JP3226002B2 - Circuit device having a backflow prevention circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a circuit which operates with a DC power supply such as a dry cell or a battery as a power supply when a voltage having a polarity opposite to the original voltage is applied due to incorrect insertion of a dry cell or incorrect connection of a battery. The present invention relates to a circuit device capable of protecting a circuit.

[0002]

2. Description of the Related Art A circuit shown in FIG. 1 is known as a circuit for protecting a circuit including a relatively expensive integrated circuit or electronic circuit element from a voltage or a current having a reverse polarity. In FIG. 1, for example, a pair of input terminals 5 and 6 of a circuit 4 to be protected such as a measuring device and an electronic application device are connected to a pair of power terminals 2 and 3 for connecting a DC power supply 1 such as a battery,
Further, a backflow preventing diode 7 is connected between the power supply terminal 2 and the input terminal 5 of the circuit 4 to be protected. For this reason,
As shown by the dotted line in FIG. 1, even if the power supply 1 is connected to the opposite polarity, no reverse current flows through the circuit 4 to be protected.

A circuit shown in FIG. 2 is also known as another conventional protection circuit. In FIG. 2, a fuse 8 is connected between the DC power supply 1 and the circuit 4 to be protected, and a diode 9 is connected in anti-parallel to the DC power supply 1 via the fuse 8. In the circuit shown in FIG. 2, when the power supply 1 is connected in reverse polarity, a current flows through the circuit of the diode 9 and the fuse 8, and the fuse 8 is blown to protect the protection target circuit 4.

[0004]

By the way, in the conventional circuit shown in FIG. 1, the forward voltage VF of the diode 7 during normal use.
(Approximately 1 V for a PN junction diode, and approximately 0.5 V for a Schottky barrier diode), and a power loss occurs here, resulting in a decrease in the efficiency of the electronic device. Also,
In the circuit of FIG. 2, when the power supply 1 is connected in reverse polarity, an excessive current flows through the diode 9 until the fuse 8 is blown. Therefore, an element having a large current capacity must be used for the diode 9.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a circuit device having a backflow prevention circuit having a relatively simple structure and having a small voltage drop and a small power loss during normal operation.

[0006]

SUMMARY OF THE INVENTION To achieve the above object, the present invention comprises a pair of power supply terminals for connecting a DC power supply, a circuit to be protected which must prevent reverse current from flowing, Between the one of the pair of power terminals and the circuit to be protected so that when the DC power source is connected with the normal polarity between the pair of power terminals, a current flows from the source to the drain. Connected field effect transistor and emit light when the DC power supply has reverse polarity
Connected between the pair of power terminals with
A light emitting diode, and optically coupled to the light emitting diode;
Connected between the gate and source of the field effect transistor
And a phototransistor . As described in claim 2 , a light emitting diode for displaying reverse polarity can be connected in series with the light emitting diode for detecting reverse polarity.

[0007]

According to the present invention, the backflow prevention circuit is constituted by the field effect transistor, so that the voltage drop and the power loss during normal use are greatly reduced. In addition, the field effect transformer at the reverse polarity
The off operation of the transistor can be reliably achieved. Ma
Also, the circuit to be protected is reliably protected by a relatively simple circuit.
You. Further, according to the second aspect of the present invention, whether or not the polarity is opposite can be easily known by the light emitting diode for displaying the reverse polarity.

[0008]

First Embodiment Next, an electronic circuit device according to a first embodiment of the present invention will be described with reference to FIG. This electronic circuit device has a pair of power supply terminals 2 and 3 for connecting a DC power supply 1 composed of a battery, similarly to the conventional circuits of FIGS. 1 and 2,
The protection target circuit 4 is connected here. One power supply terminal 2 which is positive when normal is a positive input terminal 5 of the circuit 4 to be protected.
A field effect transistor, ie, FET 10, which functions as a backflow prevention circuit, is connected between the other power supply terminal 3, which is normally negative, and the negative input terminal 6 of the circuit 4 to be protected. I have. However, the FET 10 is connected so that a current flows from the source to the drain in a normal state. More specifically, the drain D is connected to the negative power supply terminal 3, and the source S is connected to the input terminal 6 of the circuit 4.

The FET 10 is an N-channel enhancement-type insulated gate (MOS) field-effect transistor having a structure in which a source S is connected to a substrate (bulk), and is incorporated in addition to a switch SW between the source S and the drain D. PN junction diode D1. This FE
T10 has a drain-source on-resistance RDS of about 6.5 mΩ when a voltage of about 12 V is applied between the gate and the source, and the voltage drop here is about 15 A even if the current flowing therethrough is 15 A × 0.0065Ω = 0.098V. The FET 10 has a gate-source voltage VGS.
Is lower than the threshold value VTH (about 2 to 4 V), the drain current ID becomes substantially zero, and the drain
Resistance RDS (ON) is sufficiently small (preferably 20
mΩ or less) is required. In short, an FE having an on-resistance such that the voltage drop of the FET 10 can be made smaller than the forward voltage drop of the diode in a normal state.
It is necessary to use T. As this FET 10, for example, 2SK1380 made by Toshiba can be used.

The control circuit of the FET 10 comprises a resistor 11, a light emitting diode 12, a resistor 13, a phototransistor 14, and a Zener diode 15. The gate G of the FET 10 is connected to the positive power supply terminal 2 via a 3 kΩ resistor 11. As shown by a dotted line in FIG. 3, a series circuit of a light emitting diode 12 for reverse polarity detection and a resistor 13 of 2 kΩ is provided as a reverse polarity detection circuit for detecting that the DC power supply 1 is erroneously connected to the reverse polarity. This is connected between the power supply terminals 2 and 3. Note that the light emitting diode 12 has a direction in which light is emitted when the DC power supply 1 has the opposite polarity. A phototransistor 14 optically coupled to the light emitting diode 12 is connected between the gate and the source of the FET 10 and functions as a switch for short-circuiting the same. A Zener diode 15 having a Zener voltage of about 12.4 V is connected between the gate and the source of the FET 10. The Zener diode 15 is for preventing a voltage higher than a certain voltage from being applied between the gate and the source.

In FIG. 3, for example, a 12 V
When the DC power supply 1 is connected to a normal polarity as shown by a solid line, the light emitting diode 12 is in a reverse bias state and does not emit light, and the phototransistor 14 is also kept off. For this reason, the gate-source voltage V
GS becomes about +12 V, and the conduction between the drain and the source becomes conductive, so that a forward current can flow normally to the protection target circuit 4 as a load. As described above, the on-resistance of the FET 10 is extremely small, for example, about 6.5 mΩ, so that the voltage drop here is 0.098 V even when the load current is 15 A, and the conventional diode 7 shown in FIG. 1/10 to 1/5 of the case of That is, FIG.
When the diode 7 is a PN junction diode and a current of, for example, 12 A flows, a voltage drop of about 1.0 V occurs. When the diode 7 is a Schottky barrier diode and a current of 15 A flows, a voltage drop of about 0.5 V occurs. A voltage drop occurs. Therefore, the FET1 of the circuit of FIG.
The voltage drop of 0 is much smaller than in the past. Also, FE
The power loss at T10 is also significantly smaller than the power loss of the diode 7 of the conventional circuit of FIG. The relationship between the current and the voltage drop of the FET 10 of FIG. 3 and the diode 7 of FIG. 1 is such that the voltage drop increases as the current increases. Therefore, the FET according to the present invention in any current region
The voltage drop and power loss of 10 are smaller than before.

In FIG. 3, when the DC power supply 1 is erroneously connected as shown by a dotted line and has the opposite polarity, the potential of the gate of the FET 10 becomes lower than that of the drain and the source.
The light emitting diode 12 becomes forward biased and emits light,
Since the phototransistor 14 is turned on and the gate and source of the FET 10 are short-circuited, the FET 10 is reliably turned off. As a result, the protection target circuit 4 is electrically disconnected from the DC power supply 1, and no reverse current flows here. Thus, the protection target circuit 4 including expensive electronic components is reliably protected by a relatively simple circuit.

[0013]

Second Embodiment Next, an electronic circuit device according to a second embodiment will be described with reference to FIG. However, in FIG. 4, substantially the same parts as those in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted. The circuit of FIG. 4 has the same configuration as that of FIG. 3 except that a display light emitting diode 16 is added to the circuit of FIG. Since the display light emitting diode 16 is connected in series to the reverse polarity detecting light emitting diode 12, the light emitting diode 16 emits light at the same time as the reverse polarity detecting light emitting diode 12 and has a reverse polarity connection to the person connected to the DC power supply 1. Notify. Therefore, the display light emitting diode 16 is disposed at a position that can be seen from outside the circuit device. It is to be noted that the second embodiment has the same operation and effect as the first embodiment.

[0014]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The FET 10 can be a P-channel FET, and this P-channel FET can be connected between the power supply terminal 2 and the power supply input terminal 5 of the circuit device 4. In other words, FIGS. 3 4
, Power supply 1, FET 10, light emitting diode 12, 1
6, 19, the phototransistor 14 , the diode 17, and the Zener diode 15 can all have opposite polarities. (2) Various other types of FETs can be used as the FET 10. (3) The power supply 1 is not limited to a battery, but may be a rectifier circuit. (4) The protection target circuit 4 may be one or more electronic circuit elements.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a conventional circuit device.

FIG. 2 is a circuit diagram showing another conventional circuit device.

FIG. 3 is a circuit diagram showing the electronic circuit device of the first embodiment.

FIG. 4 is a circuit diagram showing an electronic circuit device according to a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2, 3 Power supply terminal 10 FET 12 Light emitting diode 14 Phototransistor

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02J ^7/ 00-7/12 H02J ⁷ /34-7/36 H02H 11/00 H02J 1/00 309

Claims (2)

(57) [Claims] 1. A pair of power supply terminals for connecting a DC power supply, a circuit to be protected that must prevent a reverse current from flowing, and a DC power supply having a normal polarity between the pair of power supply terminals. A field-effect transistor connected between one of the pair of power supply terminals and the circuit to be protected so as to have a direction in which a current flows from the source to the drain when connected, and the DC power supply has a reverse polarity. Sometimes with the direction to emit light
A light emitting diode connected between the pair of power terminals; and a light emitting diode optically coupled to the light emitting diode and connected to the field effect transistor.
A phototransistor connected between the gate and the source of the transistor. 2. The circuit device according to claim 1, further comprising a light emitting diode for displaying a reverse polarity connected in series with said light emitting diode.