Abstract:
Apparatus for opening and closing electrical circuits including one or more normally-open switches and normally-closed switches. A normally-open switch includes at least one MOSFET assembly consisting of a plurality of MOSFETs of different current-carrying capacities connected in parallel and circuitry for turning on the MOSFETs in time sequence. The normally-closed switch includes a pnp bipolar transistor, an npn bipolar transistor, and a circuit for short-circuiting the emitter-base junctions of the two transistors. The base and the collector of the npn transistor are connected respectively to the collector and the base of the pnp transistor. The normally-open and normally-closed switches both include a means for accommodating a current flow in either direction through the switch terminals. The apparatus includes two relay control terminals for receiving a relay control voltage from an external source and includes a means for transforming the relay control voltage into the switch control voltage through one or more transformation stages where at least one transformation stage is coupled to the next transformation stage by a transformer.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to relays and more specifically to solid-state relays having high current-carrying capacities. 
     Relays are widely used in railway traction systems as a means of remote control for a wide variety of applications. Mechanical relays have been an important component in traction systems. However, they contain moving parts which are subject to wear and tear, especially in busy systems in which the frequency of operation is high. Solid-state relays, which do not have mechanical moving parts and are therefore likely to offer improved reliability, are candidates to replace mechanical relays if they can be designed to meet the performance requirements of railway traction systems. 
     Solid-state relays are well-known in the electronics industry and their designs are based on various approaches. Many of them are AC types which are made up of thyristors or similar components. The AC type of solid-state relay cannot switch off by itself—it must be aided by the diminishing current in an AC system. 
     There are a few types of DC solid-state relays on the market which are able to switch off by themselves. However, none of them can meet the requirements of a traction system. Their leakage current is very often too high, or the current rating of the switch is too low. On the other hand, all of these solid state relay switches are of the “normally open” (NO) type. An NO switch remains open (non-conducting) when the solid-state relay is not energized. Also required in a railway traction system is the “normally closed” (NC) type switch. The NC switch remains closed (conducting) when the solid-state relay is not energized. Generally, a single package of two NO-type switches and two NC-type switches are required for traction applications. 
     SUMMARY OF THE INVENTION 
     The invention is an improved device for opening and closing an electrical circuit and may include one or more switches of two types: normally-open switches and normally-closed switches. A normally-open switch includes at least one MOSFET assembly consisting of a plurality of MOSFETs numbered from one to N connected in parallel, the n&#39;th MOSFET having a greater current-carrying capacity than the (n−1)&#39;th MOSFET, n taking on values between 2 and N. A switch control voltage in a normal voltage range causes the MOSFETs in the normally-open switch to be turned off while a switch control voltage in an activating voltage range causes the MOSFETs to be turned on. The normally-closed switch includes a pnp bipolar transistor, an npn bipolar transistor, and a means for short-circuiting the emitter-base junctions of the two transistors. The base and the collector of the npn transistor are connected respectively to the collector and the base of the pnp transistor. A switch control voltage in a normal voltage range causes the transistors to be turned on, and a switch control voltage in an activating voltage range causing the transistors to be turned off by short-circuiting the base-emitter junctions of the two transistors. The normally-open and normally-closed switches both include a means for accommodating a current flow in either direction through the switch terminals. 
     The normally-open switch includes a means for turning on the MOSFETs in time sequence, the n&#39;th MOSFET being turned on after the (n−1)&#39;th MOSFET, n taking on values between 2 and N and N being the number of MOSFETs in each MOSFET assembly. The normally-open switch also includes a means for discharging the gates of the MOSFETs when the switch control voltage enters the normal voltage range after having been in the activating voltage range. 
     When the normally-open switch comprises a single MOSFET assembly, the sources and the drains of the MOSFETs are connected through a diode bridge circuit to two switch terminals for connection to an external electrical circuit. When the normally-open switch comprises two MOSFET assemblies, the sources of the MOSFETs in the two MOSFET assemblies are connected together, and the drains are connected to two switch terminals for connection to an external electrical circuit. 
     When the switch is a normally-closed switch, the emitters of the pnp transistor and the npn transistor are connected through a diode bridge circuit to two switch terminals for connection to an external electrical circuit. The means for short-circuiting the base-emitter junction of the npn transistor is a MOSFET while the means for short-circuiting the base-emitter junction of the pnp transistor includes a second pnp transistor bridged across the base-emitter junction of the first pnp transistor. 
     The solid-state relay includes two relay control terminals for receiving a relay control voltage from an external source. The relay control voltage has a normal voltage range and an activating voltage range corresponding respectively to the normal voltage range and the activating voltage range of the switch control voltage. The solid-state relay also includes a means for transforming the relay control voltage into the switch control voltage through one or more transformation stages, at least one transformation stage being coupled to the next transformation stage by a transformer. 
     One of the transformation stages is a DC/AC converter stage which converts an input DC voltage to the DC/AC converter stage to an AC voltage when the relay control voltage is in the activating voltage range. The DC/AC converter stage transforms the input DC voltage to an AC voltage beginning at a predetermined time period after the relay control voltage enters the activating voltage range. 
     The solid-state relay includes a means for suppressing the transmission of switching noise resulting from the operation of the DC/AC converter stage out of the relay&#39;s enclosure via conductors connecting to the relay control terminals. 
     The power required to operate the relay is supplied by the external source through the relay control terminals. The relay further comprises a means for maintaining the current from the external source at a constant level while the relay control voltage is in the activating voltage range. 
     One of the transformation stages is an AC/DC converting stage which converts an AC relay control voltage or a DC relay control voltage of arbitrary polarity to a DC voltage having a predetermined polarity when the relay control voltage is in the activating voltage range. Another of the transformation stages converts the relay control voltage to a constant DC voltage when the relay control voltage is in the activating voltage range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of the invention. 
     FIG. 2 shows the circuit diagram of the constant current supply &amp; ON/OFF control unit. 
     FIG. 3 shows the circuit diagram of the CMOS oscillator &amp; isolating transformers unit. 
     FIG. 4 shows the circuit diagram of the normally-open switch. 
     FIG. 5 shows the circuit diagram of the normally-closed switch. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment  1  of the present invention is shown in FIG.  1 . It consists of a constant-current supply &amp; on/off control unit  3 , a CMOS oscillator &amp; isolating transformers unit  5 , two NO switches  7 ,  8  and two NC switches  9 ,  10 . Each switch  7 ,  8 ,  9 , and  10  is driven by its own switch control voltage and connects to external electrical apparatus via its own pair of switch terminals  11 . The solid-state relay is controlled by a relay control voltage supplied by an external source at relay control terminals  13 . 
     The circuit diagram for the constant-current supply &amp; on/off control unit  3  is shown in FIG.  2 . The unit is powered by the relay control voltage that appears on relay control terminals  13 . The upper section  15  utilizes the relay control voltage, when it is in an activating voltage range, in providing a constant current at a constant voltage to all of the control circuitry in the solid-state relay and enables the solid-state relay to operate properly with a large activating voltage range. The lower section  17  generates a signal with a predetermined delay that enables the operation of the CMOS oscillator in the CMOS oscillator &amp; isolating transformers unit  5 . 
     The current supplied to the unit at relay control terminals  13  flows through fuse F 1 , resistors R 1  (150Ω) and R 2  ( 150 Ω), choke T 1 , and either diodes D 1  and D 4  or diodes D 2  and D 3  (all diodes 1N4007), depending on the polarity, and charges up capacitor C 1  (1 μF). Fuse F 1  isolates the circuit in case of short-circuits or faults. Resistors R 1  and R 2  limit the surge current that results when a step rise in the relay control voltage occurs. The common-mode choke T 1  serves to reduce the magnitude of the switching noise that is coupled to the outside world via the conducting lines connecting to the relay control terminals  13 . The diodes D 1 , D 2 , D 3 , and D 4  establish a voltage of predetermined polarity across capacitor C 1 , regardless of the polarity of the relay control voltage. Thus, the relay control voltage may be either an AC voltage or a DC voltage of arbitrary polarity. 
     Varistor VAR 1  (Marcon TNR9G361k) acts as a transient voltage arrestor, tending to suppress any voltage pulses that may occur on the lines. The resistor R 38  sets the minimum operational current, and the resistor R 3  (75 kΩ) discharges capacitor C 1  when the relay control voltage falls below the activating voltage range. 
     When the voltage across capacitor C 1  rises above 9.1 V, the Zener diode DZ 1  (9.1 V) breaks down and serves to maintain the gate-source voltage of the MOSFET M 1  at a constant value, resulting in a constant current through the MOSFET while the relay control voltage is in the activating voltage range. The magnitude of the current is determined by the breakdown voltage of the Zener diode DZ 1  and the value of resistor R 6  (390Ω). The resistor R 4  (82 kΩ) limits the current that flows through the Zener diode DZ 1 . The Zener diode DZ 2  (15 V) defines the voltage of the Vcc line. The capacitors C 5 , C 6 , and C 7  (each 0.1 μF) are decoupling capacitors for the circuit. 
     The lower section  17  utilizes the voltage across capacitor C 1  to charge up capacitor C 2  (1 μF) through the Zener diode DZ 3  (15 V) and resistor R 7  (270 kΩ) to a voltage determined by R 7 , R 8  (150 kΩ), and R 9  (680 kΩ). The diode D 5  (1N4148) provides a fast discharge path for C 2  when power is removed from the switch terminals  11 . When the voltage across capacitor C 2  reaches a reference voltage defined by resistors R 10  (820 kΩ) and R 11  (820 kΩ), the quad comparator U 4  (LM239) together with the resistors R 12  (30 kΩ), R 13  (330 kΩ), R 14  (30 kΩ), and R 15  (3.3 kΩ) cause a voltage to appear on the ONF line and the LED DL 1  to light up. 
     The relay control voltage activates the switches by transitioning from a normal voltage range extending from 0 to 28 V to an activating voltage range extending from 50 to 138 V. The ONF voltage transitions from 0 V to 15 V approximately 40 ms after the relay control voltage transitions. The ONF voltage transitions from 15 V to 0 V approximately 10 ms after the relay control voltage transitions from the activating range to the normal range. 
     The CMOS oscillator &amp; isolating transformers unit  5  converts Vcc (when ONF reaches 15 V) into an AC voltage and then transfers this AC voltage by means of transformers to a full-wave rectifier for conversion back to DC voltages. The circuit diagram for the CMOS oscillator &amp; isolating transformers unit  5  is shown in FIG.  3 . 
     The conversion of Vcc into an AC voltage is accomplished by the circuit  19  consisting of the quad NAND gate U 2  (4011), resistors R 16  (16 kΩ)and R 17  (16 kΩ), and capacitor C 8  (68 pF). The circuit  19  is enabled by ONF and produces a square wave. 
     The circuit  21  consisting of the quad NAND gates U 1  and U 3  (both  4011 ), resistors R 19 , R 21 , R 23 , R 25 , R 27 , R 29 , R 31 , and R 33  (all 56 kΩ), and capacitors C 9 , C 10 , C 11 , and C 12  (all 10 nF) acts as a buffer between circuit  19  and circuit  23 . 
     The circuit  23  converts the four square-wave AC voltages supplied by circuit  21  to DC voltages by means of the diode bridges consisting of diodes D 6 , D 7 , D 8 , and D 9 , diodes D 10 , D 11 , D 12 , and D 13 , diodes D 14 , D 15 , D 16 , and D 17 , and diodes D 18 , D 19 , D 20 , and D 21 ( all 1N4148). The toroid transformers T 2 , T 3 , T 4 , and T 5  provide complete electrical isolation of circuit  23  and the switches which are driven by circuit  23  from circuit  19 , circuit  21 , and constant-current supply &amp; on/off control unit  3 . The resistors R 34 , R 35 , R 36 , and R 37  (all 20 kΩ) provide light loads to the diode rectifiers. 
     The outputs (Ja+, Ja−), (Jb+, Jb−), (Jc+, Jc−), and (Jd+, Jd−) from circuit  23  are switch control voltages which drive two normally-open (NO) switches and two normally-closed (NC) switches. The normal voltage range of the switch control voltages extends from 0 V to the threshold voltages of the MOSFETs. The activating range of the switch control voltages extends downward from 15 V to a voltage sufficient to support the amount of current that is to flow through the switch. The actual switch control voltages, because of the design, will be either close to 0 V or close to 15 V. 
     It is well known that turning on a power MOSFET requires a gate drive circuit strong enough to charge up the gate capacitance and handle the charge flow resulting from the drain-gate voltage change. This requirement is of particular importance when high voltages are involved since the charge associated with the drain-gate capacitance is large in the case of high-voltage power MOSFETs. Such MOSFETs are almost impossible to drive with weak CMOS-logic integrated circuits. This invention solves the drive problem by using a parallel combination of power MOSFETs with increasing current-carrying capacities and turning on the power MOSFETs in the sequence of their current-carrying capacities. 
     The circuit diagram of the NO switch  25  is shown in FIG.  4 . The MOSFETs M 2  and M 4  constitute a first MOSFET assembly and MOSFETs M 3  and M 5  constitute a second MOSFET assembly. MOSFETs M 2  and M 3  (both IRF830) are of lower current-carrying capacity and have lower capacitances than MOSFETs M 4  and M 5  (both IRF740). The drains of the MOSFETs connect to switch terminals  27  which can be used to connect the switch into an electrical circuit. 
     The reason for having two MOSFET assemblies connected back to back is to permit the voltage appearing across the switch terminals  27  to be of arbitrary polarity. When the MOSFETs are turned off, one or the other MOSFET assembly blocks the flow of current, regardless of the polarity of the switch-terminal voltage. 
     Polarity independent performance could also be attained with a single MOSFET assembly by incorporating a diode bridge as described below in connection with the NC switch. 
     The fuse F 3  (15 A) isolates the switch from the electrical circuit to which it is connected in case of a short-circuit or some other fault creating a high-current condition. 
     The varistor VAR 2  (TNR15G271K) absorbs any transient voltages which may be generated by inductive elements in the electrical circuit to which the switch is connected. 
     The switch control voltage enters NO switch  25  at switch control terminals  29 . When the switch control voltage transitions from the normal voltage range to the activating voltage range, the transition appears almost immediately at the gates of MOSFETs M 2  and M 3 . These MOSFETs are of a low-current type with small capacitances, and they are turned on relatively easily by weak gate drives. But these MOSFETs do not have enough current-carrying capacity to carry the full switch current continuously. However, they discharge the drain-gate capacitances of MOSFETs M 4  and M 5  which causes the drain-source voltage of the two MOSFETs to be zero momentarily 
     The resistor R 39  (3.3 kΩ) delays the appearance of the switch control voltage transition at the gates of MOSFETs M 4  and M 5  until the drain-source voltages have reached zero, and the two MOSFETs turn on with zero drain-source voltage. Thus, the MOSFET pair is turned on by the relatively weak CMOS integrated circuits. This method works well with discrete MOSFET pairs, but it would be ideal to have the MOSFETs fabricated on a single chip. On the chip level, the number of MOSFETs in parallel is not restricted, and it would be possible to optimize the delays for groups of small MOSFET cells so as to optimize the switching. 
     The resistors R 41 , R 42 , R 43 , and R 44  (all 50Ω) prevent parasitic oscillations. Diode D 23  (1N4148) and transistor Q 5  ( 9012 ) provide a discharge path for the gates of the MOSFETs when the switch is opened. The Zener diode DZ 5  (18 V) limits the gate-source voltage of the MOSFETs. The diode D 22  (1N4148) turns off the transistor Q 5  when the switch is closed. The resistor R 38  provides a light load which causes the voltage across switch terminals  29  to go to zero when the switch is not connected. 
     The circuit diagram of the NC switch  31  is shown in FIG.  5 . The pnp bipolar transistor Q 3  (MJ15025) and the npn bipolar transistor Q 4  (MJ15024) form the main part of the normally-closed switch. The emitters of the two transistors connect through the diode bridge consisting of diodes D 24 , D 25 , D 26 , and D 27  (all  6 A 4 ) to the switch terminals  33 . The switch terminals  33  provide the means for connecting the switch into an electrical circuit. The purpose of the diode bridge is to convert a voltage across the switch terminals  33  of arbitrary polarity into a voltage of fixed polarity across transistors Q 3  and Q 4 , the Q 3  emitter being positive with respect to the Q 4  emitter. 
     The fuse F 2  (15 A) isolates the switch from the electrical circuit to which it is connected in case of a short-circuit or some other fault creating a high-current condition. 
     The varistor VAR 3  (TNR23G181K) absorbs any transient voltages which may be generated by inductive elements in the electrical circuit to which the switch is connected. 
     The driving circuit for the NC switch  31  consists of the MOSFET M 6  (MTP50N05E), the bipolar transistors Q 1 ( 9012 ) and Q 2  (MPSA42), and the resistors R 45  (680 kΩ), R 46  (680 kΩ), R 47  (20 kΩ), R 48  (560 kΩ), and R 49  (1 MΩ). 
     If the switch control voltage at switch control terminals  35  is zero, MOSFET M 6  is turned off. The base-emitter junction of transistor Q 2  is short-circuited thereby causing Q 2  to be turned off. Since no current is flowing through Q 2 , the voltage across the base-emitter junction of transistor Q 1  is zero, and Q 1  is also turned off. Under these conditions any voltage at terminals  33  will cause current to flow through transistors Q 3  and Q 4 . Thus, switch  31  is normally closed. The resistor R 49  enhances the triggering sensitivity of the switch when the switch control voltage is in the normal voltage range. 
     When the switch control voltage at switch control terminals  35  transitions from the normal voltage range to the activating voltage range, MOSFET M 6  begins to conduct and short-circuits the base-emitter junction of transistor Q 4 , thereby causing Q 4  to stop conducting current. At the same time, Q 2  begins to conduct causing Q 1  to conduct and short-circuit the base-emitter junction of transistor Q 3  which causes Q 3  to also stop conducting current. Thus, the switch control voltage, in moving to the activating voltage range, causes the normally-closed switch  31  to close.