JP3003825B2 - Surge voltage protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surge voltage protection circuit for protecting an electric circuit from transient high voltages such as static electricity and surge voltage.

[0002]

2. Description of the Related Art Generally, electric circuits such as various LSIs include:
Although protection circuits are provided for high voltage such as static electricity and surge voltage from the outside, for example, wired telephones used in ordinary households are often greatly affected by static electricity or lightning charged on the human body. LSI used for
Are more susceptible to surge damage, so a more effective protection circuit is needed.

FIG. 5 shows an example of a conventional surge voltage protection circuit, which is disclosed in US Pat. No. 4,377,832. This circuit is intended to protect an electric circuit 12, for example, a subscriber line interface circuit of a telephone. The electric circuit 12 is connected via a resistor 14 to a power supply terminal 11 to which a predetermined power supply voltage (V _CC ) is applied. One end of the resistor 14 has a diode 15 connected in parallel and an SCR which is a trigger-controlled thyristor.
(Silicon Controlled Rectifier) 16. In this figure, the SCR 16 is equivalently represented by two transistors 17 and 18. In this equivalent circuit, the base of the transistor 17 is the transistor 1
8 and the emitter of the transistor 17 is grounded. The emitter of the transistor 18 is connected to the resistor 14. The collector of the transistor 17 and the base of the transistor 18 are connected to each other, and are connected to a connection point between the voltage dividing resistors 19 and 20. The other end of the voltage dividing resistor 20 is connected to a terminal 13 to which a constant voltage of −48 V is applied via a diode 21 connected in the forward direction. Connected between them.

The operation of the conventional surge voltage protection circuit having the above configuration will be described. In the normal state, a negative voltage is applied to the power supply terminal 11 and the diode 15 is in a cut-off state. However, when a positive surge voltage is applied to the power supply terminal 11, the surge current is reduced. Bypass through 14 and diode 15 to ground, the electrical circuit 12 is protected.

On the other hand, when a negative surge voltage is applied to the power supply terminal 11, a current is supplied from the terminal 13 to the base of the transistor 18 via the diode 21 and the voltage dividing resistor 20, and the transistor 18 is turned on. 1
7, a base current flows, and the transistor 17 is turned on.
As a result, the transistors 17 and 18 are both turned on and in a self-holding state, and a negative surge current flows from the ground via the transistors 17 and 18 and the resistor 14 to the power supply terminal 11.
And the electric circuit 12 is protected.

[0006]

As described above, in the conventional surge voltage protection circuit, when a negative surge voltage is applied, the SCR 16 itself serves as a bypass path for the surge current, and the SCR 16 itself becomes large from the ground point. Since current flows through the base-emitter junction of the transistor 18 via the emitter-collector junction of the transistor 17, the transistor 18 is easily broken. That is, although the internal electric circuit can be protected, there is a problem that the protection circuit itself is destroyed at the same time.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and provides a surge voltage protection circuit capable of effectively protecting an internal electric circuit without being destroyed by a high surge voltage. The purpose is to:

[0008]

Surge voltage protection circuit according to the present invention, in order to solve the problems] includes a threshold level detector means for detecting that the voltage applied to the external terminals reaches a threshold level, current bypass
Separately provided with means, have a holding means for maintaining the conductive state of the current bypass means, the threshold level detection
Means for detecting that the applied voltage has reached a threshold level
Then, when the current bypass means is made conductive,
In addition, the holding means is operated .

The surge voltage protection circuit according to the present invention is characterized in that, in the first aspect, a Zener diode is used as the threshold level detecting means, and a thyristor is used as the holding means.

[0010]

In the surge voltage protection circuit according to the present invention, when the voltage applied to the external terminal exceeds a predetermined threshold level, the current bypass means is turned on to bypass the surge current and prevent the flow into the internal circuit. At the same time, even after the voltage applied to the external terminal falls below a predetermined threshold level, the conduction state of the current bypass unit is maintained by the holding unit.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to embodiments.

FIG. 1 shows a surge voltage protection circuit according to an embodiment of the present invention. In this figure, an electric circuit 38 to be protected is operated by a power supply voltage (V _CC = 30 to 40 V) applied between the power supply terminals 31 and 32. Here, the power supply terminal 31 is a plus terminal and the power supply terminal 32 is a minus terminal. A diode 33, a holding circuit 35, and a transistor 37 are connected in parallel with the electric circuit 38 between these power terminals. The diode 33 is reverse-biased with respect to the polarities of the power terminals 31 and 32, and the collector and the emitter of the transistor 37 are connected to the power terminals 31 and 32, respectively. A Zener diode 36 is connected in reverse bias between the power supply terminal 31 and the base of the transistor 37.

The holding circuit 35 has two transistors 41,
42 and three resistors 43, 44, 45. The emitter of the transistor 41 is connected to the power supply terminal 31. The base of the transistor 41 is connected to the power supply terminal 31 via the resistor 43 and the transistor 4
2 is also connected to the collector. The collector of the transistor 41 is connected to the base of the transistor 42 via the resistor 44 and is connected to the power supply terminal 32 via the resistor 45.
, And further connected to the anode of the Zener diode 36 and the base of the transistor 37. The emitter of the transistor 42 is connected to the power supply terminal 32.

The operation of the surge voltage protection circuit having the above configuration will be described with reference to FIG.

First, when a negative surge voltage is applied to the power supply terminal 31, the surge current is bypassed from the power supply terminal 32 through the diode 33 to the power supply terminal 31, and the electric circuit 3
8 is protected.

On the other hand, a positive surge voltage (10 to 20 kV) as shown by a broken line 61 in FIG. 2 is applied to the power supply terminal 31, and the breakdown voltage V _ZD is applied across the Zener diode 36.
Is applied, the Zener diode 36
Are turned on, and a part of the surge current from the power supply terminal 31 is supplied to the base of the transistor 37. As a result, the transistor 37 is turned on, and most of the surge current flows between the collector and the emitter of the transistor 37,
The power supply terminal 32 is bypassed. A part of the surge current flowing through the Zener diode 36 flows to the power supply terminal 32 via the resistor 45, and is also supplied to the base of the transistor 42 via the resistor 44. As a result, the transistor 42 is turned on, so that the transistor 41 is also turned on, and part of the surge current is supplied to the power supply terminal 31.
From the transistor 41 to the power supply terminal 32 via the emitter-collector of the transistor 41 and the resistor 45. This state is
The voltage is maintained even after the voltage across the Zener diode 36 becomes equal to or lower than the breakdown voltage V _ZD and the Zener diode 36 is cut off, so that the current is continuously supplied to the base of the transistor 37 from the collector of the transistor 41. As a result, the surge current bypass operation continues.

If the transistor 37 is controlled only by the Zener diode 36 without providing the holding circuit 35, the voltage between both ends of the Zener diode 36 becomes the breakdown voltage V _ZD
After that, the transistor 37 is turned off.
The voltage applied between the power terminals 31 and 32 (that is, the electric circuit 3
8) changes at a substantially constant limiting voltage V _L (broken line 62 in FIG. 2) expressed by the following equation (1).

V _L = V _ZD + V _BE37 + α (1) Here, V _BE37 is a base-emitter voltage of the transistor 37, and α is a voltage drop due to line impedance.

The value of V _L is usually 50-60 V,
The electric circuit 38 does not break down at such a voltage, but since this voltage is maintained, power consumption is large and the amount of heat generated is large, and the electric circuit 38 may be thermally broken.

On the other hand, in the present embodiment, the provision of the holding circuit 35 allows the transistor 37 to remain on even after the voltage across the Zener diode 36 becomes equal to or lower than the breakdown voltage V _ZD to bypass the surge current. Therefore, the voltage applied to the electric circuit 38 rapidly decreases as shown by the solid line 63 in FIG. 2, and becomes the voltage V _R as shown in the following equation (2) or (3).

V _R = V _CE41 + V _BE42 (2) V _R = V _BE41 + V _CE42 (3) As described above, in this embodiment, most of the surge current is bypassed via the transistor 37. Therefore, the current flowing between the base and the emitter of the transistor 42 is reduced as compared with the related art, and the breakdown of the transistor 42 can be avoided.

It is needless to say that the same effect can be obtained by using a single SCR 48 instead of the holding circuit 35 as shown in FIG. However, if the resistor 44 is connected to the base of the transistor 42 as in the above-mentioned embodiment, the protection of the base-emitter junction of the transistor 42 becomes more reliable, and between the collectors of the transistors 41 and 42 and the power supply terminal, respectively. If the resistors 43 and 45 are connected, there is an effect that leakage can be prevented.

The role of these resistors 43 and 45 will be described below.

Normally, in a transistor, no current (I _CE ) flows between the collector and the emitter unless a base current flows. But in practice there is also flowing slight leakage current [Delta] I _CE in the base current I _B is zero by variations in element.

For example, when a small current ΔI _CE42 flows due to leakage in the transistor 42, without the resistor 43, ΔI _CE 42 becomes the base current of the transistor 41 as is apparent from FIG.

The _{ΔI CE42 = I B41 ... (4} ) Therefore, I _CE of the transistor 41 is amplified by the current amplification factor h _Fe41 times the transistor 41 becomes the following equation (5).

I _CE41 = ΔI _CE42 × h _fe41 (5) Accordingly, the base current of the transistor 42 is expressed by the following equation (6).

I _B42 = ΔI _CE42 × h _fe41 (6) Therefore, ΔI _CE42 becomes a value shown by the following equation (7) by _passing through the transistor 41 as a result.

ΔI _CE42 = h _fe42 × (ΔI _CE42 × h _fe41 ) (7) Normally, since h _fe has a value of several hundred, for example, h _fe41 = h
_{If fe42} = 100, the original minute leakage current ΔI
_CE42 is amplified 10,000 times, and the protection circuit malfunctions.

On the other hand, the case where the resistors 43 and 45 are provided is as follows.

[0031] That is, even in the same manner as described above is [Delta] I _CE42 occurs, [Delta] I _CE42 flows transistor to the resistor 43 41
_Does not _{turn on} , so _ICE41 does not flow. Transistor 4
In order for 1 to turn on, the condition of the following equation (8) is required.

V _BE41 ≒ ΔI _CE42 × R ₄₃ (8) Therefore, by selecting the value R _{43 of the} resistor 43, malfunction of the protection circuit can be prevented.

The role of the resistor 45 is the same.

For the reasons described above, the circuit can be stabilized by adding the resistors 43 and 45.

When the breakdown voltage V _ZD of the Zener diode 36 does not match the withstand voltage of the electric circuit 38, as shown in FIG. What is necessary is just to adjust the voltage applied to both ends of the diode 36.

Further, since the protection circuit of this embodiment can be composed of only easily integrated elements such as a transistor, a resistor, and a Zener diode, it can be easily integrated with an electric circuit into one chip.

[0037]

As described above, according to the present invention,
When the voltage applied to the external terminal exceeds a predetermined threshold level, the current bypass means is turned on to bypass the surge current, and the holding means is provided even after the voltage applied to the external terminal becomes lower than the predetermined threshold level. As a result, the conduction state of the current bypass means is maintained, so that the components of the protection circuit are not destroyed even with a high surge voltage,
In addition, there is an effect that the protection effect of the internal circuit is improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a surge voltage protection circuit according to one embodiment of the present invention.

FIG. 2 is an explanatory diagram for explaining an operation of the circuit of FIG. 1;

FIG. 3 is a circuit diagram showing a surge voltage protection circuit according to another embodiment of the present invention.

FIG. 4 is a circuit diagram showing a surge voltage protection circuit according to another embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a conventional surge voltage protection circuit.

[Explanation of symbols]

 31, 32 Power supply terminal 35 Holding circuit 36 Zener diode 38 Electric circuit 37 Transistor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-228309 (JP, A) JP-A-55-27794 (JP, A) Fully open Showa 55-19436 (JP, U) Really open Showa 53- 73855 (JP, U) (58) Fields studied (Int. Cl. ⁷ , DB name) H02H 9/04 H03K 17/00-17/10

Claims (3)

(57) [Claims] 1. A surge voltage protection circuit having current bypass means for bypassing a surge current flowing from an external terminal to protect an internal circuit from an external surge voltage, wherein a voltage applied to the external terminal is a threshold level. and a threshold level detection means for detecting that reached, provided separately from said current bypass means, have a, a holding means for maintaining the conductive state of the current bypass means, the threshold level detection means, said Applied voltage is at threshold level
When the current bypass is detected, the current bypass unit is activated.
A surge voltage protection circuit, wherein the surge voltage protection circuit is turned on and the holding means is operated . 2. The surge voltage protection circuit according to claim 1, wherein said threshold level detection means is a Zener diode, and said holding means is a thyristor. 3. The surge voltage protection circuit according to claim 2,
The bypass means is a transistor, and the Zener diode has one end connected to the transistor.
Connected to the base of the thyristor and the gate of the thyristor,
The Zener diode breaks down according to the applied voltage
The transition at the one end when the
And both the thyristor and the thyristor are turned on.
And a surge voltage protection circuit.