JPH09311731A - Dropper power supply - Google Patents

Abstract

Kind Code: A1 Abstract: A DC power supply device using a semiconductor device in series with a load.
ET) is used to provide a dropper power supply. An input power supply 2 is connected to both ends of a load 3, a FET 1 is inserted in a return path on the negative potential side, and an output voltage Vo for a load 3 and a reference voltage Vref are compared and changed according to a voltage difference. Output voltage detecting means 4 for generating a current If and a current If if the output current If flows to the primary side.
And the secondary current Ic that generates a secondary current Ic proportional to the primary current If on the secondary side via the optical output based on
The gate-source voltage Vgs that decreases according to the
And the FET control means 6 for controlling the output voltage Vo at the load 3 based on the change in the drain-source voltage Vds of the FET 1 corresponding to the change in the gate-source voltage Vgs. Make up.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC power supply device using a semiconductor device in series with a load as a final adjustment device, and more particularly to a field effect transistor (FE) as a semiconductor device.
T) is used for the dropper power supply.

A dropper power supply is a device for stepping down an input side voltage by a loss in a main transistor to obtain a desired output voltage, and is widely used as a power supply device capable of easily obtaining a stable output voltage. ing.

In the dropper power supply, it is required to minimize the voltage difference between the power supply input voltage and the output voltage.

[0004]

2. Description of the Related Art FIG. 11 is a circuit diagram showing an example of a conventional dropper power source, where Tr1 is a main transistor and Tr2 is a main transistor.
Is an auxiliary transistor Darlington connected to the main transistor Tr1, Tr3 is a control transistor for controlling the auxiliary transistor Tr2, and Dz is a constant voltage element for generating a constant voltage Vz.

In the circuit shown in FIG. 11, the input voltage Vi is dropped by the main transistor Tr1.
Obtain the output voltage Vo. The output voltage Vo at this time is expressed by the following equation, and a stable voltage can be supplied to the load. Vo = (Vz + Vbe) (1 + R1 / R2) (1)

[0006]

In the conventional dropper power supply, since the bipolar element is used as the main transistor, Vce (sat) of about 0.6 V is generated due to the characteristics of the element. Further, when the transistor is connected in Darlington in order to output a large current, this becomes about 1.2V.

As described above, in the conventional dropper power supply, when trying to obtain a large current output, the input power supply Vi and the output power supply Vo
There is a problem in that the voltage difference between and is required to be at least about 1.2V. Vi-Vo> 1.2V (2)

SUMMARY OF THE INVENTION The present invention is intended to solve such a problem of the prior art, and an object thereof is to provide a dropper power supply capable of reducing a voltage difference between a power supply input voltage and an output voltage. I am trying.

[0009]

FIG. 1 shows a principle structure of the present invention, in which 1 is an FET, 2 is an input power source, 3 is a load, 4 is output voltage detecting means, and 5 is an optical source. Joint,
6 is a FET control means.

The dropper power supply of the present invention uses FET1 as a main transistor as shown in FIG. The FET input / output voltage difference Vds is expressed by the following equation with respect to the output current Io. Vds = Rds × Io (3)

On the other hand, the minimum resistance value of the FET is Rds (o
n) is about 0.2Ω for a large current. Therefore, when the output current is 1 A, the input / output voltage difference Vds is as follows. Vds = Rds (on) × Io = 0.2Ω × 1A = 0.2V (4)

Thus, since the output current = 1A and the input / output voltage difference Vds = 0.2V, the input / output voltage difference becomes significantly smaller than that in the case where a bipolar transistor is used, and the performance of the power supply device is reduced. Moreover, it is very advantageous.

To drive the FET, Vgs (th)
Since there is a constraint that (the threshold voltage of the FET) is 2 to 4V, it cannot be driven unless there is a power source higher than the S (source) potential of the FET by 4V or more. Therefore, in the present invention, the FET 1 is arranged on the negative potential side of the input power source 2, and the FET control means 6 secures Vgs (th).

As a result, if the input voltage is at least 4 V, the FET can be driven regardless of the input / output voltage difference. Further, by arranging the FET on the negative potential side, the S (source) potential of the output voltage detecting means 4 and the FET are not the same potential, and the control becomes unstable. The output voltage detecting means 4 is provided.
And the FET control means 6 are optically coupled,
The operation is stabilized by making the reference potential irrelevant.

Specific means for solving the problems of the present invention will be given below.

(1) The dropper power supply of the present invention is an input power supply 2
Is connected to both ends of the load 3, the field effect transistor (FET) 1 is inserted in the return path on the negative potential side, and the load 3
Output voltage Vo and reference voltage Vref are compared with each other to generate an output current If which changes according to the voltage difference, and an output current If flows to the primary side.
An optical coupling section 5 for generating a secondary current Ic proportional to the primary current If on the secondary side via an optical output based on f, and a secondary current I
Gate-source voltage Vgs that decreases according to c
1 is provided for controlling the output voltage Vo of the load 3 to be constant based on the change of the drain-source voltage Vds of the FET 1 corresponding to the change of the gate-source voltage Vgs.

(2) In the case of (1), the output voltage detecting means 4
Is an amplifier 21 having a voltage obtained by dividing the output voltage Vo at one input and a reference voltage Vref at the other input.
The optical coupling section 5 comprises a light emitting diode 23 on the primary side.
And a photocoupler 22 having a phototransistor 24 on the secondary side.
Is connected to the negative potential side of the input power source 2 through the resistor R5 and the phototransistor 24, and the resistor R5
And the phototransistor 24 are connected to the gate of the FET1.

(3) In the case of (1) or (2), the output voltage detecting means 4 has a phase compensating circuit 7 for controlling the delay of the phase between the input and the output.

(4) In the case of (3), the phase compensation circuit 7 is composed of a series circuit of a resistor Rx and a capacitor Cx connected between the input and output of the amplifier 21, and a parallel circuit of a resistor R3. .

(5) In the case of (1) or (2), the output current I
When o becomes an overcurrent state, it is provided with an overcurrent control means 8 for causing the output voltage to drop by increasing the current on the primary side of the optical coupling section 5.

(6) In the case of (5), the overcurrent control means 8 is
An amplifier 25 that generates an output when the voltage drop Rs.multidot.Io in the resistor Rs inserted in the return path on the negative potential side exceeds the reference voltage Vrefa, and a current If of the light emitting diode 23 that is turned on when the output of the amplifier 25 occurs. And a transistor Trc for increasing

(7) In the case of (1) or (2), when the input power supply is turned on, the gate-source of the FET1 is short-circuited, and then gradually opened according to a predetermined time constant, so that the output voltage Vo is exceeded. A soft start means 9 for preventing a chute is provided.

(8) In the case of (7), the soft start means 9
Is a circuit that generates an output voltage that immediately rises when the input power is turned on and then gradually decreases according to a predetermined time constant,
The transistor Trs, which is connected between the gate and the source of the FET 1 and which initially causes a large current to flow in accordance with the output voltage and then gradually decreases the current, is configured.

(9) In the case of (1) or (2), the output voltage V
When o becomes higher than a predetermined value, the overvoltage protection means 10 is provided to cut off the output voltage Vo by short-circuiting the gate and source of the FET1.

(10) In the case of (9), the overvoltage protection means 10
However, an amplifier 27 that generates an output when the voltage obtained by dividing the output voltage Vo becomes higher than the reference voltage Vrefb, a transistor Tro that is turned on according to this output, and a light emitting diode 28 that emits light when the transistor Tro is turned on. FE
A solid state relay SSR1 including a photothyristor 29 connected between the gate and source of T1 and turned on via this light output is provided.

(11) In the case of (1) or (2), when the output voltage Vo becomes lower than a predetermined value, the gate of the FET 1
There is provided a low voltage protection means 11 for shutting off the output voltage Vo by short-circuiting the sources.

(12) In the case of (11), the low voltage protection means 11
Is connected between the amplifier 30 that generates an output when the voltage obtained by dividing the output voltage Vo becomes lower than the reference voltage Vrefc, the light emitting diode 31 that emits light according to this output, and the gate-source of the FET 1 And a solid state relay SSR2 including a photothyristor 32 that is turned on via the.

(13) In the case of (1) or (2), when the input voltage Vi is higher than a predetermined value, the gate-source of the FET1 is opened, and when the input voltage Vi is lower than the predetermined value, the FET1 is opened. The starting / stopping means 12 for performing the operation of short-circuiting between the gate and the source with a hysteresis is provided.

(14) In the case of (13), the starting / stopping means 12
Compares the voltage obtained by dividing the input voltage Vi with the reference voltage Vrefd and generates an output when the divided voltage becomes higher than the reference voltage Vrefd, and the divided voltage becomes lower than a predetermined voltage lower than the reference voltage. An amplifier 33 that constitutes a hysteresis comparator that cuts off the output when it is turned on and a transistor 34 that opens or shorts the gate and the source of the FET 1 according to the presence or absence of the output of the amplifier 33 are provided.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows an embodiment (1) of the present invention, in which the same components as those in FIG. 1 are designated by the same reference numerals. As shown in the figure, the output voltage detecting means 4 is composed of an error amplifying circuit using an amplifier 21, and a photocoupler 22 is used for the optical coupling section 5, and an FET is used.
The control means 6 has a structure in which the gate of the FET 1 is dropped to the source on the secondary side of the photocoupler 22 via the bleeder resistor R5.

Reference numeral 21 denotes an amplifier (operational amplifier), 22
Is a photocoupler, 23 is a light emitting diode which constitutes the primary side of the photocoupler 22, 24 is a phototransistor which constitutes the secondary side of the photocoupler 22, R1 to R5 are resistors,
Vref is a reference voltage.

When the output voltage Vo rises, the amplifier 21
Detects that the voltage obtained by dividing the output voltage Vo by the voltage divider composed of the resistors R1 and R2 is higher than the reference voltage Vref, and causes the current If to flow through the light emitting diode 23 on the primary side of the photocoupler 22. Light up. As a result, the phototransistor 2 on the secondary side of the photocoupler 22
At 4, a current Ic = hfe.If is produced. Where hfe is
This is the CTR (current amplification factor) of the photocoupler 22. When the current Ic flows to the secondary side of the photocoupler 22, the gate-source voltage Vgs of the FET 1 is lowered,
Therefore, the drain-source voltage Vds of the FET 1 is increased and the output voltage Vo is decreased.

When the output voltage Vo drops, conversely, F
By increasing the gate-source voltage Vgs of the ET1 and decreasing the drain-source voltage Vds, the ET1 operates to increase the output voltage Vo. In this way, the output voltage Vo can be stabilized. On this occasion,
Since the drain-source voltage Vds of FET1 is small,
A dropper power supply with a small input / output potential difference can be realized.

In the circuit of FIG. 2, the current If flowing in the primary side of the photocoupler 22 is given by the following equation, where the voltage on the primary side of the photocoupler 22 is Vf. If = 1 / R4 [Vo-R3 {(Vref / R1)-(Vo-Vref) / R2} -Vref-Vf] (5)

On the other hand, the gate-source voltage V of the FET1
gs looks like this: Vgs = Vi-hfe.If.R5 (6) From this, the output current Io, the input voltage Vi, the output voltage V
The relationship of o is as follows.

Io = Gm.Vgs = Gm [Vi-hfe.R5 / R4 [Vo-R3 {(Vref / R1)-(Vo-Vref) / R2} -Vref-Vf]] (7) Here, Gm: Conductance of FET1

FIG. 3 shows an embodiment (2) of the present invention, in which the same elements as those in FIG. 2 are indicated by the same numbers. In this embodiment, the error amplification circuit is provided with the phase compensation circuit 7. In the phase compensation circuit 7, Rx
Is a resistor and Cx is a capacitor.

The resistor Rx and the capacitor Cx are provided for the purpose of controlling the phase delay advance, and this prevents the dropper power supply from performing an unstable operation such as oscillation.

The phase compensation by the delay / lead control is performed according to the following equation. Delay frequency f = 1 / {2πCx (Rx + R3)} (8) Lead frequency f = 1 / {2πCx Rx} (9)

The embodiment (2) shows an example of phase compensation, and the configuration of the phase compensation circuit is not limited to this circuit.

FIG. 4 shows an embodiment (3) of the present invention, in which the same components as those in FIG. 2 are designated by the same reference numerals. In this embodiment, the overcurrent control means 8 is provided on the output side of the FET.
It can also be provided on the input side of. In the overcurrent control means 8, 25 is an amplifier (operational amplifier), 26 is a reverse current prevention diode, Trc is a transistor, Rs is a current detection resistor, and Vrefa is a reference voltage.

The output current Io increases and Vrefa = Rs.multidot.
At Io, the transistor Trc is turned on, and the current If of the light emitting diode 23 on the primary side of the photocoupler 22 increases, so that the current Ic of the phototransistor 24 on the secondary side of the photocoupler 22 increases, and as a result, In order to decrease the gate-source voltage Vgs of the FET1 and increase the drain-source voltage Vds of the FET1, the output voltage V
o drops and the dropper power supply is protected against overcurrent.

FIG. 5 shows an embodiment (4) of the present invention, in which the same components as those in FIG. 3 are designated by the same reference numerals. In this embodiment, the soft start means 9 is provided for preventing the overshoot of the output voltage Vo when the dropper power supply is activated. Soft start means 9
, Trs is a transistor, R6 and R7 are resistors, Cy
Is a capacitor. SW1 is a power switch.

When the power supply switching SW1 is closed and the power supply input voltage Vi is applied to the dropper power supply, if the response speed of the feedback loop that constitutes the dropper power supply circuit is slow, it cannot follow up and the transient response causes the output voltage Vo to rise to the output voltage Vo. Overshoot occurs.

In the soft start means 9, the transistor Trs is turned on by the increase of the base potential generated in the resistor R6 based on the charging of the capacitor Cy at the time of startup, short-circuiting between the gate and source of the FET1, and the capacitor By turning off in accordance with the drop of the base potential according to the time constants of Cy and the resistor R7, the gate of the FET1
Since the sources are gradually opened, it is possible to prevent the occurrence of overshoot in the output voltage Vo based on the transient response at the time of startup.

FIG. 6 exemplifies the output characteristic in the case of the embodiment (4), and illustrates the change of the output voltage Vo of the load 3 with respect to time t when the power is turned on. In the figure, A indicates a case where the soft start means 9 is not provided,
It is shown that overshoot has occurred as shown in a. Further, B shows the case where the soft start means 9 is provided, and it is shown that a smooth rising characteristic follows the time constant of the capacitor Cy and the resistor R7.

FIG. 7 shows an embodiment (5) of the present invention, in which the same components as those in FIG. 2 are designated by the same reference numerals. In this embodiment, the overvoltage protection means 10 is provided for detecting the abnormal rise of the output voltage and shutting off the power output when the power fails. In the overvoltage protection means 10, 27 is an amplifier (operational amplifier), Tro is a transistor, SSR1 is a solid state relay, R8 and R9 are resistors, and Vrefb is a reference voltage. In the solid-state relay SSR1, 28 is a light emitting diode on the primary side and 29 is a photothyristor on the secondary side.

The output voltage Vo rises, and Vo> (1 + R
When 9 / R8) Vrefb is reached, the transistor Tro is turned on based on the output of the amplifier 27, and a current flows through the light emitting diode 28 on the primary side of the solid state relay SSR1 to emit light. As a result, the photothyristor 29 on the secondary side of the solid state relay SSR1 is turned on, the gate-source voltage Vgs of the FET1 is short-circuited, and the FET1 is turned off, thereby cutting off the output of the dropper power supply. Once the photothyristor 29 is turned off, the output voltage Vo drops and the transistor T
Even if ro is turned off, it will not be turned on, and the output of the dropper power supply remains cut off.

FIG. 8 shows an embodiment (6) of the present invention, in which the same components as those in FIG. 2 are designated by the same reference numerals. In this embodiment, the low voltage protection means 11 is provided to detect the abnormal decrease in the output voltage when the power fails and to shut off the power output. In the low voltage protection means 11, 30 is an amplifier (operational amplifier), SSR2 is a solid state relay, R1
0 and R11 are resistors, Cz is a capacitor, and Vrefc is a reference voltage. In the solid state relay SSR2, 31 is a light emitting diode on the primary side and 32 is a photothyristor on the secondary side.

The output voltage Vo decreases and Vo <(1 + R
11 / R10) When Vrefc is reached, a current flows through the light emitting diode 31 on the primary side of the solid state relay SSR2 based on the output of the amplifier 30 to emit light. As a result, the photothyristor 32 on the secondary side of the solid state relay SSR2 is turned on, the gate-source voltage Vgs of the FET1 is short-circuited, and the FET1 is turned off, thereby cutting off the output of the dropper power supply. Once the photothyristor 29 is turned off, even if the transistor Tro is turned off due to the decrease in the output voltage Vo, it is not turned on, and the output of the dropper power supply remains cut off.

The output voltage of the dropper power supply is always in a low voltage state at startup, but in this state, the capacitor C
Since the voltage of the resistor R10 rapidly rises due to the charging current of z, the amplifier 30 does not generate an output and therefore the solid state relay SSR2 does not turn on, so the output of the dropper power supply is not cut off. In this way, the low voltage protection means 11 is configured so that the operation at the time of startup is masked.

FIG. 9 shows an embodiment (7) of the present invention, in which the same components as those in FIG. 2 are designated by the same reference numerals. In this embodiment, the dropper power supply is started / stopped to start when the input power supply voltage becomes a sufficient voltage and to stop when the input power supply voltage becomes a low voltage that interferes with the power supply operation. Means 12 are provided. Start-up·
In the stopping means 12, 33 is an amplifier (operational amplifier),
34 is a transistor, R12 to R15 are resistors, and Vrefd is a reference voltage.

In the start / stop means 12, the amplifier 33
Compares the voltage obtained by dividing the input power supply voltage Vi by the resistors R12 and R13 with the reference voltage Vrefd, generates an output according to the comparison result, turns on or off the transistor 34, and outputs F
By turning ET1 off or on, the required start / stop operation is performed. At this time, the amplifier 33 operates as a hysteresis comparator, and exhibits an operation characteristic having a certain hysteresis to prevent malfunction (pumping) near the operating voltage.

The starting voltage and the stopping voltage in the starting / stopping means 12 are determined as follows. Start voltage Vth (H) = (1 + R13 / R12) (1 + R14 / R15) Vrefd ... (10) Stop voltage Vth (L) = (1 + R13 / R12) {(1 + R14 / R15) Vrefd- (R14 / R15) VH} ... (11) Here, VH is the voltage when the output of the amplifier 33 is "H".

FIG. 10 shows an example of the configuration of a dropper power supply to which the present invention is applied, and the same components as those in the above figures are indicated by the same numbers. The configuration of FIG. 10 is similar to that of FIG.
For the basic configuration shown in FIG. 4 (including the phase compensation circuit shown in FIG. 3), the overcurrent control means 8, the soft start means 9, and the overvoltage protection shown in the embodiments of FIGS. It is a combination of the means 10, the low voltage protection means 11, and the starting / stopping means 12, and the function of each added means is the same as that of each embodiment.

[0056]

As described above, according to the present invention, in the dropper power supply, the FET is used as the main transistor for the voltage drop, the output voltage detecting means for detecting the output voltage value, and the ON / OFF of the FET. By optically coupling with the FET control means for controlling the OFF, the input-output voltage can be lowered and stable operation can be performed.

Further, as shown in FIG. 10, an overcurrent control means, a soft start means, an overvoltage protection means, and a low voltage protection are applied to a dropper power source having an output voltage detection means, an optical coupling section, and an FET control means. By adding the means and the start / stop means, it becomes possible to provide a more reliable dropper power supply device.

[Brief description of drawings]

FIG. 1 is a diagram showing a principle configuration of the present invention.

FIG. 2 is a diagram showing an embodiment (1) of the present invention.

FIG. 3 is a diagram showing an embodiment (2) of the present invention.

FIG. 4 is a diagram showing an embodiment (3) of the present invention.

FIG. 5 is a view showing an embodiment (4) of the present invention.

FIG. 6 is a diagram illustrating an output characteristic in the case of the embodiment (4).

FIG. 7 is a view showing an embodiment (5) of the present invention.

FIG. 8 is a diagram showing an embodiment (6) of the present invention.

FIG. 9 is a diagram showing an embodiment (7) of the present invention.

FIG. 10 is a diagram showing a configuration example of a dropper power supply to which the present invention is applied.

FIG. 11 is a circuit diagram showing an example of a conventional dropper power supply.

[Explanation of symbols]

 1 FET 2 Input Power Supply 3 Load 4 Output Voltage Detection Means 5 Optical Coupling Unit 6 FET Control Means 7 Phase Compensation Circuit 8 Overcurrent Control Means 9 Soft Start Means 10 Overvoltage Protection Means 11 Low Voltage Protection Means 12 Start / Stop Means

Claims (14)

[Claims] 1. An input power supply is connected to both ends of a load, a field effect transistor (FET) is inserted in a return path on the negative potential side, and an output voltage for the load and a reference voltage are compared to determine a voltage difference. Output voltage detecting means for generating a current that changes according to the present invention, and a light for generating a secondary current proportional to the primary current on the secondary side through an optical output based on the primary current when the output current flows on the primary side. A FET is provided with a coupling part and a gate-source voltage that decreases in accordance with the secondary current to the FET, and changes in the drain-source voltage of the FET corresponding to the change in the gate-source voltage. Based on this, the dropper power supply is characterized in that the output voltage at the load is kept constant. 2. The output voltage detecting means comprises an amplifier in which a voltage obtained by dividing the output voltage is applied to one input and a reference voltage is applied to the other input, and the optical coupling section is
It is composed of a photocoupler having a light emitting diode on the primary side and a phototransistor on the secondary side, and the FET control means connects from the positive potential side of the input power source to the negative potential side of the input power source through the resistor and the phototransistor. With
The dropper power supply according to claim 1, wherein a connection point between the resistor and the phototransistor is connected to a gate of the FET. 3. The dropper power supply according to claim 1, wherein the output voltage detecting means has a phase compensating circuit for controlling the delay of the phase between the input and the output. 4. The dropper power supply according to claim 3, wherein the phase compensation circuit comprises a parallel circuit of a series circuit of a resistor and a capacitor connected between the input and output of the amplifier and a resistor. 5. The dropper power supply according to claim 1, wherein, when the output current is in an overcurrent state, the output current is drooped by increasing the primary side current of the optical coupling section. A dropper power supply characterized by having means. 6. The amplifier for generating an output when the overcurrent control means generates an output when a voltage drop in a resistor inserted in a return path on the negative potential side becomes larger than a reference voltage, and is turned on when an output of the amplifier is generated. The dropper power supply according to claim 5, further comprising a transistor for increasing a current of the light emitting diode. 7. The dropper power supply according to claim 1, wherein when the input power supply is turned on, the gate-source of the FET is short-circuited and then gradually opened according to a predetermined time constant, so that the output voltage is exceeded. Dropper power supply characterized by having a soft start means for preventing a chute. 8. The soft start means is connected between a circuit for generating an output voltage which rises immediately when the input power is turned on and then gradually decreases according to a predetermined time constant, and is connected between the gate and the source of the FET and responds to the output voltage according to the output voltage. 8. The dropper power supply according to claim 7, further comprising a transistor that initially supplies a large current and then gradually decreases the current. 9. The dropper power supply according to claim 1, wherein when the output voltage becomes higher than a predetermined value, the F
A dropper power supply comprising an overvoltage protection means for cutting off an output voltage by short-circuiting a gate and a source of ET. 10. The overvoltage protection means generates an output when a voltage obtained by dividing the output voltage becomes higher than a reference voltage, a transistor which is turned on in response to the output, and light emission when the transistor is turned on. 10. The dropper power supply according to claim 9, further comprising a solid-state relay including a light-emitting diode and a photothyristor connected between the gate and the source of the FET and turned on through the light output. 11. The dropper power supply according to claim 1, further comprising a low voltage protection means for shutting off the output voltage by short-circuiting the gate and source of the FET when the output voltage becomes lower than a predetermined value. Dropper power supply characterized by having. 12. An amplifier which generates an output when the voltage obtained by dividing the output voltage becomes lower than a reference voltage, the light emitting diode which emits light according to the output, and the gate-source of the FET. 2. A solid state relay comprising a photothyristor connected between the photothyristors and being turned on through the light output.
The dropper power supply described in 1. 13. The dropper power supply according to claim 1, wherein the gate-source of the FET is opened when the input voltage becomes higher than a predetermined value, and the input voltage becomes lower than the predetermined value. A dropper power supply comprising start / stop means for performing a short-circuit operation between a gate and a source of an FET with hysteresis. 14. The starting / stopping means compares an input voltage-divided voltage with a reference voltage and generates an output when the divided voltage becomes higher than the reference voltage. An amplifier that forms a hysteresis comparator that shuts off the output when the voltage becomes lower than a predetermined voltage lower than the reference voltage, and a transistor that opens or shorts the gate-source of the FET according to the presence or absence of the output of the amplifier. The dropper power supply according to claim 13, wherein: