Abstract:
A system and method for quickly discharging an AC relay is provided. In one embodiment, the invention relates to a circuit for discharging a relay coil a relay, the circuit including relay circuitry having a relay coil disposed across a rectifier circuit, wherein the relay coil is configured to actuate at least one load switch when sufficiently energized, relay release circuitry including suppression circuitry coupled across the relay coil, and isolation circuitry in series between the relay coil and the rectifier circuit, and control circuitry configured to provide a voltage to the rectifier circuit to energize the relay coil, wherein the isolation circuitry is configured to isolate the relay coil and suppression circuitry based on a signal from the control circuitry.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of Provisional Application No. 61/048,552, filed Apr. 28, 2008, entitled “SYSTEM AND METHOD FOR QUICKLY DISCHARGING AN AC RELAY”, the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND TO THE INVENTION 
     The present invention relates generally to a system and method for quickly discharging an alternating current (AC) relay. More specifically, the present invention relates to a system for minimizing the amount of time expended in discharging a direct current (DC) relay coil charged using an AC power source. 
     Relay coils are inductors and oppose changes in current flow. DC coils are often used within an AC relay to generate a switching force capable of actuating one or more load switches. In such case, an AC voltage is rectified and then applied to the DC coils which store the applied energy and generate the switching force. Once a voltage or energy threshold has been met, load switches are actuated by the switching force of the DC coil. As the supply voltage to the coil is switched off, high voltage peaks are generated due to the inductance of the coil. Such high voltage peaks can damage control logic, power sources and switch contacts. 
     AC relays often include rectifier circuits, such as full wave or half wave rectifier circuits, that convert AC voltage to DC voltage which is used to charge DC coils. A full wave rectifier circuit generally includes four diodes in a bridge configuration. In such case, a DC coil is often coupled across the diode bridge. After the DC coil has been sufficiently charged as to provide the switching force, the AC supply voltage is removed. The energy stored in the DC coil is dissipated by the diodes over a period of time. However, the period of time needed to dissipate the energy stored in the DC coil can be substantial. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention relate to a system and method for quickly discharging an AC relay. In one embodiment, the invention relates to a circuit for discharging a relay coil, the circuit including a power source configured to generate an alternating current signal for energizing the relay coil, a rectifier circuit coupled to the power source, the rectifier circuit having at least one diode, and a relay release circuit including a switch coupled to the rectifier circuit, the switch in series with the relay coil, where the relay coil is coupled to the rectifier circuit, and a suppression circuit coupled in parallel to the relay coil, the suppression circuit including a second diode in series with a zener diode, where the relay coil, when sufficiently energized, is configured to provide a switching force sufficient to actuate at least one load switch coupled to at least one switched power line, where the suppression circuit is configured to discharge the energy stored in the relay circuit, and where the rectifier circuit is configured to rectify the alternating current signal. 
     In another embodiment, the invention relates to a circuit for discharging a relay coil, the circuit including a relay circuit having the relay coil disposed across a rectifier circuit, where the relay coil is configured to actuate at least one load switch when sufficiently energized, a relay release circuit including a suppression circuit coupled across the relay coil, the suppression circuit including a zener diode in series with a diode, and an isolation circuit in series between the relay coil and the rectifier circuit, and a control circuit configured to provide an alternating current signal to the rectifier circuit to energize the relay coil, where the isolation circuit is configured to isolate the relay coil and the suppression circuit based on a signal from the control circuit, and where the rectifier circuit is configured to rectify the alternating current signal. 
     In yet another embodiment, the invention relates to a circuit for discharging a relay coil, the circuit including a power source configured to energize the relay coil, a rectifier circuit coupled to the power source, the rectifier circuit including at least one diode, a relay release circuit including a switch coupled to the rectifier circuit, the switch in series with the relay coil, where the relay coil is coupled to the rectifier circuit, and a suppression circuit coupled in parallel to the relay coil, the suppression circuit including a second diode in series with a zener diode, where the relay coil, when sufficiently energized, is configured to provide a switching force sufficient to actuate at least one load switch coupled to at least one switched power line, where the suppression circuit is configured to discharge the energy stored in the relay circuit, and where the rectifier circuit includes a bridge rectifier circuit including four diodes in a bridge configuration. 
     In still yet another embodiment, the invention relates to a circuit for discharging a relay coil, the circuit including a relay circuit including a relay coil disposed across a rectifier circuit, where the relay coil is configured to actuate at least one load switch when sufficiently energized, a relay release circuit including a suppression circuit coupled across the relay coil, the suppression circuit including a zener diode in series with a diode, and an isolation circuit in series between the relay coil and the rectifier circuit, and a control circuit configured to provide a voltage to the rectifier circuit to energize the relay coil, where the isolation circuit is configured to isolate the relay coil and suppression circuit based on a signal from the control circuit, and where the rectifier circuit includes a bridge rectifier circuit including four diodes in a bridge configuration. 
     In another embodiment, the invention relates to a circuit for discharging a relay coil, the circuit including a relay circuit including a relay coil disposed across a rectifier circuit, where the relay coil is configured to actuate at least one load switch when sufficiently energized, a relay release circuit including a suppression circuit coupled across the relay coil, and an isolation circuit in series between the relay coil and the rectifier circuit, and a control circuit configured to provide a voltage to the rectifier circuit to energize the relay coil, where the isolation circuit is configured to isolate the relay coil and suppression circuit based on a signal from the control circuit, where the isolation circuit is a MOSFET configured as a switch, and where a gate of the MOSFET is coupled to an RC circuit in series with a zener diode and a resistor, where the RC circuit, the zener diode and the resistor are coupled across the rectifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a power control system including an AC relay circuit in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of an AC relay circuit including a full wave rectifier and a fast release circuit in accordance with an embodiment of the present invention. 
         FIG. 3  is a flow chart of a process for operating an AC relay circuit having a fast release circuit in accordance with an embodiment of the present invention. 
         FIG. 3   a  is a flow chart of a sequence of actions performed by an AC relay circuit having a fast release circuit in accordance with an embodiment of the present invention. 
         FIG. 4  is a schematic diagram of an AC relay circuit including a full wave rectifier and a fast release circuit in accordance with an embodiment of the present invention. 
         FIG. 5  is a schematic diagram of an AC relay circuit including a half wave rectifier and a fast release circuit in accordance with an embodiment of the present invention. 
         FIG. 6  is a schematic diagram of an AC relay circuit including a full wave rectifier and a fast release circuit in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, embodiments of systems and methods for quickly discharging an AC relay are illustrated. The AC relays generally include DC coils that provide a switching force when sufficient voltage is applied by a rectifier circuit. Rectifier circuits convert energy from an AC control power source to DC. Fast release circuits coupled to the rectifier circuits isolate the DC coils and quickly dissipate the energy stored in the DC coils when the AC power source is switched off. In several embodiments, the fast release circuit includes a switch in series with the DC coil and a suppression circuit including a conventional diode and a zener diode in series, where the suppression circuit is coupled in parallel across the DC coil. 
     In some embodiments, the fast release circuits are used in conjunction with full wave bridge rectifier circuits. In other embodiments, the fast release circuits are used with half wave rectifier circuits. For the full wave bridge rectifier circuits, energy stored in the DC coil when the power is switched off can be dissipated via bridge diodes. However, the time period required for sufficient dissipation of the stored energy to change the position of relay armature, after the coil energizing voltage has been switched off, or release time, can be too long for some applications. In one embodiment, for example, a release time of 20 milliseconds (ms) or more is too long. Using the fast release circuit, the release time can be substantially reduced. In one embodiment, for example, the release time can be reduced to 10 ms or less. In some embodiments, the release time is reduced by 50 to 500 percent. 
     In one embodiment, the AC relays having a fast release circuit can be used to control the distribution of power in an aircraft electrical system. Power can be distributed using any of DC or AC (single, two or three phase) systems, or combinations thereof. In a number of embodiments, the AC relay has one load switch that switches a DC power source. In several embodiments, the DC power sources operate at 28 volts, 26 volts or 270 volts. In one embodiment, the DC power sources operate in the range of 11 to 28 volts. In other embodiments, the AC relay includes three load switches that switch different phases of an AC power source. In one embodiment, the AC power source operates at 115 volts and at a frequency of 400 hertz. In other embodiments, the AC relays having a fast release circuit have more than a single load switch, where each load switch can switch a DC power source or a single phase of an AC power source. In other embodiments, the power sources operate at other voltages and other frequencies. In one embodiment, the DC power sources can include batteries, auxiliary power units and/or external DC power sources. In one embodiment, the AC power sources can include generators, ram air turbines and/or external AC power sources. 
       FIG. 1  is a schematic block diagram of a power control system  100  including an AC relay circuit  104  in accordance with an embodiment of the present invention. The power control system  100  includes a power source  102  coupled to the relay circuit  104 . The relay circuit  104  is also coupled to a load  106  and a control circuit  108 . 
     In operation, the relay circuit  104  controls the flow of current from the power source  102  to the load  106  based on input received from the control circuit  108 . In one embodiment, the power source is an AC power source used in an aircraft. In such case, the load is an aircraft load such as, for example, aircraft lighting or aircraft heating and cooling systems. 
     In several embodiments, the relay circuit  104  includes a DC coil and a fast release circuit. The fast release circuit can isolate the DC coil and quickly dissipate the energy stored in the DC coil when power provided by the control circuit  108  is switched off or removed. 
       FIG. 2  is a schematic diagram of an AC relay circuit  200  including a full wave rectifier circuit and a fast release circuit in accordance with an embodiment of the present invention. The AC relay circuit further includes a power source  202  coupled with a load switch  203 . The position of the load switch  203  is controlled by a switching force generated in a DC coil  218 . The load switch  203  is also coupled to a load  206 . 
     An AC control power source  208  is coupled by a first switch  226  to the full wave rectifier. The full wave rectifier includes four diodes ( 210 ,  212 ,  214  and  216 ) in a diode bridge rectifier configuration. Diodes  210  and  216  are coupled to AC control  208 . Diodes  212  and  214  are coupled to the AC control  208  via switch  226 . The cathodes of diode  210  and diode  212  are coupled by a node  211 . The anodes of diode  214  and diode  216  are coupled by a node  215 . A fast release control switch  220  and the DC coil  218  are coupled in series across the diode bridge, or between node  211  and node  215 . A diode  222  and a zener diode  224  are coupled in a back to back configuration, e.g., where the anodes of both diodes are coupled together, in parallel to the DC coil  218 . In another embodiment, the cathodes of diode  222  and zener diode  224  are coupled together. In one embodiment, the control switch  220 , diode  222 , zener diode  224  and DC coil  218  form a fast release circuit. 
       FIG. 3  is a flow chart of a process for operating an AC relay circuit having a fast release circuit in accordance with an embodiment of the present invention. In particular embodiments, the process is performed in conjunction with the fast release circuit of  FIG. 2 . In block  302 , the process begins by closing switch S 1  and switch S 2  to charge the DC coil using the AC control source. In block  304 , the process determines whether the DC coil has been sufficiently charged as to generate the switching force necessary to actuate the load switch. If the DC coil has not been sufficiently charged, then the process returns to block  302  and continues to charge the DC coil. If the DC coil has been sufficiently charged, then the process continues to block  306 . In block  306 , the process opens switch S 1  which isolates the rectifier from the AC control source. In block  308 , the process opens switch S 2  to isolate the DC coil from the rectifier. In a number of embodiments, a back voltage or back electromotive force (EMF) is generated by the DC coil in response to the sudden loss of current supplied by the AC control source. In block  310 , the process discharges energy stored in the DC coil (e.g., the back voltage) using the fast release circuit. 
     In the embodiment illustrated in  FIG. 2 , the fast release circuit includes diode  222  and zener diode  224  in the back to back configuration. In several embodiments, if the back EMF generated in the DC coil is greater than the breakdown voltage of the zener diode, the zener diode operates in a reverse biased mode and permits a controlled amount of current to flow through the zener diode and thus through the conventional diode. In such case, both diodes dissipate energy as current flows through both diodes and returns to the DC coil. This dissipation cycle can repeat until the DC coil is fully discharged. In some embodiments, the DC coil is discharged in a single cycle. In several embodiments, the value of the zener diode, the zener or breakdown voltage, is chosen to enable a particular release time. For example, in one embodiment, a 200 volt zener diode enables a release time of less than 10 ms. 
     In one embodiment, the process can perform the illustrated actions in any order. In another embodiment, the process can omit one or more of the actions. In some embodiments, the process performs additional actions in conjunction with the process. In other embodiments, one of more of the actions are performed simultaneously. 
       FIG. 3   a  is a flow chart of a sequence of actions performed by an AC relay circuit having a fast release circuit in accordance with an embodiment of the present invention. In particular embodiments, the process is performed in conjunction with the fast release circuit of  FIG. 2 . In block  320 , the circuit begins by receiving energy via a charging voltage. In one embodiment, the charging voltage is provided by an AC control source. In block  322 , the circuit stores the received energy in a relay coil. In block  324 , the circuit generates a switching force sufficient to actuate one or more load switches. In block  326 , the circuit generates a back EMF when the charging voltage is switched off. In several embodiments, the relay coil generates the back EMF. 
     In block  328 , the circuit isolates the relay coil and the suppression circuit using isolation circuitry. In block  330 , the circuit allows the back EMF to increase to a predetermined level such that the release time associated with the relay coil is substantially reduced. In some embodiments, circuit decreases the release time for the AC relay by 50 percent to 500 percent. In block  332 , the circuit suppresses the back EMF after it has increased to the predetermined level. In one embodiment, the predetermined level is 200 volts. In block  334 , the circuit prevents arcing across the isolation circuitry. In one embodiment, the suppression circuit includes a conventional diode in series with a zener diode. In several embodiments, the value, or breakthrough voltage, of the zener diode is selected such that it is less than an arcing voltage across the isolation circuitry. In such case, the zener diode will conduct before arcing across the isolation circuitry can take place. 
     In one embodiment, the circuit can perform the illustrated actions in any order. In another embodiment, the circuit can omit one or more of the actions. In some embodiments, the circuit performs additional actions. In other embodiments, one of more of the actions are performed simultaneously. 
       FIG. 4  is a schematic diagram of an AC relay circuit  400  including a full wave rectifier and a fast release circuit in accordance with an embodiment of the present invention. The AC relay circuit  400  further includes a power source  402  coupled with a load switch  403 . The position of the load switch  403  (e.g., position of armature of the load switch) is controlled by a switching force generated in a DC coil  418 . The load switch  403  is also coupled to a load  406 . 
     An AC control power source  408  is coupled by a first switch  426  to the full wave rectifier. The full wave rectifier includes four diodes ( 410 ,  412 ,  414  and  416 ) in a diode bridge rectifier configuration. Diodes  410  and  416  are coupled to AC control  408 . Diodes  412  and  414  are coupled to the AC control  408  via switch  426 . The cathodes of diode  410  and diode  412  are coupled by a node  411 . The anodes of diode  414  and diode  416  are coupled by a node  415 . A fast release control switch  420 , implemented here using a metal oxide semiconductor field effect transistor (MOSFET), and the DC coil  418  are coupled in series across the diode bridge, or between node  411  and node  415 . A diode  422  and a zener diode  424  are coupled in a back to back configuration, e.g., where the anodes of both diodes are coupled together, in parallel to the DC coil  418 . In another embodiment, the cathodes of diode  422  and zener diode  424  are coupled together. 
     In several embodiments, the control switch  420 , diode  422 , zener diode  424 , and DC coil  418  form a fast release circuit. In one embodiment, the value or breakdown voltage of the zener diode is selected such that it is just lower than the breakthrough voltage of the parasitic diode of the MOSFET switch  420 . In such case, circuit operates such that the zener diode conducts before the MOSFET switch allows reverse conduction. In other embodiments, the value of the zener diode can be chosen based on other circuit characteristics. In some embodiments, the value of the zener diode is selected such that arcing between switch contacts is prevented. 
     In some embodiments, while the back EMF of the DC coil is greater than the breakdown voltage of the zener diode, the zener diode operates in a reverse biased mode and permits a controlled amount of current to flow through the zener diode and thus through the conventional diode. In such case, both diodes dissipate energy as current flows through both diodes and returns to the DC coil. This dissipation cycle can repeat until the DC coil is fully discharged. In several embodiments, the value of the zener diode, the zener or breakdown voltage, and the characteristics of the MOSFET (e.g., value of breakthrough voltage of the parasitic diode) are chosen to enable a particular release time. For example, in one embodiment, a zener diode having a breakdown voltage of 200 volts enables a release time of less than 10 ms. In such case, a MOSFET having a breakthrough voltage of the parasitic diode of greater than 200 volts can be used. In one embodiment, for example, the breakthrough voltage for the parasitic diode is 500 V. In another embodiment, a separate zener diode is used instead of the depicted parasitic (zener) diode in a parallel configuration across the MOSFET  420 . 
       FIG. 5  is a schematic diagram of an AC relay circuit  500  including a half wave rectifier and a fast release circuit in accordance with an embodiment of the present invention. The AC relay circuit  500  further includes a power source  502  coupled to a load switch  503 . The position of the armature of the load switch  503  is controlled by a switching force generated in a DC coil  514 . The load switch  503  is also coupled to a load  506 . 
     An AC control power source  508  is coupled by a half wave rectifier diode  510  to the DC coil  514 . The AC control power source  508  is also coupled by a MOSFET switch  512  to the DC coil  514 . A diode  516  and a zener diode  518  are coupled in a back to back series configuration, e.g., where the anodes of both diodes are coupled together, across (e.g., in parallel to) the DC coil  514 . In an alternative embodiment, the cathodes of diode  516  and zener diode  518  are coupled together. 
     In operation, the AC relay circuit  500  can operate as described in  FIG. 3 . In several embodiments, the control switch  512 , diode  516 , zener diode  518  and DC coil  514  form a fast release circuit. In one embodiment, the value or breakdown voltage of the zener diode is selected such that it is lower than the breakthrough voltage of the parasitic diode of the MOSFET switch  512 . In such case, circuit operates such that the zener diode conducts before the MOSFET switch allows reverse conduction. In such case, arcing across the MOSFET switch is prevented. In other embodiments, the value of the zener diode can be chosen based on other circuit characteristics. In a number of embodiments, the value of the zener diode is selected such that arcing between switch contacts is prevented. 
     In some embodiments, while the back EMF of the DC coil is greater than the breakdown voltage of the zener diode, the zener diode operates in a reverse biased mode and permits a controlled amount of current to flow through the zener diode and the conventional diode. In such case, both diodes dissipate energy as current flows through both diodes and returns to the DC coil. This dissipation cycle can repeat until the DC coil is fully discharged. 
     In some embodiments, the DC coil is discharged in a single cycle. In several embodiments, the value of the zener diode, the zener or breakdown voltage, and the characteristics of the MOSFET (e.g., value of breakthrough voltage of the parasitic diode) are chosen to enable a particular release time. For example, in one embodiment, a zener diode having a breakdown voltage of 200 volts enables a release time of less than 10 ms. In such case, a MOSFET having a breakthrough voltage for the parasitic diode of greater than 200 volts can be used. In one embodiment, for example, the breakthrough voltage of the parasitic diode is 500 V. In another embodiment, a separate zener diode is used instead of the depicted parasitic (zener) diode. In such case, the separate zener diode can improve MOSFET switch response to back EMFs and/or protect circuitry from other surges (e.g., lightning). 
     In some embodiments, the fast release circuit decreases the release time for the AC relay by 50 percent to 500 percent. In such case, the AC relay having a fast release circuit operates anywhere from 50 to 500 percent faster than a conventional AC relay. 
       FIG. 6  is a schematic diagram of an AC relay circuit  600  including a full wave rectifier and a fast release circuit in accordance with an embodiment of the present invention. The AC relay circuit  600  includes an AC control source  608  coupled to a diode bridge rectifier having a fast release circuit including a DC coil coupled across the diode bridge rectifier. The diode bridge rectifier includes four diodes ( 610 ,  612 ,  614  and  616 ) in a diode bridge rectifier configuration. Diodes  610  and  616  are coupled to AC control  608 . Diodes  612  and  614  are coupled to the AC control  608 . The cathodes of diode  610  and diode  612  are coupled by a node  611 . The anodes of diode  614  and diode  616  are coupled by a node  615 . 
     A fast release control switch  620 , implemented here using MOSFET, and the DC coil  618  are coupled in series across the diode bridge, or between node  611  and node  615 . A diode  622  and a zener diode  624  are coupled in a front to front configuration, (e.g., where the cathodes of both diodes are coupled is series together), across the DC coil  618 . In another embodiment, the anodes of diode  622  and zener diode  624  are coupled together. A resistor  626  is coupled to node  611  and a cathode of a second zener diode  628 . The anode of the zener diode  628  is coupled to the gate of the MOSFET switch  620 , to a capacitor  630 , and to a resistor  632 . The capacitor  630  and the resistor  632  are also coupled to node  615  which is coupled to a ground. In the illustrated embodiment, a drain of MOSFET switch  620  is coupled to diode  622  and DC coil  618 . A source of MOSFET switch  620  is coupled to node  615 . In the illustrated embodiment, the MOSFET switch  620  includes a body zener diode, or inherent diode, having a cathode coupled to the drain and an anode coupled to the source. In other embodiments, a separate zener diode is coupled in a similar polarity across the drain and source of the MOSFET switch  620 . 
     In several embodiments, the values for resistor  626 , zener diode  628 , capacitor  630  and resistor  632  are chosen such that MOSFET switch  620  is turned on at approximately the same time as that the voltage applied to DC coil  618  reaches a level appropriate for the DC coil to generate the switching force sufficient to actuate the armature of the relay (not shown). In such case, the MOSFET switch  620  opens and isolates the DC coil  618  and transient suppression components (zener diode  624  and diode  622 ). The RC circuit including capacitor  630  and resistor  632  maintain the gate voltage of the MOSFET switch  620  for a period of time sufficient to allow the transient suppression components to fully discharge the DC coil. In several embodiments, zener diode  624  has a relatively high breakdown voltage such that a large back EMF is generated and quickly dissipated. In such case, the release time for the DC coil is substantially decreased as compared to a conventional relay. 
     In one embodiment, zener diode  624  has a breakdown voltage of 200 volts while zener diode  628  has a breakdown voltage of 12 volts. In other embodiments, zener diodes having different breakdown voltages can be used. 
     In a number of embodiments, additional characteristics of an AC relay having a fast release circuit are designed to accommodate a particular intended back EMF. For example, in several embodiments, the separation of traces on a printed circuit board of the AC relay is implemented such that arcing between traces at the intended back EMF is prevented. In other embodiments, the material and thickness of coating(s) applied to the DC coil are selected such that arcing between windings at the intended back EMF and/or damage to coatings based on the magnitude of the back EMF are prevented. 
     While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as examples of specific embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.