JPH0262051B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent detection circuit for an electronic switch, such as a proximity switch.

Conventionally, this type of overcurrent detection circuit has an emitter resistor _R1 inserted into the emitter of the output transistor _Tr1 , as shown in Figures 1 and 2, and the voltage across this
Detecting Vr ₁ using an overcurrent detection transistor Tr ₂ and thereby detecting an overcurrent is a simple method and is used in many applications. Although this method is easy to implement, it has the disadvantage that the residual voltage at the output terminal when it is turned on becomes extremely large. In other words, the remaining voltage Von when the transistor Tr ₁ is ON depends on the output current value and the output current capacity of the transistor Tr ₁ , but if Von is Von = 0.2 [V] and the output current It = 150 [m
A], including the overcurrent detection circuit, and 200 [m
A) When a current of more than 100% flows, this is considered an overcurrent. When the base-emitter voltage Vbe of the transistor Tr ₂ is detected to be 0.7 [V] or more, the value of the emitter resistance R ₁ becomes R ₁ =0.7 [V]/0.2 [V] = 3.5 [Ω]. Also, the voltage Vr ₁ across the emitter resistor R ₁ when the output current It = 150 [mA] is Vr ₁ = 3.5 × 0.15≒0.525 [V], and therefore the series circuit of the transistor Tr ₁ and the emitter resistor R ₁ The voltage Vx is Vx=Von+Vr ₁ =0.2+0.525=0.725 [V]. By inserting the overcurrent detection circuit in this way, the remaining output voltage when turned on as an electronic switch is more than tripled, and as is clear from the calculation formula, the output power capacity of the output transistor Tr ₁ is increased and The drawback is that no matter how much the ON output residual voltage Von is reduced, the ON output residual voltage including the overcurrent detection circuit hardly decreases.

Further, in the conventional overcurrent detection circuit, since the base-emitter voltage of the transistor _Tr1 was the overcurrent detection voltage, it had a large negative temperature coefficient.

The present invention aims to eliminate such conventional drawbacks, and the first object of the present invention is to reduce the detection voltage compared to the one that detects overcurrent using the voltage between the base and emitter of a transistor. This aims to reduce the overcurrent detection resistance of the electronic switch and reduce the remaining voltage when the switch is turned on.

A second object of the present invention is to enable overcurrents of both current sink and current source output types to be detected by the same overcurrent detection circuit.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram when the overcurrent detection circuit according to the present invention is applied to a proximity switch, and this proximity switch 10 includes a proximity sensor IC circuit 1 and a current switching element for supplementing the output current capacity of this IC circuit. 26, for example, an output transistor. The IC circuit 1 has an oscillation circuit 2, a comparator 3, an integration circuit 4, a comparator 5, an output circuit 6, a constant voltage circuit 7, a power supply reset circuit 8, an output controller 14, and an overcurrent detection circuit 16. , the output circuit 6, the power supply reset circuit 8, and the output controller 14 constitute an electronic switch controller 17. Further, a detection coil L ₁ , a resonance capacitor C ₁ , a variable resistance for sensitivity adjustment R ₂₂ , a bypass capacitor C ₂₃ , an integrating capacitor C ₂₄ , a power supply reset capacitor C ₂₅ and a load 9 are attached externally.

Therefore, when the metal body approaches or separates from the detection coil _L1 , the oscillation circuit 2 starts or stops oscillation, and the output circuit 6 obtains a logic L or H output.

An overcurrent detection circuit 16, which will be described later, is provided between the current switching element 26 and the output controller 14.

FIG. 4 is a circuit diagram showing the main part of the present invention, which basically includes four transistors, namely, a first transistor 22, a second transistor 23, a third transistor 24, and a fourth transistor 25.
An overcurrent detection circuit 16 is constituted by a block consisting of resistors 32 and 34 and a current source 20. Overvoltage protection circuit 18 with diode 27 and resistor 35
is configured to protect overcurrent detection circuit 16 when a large voltage is applied to node 41. This overvoltage protection circuit is for a current sink, but the circuit composed of the diode 28 and the resistor 36 is an overvoltage protection circuit for a current source. 19 is a power source. Note that the current switching element 26 controls the current flowing to the load 9, and is composed of, for example, a transistor. Connection terminals b and d are drawn out to the connection points between each transistor 24 and 25 and each resistor 32 and 34, and the transistor 2
A signal output terminal G is provided at the connection point between 4 and 25, and this output terminal is further connected to the input terminal of the controller 17 that controls the current switching element 26, thereby controlling the output state of the controller 17. is being used. In addition, in FIG. 4, the overcurrent detection circuit 16 and the overvoltage protection circuit 18
is converted to IC.

Now, in Fig. 4, the current I ₀ from the current source 20 is
I ₀ = 5 [μA], resistance values of resistors 32 and 34 R ₃₂ and R ₃₄
When R ₃₂ = R ₃₄ = 10 [KΩ], the voltages V ₄₁ and V 42 at nodes 41 and ₄₂ become the overcurrent detection voltage, and the value is V ₄₁ = V ₄₂ = I ₀ × R ₃₁ = I ₀ × R ₃₄ =5×
10=50 [mV]. Because node 41,
If both 42 and 42 are open, transistor 24
and 25 are in balance, and the output node 46 is in a critical state.

Now, when node 41 is connected to output current detection resistor _R1 , if the voltage generated across this resistor is less than 50 [mV], current flows from node 41 to resistor _R1 through resistor 35. Therefore, the current flowing through the resistor 35 decreases, the emitter voltage of the transistor 25 decreases, and the current flowing through the resistor 35 decreases.
The balance between nodes 4 and 25 is lost, and the potential of node 46 becomes L. At this time, the output of the electronic switch controller 17 becomes, for example, H, and the current switching element 26 is in the on state.

Also, for some reason, the current switching element 26
When the current flowing through the resistor R1 exceeds a predetermined value, the voltage generated across the resistor _R1 exceeds 50 [mV], and the voltage at the node 46 exceeds 50 [mV].
The voltage becomes H. At this time, the output of the electronic switch controller 17 becomes, for example, L, and the current switching element 26 is substantially turned off. Therefore, the current flowing through the resistor _R1 decreases, the potential of the energizing node 41 decreases, and the potential of the node 46 also decreases. Eventually, when the potential of the node 46 reverses to L, the output of the electronic switch controller 17 becomes H.
Current switching element 26 is turned on.

Further, when the voltage across the resistor _R1 is 50 [mV], no current flows through the resistor 35, and this becomes the threshold voltage.

In this way, the voltage generated across the resistor _R1 is 50 [m
V], the overcurrent detection circuit 16 detects that point.

The above explanation is an example of the current sink output format, but in the case of the current source output format as well, the node 46 becomes H when an overcurrent is detected. An example of the output format of the current source is shown in FIG. In this figure, external connection terminal d is used.

The circuit shown in FIG. 6 is a current source output type circuit similar to that shown in FIG. 5, but the difference from the circuit shown in FIG. 5 is that the current switching element 26 is
It is constructed using PNP type transistors.

FIG. 7 shows a modification of the current mirror circuit, and the modifications shown in FIG. 7(a) and FIG. 7(c) are well known.

In Figure 4, if the detection voltage is 50 [mV] and the overcurrent is 200 [mA], the resistance R ₁ is R ₁ = 50 mV/200 mV = 0.25 [Ω], and therefore the output current It = 150 [mA]. The ON remaining voltage Vx at the output terminal is Vx = Von + Vr ₁ = 0.2 + 0.25 x 0.15 = 0.2 + 0.0375 = 0.2375 [V], which is a great improvement over the conventional one.

As is clear from this calculation example, in the conventional example, the resistance R ₁ is smaller than the remaining voltage Von when transistor Tr ₁ is ON.
However, according to the present invention, the voltage Vr ₁ across the resistor R ₁ is considerably smaller than the Von remaining when the current switching element 26 is ON, and therefore the voltage Vr ₁ remaining when the current switching element 26 is ON is quite large. If a transistor with a large capacity is used to reduce the voltage Von, it is possible to further reduce the remaining voltage Vx when the output terminal is ON.

[Brief explanation of drawings]

Figures 1 and 2 are circuit diagrams showing conventional overcurrent detection circuits, Figure 3 is a block diagram when the overcurrent detection circuit of the present invention is applied to a proximity switch, and Figure 4 is a circuit diagram of the overcurrent detection circuit of the present invention. A circuit diagram showing one embodiment of the detection circuit, FIG. 5 is a circuit diagram showing a second embodiment of this invention, FIG. 6 is a circuit diagram showing a third embodiment of this invention, and FIG. 7 is a circuit diagram showing a third embodiment of this invention. FIG. 7 is a circuit diagram showing a modification of the mirror circuit. 1... IC circuit, 2... Oscillation circuit, 3... Comparator, 4... Integrating circuit, 5... Comparator, 6... Output circuit, 7... Constant voltage circuit, 8...
Power supply reset circuit, 9...load, 10...switch, 14...output controller, _L1 ...detection coil, _R22 ...variable resistor, _C1 ...resonant capacitor, _C24 ...integrating capacitor, _C25 ... Capacitor, 16 ... Overcurrent detection circuit, 17 ... Electronic switch controller, 18 ... Overvoltage protection circuit, 1
9...Power supply, 20...Current source, 22-25...Transistor, 26...Current switching element, 2
7...Diode, 28...Diode, 32,
34, 35, 36...Resistance, R ₁ ...Resistance, 41,
42... Node, 46... Output node.

Claims (1)

[Claims] 1. A load, a current switching element for controlling the current flowing through the load, and an overcurrent detection resistor are connected in series to a power source, and a first terminal is connected to one terminal of the power source. Connect the emitters of the transistors, connect the emitters of the second transistor to the other terminal of the power supply, connect the collectors of both transistors to each other via a current source, and connect the emitters of the second transistor to the other terminal of the power supply. The emitter of the third transistor is connected to the other terminal of the power source through the resistor, and the emitter of the fourth transistor is connected to the other terminal of the power source through the resistor. The first transistor and the third transistor are connected to each other, and the second transistor and the fourth transistor are connected to form a current mirror circuit, respectively, and the third and fourth transistors are connected to each of the above resistors. A connection terminal is drawn out from a connection point between the current switching element and the overcurrent detection resistor, and this terminal is selectively connected to a connection point between the third transistor and the fourth transistor. An overcurrent detection circuit for an electronic switch, comprising a signal output terminal, the output terminal being connected to an input terminal of a controller that controls the current switching element, and controlling the output state of the controller. 2. An overcurrent detection circuit for an electronic switch according to claim 1, wherein an overvoltage protection circuit is connected between one of the connection terminals and a connection point between the overcurrent detection resistor and the current switching element.