SWITCHING SYSTEM FOR CAPACITOR CHARGING/DISCHARGING

Abstract: First and second switches such as transistors are connected to a charge storage member such as a capacitance. The capacitance is charged through the first transistor from a positive supply when the transistor becomes conductive. The capacitances is discharged through the second transistor to a reference potential such as ground when the second transistor becomes conductive. The conductivities of the first and second transistors are controlled by pulses from a pulse source such as a transformer. The transformer primary produces a pulse of one polarity upon the occurrence of the leading edge of an input signal and a pulse of an opposite polarity upon the occurrence of the trailing edge of the input signal. Two secondary windings are respectively connected in opposite polarities to the bases of the first and second transistors to provide for the conductivity of only one of the transistors at any one time. The transformers saturate the conductive transistor to prolong on a limited basis the time during which the transistor remains conductive. The other one of the transistors is made instantaneously non-conductive by providing for an instantaneous discharge of the stored base charge between the base and emitter of the transistor. A constant current source is electrically in parallel with the first transistor to sustain the capacitance charge at a high level after the limited period of time in which the first transistor remains conductive. A low frequency transistor is connected in parallel with the second transistor to sustain the discharge of the capacitance after the limited period of time in which the second transistor remains conductive.

Description: The present invention relates to an electronic switch controlling the charging and discharging of a charged storage means according to the precharacterising part of claim 1. Switching systems have been in use for many years to control the operation of output members. For example, switches may be used to control the introduction of energy to a wide variety of members such as waveguides, heaters and computers. As technology has advanced, the variety of uses for switches has progressively increased. Furthermore, the specifications for defining the operation of the switches have become progressively tightened. Uses have been developed for providing switching systems in which the systems have to operate instantaneously in response to an input signal to introduce large magnitudes of voltage to an output member. For example, the output member may be an optical waveguide or an electro-optically active crystal which operates only at selected times indicated by the input signal to deliver optical energy through an optical circuit. The term "instantaneously" is intended to mean that the switching system has to respond in a nanosecond or less to close a switch which will provide for the delivery of a stored charge to the output member. As will be appreciated, it is not easy to operate a switch in a nanosecond or less. The problem is considerably magnified when a large amount of a stored charge has to be delivered through the switch when the switch becomes closed. Since a large amount of charge has to be delivered in a short time, a high current flows into and out of the capacitance upon the successive closures of the switch. The requirement to charge and discharge the capacitance slows the time for delivering the charge in the capacitance to the output member when the switch is closed. Considerable efforts have been made, and significant amounts of money have been expended in such efforts, to provide a switching system which will meet the objectives specified in the previous paragraph. Such efforts have not been entirely successful. It is still difficult, if not impossible, to deliver in a nanosecond or less the charge stored in a capacitance. This is particularly true when the magnitude of the charge stored in the capacitance is large. DE-3 119 450 describes an electronics switch means for providing a capacitive load with binary signals. The circuit comprises first switching means which upon receipt of an input signal connects the capacitive load to a supply voltage and second switching means which upon receipt of an opposite input signal connects the capacitance to a reference voltage. Both switching means comprise thyristor means for fast charge and discharge operations. The first switching means establishes connection between the supply voltage and the capacitive load until a sufficient amount of charge has been accumulated within the capacitive load leading to a charge current which is below the holding current of the respective thyristor means. Equally during a discharge operation the second switching means connects the capacitive load to the reference potential until the discharge current falls below the holding current of the respective thyristor means. In a parallel connection to the switching means are provided charge and discharge means for providing respective charge and discharge currents for a predetermined period of time after the termination of the operation of the thyristor means. From US-3 728 559 a MOS clock driver is known in which a binary input signal is amplified by a differential amplifier which in turn is coupled through a transformer circuit to a first and second switching means. The two switching means connect a load element to a supply or reference voltage, respectively. The second winding of the transformer consists of two windings wound in opposite directions to each other. One of the second windings is connected to the first switching means, the other to the second switching means thereby allowing either the first or the second means to be in the conductive state. It is the object of the present invention to provide an improved electronic switch controlling charge and discharge of a charge storage means at a high speed even if the amount of charge is considerably high. This object is solved by the features of claim 1. This invention provides a switching system which meets the objectives discussed in the previous paragraphs. The system of this invention is able to switch an external charge storage means such as a capacitance between two particular voltage levels in times comparable with the reciprocal of the cut-off frequency of switches such as transistors included in the switching system. The reciprocal of this cut-off frequency may be in the order of one (1) nanosecond or less. The two particular voltage levels may be limited only by the breakdown voltages of the switches. When the switches constitute transistors, this breakdown voltage may be between the collector and the base of the transistors. The system meets the above objectives even while dissipating only a minimal amount of energy. In one embodiment of the invention, first and second switches such as transistors are connected to a charge storage member such as a capacitance. The capacitance may be external to the embodiment of this invention. The capacitance is charged through the first transistor from a positive supply when the transistor becomes conductive. The capacitance is discharged through the second transistor to a reference potential such as ground when the second transistor becomes conductive. The conductivities of the first and second transistors are controlled by pulses from a pulse source such as a transformer. The transformer primary produces a pulse of one polarity upon the occurrence of the leading edge of an input signal and a pulse of an opposite polarity upon the occurrence of the trailing edge of the input signal. Two secondary windings are respectively connected in opposite polarities to the bases of the first and second transistors to provide for the conductivity of only one of the transistors at any one time. The transformers saturate the conductive transistor to prolong on a limited basis the time during which the transistor remains conductive. The other one of the transistors is made instantaneously non-conductive by providing for an instantaneous discharge of the stored base charge between the base and emitter of the transistor. A constant current source is electrically in parallel with the first transistor to sustain the capacitance charge at a high level after the limited period of time in which the first transistor remains conductive. A low frequency transistor is connected in parallel with the second transistor to sustain the discharge of the capacitance after the limited period of time in which the second transistor remains conductive. In the drawings: Figure 1 is a circuit diagram of the invention; and Figure 2 schematically illustrates voltage waveforms at strategic terminals in the circuit diagram shown in Figure 1. In one embodiment of the invention, circuitry generally shown at 10 is illustrated in Figure 1 for controlling the charge and discharge of a charge storage member such as a capacitance 12. One terminal of the capacitance 12 is common with a reference potential such as a ground 14 and the other terminal of the capacitance 12 has a common connection with a terminal of a switch, such as the emitter of an npn transistor 16, and with a terminal of another switch, such as the collector of an npn transistor 18. The collector of the transistor 16 receives a positive potential from a source 20a which may be variable. The emitter of the transistor 18 is common with the reference potential such as ground. A constant current source 22 is connected between the collector and the emitter of the transistor 16. The collector and emitter of an npn transistor 24 are respectively common with the collector and the emitter of the transistor 18. A resistance 25 may be connected between the collector of the transistor 24 and the emitter of the transistor 16. As will be appreciated, the transistor 24 is only one embodiment of a switch which may be used. Pulsing means such as a winding 26 is connected between the base and the emitter of the transistor 16. Similarly, connections are made between the end terminals of pulsing means such as a winding 28 and the base and the emitter of the transistor 18. The winding 26 preferably has an opposite polarity to the winding 28 so that only one of the transistors 16 and 18 will be triggered to a state of conductivity at any one time by pulses simultaneously produced in the windings 26 and 28. The windings 26 and 28 and a winding 30 may constitute three (3) single turn windings of small diameter wire on a small ferrite bead. The windings 26 and 28 constitute secondary windings which are magnetically coupled to the primary winding 30. One terminal of the primary winding 30 is connected to the voltage source 20 and the other terminal of the winding 30 is connected to a switch such as to the collector of an npn transistor 32. The base of the transistor 32 receives on a line 34 a potential biasing the transistor to a state of conductivity. The emitter of the transistor 32 has a common connection with one terminal of a resistance 36, the other terminal of which receives a negative potential from a source 38. The emitter of a transistor 40 is common with the emitter of the transistor 32. The base of the transistor 40 receives on a line 42 an input signal 44 preferably defined by sharp leading and trailing edges. A resistance 46 is connected between the collector of the transistor 40 and the voltage source 20. The output on the collector of the transistor 40 is introduced through a parallel combination of a resistance 48 and a capacitance 50 to the base of the transistor 24. The base of the transistor 24 is also connected to one terminal of a resistance 52, the other terminal of which receives the negative potential from the source 38. The transistor 32 is normally conductive because of the bias potential on its base. This causes current to flow through a circuit including the voltage source 20, the primary winding 30, the transistor 32, the resistance 36 and the voltage source 38. Since this current is a direct current, no voltage is induced in the winding 30. The flow of current through the resistance 36 produces on the emitter of the transistor 40 a voltage which biases the transistor to a state of non-conductivity. The voltage from the source 20 is accordingly introduced to the base of the transistor 24 to maintain the transistor conductive. This causes the circuit including the capacitance 12 and the transistor 24 to become conductive so that the capacitance remains in a discharged state. When the leading edge of the input signal 44 is introduced to the base of the transistor 40, the transistor becomes conductive. Current accordingly flows through a circuit including the voltage source 20, the resistance 46, the transistor 40, the resistance 36 and the voltage source 30. The resultant voltage drop across the resistance 46 causes the transistor 24 to become non-conductive. At the same time, the voltage produced across the resistance 36 biases the transistor 32 to a state of non-conductivity. Because of the interruption of the current through the winding 30, a pulse is induced in the winding. This pulse is indicated at 62 in Figure 2. The resultant negative pulse in the winding 28 causes the transistor 18 to become non-conductive. This pulse is of such a magnitude that the distributed charge between the base and emitter of the transistor 18 discharges instantaneously through a circuit including the distributed capacitance and the base and emitter of the transistor. A positive voltage pulse is induced in the winding 26 at the same time that the voltage pulse is induced in the winding 28. This pulse is indicated at 64 in Figure 2. The voltage pulse in the winding 26 causes the transistor 16 to become conductive. Current accordingly flows through a circuit including the variable voltage source 20a, the transistor 16 and the capacitance 12 to charge the capacitance. This current may have a high amplitude as a result of the saturation of the transistor 16. Because of the saturation of the transistor 16 and the opposition of the winding 26 to the cut-off of current through the winding 30, the voltage of the base of the transistor remains positive for a period of time even after the disappearance of the voltage on the collector of the transistor 18. The voltage pulse on the base of the transistor 16 is indicated at 64 in Figure 2. In this way, the transistor 16 is maintained conductive even after the production of a pulse 62 on the collector of the transistor by the interruption in the flow of current through the transistor. The constant current source 22 is included to insure that the capacitance 12 remains fully charged even after the termination of the pulse 64 on the base of the transistor 16. By including the constant current source 22, the voltage across the capacitance 12 is maintained substantially constant even if leakage current flows through the transistors 18 and 24. This is indicated at 68 in Figure 2. If the constant current source 22 were not included, the voltage across the capacitance 12 would have a waveform such as indicated at 66 in Figure 2. This would result from the tendency of the capacitance 22 to discharge slowly through the transistor 18 after the termination of the pulse 64 because of leakage current between the collector and the emitter of the transistor 18. When the trailing edge of the input signal 44 occurs, the transistor 40 becomes non-conductive and the transistor 32 becomes conductive because of the bias on the line 34. The primary winding 30 responds to the change in the circuit including the winding 30 and the transistor 32 by producing a voltage pulse in the winding. The resultant pulse in the winding 28 causes the transistor 18 to become conductive and the resultant pulse in the winding 26 causes the transistor 16 to become non-conductive. The pulses are of relatively high amplitude so that the charge in the distributed capacitance between the base and emitter of the transistor 16 discharges instantaneously through a circuit including the base and emitter of the transistor. Because of the action of the winding 30 in opposing the flow of current through the transistor 32, a pulse somewhat prolonged in duration is introduced to the base of the transistor 18. This pulse is indicated at 70 in Figure 2. The prolonged duration of this pulse causes the capacitance 12 to become fully discharged through the transistor 18. The saturating characteristics of the transistor 18 as a result of the high voltage introduced to the base of the transistor also facilitates the full discharge of the capacitance 12 through the transistor. The discharge of the capacitance 12 is also facilitated by the state of conductivity of the transistor 24, which responds at a relatively low frequency in comparison to the frequency of response of the transistor 18. The conductivity of the transistor 24 results from the current introduced to the base of the transistor from the collector of the transistor 40 when the transistor 40 becomes non-conductive. This voltage at the collector of transistor 40 overcomes the bias introduced to the base of the transistor 24 through the resistance 52 from the voltage source 38 to make the transistor normally non-conductive. The system described above has certain important advantages. The system provides for the charge and discharge of the capacitance 12 at times within one nanosecond (1 ns.) or less of the occurrence of the leading and trailing edges of the input signal 44. These times are comparable with the reciprocal of the cut-off frequency of the switching elements such as the transistors included in the system. The system provides for the charge and discharge of the capacitance 12 within such precise time intervals even though the voltage swing across the capacitance may be quite large. This voltage may be limited only by the collector-base breakdown voltages of the transistors included in the system. The system provides these advantages in a simple and reliable manner and with a minimal dissipation of energy. The system may be incorporated on a hybrid incorporating an integrated circuit chip and discrete elements.

Claims: Electronic switch controlling the charging and discharging of charge storage means (12) having first and second terminals, comprising: means (14) for providing a reference voltage, means (20a) for providing an energizing voltage (V₁), first switching means (16) connected between the first terminal of the charge storage means (12) and the energizing voltage means (20a) and having open and closed states and operative in the open state to prevent the energizing voltage (+V₁) from being introduced to the charge storage means (12) and operative in the closed state to provide for the introduction of the energizing voltage (+V₁) to the charge storage means (12) to charge the charge storage means (12); second switching means (18) connected between the first terminal of the charge storage means (12) and the reference voltage means (14) and having open and closed states and operative in the open state to prevent the discharge of the charge storage means (12) and operative in the closed state to provide for the discharge of the charge storage means (12) through the reference voltage means (14), means for providing an input signal (44) having leading and trailing edges; means (30, 26, 28,) responsive to the leading edge of the input signal (44) for instantaneously pulsing the first switching means (16) to the closed state and the second switching means (18) to the open state and for maintaining the first switching means (16) in the closed state and the second switching means (18) in the open state for a period of time after the leading edge of the input signal (44); means (22) for maintaining a substantially constant flow of current to the charge storage means (12) after the period of time in which the first switching means (16) is closed; the pulsing means (26, 28, 30) being responsive to the trailing edge of the input signal (44) for instantaneously pulsing the first switching means (16) to the open state and the second switching means (18) to the closed state and for maintaining the first switching means (16) in the open state and the second switching means (18) in the closed state for the period of time after the trailing edge of the input signal (44); third switching means (24) having slower response characteristics than the second switching means (18) for maintaining the discharge of the charge storage means (12) to the reference voltage means (14) after the period of time in which the second switching means (18) is closed; characterised in that said period is determined by said pulsing means, said pulsing means includes a transformer having a primary winding (30) and two secondary windings (26,28); the primary winding (30) being responsive to the input signal (44) to produce signals in the secondary windings (26,28) and the secondary windings (26,28) being respectively connected to the first and second switching means (16,18). Electronic switch as set forth in claim 1 wherein each of the first and second switching means (16,18) have characteristics of providing a saturating current; the pulsing means (26,28,30) having characteristics to produce a flow of a saturating current through the first switching means (16) upon a pulsing of the first switching means (16) to the closed state and having characteristics to produce a flow of a saturating current through the second switching means (18) upon a pulsing of the second switching means (18) to the closed state. Electronic switch as set forth in any one of the preceding claims further comprising fourth and fifth switching means (40,32), the fourth switching means (40) having open and closed states and being normally operative in the open state and being responsive to the input signal (44) to become operative in the closed state and to remain in the closed state during the occurrence of the input signal (44); the fifth switching means (32) being biased to operate in the closed state and being responsive to the operation of the fourth switching means (40) in the closed state and to become operative in the open state; the pulsing means (26,28,30) being responsive to the operation of the fifth switching means (32) in the open and closed states for pulsing the first and second switching means (16,18). Electronic switch as set forth in any one of the preceding claims wherein: the first switching means comprises a first transistor (16) having conductive and non-conductive states; the second switching means comprises a second transistor (18) having conductive and non-conductive states, said means for maintaining a substantially constant flow of current (22) being responsive to the pulsing of the first transistor (16) to the conductive state for prolonging the charging of the charge storage means (12) after such pulsing, said third switching means (24) being responsive to the pulsing of the second transistor (18) to the conductive state for prolonging the discharge of the charge storage means (12) after the said period of time. Electronic switch as set forth in claim 4, comprising means (46,40,36) responsive to the input signal for producing a first voltage during the occurrence of the input signal (44) and for producing a second voltage in the absence of the input signal (44), and means (50,48) responsive to the second voltage from the last mentioned means (46,40,36) for introducing such voltages to the third switching means (24) to control the operation thereof. Electronic switch according to claim 5 wherein fifth means (32) has conductive and non-conductive states and, being biased to a state of conductivity, responds to the production of the second voltage to become operative in a state of non-conductivity, and wherein the fourth means (40), in response to the initial operation of the fifth means (32) in the conductive state, biasses the third means (24) to the conductive state and responds to the occurrence of input signal (44) by biassing the third means (24) to the non-conductive state. Electronic switch as set forth in any one of the preceding claims wherein the charge storing means is a capacitance (12).

Inventors: KATZENSTEIN, HENRY S
Applicants: BROOKTREE CORPORATION
Application year: 1988
Main classification: H03K17
IPC codes: H03K17