WO1994013049A1

WO1994013049A1 - Arrangement in a current limited circuit for transferring overload power

Info

Publication number: WO1994013049A1
Application number: PCT/FI1993/000509
Authority: WO
Inventors: Jarmo Loukusa; Kauko Varanka
Original assignee: Nokia Telecommunications Oy
Priority date: 1992-11-30
Filing date: 1993-11-30
Publication date: 1994-06-09
Also published as: AU5564694A; FI925452A; FI925452A0

Abstract

The invention relates to an arrangement in a current limited circuit for transferring overload power. The arrangement comprises a current limited semiconductor switch (Q17), and in the arrangement the current limiting of the semiconductor switch (Q17) is implemented by a current limiter (Z4) contained in the arrangement, and the arrangement further comprises a resistor (R49), which is coupled from the second electrode on the main current path to the signal path (A), a resistor (R48) being coupled between the electrodes on the main current path of the current limited semiconductor switch, said resistor being arranged - on account of a voltage drop brought about over said resistor (R48) by a decrease in the control current caused by a current limiter (Z4) - to take part of the current of said semiconductor switch (Q17) in overload conditions.

Description

Arrangement in a current limited circuit for transfer¬ ring overload power.

The invention relates to an arrangement in a current limited circuit for transferring overload power, the arrangement being used in transferring current e.g. from a source of potential to a signal path, the arrangement comprising a current limited semiconductor switch, which comprises a control electrode and main current path electrodes, through which main current path electrodes current can be transferred to the signal path; current limiting of the semiconductor switch being implemented in the arrangement by a current limiter contained in the arrangement, the limiter being coupled from the control electrode of the semiconductor switch to the signal path, and the arrangement further com¬ prising a resistor coupled from the second electrode on the main current path to the signal path.

In this connection, a circuit means e.g. switches. Semiconductor switches can be used e.g. for switching on a current. When switching on a current, the semiconductor switch is only capable of feeding a pre¬ determined maximum current, which depends on the size of the switch and current feeder power as well as other factors. The switches should normally be protected against overload and short circuit. They can be used e.g. to couple ground potential to a signal path, such as a dialling line in devices of telecommunication technology. In a prior art solution, the connection used is single transistor connection, where the current of the transistor coupling ground potential to the line is limited by a zener diode. In overload conditions, power thereby accumulates in the switching transistor itself. In the prior art solution, big, well encapsulated tran- sistors with good current and power tolerance must be used. In some cases, cooling elements have also been used for enhancing power tolerance. If the signal path to which ground potential is to be coupled has a maximum current of 100 mA and a voltage of 60 V, a conventional transistor undergoes a voltage drop of 6 W, whereby a power transistor is needed.

The object of the present invention is to pro¬ vide a new kind of arrangement in a current limited circuit for transferring overload power, avoiding the problems involved with the known solutions.

This is achieved with an arrangement according to the invention, which is characterised in that between the electrodes on the main current path of the current limited semiconductor switch is coupled a resistor which on account of a voltage drop brought about over said resistor by a decrease in the control current caused by a current limiter is arranged to take part of the current of said semiconductor switch in overload condi- tions.

Several advantages are achieved with the solu¬ tion according to the invention. The solution of the invention enables the use of less expensive components that require less space. The solution of the invention can be applied to all electronic apparatuses where power tolerant direct-current switches that require little space and must tolerate even heavy overload are needed. In conventional connections, increase in voltage means that a transistor experiences more stress, whereas in the new solution the transistor experiences less stress since the resistor takes a larger part of the current. The solution of the invention also improves reliability since the switches are not subjected to overload condi¬ tions. In the new solution, a hybrid resistor or power resistor is used, and the maximum power that the switch¬ ing transistor can take does not exceed 0.5 W.

In the following the invention will be described in greater detail with reference to the attached drawings, wherein

Fig. 1 shows a circuit diagram of the arrange¬ ment according to the invention,

Fig. 2 illustrates the current of the switch as a function of the collector-emitter voltage of the switch.

Fig. 1 shows the arrangement according to the invention in a current limited circuit for transferring overload power. The arrangement is used for transferring current e.g. from a source of potential G to a signal path A. The arrangement may be e.g. a switching element in a direct-current signal device, by which element e.g. ground potential G is coupled via resistors e.g. to a dialling line, such as the signal path A in Fig. 1.

The arrangement comprises a current limited semiconductor switch Q17, which comprises a control electrode 1 or a base B, and main current path electrodes 2 and 3 or a collector C and an emitter E, current being transferable via said main current path electrodes 2 and 3 to the signal path A. In the arrange- ment, the current limiting of the semiconductor switch Q17 is implemented by a current limiter or advantageous¬ ly a zener diode Z4 contained in the arrangement, the limiter being coupled from the control electrode 1 or base B of the semiconductor switch Q17 to the signal path A. The arrangement further comprises a resistor R49 coupled from the second electrode 2 or emitter E on the main current path to the signal path A. In the inven¬ tion, between the electrodes 2 and 3 on the main current path of the current limited semiconductor switch Q17 is coupled a resistor R48 which on account of a voltage drop U(CE) brought about over said resistor R48 by a decrease in the control current I_B caused by a current limiter Z4 is arranged to take part of the current of said semiconductor switch Q17 in overload conditions. The decrease in the control current naturally reduces the current in the collector circuit, i.e. the current on the current path called a main current path in normal conditions. The decrease in the control current I_B naturally reduces the collector current I_c or in normal conditions the current on the main current path. The reduction in the collector current caused by the decrease in the control current thus produces a voltage difference over the transistor or the resistor R48.

In an advantageous embodiment of the invention the arrangement comprises a second semiconductor switch Q16, which comprises a control electrode 4 or a base B and main current path electrodes 5 and 6 or a collector C and an emitter E. Said second semiconductor switch Q16 is current unlimited, and it is connected in series with the first current limited semiconductor switch Q17, and it is further characterised in that the current unlimitedness of the second semiconductor switch Q16 is implemented by switching said semiconductor switch to a saturation state. This enables e.g. coupling of ground potential 7 by a current unlimited switch Q16, a current limited switch Q17 provided with a resistor R48 being connected above it in series as shown in Fig. 1. The switches Q16 and Q17 or advantageously transistors are connected in series such that the main current path of the current unlimited switch Q16 is connected with the main current path of the current limited switch Q17 and they are advantageously separated by a power resistor R47. The emitter E of the current unlimited switching transistor Q16 is thus coupled via a low ohmage resistor R47 to the collector C of the first switching transistor Q17.

In an advantageous embodiment for switching a current unlimited second semiconductor switch Q16 to a saturation state the arrangement comprises a third semi¬ conductor switch Q15, which comprises a control electrode or a base B and main current path electrodes 10 and 11 or a collector C and an emitter E, the switch Q15 being coupled at the collector C or electrode 10 to control the switching operation of the current unlimited second semiconductor switch Q16. The third semiconductor switch is connected at the base or electrode 9 to a control line 12, by which the base can be activated for a switching operation. For example, ground potential can be taken from the control line 12 to the base or electrode 9. The control line 12 can be used for coupling positive current from a source of potential 13 to the main current path or collector or electrode 10 of the third switch Q15, the potential of the source being +5 V (5 volts positive). This is an advantageous way of switching the current unlimited switch Q16 to a saturation or short circuit state in overload condi¬ tions. In a saturation state, the current I_τoτ flowing from the ground potential G via the current unlimited switch Q16 and the resistors R47, R48 and R49 to the signal path A is high. The resistance on this current path is very low, i.e. the sum of the resistances R47, R48 and R49, which makes 839 ohm based on the values indicated in Fig. 1. In a preferred embodiment, the resistance (550 ohm) of the shunt resistor of the current limited switch Q17 provides for the major part of the resistance on said current path. The power thereby remains at the shunt resistor R48. As regards the components, in a preferred embodiment the current unlimited semiconductor switch Q16, and also Q17, is an npn transistor, and the third semiconductor switch Q15 controlling the switch Q16 is of the opposite type, i.e. a pnp transistor. A change in the voltage set between B and A reverses the situ¬ ation.

With regard to the operation, the following is stated. When the circuit (switches) is active or switched on, the base of the switch Q15 receives ground potential from the control line 12, the ground potential establishing control current at the base. This produces a main current path from the potential 13 via the resistor R43 to the collector 10 of the switch Q15 or transistor, and it passes the control current to the base of the switch Q16. In overload conditions, Q16 undergoes saturation or short circuit. In short circuit or saturation conditions, hardly any power remains in the transistor Q16. Current is then set up from ground potential G to flow via the collector 5 and the emitter 6 and via the resistor R47 through the current limited switch Q17 to the resistor R49 and further to the point to be controlled, i.e. the signal path A. The current flowing on the path brings about a voltage drop in the resistors R49 and R47. The voltage in the resistor R47 maintains the current limiting circuit in a conductive state. A second voltage drop in the resistor R49 causes current limiting such that when the voltage drop grows sufficiently great (threshold voltage of the zener diode e.g. 4.7 V), i.e. when the current in the resistor R49 is sufficiently high, the zener diode Z4, which is coupled between the resistor R49 and the base of the switching transistor Q17, begins to limit the voltage and current I_B flowing to the base. The current I_c of the transistor Q17 is thereby limited to a predetermined maximum. When limiting of the current of the transistor Q17 begins, the voltage drop U(CE) over the transistor Q17 establishes current in the resistor R48. The higher the voltage on a signal path A, such as a signal line, the larger the portion of the current that flows via the resistor R48 and the smaller the portion that flows through the transistor as current I_c. The resistor R49 also measures the current of the resistor R48; the total current I_τoτ remains the same as shown by the direct por- tion in Fig. 2, but the power remains in the resistor R48. The essential point is that in overload conditions the resistor R48 of the current limited switch (circuit) takes the power to itself. The resistor R49 limits current, i.e. determines the maximum current that can pass through the transistor Q17. The zener diode Z4 on the other hand limits the control current coming to the base and thereby makes the current flow via the resistor R48.

In Fig. 1 the upper switch or switch Q17 is a current limited circuit by operation, operating such that the current flowing to the signal path A grows to be sufficiently high, i.e. so high that the voltage of the emitter resistor R49 reaches a voltage that is equal to the threshold voltage of the zener diode minus the base-emitter voltage of the transistor Q17; at this stage the potential in this point is so high that the zener diode Z4 can draw base voltage downward, whereby limiting of the current of the transistor begins. In other words, the base current is limited by the zener diode. Thus, as the collector current tends to grow, the zener diode prevents base control, whereby the current flowing through the transistor Q17 is limited thereto. The current of the transistor Q17 thereby starts to transfer to the power resistor R48, as shows in Fig. 2. In its simplest form - without the switch Q16 - the solution according to the invention operates as a current limiter in which the transistor Q17 is not loaded. The collector of the current limited switch Q17 is then coupled directly to ground potential G and not to the switch Q16 as in Fig. 1.

The arrangement according to the invention as shown in Fig. 1 is based on the idea that the current of the switch Q16 is not limited in any way but the switch is an unlimited switch, i.e. at a saturation state, and is connected in series with a current limited switch Q17. In the Fig., voltage dividing resistors R45 and R46 are arranged between the base 4 of the current unlimited switch Q16 and the base 1 of the current limited switch Q17.

Although the invention is described above with reference to the embodiments illustrated in the attached drawings, it is to be understood that the invention is not restricted thereto but that it can be modified in many ways without deviating from the inventive idea disclosed in the attached claims.

Claims

1. An arrangement in a current limited circuit for transferring overload power, the arrangement being used in transferring current e.g. from a source of potential (G) to a signal path (A) , the arrangement comprising

- a current limited semiconductor switch (Q17), which comprises a control electrode ( 1 ) and main current path electrodes (2,3), through which main current path electrodes (2,3) current can be transferred to the signal path (A); current limiting of the semiconductor switch (Q17) being implemented in the arrangement by

- a current limiter (Z4) contained in the arrangement, the limiter being coupled from the control electrode (1) of the semiconductor switch (Q17) to the signal path (A), and the arrangement further comprising a resistor (R49 ) coupled from the second electrode (3) on the main current path to the signal path (A) , c h a r a c t e r i s e d in that between the electrodes (2,3) on the main current path of the current limited semiconductor switch (Q17) is coupled a resistor (R48) which on account of a voltage drop (U(CE)) brought about over said resistor (R48) by a decrease in the control current (I_B) caused by a current limiter (Z4) is arranged to take part of the current of said semi¬ conductor switch (Q17) in overload conditions.

2. The arrangement according to claim 1, c h a r a c t e r i s e d in that the arrangement com- prises a second semiconductor switch (Q16), which com¬ prises a control electrode (4) and main current path electrodes (5,6), said second semiconductor switch (Q16) being current unlimited, and that said current unlimited semiconductor switch (Q16) is connected in series with the current limited semiconductor switch (Q17), and that the current unlimitedness of the second semiconductor switch (Q16) is implemented by switching said semi¬ conductor switch (Q16) to a saturation state.

3. The arrangement according to claim 2, c h a r a c t e r i s e d in that to switch the current unlimited second semiconductor switch (Q16) to a satura¬ tion state, the arrangement comprises a third semi¬ conductor switch (Q15), which comprises a control electrode (9) and main current path electrodes (10,11), said semiconductor switch (Q15) being coupled such that it controls the switching operation of the current unlimited second semiconductor switch (Q16).

4. The arrangement according to claim 3, c h a r a c t e r i s e d in that the current unlimited semiconductor switch (Q16) is an npn transistor and the third semiconductor switch (Q15) controlling it is of the opposite type, i.e. a pnp transistor.