JPH0956058A - Circuit device having rush current preventing function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of a rush current when power supply is turned on with a simple circuit configuration. SOLUTION: An FET is connected between a power source circuit 1 and a circuit 9 to be supplied with electric power having a smoothing capacitor 10. In order to control the FET 12, a serial circuit of first and second resistors 13 and 14 is connected between the paired output lines 7 and 8 of the power source circuit 1. Then a capacitor 15 is connected in parallel with the second resistor 14 and the capacitor 15 is connected between the gate and source of the FET 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit device having a function of preventing a rush current such as a smoothing capacitor or an auxiliary power supply capacitor.

[0002]

2. Description of the Related Art Inrush current, that is, surge current flows through a smoothing capacitor connected to an output stage of a rectifier circuit when the power is turned on. As a method to prevent this surge current, a parallel circuit of a resistor and a thyristor is connected to the power supply line, the charging current of the capacitor flows through the resistor when the power is turned on, then the thyristor is turned on and the load current passes through this thyristor. There is a method of flowing. Another method of preventing inrush current is to connect an NTC type thermistor whose resistance value decreases as the temperature rises. In this method, the temperature of the thermistor is low when the power is turned on, so the thermistor shows a high resistance value. As a result, inrush current is prevented by the thermistor. Then, when the temperature of the thermistor rises due to the load current, the resistance of the thermistor decreases and the voltage drop and power loss decrease.

[0003]

By the way, the former method using a thyristor requires a control circuit for turning on the thyristor after the inrush current is prevented, which makes the circuit complicated and expensive. Also, some surge current flows when the thyristor is turned on. In the latter method using the NTC type thermistor, the resistance value of the thermistor does not become sufficiently small after the inrush current is prevented, so that the power loss in the thermistor becomes relatively large. Further, if the temperature of the thermistor is not lowered at the time of restart, the inrush current preventing action does not occur. The problem as described above is not limited to the circuit having the smoothing capacitor, but also occurs in the circuit including the auxiliary power supply capacitor.

Therefore, the present invention is to provide a circuit device capable of reliably preventing an inrush current with a simple circuit.

[0005]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a DC power supply circuit for supplying a DC voltage and a power supply connected to the DC power supply circuit in which an inrush current may flow. A target circuit, a series circuit of a first resistance and a second resistance connected between a pair of output terminals of the DC power supply circuit, a capacitor connected in parallel to the second resistance, and the DC power supply A field effect transistor connected in series to a power supply line between the circuit and the circuit to be supplied with power, a gate electrode connected to one end of the capacitor, and a source electrode connected to the other end of the capacitor. Circuit device. The temperature detection switch can be connected in parallel with the capacitor as described in claim 2. Also, as shown in claim 3,
A power supply on / off control switch can be connected in parallel with the capacitor. Moreover, as shown in claim 4,
Switch means may be provided to disconnect the first resistor from the first resistor during the off period of the field effect transistor. Further, as described in claim 5, the DC power supply circuit can be a rectifier circuit, and the power supply target circuit can be a circuit including a smoothing capacitor.

[0006]

According to the invention of each claim, the first
Further, the inrush current can be surely prevented by a simple circuit including a combination of the second resistor, the capacitor and the field effect transistor. Further, since the resistance of the field effect transistor gradually decreases at the start of power supply, abrupt current fluctuation due to the turning on of the field effect transistor does not occur. According to the second aspect of the present invention, protection in the abnormal temperature state of the circuit device and inrush current protection can be performed using the same field effect transistor, and simplification and cost reduction of the circuit device are achieved. . According to the invention of claim 3,
On / off control of power supply and rush current prevention can be performed using the same field effect transistor, and simplification and cost reduction of the circuit device can be achieved. According to the invention of claim 4, since the first resistor is separated from the second resistor by the switch means, the power loss in the first and second resistors can be reduced. According to the invention of claim 5,
Inrush current to the smoothing capacitor can be easily and reliably prevented.

[0007]

[First Embodiment] Next, a circuit device according to a first embodiment will be described with reference to FIGS. The DC power supply circuit 1 of the circuit device of FIG. 1 includes a pair of terminals 3 and 4 for connecting a commercial AC power supply 2 and a diode rectifier circuit 6 connected to the terminals 3 and 4 via a power switch 5. The pair of power supply lines 7 and 8 connected to the pair of output terminals of the rectifier circuit 6 are connected to the power supply target circuit 9. The power supply target circuit 9 includes an electrolytic capacitor 10 for smoothing and compensating for an instantaneous power failure, and a load 11 connected to the capacitor 10.
Consisting of The load 11 is, for example, a circuit of a computer system connected to the capacitor 10 via a constant voltage control circuit.

In order to prevent the inrush current of the capacitor 10, an N-channel enhancement type MO transistor, which is a kind of insulated gate field effect transistor, is connected in series with the line 8.
The SFET 12 is connected. Since the FET 12 has a structure in which the source S is connected to the substrate (bulk), a diode is built in between the source S and the drain D. In order to use the FET 12 for preventing the inrush current, a series circuit of the first resistor 13 and the second resistor 14 is connected between the pair of power source lines 7 and 8 on the power source side of the FET 12, and the second resistor 14 is connected. 1 in parallel with
5 is connected, and one end of this capacitor 15 is FET 12
Is connected to the gate G of the
12 sources S are connected. In addition, capacitor 1
5 and a zener diode 16 for voltage limiting are connected in parallel between the gate and the source.

To illustrate the constants of the respective parts in FIG. 1, the voltage of the power supply 2 is 100 V and the capacity of the capacitor 10 is 1000 μ.
F, the first resistor 13 is 1 MΩ, the second resistor 14 is 100
KΩ, the capacitor 15 is 10 μF, the ON resistance of the FET 12 is about 0.3Ω, and the gate threshold voltage is about 2 to 4V.

FIG. 2 is for explaining the operation of the circuit of FIG. 1. FIG. 2 (A) shows the change in the gate-source voltage VGS of the FET 12, and FIG. 2 (B) shows the drain of the FET 12.・ Shows the change of resistance RDS between sources. When the capacitor 15 is completely discharged, t0 in FIG.
When the power switch 5 is turned on at this point, the first resistor 13
A charging current flows through the capacitor 15 via the capacitor 15, and the charging voltage of the capacitor 15 gradually increases with the passage of time, and at the same time, the gate-source voltage gradually increases as shown in FIG. When the gate-source voltage becomes equal to or higher than the threshold value VTH at time t1, the FET 12 becomes conductive and the drain current starts to flow. The drain-source resistance of the FET 12 changes in inverse proportion to the gate-source voltage. That is,
As shown in FIG. 2A, when the gate-source voltage gradually increases with time based on the voltage of the capacitor 15, conversely, the drain-source resistance of the FET 12 gradually decreases with time. As a result, the current flowing through the closed circuit composed of the power supply circuit 1, the smoothing capacitor 10, the load 11, and the FET 12 is limited by the FET 12 at the initial stage of turning on the power switch 5, and no rush current flows through the capacitor 10. When the voltage of the capacitor 15 rises with the passage of time, the current of the FET 12 also gradually increases. When the gate-source voltage finally becomes 12V limited by the Zener diode 16, the on-resistance of the FET 12 becomes a very small value (for example, 0.3Ω). Note that the gate-source voltage of 12 V is a value at which the FET 12 is fully saturated. When the FET 12 is completely turned on, the FE during power supply to the capacitor 10 and the load 11
The power loss at T12 is extremely small. afterwards,
Since the voltage of the capacitor 15 is kept almost constant, FE
The on resistance of T12 is also kept substantially constant. When the power switch 5 is turned off, the electric charge of the capacitor 15 changes to the resistance 1
It is released via 4. Therefore, when the power switch 5 is turned on again, the capacitor 15 is gradually charged again, and F
The drain-source resistance of the ET12 changes as shown in FIG. 2 (B), and the inrush current is prevented.

As is apparent from the above, according to this embodiment, the FET 12, the two resistors 13 and 14, and the capacitor 1 are provided.
A simple circuit consisting of 5 and 5 reliably prevents inrush current. As a result, voltage fluctuations are suppressed, and protection of the capacitor 10, the rectifying circuit 6, etc. is achieved. Also,
Because the current passing through the FET12 changes gradually, FET1
Noise is not generated based on the fact that the current is controlled by 2. Moreover, since the Zener diode 16 is provided,
The voltage of the capacitor 15 can be limited to a predetermined value or less.

[0012]

[Second Embodiment] Next, a circuit device according to a second embodiment will be described with reference to FIGS. However, in FIG.
The same reference numerals are given to substantially the same portions as those described above, and the description thereof is omitted. The circuit device of FIG. 3 is provided with a power supply circuit 1a including a DC power supply 2a composed of a battery and a power switch 5 in place of the power supply circuit 1 of FIG. Other than that, the configuration is the same as that of FIG. The DC power supply 2a has terminals 3a and 4
A DC voltage is supplied to the pair of lines 7 and 8 via a and the power switch 5. The capacitor 10 in FIG. 3 is used as an auxiliary power source. That is, the capacitor 10 is provided to continue supplying power to the load 11 for a short time after the power switch 5 is turned off.

Since the inrush current prevention circuit composed of the FET 12, the resistors 13 and 14 and the capacitor 15 has the same structure as in FIG. 1, when the power switch 5 is turned on at the time t0 in FIG. 4, the gate-source voltage of the FET 12 is increased. VGS is Figure 4
As shown in FIG. 4A, the drain-source resistance RDS gradually decreases, and as shown in FIG. 4B, the gate-source voltage VGS gradually decreases from the time t1 when the threshold VTH is reached. The input current I gradually increases from the time t1 as shown in FIG. When the drain-source resistance RDS becomes substantially constant on-resistance Ron at time t2, the current I also becomes substantially constant. Therefore, the same working effect as that of the first embodiment can be obtained by the second embodiment.

[0014]

[Third Embodiment] Next, a circuit device of a third embodiment shown in FIG. 5 will be described. However, FIG. 5 and FIGS.
In FIG. 3, parts that are substantially the same as those in FIGS. 1 and 3 are given the same reference numerals, and explanations thereof are omitted. The circuit device of FIG.
The temperature sensor 20 detects an abnormal temperature of the load 11 in the power supply target circuit 9. This temperature sensor 20 is a temperature detection switch 2 which is turned on when the temperature exceeds a predetermined temperature.
1, the temperature detection switch 21 includes a capacitor 15
Are connected in parallel.

In this circuit device, when an abnormal temperature is detected while power is being supplied to the load 11 and the temperature detection switch 21 is turned on, the electric charge of the capacitor 15 is discharged through the switch 21 and the FET 12 is turned off. The power supply to the capacitor 10 and the load 11 is cut off, and the protection of the load 11 is achieved.

The circuit device of FIG. 5 has the same effects as those of the first and second embodiments, and the circuit structure is simplified by using the FET 12 for both inrush current prevention and temperature protection. Also has.

[0017]

Fourth Embodiment A circuit device of a fourth embodiment shown in FIG. 6 is similar to the circuit device of FIG. 3 except that the power switch 5 is omitted from the circuit device of FIG. 3 and a switch 5a is connected in parallel to a capacitor 15. It has the same configuration as. The switch 5a is kept on when power is not supplied to the power supply target circuit 9. As a result, a discharge circuit for the capacitor 15 is formed via the switch 5a, and the FET 12 is kept off. When the switch 5a is turned off, the capacitor 15 is charged, and the effect of preventing inrush power is obtained as in the circuit device of FIG.

The circuit device shown in FIG. 6 has the same effects as the circuit device shown in FIG. 3, and also has the effect that the current capacity of the switch 5a can be reduced. That is, although the load current flows through the power switch 5 in FIG. 3, the discharge current of the capacitor 15 and the first resistor 13 flow through the switch 5a in FIG.
The current is limited by the current, and these are smaller than the load current. Therefore, it is possible to reduce the cost by using the switch 5a with a small capacity. Further, since the load current is turned on / off by the FET 12, the load current can be turned on / off with a non-contact switch without sparks. Further, in FIG. 6, since the FET 12 is used for both inrush current prevention and on / off of load current, cost reduction of the circuit device is achieved.

[0019]

[Fifth Embodiment] The fifth embodiment shown in FIG. 7 is different from that of FIG. 6 except that a changeover switch 5b is provided instead of the switch 5a of FIG.
It has the same configuration as that of FIG. The common movable contact 22 of the changeover switch 5b has the first and second fixed contacts 23 and 24.
Selectively contact. The movable contact 22 has a second resistor 14
And the first fixed contact 23 is connected to the lower end of the capacitor 15, and the second fixed contact 24 is connected to the lower end of the first resistor 13. Figure 7
In this circuit, when the movable contact 22 is inserted in the first fixed contact 23, the discharge circuit of the capacitor 15 is formed and the FET 12 is controlled to be off. When the movable contact 22 is turned on to the second fixed contact 24, the capacitor 15
A charging current flows through the same, and the same operation as the circuit device of FIG. 3 occurs.

The circuit device of FIG. 7 has the same effects as the circuit devices of FIGS. 3 and 6, and has the effect that the current passing through the resistor 13 can be completely cut off by the switch 5b during the OFF period of the FET 12.

[0021]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) In the circuit device of FIGS. 5, 6 and 7, the power supply circuit 1a can be a rectifier circuit like the power supply circuit 1 of FIG. (2) The circuit device shown in FIGS. 3, 5, 6 and 7 may be provided with the same device as the Zener diode 16 shown in FIG. (3) The circuit device shown in FIGS. 1, 6 and 7 may be provided with a component corresponding to the temperature sensor 20 shown in FIG. (4) The polarity of the FET 12 can be changed by transforming the line 8 into a positive power source line and the line 7 into a negative power source line. (5) The type of the FET 12 can be changed. (6) Instead of turning on / off the power supply with the switch 5, a plug can be inserted into an outlet. (7) Instead of the changeover switch 5b of FIG. 7, two switches that operate in the opposite directions can be provided. (8) The present invention can be applied not only to the inrush current of the capacitor 10 but also to prevent the inrush current of a circuit element whose resistance or impedance is low at the time of starting an incandescent lamp or the like and then becomes high.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a circuit device of a first embodiment.

FIG. 2 is a waveform diagram showing a state of each part of FIG.

FIG. 3 is a circuit diagram showing a circuit device according to a second embodiment.

FIG. 4 is a waveform diagram showing a state of each part of FIG.

FIG. 5 is a circuit diagram showing a circuit device according to a third embodiment.

FIG. 6 is a circuit diagram showing a circuit device of a fourth embodiment.

FIG. 7 is a circuit diagram showing a circuit device of a fifth embodiment.

[Explanation of symbols]

 9 Power supply target circuit 12 FET 15 Capacitor

Claims (5)

[Claims] 1. A DC power supply circuit for supplying a DC voltage, a power supply target circuit connected to the DC power supply circuit, in which a rush current may flow, and a pair of output terminals of the DC power supply circuit. First connected
A series circuit of a resistor and a second resistor, a capacitor connected in parallel to the second resistor, a power supply line between the DC power supply circuit and the power supply target circuit, and A circuit device comprising a field effect transistor having a gate electrode connected to one end of the capacitor and a source electrode connected to the other end of the capacitor. 2. A temperature detection switch for detecting the temperature of the power supply target circuit and turning on when the temperature reaches a predetermined value, the temperature detection switch being connected in parallel to the capacitor. Claim 1 characterized in that
Circuit device according to. 3. A switch for controlling ON / OFF of power supply from the DC power supply circuit to the power supply target circuit is connected in parallel to the capacitor. Circuit device according to. 4. The circuit according to claim 3, further comprising switching means for disconnecting the first resistor from the second resistor when the capacitor is short circuited by the switch. apparatus. 5. The DC power supply circuit is a rectifier circuit connected to an AC power supply, and the power supply target circuit is a circuit including a smoothing capacitor for smoothing an output of the rectifier circuit. The circuit device according to 2 or 3 or 4.