level at the inverting input terminal of the operational amplifier 43 and current starts to flow from the collector to the emitter of the transistor 44 via the diode 46, This is a command signal for starting overcurrent protection from the overcurrent protection circuit 32 toward the control IC 23. As shown in the waveforms at points A and B at the start of the overcurrent protection operation in FIG. The conduction width of the pulse drive signal to the switching elements 7 and 8 is gradually narrowed, and the output voltage Vo is lowered.

  At this time, in the drooping characteristic correction circuit 51, the power supply voltage of the current mirror circuit 57 at the point C gradually decreases, and the amount of current of the correction signal to the current mirror circuit 57 at the point D also decreases, so that the diodes 35A to 35D The current flowing from the output terminal through the diode 39 to the non-inverting input terminal of the operational amplifier 43 increases, and the impedance at the point F at the non-inverting input terminal of the operational amplifier 43 increases. That is, the voltage waveform at the point E shown in FIG. 2 is a current detection signal detected by the current transformer 31 when the conduction width of the pulse drive signal to the switching elements 7 and 8 is narrowed by the command signal from the overcurrent protection circuit 32. Is further increased by adding a voltage correction amount considering the current amount of the correction signal generated by the drooping characteristic correction circuit 51 to the voltage waveform based on (the hatched portion in the waveform diagram at point E) (solid line in the waveform diagram at point E) And wavy lines).

  In this way, when an operation for changing the impedance is performed on the current information of the output current Io given to the overcurrent protection circuit 32 using the pulse drive signal generated by the switching power supply device itself, the output voltage Vo is The constant current drooping characteristic in which the output voltage Vo decreases while the output current Io is maintained at a constant current value is shown instead of the conventional constant power drooping characteristic in which the output current Io increases as the voltage decreases. Become. As an example, FIG. 3 shows the relationship between the output current Io and the output voltage Vo when the conventional drooping characteristic correction circuit 51 is not provided and when the drooping characteristic correction circuit 51 in this embodiment is provided. Yes. When the drooping characteristic correction circuit 51 is added, it can be seen that the output voltage Vo decreases without increasing the output current Io at the time of overcurrent. This is because the drooping characteristic correction circuit 51 changes the current amount of the correction signal flowing into the current mirror circuit 57 using the pulse drive signal of the switching power supply device itself, and substantially variably operates the resistance value at the F point. Realized.

As described above, in the present embodiment , the switching elements 7 and 8 are switched by the pulse drive signal given to the switching elements 7 and 8 from the control IC 23 which is the control means, and the output voltage Vo is supplied to the load Z. In order to stabilize the output voltage Vo, the output voltage Vo is detected by the output voltage detection circuit 22, and the control IC 23 varies the conduction width of the pulse drive signal based on the detection signal from the output voltage detection circuit 22. is provided to the switching power supply device for controlling a diode 35A~35D for rectifying a current detection signal from the current detector serving the current transformer 31 for detecting an output current Io is a current to the load Z, on the switching elements 7 and 8 A capacitor 38 that is charged by the flow of the current of the rectified current detection signal during the period, and a capacitor 8 and a resistor 37 that discharges the capacitor 38 during the off period of the switching elements 7 and 8. When the voltage level across the capacitor 38 becomes higher than the reference voltage level, pulse driving is performed. In the overcurrent protection circuit 32 that causes the output voltage Vo, which is a voltage to the load Z, to flow toward the control IC 23 by flowing a command signal that narrows the conduction width of the signal, the level according to the conduction width of the pulse drive signal For example, a voltage generation circuit 56 that generates the voltage between both ends of the capacitor 65, and a voltage obtained by the voltage generation circuit 56 as an input, and a current that changes according to the conduction width of the pulse drive signal is generated as a correction signal. The current mirror circuit 57 as a variable current source that adjusts the amount of current flowing to the capacitor 38 from the rectified current detection signal by the current of the correction signal. When the conduction width of the pulse drive signal is narrowed, the amount of current of the correction signal is decreased to increase the amount of current flowing through the capacitor 38, and the voltage level between both ends of the capacitor 38 is the reference voltage. By exceeding the level, a drooping characteristic correction circuit 51 that droops the output voltage Vo while keeping the output current Io to the load Z constant is provided.

In this case, when an overcurrent flows through the load Z, the drooping characteristic correction circuit 51 uses the pulse drive signal supplied to the switching elements 7 and 8 to generate a correction signal corresponding to the change in the pulse drive signal, By adjusting the amount of current flowing to the capacitor 38 from the rectified current detection signal, the output voltage Vo can be drooped while maintaining a constant current value without further increasing the output current Io to the load Z. become. Since this can be realized simply by adding the drooping characteristic correction circuit 51 to the existing circuit configuration and originally using the pulse drive signal to the switching elements 7 and 8, the existing circuit configuration is modified. Therefore, the constant current drooping characteristic can be easily obtained at the time of overcurrent.

Moreover, by utilizing to increase or decrease the conduction width of the pulse drive signal, the voltage generating circuit 56 generates a voltage level varying, the current mirror circuit 57 on the basis of this voltage, the capacitor 38 from the rectified current detection signal since adjusting the amount of current flowing in can be easily configured the drooping characteristic correction circuit 51 by the voltage generating circuit 56 and the current mirror circuit 57.

  In addition, this invention is not limited to the said Example, A various deformation | transformation implementation is possible in the range of the summary of this invention. The overcurrent protection circuit in this embodiment is incorporated in a so-called switching power supply device, but the above-described overcurrent protection circuit is incorporated in any circuit configuration that can change the load current by a pulse drive signal applied to the switching element. It is possible to apply.

It is a circuit block diagram of the switching power supply device provided with the overcurrent protection circuit in one preferable Example of this invention. 2 is a waveform diagram of each part in the circuit of FIG. It is a graph which shows the relationship between the output current and output voltage in a conventional and a present Example same as the above. It is a circuit block diagram of the switching power supply device provided with the conventional overcurrent protection circuit. FIG. 5 is a waveform diagram of each part in the circuit of FIG.

7,8 Switching element
23 Control IC (control means)
31 Current transformer (current detector)
32 Overcurrent protection circuit
35A-35D diode
37 Resistance
38 Capacitor 51 Drooping characteristic correction circuit 56 Voltage generation circuit 57 Current mirror circuit (variable current source)
Z load

Claims (1)

In accordance with a pulse drive signal given from the control means to the switching element, the switching element is switched to supply an output voltage to the load, and the output voltage is detected by an output voltage detection circuit in order to stabilize the output voltage. The control means is provided in a switching power supply device that variably controls the conduction width of the pulse drive signal based on a detection signal from the output voltage detection circuit,
A diode for rectifying a current detection signal from the current detector for detecting an output current flowing through the load,
A capacitor that is charged when a current of a current detection signal rectified from the diode flows during an on period of the switching element;
A resistor connected between both ends of the capacitor, and discharging the capacitor during an off period of the switching element,
When the voltage level between both ends of the capacitor becomes higher than the reference voltage level, an overcurrent protection circuit that causes a command signal to narrow the conduction width of the pulse drive signal to the control means to drop the output voltage In
A voltage generation circuit that generates a voltage at a level corresponding to the conduction width of the pulse drive signal, and a voltage obtained by the voltage generation circuit as an input, and a current that changes according to the conduction width of the pulse drive signal as a correction signal And a variable current source that adjusts the amount of current flowing from the rectified current detection signal to the capacitor by the current of the correction signal,
When the conduction width of the pulse drive signal is narrowed, the amount of current of the correction signal is decreased, the amount of current flowing through the capacitor is increased, and the voltage level across the capacitor exceeds the reference voltage level. An overcurrent protection circuit comprising a drooping characteristic correction circuit that droops the output voltage when the output current is a constant current .

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