JP6858692B2 - Low voltage detector - Google Patents

Description

In the present invention, the AC input is converted to DC and a DC voltage is supplied to the load. For a power supply device in which an X capacitor for a noise filter is connected in the power supply line, a low voltage (power failure or voltage drop) of the AC input is applied. The present invention relates to a low voltage detector for detection.

FIG. 6 is a circuit diagram showing a conventional low voltage detection device (see, for example, Patent Document 1). In this conventional low voltage detection device, the full-wave rectified waveform from the rectifier 2 is divided by the resistor R1 and the resistor R2 and input to the comparator 5 in the pulse forming unit 4. The comparator 5 outputs a rectangular wavy pulse having a time width in which the input waveform exceeds the threshold value, using the reference voltage determined by the Zener diode Z1 as a threshold value (see FIG. 7). The microcomputer 7 of the input voltage monitoring means 6 monitors the input pulse, determines that the power supply is cut off when the input pulse is interrupted for 10 ms or more, and determines that the power supply voltage is overvoltage when the time width of the input pulse is equal to or greater than the upper limit value (Max). When the time width of the input pulse is equal to or less than the lower limit value (Min), the power supply voltage is determined to be low (see FIG. 8).

Japanese Unexamined Patent Publication No. 2007-322192

However, in the above-mentioned conventional example, when the load is light load or no load and the amount of current is small, there is a problem that the discharge speed of the X capacitor is slow when the AC input becomes a low voltage. The X capacitor is interposed in the input line together with the choke coil as an element of the line filter for noise reduction. If the AC input is normal and the positive voltage and negative voltage are alternately repeated in a predetermined period in a sinusoidal shape, the X capacitor does not accumulate more charge than necessary, but the low voltage of the AC input is If the alternating current of positive voltage and negative voltage is stopped (or a voltage drop occurs), and the load is light or no load and the amount of current is small, energy is stored in the X capacitor, and that The state is maintained.

That is, the voltage input to the comparator 5 in the pulse forming unit 4 (detection voltage of the AC input) remains high, and the collector 5 cannot output the pulse signal repeating “H” and “L”. A signal that remains high at the "H" level is sent to the microcomputer 7. As a result, the microcomputer 7 cannot detect the low voltage of the AC input with respect to the low voltage of the AC input at the time of light load (no load).

The present invention has been made in view of such circumstances, and the present invention relates to a power supply device in which an X capacitor for a noise filter is connected in the power supply line, and the low voltage detection device for detecting the low voltage of the AC input has a light load. The purpose is to be able to quickly detect the low voltage of the AC input even if the low voltage of the AC input is generated at the time of (no load) and the voltage of the X capacitor remains high.

The present invention solves the above problems by taking the following measures.

The low voltage detection device according to the present invention
The AC input is converted to DC and a DC voltage is supplied to the load. For a power supply device to which an X capacitor for a noise filter is connected in the power supply line, when the low voltage of the AC input is detected, a low voltage detection signal is output. It is a voltage detector
When the rectified voltage of the AC input exceeds a predetermined threshold voltage, the first state is detected and the output of the low voltage detection signal is prohibited, while when the rectified voltage becomes equal to or less than the threshold voltage, the second state is detected and A voltage drop detection circuit that outputs the low voltage detection signal when the second state exceeds a predetermined first threshold time, and a voltage drop detection circuit.
It is provided with a light load state detection circuit that inverts the state of the voltage drop detection circuit and outputs the low voltage detection signal when the first state continues beyond a predetermined second threshold time. It is a feature.

According to the above configuration of the present invention, the following actions are exhibited.

In the voltage drop detection circuit, the rectified voltage of the AC input is compared with a predetermined threshold voltage, and when the rectified voltage exceeds the threshold voltage, the first state is set and the output of the low voltage detection signal is prohibited. When the rectified voltage is equal to or less than the threshold voltage, the second state is set. When the AC input is in a normal state without a power failure or a voltage drop (hereinafter referred to as "low voltage"), the first state and the second state are repeated at a constant cycle.

Then, the voltage drop detection circuit compares the duration of the second state with the predetermined first threshold time. The reaction when a low voltage is generated at the AC input differs depending on whether the load state is heavy load, light load, or no load (hereinafter, referred to as light load (no load)).

In the case of a heavy load, since the current that flows to the load and is consumed is relatively large, the charge is quickly released from the X capacitor due to the generation of a low voltage at the AC input, and the rectified voltage is linked to the base level quickly. Converges to. That is, the rectified voltage becomes equal to or less than the threshold voltage, and the result of the second state is obtained. Needless to say, in the second state, the first state is canceled and the output prohibition state of the low voltage detection signal is released. If the low voltage state of the AC input continues, the duration of the second state in which the rectified voltage is equal to or lower than the threshold voltage exceeds the first threshold time, and the voltage drop detection circuit represents the low voltage of the AC input. Outputs a voltage detection signal.

In the case of a light load (no load), the current flowing to the load is small, so even if a low voltage at the AC input occurs, the charge release from the X capacitor is greatly delayed, and the rectified voltage continues to be in a relatively high voltage state. To do. That is, the state in which the rectified voltage exceeds the threshold voltage continues, and the first state in which the rectified voltage exceeds the threshold voltage continues. As long as the first state continues, the output of the low voltage detection signal is prohibited in the voltage drop detection circuit. The result of the first state is sent to the light load state detection circuit.

On the other hand, in the light load state detection circuit, the result of the first state (the rectified voltage exceeds a predetermined threshold voltage) is input from the voltage drop detection circuit, and the duration of the first state is set to the predetermined second state. When the duration of the first state exceeds the second threshold time as compared with the threshold time, the state of the voltage drop detection circuit, which is in the low voltage detection signal output prohibited state until immediately before that, is inverted and lowered. Output a voltage detection signal. That is, when the AC input becomes a low voltage, even if the charge discharge from the X capacitor is delayed in a light load (no load) state and the rectified voltage tends to remain high, the light load state detection circuit The low voltage detection signal is forcibly output by the function.

As described above, it is possible to quickly detect the low voltage of the AC input in spite of the delay of charge release from the X capacitor, not only under heavy load but also under light load (no load). ..

The low voltage detection device of the present invention having the above configuration has the following preferable modes or changes / modifications.

There is an embodiment in which the second threshold time is set to be equal to or higher than the first threshold time. If the second threshold time is set shorter than the half cycle of the AC input, the switching is repeatedly turned on and off, and as a result, the first threshold time becomes shorter when the AC input becomes a low voltage. It may not be possible to hold the first threshold time for more than half the cycle of the AC input. As a countermeasure, this inconvenience can be avoided by setting the second threshold time to be equal to or higher than the first threshold time.

Further, the voltage drop detection circuit is
A first comparator that compares the rectified voltage at which the AC input is rectified with the predetermined threshold voltage, and generates and outputs a pulse signal according to the comparison result.
A first time constant circuit that inverts the output after waiting for the duration of the second state indicated by the pulse signal output from the first comparator to exceed the predetermined first threshold time.
It has a detection signal output circuit that generates and outputs the low voltage detection signal in response to the inversion operation of the first time constant circuit.
The light load state detection circuit waits for the first state indicated by the pulse signal output from the first comparator to exceed the second threshold time and then inverts the output, thereby causing the first state. There is an embodiment in which the time constant circuit has a second time constant circuit that outputs the low voltage detection signal from the detection signal output circuit by inverting the input state received from the first comparator.

According to this aspect, the following actions are exhibited.

The voltage drop detection circuit inputs the rectified voltage of the AC input in the first comparator and compares it with a predetermined threshold voltage. When the rectified voltage exceeds the threshold voltage, the first state is set (the output of the low voltage detection signal is prohibited), and when the rectified voltage is equal to or less than the threshold voltage, the second state is set. In the normal state where no low voltage is generated at the AC input, the first state and the second state are repeated at a constant cycle, and by this inversion operation, the first comparator alternates between "H" and "L". Generates and outputs a pulse signal that repeats in.

The first time constant circuit in the voltage drop detection circuit waits for the duration of the second state indicated by the pulse signal output from the first comparator to exceed the first threshold time. While the AC input is normal and the pulse signal is continuously output from the first comparator, the second state does not exceed the first threshold time. Therefore, the output state of the first time constant circuit is not inverted. However, when a low voltage is generated at the AC input, the output state of the first time constant circuit changes. This change depends on whether the load condition is heavy load, light load or no load.

In the case of a heavy load, since the current that flows to the load and is consumed is relatively large, the low voltage of the AC input causes the charge to be released from the X capacitor quickly, and the rectified voltage quickly falls below the threshold voltage. The result of the second state is obtained as the output state of the comparator of the above, and the output prohibition state of the low voltage detection signal is released. When the duration of the second state exceeds the first threshold time, the first time constant circuit inverts the output state.

In response to the inversion operation of the first time constant circuit, the detection signal output circuit outputs a low voltage detection signal. As a result, the low voltage of the AC input under heavy load is quickly detected.

On the other hand, in the case of a light load (no load), the current flowing to the load is small, so even if the AC input becomes a low voltage, the charge release from the X capacitor is greatly delayed and the rectified voltage is also in a relatively high voltage state. continue. That is, the state in which the rectified voltage exceeds the threshold voltage continues, and the output state of the first comparator continues in the first state (state in which the output voltage remains high). That is, even though the AC input has a low voltage, the second state does not appear, and therefore the output state of the first time constant circuit is not inverted. That is, the detection signal output circuit does not work and the output of the low voltage detection signal remains prohibited (assuming there is no light load detection circuit).

However, in the case of the present embodiment, a light load state detection circuit having a second time constant circuit is provided. In the light load state detection circuit, the second time constant circuit that inputs the result of the first state (state in which the output voltage remains high) from the first comparator of the voltage drop detection circuit determines the duration of the first state. Compare with the second threshold time. Then, when the duration of the first state exceeds the second threshold time, the low voltage detection signal is output via the inversion operation of the first time constant circuit and the detection signal output circuit. That is, when the AC input becomes a low voltage, even if the charge discharge from the X capacitor is delayed in a light load (no load) state and the rectified voltage tends to remain high, the light load state detection circuit is used. The low voltage detection signal is forcibly output by the function of the second time constant circuit.

As described above, it is possible to quickly detect the low voltage of the AC input in spite of the delay of charge release from the X capacitor, not only under heavy load but also under light load (no load). ..

According to the present invention, it is possible to quickly detect the low voltage of the AC input in spite of the delay of charge release from the X capacitor, not only under heavy load but also under light load (no load). it can.

A circuit diagram showing a configuration of a low voltage detection device according to an embodiment of the present invention. Waveform diagram showing the operation of the low voltage detection device in the embodiment of the present invention under heavy load. Waveform diagram showing the operation of the low voltage detection device in the embodiment of the present invention under a light load (no load). Waveform diagram showing the operation of another mode of the low voltage detection device in the embodiment of the present invention when the load is light (no load). A circuit diagram showing a configuration of a low voltage detection device according to another embodiment of the present invention. A circuit diagram showing the configuration of a conventional low voltage detection device Explanatory diagram of input / output waveform of comparator before and after power off of conventional low voltage detection device Explanatory drawing of the input / output waveform of the comparator when the power supply voltage of the conventional low voltage detection device fluctuates.

Hereinafter, embodiments of the low voltage detection device of the present invention having the above configuration will be described in detail at the level of specific examples.

FIG. 1 is a circuit diagram showing a configuration of a low voltage detection device according to an embodiment of the present invention. In FIG. 1, 8 is a power supply device that generates a DC output from an AC input and supplies it to a load 9, 10 is a low voltage detection device as an object of invention, 20 is a voltage drop detection circuit, 21 is a comparison circuit, and 22 is a first. 23 is a detection signal output circuit, 30 is a light load state detection circuit, and 31 is a second time constant circuit.

The low voltage detection device 10 is for detecting a low voltage (power failure or voltage drop) of the AC input of the power supply device 8. The low voltage detection device 10 includes a voltage drop detection circuit 20 and a light load state detection circuit 30. The voltage drop detection circuit 20 includes a comparison circuit 21, a first time constant circuit 22, and a detection signal output circuit 23. The light load state detection circuit 30 has a second time constant circuit 31.

8a as a component of the power supply device 8 is an AC input unit, 8b is a choke coil for noise reduction composed of a first coil L1 and a second coil L2, C11 is an X capacitor for noise reduction, and 8c is a diode bridge. It is a rectifying smoothing circuit composed of a DB and a smoothing capacitor (electrolytic capacitor) C12.

First, the voltage drop detection circuit 20 will be described.

The comparison circuit 21 in the voltage drop detection circuit 20 includes a first comparator U1, backflow prevention diodes D5 and D6, and resistance elements R21, R22, R23, R4, R5 and R6. The anode of the backflow prevention diode D5 is connected to the first line (Live line, L line) 8d of the power supply device 8, and the anode of the backflow prevention diode D6 is the second line (neutral line, N line). ) It is connected to 8e. The cathodes of the backflow prevention diodes D5 and D6 are connected to each other, and the series circuit of the resistance elements R21 and R22 is connected between the connection point and the ground GND. The positive electrode and negative electrode power supply terminals (VCC, VEE) of the first comparator U1 are connected to the positive electrode terminal and the negative electrode terminal of the DC power supply V1, respectively. The connection points of the resistance elements R21 and R22 are connected to the non-inverting input terminal (+) of the first comparator U1 via the resistance element R5. The connection points of the resistor elements R23 and R4 connected in series are connected to the inverting input terminal (−) of the first comparator U1. One end of the resistance element R23 on the high potential side is connected to the positive electrode terminal of the DC power supply V1, and one end of the resistance element R4 on the low potential side is connected to the negative electrode terminal of the DC power supply V1, that is, the ground GND. A resistance element R6 is connected between the output terminal of the first comparator U1 and the non-inverting input terminal (+).

The first time constant circuit 22 in the voltage drop detection circuit 20 includes a switching transistor Q1, a capacitor C3 for charging / discharging used for time measurement, backflow prevention diodes D7, D8, and resistance elements R7, R8, R9, R10, R11. I have. A series circuit of the resistance element R10 and the switching transistor Q1 is connected between both terminals of the DC power supply V1. The switching transistor Q1 is composed of an NPN type bipolar transistor. As a bias of the switching transistor Q1, a series circuit of the resistance elements R7, R8, and R9 is connected between both terminals of the DC power supply V1, and the connection points of the resistance elements R8 and R9 are connected to the base of the switching transistor Q1. The connection points of the resistance elements R7 and R8 are connected to the anode of the backflow prevention diode D7, and the cathode thereof is connected to the output terminal of the first comparator U1 in the comparison circuit 21. A series circuit of a backflow prevention diode D8 and a charging / discharging capacitor C3 is connected between the collector and the emitter of the switching transistor Q1. A resistance element R11 is connected between both terminals of the backflow prevention diode D8.

The detection signal output circuit 23 in the voltage drop detection circuit 20 includes a second comparator U2, a photocoupler PC, and resistance elements R12, R13, R14, R15, and R16. The positive electrode and negative electrode power supply terminals (VCC, VEE) of the second comparator U2 are connected to the positive electrode terminal and the negative electrode terminal of the DC power supply V1, respectively. The series circuit of the resistance elements R12 and R13 is connected to the positive electrode terminal and the negative electrode terminal of the DC power supply V1. The connection points of the resistance elements R12 and R13 are connected to the non-inverting input terminal (+) of the second comparator U2 via the resistance element R14. The positive electrode terminal of the charging / discharging capacitor C3 in the first time constant circuit 22 is connected to the inverting input terminal (−) of the second comparator U2. A resistance element R15 is connected between the output terminal of the second comparator U2 and the non-inverting input terminal (+). A series circuit of the resistance element R16 and the light emitting diode LD in the photocoupler PC is connected between the positive electrode terminal of the DC power supply V1 and the output terminal of the second comparator U2. The phototransistor PT in the photocoupler PC is an open collector, and its emitter is connected to the negative electrode terminal of the DC power supply V1. The collector of the phototransistor PT is a terminal that outputs a low voltage detection signal Sd, and is connected to a control unit such as a microcomputer (not shown).

Next, the light load state detection circuit 30 will be described. The light load state detection circuit 30 is composed of a second time constant circuit 31. The second time constant circuit 31 includes a switching transistor Q2, a charging / discharging capacitor C4 used for time measurement, a Zener diode ZD1, a backflow prevention diode D9, and resistance elements R17 and R18.

The collector of the switching transistor Q2 is connected to the connection points of the resistance elements R7 and R8 and the backflow prevention diode D7 in the first time constant circuit 22, and its emitter is connected to the ground GND. A series circuit of the resistance element R17 and the charging / discharging capacitor C4 is connected between both terminals of the DC power supply V1. The positive electrode terminal of the charging / discharging capacitor C4 is connected to the base of the switching transistor Q2 via the Zener diode ZD1. The anode of the Zener diode ZD1 is connected to the base of the switching transistor Q2, and the cathode thereof is connected to the positive electrode terminal of the capacitor C4 for charging / discharging. A resistance element R18 is connected between the base and the emitter of the switching transistor Q2. A backflow prevention diode D9 is connected between the positive electrode terminal of the charging / discharging capacitor C4 and the output terminal of the first comparator U1 in the comparison circuit 21.

Next, the operation of the low voltage detection device 10 configured as described above will be described.

(1) Operation under heavy load First, the operation under heavy load will be described with reference to the waveform diagram of FIG.

The AC input detection voltage Va in which a sine wave is rectified is input to the connection points of the resistance elements R21 and R22. _{When the comparator input voltage V U1} substantially proportional to the AC input detection voltage Va _{exceeds the reference voltage V ref1} applied to the inverting input terminal (−), the first comparator U1 becomes “H”. Output a "level signal". On the other hand, when the comparator input voltage V _U1 becomes a state (second state) equal to or lower than the reference voltage V _ref1 , the first comparator U1 outputs an “L” level signal. Since this operation is repeated, the time width of the rectangular wavy pulse appearing at the output terminal of the first comparator U1 corresponds to the time when _{the comparator input voltage V U1} exceeds the reference voltage V _{ref 1. SU1} is a pulse signal output by the first comparator U1.

When the comparator input voltage V _U1 is in the first state exceeding the reference voltage V _{ref1 and the pulse signal S U1} output by the first comparator U1 is at the “H” level, the node N1 (backflow prevention diode D7 and resistor) The potential of the elements R7 and R8) is at the "H" level, the switching transistor Q1 is in a conductive state, and the potential V _{Q1 of} the node N2 (the connection point between the collector of the switching transistor Q1 and the resistance element R10) is ". At the "L" level, the output voltage of the second comparator U2 is at the "H" level. Therefore, the photocoupler PC is in an inactive state, and the low voltage detection signal Sd appearing in the collector of the phototransistor PT is at the “H” level, which means that the AC input is not in a power failure (low voltage) state but in a normal state. Is shown. Since the low voltage detection signal Sd is a negative logic low active signal, its effective output of the low voltage detection signal Sd is prohibited at the “H” level.

When the switching transistor Q1 is in a conductive state, the charging / discharging capacitor C3 is discharged, and the voltage of the positive electrode terminal thereof becomes the “L” level.

When the comparator input voltage V _U1 becomes equal to or less than the reference voltage V _ref1 and the output voltage of the first comparator U1 reaches the “L” level, the switching transistor Q1 becomes non-conducting at the “L” level of the node N1. Then, the current flowing through the path of the resistance element R10 and the switching transistor Q1 in the conductive state is switched to the path of the resistance element R10, the diode D8, and the charging / discharging capacitor C3, and goes to the charging / discharging capacitor C3. Charging is done. _{Along with this, the potential V Q1 of} the node N2 (collector of the switching transistor Q1), _{that is, the comparator input voltage V U2} applied to the inverting input terminal (−) of the second comparator U2 gradually increases.

However, since the AC input is normal, the switching transistor Q1 is inverted and becomes conductive by the time _{the comparator input voltage V U2} reaches the reference voltage V _ref2. That is, once the comparator input voltage V _U1 drops to the ground level, it starts rising again, _{and when it exceeds the reference voltage V ref1} , the switching transistor Q1 turns on again. When the switching transistor Q1 is turned on, the charging / discharging capacitor C3 starts discharging. That is, the charge charge of the charging / discharging capacitor C3 is rapidly discharged via the resistance element R11 and the switching transistor Q1. As a result, the input voltage V _U2 of the second comparator U2 drops rapidly. As a result, the comparator input voltage V _U2 never reaches the reference voltage V _{ref 2.} That is, this is because the AC input is normal. In this case, without input voltage V _U2 of the second comparator U2 exceeds the reference voltage V _ref2, the low voltage detection signal Sd of low-active (negative logic) is maintained at the "H" level state of inactive.

The capacitance value of the charging / discharging capacitor C3 and the resistance values of the resistance elements R10 and R11 are determined so that the charging / discharging capacitor C3 is rapidly discharged when the switching transistor Q1 is turned on. (R11 <R10).

In the charging / discharging capacitor C4 of the second time constant circuit 31 constituting the light load state detection circuit 30, the first comparator is similar to the charging / discharging capacitor C3 of the first time constant circuit 22. Since charging and discharging are alternately performed by repeating short-period alternating “H” and “L” of the pulse signal S _{U1 output by U1, the switching transistor Q2 turns on as long as the AC input is normal.} There is no such thing.

Next, the operation when the AC input becomes a low voltage in the heavy load state will be described.

In the case of a heavy load, since the current consumed is large, the low voltage of the AC input causes the charge to be released from the X capacitor C11 quickly, and the rectified voltage is linked and quickly converges to the base level. That is, the rectified voltage becomes equal to or less than the threshold voltage, and that state (second state) continues. Needless to say, in the second state, the first state is canceled and the output prohibition state of the low voltage detection signal Sd is canceled.

When the low voltage of the AC input is generated, the _{state in which the input voltage V U1} of the first comparator U1 remains lowered to the base level, that is, the second state continues. As a result, the process in which the charging voltage of the charging / discharging capacitor C3 rises exponentially becomes long, and finally the input voltage V _U2 applied to the inverting input terminal (-) of the second comparator U2 becomes a predetermined reference. The voltage exceeds _{V ref2.} The reference voltage V _ref2 is from the timing when the potential of the charging / discharging capacitor C3 and the input voltage V _U2 of the second comparator U2 starts to rise immediately before the low voltage of the AC input to exceed the _{reference voltage V ref2.} Corresponds to the predetermined time T1 (first threshold time) of.

The first threshold time T1 is a elapsed time based on the time constants of the resistance element R10, the backflow prevention diode D9, and the charge / discharge capacitor C3 from the timing when _{the pulse signal S U1 output by the first comparator U1 starts to fall.} Set by time. Normally, since the momentary interruption time needs to be maintained at least half a cycle of the AC input cycle or more, the first threshold time T1 is set to half a cycle or more of the AC input cycle (10 ms or more in this embodiment).

When the comparator input voltage V _U2 exceeds the reference voltage V _ref2 , the output voltage of the second comparator U2 is inverted from the previous “H” level to the “L” level, and the photocoupler PC operates to operate the phototransistor PT. The low-active low-voltage detection signal Sd is output from, and is used for the control operation corresponding to the low voltage of the AC input. That is, it is possible to quickly detect the low voltage of the AC input at the time of heavy load.

In the state where the pulse signal S _U1 output by the first comparator U1 repeats "H" and "L" and the state where the "L" level is continued, the second light load state detection circuit 30 is used. The charging / discharging capacitor C4 of the time constant circuit 31 repeats charging and discharging, but the charging voltage does not rise to a level sufficient to turn on the switching transistor Q2.

(2) Operation at Light Load (No Load) Next, the operation at light load (no load) will be described with reference to the waveform diagram of FIG.

The operation when the AC input is normal is the same as the case of (1) above. Therefore, here, the operation when the AC input becomes a low voltage will be mainly described.

When a low voltage of the AC input is generated at the time of light load (no load), unlike the case of heavy load, the electric charge (energy) stored in the X capacitor C11 such as the noise filter inserted in the AC input line is quickly released. Cannot be released to. _{Therefore, the input voltage V U1} to the non-inverting input terminal (+) of the first comparator U1 stays high, and as a result, the switching transistor Q1 continues in the conductive state, and the input voltage to the second comparator U2. The V _U2 continues at the "L" level and the output voltage of the second comparator U2 continues at the "H" level.

Therefore, without the light load state detection circuit 30 added in this embodiment, the low voltage detection signal Sd maintains an inactive "H" level state. That is, even though the low voltage of the AC input is generated, the situation cannot be detected. That is, the output of the low voltage detection signal Sd will continue to be prohibited.

The light load state detection circuit 30 solves this inconvenience. Hereinafter, the operation of the light load state detection circuit 30 will be described.

In the state where the AC input is normal, the pulse signal S _U1 output by the first comparator U1 repeats "H" level and "L" level alternately in time, and this pulse signal S _U1 is "H". During the level period, there is no discharge from the charging / discharging capacitor C4, the DC power supply V1 charges the charging / discharging capacitor C4 via the resistance element R17, and the pulse signal _SU1 is at the “L” level. Then, the charge charge of the charging / discharging capacitor C4 is discharged to the output terminal (“L” level) of the first comparator U1 via the backflow prevention diode D9. That is, as described above, in the _{state where the pulse signal S U1} output by the first comparator U1 repeats “H” and “L”, the charging / discharging capacitor C4 is sufficient to turn on the switching transistor Q2. It will not be charged to just the level. Therefore, as it is, the potential of the node N1 remains high, and the potential of the charging / discharging capacitor C3 does not rise while the switching transistor Q1 remains conductive. That is, the output of the valid low voltage detection signal Sd does not occur (the low voltage detection signal Sd is maintained in the invalid "H" level state).

However, when a low voltage is generated at the AC input and the pulse signal S _U1 output by the first comparator U1 is maintained at the “H” level, the second time constant circuit 31 of the light load state detection circuit 30 The intermittent discharge from the charging / discharging capacitor C4 disappears, and the voltage V _C4 of the charging / discharging capacitor C4 starts to rise under a constant time constant. When the voltage V _C4 of the positive terminal of the capacitor C4 for the charge and discharge exceeds a predetermined reference voltage V _ref3, the zener diode ZD1 becomes conductive, the switching transistor Q2 is rendered conductive by the inverted node until it was in the non-conducting state The potential of N1 is lowered to the "L" level. Here, the predetermined reference voltage V _ref3 is the total voltage of the Zener voltage of the Zener diode ZD1 and the base-emitter voltage V _{BE of the switching transistor Q2.} The predetermined reference voltage V _ref3 is a predetermined time T2 (second threshold value) from the timing when _{the voltage V C4} of the charging / discharging capacitor C4 starts to rise immediately before the low voltage of the AC input until the switching transistor Q2 turns on. Time) is supported.

The second threshold time T2 is set by the elapsed time based on the time constant of the resistance element R17 and the charging / discharging capacitor C4 from the timing when _{the pulse signal S U1 output by the first comparator U1 starts to rise.} The second threshold time T2 is preferably set to the first threshold time T1 or higher. As shown in FIG. 4, when the second threshold time T2 is set shorter than the half cycle (TB or TA) of the AC input, the switching transistor Q2 repeatedly turns on and off (see the broken line of the Q2 waveform). When the switching transistor Q2 is turned on and the switching transistor Q1 is turned off, a voltage is applied to the inverting input terminal (−) of the second comparator U2, and the reference voltage V _ref2 is easily reached. As a result, when the AC input becomes a low voltage, the first threshold time T1 becomes short, and there is a possibility that the first threshold time T1 cannot be held for more than half the cycle of the AC input. Therefore, as described above, it is preferable to set the second threshold time T2 to be equal to or higher than the first threshold time T1.

In this way, when the voltage V _{C4 of} the charging / discharging capacitor C4 exceeds the reference voltage V _ref3 , the Zener diode ZD1 conducts, and the switching transistor Q2 conducts, the voltage drop detection circuit 20 in the voltage drop detection circuit 20 interlocks with this. The switching transistor Q1 of the time constant circuit 22 of 1 turns off, and the potential V _{Q1 of} the node N2 (collector of the switching transistor Q1) and thus the voltage of the charging / discharging capacitor C3 start to rise. When a predetermined time T1 elapses after the switching transistor Q2 is turned on and the collector potential V _Q1 of the switching transistor Q1 exceeds the reference voltage V _ref2 and the output voltage of the second comparator U2 reaches the “L” level, the photo The coupler PC operates, the phototransistor PT conducts, a low-active low-voltage detection signal Sd is output, and the AC input is subjected to a control operation corresponding to the low voltage.

As described above, at the time of low voltage of AC input at the time of light load (no load), the light load state detection circuit 30 (second time constant circuit 31) despite the delay of charge release from the X capacitor C11. With this function, the low voltage of the AC input can be detected quickly.

In the above embodiment, the voltage introduced into the non-inverting input terminal (+) of the first comparator U1 of the comparison circuit 21 in order to determine the change in the voltage level of the AC input is applied to the first line of the power supply device 8. Although it is taken in from the 8d and the second line 8e via the backflow prevention diodes D5 and D6, it is not necessarily limited to that, and it may be taken in from the output terminal of the diode bridge DB.

Further, in the above embodiment, as a circuit for supplying the reference voltage Vref1 to the inverting input terminal (-) of the first comparator U1, a series circuit of the resistance elements R23 and R4 is connected between both terminals of the DC power supply V1. However, the present invention is not necessarily limited to that, and a Zener diode may be used instead of the resistance element R4.

_{Similarly, as a circuit for supplying the reference voltage V ref2} to the inverting input terminal (-) of the second comparator U2, a series circuit of the resistance elements R12 and R13 is connected between both terminals of the DC power supply V1. It is not always necessary to limit it, and a Zener diode may be used instead of the resistance element R13.

Further, in the above embodiment, the switching transistor Q2 is used as the switching element used for time measurement in the second time constant circuit 31 of the light load state detection circuit 30, but it is not necessarily limited to that, and a comparator is used. You may. Further, the first comparator U1 and the second comparator U2 may be composed of switching transistors.

Further, in the above embodiment, the period of the low voltage detection signal Sd includes the time obtained by adding the second threshold time T2 and the first threshold time T1 after the AC input becomes low voltage. On the other hand, as shown in FIG. 5, the collector of the switching transistor Q2 may be connected to the output terminal of the second comparator U2. With this configuration, the period of the low voltage detection signal Sd can include only the second threshold time T2, and the low voltage of the AC input at the time of light load (no load) is quickly detected. can do.

The present invention relates to a low voltage detection device that detects a low voltage (power failure or voltage drop) of the AC input for a power supply device in which an X capacitor for a noise filter is connected in the power supply line, and the AC input at a light load (no load). Even if the low voltage of the AC input is generated and the discharge from the X capacitor is delayed, it is useful as a technique for quickly detecting the low voltage of the AC input as expected.

8 Power supply device 10 Low voltage detection device 20 Voltage drop detection circuit 22 First time constant circuit 23 Detection signal output circuit 30 Light load state detection circuit 31 Second time constant circuit C11 X capacitor Sd Low voltage detection signal U1 First comparator

Claims (3)

The AC input is converted to DC and a DC voltage is supplied to the load. For a power supply device to which an X capacitor for a noise filter is connected in the power supply line, when the low voltage of the AC input is detected, a low voltage detection signal is output. It is a voltage detector
When the rectified voltage of the AC input exceeds a predetermined threshold voltage, the first state is detected and the output of the low voltage detection signal is prohibited, while when the rectified voltage becomes equal to or less than the threshold voltage, the second state is detected and A voltage drop detection circuit that outputs the low voltage detection signal when the second state exceeds a predetermined first threshold time, and a voltage drop detection circuit.
It is provided with a light load state detection circuit that inverts the state of the voltage drop detection circuit and outputs the low voltage detection signal when the first state continues beyond a predetermined second threshold time. A featured low voltage detector. The low voltage detection device according to claim 1, wherein the second threshold time is set to be equal to or longer than the first threshold time. The voltage drop detection circuit
A first comparator that compares the rectified voltage at which the AC input is rectified with the predetermined threshold voltage, and generates and outputs a pulse signal according to the comparison result.
A first time constant circuit that inverts the output after waiting for the duration of the second state indicated by the pulse signal output from the first comparator to exceed the predetermined first threshold time.
It has a detection signal output circuit that generates and outputs the low voltage detection signal in response to the inversion operation of the first time constant circuit.
The light load state detection circuit waits for the first state indicated by the pulse signal output from the first comparator to exceed the second threshold time and then inverts the output, thereby causing the first state. Claim 1 or claim having a second time constant circuit that outputs the low voltage detection signal from the detection signal output circuit by inverting the input state that the time constant circuit receives from the first comparator. 2. The low voltage detection device according to 2.

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