Abstract:
A highly efficient line transient protection circuit is provided for high power loads that are designed to operate through a high line transient. The transient protection circuit for high power loads is provided with a primary leg circuit, a load circuit and a secondary circuit. The load circuit may be a switching regulator circuit, a load circuit containing an oscillator, a push-pull circuit, a boost converter circuit, a buck converter circuit or the like. The transient protection circuit is provided with a simple driver circuit to turn on a bypass n-channel MOSFET. It operates at a higher efficiency; I.E., conduction losses are minimized during normal input voltage conditions. Furthermore, the transient protection circuit provides a programmable voltage clamp which is implemented through selecting zener diode VR 1 . The transient protection may be used to protect medium to large current circuits from line transients. Additionally, the transient protection circuit it is suitable for applications in loads that have to operate through a high line transient (e.g. military, medical, etc.).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to line transient protection circuits. In particular, the present invention relates to highly efficient line transient protection circuits for high power loads that are designed to operate through a high line transient (e.g. military and medical applications, etc.).  
         [0005]     2. Background of the Invention  
         [0006]     In general, transient electric phenomena occur, for instance, when a load is suddenly changed and an appreciable time elapses before the power level and circuit adapt to the new conditions. The voltages and currents during the intermediate time are known as transient. A transient may further be described as the momentary departure of a characteristic from steady-state conditions and back to steady-state conditions as a result of a system disturbance, such as a load or line change.  
         [0007]     In high power load applications, line transient protection circuits (or voltage limiting circuits) have traditionally been designed using a power bipolar transistor. One well-known bipolar approach is the darlington power transistor transient protection circuit ( 1 ) is shown in  FIG. 1  The darlington configuration typically includes a primary circuit having an input V IN  ( 4 ), a bipolar transistor Q 1 A and Q 1 B ( 54 ) and an output V IN ′ (5). A secondary leg connected to the primary leg between V IN  and the bipolar transistor ( 54 ) includes a resistor R 1  ( 34 ). The secondary leg is then fed into the bipolar transistor ( 54 ). A zener diode VR 1  ( 28 ) is further arranged between the resistor R 1  and the bipolar transistor ( 54 ) where the anode of VR 1  is grounded ( 26 ). Additionally, the transient protected load depicted as RL circuit leg ( 50 ) is arranged between the bipolar transistor ( 54 ) and V IN ′ where the return RL leg is grounded ( 26 ). However, a major disadvantage of the darlington circuit ( 1 ) is that it is not very efficient with respect to power consumption. In particular, at mid to high current levels, the power bipolar transistor suffers from poor current gain requiring high base current and high forward collector to emitter voltage drop, thus, is typically inefficient.  
         [0008]     Another traditional approach to transient protection circuitry is to use a power MOSFET transient protection circuit as shown in  FIG. 2 . The power MOSFET transient protection circuit ( 2 ) typically includes a primary circuit having an input V IN  ( 4 ), an n-channel enhancement power MOSFET Q 1  ( 8 ) and an output V IN ′ ( 5 ). A secondary leg connected to the primary leg between V IN  and Q 1  includes a resistor R 1  ( 34 ). The secondary leg is then fed into the gate connection of Q 1 . A zener diode VR 1  is further arranged between the resistor R 1  and Q 1  where the anode end of the zener diode is grounded ( 26 ). Additionally, a load, depicted as an RL circuit leg  50  is arranged between Q 1  and V IN ′ where the return of the RL leg is grounded ( 26 ). However, a major disadvantage of the power MOSFET transient protection circuit ( 2 ) is that it is typically more inefficient with respect to power consumption than that of the darlington transient protection circuit ( 1 ). Thus, one of the overall primary disadvantages of the aforementioned designs is that they are not power efficient.  
         [0009]     It would be advantageous to provide a highly efficient line transient protection circuit for high power loads. In particular, it would be beneficial to provide a transient protection circuit that is adapted to operate through a high transient line. It would further be ideal to provide a transient protection circuit that can be used to protect medium to large current circuit from line transients.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The aforementioned disadvantages are overcome by providing a highly efficient line transient circuit for high power loads. The present invention provides a transient protection circuit which can serve as a transient protection circuit for applications that require loads to operate through a high line transient. It uses a novel approach in boosting the gate voltage of Q 1  to fully enhanced Q 1  therefore reducing conduction loss.  
         [0011]     According to one exemplary embodiment of the present invention, a transient protection circuit for high power loads is provided comprising a primary circuit leg having an input V IN  in electrical communication with a drain of a MOSFET Q 1  and a source of the MOSFET Q 1  being an output of the primary circuit. The transient protection circuit also includes a load circuit having an input V IN ′ in electrical communication with the output of the primary circuit leg, the load circuit having a winding N 1  forming a portion of one of an inductor L 1  or a transformer T 1 . Also, the transient protection circuit includes a secondary circuit in electrical communication with a gate of the MOSFET Q 1  and in electrical communication with a zener diode VR 1  and wherein the zener diode VR 1  is electrically grounded, a circuit leg V GATE  in electrical communication with a node between the gate of MOSFET Q 1  and the zener diode VR 1  being in electrical communication with a resistor R 2 , a circuit leg V B  in electrical communication with resistor R 2  and in electrical communication with a diode D 1 , a circuit leg V A  in electrical communication with diode D 1  and with a first terminal of a winding N 2  forming a portion of the one of an inductor L 1  or a transformer T 1 , and wherein another terminal of the N 2  winding is in electrical communication with the input VIN′ of the load circuit.  
         [0012]     According to another aspect of the present invention, the load circuit further comprises an output V OUT . According to another aspect of the present invention, the load circuit comprises one of a switching regulator circuit, oscillator circuit, a push-pull circuit, a boost converter circuit or a buck converter circuit.  
         [0013]     According to still another aspect of the present invention, the buck converter comprises the input V IN ′ in electrical communication with a drain of a FET Q 2 , a circuit leg V SW  in electrical communication with a source of FET Q 2  and in electrical communication with a terminal of the winding N 1  of the inductor L 1 , the output V OUT  in electrical communication with another terminal of the winding N 1 , a FET Q 3  having a drain in electrical communication with the circuit leg V SW  and source of the FET Q 3  being electrically grounded, a regulator controller U 1  in electrical communication with a gate of the FET Q 2  and with a gate of the FET Q 3  and wherein a source of the FET Q 3  is electrically grounded.  
         [0014]     Further, according to other aspects of the present invention, the transient protection circuit further comprises a capacitor C 2  in electrical communication with the input V IN ′ and further electrically grounded. Moreover, the present invention may further comprise a capacitor C 4  in electrical communication with the output V OUT  and further electrically grounded. In another aspect of the present invention, a resistor R 1  in electrical communication with the input V IN  upstream the drain of MOSFET Q 1  and in electrical communication with the secondary circuit leg upstream the node between the gate of MOSFET Q 1  and the zener diode VR 1 .  
         [0015]     According to another aspect of the present invention, the transient protection circuit further comprises a capacitor C 3  in electrical communication with the primary circuit downstream the source of MOSFET Q 1  and in electrical communication with the circuit leg V GATE  upstream the resistor R 2 . Another aspect of the present invention includes the transient protection circuit further comprising a zener diode VR 2  in electrical communication with the primary circuit downstream the source of MOSFET Q 1  and in electrical communication with the circuit leg V GATE  upstream the resistor R 2 . The transient protection circuit may further comprise a capacitor C 1  in electrical communication with the input V IN  upstream the drain of Q 1  and further electrically grounded. And according to a further aspect of the present invention, a programmable voltage clamp is implemented through zener diode VR 1 .  
         [0016]     According to another exemplary embodiment of the present invention, a transient protection circuit for high power loads comprising a primary circuit leg comprising an input V IN  in electrical communication with a drain of a MOSFET Q 1  and a source of the MOSFET Q 1  being an output of the primary circuit, and a capacitor C 1  in electrical communication with the input V IN  upstream the drain of Q 1  and further electrically grounded. The transient protection circuit further includes a load circuit comprising an input V IN ′, a winding N 1  forming a portion of an inductor L, and an output V OUT , wherein input V IN ′ is in electrical communication with the output of the primary circuit. The transient protection circuit further includes a secondary circuit in electrical communication with a gate of the MOSFET Q 1  and in electrical communication with a zener diode VR 1  and wherein the zener diode VR 1  is electrically grounded, a circuit leg V GATE  in electrical communication with a node between the gate of MOSFET Q 1  and the zener diode VR 1  and in electrical communication with a resistor R 2 , a circuit leg V B  in electrical communication with resistor R 2  and in electrical communication with a diode D 1 , a circuit leg V A  in electrical communication with diode D 1  and with a negative terminal of a winding N 2  forming a portion of the inductor L 1  and wherein a positive terminal of the N 2  winding is in electrical communication with the input VIN′, a capacitor C 3  in electrical communication with the primary circuit downstream the source of MOSFET Q 1  and in electrical communication with the circuit leg V GATE  upstream the resistor R 2 , a zener diode VR 2  in electrical communication with the primary circuit leg V GATE  upstream the capacitor C 3 , and a resistor R 1  in electrical communication with the input V IN  upstream the drain of MOSFET Q 1  and in electrical communication with the secondary circuit leg upstream the node between the gate of MOSFET Q 1  and the zener diode VR 1 .  
         [0017]     According to yet another exemplary embodiment of the present invention, a transient protection circuit for high power loads is provided comprising a primary circuit leg comprising an input V IN  in electrical communication with a drain of a MOSFET Q 1  and a source of the MOSFET Q 1  being an output of the primary circuit, and a capacitor C 1  in electrical communication with the input V IN  upstream the drain of Q 1  and further electrically grounded. The transient protection circuit further includes a load circuit comprising a buck converter including an input V IN ′ in electrical communication with a drain of a FET Q 2 , a circuit leg V SW  in electrical communication with a source of FET Q 2  and in electrical communication with a positive terminal of a winding N 1  of an inductor L 1 , an output V OUT  in electrical communication with a negative terminal of the winding N 1 , a FET Q 3  having a drain in electrical communication with the circuit leg V SW  and source of the FET Q 3  being electrically grounded, a regulator controller U 1  in electrical communication with a gate of the FET Q 2  and with a gate of the FET Q 3  and wherein a source of the FET Q 3  is electrically grounded, a capacitor C 2  in electrical communication with the input V IN ′ and further electrically grounded, and a capacitor C 4  in electrical communication with the output V OUT  and further electrically grounded. The transient protection circuit further includes a secondary circuit in electrical communication with a gate of the MOSFET Q 1  and in electrical communication with a zener diode VR 1  and wherein the zener diode VR 1  is electrically grounded, a circuit leg V GATE  in electrical communication with a node between the gate of MOSFET Q 1  and the zener diode VR 1  and in electrical communication with a resistor R 2 , a circuit leg V B  in electrical communication with resistor R 2  and in electrical communication with a diode D 1 , a circuit leg V A  in electrical communication with diode D 1  and with a negative terminal of a winding N 2  forming a portion of the inductor L 1  and wherein a positive terminal of the N 2  winding is in electrical communication with the input VIN′, a capacitor C 3  in electrical communication with the primary circuit downstream the source of MOSFET Q 1  and in electrical communication with the circuit leg V GATE  upstream the resistor R 2 , a zener diode VR 2  in electrical communication with the primary circuit leg V GATE  upstream the capacitor C 3 , and a resistor R 1  in electrical communication with the input V IN  upstream the drain of MOSFET Q 1  and in electrical communication with the secondary circuit leg upstream the node between the gate of MOSFET Q 1  and the zener diode VR 1 . Additionally, a programmable voltage clamp is implemented through zener diode VR 1 .  
         [0018]     And yet, in still another exemplary embodiment of the present invention, a portion of a transient protection circuit for high power loads is provided including a primary circuit leg having an input V IN  in electrical communication with a drain of a MOSFET Q 1  and a source of the MOSFET Q 1  being an output of the primary circuit adapted to be connected to a load circuit having an input V IN ′. The transient further includes a secondary circuit in electrical communication with a gate of the MOSFET Q 1  and in electrical communication with a zener diode VR 1  and wherein the zener diode ZR 1  is electrically grounded, a circuit leg V GATE  in electrical communication with a node between the gate of MOSFET Q 1  and the zener diode VR 1  being in electrical communication with a resistor R 2 , a circuit leg V B  in electrical communication with resistor R 2  and in electrical communication with a diode D 1 , a circuit leg V A  in electrical communication with diode D 1  and with a first terminal of a winding N 2  forming a portion of the one of an inductor L 1  or a transformer T 1 , and wherein another terminal of the N 2  winding is adapted to be in electrical communication with the input VIN′ of the load circuit.  
         [0019]     According to another aspect of the present aforementioned embodiment, the transient protection circuit further comprises a resistor R 1  in electrical communication with the input V IN  upstream the drain of MOSFET Q 1  and in electrical communication with the secondary circuit leg upstream the node between the gate of MOSFET Q 1  and the zener diode VR 1 . According to another aspect of the present aforementioned embodiment, the transient protection circuit further comprises a capacitor C 3  in electrical communication with the primary circuit downstream the source of MOSFET Q 1  and in electrical communication with the circuit leg V GATE  upstream the resistor R 2 .  
         [0020]     Furthermore, according to another aspect of the present aforementioned embodiment, the transient protection circuit further comprises a zener diode VR 2  in electrical communication with the primary circuit downstream the source of MOSFET Q 1  and in electrical communication with the circuit leg V GATE  upstream the resistor R 2 . Moreover, according to another aspect of the present aforementioned embodiment, the transient protection circuit further comprises a capacitor C 1  in electrical communication with the input V IN  upstream the drain of Q 1  and further electrically grounded.  
         [0021]     The present invention has advantages over traditional circuits by improving efficiency for medium to high power loads. Another advantage of the present invention is that it provides a simple driver circuit to turn on a bypass n-channel MOSFET. Another advantage of the present invention is that it operates at a higher efficiency; I.E., conduction losses are minimized during normal input voltage conditions. A further advantage of the present invention is that it provides a programmable voltage clamp which is implemented through selecting zener diode VR 1 . And still yet another advantage of the present invention is that it can be used to protect medium to large current circuits from line transients. Additionally, another advantage of the present invention is that it is suitable for applications in loads that have to operate through a high line transient (e.g. military, medical, etc.).  
         [0022]     Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The present invention is further described in the detailed description that follows, by reference to the noted drawings by way of non-limiting examples of preferred embodiments of the present invention, in which like reference numerals represent similar parts throughout several views of the drawings, and in which:  
         [0024]      FIG. 1  depicts a prior art darlington power transistor protection circuit, according to an aspect of the present invention;  
         [0025]      FIG. 2  depicts a prior art power MOSFET transient protection circuit, according to an aspect of the present invention;  
         [0026]      FIG. 3A  is a notational depiction of the present invention which is a highly efficient line transient protection circuit for high power loads, according to an aspect of the present invention;  
         [0027]      FIG. 3B  is an exemplary embodiment of the present invention which is a highly efficient line transient protection circuit for high power loads coupled with a load circuit, according to an aspect of the present invention;  
         [0028]      FIG. 3C  is an exemplary embodiment of the preferred embodiment of the present invention which is a highly efficient line transient protection circuit for high power loads coupled with a buck converter, according to an aspect of the present invention;  
         [0029]      FIG. 4  is an equivalent circuit between V A  and V IN ′, when Q 2  is conducting and the reactor turns ratio N 2 /N 1  and resistor divider R 2 /R 1  is set to boost voltage at the gate of Q 1  such that it will be fully enhanced, according to an aspect of the present invention;  
         [0030]      FIG. 5  is an equivalent circuit between V A  and V IN ′ when capacitor C 3  is allowed to slowly decay through R 1  and VR 2 , according to an aspect of the present invention;  
         [0031]     FIGS.  6 A-D depicts resulting waveforms from an exemplary buck converter, according to an aspect of the present invention;  
         [0032]     FIGS.  7 A-G depicts the present invention in steady state and transient operation based upon a variety selected circuit parameters, according to an aspect of the present invention;  
         [0033]     FIGS.  8 A-D depict numerous alternative load circuit topologies, according to an aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.  
         [0000]     Description of the Exemplary Transient Protection Circuit  
         [0035]      FIG. 3A  is a notational depiction of the present invention which is a highly efficient line transient protection circuit ( 3 ) for high power loads, according to an aspect of the present invention [hereinafter “transient protection circuit ( 3 )”]. In the most general embodiment, the present invention includes programmable series pass elements  100  having a input V IN    4  and output V OUT    6  coupled to a load circuit  9  including switching regulatory/oscillator  104  by coupling element  102 .  
         [0036]      FIG. 3B  is a depiction of an exemplary of the present invention which is a highly efficient line transient protection circuit ( 3 ) for high power loads coupled with a load circuit ( 9 ), according to an aspect of the present invention. The transient protection circuit ( 3 ) comprises a primary circuit, a secondary circuit, and a load circuit ( 9 ). The primary circuit includes input V IN  ( 4 ) and an enhancement n-channel power MOSFET Q 1  ( 8 ). The primary circuit may also include capacitor C 1  ( 18 ). In particular, the input V IN  is electrically connected to the drain of Q 1 . A circuit leg of the primary circuit is electrically connected to the source of Q 1  and the other end of the leg is in electrical communication with an input V IN ′ of the load circuit ( 9 ). The primary circuit also includes a circuit leg which electrically connects between V IN  and the drain Q 1 , which includes a capacitor C 1  that is grounded.  
         [0037]      FIG. 3C  is an exemplary embodiment of the preferred embodiment of the present invention which is a highly efficient line transient protection circuit ( 3 ) for high power loads coupled with a buck converter  56 , according to an aspect of the present invention.  
         [0038]     Preferably, the exemplary transient protection circuit ( 3 ) depicted in  FIG. 3C  is provided with the load circuit ( 9 ) being that of a buck converter ( 56 ) composing of input V IN ′ ( 5 ), switching FET Q 2  ( 10 ), switching FET Q 3  ( 12 ), regulator controller U 1  ( 14 ), winding N 1  ( 50 ) of inductor L 1  (or reactor) ( 16 ), capacitor C 2  ( 20 ), capacitor C 4  ( 24 ) and output V OUT  ( 6 ). In particular, the primary circuit is in electrical communication with V IN ′, which is in electrical communication with the drain of Q 2 . A circuit leg of the buck converter, V SW , is in electrical communication with the source of Q 2  and is further in electrical communication with the positive (+) terminal of winding N 1  of inductor L 1 . The negative (−) terminal of winding N 1  is in electrical communication with the output V OUT . The gate of Q 2  is in electrical communication with regulator controller U 1 . Moreover, the drain of switching FET Q 3  is in electrical communication with the circuit leg of the buck converter V SW  between the source of Q 2  and the positive (+) terminal of winding N 1 , while the source of Q 3  is grounded. The gate of Q 3  is also in electrical communication with regulator controller U 1 . Additionally, the capacitor C 2  is connected to V IN ′ and further grounded. Also, the capacitor C 4  is connected between the negative (−) terminal of winding N 1  of V OUT.    
         [0039]     A secondary circuit is further provided which includes a circuit leg in electrical communication with the gate of Q 1  and in electrical communication with a zener diode VR 1  ( 28 ) which is then grounded. The secondary circuit leg also includes a circuit leg V GATE  which is connected between the gate of Q 1  and the zener diode VR 1  and further in electrical communication with a resistor R 2  ( 36 ). Another circuit leg V B  is in electrical communication with R 2  and diode D 1  ( 38 ). Another circuit leg V A  of the secondary circuit, is in electrical communication with D 1  and is further electrically connected to a negative (−) terminal of the secondary winding N 2  of the inductor L 1 . Another circuit leg of the secondary leg is further in electrical communication with a positive (+) terminal of winding N 2  and also then tied into the load circuit ( 9 ) between the node at which C 2  is connected and the drain of Q 2 . Additionally, a capacitor C 3  ( 22 ) and a zener diode VR 2  ( 30 ) are both interconnected between the primary and secondary circuit legs between Q 1  and Q 2  on the primary circuit and to the V GATE  circuit leg upstream of R 2 . Moreover, a resistor R 1  ( 34 ) is connected between C 1  and the drain of Q 1  and between the gate Q 1  and upstream the V GATE  circuit leg.  
         [0000]     Theory of Operations of the Exemplary Transient Protection Circuit  
         [0040]     When voltage is applied to the input of the circuit V IN , the gate of the power MOSFET Q 1  is biased up to the same input voltage through resistor R 1 . This gate voltage forces Q 1  to conduct; however, the voltage at the source of Q 1  will be a VGS threshold voltage (wherein VGS is the classical specified gate to source voltage necessary to turn on, or enhance the channel of a MOSFET) below that of the gate because Q 1  will not be fully enhanced. A typical threshold voltage of an n-channel power MOSFET is 2V to 4V. Therefore, a differential voltage of 2V to 4V exists from drain to source of Q 1 . This voltage difference across power MOSFET Q 1  produces power loss when current passes through it which can be significant when current levels increase.  
         [0041]     To overcome this undesirable power loss, a voltage higher than the existing input voltage is generated at the gate to turn the MOSFET Q 1  on hard (fully enhanced) and minimize drop between the drain and source of Q 1 . To generate this higher voltage signal, the transient protection circuit ( 3 ) takes advantage of the switching waveform across the buck inductor L 1  and develops a higher voltage out of the secondary circuit. The turns ratio of the reactor of inductor L 1 , N 2 /N 1 , can be adjusted such that a higher voltage comes out of the secondary winding when N 2 &gt;N 1 . For instance, if N 2 /N 1 =2, then V 2  is twice that of V 1 , because winding N 2  is equal to twice that of N 1 .  
         [0042]     It is noted that in operational steady-state, V 1 , is in phase with V 2 . When V 1  is high, V 2  is also high. When V 1  is low, V 2  is also low. However, the un-dotted terminal of the secondary of the reactor, V A , is opposite polarity to that of V 2 . That is because V A =V IN ′−V 2 . A progression of the waveforms showing V 1 , V 2 , and V A  is shown in FIGS.  6 A-D.  
         [0043]     The input-output switching relationship for the buck converter is given by  
       D   =       Vout   Vin     .         
 
 The buck converter top FET, Q 2 , is on for time duration DT, where D is the duty ratio and T is the switching period. During Q 2 &#39;s on time, V A  is low, its magnitude given by:
 
 VA=V in′− V   2  0 ≦t≦DT Q   2 :ON
 
         [0044]     When top FET, Q 2  is off for duration (1−D)T, V A  is high, its magnitude given by:
 
 VA=V in′− V   2   DT≦t≦T Q   2 :OFF
 
         [0045]     When V A  is higher than the existing gate voltage, V GATE , the diode D 1  conducts charging capacitor C 3  through the resistor divider form by R 1  and R 2 . The resistor divider sets the upper voltage limit at which the gate is charged. Preferably, the reactor turns ratio N 2 /N 1  and resistor divider R 2 /R 1  is set to boost the voltage at the gate of MOSFET Q 1  such that it will be fully enhanced. The equivalent circuit at this state is shown in  FIG. 4 . It is noted that the ratio N 2 /N 1  allows a high voltage to charge up capacitor C 3  during Q 2 &#39;s off time, (1−D)T, to turn on the gate of MOSFET Q 1 .  
         [0046]     When V A  is lower than the existing V GATE , the diode D 1  is off. The capacitor C 3  is allowed to slowly decay through R 1  and VR 2  as shown in equivalent circuit in  FIG. 5 . However, in an alterative embodiment, if C 3  and R 1  are designed large enough such that its gate voltage decay is minimized during Q 2 &#39;s on time, then the MOSFET Q 1  would remain in the fully enhanced state for the remainder of the period.  
         [0000]     An Exemplary Transient Protection Circuit with Modeled Parameters  
         [0047]     For illustrative purposes of the present invention, the exemplary transient protection circuit ( 3 ) depicted in FIGS.  3 A-C is modeled according to a chosen set design parameters. It is noted that the buck converter components are idealized for simplification. For exemplary purposes, the assumed set of design parameters may be as follows:
 
V IN= 28V
 
I OUT =3A
 
F=200 kHz
 
T=5 uS
 
V OUT =7V
 
 D=V   OUT   /V   IN =7V/28V=0.25
 
DT=1.25 uS FET ON TIME
 
(1 −D ) T =3.75 uS FET OFF TIME
 
 N   2 / N   1 =2
 
         [0048]     From the equivalent circuit depicted in  FIG. 5 , one can write the following equations:
 
0≦t≦DT
 
 V   GATE   =V   ∞   e   −t/τ1 
 
τ1= R   1 * C   3 
 
Q 2 :ON
 
C 3 :Discharge
 
         [0049]     From the equivalent circuit depicted in  FIG. 4 , one can write the following equations:
 
DT≦t≦T
 
 V   GATE   =V   ∞ (1 −e   −t/τ2 )
 
τ 2=(   R   1 // R   2 )* C   3 
 
Q 2 :OFF
 
C 3 :Charge
 
         [0050]     It is noted that V ∞  is the peak charge voltage on the gate of MOSFET Q 1  and V ∞  is set to 35V to fully enhance the MOSFET Q 1  (VGS=7V).  
         [0051]     Furthermore, it is preferable that, τ2&lt;τ1. The time constant during the charge interval preferably is smaller than the time constant during the discharge interval. This is done to ensure that V GATE  will charge up to V ∞  to fully turn on MOSFET Q 1  faster than it discharges its voltage because a smaller time constant equates to faster charging time. In other words, preferably C 3  charges up faster than it discharges, before Q 1  can be fully enhanced. Otherwise, Q 1  will not reach its fully enhanced mode.  
         [0052]     Once the transient protection circuit ( 3 ) reaches its steady state condition (when Q 1  is fully enhanced), it is preferable to hold the V GATE  to a value greater than V IN  for the entire discharge interval. This is implemented to make sure that Q 1  does not exit its fully enhance mode. This may be accomplished by setting τ2&lt;&lt;τ1.  
         [0053]     The V GATE  can be set to charge up to V ∞  of 35V by setting resistor values for R 1  and R 2 . Analyzing the circuit for DT≦t≦T in steady state we get the following set of equations.
 
DT≦t≦T
 
Vin′≈Vin=28V (Q 1 =FULLYENHANCED)
 
 V   GATE   =V   in   +V   GS =28V+7V=35V
 
 V   1   =V   SW   −V   out =0V−7V=−7V
 
  
         V   2     =           N   2       N   1       *     V   1       =       2   *     -   7     ⁢   V     =       -   14     ⁢   V             
  V   A   =V in′− V   2 =28V−(−14V)=42V
 
         [0054]     By applying the voltage V A  and V IN ′ to  FIG. 4 , one can clearly see that V GATE  can be set to charge to V ∞  of 35V by setting R 1 =1 k and R 2 =1 k.  
         [0055]     In the instant example, the capacitor C 3  will discharge for 1.25 uS in one period. The value of C 3  is determined by choosing a capacitor such that its discharge voltage is insignificant for this time interval. If C 3 =10 uF, then the gate voltage will discharge to  
         35   ⁢     ⅇ         -   1.25     ⁢   uS       1   ⁢   K   *   10   ⁢   uF           =     34.995   ⁢   V         
 
 at the end of that interval. This is clearly above the input voltage of 28V. Therefore, it is assured that the average of the gate voltage is around 35V and the MOSFET Q 1  remains in the fully enhanced mode. The zener diode VR 2  is inserted to protect the gate-source junction of Q 1 . If VGS of Q 1  exceeds 15V, then VR 2  is on; otherwise, VR 2  is off. 
 
         [0056]     The waveforms for the aforementioned example, V 1 , V 2 , V A , are depicted in  FIGS. 6A through 6D . The waveform V GATE  is illustrated in an exaggerated fashion to indicate its charge/discharge phenomenon.  FIG. 6A  shows the voltage V 1  is across the primary inductor L 1 . Its voltage swing is from +21V during Q 2 &#39;s on time (DT) to −7V during Q 2 &#39;s off time T(1−D).  FIG. 6B  shows the voltage across the secondary of inductor L 1  is V 2 . Since N 2 /N 1 =2, V 2  will be twice that of V 1 .  FIG. 6C  is a single ended waveform (relative to ground) of V A  which is V IN ′−V 2 .  FIG. 6D  shows the charge and discharge of the gate voltage. The waveform is illustrated in an exaggerated fashion to indicate its charge/discharge phenomenon.  
         [0057]     One aspect of the present invention is that the voltage at V IN ′ is clamped by programmable a zener diode voltage. In particular, the transient protection circuit ( 3 ) depicted in FIGS.  3 A-C serves as a line transient protection circuit by the addition of a zener diode VR 1  at the gate of MOSFET Q 1 . This enables the transient protection circuit ( 3 ) to clamp a line transient to a maximum set voltage. The maximum clamping voltage is set by selecting the zener voltage of VR 1 . The second zener diode, VR 2 , connected from gate to source of MOSFET Q 1  is used to protect the gate-source of the MOSFET Q 1 . When a transient is encountered that exceeds the maximum voltage rating of zener diode VR 1 , zener diode VR 1  turns on clamping the gate of MOSFET Q 1  to the maximum set voltage of the zener diode VR 1 . With MOSFET Q 1  fully enhanced, the source voltage of Q 1  at node V IN ′ will be clamped to the clamped voltage of VR 1  minus VGS. V IN ′ (clamped) is the voltage at node V IN ′ during the onset of a transient. The aforementioned relationship may be described as follows:
 
 V   IN ′(clamped)= VR   1 (programmable clamped voltage)− VGS 
 
         [0058]     For example, in  FIG. 7E , a 76V transient is injected into the line input. The programmable clamped voltage of VR 1  is set at 43V. Therefore, at the onset of a transient, V GATE  is clamped to 43V. V IN ′ is then clamped at V GATE −VGS=38V.  FIG. 7F  and  FIG. 7G  shows the close up of the rising and falling edge of the transient respectively.  
         [0059]     The transient protection circuit ( 3 ) is also more efficient because the MOSFET Q 1  operates in the fully enhanced mode. The power saving of this circuit can be significant in comparison to the prior art transient protect circuits. To illustrate the present invention&#39;s power efficiency characteristics compared to the power efficiency characteristics of prior art transient protection circuits, a comparison test is herein presented below.  
         [0000]     Comparison of Prior Art Transient Protection Circuits to the Present Invention Transient Protection Circuit  
         [0060]     To illustrate the power efficiency gains that the present invention has over the prior art, it is beneficial to compare each transient protection circuit (darlington from  FIG. 1 ; power MOSFET from  FIG. 2 , and the present invention transient protection circuit ( 3 ) from FIGS.  3 A-C). To accomplish this comparison test, a common set of input parameters are utilized.  
         [0000]     a. Darlington Type Transient Protection Circuit ( FIG. 1 )  
         [0061]     A calculation of the power dissipation of the darlington circuit ( 1 ) with an example parameter reveals that the power dissipation of the darlington transistor is close to 10W. The parameters and calculations are as follows:
 
Vin=28V
 
Io=5A
 
Hfe_Q 1 (typical)=500
 
  
       Ib   =       Ic   Hfe     =         5   ⁢           ⁢   A     500     =     10   ⁢           ⁢   mA             
 Vbe_on=1.4V(typical)
 
Vc=28V
 
 Vb=Vc −( Ib*R   1 )=28*(0.010A*50 ohms)=27.5V
 
 Ve=Vb−Vbe _on=27.5V−1.4V=26.1V
 
 Vce=Vc−Ve =28V−26.1V=1.9V
 
PowerDissipation —   Q   1 =( Vce*Ic )+( Vbe*Ib )=(1.9V*5A)+(1.4V*0.010A)=9.514W
 
 b. Power MOSFET Type Transient Protection Circuit ( FIG. 2 ) 
 
         [0062]     A calculation of the power dissipation of the power MOSFET transient protection circuit ( 3 ) with an example parameter set reveals that the power dissipation of the power MOSFET transient protection circuit is 20W. The dissipation is at 20W because the FET is not fully enhanced. Applying the same parameters as in the last example, the power consumption was calculated as follows:
 
Vin=28V
 
Io=5A
 
Vg=28V
 
Vgatethreshold=4V
 
 Vin′=Vg−V gatethreshold=28V−4V=24V
 
PowerDissipation —   Q   1 =( V in− V in′)* Id =4V*5A=20W
 
 c. The Present Invention Transient Protection Circuit ( FIG. 3 ) 
 
         [0063]     A calculation of the power dissipation of the present invention transient protection circuit ( 3 ) with the same example parameter set reveals that the power dissipation of the present invention transient protection circuit ( 3 ) is only 0.375 W. This power savings is realized because the MOSFET Q 1  is fully enhanced. Its dissipation is purely dependent on the channel resistance (r DS     —     ON ) of the MOSFET. By applying the same parameters as in the last two examples, the power consumption was calculated as follows:
 
Vin=28V
 
Io=5A
 
Vg=35V
 
Rds_on=0.015 ohms(typical)
 
 V in′= V in−( Io*Rds _on)=28−(0.015 ohm*5A)=27.925V
 
PowerDissipation —   Q   1 =( Io   2   *Rds _on)=25*0.015=0.375W
 
 Waveforms Taken from the Present Invention Transient Protection Circuit with Exemplary Parameters 
 
         [0064]     FIGS.  7 A-G depict a series of waveforms which are taken from an exemplary modeled transient protection circuit ( 3 ) from FIGS.  3 A-C utilizing the following parameters:
 
V IN =+28V (50 mS of transient peak at 76V)
 
V OUT =7V@3A
 
L 1 =20 uH
 
 N   2 / N   1 =2
 
R 1 =1K
 
R 2 =1K
 
C 3 =68 uF
 
VR 1 =43V zener voltage
 
VR 2 =15V zener voltage
 
         [0065]      FIG. 7A  shows the voltage waveform V SW  relative to ground. FIGS.  7 B-D shows the progression of voltage waveforms from the switching voltage across primary inductor L 1  to the boosted DC voltage at the gate to fully enhance the MOSFET Q 1 .  
         [0000]     Alternative Load Circuit Embodiments  
         [0066]     The advantages of the load circuit ( 9 ) described herein can alternatively be gained with the load being that of any other equivalent load circuit that takes the form of a variety of topology of load circuits known in the art. FIGS.  8 A-D depict various exemplary embodiments of the highly efficient line transient protection circuit ( 3 ) for high power loads using alternative load circuits ( 9 ), including a simple oscillator circuit, and various switching regulator circuits. For example, the load circuit ( 9 ) may be oscillator and transformer circuit ( 58 ; see  FIG. 8A ), boost converter ( 60 ; see  FIG. 8B ), push-pull and transformer circuit ( 62 ; see  FIG. 8C ), or a push-pull circuit and tertiary winding on an inductor circuit ( 64 ; see  FIG. 8D ).  
         [0067]      FIG. 8A  depicts the load circuit ( 9 ) of the present invention as a simple oscillator circuit and a transformer ( 58 ). A switching waveform is generated across the primary of the transformer T 1  ( 16 ). In this case an oscillator U 1  ( 14 ) is provided instead of a regulator controller. A switching waveform is then developed on the secondary winding N 2 , which is used to generate a voltage to fully enhance the MOSFET Q 1  ( 8 ). The oscillator circuit ( 58 ) also includes a capacitor C 2  ( 20 ) and switching FET Q 3  ( 12 ).  
         [0068]      FIG. 8B  depicts the load circuit ( 9 ) of the present invention as a boost converter ( 60 ). As previously discussed in the specification, the circuit ( 3 ) takes advantage of the switching voltage waveform developed across a boost inductor, utilizing a tertiary winding to generate a voltage to fully enhance the MOSFET Q 1  ( 8 ). The boost converter ( 60 ) circuit also includes a capacitor C 2  ( 20 ), regulator controller U 1  ( 14 ), switching FET Q 3  ( 12 ), diode D 2  ( 51 ), and capacitor C 4  ( 24 ).  
         [0069]      FIG. 8C  embodies the invention using a push-pull circuit ( 62 ) and a transformer T 1  ( 16 ). A switching waveform is generated across the primary winding N 1  of the transformer T 1  ( 16 ). A switching waveform is then developed on the secondary winding N 2 , which is used to generate a voltage to fully enhance the MOSFET Q 1  ( 8 ). The push-pull circuit ( 62 ) also includes a capacitor C 2  ( 20 ), regulator controller U 1  ( 14 ), switching FETs Q 2  ( 10 ) and Q 3  ( 12 ), diodes D 2  ( 51 ), D 3 A ( 53 ), D 3 B ( 55 ), inductor L 1  and capacitor C 4  ( 24 ).  
         [0070]      FIG. 8D  embodies the invention using a push-pull circuit ( 64 ) and a tertiary winding on an inductor L 1 . As discussed previously in the specification, the circuit ( 3 ) takes advantage of the switching voltage waveform developed across the inductor L 1 , utilizing a tertiary winding to generate a voltage to fully enhance the MOSFET Q 1  ( 8 ). The push-pull circuit ( 64 ) also includes a capacitor C 2  ( 20 ), regulator controller U 1  ( 14 ), switching FETs Q 2  ( 10 ) and Q 3  ( 12 ), diodes D 3 A ( 53 ), D 3 B ( 55 ), and capacitor C 4  ( 24 ).  
         [0071]     It is even further noted that the scope of the present invention is not and should not be limited to the embodiments illustrated in FIGS.  3 A-C and FIGS.  8 A-D. Rather, FIGS.  3 A-C and FIGS.  8 A-D depict a few of many forms or topologies of various of load circuits ( 9 ) which may be incorporated as a feature of the present invention. Persons knowledgeable in the art will be able to embody the present invention using the exemplary embodiments of load circuits ( 9 ) or other exemplary embodiments of load circuits not specifically depicted in the instant application, such as, e.g. single-ended forward converters, half-bridge converters, full-bridge converters, and various switching regulator circuits.  
         [0072]     Although the invention has been described with reference to several exemplary embodiments, it is understood that the words that have been used are words of description and illustration, rather than words of limitation. Changes may be made within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the invention in its aspects. Although the invention has been described with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed; rather, the invention extends to all functionally equivalent structures, methods, and uses such are within the scope of the appended claims.