JP2002325361A - Power source circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power source circuit which electronic devices can initiate safely and rapidly, using effectively residual charges of a capacitive element used as an auxiliary power source for the power source circuit of the electronic devices. SOLUTION: A power source circuit 100 comprises a power source 5, a capacitive element 8, SW1 provided between the power source 5 and the capacitive element 8, SW2 provided between the capacitive 8 and load, a detector means (A/D converter 11) detecting voltage of the power source 5, and control means (a microprocessor 1 and a DC/DC converter 7) for controlling the on/off of SW1, SW2, based on power source voltage detected and turns off the switches SW1, SW2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit for suppressing electric charge leakage from a capacitive element used as an auxiliary power supply in a power supply circuit of an electronic device.

[0002]

2. Description of the Related Art Recently, a power supply circuit of an electronic device is connected to a capacitive element as an auxiliary power supply in addition to a battery as a main power supply, and the capacitive element is charged by a battery. The electronic device is configured to supply power to the electronic device by using the charged charges charged in the capacitive element.

An example of this type of power supply is disclosed in Japanese Patent Laid-Open No. 11-2
No. 52807 discloses this. JP-A-11-25
The power supply device disclosed in Japanese Patent No. 2807 discloses a DC current source, switching means for switching a current output from the DC current source, smoothing the current switched by the switching means, and converting the smoothed current. Output means for supplying to the charging target, and controlling the switching means to keep the average current output from the output means constant based on a voltage difference between the voltage generated from the DC current source and the voltage of the charging target. Control means for performing the control.

[0004] The power supply device disclosed in Japanese Patent Application Laid-Open No. H11-252807 discloses a direct current source, switching means for switching a current output from the direct current source, and a current switched by the switching means. Output means for smoothing and supplying the smoothed current to a charging target and a capacitive element receiving power from the output means are connected via a circuit.

As the DC power source, a dry battery such as a lithium battery is used, and since the charge of this battery is consumed and decreases, when the charge is consumed, the battery is re-used. It needs to be charged or replaced with a new battery.

When the battery is recharged or replaced with a new battery, the battery is removed from the electronic device, and the battery and the power supply circuit of the electronic device are electrically disconnected.

On the other hand, even after the battery is removed, the electric charge remains in the capacitive element, and the electric charge leaks out, causing malfunction of the electronic device, which may cause an accident.

In order to solve this problem, it is conceivable to provide a dedicated discharge circuit for discharging the charge remaining in the capacitive element. However, in such a configuration, when a new or fully charged battery is connected, the capacitive element is first charged by the battery, so that the capacity of the battery is wastefully consumed by this charged charge. I will. In addition, since the capacitive element has a large electric capacity and requires a relatively long time for charging, there is a problem that the startup time until the device becomes usable after battery replacement becomes long.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a power supply circuit having a capacitive element used as an auxiliary power supply in a power supply circuit of an electronic device. The purpose is to obtain a power supply circuit that can be used.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a power supply circuit that suppresses charge leakage from a capacitive element used as an auxiliary power supply in a power supply circuit of an electronic device and shortens a startup time after battery replacement. And

[0011]

SUMMARY OF THE INVENTION In order to achieve the above object, a power supply circuit according to the present invention has a power storage element functioning as an auxiliary power supply and supplies power to a load connected to the capacitive element. A first switch provided between the power supply and the capacitive element, a second switch provided between the capacitive element and the load, and a voltage of the power supply. Detecting means for detecting, and control means for performing on / off control of the two switch means based on a detection signal from the detecting means, the control means,
When the two switch means are turned on to charge the capacitive element and supply power to the load, and the detecting means detects that the voltage of the power supply has become equal to or less than a first predetermined voltage value. Is characterized in that the two switch means are turned off to electrically insulate the capacitive element from the power supply and the load.

Further, the detecting means detects the voltage of the capacitive element in addition to the voltage of the power supply, and the control means detects a voltage of the capacitive element.
Turning on the first switch means, turning on the second switch means when the voltage of the capacitive element becomes equal to or higher than a second predetermined voltage value by starting charging of the capacitive element, and If the power supply voltage falls below a first predetermined value after turning on the second switch means,
Is desirably turned off.

[0013] The first and second switch means may be an MO.
It is preferable that the parasitic diodes are composed of SFETs and connected so that the forward direction of each parasitic diode is directed toward the capacitive element.

In addition, a third switch means is provided between the first switch means and the capacitive element, and the control means performs switching control of the third switch means to intermittently charge the capacitive element. It is desirable to do.

It is preferable that the capacitive element is an electric double layer capacitor.

[0016] A current limiting element for preventing an overcurrent from the power supply to the capacitive element may be provided.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a main configuration of a power supply circuit according to an embodiment of the present invention.

In FIG. 1, a power supply circuit 100 includes a power supply 5.
And loads 3 and 4. The power supply 5 is a main power supply, for example, a primary battery or a secondary battery such as a lithium battery, a DC power supply, or the like.

The power supply circuit 100 has a function of supplying electric power to the load 3 by the switching operation of the DC / DC converter 6 and a function of supplying electric power to the load 4 with the charge of the capacitive element 8. Here, the switching operation of the DC / DC converter 6 means that the DC / DC converter 6 is continuously turned on / off. The load 2 is an LED (light emitting diode for displaying a power supply low voltage warning), and the load 2 is supplied with power from a backup power supply 13.

The DC / DC converter 6 includes a backflow prevention element 9
The power supply 5 is not connected to the power supply 5 through the power supply 5 and has no capacitance.

The capacitive element 8 includes a backflow preventing element 9, a switch (first switch means) SW1, and a current limiting element 1.
0, connected to the power source 5 via a switch (third switch means) SW3, and further connected to a switch (second switch means)
Connected to load 4 via SW2. The switch (third switch means) SW3 is controlled by the DC / DC controller 7, and the DC / DC controller 7, the switches SW1, S
W2 is controlled by the microcomputer 1. Note that an electric double layer capacitor is used as the capacitive element 8.

The microcomputer 1 controls the operations of the power supply circuit 100, such as operation control of the loads 2, 3 and 4, control of the DC / DC converter 6 and DC / DC controller 7, and control of the switches SW1 and SW2. It is. An A / D converter 11 for monitoring the voltage of the power supply 5 and an A / D converter 12 for monitoring the terminal voltage of the capacitive element 8 are connected to the microcomputer 1. Further, the microcomputer 1 is connected to a main switch 14 and a backup power supply 13 that are turned on when the electronic device is used, and the electronic device is in a standby state (main switch 1).
Even when the power supply 5 is unplugged, or when the power supply 5 is disconnected, power is supplied from the backup power supply 13 and the power supply 5 is maintained in an operable state at all times. The DC / DC controller 7 controls the switch SW under the control of the microcomputer 1.
3 is turned on and off, and the capacitive element 8 is charged by the current supplied from the power supply 5 via the current limiting element 10.

The A / D converter 11 constitutes detecting means for detecting the voltage of the power supply 5, and the A / D converter 12 constitutes detecting means for detecting the voltage of the capacitive element 8. In addition, two A / D converters 11 and A / D converters 12 are installed, but one A / D converter is used in a time-divided manner, and the voltage of the power supply 5 is controlled by one A / D converter. And the voltage of the capacitive element 8 may be switched for detection.

The microcomputer 1 and the DC / DC controller 7 provide control means for performing on / off control of the two switches SW1, SW2 and SW3 based on detection signals from the A / D converter 11 and the A / D converter 12. Make up. The microcomputer 1 turns on the two switches SW1 and SW2 to charge the capacitive element 8 and supply electric power to the loads 3 and 4, and the A / D converter 11 switches the voltage of the power supply 5 to the cutoff voltage ( When it is detected that the voltage has become equal to or less than the first predetermined voltage value, the two switches SW1 and SW2 are turned off to electrically connect the capacitive element 8 with the power supply 5 and the loads 2, 3, and 4. Insulation. Here, the end voltage means a minimum voltage value for supplying power to the load side.

The microcomputer 1 turns on the switch SW1 based on the detection signal of the A / D converter 12 to start charging the capacitive element 8, and the voltage of the capacitive element 8 is equal to or higher than a predetermined threshold. Is turned on, the switch SW2 is turned on.
The microcomputer 1 also causes the DC / DC controller 7 to perform switching control of the switch SW3, and
Charge intermittently. Here, the switching control of the switch SW3 by the DC / DC controller 7 refers to DC / DC
Switch SW3 is continuously turned ON / OFF by controller 7.
It means to operate. Here, the threshold value is set to the minimum operation voltage of the load 4 (> end voltage).

The microcomputer 1 further includes a switch SW2
If the voltage of the power supply 5 becomes equal to or lower than the end voltage after turning on the power, control is performed to turn off the two switches SW1 and SW2.

The operation of the power supply circuit 100 shown in FIG. 1 will be described with reference to FIG. FIG. 2 is a control operation flowchart of the microcomputer 1 in the power supply circuit 100. In step S1, it is determined whether or not the main switch 14 is ON and whether or not the restart is being performed. Here, restart is
When the 0FF processing is performed electrically due to the detection of the cutoff voltage or the absence of the input of the A / D converter 11, the main switch 14 is turned off once because the main switch 14 is mechanically kept at the ON position. And then turn it on again.

In step S2, the main switch 1
When the microcomputer 4 is detected to be ON, the microcomputer 1
Reads the output value of the A / D converter 11 and detects the power supply voltage of the power supply 5 (step S3).

In step S3, the microcomputer 1
Determines the relationship of the output of the A / D converter 11 ≦ the cut-off voltage. If the output of the A / D converter 11 is larger than the cut-off voltage (first predetermined value), the process proceeds to step S5.

If the output of the A / D converter 11 is equal to or lower than the final voltage, it is determined whether or not the output of the A / D converter 11 is greater than 0 V. If the output of the A / D converter 11 is larger than 0 V, The microcomputer 1 judges that the power supply 5 is loaded,
(Load 2) flashes to warn that the power supply 5 is at a low voltage, and the process proceeds to step S4. On the other hand, when the output of the A / D converter 11 is 0 V, the microcomputer 1 determines that the power supply 5 has been disconnected, skips the low-voltage warning display process, and proceeds to step S4. The / D converter 11 is maintained in the OFF state, and enters a standby state in which it operates in a so-called low power consumption mode.

In step S3, the microcomputer 1
When it is determined that the output of the / D converter 11 is higher than the end voltage (step S3: YES), the process proceeds to step S5.

In step S5, the microcomputer 1 confirms that the power is turned on, and shifts from the low power consumption mode to the normal operation mode. At this time, if the low-voltage warning display process is performed by blinking the LED (load 2),
This low voltage warning display processing is stopped. Next, the switch SW1 on the power input side of the capacitive element 8 is turned on, and the DC / D
The switching operation of the switch SW3 by the C converter 6 and the DC / DC controller 7 is started (step S6). When the DC / DC converter 6 is activated, power is supplied to the load 3 in FIG.

The microcomputer 1 continuously operates the A / D converter 11
To determine whether the detected voltage has reached a predetermined end voltage, that is, a minimum voltage value for supplying power to the load including the microcomputer 1 (step S7).

If the power supply voltage of the power supply 5 has reached the end voltage, the operation of each unit is terminated (step S13).

On the other hand, if it is determined that the power supply voltage is higher than the cut-off voltage, the process proceeds to step S8.

In this case, the microcomputer 1 includes the A / D converter 12
Is turned on to check the charging voltage level of the capacitive element 8 (step S8). The A / D converter 12 outputs the charging voltage of the capacitive element 8 as a detection signal, and the microcomputer 1 reads the output value of the A / D converter 12 and
When the charging voltage of the capacitive element 8 is a predetermined threshold (a second predetermined value)
Is detected (step S8).

In step S9, the microcomputer 1 determines that the charging voltage level of the capacitive element 8 is equal to the threshold value (second predetermined value).
If not, the charging of the capacitive element 8 is continued (step S8: NO). The threshold is set to the minimum operation voltage of the load 4 (> the end voltage). Until the threshold is reached, the switch SW2 is off, and the power supply 5 and the capacitive element 8 are not connected to the load 4.

In step S9, when the microcomputer 1 determines that the charge level of the capacitive element 8 has reached the threshold value, that is, the minimum operation voltage of the load 4, the microcomputer 1 turns on the switch SW2 on the power output side of the capacitive element 8. , Power supply 5 and capacitive element 8
Is started to the load 4 in FIG. 1 (step S10). As a result, power is supplied to all the loads 3 and 4, and the operation of the electronic device in a normal use state is performed (step S11).

The microcomputer 1 detects whether the main switch 14 of the microcomputer 1 has been turned off (step S12).

The microcomputer 1 has a main switch 1
Steps S7 to S12 are repeatedly executed as long as the switch 4 is maintained in the ON state. When the main switch 14 is turned off, the switches SW1, SW2, DC
/ DC converter 6 and DC / DC controller 7, A
/ D converters 11 and 12 are switched to an OFF state,
4 is completely stopped (step 13).
When the switches SW1 and SW2 are turned off, the capacitive element 8
Is insulated from other circuits, is prevented from flowing out to other circuits, and holds the charge in the capacitive element 8.

Next, during use of the electronic device (steps S7 to S7).
The case where the power supply 5 is taken out during the processing of repeating S12) will be described. When the power supply 5 is taken out, the output of the A / D converter 11 becomes the final voltage value (first predetermined value).
Therefore, as in the case where the voltage of the power supply 5 reaches the cutoff voltage in normal use, the process proceeds from step S7 to step S13, and the switches SW1 and SW2 are turned off, so that the capacitance is reduced. The element 8 is insulated from other circuits, is prevented from flowing out to other circuits, and retains the charge in the capacitive element 8.

FIG. 3 is a block diagram showing a main configuration of another embodiment of the power supply circuit 200 according to the embodiment of the present invention. 3, the same reference numerals are given to the same components as those of the power supply circuit 100 of the embodiment shown in FIG. A diode as the backflow prevention element 9 and a limiting resistor as the current limiting element 10 in FIG. 3 are used, and these are denoted as a diode 9 and a limiting resistor 10, respectively. The diode 9 prevents the power supply 5 from being charged by mistake.

A plus terminal Vin connected to the power supply 5 via the diode 9 is connected to a Vin terminal of the voltage detector (corresponding to the A / D converter 11 in FIG. 1), and a GND terminal of the voltage detector 111. Is connected to the ground potential. The Vo terminal of the voltage detector 111 is connected to the emitter of the transistor 15, the collector of the transistor 15 is connected to the input terminal of the microcomputer 1 via the resistor 17a, and the collector of the transistor 15 is connected to the ground potential via the resistor 17b. It is connected to the. A resistor 18a is connected between the base and the emitter of the transistor 15, and the base of the transistor 15 is connected via a resistor 18b to the collector of a transistor 16 of a different conductivity type.
A resistor 19a is connected between the base and the emitter of the transistor 16, the emitter of the transistor 16 is connected to the ground potential, and the base of the transistor 16 is connected to the control terminal of the microcomputer 1 via the resistor 19b.

Therefore, when a control signal is input from the microcomputer 1 to the base of the transistor 16, the transistor 1
6 is turned on, thereby turning on the transistor 15.
Accordingly, a detection signal regarding the power supply voltage of the power supply 5 detected by the voltage detector 111 is input to the input terminal of the microcomputer 1.

The microcomputer 1 detects the power supply voltage of the power supply 5 based on the output signal from the voltage detector 111, and if the voltage has reached the end voltage value (first predetermined value), the microcomputer 1 outputs the start signal to DC. / DC converter 6.

The DC / DC converter 6 has a Vcc terminal connected to a plus terminal Vin connected to the power supply 5, and a G terminal connected to the ground potential. Capacitors 20a and 20b are connected between the Vcc terminal of the DC / DC converter 6 and the ground potential. The EBL terminal of the DC / DC converter 6 is connected to the output terminal of the microcomputer 1, and starts the switching operation based on the start signal from the microcomputer 1. In addition, the DC / DC converter 6
A smoothing circuit including a diode 21a, an inductor 21d, and capacitors 21b and 21c is connected to the SW terminal.
The load 3 is connected to the output side of the smoothing circuit.

Therefore, the DC / DC converter 6 performs a switching operation based on a start signal from the microcomputer 1 and outputs power to be supplied from the SW terminal, and the power is smoothed by the smoothing circuit and controlled by the microcomputer 1. Load (resistance)
3 is supplied.

The positive terminal of the capacitive element 8 is connected to the Vin terminal of the voltage detector (corresponding to the A / D converter 12 in FIG. 1) 112, and the GND terminal of the voltage detector 112 is connected to the ground potential. ing. The Vo terminal of the voltage detector 112 is connected to the emitter of the transistor 22, the collector of the transistor 22 is connected to the input terminal of the microcomputer 1 via the resistor 24a, and the collector of the transistor 22 is connected to the ground potential via the resistor 24b. It is connected to the.
A resistor 23a is connected between the base and the emitter of the transistor 22, and the base of the transistor 22 is connected to a collector of a transistor 25 of a different conductivity type via a resistor 23b. A resistor 26a is connected between the base and the emitter of the transistor 25, the emitter of the transistor 25 is connected to the ground potential, and the base of the transistor 25 is connected to the control terminal of the microcomputer 1 by the resistor 26a.
b.

Therefore, when a control signal is input from the microcomputer 1 to the base of the transistor 25,
5 turns on, thereby turning on the transistor 22.
Accordingly, a detection signal of the charging voltage of the capacitive element 8 detected by the voltage detector 112 is input to the input terminal of the microcomputer 1.

A transistor 27, a P-channel type field effect transistor (MOSFET) 28, and a resistor 29
a, 29b, and 30 correspond to the switch SW1 in FIG. The transistor 27 has a base connected to the control terminal of the microcomputer 1 via the resistor 29b.
The resistor 29a is connected between the base and the emitter of the transistor 27, and the emitter of the transistor 27 is connected to the ground potential. The gate of the MOSFET 28 is the transistor 2
7, the drain of the MOSFET 28 is connected to the plus terminal Vin connected to the power supply 5, and the
The source of the SFET 28 is connected to the collector of the transistor 27 via the resistor 30 and is connected to the Vin terminal of the DC / DC controller 7, and the Vin terminal of the DC / DC controller 7 is connected to the ground potential via the capacitors 31a and 31b. It is connected.

A transistor 37, a P-channel type field effect transistor (MOSFET) 38, and a resistor 4
0, 39a, and 39b correspond to the switch SW2 in FIG. The transistor 37 has a base connected to the control terminal of the microcomputer 1 via the resistor 39a,
7, a resistor 39b is connected between the base and the emitter,
The emitter of the transistor 37 is connected to the ground potential. The gate of the MOSFET 38 is connected to the collector of the transistor 37, the source of the MOSFET 38 is connected to the collector of the transistor 37 via the resistor 40, and the source of the MOSFET 38 is connected to the plus terminal of the capacitive element 8. Also MOSFET
The drain 38 is connected to one end of a load (resistance) 4 controlled by the microcomputer 1, and the other end of the load (resistance) 4 is connected to the ground potential.

The P-channel field-effect transistor (MOSFET) 32 and the diode 33 are formed as shown in FIG.
Corresponding to the switch SW3. The gate of the MOSFET 32 is connected to the Pgate terminal of the DC / DC controller 7, the source of the MOSFET 32 is connected to the SENSE terminal of the DC / DC controller 7, and the MOSFET is
The source of T32 is connected to the Vin terminal of the DC / DC controller 7 via the current limiting resistor 10. Also MO
The drain of the SFET 32 is connected to the ground potential via the diode 33.

The drain of the MOSFET 32 is connected to the capacitive element 8 via a smoothing circuit including an inductor 34 and capacitors 35a and 35b. DC / DC
The FB terminal of the controller 7 is connected to the ground potential via the resistor 36b, and is connected to the plus terminal of the capacitive element 8 via the resistor 36a.

Further, as shown in FIG. 3, the MOSFETs 28 and 38 are connected so that the forward direction of the parasitic diodes 28a and 38a is directed toward the capacitive element 8, respectively. Therefore, when the MOSFETs 28 and 38 are off, from the viewpoint of the capacitive element 8, even if an electric charge flows from the capacitive element 8, the parasitic diode 28
Since a and 38a are reverse-biased with respect to the charge of the capacitive element 8, the charge from the capacitive element 8 is prevented from being discharged through the parasitic diodes 28a and 38a.

In FIG. 3, when a control signal is input from the microcomputer 1 to the base of the transistor 27, the transistor 27 is turned on, and accordingly, the MOSFET 28 is turned on.
And the power from the power supply 5 is Vin of the DC / DC controller 7.
Input to the terminal.

Here, the microcomputer 1 is connected to the main switch 1
When the power supply is confirmed when the power supply 4 is switched from OFF to ON, the microcomputer 1 controls the DC / DC controller 7
The start signal is output to the EBL terminal.

The microcomputer 1 detects the charging voltage of the capacitive element 8 based on the output signal from the voltage detector 112, and if the voltage has not reached the threshold, continues charging.
If the threshold is reached, the MOSFET 38 is turned on.

The DC / DC controller 7 includes the microcomputer 1
MOSFET3 from Pgate terminal based on start signal from
2 to output a control signal. With this, MOSFET3
2 starts a switching operation, and the power of the power supply 5 is input from the DC / DC controller 7 to the capacitive element 8 via the smoothing circuit, whereby the capacitive element 8 is switched to the switch SW3.
Is intermittently charged in conjunction with the switching operation of.

When the charging voltage of the capacitive element 3 reaches the threshold value and a control signal is input from the microcomputer 1 to the base of the transistor 37, the transistor 37 is turned on.
The OSFET 38 is turned on, the charge from the capacitive element 8 flows out to the load 4 side, and the charge of the capacitive element 8 and the power supply 5
The power is supplied to the load 4 with the output of.

The operation of power supply circuit 100 shown in FIG. 3 operates according to the flowchart of FIG.

As described above, even if the power supply (battery) 5 is removed from the electronic device and the capacitive element 8 is present in the power supply circuit of the electronic device, the power input side and the power output side of the capacitive element 8 The switches SW1 and SW2 provided in the above can prevent the charge from flowing out of the capacitive element 8 to the load side.
A malfunction of the load due to the charge from the capacitive element 8 can be prevented.

Further, since a dedicated discharge circuit for discharging the electric charge remaining in the capacitive element 8 is not provided, the electric charge remaining in the capacitive element 8 is not wastefully discharged, and the battery is replaced and restarted. Only the spontaneous discharge from the capacitive element 8 needs to be charged, and the capacitive element can be charged to a full charge state instantaneously, which not only improves the use efficiency of the electronic device, but also increases the circuit scale. Increase and cost can be prevented.

[0064]

As described above, according to the present invention, it is possible to suppress the outflow of electric charge from the capacitive element used as the auxiliary power supply in the power supply circuit of the electronic device, and to reduce the start-up time after replacing the battery. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a power supply circuit according to an embodiment of the present invention.

FIG. 2 is a control operation flowchart of a microcomputer in the power supply circuit according to the embodiment shown in FIG. 1;

FIG. 3 is a circuit diagram showing a power supply circuit according to another embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 1 microcomputer 2 3 4 load 5 power supply 6 DC / DC converter 7 DC / DC controller 8 capacitive element 9 backflow prevention element (diode) 10 current limiting element (current limiting resistor) 11 12 A / D converter SW1 SW2 SW3 switch 27 37 Transistor 28 38 P-channel field-effect transistor (M
OSFET) 111 112 Voltage detector

Claims (6)

[Claims] 1. A power supply circuit having a capacitive element functioning as an auxiliary power supply and supplying power to a load connected to the capacitive element, provided between the power supply and the capacitive element. A first switch, a second switch provided between the capacitive element and the load, a detector for detecting a voltage of the power supply, and a detection signal from the detector. Control means for performing on / off control of the two switch means. The control means turns on the two switch means to charge the capacitive element and supply power to the load. When the means detects that the voltage of the power supply has become equal to or less than the first predetermined voltage value, the two switch means are turned off to electrically connect the capacitive element with the power supply and the load. Insulation A power supply circuit characterized by: 2. The power supply circuit according to claim 1, wherein said detection means detects a voltage of said capacitive element in addition to a voltage of said power supply, and said control means turns on said first switch means. Then, when the charging of the capacitive element is started and the voltage of the capacitive element becomes equal to or higher than a second predetermined voltage value, the second switch is turned on and the second switch is turned on. A power supply circuit that turns off the first and second switch means when the power supply voltage becomes equal to or lower than a first predetermined value after the power supply voltage is reduced. 3. The power supply circuit according to claim 1, wherein said first and second switch means are MOSFETs, and are connected so that a forward direction of each parasitic diode is directed to said capacitive element. A power supply circuit, characterized in that: 4. The power supply circuit according to claim 1, wherein a third switch is provided between said first switch means and said capacitive element.
A power supply circuit, wherein the control means controls switching of the third switch means to intermittently charge the capacitive element. 5. The power supply circuit according to claim 1, wherein said capacitive element is an electric double-layer capacitor. 6. The power supply circuit according to claim 1, further comprising a current limiting element for preventing an overcurrent from flowing from said power supply to said capacitive element.