Abstract:
A transistor driven load circuit includes a gate driver transistor includes an internal voltage clamp, a controller providing a gate control signal operable to control a state of said gate driver, a load connected to said gate driver, such that said gate driver allows power to flow through the load when the gate driver is in an on state and prevents power from flowing through the load when the gate driver is in an off state, a clamp assist circuit connected in electrical parallel to the load, wherein the clamp assist circuit is operable to dissipate energy flowing through the load during a high energy event in a recirculating device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present disclosure relates generally to voltage clamps for control devices such as transistors and, more particularly, to a clamp assist circuit for the same. 
         [0002]    Control devices, such as those used in power train high side and low side control circuits, are frequently required to dissipate energy within the control device. In some instances, the level of dissipation required exceeds the energy dissipation ability of an internal voltage clamp of control device, potentially damaging the internal electronics of the control device. 
         [0003]    To address the energy dissipation needs, existing control devices utilize a recirculation diode that recirculates current and, in the process, dissipates energy within the recirculation diode. The recirculation diode is arranged in a voltage clamp configuration, and only recirculates current when the clamp voltage is exceeded. Existing recirculation diodes and clamp circuits typically have a relatively low clamp voltage, thereby limiting the maximum voltage that can pass through the control device, and thus be absorbed by the control device, to a low magnitude. 
         [0004]    At the same time, in some control devices, a high clamping voltage is required in order to guarantee a fast dissipation of the stored energy. The voltage clamp including a recirculation diode utilized in existing systems lowers the clamping voltage, thereby slowing the rate at which the energy is dissipated. 
       SUMMARY OF THE INVENTION 
       [0005]    Disclosed is a transistor driven load circuit having a gate driver transistor including an internal voltage clamp, a controller providing a gate control signal operable to control a state of said gate driver, a load connected to said gate driver, such that said gate driver allows power to flow through the load when the gate driver is in an on state and prevents power from flowing through the load when the gate driver is in an off state, a clamp assist circuit connected in electrical parallel to the load, wherein the clamp assist circuit is operable to dissipate energy flowing through the load during a high energy event in a recirculating device. 
         [0006]    Also disclosed is a method for dissipating energy in a transistor driven load circuit including the steps of: activating a clamp assist circuit when a voltage threshold is exceeded, delaying activation of a recirculating device within the clamp assist circuit when the clamp assist circuit is activated using a delay network such that an internal voltage clamp of a gate driver can activate, dissipating energy within the recirculating device in the clamp assist circuit, thereby preventing an energy dissipation within the internal voltage clamp from exceeding a rated value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically illustrates a low side driven control device, load, and a clamp assist circuit in conjunction with a low side driven control device and a load. 
           [0008]      FIG. 2  schematically illustrates an alternate example clamp assist circuit in conjunction with a low side driven control device and a load. 
           [0009]      FIG. 3  illustrates a plot of energy dissipation within the circuit of  FIG. 2  during a high energy event. 
           [0010]      FIG. 4  illustrates a clamp assist circuit in conjunction with a high side driven control device and a load. 
           [0011]      FIG. 5  schematically illustrates a low side driven control device in conjunction with a load and a pulsed clamp assist circuit. 
           [0012]      FIG. 6  illustrates a plot of energy dissipation within the circuit of  FIG. 5  during a high energy event. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a high voltage clamp assist circuit  10  that aids an internal clamp  22  of a low side driven control device (MOSFET  20 ) in dissipating energy. The MOSFET  20  is controlled by a gate control circuit  30  using a known gate control scheme. The MOSFET  20  drives a load  32  and controls the flow of power through the load  32  from a voltage source  34 . While the load  32  can be any load type, one of skill in the art will appreciate that a majority of loads include at least an inductive and a resistive component and will therefore be an inductive/resistive (IR) load. 
         [0014]    The clamp assist circuit  10  is connected to the load  32  and includes three primary components: a recirculation device  40 , a voltage divider  50  and a clamp delay network  60 . In the illustrated example, the recirculating device  40  is a bipolar junction transistor (BJT). In alternate examples, any suitable recirculating device, such as a transistor of another transistor topology, can be utilized to the same effect. When the recirculation device  40  is on, energy is routed through the recirculation device  40  allowing some of the energy to be dissipated within the recirculation device  40 . 
         [0015]    A control signal for the recirculation device  40  is provided by the voltage divider  50  and the delay network  60 . The voltage divider  50  includes a pair of resistors  52 ,  54 . The first resistor  52  is connected between the MOSFET  20  and the clamp delay network  60 , and a second resistor  54  is connected between the clamp delay network  60  and a ground  70 . The specific resistances of the resistors  52 ,  54  define a threshold voltage at which power begins to be provided to the control input of the recirculating device  40  from the node connecting the resistors  52 ,  54 . One of skill in the art, having the benefit of this disclosure, would be able to determine a particular resistance for each resistor  52 ,  54  required to achieve a desired voltage threshold above which the clamp assist circuit  10  begins to operate. 
         [0016]    The clamp delay network  60  includes a resistor  62  and two capacitors  64 ,  66 . The first capacitor  64  connects the control input of the recirculation device  40  to ground  70 , and the second capacitor  66  connects the resistor  62  and the voltage divider  50  to ground  70 . During operation, when the voltage threshold set by the voltage divider  50  is initially exceeded, power flows into the delay network  60 . The power in the delay network  60  is prevented from reaching the control input of the recirculation device  40  for a period of time determined at least partially by the capacitance values of the first and second capacitors  64 ,  66  thereby delaying the activation of the recirculation device  40 . This delay in activating the recirculating device  40  provides time for the internal voltage clamp  22  of the MOSFET  20  to act on its own before introducing the clamp assist circuit  10 . 
         [0017]    With continued reference to  FIG. 1 , and with like numerals indicating like elements,  FIG. 2  illustrates an alternate example clamp assist circuit  100 . The alternate clamp assist circuit  100  illustrated in  FIG. 2  includes the same elements described previously and illustrated in  FIG. 1 . The example in  FIG. 2  further incorporates an additional parallel clamp assist circuit  110 . The parallel clamp assist circuit  110  is connected to the load  32  in parallel with the primary clamp assist circuit  10 . As with the primary clamp assist circuit  10 , the parallel clamp assist circuit  110  includes a voltage divider circuit  150  and a delay network  160  arranged similarly to the voltage divider  50  in the delay network  60  described above. The voltage divider circuit  150  and the delay network  160  operate in the same manner as the voltage divider  50  and the delay network  60  described above. 
         [0018]    Inclusion of the parallel clamp assist circuit  110  provides redundant clamp assist properties, and further increases the ability of the overall clamp assist configuration to absorb excess energy. Further, one of skill in the art, having the benefit of this disclosure, would appreciate that any number of parallel clamp assist circuits  110  can be utilized in an arrangement similar to the parallel arrangement illustrated in  FIG. 2  and further magnify the recognized benefits of the parallel clamp assist circuits. 
         [0019]      FIG. 3  illustrates an example plot  200  demonstrating the amount of energy dissipated in the MOSFET  20  internal clamp  22  of  FIG. 2  with respect to time at line  210 , the amount of energy dissipated in the primary clamp assist circuit  10  of  FIG. 2  with respect to time at line  220 , and the amount of energy dissipated in the secondary clamp assist circuit  110  of  FIG. 2  with respect to time at line  230 . In the example plot  200 , an energy spike (high energy event) occurs at time t 0 . Once the voltage divider thresholds are exceeded, the delay networks in the clamp assists circuits  10 ,  100  begin working, allowing time for the internal clamp  22  to activate and work at t 2 . As can be seen from the plot  200 , significant amounts of energy can be dissipated in a relatively short time period with a relatively high voltage clamp level  240  using the clamp assist circuits  10 ,  100  described above. 
         [0020]      FIG. 4  illustrates a clamp assist circuit  300  for use in conjunction with a high side gate driver arrangement  310 . As with the circuit of  FIG. 1 , a gate control circuit  330  controls a mosfet  320  using a known high side control scheme, and the MOSFET  320  drives a load  340 . The load  340  is connected to a voltage source  334 . The load  340  can be any load, however, it is understood that most loads  340  will be inductive/resistive (IR) loads  340 . 
         [0021]    The clamp assist circuit  300  includes two recirculating devices  352 ,  354  arranged as a recirculation circuit  350 . Connected to the high side of the load and the recirculation circuit  350  is a voltage divider  360 . Similarly, connected to the voltage divider  360  and ground is a delay circuit  370 . In the illustrated example of  FIG. 4 , the delay circuit  370  is a capacitor. 
         [0022]    Each of the three components  350 ,  360 ,  370  of the clamp assist circuit  300  functions as described above with regard to the low side clamp assist circuits  10 ,  100  and illustrated in  FIGS. 1 and 2 . Similarly, as in the example of  FIG. 2 , additional clamp assist circuits can be connected in parallel to the primary clamp assist circuit  300  and provide the previously described additional benefits of parallel clamp assist circuits. 
         [0023]    In some example systems, increasing the magnitude of the energy that can be dissipated by the clamp assist circuit(s) is a higher priority than increasing the speed of the energy dissipation. Each of the previously described clamp assist circuits  10 ,  100 ,  300  dissipates energy quickly at the expense of a total magnitude of energy that can be dissipated.  FIG. 5  illustrates a pulsed clamping assist circuit  400  that increases the magnitude of energy dissipation. 
         [0024]    As with the previous circuits, a gate control  410  controls a drive transistor  420  with an internal clamping circuit  422  (a zener diode). The drive transistor  420  is connected to a low side of a load  430 . The load  430  is connected to a voltage  434 . The load  430  can be any load type, however, one of skill in the art will appreciate that a typical load is an inductive/resistive (IR) load. The pulsed clamp assist circuit  400  includes a voltage divider  440 , an oscillator circuit  450 , a diode  460  and a recirculating device  470 . 
         [0025]    Within the oscillator circuit  450  is a standard Schmidt trigger circuit  452 . The Schmidt trigger circuit  452  connects two resistors  454 ,  455  and a capacitor  456  to a recirculation device  470  gate control switch  458 . In particular, the resistances and capacitance of the resistors  454 ,  455  and capacitor  456  control the rate at which the Schmidt trigger  452  pulses according to known principles. The pulsing of the Schmidt trigger in turn controls the pulsing of the switch  458  and the on/off pulsing of the recirculation device  470 . The energy dissipation is alternated between the clamping assist circuit  400  and the drive transistor  420  internal clamping circuit  422 . 
         [0026]    The voltage divider  440  includes two resistors  442 ,  444  with the resistance values of the resistors  442 ,  444  determining a voltage threshold at which the pulse clamp assist circuit  400  begins operating. The diode  460  is placed between the oscillator  450  and the load  430 , and prevents the oscillator  450  from being back-charged. 
         [0027]    While the circuit configuration of  FIG. 5  is that of a low side driver, one of skill in the art having the benefit of this disclosure could adapt the pulsed clamp assist circuit  400  to operate as a high side driver pulsed clamp assist circuit in a similar manner to the clamp assist circuit  300  illustrated in  FIG. 4 . 
         [0028]    Furthermore, as with the previous examples, the pulsed clamp assist circuit  400  of  FIG. 5  can include multiple clamp assist circuits  400  arranged in parallel, thereby achieving additional benefits. 
         [0029]      FIG. 6  illustrates a plot  500  of the energy dissipation in a low side driver pulsed clamp assist circuit such as the pulsed clamp assist circuit  400  of  FIG. 5 . Line  510  indicates the amount of energy dissipated with respect to time by the pulsed clamp assist circuit. Each time the clamp assist circuit  400  pulses, a corresponding energy pulse occurs in the clamp assist circuit. The energy then dissipates and another pulse occurs. As can be appreciated, the pulsing significantly increases the magnitude of energy absorbed by the clamp assist circuit, thereby decreasing the amount of energy required to be absorbed by the internal clamp of the driver switch. 
         [0030]    In a further example, any of the previously described clamp assist circuits  10 ,  100 ,  300 ,  400  can be utilized in conjunction with any other of the previously described clamp assist circuits  10 ,  100 ,  300 ,  400  in the parallel clamp assist configuration and provide further benefits. 
         [0031]    It is further understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.