Abstract:
A power converter apparatus, such as a DC—DC converter, includes a switch that controls current transfer between an input port and an inductance. A control circuit is operative, while current is being transferred between the inductance and a clamping circuit, to control the switch responsive to a current in the inductance. For example, the control circuit may include a current sensor configured to be coupled in series with the inductance and a switch control circuit operative to control the first switch responsive to a current sense signal generated by the current sensor. The switch control circuit may be operative to prevent transition of the switch from the first state to the second state until the current sense signal meets a predetermined criterion, e.g., a signal state indicative of a desired current condition, such as a current approximating zero or a current reversal. Related operating methods are also discussed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to power converter apparatus and methods, and more particularly, to clamped converters, asymmetrical half-bridges, and similar power conversion apparatus that use a clamped inductance. 
     DC—DC converters and other power conversion apparatus often use “clamped converter” and “asymmetrical half-bridge” configurations. A common feature of such devices is the use of a power conversion cycle in which a transformer winding, inductor or other inductance is energized in an “on” phase by application of an input voltage (directly or via magnetic coupling) and then “clamped” during an “off” phase using a capacitor and/or other circuitry that receives magnetizing energy from the inductance. Examples of such converter configurations may be found in U.S. Pat. No. 4,441,146 to Vinciarelli; U.S. Pat. No. 4,959,764 to Bassett; U.S. Pat. No. 5,291,382 to Cohen; “Small-Signal Modeling of Soft-Switched Asymmetric Half-Bridge DC/DC Converter,” by Korotkov et al, IEEE Applied Power Electronics Conference, Record, 1995, p. 707-711. 
     Many conventional clamped converter and asymmetrical half-bridge designs use a capacitor to receive energy during the “off” phase. A potential drawback of such circuits is that an abrupt change in the converter&#39;s duty cycle can lead to an incomplete energy transfer during the “off” phase due to premature entry into the “on” phase. This can lead to undesirably large peak currents in the inductance. For example, in a transformer-type clamped converter, an abrupt change in duty cycle may lead to excessive magnetizing current in the transformer, which can, in turn, lead to saturation of the transformer. In circuits that use a transistor with an integral body diode to switch the clamping circuit, such premature entry into the “on” phase can also damage the transistor through uncontrolled reverse recovery of the body diode. 
     SUMMARY OF THE INVENTION 
     In some embodiments of the invention, a power converter apparatus, such as a DC—DC converter, power supply, or the like, includes an input port, an output port, an inductance, a clamping circuit coupled to the inductance and an output circuit coupled to the inductor and the output port. The inductance may include, for example, a transformer winding and/or a discrete inductor. The apparatus also includes a switch operative to control energy transfer between the input port and the inductance. The apparatus further includes a control circuit operative to control the switch responsive to a current in the inductance while current is being transferred between the inductance and the clamping circuit. For example, the control circuit may include a current sensor configured to be coupled in series with the inductance while current is being transferred between the inductance and the clamping circuit and operative to generate a current sense signal indicative of the current in the inductance, along with a switch control circuit operative to control the first switch responsive to the current sense signal. The switch control circuit may be operative to prevent transition of the switch from the first state to the second state until the current sense signal meets a predetermined criterion, e.g., a signal state indicative of a desired current condition, such as a current approximating zero or a current reversal. 
     In further embodiments of the invention, the switch includes a first switch. The clamping circuit includes an impedance, such as a capacitor, a second switch operative to control current flow between the impedance and the inductance, and a clamping control circuit operative to control the second switch. The second switch may include a transistor that is responsive to a clamping control signal, and a diode, such as a transistor body diode, coupled in parallel with the transistor. A current limiting circuit may be provided to limit current in the second switch. In some embodiments, the current limiting circuit may be asymmetrical, i.e., may provide a variable impedance responsive to the direction of the current between the impedance and the inductance. 
     In other embodiments of the invention, a power converter apparatus includes an input port, an output port, and an inductance. A first switch is coupled to the input port and the inductance and controls current flow between the input port and the inductance. A second switch is coupled to an impedance and the inductance, and controls current flow between the impedance and the inductance. A control circuit operates the first and second switches in a substantially complementary fashion to provide energy transfer between the inductance and respective ones of the input port and the impedance, and is further operative to control operation of the first switch responsive to a current in the inductance. An output circuit couples the inductance to the output port. 
     In method embodiments of the invention, a power converter apparatus that transfers energy from a power source to a load by cyclically energizing an inductance is operated. The power source is decoupled from the inductance. The inductance is then clamped while sensing a current therein. The power source is then coupled to the inductance responsive to the sensed current. 
     Embodiments of the invention may provide significant advantages over convention converter configurations. In particular, by controlling coupling of a clamped inductance to a power source responsive to current in the inductance while it is being clamped, e.g., responsive to a sensed current in the clamping circuit, the present invention may limit peak current generated in the inductance during transient conditions when the charging/clamping cycle of the inductance abruptly changes and, thus, may prevent saturation of the inductance. In some converter configurations, the invention may also reduce damaging effects, such as uncontrolled reverse recovery of switching diodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a clamped converter apparatus according to embodiments of the invention. 
     FIG. 2 is a schematic diagram of a clamped converter apparatus according to other embodiments of the invention. 
     FIG. 3 is a schematic diagram illustrating a clamped converter apparatus with an exemplary control circuit configuration according to some embodiments of the invention. 
     FIGS. 4A and 4B are waveform diagrams illustrating exemplary operations of the converter apparatus of FIG. 3 according to embodiments of the invention. 
     FIG. 5 is a schematic diagram illustrating a clamped converter apparatus with an exemplary current limiting circuit configuration according to some embodiments of the invention. 
     FIG. 6 is a schematic diagram illustrating a power converter apparatus according to still further embodiments of the invention. 
     FIG. 7 is a schematic diagram illustrating a power converter apparatus with an exemplary current limit/current sense circuit according to some embodiments of the invention. 
     FIG. 8 is a schematic diagram illustrating still another power converter configuration according to embodiments of the invention. 
     FIG. 9 is a schematic diagram illustrating a power converter apparatus with an exemplary current limit circuit according to still further embodiments of the invention. 
    
    
     DETAILED DESCRIPTION 
     Specific embodiments of the invention now will be described more fully with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     FIG. 1 illustrates a power converter apparatus  100  according to embodiments of the invention. The apparatus  100  includes an input port  110   a ,  110   b  at which a voltage v in , for example, a DC voltage produced by a rectifier, may be applied. The apparatus  100  also includes an output port  140   a ,  140   b , an inductance in the form of a primary winding  122  of a transformer  122 , a clamping circuit  170  and an output circuit  130 , here shown as including a secondary winding  124  of the transformer  120 , coupled to the inductance  122  and the output port  140   a ,  140   b . The apparatus further includes a switch  150  that is operative to couple and decouple the input port  110   a ,  110   b  and the inductance  122  to selectively apply the input voltage v in  thereto. The apparatus  100  further includes a control circuit  160 , here shown as including a current sensor  162  coupled in series with the clamping circuit  170  and a switch control circuit  164  that is responsive to the current sensor  162 . The control circuit  160  is operative to sense a current in the inductance  122  while the clamping circuit  170  receives current from the inductance  122 . The control circuit  160  is further operative to control the switch  150  responsive to the current in the inductance  122 . 
     It will be understood that, in a particular application, the converter apparatus  100  will typically include other components. In particular, the control circuit  160  and/or the clamping circuit  170  may be further controlled responsive to, for example, a voltage and/or current at the output port  140   a ,  140   b , or to another circuit state, such as a voltage and/or current of additional circuitry coupled to the apparatus. For purposes of the generality of description, detailed discussion of such voltage and/or current feedback control techniques will not be provided herein. 
     It also will be appreciated that the configuration of FIG. 1 may be modified within the scope of the invention. For example, rather than using a current sensor  162  coupled in series with a clamping circuit  170  as shown in FIG. 1, other current sensing techniques can be used with the invention, including, for example, a current sensor coupled in series with the inductance  122 . 
     It will also be understood that the invention is not limited to the “clamped converter” configuration shown in FIG.  1 . In general, the invention is also applicable to a variety of power converter configurations, including configurations that use types of inductances other than transformer windings. The invention is also generally applicable to configurations using a variety of different types of clamping circuits, including, but not limited to, resonant (e.g., capacitive) clamping circuits, dissipative (e.g., resistive) clamping circuits, and combinations thereof. Moreover, the invention may be embodied in a variety of different types of devices, such as DC—DC converters, power supply devices, uninterruptible power supply (UPS) devices, and the like. The invention generally may be implemented using discrete electrical components, integrated circuits, and combinations thereof. 
     FIG. 2 illustrates a power converter apparatus  200  according to other embodiments of the invention. The apparatus  200  includes an input port  210   a ,  210   b , an output port  240   a ,  240   b , an inductance in the form of a primary winding  222  of a transformer  220 , and an output circuit  230 , here shown as including a secondary winding  224  of the transformer  220 , coupled to the inductance  222  and the output port  240   a ,  240   b . A switch  250 , here shown as including a transistor Q and associated body diode DB, is operative to couple and decouple the input port  210   a ,  210   b  and the inductance  222  to selectively apply an input voltage v in  thereto. A clamping circuit  270  includes a capacitor C and second switch  272 , here shown as including a transistor Q and a body diode D B , that is operative to control current flow between the capacitor C and the inductance  222 . 
     A current sensor  262  is coupled in series with the switch  272  and is operative to sense a current in the inductance  222  while the switch  272  couples the clamping capacitor C across the inductance  222 . A switch control circuit  264  generates respective control signals that are applied to respective ones of the switches  250 ,  272 . In particular, the switch control circuit  264  is operative to control the switch  250  responsive to a current sense signal  263  generated by the current sensor  262 . 
     As illustrated in FIG. 3, a power converter apparatus  300  according to other embodiments of the present invention is similar to the apparatus  200  of FIG. 2, with like components being indicated by like reference numerals, description of which is provided in the foregoing discussion of FIG.  2 . The apparatus  300  includes a switch control circuit  264 ′ including a switching signal generator circuit  310  that generates first and second switch control signals S 1 , S 2 . The switch control signal S 1  is applied to an AND gate circuit  320 , which also receives a current sense signal SCS generated by a current sensor  262 ′ coupled in series with a clamping circuit  270 . The AND gate  320  generates a control signal S 1 ′ that is applied to the switch  250 , which controls current flow between the inductance  222  and the input port  210   a ,  210   b  responsively thereto. 
     Exemplary operations of the apparatus  300  may be understood by reference to FIGS. 4A and 4B. In the embodiments illustrated in FIGS. 3,  4 A and  4 B, the first and second drive signals S 1 , S 2  transition in a substantially complementary fashion, i.e., in a complementary fashion that may incorporate a small amount of “dead time” such that signal S 1  delays transition to a “high” state for a short period after transition of the signal S 2  to a “low” state, and/or vice versa. Generation of the control signals S 1 , S 2  may be achieved via any of a number of conventional control techniques commonly used in clamped converter apparatus, for example, using voltage and/or current feedback techniques. 
     Prior to a time t 1 , it is assumed that the first and second signals S 1 , S 2  transition at substantially constant complementary duty cycles such that the first signal S 1  has a duty cycle approaching 0% and such that the second signal S 2  has a duty cycle approaching 100%, i.e., such that the second signal S 2  is at nearly a continuous “high” state while the first signal is at nearly a continuous “low” state. As a result, the switch  272  of the clamping circuit  272  is “on” substantially more than the switch  250 . Accordingly, the current i 1  in the inductance  222  remains relatively low and, consequently, the voltage v C  across the clamping capacitor C remains relatively low. Such a condition might occur, for example, when the apparatus  300  is lightly loaded at the output port  240   a ,  240   b.    
     At time t 1 , however, the duty cycles of the signals S 1 , S 2  abruptly change such that the duty cycle of the signal S 1  abruptly increases to around near 50% and the duty cycle of the switch S 2  abruptly decreases to around 50%. Such a change might occur, for example, in response to an increase in load at the output port  240   a ,  240   b . In a first “on” interval of the switch  250  from time t 1  to time t 2 , the current i 1  ramps up to a relatively high level, such that, when the switch  250  is turned off at time t 2  and the switch  272  turns “on” by forward biasing of the body diode D B  shortly thereafter, a relatively large current i 2  begins to flow from the inductance  222  to the capacitor C. Because the decay time for this large initial current is relatively long due to the highly discharged state of the capacitor at time t 2 , the current i 2  remains relatively high when the signal S 1  goes “high” again at time t 3 . However, the current sense signal SCS remains “low” due to the positive, nonzero level of the current i 2 , maintaining the switch  250  in an “off” state until the current i 2  falls to near zero at time t 4 , several cycles of the signals S 1 , S 2  later. For the operations illustrated in FIGS. 4A and 4B, this current limiting action continues for subsequent cycles of the signals S 1 , S 2 . However, assuming that the duty cycles of the signals S 1 , S 2  remain relatively constant, the converter may approach a steady state, wherein the current i 2  reaches zero before each new rising edge of the signal S 1  and the voltage v C  remains relatively constant. The action of the current sense signal SCS serves to limit the peak value of the current generated in the inductance  222  during the transient period following the abrupt change in the substantially complementary duty cycles of the signals S 1 , S 2  at time t 1 . This can prevent saturation of the transformer  220 . The action of the current sense signal SCS can also provide a more controlled reverse recovery of the body diode D B  of the switch  272 . 
     It will be understood that apparatus and operations described with reference to FIGS.  3  and  4 A- 4 B may be modified within the scope of the invention. For example, rather than configure the current sensor  262 ′ to transition the current sense signal SCS when the current i 2  is approximately zero, the current sensor  262 ′ could be configured to transition the current sense signal SCS at some other current level, such as a positive level that can still provide saturation protection, or a negative level that can provide better reverse recovery for the body diode D B  of the switch  272 . 
     FIG. 5 illustrates a converter apparatus  500  according to other embodiments of the invention. The converter apparatus  500  is similar to the apparatus  200  of FIG. 2, with like components indicated by like reference numerals, description of which is provided in the foregoing description of FIG.  2 . The converter apparatus  500  further includes an asymmetrical current limiting circuit  280  coupled in series with the clamping circuit  270 . Here shown as including a current limiting resistor R CL  connected in parallel with a bypass diode D BP , the asymmetrical current limiting circuit  280  serves to limit current in the switch  272  of the clamping circuit  270  in an asymmetrical fashion. In particular, the current limiting circuit  270  allows relatively large currents to flow from the inductance  222  to the clamping capacitance C through the forward biasing of the bypass diode D BP , but limits reverse current through the action of the current limiting resistor R CL . This latter characteristic may be particularly advantageous in limiting currents in the switch  272  during transients in which the switch  250  transitions abruptly from a relatively high duty cycle, e.g., near 100% (corresponding to a heavily loaded condition) to a substantially lower duty cycle, with concomitant transitioning of the switch  272  from a relatively low duty cycle, e.g., near 0%, to a substantially higher duty cycle. Although the bypass diode D BP  could be omitted, its presence can reduce unnecessary power dissipation in comparison to use of the current limiting resistor R CL  alone. 
     As noted above, the invention is not limited to “clamped converter” embodiments, and is generally applicable to many types of converter configurations that cyclically charge a transformer winding, inductor, or other inductance and “clamp” the charged inductance using a resonant, dissipative or other type of clamping circuit. For example, as illustrated in FIG. 6, a converter  600  according to embodiments of the invention may have a structure like that found in an asymmetrical half-bridge converter. As shown, the converter  600  includes a first switch  620  that control current flow between and inductance L and an input port  610   a ,  610   b  at which an input voltage v in  is applied. As shown, the first switch  620  includes a transistor Q and associated body diode D B . Current flow between the inductance L and a clamping capacitance C is controlled by a second switch  630 , here also shown as including a transistor Q and associated body diode D B . The inductance L may be coupled to an output port (not shown for purposes of generality of illustration) in a number of different ways, including, for example, via magnetic coupling (as in a transformer) or electrical coupling to the inductance L. 
     A switch control circuit  664  controls the first and second switches  620 ,  630 . In particular, the switch control circuit  664  controls the first switch  620  responsive to a current sense signal generated by a current sensor  662  coupled in series with the clamping capacitor C. Much like the embodiments described above with reference to FIGS. 1-5, the switch control circuit  664  operates the switches  620 ,  630  in a substantially complementary fashion. The switch control circuit  664  is further operative to condition closure of the switch  620  responsive to the current in the inductance L while the capacitor C is still coupled to the inductance L. In this manner, peak current in the inductance L can be limited, and reverse recovery of the body diode DB of the switch  630  can be controlled. 
     FIG. 7 illustrates a converter apparatus  700  according to other embodiments of the invention. The apparatus  700  is similar to the apparatus  600 , with like components illustrated by like reference numerals, description of which is provided in the foregoing description of FIG.  6 . The apparatus  700  includes a combined current limiting/current sensing circuit including a current limiting resistor R CL , a bypass diode D BP , and a current sense diode D CS  coupled in series with the current limiting resistor R CL . A voltage v CS  at a node  680  at which the current limiting resistor R CL  is coupled to the clamping capacitor C serves as a current sense signal provided to a switch control circuit  664 ′ that controls the first and second switches  620 ,  630 . Along the lines of the switch control circuit  664  of FIG. 6, the switch control circuit  664 ′ is operative to condition closure of the switch  620  responsive to the current sense signal v CS , which is representative of the current in the inductance L while the capacitor C is coupled to the inductance L. 
     In particular, assuming the voltage at the second terminal  610   b  of the input port is signal ground (zero volts), when the current i C  in the clamping capacitor C is positive (in the sense defined by the arrow), the voltage v CS  is approximately one diode drop (e.g., 0.6 volts) positive due to the forward biasing of the bypass diode D BP . However, when the current ic approaches zero and passes to a negative value, the bypass diode becomes reversed biased, and the current sense diode D CS  becomes forward biased. This causes the current sense voltage v CS  to transition to at least one diode drop negative (e.g., −0.6 volts or lower). This change in voltage can be detected by the switch control circuit  664 ′, which may responsively enable closure of the first switch  620 . For example, the switch control circuit  664 ′ may include, for example, comparator and/or other signal detection circuitry that detects such a transition of the current sense voltage v CS . In this manner, saturation of the inductance L and/or reverse recovery of the body diode D B  of the switch  630  can be controlled. 
     FIG. 8 illustrates yet another possible converter topology according to embodiments of the invention. The converter apparatus includes an inductance L and a clamping capacitance C. As with the converter apparatus of FIGS. 6 and 7, the inductance L may be coupled to an output port (not shown for purposes of generality of illustration) in a number of different ways, including magnetic and electrical coupling. A first switch  820 , including a transistor Q and associated body diode D B , is operative to control current flow between the inductance L and an input port  810   a ,  810   b  at which an input voltage v in  is applied. A second switch  830 , also including a transistor Q and body diode D B , is operative to control current flow between the clamping capacitor C and the inductance L. A switch control circuit  864  operates the first and second switches  820 ,  830  in a substantially complementary fashion, and is further operative to condition operation of the switch  820  on a current sense signal v CS  generated at a node  880  at which the second switch  830  is connected to a current limit/current sense circuit including a current limiting resistor R CL , a bypass diode D BP , and a current sense diode D CS . The current limit/current sense circuit can operate in a manner similar to that described with reference to FIG.  7 . 
     FIG. 9 illustrates a converter apparatus  900  according to yet other embodiments of the invention. The apparatus  900  is similar to the apparatus  800  of FIG. 8, with like elements indicated by like reference numerals, description of which is provided above with reference to FIG.  8 . The apparatus  900  differs from the apparatus  800  in that the current limiting resistor R CL  and bypass diode D BP  are moved to the other side of the transistor switch  830 . This allows the switch  830  to operate in a linear, current limiting manner when current i C  in the clamping capacitance C becomes excessive in the negative direction. A current sensor  862  coupled in series with the switch  830  provides a current sense signal to a switch control circuit  864 ′ that controls the first and second switches  820 ,  830 . 
     In the drawings and foregoing description thereof, there have been disclosed typical embodiments of the invention. Terms employed in the description are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.