Abstract:
A method for limiting a switching current and limiting a voltage drop across a circuit includes the steps of limiting a current sent from a power source to a switching device during a turn on time of the switching device by disposing an inductor device in series between the power source and said switching device and returning flux energy stored in said inductor device to the power source during a turn off time of the switching device.

Description:
FIELD  
         [0001]    The invention relates to a method for limiting the switching current of a switching device and therefore limiting the power dissipated by the switching device at turn-on time, and further returning energy to a power receiving circuit when the switching device disconnects from the power source.  
         BACKGROUND  
         [0002]    During the turn-on time for a switching device, which may include, but is not limited to, an FET or bipolar transistor, wasteful transition power is dissipated during a transition turn-on time. Thus, the longer the transition turn-on time lasts, the more wasteful power is dissipated.  
           [0003]    [0003]FIG. 1 shows, for example, a MOSFET switching device  10  which switches a voltage from  + V to the input of inductor  20  in a simple buck regulator. It is known to provide a buck regulator, which includes an inductor  20 , capacitor  30  and freewheel diode  50  and switching device  10  for producing a first regulated output voltage across the capacitor  30  from a pulsed input supply. Controller  60  is provided to control the duty cycle of the switching device  10 . During the transition turn-on time required for the switching device  10  (which may be, but is not limited to, an FET or bipolar transistor), transition power is dissipated, and the longer the transition, in relation to the repetition period of the transition, the more power is dissipated, as further illustrated in this example.  
           [0004]    More particularly, the disadvantageous example embodiment of FIG. 1 shows a buck regulator which utilizes a switching device  10 , which is used to switch the input of inductor  20  to the input voltage  + V for a period of time, and then to disconnect inductor  20  from  + V for a subsequent period of time. If switching device  10  has to switch 15A (i.e., 15 amps), the power being switched will be given by the formula 15× + V. If V were 20V, then the power switched would be 300 watts, since (15A)(20V)=300 watts. That is, as an example, when 15A are transmitted from source  + V, and when transistor  10  is turned on, the left side of inductor  20  charges up to  + V, which is the voltage level from the power source. Current begins to increase through inductor  20  and capacitor  30  is charged up. In a stable state, the charge on capacitor  30  rises above the output voltage and then back down below the output voltage, producing a ripple. In this example, then, the transistor  10  would be immediately switching 15A at turn-on time, and therefore would be wasting a significant amount of power during turn-on transition time.  
         SUMMARY  
         [0005]    According to the invention, a method, including the steps of limiting a current sent from a power source to a switching device, and system are provided for limiting a switching current in a switching device during the transition turn-on time of the switching device by disposing an inductor device in series between the power source and the switching device, and returning flux energy stored in the inductor device to a power receiving circuit at a turn-off time of the switching device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The foregoing and a better understanding of the present invention will become apparent from the following detailed description of example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written disclosure focus on disclosing example embodiments of this invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto. The spirit and scope of the present invention are limited only by the terms of the appended claims.  
         [0007]    The following represents brief descriptions of the drawings, wherein:  
         [0008]    [0008]FIG. 1 shows a disadvantageous embodiment of a switching device;  
         [0009]    [0009]FIG. 2 shows a switching device according to an example embodiment of the present invention;  
         [0010]    [0010]FIG. 3 shows a waveform of the current of a switch in the disadvantageous embodiment shown in FIG. 1;  
         [0011]    [0011]FIG. 4 shows a waveform of the current of a switch in the example embodiment of the present invention shown in FIG. 2;  
         [0012]    [0012]FIG. 5 shows a voltage waveform across a switch in the disadvantageous embodiment shown in FIG. 1; and  
         [0013]    [0013]FIG. 6 shows a voltage waveform across a switch in the example embodiment of the present invention shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0014]    Before beginning a detailed description of the invention, it should be noted that, when appropriate, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing figure drawings. Further, in the detailed description to follow, example embodiments and values may be given, although the present invention is not limited thereto. Further, while example embodiments of the present invention will be described in conjunction with a buck regulator as an example, practice of the present invention is not limited thereto, i.e., the present invention can be implemented in conjunction with any switching device where power is wasted during a turn-on transition time, or produces and/or is connected to a circuit having a rapid dV/dt.  
         [0015]    As shown in the disadvantageous embodiment of FIG. 1, a significant amount of power is dissipated by the example buck regulator which utilizes a switching device  10  to switch the input of inductor  20  to the input voltage  + V for a period of time, and then to disconnect inductor  20  from  + V for a subsequent period of time. When switching device  10  switches, for example, 15A, the power being switched will be 15×( + V). If  + V were 20V, then the power switched would be 300 watts, since (15A)(20V)=300 watts. If switching device  10  further requires, for example, 300 nS to switch on and to switch off, and has a repetitive period of ten microseconds (10 μS), then the switching device would consume 3.79 watts of power in switching losses. Since the load being switched in the disadvantageous embodiment is a constant current, the loss of power is calculated by a two-part formula:  
           P={Iτ   1  ( E   max +2 E   min )/6 T}+{E   max    Iτ   2 /2 T},   (a) 
         [0016]    wherein the first part of the formula Iτ 1  (E max +2E min )/6T is determined during the first 50 nS when the current rises from 0A to 15A. E max =20V and E min =16.67V as shown in FIG. 5, which shows the voltage waveform across the switch in the disadvantageous embodiment of FIG. 1; further, I=15A, T 1 =50 nS. The second part of the formula, E max  Iτ 2 /2T, occurs after the first 50 nS of the 300 nS switching period of the switching device  10 , and further still, T 2 =250 nS, and T=10 μS.  
         [0017]    [0017]FIG. 3 shows an example current waveform for switching device  10 , and as set forth above, FIG. 5 shows an example voltage waveform across switching device  10  of the disadvantageous embodiment of FIG. 1. In accordance with the disadvantageous embodiment, the switching device  10  is fully turned on at 300 nS, switching the full device current of 15A in 50 nS, and switching completely on in 300 nS, so that the voltage across the switching device  10  will drop from 20V (E max ) to 0V in 300 nS, resulting in 3.79 watts of wasteful power being dissipated at the switching device during that transition turn-on time.  
         [0018]    The present invention, an example embodiment of which is shown in FIG. 2, provides a switching device  10  which switches a voltage from  + V to the input of inductor  20  in an example buck regulator, as well. The buck regulator of the example embodiment of FIG. 2 includes an inductor  20 , capacitor  30  and freewheel diode  50 , controller  60  for controlling the duty cycle and switching device  10  for producing a first regulated output voltage across the capacitor  30  from a pulsed input supply. However, unlike the disadvantageous embodiment shown in FIG. 1, the example embodiment of the present invention shown in FIG. 2 includes the voltage source in series with a transformer  70  and a diode  80 . Transformer  70  includes a primary inductor  71 , and a secondary inductor  72  which is in series with a diode  80 . Transformer  70  may also be a toroid with a straight wire passing through in series with the source or drain of switching device  10 , although the transformer  70  is not limited to either implementation described above, i.e., all that is required is some type of current-delay/power-storage arrangement.  
         [0019]    The primary inductor  71  of transformer  70  is in series with switching device  10 . Although the present embodiment shows the primary inductor  71  of the transformer  70  is in series with the drain terminal of switching device  10 , it could also be in series with the source terminal of switching device  10 . Although switching device  10  is shown in the example embodiment of FIG. 2 as a MOSFET, practice of the present invention is certainly not limited to such implementation. That is, by the present invention, any type of switching device can be utilized, including, but not limited to, a synchronous MOSFET in a buck converter. So, regardless of the type of switching device, the implementation of transformer  70 , as shown in FIG. 2 for example, slows down the rate of change of current on the drain or source of the switching device, thus reducing switching losses and any “shoot-through” thereat caused by a rapid rate of change of voltage across any device in the circuit of the switching device.  
         [0020]    Similar to the example provided above, when the controller  60  provides, for example, a 10% duty cycle waveform turning on the switching device  10 , the switching device  10  can take an extended period of time to turn on. The primary winding  71  is an inductor which provides a slow transfer of energy to the switching device  10 . Thus, while the switching device  10  is turning on, the primary inductor  71  provides an inductance which limits the current during the turn-on time so that transistor  10  turns on with little current and a subsequent rapid voltage drop across the switching device, further reducing power loss. The current continues to increase through the inductor  71  to either the point of saturation of transformer  70  or the current being limited through inductor  20  and capacitor  30  into the load  40 . That is, the transformer  70  either saturates or increases the current flow until it is limited by the circuit being switched, which is the buck converter inductor in the example embodiment of FIG. 2.  
         [0021]    As seen in the example current waveform of FIG. 4 and the example voltage waveform of FIG. 6, both corresponding to the example embodiment of FIG. 2, after the switching device  10  is turned on at 0S (zero seconds), since this embodiment also has a constant current being switched, the voltage across the switching device  10  goes to essentially 0V in 100 nS, as shown in FIG. 6. Therefore, the time that transition power is dissipated (τ) is only 100 nS. Thus, according to the calculation of power dissipated P=(τEI)/(6T), P={(100×10 −9 )(20)(5)}/{6(10×10 −6 )}=0.167 watts of power in switching losses. Thus, the loss of power according to the buck regulator provided in correspondence with the example embodiment of FIG. 2 is over 95% less than that of the same buck regulator provided in correspondence with the disadvantageous embodiment of FIG. 1, which dissipates 3.79 watts of power during the turn-on time.  
         [0022]    Furthermore, according to the example embodiment of the present invention in FIG. 2, flux energy is stored in the secondary inductor  72  of transformer  70 . Thus, when transistor  10  is turned off, the secondary inductor  72  transfers an energy level to a power receiving circuit, in this case  + V, as the stored energy in primary inductor  71  is transferred to the secondary inductor  72 . Accordingly, a flyback effect occurs whereby the stored flux energy is then returned to the source voltage, in the present embodiment, via diode  80 . The present invention is not limited to using a diode to feed the stored energy back to a power receiving circuit. That is, any means which will accomplish this function is valid for this purpose, including a synchronous rectifier.  
         [0023]    Furthermore, it is noted that, although transformer  70  and diode  80  dissipate power themselves, such power dissipation is nominal relative to the power dissipated by the switch  10  in disadvantageous embodiments, an example of which is described above in reference to FIGS. 1, 3 and  5 .  
         [0024]    This concludes the description of the example embodiments. Although the present invention has been described with reference to illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope and spirit of the principles of the invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings and the appended claims without department from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.