Device for limiting the working voltage for mechanical switches in telephony

Abstract: Device for limiting the working voltage for mechanical switches in telephony includes terminals (20,21) for connection to a telephone line, a connection and power supply branch for a control circuit (24) extending from a first terminal (21), the branch having a first switch (25), the cathode terminal of a first Zener diode (26) and the source terminal of a first MOSFET transistor (27) being connected to the output terminal of the first switch (25), the gate terminal of the first MOSFET transistor (27) being connected, through the anode terminal of the Zener diode (26), to the first terminal. The current absorbed by the device may be adjusted.

Description: The present invention relates to a device for limiting the working voltage for mechanical switches in telephony. As is known, a switch, technically termed a "hook", is used in telephone sets to connect the telephone set itself to the telephone line when the handset is lifted. This hook carries, when activated, a line current which in some cases reaches a value of 120 milliamperes. Traditionally, in electromechanical sets the hook comprised an actual mechanical switch, as better explained hereinafter. In recent times the mechanical switch has been completely or partially replaced with electronic switches, such as bipolar transistors or MOSFET transistors, due both to their lower costs and to their higher reliability, which is the result of the elimination of problems related to the oxidation of the contacts of the switch due to sparking, and other minor problems. However, in order to drive these electronic switches it is still necessary to use a mechanical or magnetic switch to detect the lifting of the handset from its normal on-hook state. In this case one obtains the simplification of the mechanical switch, of the simple-contact type, as well as the reduction of the currents which flow through the switch itself, which can be estimated in the range of a few microamperes. The voltage that in any case remains applied across these switches, when the switch is open, is equal to the voltage present on the telephone line. In some cases, this line voltage can be of several hundred volts, for example in the case of a call as signal for the ringer or during tests performed on telephone lines, and is limited only by the value set by the circuit which protects against atmospheric discharges, i.e. lightning. In the presence of such high voltage values it is currently necessary to follow precautions, since the humidity of the air can cause the false closure of the switch due to a significant reduction in insulation and thus produce a false closure detection which thus causes an associated actual closure of the electronic line switch. From DE-A-3 623 017 it is known a device adapted to drive the on-hook/off-hook switch of a telephone set. A transistor is provided in order to enable the closure of the loop; the transistor is driven by a low resistance signal provided upon the lifting of the telephone handset. An object of the present invention is to eliminate or substantially reduce the problems described above by providing a device for limiting the working voltage for mechanical switches in telephony, which device limits the voltage across the mechanical switches present in electronic sets in order to eliminate false closure signals. Another object of the present invention is to provide a limiting device that allows the use of low voltage/current switches. Yet another object of the present invention is to provide a limiting device which is highly reliable, relatively easy to manufacture and has a competitive cost. The objects mentioned and others which will become apparent hereinafter are achieved by a device for limiting the working voltage for mechanical switches in telephony, as defined in appended claim 1. Further characteristics and advantages of the invention will become apparent from the description of some preferred but not exclusive embodiments of a device for limiting the working voltage for mechanical switches in telephony, according to the invention, which are illustrated only by way of non-limitative example in the accompanying drawings. Figure 1a is a known circuit diagram of an electromechanical circuit with mechanical connection switch for telephony; Figure 1b is a known circuit diagram of a circuit with an electronic connection switch for telephony; Figure 2 is a circuit diagram of a first embodiment of a limiting device according to the invention; Figure 3 is a circuit diagram of a second embodiment of a limiting device according to the invention; and figure 4 is a sectional front elevation view of a portion of a MOSFET depletion transistor. With reference to figures 1a and 1b, a mechanical switch 1 was traditionally used for the hook of a telephone; said switch alternatively selected connection between the terminals 2 and 3 of a telephone line or to the terminals 4 and 5 of a talk circuit or to a ringer 6. As illustrated in figure 1b, by using electronic switches, coupled with simple mechanical switches, the telephone line, at terminals 2 and 3, is connected to a "start" circuit 60 by means of a simple switch 7 which, through a simple switch 9 which cooperates with a control circuit 8, allows for connection of the talk circuit at terminals 4 and 5. With reference to figure 2, a device for limiting the working voltage for mechanical switches in telephony in accordance with the present invention comprises terminals 20 and 21 for connection to a telephone line and terminals 22 and 23 for connection to a talk circuit. A connection and power supply branch for a control circuit 24 extends from a first terminal 21 and has a first switch 25. The cathode terminal of a first Zener diode 26 and the source terminal of a first MOSFET transistor 27 are connected to the output terminal of said first switch 25. The gate terminal of said first MOSFET transistor is connected, through the anode terminal of the first Zener diode 26, to the first terminal 21. Means are provided for adjusting the current absorbed by the device, as will become apparent hereinafter. The first MOSFET transistor 27 is a depletion transistor, technically termed "depletion MOSFET". The same reference numerals are kept hereinafter for identical components. In the first embodiment illustrated in figure 2, the means for adjusting the absorbed current comprise a first resistor 28 that is coupled in series with the drain terminal of the first transistor 27 and a second resistor 29 that is coupled in series with the first resistor 28 and is connected to a second terminal 20 of the telephone line. The anode terminal of a second Zener diode 30 is connected between the first resistor 28 and the second resistor 29 and the cathode terminal of Zener diode 30 is connected to the second terminal 20 of the telephone line. The gate terminal of a second MOSFET transistor 31 is connected to the anode terminal of the second Zener diode 30 and a terminal of the control circuit 24 is connected to the drain terminal of said second MOSFET transistor. The control circuit 24 controls a second switch 32 which comprises an appropriately connected transistor. The first switch 25, which is mechanical, is sized, when open, for a working voltage set by the first Zener diode 26, and no current can flow into the first transistor 27 and thus the second transistor 31 is off. This situation is optimum, since no absorption is tolerable in the on-hook state. The value of the first Zener diode 26 must be such as to ensure the off state of the first transistor 27. When the first switch 25 is closed, i.e. when the handset is lifted, i.e. in the off-hook state, the first transistor 27 starts to conduct and causes a drop in the voltage across the second resistor 29 and thus the conduction of the second transistor 31. This situation, once processed by the control circuit 24, causes the conduction of the second switch 32, which as mentioned earlier comprises a transistor. The value of the current absorbed by the telephone line is mainly determined by the voltage applied to the line and by the value of the sum of the resistances of the first resistor 28 and of the second resistor 29, in the initial state, and subsequently, in the steady state, by the value of the resistance of the first resistor 28 alone. The current is practically not limited through the second transistor 31 except by the value of the voltage drop across the second resistor 29 and by the constructive characteristics of the second transistor 31. From a practical point of view, the second transistor 31 acts as a switch. In a second embodiment, the second transistor 31, arranged between the terminals A, B and C of the circuit of figure 2, can be replaced with a first PNP bipolar transistor 33 without altering the above described functions of the circuit. In a third embodiment, as illustrated in figure 3, a third resistor 34 is arranged in series to the source terminal of the first transistor 27 and to the cathode terminal of the first Zener diode 26, and is connected to the first switch 25. In this situation, it is possible to eliminate the first resistor 28 and the second resistor 29, advantageously replacing them with a third MOSFET transistor 35 wherein the drain terminal is connected to the drain terminal of the first transistor 27, the source terminal is connected to the second terminal 20 of the telephone line and the gate terminal is connected to the drain terminal and to the gate terminal of the second transistor 31. In this case, the current which flows in the first transistor 27, which is a function of the threshold voltage of said first transistor and of the resistance of the third resistor 34, also flows in the second transistor 31 and in the third transistor 35, and except for possible different area ratios among the transistors, obtains a substantially constant current consumption, i.e. dissipated power level, of the telephone line for any value of the voltage present on the terminals 20 and 21. Without loss of generality, the second MOSFET transistor 31, at terminals A, B and C, can be replaced with the first PNP bipolar transistor 33, while the third MOSFET transistor 35, at terminals D, E and F, can be replaced with a second PNP bipolar transistor 36, while maintaining the above described functions. In particular, the first MOSFET transistor 27 can be produced, so that it is integrated with the other above described components, with the Multipower BCD250 process, for example, deriving it from a lateral power MOSFET transistor, technically termed "LDMOS", wherein doping with a dopant, preferably arsenic, is performed prior to gate oxidation in an appropriate dose in order to provide negative threshold control of transistor 27. With reference to figure 4, the doping is designated, in the standard structure of the LDMOS, by the reference numeral 38. Accurate control of the dose of the doping, which is normally performed by implantation, and of the diffusion in the body of the device is necessary in order to ensure negative threshold voltages in the range of a few volts without compromising the breakdown voltage of said first transistor 27. The threshold voltage of the first transistor 27 must be smaller, in absolute value, than the Zener voltage of the first diode 26 in order to ensure proper functioning of the circuits described earlier with reference to figures 2 and 3. In particular, the so-called "P-well" diffusion 40 of the BCD250 process has been chosen as the diffusion. This diffusion is produced prior to the deposition of a layer 39 of polysilicon that defines the gate terminal of the first transistor 27. The first transistor 27 thus has a body diffusion 40 that is not self-aligned with the layer 39 gate terminal. This diffusion, which differs from the conventional diffusion of transistors produced with the BCD250 process, allows low arsenic doping doses in order to control low thresholds. This leads to particularly high breakdown voltages of the transistor even for negative threshold voltages in the range of a few volts. Practical tests have shown that the invention achieves the intended aim and objects, making the dissipated power practically constant, independently of the voltage across the terminals 20 and 21 of the telephone line, furthermore advantageously allowing correct operation in the presence of any biasing of the telephone line. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs

Claims: A device for limiting the working voltage for mechanical switches in telephony comprising a first and second terminals (20, 21) for connection to a telephone line, third and fourth terminals (22,23) for connection to a talk circuit, a first mechanical switch (25) adapted to reflect an on-hook or off-hook condition of an handset and connected between said terminals (20, 21), the device further comprising an electronic switch (32) connected between said first and third terminals (20, 22), characterized in that a cathode terminal of a first Zener diode (26) and a source terminal of a first MOSFET transistor (27) are connected to an output terminal of said first switch (25), a gate terminal of said first MOSFET transistor (27) being connected, through an anode terminal of said Zener diode (26), to said first terminal (21), and in that a second transistor (31) is connected with its terminals respectively to the first terminal (20) and to a control circuit (24), said control circuit being adapted to control said electronic switch (32), absorbed current limiting means (28, 29, 30; 35, 36) being connected between the body command terminal of said second transistor (31) and the drain terminal of the first MOSFET transistor (27) and between said body command terminal of said second transistor (31) and said first terminal (20). A device for limiting the working voltage for mechanical switches in telephony according to claim 1, characterized in that said second transistor (31) is a MOSFET transistor, the drain and source terminals thereof being respectively connected to the first and second terminals (21, 20), the gate terminal thereof being connected to said absorbed current limiting means (28, 29, 30; 35, 36). A device for limiting the working voltage for mechanical switches in telephony according to claim 1, characterized in that said second transistor (31) is a bipolar transistor, the collector and emitter terminals thereof being respectively connected to said first terminal (21) and to the control circuit (24), the base terminal thereof being connected to said absorbed current limiting means (28, 29, 30; 35, 36). A device for limiting the working voltage for mechanical switches in telephony according to claim 2, characterized in that said absorbed current limiting means comprise a first resistor (28) arranged in series to the drain terminal of said first transistor (27) and a second resistor (29) which is arranged in series to said first resistor (28) and is connected to a second terminal (20) of said telephone line, the anode terminal of a second Zener diode (30) being connected between said first resistor (28) and said second resistor (29), the cathode terminal of said second Zener diode (30) being connected to said second terminal (20) of said telephone line, the gate terminal of a second MOSFET transistor (31) being connected to said anode terminal, a terminal of said control circuit (24) being connected to the drain terminal of said second MOSFET transistor (31), the source terminal of said second MOSFET transistor (31) being connected to said second terminal (20) of said telephone line. A device for limiting the working voltage for mechanical switches in telephony according to claim 3, characterized in that said absorbed current limiting means comprise a first resistor (28) which is arranged in series to the drain terminal of said first transistor (27) and a second resistor (25) which is arranged in series to said first resistor (28) and is connected to a second terminal (20) of said telephone line, the anode terminal of a second Zener diode (30) being connected between said first resistor (28) and said second resistor (29), the cathode terminal of said second Zener diode (30) being connected to said second terminal (20) of said telephone line, the base terminal of a first bipolar transistor (33) being connected to said anode terminal, a terminal of said control circuit (24) being connected to the collector terminal of said first bipolar transistor (33), the emitter terminal of said first bipolar transistor (33) being connected to said second terminal (20) of said telephone line. A device for limiting the working voltage for mechanical switches in telephony according to claim 1, characterized in that said absorbed current limiting means comprise a third MOSFET transistor (35) wherein the drain terminal is connected to the drain terminal of said first MOSFET transistor (27), the source terminal is connected to said second terminal (20) of said telephone line and the gate and drain terminals are connected to the gate terminal of a second MOSFET transistor (31) wherein the drain terminal is connected to said control circuit (24) and the source terminal is connected to said second terminal (20) of said telephone line. A device for limiting the working voltage for mechanical switches in telephony according to claim 1, characterized in that said absorbed current limiting means comprise a second bipolar transistor (36) wherein the collector terminal is connected to the drain terminal of said first MOSFET transistor (27), the emitter terminal is connected to said second terminal (20) of said telephone line and the base and collector terminals are connected to the base terminal of a first bipolar transistor (33) wherein the collector terminal is connected to said control circuit (24) and the emitter terminal is connected to said second terminal (20) of said telephone line. A device for limiting the working voltage for mechanical switches in telephony according to claims 6 and 7, characterized in that a third resistor (34) is connected between the cathode terminal of the Zener diode (26) and the output terminal of said first switch (25). A device for limiting the working voltage for mechanical switches in telephony according to claim 1, characterized in that said first MOSFET transistor (27) is a depletion transistor. A device for limiting the working voltage for mechanical switches in telephony according to one or more of the preceding claims, characterized in that said first MOSFET transistor (27) is produced with the Multipower BCD250 process wherein, prior to gate oxidation, doping with an N-type dopant (38) is performed in order to provide negative threshold control of said transistor (27). A device for limiting the working voltage for mechanical switches in telephony according to one or more of the preceding claims, characterized in that said first MOSFET transistor (27) is a depletion transistor suitable to work without compromising its maximum working voltage.

Inventors: ANDREINI ANTONIO, CONSIGLIO PIETRO, ERRATICO PIETRO, RAVANELLI ENRICO MARIA ALFONSO, ANDREINI, ANTONIO, CONSIGLIO, PIETRO, ERRATICO, PIETRO, RAVANELLI, ENRICO MARIA ALFONSO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H04M1
IPC codes: H04M1