JPS6118312A - Overcurrent protecting circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an overcurrent protection circuit, and particularly to an overcurrent protection circuit suitable for application to a power supply circuit forming each subscriber circuit in an exchange.

An overcurrent protection circuit serves to protect a certain circuit from excessive current when a load current that significantly exceeds the rated current of the circuit flows. The most typical example is Hughes. The fuse blows due to the large current generated during a short circuit fault, protecting the vortex circuit. In addition to heaters, various overcurrent protection circuits have been proposed and put into practical use.

[Conventional technology]

There are two types of overcurrent protection, one using a fusing element such as the above-mentioned fuse, and the other using a semiconductor element.

Those using semiconductor elements are constructed from transistors and the like, and there are known current-limiting types and current-cutting types.

As is well known, the heater is inserted in series with the current carrying path through which the load current flows. On the other hand, the following circuits are known as protection circuits using transistors. FIG. 10 is a circuit diagram showing an example of a current limiting type overcurrent protection circuit. The overcurrent protection circuit 10 shown in the figure is used, for example, to protect a power supply circuit in an exchange. 11 is a resistance of a subscriber line, 12 is a load such as a telephone terminal, Vi is an input voltage,
Vo is the output voltage. As shown in the figure, this overcurrent protection circuit 10 has a configuration consisting of a transistor Q1, a diode D1, and a resistor R, and is a graph showing a predetermined load current value.

As shown in this graph, ■ due to short circuit, etc. is the predetermined load current value■. Once you exceed that amount, there will be a certain price increase. Limited to 8.

FIG. 12 is a circuit diagram showing another example of a current limiting type overcurrent protection circuit, which is called a holdback type. This overcurrent protection circuit 20 is made up of two transistors Q and a resistor R as shown in the figure, and has a function of reducing the load current when it exceeds a predetermined load current value. Figure 13 shows the load current diagram in Figure 12. and output voltage V0
This is a graph showing the relationship between sb, and the connection shown in this graph is caused by short circuits, etc. is the predetermined load current value■. If it exceeds 7, then ■. Yayo) Small constant value. I am brought back to 8.

FIG. 14 is a circuit diagram showing an example of a current cutoff type overcurrent protection circuit. As shown in the figure, this overcurrent protection circuit 30 consists of a transistor Q, a thyristor S, a diode D, a resistor R, and a reset switch R8T.When the thyristor S is turned on due to an overcurrent such as a short circuit, the transistor Q is cut off, Cuts off the load current.

At the time of recovery, when the reset switch R8T is turned on, the thyristor S is turned off and the transistor Q is turned on again.

[Problem that the invention seeks to solve]

First, when looking at the overcurrent protection circuit 10 shown in FIG. 10 and the overcurrent protection circuit 20 shown in FIG. This is the problem. This is undesirable because it reduces power supply efficiency. For example, the collector-emitter voltage of the transistor Q.

is 0.2V, and the base-emitter voltage vEE is 0.6V.
, load current■. is 100 mA, and the voltage at the time of overcurrent detection occurring across the resistor R inserted in series in the current path is 0.8 V, the insertion power loss is 160 (=1
．． 6VxlOOrnA)mW. Moreover, if the resistance 11 of the subscriber line is set to 0.28 Ω/m, the length of the subscriber line is apparently shortened by inserting the overcurrent protection circuit (10, 20).

Generally, the maximum subscriber line length of a small private branch exchange (PBX) is about 500 m, so a reduction of 57 m will result in a significant loss. Furthermore, heat generation in a load short-circuited state is also large, which poses an obstacle to miniaturization and cost reduction of the overcurrent protection circuit. Further, the above-mentioned insertion power loss cannot be avoided in the overcurrent protection circuit 30 shown in FIG. 14 either. Furthermore, in the case of overcurrent protection circuits using the above-mentioned fuses, each time a fuse blows, it must be replaced with a new one, a new one must always be stocked, and the fuse must be visible from the outside. , there are problems such as the need to consider a layout that allows easy replacement with a new product.

The above problems are inherent in conventional overcurrent protection circuits.

[Means for solving problems]

The present invention provides an overcurrent protection circuit that solves the above-mentioned problems, and includes a first transistor that switches the load current and an integration circuit that integrates the collector-emitter voltage of the first transistor. and a second transistor that is controlled on and off by the output voltage of the integrating circuit and bypasses the base current to the first transistor when turned on; It consists of a backup section that backs up the operation of the overcurrent limiter.

[Effect]

When an overcurrent is applied to the first transistor, the first transistor with a common emitter operates in its active region, and its collector-emitter voltage V. E increases. This V.

When the second transistor is turned on in response to an increase in Ii, the base current flowing to the first transistor is cut off, and the first transistor is turned off to cut off the overcurrent. In this case, the second transistor is turned on with a certain delay by the integrating circuit,
It does not respond to instantaneous inrush current. In this way, the overcurrent limiting section is formed.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing the basic form of an overcurrent protection circuit according to the present invention. In this figure, the resistance 11 of the subscriber line,
The load 12 (shown as an equivalent circuit of a telephone terminal) has already been described. The overcurrent protection circuit 40, which forms the basic form of the present invention, includes an overcurrent limiting section 41 and a backup section (BU) 42 shown by a dotted line. Backup section 42
cooperates with the overcurrent limiter 41 to back up its operation (described later). First, regarding the overcurrent limiting section 41,
This is the load current ■. The first transistor Q□ opens and closes the
and a resistor RB and a capacitor CB for integrating the collector-emitter voltage VCE of the first transistor Q1.
and a second transistor Q2 whose base receives the output voltage of the integrator circuit CR. When the second transistor Q2 is turned on, the base current that should be passed through the resistor Rs to the first transistor Q1 is
It functions to bypass 8.

The base current regulator 8 is set such that the first transistor Q1 operates in a sufficiently saturated region during normal operation.

It is better to set an appropriate resistance Rs in advance. During operation in this saturation region, the voltage of the first transistor Q1. B(SA
T) (Saturation voltage between collector and emitter) is O8 several V
It is. Such a low VCE(SAT) is not enough to turn on the second transistor Q2. Furthermore, power loss in the first transistor Q1 is also small.

However, if a short circuit occurs, for example, at terminal TI-T or on the high load side, normal load current control will occur. An overcurrent that significantly exceeds the current flows. When such an overcurrent is applied to the first transistor Q□, the first transistor Q
1 will operate in the active region out of the saturation region. In this active region, ■ of the first transistor Q□. E increases in proportion to the magnitude of the overcurrent. If the overcurrent is 1 or 2 (I!<I,), the increasing VcF
The increment of , respectively becomes ΔVCE□ or ΔVC1i, 2 (ΔVCE<'VCE2) o Then, "CE
(SAT) “The level of VCEz (also ∆VCE2) is the pace-emitter voltage Vnh of the second transistor Q!
(qz) and switches the second transistor Q from off to on.

When the second transistor Q2 turns on slightly, the first
The base current I, which was flowing to the transistor Q□, is
A portion is bypassed to the second transistor Q. Thus, the factor 8 to Ql is slightly reduced. This slight decrease in kW is again due to the V of Ql. This causes an increase in E. This causes Q to turn on even more deeply. On the other hand, the work to Q□8
will continue to decrease.

Then, the VCE of Ql increases more and more, Q2 becomes completely on, and ultimately the supply of IB to Ql is interrupted. At this point, the overcurrent is sufficiently limited and reaches zero. In this way, since the off-state of Ql and the on-state of Q are controlled by positive feedback, it is possible to interrupt an overcurrent without error. in this case,
Since the transition of Ql from off to on and from on to off of Ql is completed in a very short time, the time that the first transistor Q1 is in the active region is very short. This means that although an excessive current flows through the first transistor Q1 (however, the current amplification factor of Ql is less than 8 based on the current amplification factor βXX), this time is very short, so the heat resistance characteristics of the first transistor Q1 are strictly required. This means that no expensive transistor is required as the transistor Q1.

Further, when the second transistor Q2 is apparently turned on (the first transistor Q is off), it continues to maintain that state and cannot reverse its state by itself. In other words, once an overcurrent flows, the Q
Since l remains off (in the new region) and Q2 remains on (in the saturation region), there is extremely little heat generation during short-circuit failures, which is effective for miniaturization and economicalization. In this case, if the load 12 causing the short circuit fault is removed and the base current path of the second transistor Q2 is cut off, Q,
, Q2 automatically returns to the initial state. Therefore, when a normal load 12 is connected again, Q, Q, and Q start supplying power and perform the overcurrent protection operation as described above.

Considering the integrating circuit CR in the overcurrent limiting section 41, this is convenient for obtaining characteristics similar to those of a general fuse. It is well known that when a general fuse is used, it does not respond to instantaneous overcurrent. The clamp will not melt unless an overcurrent exceeding a predetermined level is applied for a certain period of time. Such characteristics unique to heaters are extremely convenient for overcurrent protection in power supply circuits.

In FIG. 1, terminal T1. T, , T, and T
The block labeled 4 forms part of the subscriber circuit package within the switch. In response to the addition of the load 12, a new subscriber circuit package has these terminals T□
~Plugged into T4. At this time, an inrush current flows to charge isometric capacitors and other stray capacitances within the load 12. This inrush current is the normal load current. However, unlike overcurrent caused by a fault, it only flows instantaneously.

Therefore, in order to prevent response to such instantaneous rush current, an integrating circuit CR is provided.

Also, the construction worker uses MDF (Matn I) tributary
ion Frame) If you accidentally touch the terminal board with a screwdriver, etc., an instantaneous overcurrent will flow, but it will not respond to this and will selectively cut off only persistent overcurrents due to short circuit failures, etc. This integration circuit CR has made this possible. Furthermore, depending on how the time constant of CB of this integrating circuit is selected, the time period during which the second transistor is turned on can be arbitrarily set.

FIG. 2 is a graph showing the voltage-current characteristics of the overcurrent limiting section 41 in FIG. 1. VCE(SAT)'a for the overcurrent 11r12+collector-emitter voltage V which appears in the operation description of the overcurrent limiter 41 described above; its increment ΔVCF, 1 rΔvcE2. The voltage between the base and emitter of the second transistor Q2 is V'+74
1) Run BE (Q2) The relationship between the base current of transistor Q and ■8 can be seen at a glance.

Note that I is the rated current.

Further, the operation of the overcurrent limiting section 41 will be explained more qualitatively as follows. This overcurrent limiter 41 can detect an overcurrent and cut it off by a combined resistance on the load side (combined resistance when looking at terminals T, -T, and load 12 in FIG. 1) Rr. is determined by the following equation (1).

Here, VcE(SAT), ``BE(Q2') I have the same meaning as described above, E is the power supply voltage (the absolute value of 24V in Figure 1), and β is the 11 transistor Q1. is the DC current amplification factor.

For example, if a short-circuit fault occurs, the delay time Toff until the resulting overcurrent is cut off is as follows:
It can be roughly determined from the following equation (2).

Here ~ time constant τ1ΔVCE(Ql)'ΔVBE(Q is 1τ=RBxCB(3) ΔVCE(Qs) = E-VCF(SAT)-(βx
I, X Rr) (4)”EB(Qn) =v
BE(Qz) −vCE(SAT) (5)

Try substituting specific numerical values into each of the above equations. That is,
E = 24 V”CIC(SAT)” O’2 V”
BE (Q = 0.7 V, β = 210, I B =
1.2 mh % RB = 200Ω, cB = -io
Substitute pF'.

According to the above formula (1), the range of Rr(2) is 91>Rr≧
0(6) According to equation (2) above, delay time T. ff(SeC) is T
oH average (200xlO"xlOxlO-').

FIG. 3 is a graph showing the cut-off behavior of the overcurrent protection circuit according to the present invention, and the horizontal axis is T. ff and is displayed on a logarithmic scale. The vertical axis shows the combined resistance Rr and the load current.

Take. 8 is a virtual current obtained by converting the combined resistance Rr into a current value. From this graph, it can be seen that the time required to shut off changes depending on the magnitude of the overcurrent, which can be short or long, and its characteristics are very similar to those of a fuse. Note that the curve T in this graph. ff is τ
The curves obtained from the above formula when τ = 2 seconds are shown, while the curves τ1 and τ2 are plots of actual measured values when τ = 1 second and τ = 2 seconds, respectively.

Returning to FIG. 1, the backup section 42 in this figure compensates for the operation that the overcurrent limiting section 41 cannot perform. The motion to be supplemented may be appropriately selected as necessary.

The embodiments described below show examples of individually realizing various operations to be compensated, but some of these can also be used in combination.

FIG. 1 is a circuit diagram showing a first embodiment of the backup section shown in FIG. 1. In addition, the same reference number or symbol is attached and shown to the same component throughout all the figures. A first embodiment of the backup section constituting the overcurrent protection circuit 50 is shown as a capacitor C8 in the figure, and is connected between the base and emitter of the first transistor Q1. As described above, the block divided by the terminals T1 to T4 constitutes a subscriber circuit package, and when newly installed, it is plugged into these terminals T, to T4. At this time, sparks generated at these terminals (indicated by arrow SP in the figure) become a problem.

This spark does not cause noise to adjacent subscriber circuit packages that are already in operation, causing malfunctions;
This is because the T4 itself is subject to burnout due to sparks. Such a spark SP is generated because an excessive inrush current flows through these terminals T and T4 to charge the capacitor in the load 12 and other stray capacitances attached to the line. Generally, one subscriber circuit package has multiple
For example, since eight subscriber circuits are housed together, the spark generated at terminals T and -T, which are shared by these subscriber circuits, is enormous.

The backup section (Cs) of the first embodiment prevents such an inrush current from flowing instantaneously when the subscriber circuit package is plugged into the terminals T1 to T4. That is, the first transistor Q1 is turned off at the beginning of plug-in, and then turned on. The focus is on the fact that even if an excessive inrush current flows, sparks will not occur after plug-in is completed.

That is, the base N flow 3 to the first transistor QI does not flow into the base of Ql immediately after plug-in;
Absorbed by capacitor Cs. Then, Ql turns on for the first time after a time constant determined by resistor RS and capacitor Cs has elapsed. The slow movement of the knob Q1 is realized by Cs.

In this case, the time constant τB (“CBx RB
) and the setting of the time constant t″5 (=C8″R8). However, τS must not exceed τS.

This is because the original role of the integrating circuit CR is lost. Practically speaking, it is desirable that τS = -Rain - τB.

FIG. 5 is a circuit diagram showing a second embodiment of the backup section shown in FIG. 1. A second embodiment of the backup section constituting the overcurrent protection circuit 60 is configured as a first switch SW1 and a second switch SW2 in the figure. The figure shows an example configured with photocouplers. Overcurrent limiting section 4 shown in FIG.
1 turns off the first transistor Q□ (Q is on)
After the load 12 is removed, the state cannot be reversed by the positive feedback described above, and the state can be returned to the initial state by removing the load 12. However, in this embodiment, the load 12 Instead of removing the first switch Swiμ2
The switch SW allows for on/off control of the Q-engine and the pump.

There is a short circuit fault, the second transistor Q2 is on, the first
After the transistor Q is turned off, the first pulse P1 is applied to recover from the failure and restart power supply. This turns on the first switch sw, and the first transistor Q2
The base current to is cut off. On the other hand, the second transistor Q
2, the supply of base current to the first transistor Q1 is restarted, and it is turned on. At this point, power supply is resumed, and this state is maintained until an overcurrent occurs.

Conversely, when there is a request to cancel this power supply state, the second pulse P is applied. This is multi-tube 2 switch SW
2 is turned on, turning on the second transistor Q2. This cuts off the supply of base current to the first transistor Q1, turning it off. At this point, power supply is interrupted.

FIG. 6 is a circuit diagram showing a third embodiment of the buffer lab section shown in FIG. 1. The third embodiment of the backup section constituting the overcurrent protection circuit 7o realizes a "disconnection" display function. If the overcurrent protection circuit uses a fuse, it is possible to know whether the fuse is blown or not by directly monitoring whether the fuse is blown or not.Especially, in exchangers, etc., failure can occur. Rapid removal is important, and for this it is first necessary to discover which subscriber circuit is faulty.
, is on (Q, is off). Taking advantage of this fact, a light emitting element LED is connected to the collector side. Then, when the circuit 70 is "off", the light emitting element LED will not light up, and the faulty system can be instantly found.

In order to prevent the first transistor Q1 from accidentally turning on when the second transistor Q2 is turned on, an element is required to cancel the forward voltage of the light emitting element LED. , Zener diode ZD
, inserted in series to the pace of 〈. - FIG. 7 is a circuit diagram showing a modification of the backup section of FIG. 6. In this overcurrent protection circuit 70', another light emitting element LED' is provided in place of the Zener diode ZD shown in FIG. Since this light emitting element LED' lights up when the first transistor Q□ is turned on, its lighting indicates that the overcurrent protection circuit is "connected" (in normal operation).

FIG. 8 is a circuit diagram showing a first embodiment of the backup section shown in FIG. 1. The overcurrent protection circuit 80 is connected to Fl and F,
- The current detection units DET□ and DET2 are provided in the parts shown in the figure. Specifically, the LED and LED' in Figure 7
It can be formed using photocouplers that form a pair with each other.

In this way, when the "connection"/""disconnection" status signal of the power supply circuit is output, it is possible to control the single connection/"disconnection" of the power supply circuit from the outside as in the second embodiment. This allows easy control from a control device such as a microconbeater, and enables maintenance, automation of operation, and remote control.

FIG. 9 is a circuit diagram showing an application example of the overcurrent protection circuit according to the present invention, and shows a part of a telephone switching system. In this figure, 12 is the load mentioned above, such as a telephone terminal. The figure shows a so-called multi-function telephone. load 12
A line 91 connected to the line 91 corresponds to the resistor 11 of the subscriber line described above, and is a power supply line as well as a control line. Line 92 forms a communication line through which voice signals flow. An exchange main body 93 is connected to the other ends of these lines 91 and 92. It consists of an interface card 94 and a communication path and control device 95. Overcurrent protection circuit of the present invention (40, 50,
60, 70, 70', 80) are housed as blocks 96 within an interface card 94.

The four-piece coil CH and capacitor CD in the load 12 form a low-pass filter, and the OO/DC converter (CONV)
It receives DC (D24V) and lowers it to 5V. This 5V becomes the power source for the communication circuit and control circuit constructed with 0MO8 IC, etc. Note that Tr in the figure is a transformer for impedance matching.

〔Effect of the invention〕

As described above in detail, according to the present invention, the base current to the first transistor that controls the opening and closing of the load current conduction path is bypassed, and the opening and closing control of the second transistor that cuts off the first transistor is performed by the integrating circuit. This can be done by output voltage.

Therefore, the second transistor performs opening/closing control with a delay time determined by the time constant of the integrating circuit, and does not malfunction even in response to transient changes in load current, which has the advantages of a fuse. . Furthermore, once an overcurrent flows through the first transistor and the second transistor, they continue to be turned off and turned on, respectively, so they have the advantage of a fuse in this respect as well.

Further, by selecting the values of R and C of this integrating circuit, the cut-off time of the load current carrying path can be arbitrarily set. Also, while the transient current or overcurrent is being limited, the first transistor is in the active region and the collector loss increases, but the duration of this does not exceed the time determined by the constant of the integrating circuit, so it is extremely It is possible to create a protection circuit that generates less heat, allowing for high-density packaging (miniaturization). In addition, as a characteristic of general transistors, the single-shot maximum allowable collector loss as mentioned above is more than 10 times that of the continuous case, so it is possible to use a transistor with small collector loss, which is economical. .

Furthermore, between the base and emitter of the first transistor,
Since a capacitor is provided to absorb inrush current, sparks will not be generated at the connector even if the package is inserted while the current is being applied, and therefore there will be no disturbance due to burnout of the connector or induced noise to adjacent packages. There is no fear of
Maintenance work such as adding packages can be easily performed.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the basic form of the overcurrent protection circuit according to the present invention, FIG. 2 is a graph showing the voltage-current characteristics of the overcurrent limiting section 41 in FIG. 1, and FIG. 27. Showing the interruption characteristics of the current protection circuit. Figure 1 is the first
FIG. 5 is a circuit diagram showing a first embodiment of the backup section shown in the figure, and FIG. 5 is a circuit diagram showing a second embodiment of the backup section shown in FIG. 6 is a circuit diagram showing a third embodiment of the backup section shown in FIG. 1, FIG. 7 is a circuit diagram showing a modification of the backup section shown in FIG. 6, and FIG. 8 is a circuit diagram showing the backup section shown in FIG. 1. 9 is a circuit diagram showing an application example of the overcurrent protection circuit based on the present invention, FIG. 10 is a circuit diagram showing an example of a current limiting type overcurrent protection circuit, 11 is a graph showing the relationship between load current I0 and output voltage V0 in FIG. Load current engineer. FIG. 14 is a circuit diagram showing an example of a current interrupt type overcurrent protection circuit. 11...Resistance of subscriber line, 12...Load, 40.
50.60, 70.70', 80... Overcurrent protection circuit, 41... Overcurrent limiting section, 42... Backup section, Ql... First transistor, Qz... Second transistor, CR...integrator circuit, IB...base current,
CBIC8...Capacitor, RB, R8...Resistor,
SW...first switch, SW,...second switch,
LED, LED'...Light emitting element, DET engineering, I)E
T, . . . current detection element.

Claims (1)

[Claims] 1. A first transistor that opens and closes a load current conduction path;
an integrating circuit that integrates the voltage between the collector and emitter of the first transistor; and a second transistor whose on/off is controlled by the output voltage of the integrating circuit, and by turning on the second transistor, the base of the first transistor An overcurrent protection circuit configured to bypass current. 2. The scope of the claim is that the integrating circuit is constituted by a capacitor and a resistor, and the response speed of the second transistor when turned on is arbitrarily set by selecting a time constant determined by the capacitor and the resistor. The overcurrent protection circuit described in item 1. 3. The overcurrent protection circuit according to claim 1, wherein a capacitor for absorbing inrush current is connected between the base and emitter of the first transistor. 4. A time constant τ_B determined by a capacitor and a resistor constituting the integrating circuit, and a time constant determined by a capacitor connected between the base and emitter of the first transistor and a resistor connected in series with the base of the first transistor. The overcurrent protection circuit according to claim 3, wherein τ_S is set to τ_S<τ_B.