Patent Publication Number: US-9412544-B2

Title: System and method for driving a relay circuit

Description:
This application is a continuation of copending U.S. patent application Ser. No. 12/892,745, filed on Sep. 28, 2010, the contents of which are incorporated by reference herein. 
    
    
     Embodiments of the invention relate generally to electrical systems and methods and, more particularly, to systems and methods for driving a relay circuit. 
     A relay circuit provides electrical isolation between different circuits. Using a relay circuit, a low current circuit can be used to control a high current circuit while the low current circuit is electrically isolated from the high current circuit by the relay circuit. A relay driver circuit is usually used to drive a relay circuit. However, characteristics of the relay circuit such as turn-off speed and lifetime can be affected by the relay driver circuit. 
     A system and method for driving a relay circuit involves driving a relay circuit using a first driver circuit if a voltage of a battery supply for the relay circuit is lower than a voltage threshold and driving the relay circuit using a second driver circuit if the voltage of the battery supply for the relay circuit is higher than the voltage threshold. 
     In an embodiment, a method for driving a relay circuit involves driving a relay circuit using a first driver circuit if a voltage of a battery supply for the relay circuit is lower than a voltage threshold and driving the relay circuit using a second driver circuit if the voltage of the battery supply for the relay circuit is higher than the voltage threshold. 
     In an embodiment, a driver circuit system for driving a relay circuit includes a first driver circuit configured to drive a relay circuit using a first driving mechanism, a second driver circuit configured to drive the relay circuit using a second driving mechanism, and a switch circuit configured to switch off the first driver circuit and to switch on the second driver circuit if a voltage of a battery supply for the relay circuit is higher than a voltage threshold. The second driving mechanism is different from the first driving mechanism. 
     In an embodiment, a driver circuit system for driving a relay circuit includes a first switch connected to a relay circuit, a second switch connected to a battery supply for the relay circuit, a voltage source, a comparator, a first diode, a second diode, a third diode, and a driver transistor. The comparator includes a first input terminal connected to the battery supply for the relay circuit, a second input terminal connected to the voltage source, and an output terminal connected to the first switch and the second switch. The cathode of the first diode is connected to the first switch, the anode of the first diode is connected to the anode of the second diode, and the cathode of the third diode is connected to the second switch. The cathode of the second diode is connected to the gate of the driver transistor and the anode of the third diode is connected to the driver transistor. 
    
    
     
       Other aspects and advantages of embodiments of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, depicted by way of example of the principles of the invention. 
         FIG. 1  is a schematic block diagram of an electrical circuit in accordance with an embodiment of the invention. 
         FIG. 2  depicts an embodiment of the electrical circuit of  FIG. 1 . 
         FIG. 3  depicts another embodiment of the electrical circuit of  FIG. 1 . 
         FIG. 4  is a process flow diagram of a method for driving a relay circuit in accordance with an embodiment of the invention. 
     
    
    
     Throughout the description, similar reference numbers may be used to identify similar elements. 
     It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment. 
     Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
       FIG. 1  is a schematic block diagram of an electrical circuit  100  in accordance with an embodiment of the invention. The electrical circuit may be used for various applications in which an isolated circuit is controlled by another circuit. In some embodiments, the electrical circuit is used for automobile applications such as controlling modules such as engine, rain wipers, window, roof, doors, and/or brakes of a motor vehicle. 
     In the embodiment depicted in  FIG. 1 , the electrical circuit  100  includes a driver circuit system  102 , a relay circuit  104 , and an isolated circuit  106 . Although the electrical circuit is depicted and described with certain components and functionality, other embodiments of the electrical circuit may include fewer or more components to implement less or more functionality. 
     The driver circuit system  102  of the electrical circuit  100  is configured to drive the relay circuit  104  to control the isolated circuit  106 . In the embodiment depicted in  FIG. 1 , the driver circuit system includes a first driver circuit  108 , a second driver circuit  112 , and a switch circuit  110 . Although the driver circuit system is shown in  FIG. 1  as including only two driver circuits, the driver circuit system may include more than two driver circuits in other embodiments. 
     In the embodiment depicted in  FIG. 1 , the first driver circuit  108  of the driver circuit system  102  is configured to drive the relay circuit using a first driving mechanism. The second driver circuit  112  of the driver circuit system is configured to drive the relay circuit using a second driving mechanism, which is different from the first driving mechanism. 
     The first driver circuit  108  and the second driver circuit  112  may share a semiconductor device. The shared semiconductor device may be any type of semiconductor device. In an embodiment, the first driver circuit and the second driver circuit share a driver transistor. 
     The switch circuit  110  of the driver circuit system  102  is configured to switch off one of the first and second driver circuits  108 ,  112  and to switch on another one of the first and second driver circuits if a certain relationship between a voltage of a battery supply for the relay circuit  104  and a voltage threshold is met. In an embodiment, when a circuit is switched off, at least a part of all components in the circuit is disabled and dysfunctional. In this case, when a circuit is switched on, all components in the circuit are enabled and functional. 
     In an embodiment, the switch circuit  110  switches off the first driver circuit  108  and switches on the second driver circuit  112  if the voltage of the battery supply for the relay circuit is higher than the voltage threshold. In this case, the relay circuit  104  is driven using the second driver circuit if the voltage of the battery supply for the relay circuit is higher than the voltage threshold. The switch circuit switches off the second driver circuit and switches on the first driver circuit if the voltage of the battery supply for the relay circuit is lower than the voltage threshold. In this case, the relay circuit is driven using the first driver circuit if the voltage of the battery supply for the relay circuit is lower than the voltage threshold. 
     The relay circuit  104  of the electrical circuit  100  provides electrical isolation between the driver circuit system  102  and the isolated circuit  106 . In the embodiment depicted in  FIG. 1 , the relay circuit is configured to be energized by the driver circuit system to control the isolated circuit. 
     The isolated circuit  106  of the electrical circuit  100  is isolated from the driver circuit system  102  by the relay circuit  104 . The isolated circuit usually differs from the driver circuit system in circuit characteristics. For example, the isolated circuit is a high voltage circuit and the driver circuit system is a low voltage circuit. In another example, the isolated circuit is a high current circuit and the driver circuit system is a low current circuit. 
     Switching off one of the first and second driver circuits  108 ,  112  and switching on another one of the first and second driver circuits when a certain relationship between the voltage of the battery supply for the relay circuit  104  and the voltage threshold is met enables driving the relay circuit using a particular driver circuit under the certain relationship between the voltages. Therefore, a driver circuit that achieves a particular benefit or has a specific characteristic when there is a certain relationship between the voltage of the battery supply for the relay circuit and the voltage threshold can be chosen from multiple driver circuits to drive the relay circuit. 
     In some applications, the relationship between the voltage of the battery supply for the relay circuit  104  and a predefined voltage threshold is fixed. For example, in some automotive applications, the voltage of the battery supply is smaller than the voltage threshold in most of the lifetime of the relay circuit. 
     Therefore, a driver circuit can be selected to achieve a particular benefit or to exhibit a specific characteristic under the fixed relationship. When the relationship between the voltage of the battery supply and the predefined voltage threshold changes, a different driver circuit can be chosen to achieve another particular benefit or to exhibit another specific characteristic. 
     In an embodiment, one of the first and second driver circuits  108 ,  112  is an active clamping driver circuit and another one of the first and second driver circuits is a free-wheel diode driver circuit. Two of such embodiments of the electrical circuit  100  of  FIG. 1  are depicted in  FIGS. 2 and 3 . 
     The electrical circuits  200 ,  300  in the embodiments depicted in  FIGS. 2 and 3  can be used in automotive applications where the battery supply for the relay circuit is a 12 volt battery supply. The electrical circuits may be used for central body control modules, rain wipers, window lifters, roof modules, power sliding doors, anti-lock braking system (ABS), Electronic stability Programme (ESP), and engine control of a motor vehicle. For example, when the ignition switch of a motor vehicle is turned on, approximately 12 volts is applied to the starter solenoid of the motor vehicle, the coil of the starter solenoid is energized, and the battery voltage is delivered through switch contacts to the starter motor of the motor vehicle. 
       FIG. 2  depicts an embodiment of the electrical circuit  100  of  FIG. 1  in which one of the first and second driver circuits  108 ,  112  is an active clamping driver circuit and another one of the first and second driver circuits is a free-wheel diode driver circuit. In the embodiment depicted in  FIG. 2 , the electrical circuit  200  includes a driver circuit system  202 , a relay circuit  204 , and an isolated circuit  206 . The driver circuit system includes a switch circuit  210 , an active clamping driver circuit  208 , a free-wheel diode driver circuit  212 , and a battery supply  214  for the relay circuit  204 . Although the driver circuit system is shown in  FIG. 2  as including the battery supply for the relay circuit, in other embodiments, the battery supply for the relay circuit may be external to the driver circuit system and not included in the driver circuit system. For example, the battery supply for the relay circuit in a motor vehicle is the main battery of the motor vehicle. 
     The switch circuit  210  of the driver circuit system  202  includes a comparator  216 , a first switch  218 , a second switch  220 , and a voltage source  222 . In the embodiment depicted in  FIG. 2 , the comparator of the switch circuit includes a first input terminal  224  connected to the battery supply  214  for the relay circuit  204 , a second input terminal  226  connected to the voltage source, and an output terminal  228  connected to the first switch and the second switch. The first switch of the switch circuit is configured to switch on or to switch off the active clamping driver circuit  208  under the control of the comparator. The second switch of the switch circuit is configured to switch on or to switch off the free-wheel diode driver circuit  212  under the control of the comparator. The voltage source of the switch circuit is configured to have a voltage value that is equal to the voltage threshold. 
     In an embodiment, the battery supply  214  for the relay circuit  204  is an automotive 12 volt battery supply and the operating range of the battery supply for the relay circuit is from 5 volts to 18 volts. In this case, the voltage threshold of the voltage source  222  is set to 18 volts. However, in some situations, the voltage value of the battery supply for the relay circuit can rise to be above the voltage threshold of the voltage source. For example, during a vehicle jump start, the voltage value of the battery supply can rise to between 18 volts and 28 volts. During a vehicle load dump, the maximum voltage value of the battery supply can be higher than 28 volts. 
     The active clamping driver circuit  208  of the driver circuit system  202  includes a driver transistor  230 , a first diode  232 , and a second diode  234 . The active clamping driver circuit limits the output voltage across the driver transistor to a safe value. The driver transistor can be any type of semiconductor transistor. In the embodiment depicted in  FIG. 2 , the driver transistor is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). In the embodiment depicted in  FIG. 2 , the first diode  232  is a Zener diode and the second diode  234  is a normal diode. As depicted in  FIG. 2 , the cathode  236  of the first diode  232  is connected to the first switch  218 , the anode  238  of the first diode  232  is connected to the anode  240  of the second diode  234 , and the cathode  242  of the second diode  234  is connected to the gate  244  of the driver transistor. In the embodiment depicted in  FIG. 2 , the driver transistor is connected to ground. 
     The free-wheel diode driver circuit  212  of the driver circuit system  202  shares the driver transistor  230  with the active clamping driver circuit  208 . In the embodiment depicted in  FIG. 2 , the free-wheel diode driver circuit includes the driver transistor  230  and a third diode  246 . As depicted in  FIG. 2 , the anode  248  of the third diode  246  is connected to the driver transistor and the cathode  250  of the third diode  246  is connected to the second switch  220 . In this configuration, the third diode  246  is connected in parallel with the relay circuit  204  to limit the voltage across the driver transistor and to prevent breakdown of the driver transistor. 
     Compared to the free-wheel diode driver circuit  212 , the active clamping driver circuit  208  significantly increases the turn-off speed of the relay circuit  204  at low supply voltages. Because the lifetime of relay switch contacts in the relay circuit can be determined by the duration of the arc between the relay switch contacts during the turn-off of the relay circuit, the fast turn-off of the relay circuit can increase the lifetime of the relay switch contacts. In addition, compared to the free-wheel diode driver circuit, the active clamping driver circuit increases the dissipation in the driver transistor  230  during the turn-off of the relay circuit. At high supply voltages, the turn-off speed advantage of the active clamping driver circuit disappears and the increase of the dissipation in the driver transistor can be significant enough to threaten the function of the driver transistor. To accommodate the active clamping driver circuit under high supply voltages, the chip area for the driver transistor has to be significantly increased to distribute the increased dissipation in the driver transistor. Furthermore, for the active clamping driver circuit, the clamping voltage should always be higher than the voltage of the battery supply  214  to guarantee to be able to turn off the relay circuit during a load dump. 
     Compared to the active clamping driver circuit  208 , the cost to manufacture the free-wheel diode driver circuit  212  is lower. In addition, the free-wheel diode driver circuit incurs a lower dissipation in the driver transistor  230  during the turn-off of the relay circuit  204 . The disadvantage of the free-wheel diode driver circuit is the slow turn-off of the relay circuit under low supply voltages. 
     Therefore, using only the active clamping driver circuit  208  when the voltage of the battery supply  214  for the relay circuit  204  is lower than a predefined voltage threshold and using only the free-wheel diode driver circuit  212  when the battery supply voltage is higher than a predefined voltage threshold combines the benefit of fast turn-off of the relay circuit with the benefit of the low dissipation of the driver transistor  230 . Specifically, by using only the active clamping driver circuit when the battery supply voltage is lower than a predefined voltage threshold, the turn-off speed of the relay circuit at low supply voltages is increased, which in turn increases the lifetime of the relay contacts. In addition, using only the free-wheel diode driver circuit when the battery supply voltage is higher than a predefined voltage threshold has the benefit of low dissipation of the driver transistor while maintaining the same turn-off speed of the relay circuit compared to active clamping. As a result, the dissipation in the driver transistor at high supply voltages can be reduced, which results in a significant reduction in chip area for the driver transistor. 
     A possible drawback to using only the free-wheel diode driver circuit  212  when the voltage of the battery supply  214  for the relay circuit  204  is higher than a predefined voltage threshold is that the turn-off speed of the relay circuit is low. However, in some applications, the battery supply voltage is smaller than a predefined voltage threshold throughout most of the lifetime of the relay circuit. For example, for automotive applications where the battery supply is an automotive 12 volt battery supply, the battery supply voltage is smaller than the voltage threshold of 18 volts in most of the lifetime of the relay circuit. Typically, a vehicle jump start event, where the battery supply voltage can rise to between 18 volts and 28 volts, occurs only for 600 seconds over a 10 year lifetime. A vehicle load dump event, where the maximum battery supply voltage can be even higher than 28 volts, occurs only for 60 seconds over a 10 year lifetime. 
     The relay circuit  204  of the electrical circuit  200  provides electrical isolation between the driver circuit system  202  and the isolated circuit  206 . In the embodiment depicted in  FIG. 2 , the relay circuit includes a relay coil  252  and a relay switch  254 . The relay switch is connected to the isolated circuit and includes two relay switch contacts  256 ,  258  and a contact arm  260 . The relay switch can be any type of relay switch. In an embodiment, the relay switch is a mechanical relay switch that includes mechanical switch contacts and a mechanical contact arm. The relay coil of the relay circuit is configured to be energized by the driver circuit system to control the relay switch contacts. Specifically, when an electric current from the driver circuit system is passed through the relay coil, the resulting magnetic field connects the relay contacts with the contact arm and enables or closes the relay switch. In the embodiment depicted in  FIG. 2 , the battery supply  214  for the relay circuit is connected to one terminal  262  of the relay coil and to the second switch  220  while another terminal  264  of the relay coil is connected to the anode  248  of the third diode  246 , to the driver transistor  230 , and to the first switch  218 . The isolated circuit  206  in the embodiment depicted in  FIG. 2  is the same as or similar to the isolated circuit  106  in the embodiment depicted in  FIG. 1 . 
       FIG. 3  depicts another embodiment of the electrical circuit  100  of  FIG. 1  in which one of the first and second driver circuits  108 ,  112  is an active clamping driver circuit and another one of the first and second driver circuits is a free-wheel diode driver circuit. In the embodiment depicted in  FIG. 3 , the electrical circuit  300  includes a driver circuit system  302 , a relay circuit  204 , and an isolated circuit  206 . 
     The driver circuit system  302  of the electrical circuit  300  includes a switch circuit  310 , an active clamping driver circuit  308 , a free-wheel diode driver circuit  312 , and a battery supply  214  for the relay circuit  204 . Although the driver circuit system is shown in  FIG. 3  as including the battery supply for the relay circuit, in other embodiments, the battery supply for the relay circuit may be external to the driver circuit system and not included in the driver circuit system. 
     In the embodiment depicted in  FIG. 3 , the switch circuit  310  of the driver circuit system  302  includes a comparator  316 , a switch transistor  318  for the active clamping driver circuit  308 , a switch transistor circuit  320  for the free-wheel diode driver circuit  312 , a voltage source  322 , a resistor  324  connected between the comparator and the battery supply  214  for the relay circuit  204 , and a resistor  326  connected between the comparator and the voltage source. 
     The comparator  316  of the switch circuit  310  includes a first input terminal  328  connected to the battery supply  214  for the relay circuit  204  via the resistor  324 , a second input terminal  330  connected to the voltage source  322 , and an output terminal  332  connected to the switch transistor  318  and to the switch transistor circuit  320 . 
     The switch transistor  318  of the switch circuit  310  is configured to switch on or to switch off the active clamping driver circuit  308  under the control of the comparator  316 . The switch transistor circuit  320  of the switch circuit is configured to switch on or to switch off the free-wheel diode driver circuit  312  under the control of the comparator. In the embodiment depicted in  FIG. 3 , the switch transistor circuit  320  includes an OR gate  334 , a current source  336  connected to a fixed voltage source  338 , such as 3.3 volts, transistors  340 ,  342 ,  344 ,  346 ,  348 , a resistor  350 , capacitors  352 ,  354 , and diodes  356 ,  358 . The OR gate of the switch transistor circuit includes an input terminal configured to receive a clock signal (CLK) and another input terminal connected to the output terminal  332  of the comparator  316 . The transistors  340 ,  342 , and  344  are connected between the current source and ground. The resistor  350 , the capacitor  354 , the transistor  348 , and the diodes  356  and  360  are connected to the battery supply  214 . In the embodiment depicted in  FIG. 3 , the transistor  348  includes an internal back-gate diode  360 . In an embodiment, the current from the current source is equal to the voltage value of the fixed voltage source  338  divided by the resistance value of the resistor  350 . The voltage source  322  of the switch circuit is configured to have a voltage value that is equal to a bandgap voltage. 
     The active clamping driver circuit  308  of the driver circuit system  302  includes a driver transistor  230 , resistors  362 ,  364 , a diode  366 , transistors  368 ,  370 ,  372 , and a NOT gate  374 . The active clamping driver circuit is switched on or off by the switch transistor  318  under the control of the comparator  316  to limit the output voltage across the driver transistor to a safe value. In the embodiment depicted in  FIG. 3 , the driver transistor is driven by input signals to the NOT gate and the switch transistor  318  enables the active clamp driver circuit when the driver transistor  230  is driven high. The gate  244  of the driver transistor  230  is connected to the switch transistor  318  and the transistors  368  and  372 . The transistor  372  is connected to a fixed voltage source  376 , such as 3.3 volts. In the embodiment depicted in  FIG. 3 , the transistors  230 ,  368 , and  370  are connected to ground. 
     The free-wheel diode driver circuit  312  of the driver circuit system  302  shares the driver transistor  230  with the active clamping driver circuit  308 . The free-wheel diode driver circuit includes the driver transistor  230  and a diode  246 . In the embodiment depicted in  FIG. 3 , the anode  248  of the diode  246  is connected to the driver transistor and the cathode  250  of the diode  246  is connected to the switch transistor circuit  320 . In this configuration, the diode  246  is connected in parallel with the relay circuit  204  to limit the voltage across the driver transistor to prevent breakdown of the driver transistor. 
     Two examples of operations of the electrical circuit  300  are described below. In the first example, the battery supply  214  to the relay circuit  204  and to the resistors  324  and  326  satisfies: 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       bat 
                     
                     &lt; 
                     
                       
                         
                           V 
                           thre 
                         
                         × 
                         
                           ( 
                           
                             
                               R 
                               1 
                             
                             + 
                             
                               R 
                               2 
                             
                           
                           ) 
                         
                       
                       
                         R 
                         1 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where V bat  represents the voltage of the battery supply, V thre  represents the voltage threshold of the voltage source  322 , R 1  represents the resistance value of the resistor  326 , and R 2  represents the resistance value of the resistor  324 . In this case, the comparator output at the output terminal  332  is logic high and the active clamping driver circuit  308  is activated by the switch transistor  318 . When the input signal at the NOT gate  374  is logic ‘1’, the gate of the transistor  372  is driven to ground and the gate  244  of the driver transistor  230  is driven with the fixed voltage source  376 . The terminal  264  of the relay circuit  204  is driven low and the relay circuit is activated. When the input signal at the NOT gate  374  becomes logic ‘0’, the transistor  372  opens and the gate voltage of the driver transistor  230  starts to drop. The electric current through the driver transistor  230  and the relay coil  252  of the relay circuit decreases while the inductance of the relay coil generates a high voltage on the terminal  264  of the relay circuit. If the voltage on the terminal  264  of the relay circuit becomes higher than a voltage value, the gate  244  of the driver transistor  230  will be driven by the voltage feedback via the resistor  362 , the diode  366 , and the switch transistor  318 , which effectively clamps the voltage on the terminal  264  of the relay circuit and decreases the current through the driver transistor  230  to zero. When the current stops flowing through the driver transistor  230 , the voltage on the terminal  264  of the relay circuit will drop back to the battery supply level and the gate of the driver transistor  230  will be pulled down to ground. 
     In the second example, the battery supply  214  to the relay circuit  204  and to the resistors  324  and  326  satisfies: 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       bat 
                     
                     &gt; 
                     
                       
                         
                           V 
                           thre 
                         
                         × 
                         
                           ( 
                           
                             
                               R 
                               1 
                             
                             + 
                             
                               R 
                               2 
                             
                           
                           ) 
                         
                       
                       
                         R 
                         1 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where V bat  represents the voltage of the battery supply, V thre  represents the voltage threshold of the voltage source  322 , R 1  represents the resistance value of the resistor  326 , and R 2  represents the resistance value of the resistor  324 . The comparator output at the output terminal  332  is logic low and the active clamping driver circuit  308  is disabled. When the input signal at the NOT gate  374  makes the transition from logic ‘1’ to logic ‘0’, the current through the driver transistor  230  will immediately become zero, which results in a positive peak voltage on the terminal  264  of the relay circuit  204  caused by the inductance of the relay coil  252 . Because the comparator output at the output terminal  332  is logic low, transistors  340  and  346  are now open and the charge pump circuit builds around transistors  342 ,  344 , the resistor  350 , the capacitors  352 ,  354 , and the diodes  356  and  358  drives the transistor  348 . The current of the relay coil  252  now runs through the diode  246  of the free-wheel diode driver circuit  312  to discharge the inductance. 
       FIG. 4  is a process flow diagram of a method for driving a relay circuit in accordance with an embodiment of the invention. At block  402 , a relay circuit is driven using a first driver circuit if a voltage of a battery supply for the relay circuit is lower than a voltage threshold. At block  404 , the relay circuit is driven using a second driver circuit if the voltage of the battery supply for the relay circuit is higher than the voltage threshold. 
     Although the operations of the method herein are shown and described in a particular order, the order of the operations of the method may be altered so that certain operations may be performed in an inverse order or so that certain operations may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be implemented in an intermittent and/or alternating manner. 
     In addition, although specific embodiments of the invention that have been described or depicted include several components described or depicted herein, other embodiments of the invention may include fewer or more components to implement less or more feature. 
     Furthermore, although specific embodiments of the invention have been described and depicted, the invention is not to be limited to the specific forms or arrangements of parts so described and depicted. The scope of the invention is to be defined by the claims appended hereto and their equivalents.