JP2006033913A - Power supply output voltage monitoring circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power output voltage monitoring circuit capable of reducing the size of a power supply device by reducing the number of photocouplers.
The secondary circuit 2 controls the transistor 27 that drives the light emitting diode 29 of the photocoupler PC by combining the output of the comparator 21 that detects overvoltage and the output of the comparator 22 that detects undervoltage. Thus, the photocoupler PC is turned on when the output voltage is normally output, and is turned off when the overvoltage or undervoltage is present. The primary circuit 1 closes the AND gate 14 when the photocoupler PC is turned on and outputs a low level signal f indicating normality, and when the photocoupler PC is turned off, the primary circuit 1 outputs a high level alarm signal indicating abnormality from the AND gate 14. Is output. When the power supply device is activated, the timer 15 outputs an output signal b having a low level for a predetermined time after activation, and masks the output of an alarm signal due to detection of an undervoltage at the time of startup.
[Selection] Figure 1

Description

  The present invention relates to a power supply output voltage monitoring circuit, and more particularly to a power supply output voltage monitoring circuit in a power supply device used for communication equipment or the like that requires reliability.

  A switching power supply generally has a primary circuit on the input side and a secondary circuit on the output side, and the primary circuit and the secondary circuit are electrically isolated by a transformer. The switching power supply detects the output voltage of the secondary circuit and feeds it back to the primary circuit via a photocoupler to control the pulse width of the pulse signal supplied to the switching transistor so that a constant output voltage can be obtained. (For example, refer to Patent Document 1).

  A power supply device used for communication equipment or the like generally has a function of monitoring an output voltage in order to prevent malfunction of a load circuit. Here, the output voltage abnormality includes “overvoltage” in which the voltage rises higher than the specified voltage and “undervoltage” in which the voltage drops.

6A and 6B are explanatory diagrams of the output voltage abnormality, in which FIG. 6A is a diagram defining the output voltage abnormality, and FIG.
As shown in FIG. 6A, when the specified voltage of the output voltage Vo is 100%, the first set voltage is 120%, and the second set voltage is 80%, the output voltage abnormality is 120% or more is defined as overvoltage OV, and 80% or less is defined as undervoltage LV.

  Further, as shown in FIG. 6B, the power supply device starts up from 0%, passes through an undervoltage region of 80% or less, and has a specified output voltage of 100%. It shows a change that rises to.

FIG. 7 is a circuit diagram showing a configuration example of a conventional power supply output voltage monitoring circuit in the power supply device.
The power supply device includes a control circuit 101 and a switching transistor 102 in the primary circuit on the input side, and has a configuration for controlling the current flowing through the primary winding of the transformer 103 by these. The control circuit 101 is connected to the light receiving portions of two photocouplers PC1 and PC2. A secondary circuit on the output side of the power supply device is connected to the secondary winding of the transformer 103. This secondary circuit includes diodes 104 and 105 that perform full-wave rectification, and a choke coil 106 and a capacitor 107 that form a smoothing circuit.

  Further, the secondary circuit of the power supply device includes a power supply output voltage monitoring circuit having an overvoltage detection circuit 108 and an undervoltage detection circuit 109. The overvoltage detection circuit 108 is a voltage divider that detects the output voltage of the power supply device by two resistors 110 and 111, and a first voltage that outputs a voltage corresponding to the first set voltage from the voltage detected by the voltage divider. A comparator 113 for comparing with the reference voltage source 112, and a light emitting part of the photocoupler PC 1 and a current limiting resistor 114 connected to the output of the comparator 113 are provided.

  The undervoltage detection circuit 109 has the same configuration as that of the overvoltage detection circuit 108, and a voltage divider that detects the output voltage of the power supply device by two resistors 115 and 116, and the voltage detected by the voltage divider is the above-described first voltage. A comparator 118 that compares with a second reference voltage source 117 that outputs a voltage corresponding to a set voltage of 2, and a light emitting section of the photocoupler PC2 and a current limiting resistor 119 connected to the output of the comparator 118. Yes.

In FIG. 7, a feedback circuit in which the power supply device controls the output voltage of the secondary circuit to a specified voltage is omitted here.
In the power supply output voltage monitoring circuit having the above configuration, when the output voltage is between the first set voltage and the second set voltage, the overvoltage detection circuit 108 detects the voltage applied to the inverting input of the comparator 113. Since the voltage is lower than the voltage of the first reference voltage source 112, the output of the comparator 113 is maintained at a high level. Therefore, the photocoupler PC1 is turned off because no current flows through the light emitting portion. On the other hand, since the voltage applied to the inverting input of the comparator 118 is higher than the voltage of the second reference voltage source 117, the undervoltage detection circuit 109 maintains the output of the comparator 118 at a low level. Therefore, the photocoupler PC2 is in a lighting state because a current flows through the light emitting portion.

  Here, when the output voltage becomes higher than the first set voltage, in the overvoltage detection circuit 108, the voltage applied to the inverting input of the comparator 113 becomes higher than the voltage of the first reference voltage source 112. Is inverted to a low level, and as a result, the light emitting portion of the photocoupler PC1 is turned on. The control circuit 101 of the primary circuit controls the power supply device to stop immediately when the light receiving unit of the photocoupler PC1 responds.

  In addition, when the output voltage becomes lower than the second set voltage, the voltage applied to the inverting input of the comparator 118 becomes lower than the voltage of the second reference voltage source 117 in the undervoltage detection circuit 109. Is inverted to a high level, and as a result, the light emitting portion of the photocoupler PC2 is turned off. The control circuit 101 of the primary circuit controls the power supply device to stop immediately when the light receiving unit of the photocoupler PC2 responds.

  Further, when the power supply device is started, the output voltage rises to a specified output voltage of 100% while passing through the undervoltage region of 80% or less. However, when the output voltage is in the undervoltage region, the undervoltage detection circuit 109 is always used. Detects the undervoltage LV. Therefore, at the time of starting such a power supply device, the control circuit 101 masks the detection of the undervoltage LV by the undervoltage detection circuit 109 with a timer or the like for a certain time from the start, and after the certain time has elapsed, the undervoltage detection circuit. The detection of the undervoltage LV by 109 is made effective.

As described above, the conventional power supply output voltage monitoring circuit needs to be configured to stop immediately when an overvoltage is detected, or to stop after a certain period of time when an undervoltage is detected. The voltage detection is configured by an independent circuit, and therefore two photocouplers are required to bridge the isolated primary circuit and secondary circuit.
JP-A-9-17276 (FIG. 1)

  However, in recent years, electronic components are often integrated with the aim of reducing the size of the device. However, for photocouplers, it is necessary to ensure insulation withstand voltage between the primary circuit and the secondary circuit, so that there is a problem in the integrated circuit. In addition, the power supply output voltage monitoring circuit requires two photocouplers, which causes a problem of downsizing.

  The present invention has been made in view of these points, and an object thereof is to provide a power supply output voltage monitoring circuit capable of reducing the number of photocouplers and reducing the size of a power supply device.

FIG. 1 is a circuit diagram showing a power supply output voltage monitoring circuit that achieves the above object.
This power supply output voltage monitoring circuit includes a primary circuit 1 provided on the input side of the power supply device and a secondary circuit 2 provided on the output side. The secondary circuit 2 includes two comparators 21 and 22 and a light emitting unit of one photocoupler PC, and detects an upper limit and a lower limit of an allowable change range when the output voltage changes, Turns on the photocoupler PC, and turns off the photocoupler PC when overvoltage or undervoltage occurs.

  On the other hand, the primary circuit 1 includes a light receiving unit of the photocoupler PC, a latch circuit including NAND gates 11 and 12, an OR gate 13, an AND gate 14, and a timer 15. When the output voltage is within the allowable change range, the light receiving unit of the photocoupler PC receives light, so that the signal c becomes low level. As a result, the output signal f of the AND gate 14 becomes low level. A level normal signal is output. When the output voltage is overvoltage or undervoltage, the light receiving unit of the photocoupler PC does not receive light, so that the signal c becomes high level and the output signal b of the timer 15 is also high level. The output signal f outputs a high level alarm signal. In this case, the control circuit of the power supply device stops the power supply circuit immediately upon receiving the alarm signal.

  When the power supply device is activated, the timer 15 receives the activation signal a, outputs a low-level output signal b for a predetermined time from the activation, and outputs a high-level output signal b after a predetermined time elapses. The AND gate 14 forcibly outputs a low-level output signal f for a predetermined time from the start-up, whereby the comparator 22 for detecting an undervoltage in the secondary circuit 2 causes the output voltage to rise. The inevitable detection of undervoltage is masked.

  Since the power supply output voltage monitoring circuit of the present invention is configured to transmit an abnormality of the overvoltage or undervoltage of the output voltage to the primary circuit in the light emission mode of the photocoupler, the number of photocouplers used conventionally is reduced to one. Therefore, there is an advantage that it is possible to contribute to miniaturization of the power supply device.

1 is a circuit diagram showing a power supply output voltage monitoring circuit according to the first embodiment, FIG. 2 is a diagram showing an operation state of a secondary circuit, and FIG. 3 is a time chart showing an operation of the primary circuit.
This power supply output voltage monitoring circuit includes a primary circuit 1 provided on the input side of the power supply device and a secondary circuit 2 provided on the output side. The primary circuit 1 has a latch circuit composed of NAND gates 11 and 12. An output of the NAND gate 11 is connected to one input of the OR gate 13, and an output thereof is connected to one input of the AND gate 14. The other input of the AND gate 14 is connected to one input of the NAND gate 11, and the output constitutes the output of the power supply output voltage monitoring circuit. One input of the NAND gate 12 is configured to receive an activation signal a, and the activation signal a constitutes an input of the timer 15. The output of the timer 15 is connected to the other input of the OR gate 13. One input of the NAND gate 11 is also connected to the collector of the light receiving transistor 16 constituting the light receiving portion of the photocoupler PC and the other of the resistor 17 connected to the power source Vcc. The emitter of the light receiving transistor 16 is connected to the ground of the primary circuit 1. As described above, the primary circuit 1 includes the light receiving unit of the photocoupler PC, the logic circuit, and the timer 15.

  The secondary circuit 2 includes two comparators 21 and 22 and three resistors 23, 24, and 25 (voltage detection circuit) connected in series for detecting an output voltage. The connection point of the resistors 23 and 24 is connected to the non-inverting input of the comparator 22, and the inverting input is connected to the positive terminal of the reference voltage source 26. The connection point of the resistors 24 and 25 is connected to the inverting input of the comparator 21, and the non-inverting input is connected to the positive terminal of the reference voltage source 26. The negative terminal of the reference voltage source 26 is connected to the negative output terminal of the secondary circuit 2.

  The outputs of the comparators 21 and 22 are connected to the base of the transistor 27 and the other of the resistors 28, one of which is connected to the positive output terminal of the secondary circuit 2. The emitter of the transistor 27 is connected to the negative output terminal of the secondary circuit 2, the collector is connected to the cathode of the light emitting diode that constitutes the light emitting part of the photocoupler PC, and the anode thereof is connected to the secondary circuit 2 via the resistor 30. Connected to the positive output terminal.

  As described above, the power supply output voltage monitoring circuit according to the first embodiment integrates two photocouplers that have been used in the past into one, and the secondary circuit 2 has an overvoltage and an undervoltage. Both are monitored.

  Here, the comparator 21 constitutes a circuit for detecting an overvoltage, the comparator 22 constitutes a circuit for detecting an undervoltage, and respective outputs are connected to each other to constitute an AND circuit. When the output voltage is normal, the potential at the connection point between the resistor 24 and the resistor 25 is set lower than the potential of the reference voltage source 26, and the potential at the connection point between the resistor 23 and the resistor 24 is higher than the potential of the reference voltage source 26. Since it is set high, the outputs of the comparators 21 and 22 are at a high level, whereby the transistor 27 is turned on and the light-emitting diode 29 of the photocoupler PC is in the lit state. When the output voltage rises and the potential at the connection point between the resistor 24 and the resistor 25 becomes higher than the potential of the reference voltage source 26, the comparator 21 inverts the output state, turns off the transistor 27, and turns off the photocoupler PC. The light emitting diode 29 is turned off. Conversely, when the output voltage decreases and the potential at the connection point between the resistor 23 and the resistor 24 becomes lower than the potential of the reference voltage source 26, the comparator 22 inverts the output state and turns off the transistor 27. The light emitting diode 29 of the photocoupler PC is turned off.

  That is, as shown in FIG. 2, the secondary circuit 2 of the power supply output voltage monitoring circuit has a comparator at the time of normal output in which the output voltage is between the first set voltage Vov and the second set voltage Vlv. The outputs of 21 and 22 are at a high level, and the light-emitting diode 29 of the photocoupler PC is turned on, and in the case of an abnormal condition that causes other overvoltage or undervoltage, the output of the comparator 21 or the comparator 22 is inverted and low. When the level is reached, the light emitting diode 29 of the photocoupler PC is turned off.

  Next, the operation of the primary circuit 1 with respect to such an operation of the secondary circuit 2 will be described with reference to FIG. In FIG. 3, signals corresponding to the signals a to f appearing in the respective circuits in FIG. 1 are denoted by the same reference numerals.

  First, when the power supply device is activated, the timer 15 is activated by the activation signal a, and a high level signal b is output after a predetermined time has elapsed. At the same time, when the activation signal a is input to the NAND gate 12, the latch circuit is reset and the output signal d of the NAND gate 11 is maintained at a low level.

  Here, while the predetermined time of the timer 15 elapses, the timer 15 outputs the low level signal b, so that the AND gate 14 is supplied to one input via the OR gate 13. It is blocked by the signal e so that it is not affected by the level of the signal c supplied to the other input. This masks detection of an undervoltage that inevitably occurs when the power supply device is started.

  If the output voltage of the secondary circuit 2 rises normally while the predetermined time of the timer 15 elapses, the photocoupler PC is turned on, the light receiving transistor 16 is turned on, and the signal c becomes low level. As a result, the latch circuit is set and outputs a high level signal d to open the AND gate 14, but the AND gate 14 is forcibly blocked by the low level signal c, and the output is normal. A low level signal f representing the operation is output.

  If the output voltage of the secondary circuit 2 does not rise normally during the lapse of the predetermined time of the timer 15, the photocoupler PC remains off, so that the light receiving transistor 16 remains off, When c becomes high level, the AND gate 14 is opened. On the other hand, since the output signal b of the timer 15 becomes high level after a predetermined time of the timer 15 elapses, at that time, the high level signal e is input to the AND gate 14 via the OR gate 13. As a result, the AND gate 14 outputs a high level signal f, that is, an alarm signal ALM, at its output. When the alarm signal ALM is output from the power supply output voltage monitoring circuit, the control circuit of the power supply device controls to stop the power supply circuit immediately.

  When the output voltage of the secondary circuit 2 rises abnormally and exceeds the first set voltage Vov or falls abnormally and falls below the second set voltage Vlv during normal operation of the power supply device, the photocoupler PC Is turned off, the light receiving transistor 16 is turned off, and the signal c changes to high level. At this time, since the output signal b of the timer 15 is high level and the output signal e of the OR gate 13 is high level, the output signal f of the AND gate 14 changes to high level, Alarm signal ALM. Even in this case, the control circuit of the power supply device controls the power supply circuit to stop immediately upon receiving the alarm signal ALM.

  FIG. 4 is a circuit diagram of a power supply apparatus including a power supply output voltage monitoring circuit according to the second embodiment, and FIG. 5 is a time chart showing the operation of the power supply output voltage monitoring circuit according to the second embodiment. In FIG. 5, signals corresponding to the signals A to F appearing in the respective circuits in FIG. 4 are denoted by the same reference numerals.

  Compared with the power supply output voltage monitoring circuit according to the first embodiment, the power supply output voltage monitoring circuit according to the second embodiment performs an operation in synchronization with the switching frequency of the switching power supply so that the overvoltage and The difference is that it is configured so that it can be detected separately from the undervoltage.

  The power supply device basically electrically insulates between the control circuit 41 that performs pulse width control, the switching transistor 42 that is driven by the switching signal from the control circuit 41, and the primary circuit and the secondary circuit. It includes a transformer 43, diodes 44 and 45 that perform full-wave rectification, and a choke coil 46 and a capacitor 47 that form a smoothing circuit. The power supply device further includes a power supply output voltage monitoring circuit.

  The power supply output voltage monitoring circuit includes a NOT gate 51 whose input is connected to the output of the control circuit 41, an AND gate 52 whose one input is connected to the output of the NOT gate 51, and an output whose other input is the AND gate 52. And a NOT gate 53 connected to the. The input of the NOT gate 53 is connected to one input of the AND gate 54, and the other input is connected to the output of the control circuit 41. The output of the AND gate 54 is connected to the timer 55. Further, the input of the NOT gate 53 and one input of the AND gate 54 are a connection point between the other end of the resistor 56 having one end connected to the power supply Vcc and the collector of the light receiving transistor 57 constituting the light receiving portion of the photocoupler PC. It is connected to the. The emitter of the light receiving transistor 57 is connected to the ground of the primary circuit 1.

  The power supply output voltage monitoring circuit also has two comparators 58 and 59, and three resistors 60, 61 and 62 connected in series to detect the output voltage with both ends connected between the output terminals of the secondary circuit. is doing. The connection point of the resistors 60 and 61 is connected to the non-inverting input of the comparator 59, and the inverting input is connected to the positive terminal of the reference voltage source 63. The connection point of the resistors 61 and 62 is connected to the non-inverting input of the comparator 58, and the inverting input is connected to the positive terminal of the reference voltage source 63. The negative terminal of the reference voltage source 63 is connected to the negative output terminal of the secondary circuit.

  The output of the comparator 58 is connected to the base of the transistor 64, the collector is connected to the base of the transistor 65, and the emitter is connected to the negative output terminal of the secondary circuit. The emitter of the transistor 65 is connected to the positive output terminal of the secondary circuit, and the collector is connected to the anode of the light emitting diode 67 constituting the light emitting part of the photocoupler PC via the resistor 66. The cathode of the light emitting diode 67 is connected to the collector of the transistor 68, the emitter thereof is connected to the negative output terminal of the secondary circuit, and the base is connected to the output of the comparator 59. The connection point between the resistor 66 and the light emitting diode 67 is connected to the cathode of the diode 70 via the resistor 69, and its anode is connected to the connection point between the cathodes of the rectifying diodes 44 and 45 and the choke coil 46. Yes.

  In this configuration, when the power supply apparatus is operating, the switching signal A shown in FIG. 5 is output from the control circuit 41, and the switching transistor 42 is driven on and off by the switching signal A. By this switching operation, a pulse signal B similar to the switching signal A appears at the output of the rectifier circuit of the secondary circuit.

  Here, the comparator 58 constitutes a circuit that detects an overvoltage, and the comparator 59 constitutes a circuit that detects an undervoltage. That is, when the output voltage is normal, the potential at the connection point of the resistors 60 and 61 is higher than the potential of the reference voltage source 63, and the potential at the connection point of the resistors 61 and 62 is lower than the potential of the reference voltage source 63. The voltage dividing ratio of the resistors 60, 61, 62 is set.

  Therefore, when the output voltage is normal, the output of the comparator 58 is always at a low level, and the output of the comparator 59 is always at a high level. Therefore, the transistors 64 and 65 are off and the transistor 68 is on. Therefore, the pulse signal B flows to the light-emitting diode 67 of the photocoupler PC via the diode 70, the resistor 69, the light-emitting diode 67 of the photocoupler PC, and the transistor 68. The light-emitting diode 67 of the photocoupler PC Flashing operation is performed by the signal C having the same period.

  When this blinking operation is received by the light receiving transistor 57 of the photocoupler PC, a signal D having an inverted phase is output to its collector. Since this signal D is out of phase with the switching signal A, the AND gates 52 and 54 both output low level signals E and F.

  If the output voltage of the secondary circuit rises abnormally during normal operation of the power supply device, the comparator 58 inverts and the output becomes high level, and the output of the comparator 59 remains at high level. Therefore, the transistors 64, 65, and 68 are turned on, a smoothed current flows through the light emitting diode 67 of the photocoupler PC, and the light emitting diode 67 is turned on.

  When the light-up signal C is received by the light receiving transistor 57 of the photocoupler PC, the light receiving transistor 57 is turned on, and a low-level signal D whose phase is inverted is output to the collector. The signal D is compared with the switching signal A, whereby a signal E having a phase opposite to that of the switching signal A is output to the output of the AND gate 52. Since one input of the AND gate 54 receives the low level signal D, the low level signal F is always output.

  If the output voltage of the secondary circuit drops abnormally during normal operation of the power supply device, the comparator 59 inverts and the output becomes low level and is connected in series with the light emitting diode 67 of the photocoupler PC. The transistor 68 is turned off. Thereby, the light emitting diode 67 of the photocoupler PC is turned off. At the time of starting the power supply device, the output of the comparator 59 is at a low level until the output voltage rises. Similarly, the transistor 68 is off and the light emitting diode 67 of the photocoupler PC is turned off.

  When the light-emitting diode 67 of the photocoupler PC is in an extinguished state, the light receiving transistor 57 of the photocoupler PC is turned off, and a high-level signal D whose phase is inverted is output to its collector. Therefore, since the low level signal inverted by the NOT gate 53 is input to one input of the AND gate 52, the low level signal E is output to the output of the AND gate 52. On the other hand, since the high level signal D is inputted to one input of the AND gate 54, the same signal F as the switching signal A is outputted to the output. At the time of starting the power supply device, the timer 55 operates and the signal F is masked for a predetermined time.

  Thus, in the power supply output voltage monitoring circuit according to the second embodiment, it is possible to identify the state of whether the output voltage is an overvoltage or an undervoltage by the signals E and F. For this reason, when the signal E is output due to overvoltage detection, the control circuit 41 performs control to immediately stop the power supply device. However, when the signal F is output due to undervoltage detection, for example, the undervoltage detection is performed. If the detection is held and the power supply device is stopped when the detection continues for a predetermined time, and the recovery is performed within the predetermined time, the operation can be continued as it is.

  In the above embodiment, the circuit is more complicated than the conventional power supply output voltage monitoring circuit. However, the reduction of installation space in consideration of the withstand voltage by reducing the number of photocouplers PC that are difficult to integrate to one is great, and other electronic circuits can be miniaturized by integration. Has merit. The primary circuit is incorporated in the integrated circuit of the control circuit, and the secondary circuit is incorporated in the integrated circuit of the device that is a load of the power supply device, for example, so that the installation space for the power supply output voltage monitoring circuit excluding the photocoupler PC is installed. Can be substantially eliminated.

  Although this power supply output voltage monitoring circuit is shown as an example applied to a power supply device, the primary circuit and the secondary circuit are insulated, and the voltage level monitoring result of the secondary circuit is transmitted to the primary circuit by a photocoupler. The same applies to other devices.

1 is a circuit diagram showing a power supply output voltage monitoring circuit according to a first embodiment. FIG. It is a figure which shows the operation state of a secondary circuit. It is a time chart which shows operation | movement of a primary circuit. It is a circuit diagram of the power supply device containing the power supply output voltage monitoring circuit which concerns on 2nd Embodiment. It is a time chart which shows operation | movement of the power supply output voltage monitoring circuit which concerns on 2nd Embodiment. It is explanatory drawing of output voltage abnormality, (a) is a figure which defines output voltage abnormality, (b) is a figure which shows the change of the output voltage at the time of starting. It is a circuit diagram which shows the structural example of the conventional power supply output voltage monitoring circuit in a power supply device.

Explanation of symbols

1 Primary circuit 2 Secondary circuit 15 Timer 21 Comparator (for overvoltage detection)
22 Comparator (for undervoltage detection)
41 control circuit 42 switching transistor 55 timer 58 comparator (for overvoltage detection)
59 Comparator (for undervoltage detection)
PC photocoupler

Claims (5)

In a power supply output voltage monitoring circuit for monitoring the output voltage of a power supply device having a primary circuit on the input side and a secondary circuit on the output side that are electrically insulated from each other,
A voltage detection circuit for detecting the output voltage;
A first comparator for comparing a voltage detected by the voltage detection circuit with a first set voltage higher than a specified voltage;
A second comparator for comparing a voltage detected by the voltage detection circuit with a second set voltage lower than the specified voltage;
Connected to the outputs of the first comparator and the second comparator, the light emitting unit is turned on when the output voltage is between the first set voltage and the second set voltage, One photocoupler that turns off the light-emitting unit when one comparator detects an overvoltage higher than the first set voltage or when the second comparator detects an undervoltage lower than the second set voltage; ,
A logic circuit for outputting an alarm signal for immediately stopping the power supply device when the light receiving unit of the photocoupler is not receiving light;
A timer for masking the output of the alarm signal for a predetermined time after starting operation with the activation of the power supply device;
A power supply output voltage monitoring circuit comprising:   The logic circuit receives the output of the light receiving unit of the photocoupler on one input, receives the output of the timer on the other input, and is blocked by the output of the timer for a predetermined time from the start of the power supply device. 2. The power supply output voltage monitoring circuit according to claim 1, further comprising a gate for outputting the alarm signal in accordance with an output of the light receiving unit after a lapse of time.   The logic circuit is reset when the power supply device is activated, and is set when the output voltage is normally output and the light receiving unit receives light, and the timer outputs the same signal as the signal after a predetermined time has elapsed. 3. The power supply output voltage monitoring circuit according to claim 2, further comprising a latch circuit adapted to be applied to the other input of the power supply. In a power supply output voltage monitoring circuit for monitoring the output voltage of a power supply device having a primary circuit on the input side and a secondary circuit on the output side that are electrically insulated from each other,
A voltage detection circuit for detecting the output voltage;
A first comparator for comparing a voltage detected by the voltage detection circuit with a first set voltage higher than a specified voltage;
A second comparator for comparing a voltage detected by the voltage detection circuit with a second set voltage lower than the specified voltage;
One photocoupler having a light emitting portion and a light receiving portion;
Connected in series with the light emitting section of the photocoupler and turned on when the second comparator detects a specified voltage, and the light emitting section is turned on by a pulse after rectification of the secondary circuit. A first transistor that turns off when the second comparator detects an undervoltage lower than the second set voltage to turn off the light emitting unit;
The light emitting unit connected between the output of the smoothing circuit of the secondary circuit and the light emitting unit of the photocoupler and turned on when the first comparator detects an overvoltage higher than the first set voltage. A second transistor for turning on
State determination for determining three states corresponding to the blinking state, the extinguishing state, and the lighting state of the light emitting unit from a switching signal for driving the switching transistor of the primary circuit and an output signal of the light receiving unit of the photocoupler Circuit,
A timer for masking the output corresponding to the extinguishing state for a predetermined time after starting operation with the activation of the power supply device;
A power supply output voltage monitoring circuit comprising: The state determination circuit outputs an overvoltage detection signal synchronized with the switching signal when the light receiving unit outputs the output signal of the lighting state, and the light receiving unit outputs the output signal of the extinguishing state. 5. The power supply output voltage monitoring circuit according to claim 4, wherein the power output voltage monitoring circuit has a function of outputting an undervoltage detection signal synchronized with the switching signal.

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