JP3157042B2 - Overcurrent protection device - Google Patents

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 used for video equipment and audio equipment, and an overcurrent protection device for protecting a load circuit thereof.

[0002]

2. Description of the Related Art In recent years, video equipment and audio equipment have become widespread in homes, and it is important to ensure their safety. A power supply circuit is indispensable in each of such devices, and an overcurrent protection device is added so that smoke does not occur even if the load causes some abnormality.

FIG. 4 is a circuit diagram showing an example of a conventional overcurrent protection device. In this figure, a power switch 2, a fuse 3, and a resistor 4 are connected in series to a voltage source 1, and connected to a power supply terminal of a load 5. A decoupling capacitor 6 is connected between the power supply terminal of the load 5 and the ground terminal. Then, a PNP transistor 7 (hereinafter referred to as “op-
Below, simply abbreviated as transistor 7)
There are respectively connected, as shown in the collector
NPN transistor 8 (hereinafter simply abbreviated as transistor 8)
Based be) is connected, further capacitor 9, the resistor 10
Is connected. The emitter of the transistor 8 is grounded, and the collector is connected to the common connection terminal of the fuse 3 and the resistor 4.

[0004] The operation of the overcurrent protection device thus configured will be described. FIG. 5A is a time chart showing the normal operation. In FIG. 5, when the power switch 2 is on, the terminal voltage V ₂ of the power switch ₂ is a constant voltage equal to the voltage source 1 and the load current I ₅
Is constant. Therefore, the voltage V ₄ across the resistor 4 is
Although it is constant as shown in (c), if this resistance is set to a sufficiently small value, the transistor 7 is off. Therefore, the base voltage V _B8 of the transistor 8 is 0 V, and the collector current I _C8 of the transistor 8 does not flow.

Now, let us consider a case where some abnormality occurs in the load 5 at time t ₁ and the load current I ₅ flowing into the load ₅ increases. Load current I ₅ In this example it is assumed that the linearly increasing. When the load current I _5, as shown in FIG. 5 (b) increases, the voltage V ₄ of the resistor 4 is increased, the base-emitter forward voltage drop V _BE of time t ₂ to the transistor 7
(Approximately 0.7V). Therefore, transistor 7 is turned on at time t ₂
To turn on and charge the capacitor 9 instantaneously.
Base voltage V _B8 of the transistor 8 reaches V _BE at time t ₂ as shown in FIG. 5 (d). Therefore, as shown in FIG. 5E, the base current flows through the transistor 8, and the collector current I _C8 also flows through the collector. The collector current I _C8 has a large current value because the current is amplified by the transistor 8. The current I ₃ flowing through the fuse 3 is substantially equal to the sum of the load current I ₅ and the collector current I _C8 of the transistor 8, and
It changes as shown in FIG. Collector current I _C8 current I ₃ is also increased since a large current, the collector
Fuse from a current I _C8 begins to flow at a constant after time t ₃ 3
Is blown. Therefore, the power supply to the load 5 in which the abnormality has occurred is stopped, and the danger that the voltage source 1 or the load 5 emits smoke can be prevented.

[0006]

However, in such a conventional configuration, the current flows through the capacitor 6 when the power is turned on.
The rush current I ₆ would overcurrent protection device malfunctions, has a drawback that there is a case where the fuse 3 will be blown. FIG. 6 is a time chart showing the operation when the power is turned on. When the time t ₄ in the figure as a power switch 2 is turned on, FIG. 6 (a), (b) , the rise in the output voltage V ₂ is the time t ₄ of the power switch 2 as shown in (c), which Accordingly, the load current I ₅ flowing into the load ₅ and the current I ₆ flowing into the capacitor ₆ also rise. In general the resistance value R ₄ of the load 5 resistor is connected in series to 4, for example as small as about 1 [Omega, since the capacitance C ₆ of the capacitor 6 for decoupling is large (approximately 100 .mu.F), rush current I ₆ large current Becomes In this case the resistance 4 of the voltage drop V ₄ is influenced by the rush current I ₆ to the capacitor 6 base-emitter forward voltage drop V _BE (about 0.7
V). Thus the base voltage V _B8 also V _BE next instant of the transistor 8 at time t _4, it starts to flow the collector current I _C8 to transistors 8. The current I ₃ flowing through the fuse 3 is equal to the load current I ₅ flowing through the load ₅ and the capacitor 6.
Rush current I ₆ flowing into, and the sum of the collector current I _C8 of transistor 8. Thus at time t ₄ after a predetermined time after the time t ₅ as shown in FIG. 6 (g), even though there is no nothing abnormal load 5, so rush current I ₆ flowing into the capacitor 6 is triggered Since the transistor 8 is turned on, there is a disadvantage that the fuse 3 may be blown when the power is turned on .

SUMMARY OF THE INVENTION The present invention has been made in view of such conventional problems, and does not respond to an instantaneous large current such as a rush current, but can respond only to a steady large current. Is a technical issue.

[0008]

SUMMARY OF THE INVENTION The present invention provides a voltage source, a power switch, an overcurrent fuse is used in the power supply circuit having a first capacitor for Decoupling in g in parallel to the connected load in series with the load A protection device, comprising: a first resistor connected in series between a fuse and a load; and a first switch that is turned on when a current flowing through the load exceeds a predetermined value based on a voltage drop of the first resistor. , A second resistor connected between the output terminal of the first switch and the ground terminal, and a second resistor.
And a second switch that conducts when the terminal voltage of the second capacitor exceeds a predetermined value and connects between the fuse and the ground terminal. The time constant with the second capacitor is larger than the time constant with the first resistor and the first capacitor.

[0009]

According to the present invention having the above features, the second switch is turned on even if the first switch is turned on by detecting the overcurrent.
The second capacitor is charged through the resistor of the second type, and when the charged voltage reaches a predetermined value, the second switch is turned on to blow the fuse. Therefore, the instantaneous current flowing through the first capacitor for decoupling after power-on is obtained.
With respect to overcurrent , the second switch is not turned on due to a difference in time constant.

[0010]

FIG. 1 is a circuit diagram showing the configuration of an overcurrent protection device according to one embodiment of the present invention. In this figure, the same parts as those of the above-described conventional example are denoted by the same reference numerals, and detailed description is omitted. Also in this embodiment, a power switch 2, a fuse 3, and a first resistor 4 are connected in series to a voltage source 1, and are connected to a load 5. A first decoupling terminal is provided between the load 5 and a ground terminal. A capacitor 6 is provided. The base and the emitter of the transistor 7 as the first switch are connected to both ends of the resistor 4. In this embodiment, a second resistor 11 and a second capacitor 12 are connected in series to the collector of the transistor 7. And in the mid-point first
The base of the transistor 8 which is the switch 2 is connected. The emitter of the transistor 8 is grounded, and the collector is connected to the common connection terminal of the fuse 3 and the resistor 4. Here the time constant of resistor 11 and capacitor 12, the resistor 4 及
And the time constant of the capacitor 6 are selected.

The operation of the overcurrent protection device according to the present embodiment having the above configuration will be described. FIG. 2 is a time chart showing the operation of the present embodiment in a steady state. Power switch 2
Is on, the voltage V ₂ at its output Is a voltage equal to that of the voltage source 1 as shown in FIG. 2A, and a constant load current I5 flows through the load ₅ . The current I ₆ flowing into the capacitor 6 is zero.

[0012] some abnormality occurs in the load 5 at now time t _1, consider the case where the load current I ₅ flowing through the load 5 increases gradually. Here the load current I _5, as shown in FIG. 2 (b) it is assumed that a linearly increasing. Load current I
_{When 5} increases, the voltage drop V _{4 of the} resistor ₄ also increases,
At time t ₂ is reached between the base and emitter of the transistor 7 forward voltage drop V _BE (about 0.7 V). Transistor 7 collector current flows becomes at time t ₂ to the ON state. This collector current flows to the capacitor 12 via the resistor 11. Here, assuming that the resistance value of the resistor 11 is R ₁₁ and the capacitance of the capacitor 12 is C ₁₂ , the capacitor 12 is charged according to a time constant of R ₁₁ × C ₁₂ . Therefore, the base voltage V _B8 of the transistor 8 gradually increases after the time t _{2 as} shown in FIG. When the V _B8 time t ₃ reaches V _BE, the base current flows transistor 8 is turned on. The base current is amplified by the transistor 8, and a collector current I _C8 having a large current value flows as shown in FIG.
Fuse 3 a current I ₃ which flows is the sum of the collector current I _C8 negatively charged <br/> current I ₅ and the transistor 8 flows substantially load. Current I ₃ also becomes a large value because the collector current I _C8 is a large current as mentioned above, the fuse 3 is blown at time t ₄ after a certain time of the time t ₃ after. Therefore, the supply of power to the load 5 where the abnormality has occurred is stopped, and the danger that the voltage source 1 or the load 5 will ignite or smoke will be prevented.

FIG. 3 is a time chart showing the operation of the overcurrent protection device of this embodiment when the power is turned on. Time t ₆
3 (a) to 3 (c) when the power switch 2 is turned on.
As shown in FIG. 7, the output voltage V ₂ rises at time t ₆ , and accordingly, the load current I ₅ flowing into the load ₅ and the rush current I ₆ flowing into the capacitor ₆ also rise. Since the resistance value R ₄ of the resistor 4 inserted in series with the load 5 is small and the capacitance C ₆ of the decoupling capacitor ₆ is large, the rush current I ₆ has a considerably large value. Therefore, the voltage drop amount V ₄ of the resistor 4 is affected by the rush current I ₆ to the capacitor 6 exceeds the base-emitter forward voltage drop V _BE of the transistor 7 (approximately 0.7 V). Thus transistor 7 at time t ₆ is in the ON state.

However, in this embodiment, since the capacitor 12 is gradually charged by the collector current of the transistor 7, the base voltage V _B8 of the transistor 8 does not immediately become V _BE , but gradually as shown in FIG. To rise. Since the time constants of the resistor 11 and the capacitor 12 are larger than those of the resistor 4 and the capacitor 6, the transistor 8
Although the base voltage V _B8 of the maximum value at time t _7, V
_The voltage does not reach _BE , and the transistor 8 does not turn on. Therefore, the current I ₃ flowing through the fuse 3 is substantially the sum of the load current I ₅ flowing into the load ₅ and the rush current I ₆ flowing into the decoupling capacitor 6. Therefore, the current I ₃ does not become a large value for a long time, and the rush current I 3
_The fuse 3 is not blown by the trigger ₆ .

[0015]

As described above in detail, according to the overcurrent protection device of the present invention, by providing the overcurrent detection circuit with a time constant, it is possible to reliably respond to a steady large current. The fuse can be blown. But not respond to transient large current at power-on, not a child to blow the fuse, it is possible to realize an ideal protection operation.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of an overcurrent protection device according to one embodiment of the present invention.

FIG. 2 is a time chart showing a normal operation of the overcurrent protection device according to the embodiment.

FIG. 3 is a time chart showing an operation at the time of turning on the power of the present embodiment.

FIG. 4 is a circuit diagram showing an example of a conventional overcurrent protection device.

FIG. 5 is a time chart showing a normal operation of the conventional overcurrent protection device.

FIG. 6 is a time chart showing the operation of the conventional overcurrent protection device when the power is turned on.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Voltage source 2 Power switch 3 Fuse 4,11 Resistance 5 Load 6 Capacitor 7,8 Transistor 12 Capacitor

Claims (1)

(57) [Claims] And 1. A voltage source, a power switch, a overcurrent protection device for use in power supply circuit having a first capacitor for Decoupling in g in parallel to the fuse connected in series with the load load, the A first resistor connected in series between the fuse and the load; a first switch that is turned on when a current value flowing through the load exceeds a predetermined value based on a voltage drop of the first resistor; A series connection of a second resistor and a second capacitor connected between an output terminal of the switch and a ground terminal, the fuse being electrically connected when a terminal voltage of the second capacitor exceeds a predetermined value; And a second switch connecting between the ground terminal and the second terminal, wherein the time constant of the second resistor and the second capacitor is larger than the time constant of the first resistor and the first capacitor. Overcurrent protection feature .