Abstract:
A switch recovery circuit is disclosed for improving the efficiency of switching voltage regulators. The switch recovery circuit includes a first and second inductor, a capacitor, a first diode, and a recovery circuit. The capacitor and diode comprise an AC coupled loop circuit around the first inductor. Current flows through the loop circuit soon after the switch is opened and charges the capacitor. The recovery circuit, which includes a second inductor that is magnetically coupled to the first inductor, provides current, at least some of which discharges the capacitor (i.e. current that flows in the opposite direction to the loop current) after the loop current stops. A second diode is interposed in series with the second inductor to provide appropriate voltage offsets in the circuit and to prevent the first inductor from being shorted to ground through the second inductor.

Description:
BACKGROUND OF THE INVENTION 
     The field of this invention relates to conserving energy that is lost through the action of electrical switches (e.g., armature, semiconductor or any other suitable type of switches). More specifically, but without limiting the applicability of the present invention, this invention relates to improving the efficiency of switching voltage regulators. This invention also relates to controlling the voltage across a switch. 
     In many different electrical circuits, switching action results in various types of currents that would result in energy loss unless the energy in these currents is somehow conserved. One example is a synchronous switching voltage regulator, which includes two switching transistors that are switched ON and OFF out of phase with one another by a control circuit. The switching transistors include a main switching transistor and a synchronous switching transistor. When the synchronous switching transistor is turned OFF in each cycle, the channel current of the synchronous switching transistor moves into its body diode. A short time later, the main switching transistor turns ON, and a reverse recovery current flows through both switching transistors. The reverse recovery current increases rapidly to a large value, causing substantial power dissipation, because the body diode of the synchronous switching transistor has not yet commutated. 
     One approach to alleviating this problem involves controlling the rate of rise of the reverse recovery current, as disclosed in commonly-assigned U.S. Pat. No. 6,504,351 (hereinafter, “the &#39;351 patent”) to Eager et al., which is hereby incorporated by reference in its entirety. In the particular embodiments disclosed in the &#39;351 patent, at least one inductor is placed in the commutation path of the body diode of a switching transistor. The inductor reduces the maximum reverse recovery current through the switching transistor, which reduces power dissipation. 
     The reverse recovery current flows through the inductor, which means the inductor stores energy. It is highly desirable to transfer this energy back to the input voltage source and/or other places where it may be used. In other words, it is highly desirable to conserve this energy. Possible methods and apparatus for restoring this energy to the input voltage source are described in the &#39;351 patent and also in commonly-assigned U.S. Pat. No. 6,495,993 (hereinafter, “the &#39;993 patent”) to Eager, which is hereby incorporated by reference in its entirety. Although the energy transfer methods and apparatus described in the above patents are believed to be highly efficient, at least in some circumstances, it would be desirable to have another type of energy transfer system. 
     In addition to energy considerations, it is often necessary to control the voltage across a switch to ensure that the switch isn&#39;t damaged. The above patents describe an effective circuit configuration for ensuring that the absolute value of the voltage across a switch does not reach too high a level. In particular, embodiments are disclosed therein that show a loop comprising an inductor, a capacitor and a diode. The switch is coupled to the inductor such that the voltage across the switch is related to the voltage across the inductor. When this voltage reaches a sufficiently large value, the diode turns on, such that current flows through the loop. That is, current flows through the inductor, capacitor and diode, limiting the voltage drop across the inductor to the sum of the voltage drops across the capacitor and diode. Moreover, The voltage drop across the capacitor is controlled, which in effect controls the voltage across the switch. 
     Although the above patents disclose effective methods and apparatus for controlling the voltage across the capacitor, it would be desirable to provide an alternative to those methods and apparatus, at least in some circumstances. 
     SUMMARY OF THE INVENTION 
     The switch recovery circuit of the present invention answers the above needs. The switch recovery circuit of the present invention may be used in a circuit that includes a switch, an input energy source and current path circuitry. The current path circuitry may comprise, for example, a transistor, an inductor and an output capacitor that are part of a switching regulator circuit. 
     The switch recovery circuit of the present invention preferably includes a first inductor, a second inductor, a first capacitor, a first diode, and a recovery circuit. The recovery circuit transfers energy from the first inductor to the first capacitor and to the recovery circuit, and then back to the input energy source and/or to the current path circuitry. 
     As in the &#39;351 patent and the &#39;993 patent, the first capacitor and first diode comprise an AC coupled loop circuit around the first inductor. The first capacitor preferably has a relatively large capacitance value (e.g., 22 μF), so that the voltage across it remains relatively constant over time. 
     When the switch first opens, the current in the first inductor begins to decrease, causing the voltage across the first inductor to increase. In turn, this voltage increase causes the first diode to conduct and current flows in the loop comprising the first inductor, first capacitor and first diode. The switch is coupled to the input energy source (generally an AC ground) through the first capacitor and the diode so that the voltage across the switch is largely dictated by the voltage across the first capacitor when the diode conducts. Thus, the voltage across the switch may be kept at a relatively low value, which prevents the switch from being damaged. 
     The above mentioned loop current charges the first capacitor, which represents a transfer of energy from the first inductor to the first capacitor. The energy stored in the capacitor may increase during each switching cycle if the excess charge therein, caused by the loop current, is not returned to the input energy source and/or the current path circuitry. The recovery circuit assists in transferring this energy back into the input energy source and/or to the current path circuitry. The extent to which this energy return occurs during each cycle depends on circuit specific parameters. 
     The recovery circuit provides current that discharges the first capacitor (i.e. current that flows in the opposite direction to the loop current) after the loop current stops. The loop current stops because, as the current through the first inductor stabilizes, the absolute value of the voltage drop across the first inductor decreases, which in turn shuts off the current through the first diode. 
     The current flowing from the recovery circuit through the first capacitor may be returned to the input energy source and/or the voltage path circuitry. In either event, energy is transferred from the first capacitor. 
     The recovery circuit includes a second capacitor coupled at a node to the series combination of a third inductor and a second diode. The second diode is interposed between ground and the third inductor such that current may not flow through the third inductor to ground but only from ground through the third inductor. The third inductor and the first inductor are mutually inductive. Preferably, a voltage across the first inductor corresponds to a multiple of that voltage across the third inductor. In other words, the ratio of windings of the third inductor to the first inductor is preferably N:1, where N is greater than 1. 
     The action of the third inductor and second diode after the switch is closed depends on the voltage across the first capacitor. As previously mentioned, the voltage across the first capacitor gradually increases each switching cycle if the excess charge caused by the loop current is not returned. When the switch is open and the first diode is on, the voltage across the first inductor is almost equal to the voltage across the first capacitor. Thus, the larger the voltage across the first capacitor, the larger the voltage across the first inductor. 
     If the voltage across the first capacitor is sufficiently large, the negative voltage across the third inductor, which is equal to N times the negative voltage across the first inductor, will be sufficiently large to lower the voltage at the cathode of the second diode to turn on the second diode, such that current flows from ground through the third inductor. In this case, if the first diode is on, depending on the circumstances, current flows through the third inductor to charge the second capacitor or through the second inductor and the first diode. 
     If the first diode is off, depending on the circumstances, current flows through the third inductor to charge the second capacitor or through the first capacitor, thereby discharging the first capacitor. As previously described, the discharging of the first capacitor corresponds to a desirable return of energy to the input energy source and/or the current path circuitry. 
     In addition to returning energy from the first capacitor while the switch is open, the recovery circuit, in conjunction with the second inductor, acts to return energy from the first capacitor when the switch is closed. In this case, the voltage across the first inductor produces a voltage across the second inductor that pulls current across the second capacitor. This current flows through the first capacitor, thereby discharging the first capacitor. Discharging of the first capacitor results in a desirable return of energy from the first capacitor, as previously mentioned. 
     In addition to providing the energy return function mentioned above, the recovery circuit helps to provide current to the first inductor after the switch has opened but while the first diode is still on. This additional current helps to stabilize the voltage across the first inductor during this time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned objects and features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same structural elements throughout, and in which: 
     FIG. 1 is a schematic of a switch recovery circuit constructed in accordance with the principles of the present invention, shown as part of a circuit that includes a switch, an input capacitor, and current path circuitry; and 
     FIG. 2 is a schematic of a switch recovery circuit constructed in accordance with the principles of the present invention, shown as part of a voltage regulator circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Switch recovery circuits are described below in accordance with the principles of the present invention. Two embodiments of the present invention, as shown in FIGS. 1 and 2 and described below, operate in a similar manner and are comprised of mainly identical components. Therefore, similar aspects will only be explained in great detail when dealing with the first figure. 
     Furthermore, as used in the specification and claims hereof, a first element may be coupled to a second element even though the elements are not linked at the same node. Moreover, current may flow from a first element to a second element even if there are intervening elements between the first and second elements, and not all of the current flowing through the first element must reach the second element. 
     FIG. 1 is a schematic diagram of an embodiment of an energy conservation circuit  100  constructed in accordance with the principles of the present invention. Circuit  100  includes a switch recovery circuit  11 , shown as part of a circuit that includes a switch  17 , an input capacitor  16 , and a current source  67  of arbitrary magnitude and polarity. The input capacitor  16  represents an input energy source. It should be understood, however, that the present invention is not limited to circuits wherein the input energy source is a capacitor. 
     The combination of switch  17  and current source  67  in FIG. 1 models several different types of real world “switches.” To the extent that a real world (non-ideal) switch conducts at least some current during the time it is open, in terms of the model shown in FIG. 1, that current flows through current source  67 . Additionally, in FIG. 1, capacitor  57  at node  21  represents the effects of stray capacitance associated with the circuitry. Moreover, the circuit shown in FIG. 1 is simplified and partially idealized because, as will be appreciated by those of ordinary skill in the art, it does not show the numerous parasitic capacitances, inductances and resistances associated with real world circuit elements. 
     Switch recovery circuit  11 , shown in FIG. 1, is designed to prevent the voltage level across switch  17  from reaching a level that could potentially damage switch  17 . Furthermore, switch recovery circuit  11  also conserves energy associated with the switching operation of switch  17 . 
     Switch recovery circuit  11  includes an inductor  18  coupled in series between switch  17  and input capacitor  16 . A capacitor  20  is coupled to the inductor  18  and diode  26  such that inductor  18 , capacitor  20  and diode  26  form a loop. An inductor  22  is also coupled to both diode  26  and capacitor  20  at a node  23 . Moreover, a capacitor  40  is coupled in series to inductor  22  at a node  31  (however, capacitor  40  may preferably be omitted, depending on the circumstances, as will be further described below). Additionally, an inductor  42  is coupled to both capacitor  40  and the inductor  22  at node  31 . 
     Inductors  18  and  22  may be two windings on the same core or they may be separate inductors. If inductors  18  and  22  are wound on the same core, the polarity of the voltages induced across inductors  18  and  22  are indicated by the dots shown in FIG.  1 . The coupling factor between inductors  18  and  22  and  18  and  42  may be, for example, 80%-98%, however, other coupling factors may be used. Moreover, the turns ratio of the winding of inductor  18  and the winding of inductor  22  is generally unity. 
     Inductor  42  is magnetically coupled to inductor  22  whether or not inductor  22  is magnetically coupled to inductor  18 . Specifically, the inductor  42  and inductor  22  comprise windings in the same magnetic circuit, with the ratio of windings between the inductor  42  and inductor  22  equal to N:1, where N is greater than 1. The optimal value of N will depend on circuit specific parameters. A ratio of 5:1 has been found to produce desirable results. 
     Capacitor  20  preferably has a relatively large capacitance value (e.g., 22 μF), so that the voltage across capacitor  20  (V 20 ), which has the polarity shown in FIG. 1, changes by only a small amount over any given switching cycle (a switching cycle is the opening and closing of switch  17 ). 
     Capacitor  40 , as mentioned above, may or may not be present in the circuit, depending on the design criteria. If the capacitor  40  is present, an associated capacitance of approximately 10 μF has been shown to produce favorable results. The return point of capacitor  40  is shown connected to node  29  but could be connected to ground or many other nodes. 
     Switch recovery circuit  11 , comprising the elements described above, implements a set of functionalities. These functionalities include commutating the body diode intrinsic to a synchronous transistor (when switch recovery circuit  11  is part of a synchronous switching voltage regulator as shown in FIG.  2 ), clamping the peak voltage on node  21 , transferring energy from inductor  18  to various storage sites, including capacitor  20 , capacitor  40  and capacitor  16 , and controlling the voltage across capacitor  20 . 
     As stated above, commutation of the body diode of a synchronous transistor is one of the functionalities of switch recovery circuit  11 . Because of the existence of current source  67 , there will be a current flowing through inductor  18  during the time that switch  17  is open. When switch  17  is then closed, however, the magnitude of the current flowing through inductor  18  begins at the value flowing through current source  67  and then ramps up in accordance with the following equation: V=L*(di/dt). Current flow and corresponding voltage polarities are positive in the directions indicated by the dots shown in FIG.  1 . 
     Moreover, because of the relatively small inductance value of inductor  18  and the relatively high voltage across capacitor  16 , the race of current rise in inductor  18  is quite rapid. The time that switch  17  in FIG. 1 opens is the moment of commutation of the body diode intrinsic to transistor  82  of FIG.  2 . 
     The clamping action performed by switch recovery circuit  11  shown in FIG. 1 results in a transfer of energy stored in inductor  1 B. This energy is transferred primarily to capacitor  20  and secondarily to various energy-storing components in the circuit shown in FIG. 1, and then ultimately back to input capacitor  16 . 
     The clamping action occurs upon the opening of switch  17 . When switch  17  opens, the portion of the current in inductor  18  that is in excess of the current of current source  67 , having no path to flow in, commences to charge the stray capacitance modeled by capacitor  57 . This causes a rapid rise in the voltage at node  21  as well as a small decrease in the current flowing through inductor  18 . 
     This precipitous rise in the voltage of node  21  would, were it not for the clamping action innate to the design of the circuit shown in FIG. 1, damage switch  17 . When the voltage at node  21  rises above the voltage at node  29  by an amount equal to V 20 +V 26  (the voltage across diode  26 ), diode  26  begins conducting current, thus activating a clamp loop comprising capacitor  20  and diode  26  coupled around inductor  18 . 
     At this time, the amount of current flowing through inductor  18  that is in excess of the amount of current flowing in current source  67  begins to flow through capacitor  20  and diode  26 , and back to inductor  18 . This current charges capacitor  20 , implementing a transfer of energy from inductor  18  to capacitor  20 . 
     The current flowing through inductor  18  will then begin to decrease until it becomes equal to the current of current source  67 , at which time diode  26  commutates. At this point, the clamping action of the circuit shown in FIG. 1 is completed. 
     As described above, the transfer of energy from inductor  18  begins at the moment that switch  17  opens and continues throughout the clamping period until diode  26  commutates. Furthermore, in the above description of energy transfer, the excess energy of inductor  18  was transferred exclusively into capacitor  20 . This is only the case when inductor  18  and inductor  22  are not coupled and capacitor  40 , inductor  42  and diode  54  shown in FIG. 1 are omitted. However, with coupled inductors, the energy transfer becomes somewhat more complex. 
     When inductor  18  and inductor  22  are coupled and capacitor  40  is present, a part of the energy transferred out of inductor  18  is transferred into capacitor  20  directly, as described above. 
     The balance of the energy that is transferred out of inductor  18  is transformer coupled by the mutual inductance of inductor  18  and inductor  22  to charge capacitor  40 . 
     There is an even more indirect transfer of energy involving inductor  42  that is coupled to inductor  22  with a turns ratio. The amount of energy transferred through this path, which is dependant on the amount of voltage across capacitor  40 , charges capacitor  16 . 
     The last of the switch recovery circuit  11  functionalities listed above involves the controlling of the voltage on capacitor  20 . As excess energy is removed from inductor  18  during the clamping process, the majority of it is transferred into capacitor  20 . In the absence of a mechanism for removal of the energy stored in capacitor  20  during each switching cycle, the voltage across it would gradually increase with each switching cycle and would eventually reach an unacceptably large value. 
     When switch  17  is open and diode  26  is on, the voltage across inductor  18  is almost equal to the voltage across capacitor  20 . Therefore, the larger the voltage across capacitor  20  becomes, the larger the voltage across inductor  18  is. 
     If no path is available for the removal of energy from capacitor  20 , the voltage across it and hence the voltage that node  21  reaches when switch  17  is opened will increase until switch  17  is damaged. This is a significant reason for the clamping action requirement. 
     To prevent switch  17  from being damaged, the voltage across capacitor  20  is regulated. The method of its regulation is as follows. If the voltage across capacitor  20  is sufficiently large (e.g., larger than the voltage programmed by the turns ratio N), the negative voltage across inductor  42 , which is equal to N times the negative voltage across the inductor  18 , will be sufficiently large to lower the voltage at the cathode of diode  54  in order to turn it on. 
     With diode  54  on, current will then flow through diode  54  and inductor  42 , even though the induced voltage across inductor  42  suggests a di/dt through inductor  42  opposite the direction of current flowing from ground through the diode  54 . 
     Current flows through diode  54  and inductor  42  to adjust the charge on capacitor  40  (if present) and on through inductor  22  and diode  26  (which is also on) into input capacitor  16 . 
     When capacitor  40  is present and the voltage across it is of sufficient magnitude, the current through diode  54  will exceed that of inductor  22 , with the difference in current discharging capacitor  40 . 
     Because of the coupling between inductor  18  and inductor  22 , whenever diode  26  is conducting, the current in inductor winding  22  will always flow in a direction and with a magnitude that will tend to equalize the magnitudes of the voltages across capacitors  20  and  40 . This is due to the single-ended primary inductance converter (SEPIC) like action of the collection of parts comprising inductor  18 , capacitor  20 , diode  26 , inductor winding  22 , and capacitor  40 . 
     At the end of the clamp time, the current through inductor  18  reduces to the magnitude of the current flowing in the current source  67  so that the current through diode  26  ceases. This point in time is referred to herein as the “Diode Commutation Event.” 
     After the Diode Commutation Event, depending upon the voltage at node  21 , either some current flows from holding capacitor  40  through inductor  22  and capacitor  20  into current source  67 , or the current flows from holding capacitor  40  through inductor  22 , capacitor  20 , inductor  18  and into input capacitor  16 . 
     In either case, capacitor  20  discharges, returning a portion of the energy stored in it to input capacitor  16  and/or current source  67 . 
     The operation of the subcircuit described above, comprising capacitor  40 , inductor  42  and diode  54 , is explained in further detail below. The main function of this subcircuit (hereinafter referred to as “the recovery subcircuit”) is to keep the voltage across capacitor  20  at a fixed, programmed value. In doing so, the recovery subcircuit returns the excess energy stored in capacitor  20  to input capacitor  16  and/or the current source  67 . By either mode, the result is the discharging of capacitor  20  (i.e. removing its excess energy). 
     The ratio N+1:1 establishes the voltage at node  23  when both diodes  26  and  54  are on (the addition of 1 to N in the sum N+1 represents the winding of the inductor  22 ). If capacitor  40  is absent from switch recovery circuit  11 , inductor winding  22  is more naturally considered one of the turns of inductor winding  42 . Ignoring the voltage drop of the diodes  26  and  54 , the voltage at node  23  is N+1 times the voltage across inductor  18 , which, because diode  26  is conducting, is equal to the voltage across capacitor  20 . 
     When capacitor  40  is absent and the voltage across capacitor  20  is in excess of the voltage set up by the turns ratio N, diode  54  continues conducting current after diode  26  has commutated. Conduction of diode  54  continues with an appropriate amount of current for the right duration of time in order to restore the voltage across capacitor  20  to its controlled voltage. 
     Accordingly, when switch  17  is open, the actions of inductors  22  and  42  and the capacitor  40  result in the existence of current that removes the excess energy from capacitor  20 . 
     The operation of the recovery subcircuit when switch  17  is closed is also of significance. In addition to removing excess energy from capacitor  20  when switch  17  is open, the recovery subcircuit acts to remove such energy even when switch  17  is closed. 
     Moreover, in the case where inductor  18  and inductor  22  are coupled, and switch  17  is closed, a voltage develops across inductor  18 , which in turn induces a voltage across inductor  22 . This induced voltage moves charge from capacitor  20 , placing it into capacitor  40 . This charge movement is brought about by the induced voltage across inductor winding  22 , and is performed by a current flowing through switch  17 . This, in turn, results in a transfer of energy from the capacitor  20  to capacitor  40 . 
     In the case where inductor  18  and inductor  22  are not coupled, there is an additional AC current flowing through capacitor  20  that represents the magnetizing current of inductor  22 . This current would have the effect of causing too much charge to be extracted from capacitor  20  while switch  17  is closed, and extra charge put into capacitor  20  while the clamping action is occurring. Therefore, the recovery subcircuit acts to remove the excess energy from capacitor  20  both while switch  17  is open and while it is closed. 
     The behavior of inductor  42  and diode  54  after switch  17  is closed is as follows. When switch  17  is closed, the input voltage at node  29  is placed across inductor  18 , which acts as the primary of a transformer. 
     Inductor winding  42  will, in response to the voltage imposed on inductor  18 , produce a voltage that is N times the voltage across inductor  18 . This voltage is imposed in a direction in a manner that causes diode  54  to block the flow of current. 
     Furthermore, the function of diode  54  may now be better appreciated in light of the above discussion of the operation of the recovery circuit. In the embodiment shown in FIG. 1, diode  54  is oriented to conduct current from ground through inductor  42 . This orientation of the diode prevents current flowing from capacitor  20  through inductor  42 . 
     As described above for FIG. 1, switch  17  represents many different types of switching circuits. Switch  17  may include, without limitation, a portion of a switching regulator circuit. 
     FIG. 2 is a schematic diagram of an embodiment of an energy conservation circuit  200  constructed in accordance with the principles of the present invention. As shown in FIG. 2, switch recovery circuit  11  is coupled to a switching voltage regulator. Many of the components comprising the energy conservation circuit  200  of FIG. 2 are also found in energy conservation circuit  100  as shown in FIG. 1, with identical reference numbers, and function as described above. 
     The voltage regulator circuit of FIG. 2 also includes transistors  80  and  82 , an inductor  84 , an output capacitor  86  and a control circuit  88  that closes the transistor switches  80  and  82  out of phase with one another. Moreover, transistors  80  and  82 , inductor  84 , capacitor  86  and control circuit  88  correspond to switch  17  and current source  67  of FIG.  1 . In addition, the stray capacitance associated with node  21  shown as item  57  in FIG. 1 corresponds generally to the stray capacitance of FIG. 2 currently found in transistor  80 , the output capacitance of transistor  82 , the self capacitance of inductor  84 , and all trace capacitances associated therewith. 
     The switch recovery circuit  11  of FIG. 2 implements the same set of functionalities as in FIG.  1 . As in FIG. 1, switch recovery circuit  11  of FIG. 2 is responsible for the following: commutating the body diode that is intrinsic to transistor  82 , clamping the peak voltage on node  21 , transferring energy from inductor  18  to various storage sites including capacitors  20 ,  40  and  16 , and controlling the voltage across capacitor  20 . 
     These and other objects of the present invention are achieved with the circuit in FIG. 2 in the same manner and with the same components as found in FIG.  1 . Energy is delivered to the output capacitor  86  of the switching power supply detailed in FIG. 2 in essentially the same manner that energy flows into current source  67  from capacitor  20  in FIG.  1 . 
     Persons skilled in the art will further recognize that the circuitry of the present invention may be implemented using circuit configurations other than those shown and discussed above. For example, capacitor  20  may be eliminated from the circuits of FIGS. 1 and 2. As another example, diodes  26  and  54  of FIGS. 1 and 2 may each be replaced with a switching device such as a transistor. All such modifications are within the scope of the present invention, which is limited only by the claims which follow.