PATENT DOCUMENT

Publication Number: US-9923472-B1
Application Number: US-201715405901-A
Country: US
Kind Code: B1

Title: Fixed frequency series-parallel mode (SPM) active clamp flyback converter

Abstract:
A flyback converter can include a series-parallel mode (SPM) active clamp circuit. The active clamp circuit, coupled in parallel with the primary coil, may include multiple networks in parallel. The first network, comprising a switch, one or more snubber capacitors, and one or more diodes, may be configured to absorb and retain the leakage energy from the leakage inductance of the flyback converter. The second network, comprising another switch and a diode, may be configured to create a circulating circuit for the flow of current through the primary coil in a reverse direction and clamp the current to a threshold level. With the active clamp circuit, the flyback converter may first re-capture the leakage energy in the active clamp circuit and then recover it back to the power source.

Claims:
The invention claimed is: 
     
       1. A power conversion apparatus, comprising:
 a primary coil configured to receive an input voltage; 
 a secondary coil electromagnetically coupled to the primary coil and configured to provide an output voltage; 
 a first switch coupled to the primary coil and configured to control a flow of current through the primary coil based on a first control signal; 
 a second switch configured to control, based on a second control signal, a flow of current through a snubber network of an active clamp circuit, the snubber network comprising two or more capacitors configured to charge in series and discharge in parallel, the snubber network coupled in parallel with the primary coil and configured to absorb a leakage energy from a leakage inductance associated with the primary coil and the secondary coil by charging the two or more capacitors; 
 a third switch configured to control, based on a third control signal, a flow of current through a reverse circulating network of the active clamp circuit, the reverse circulating network coupled in parallel with the primary coil and configured to clamp the current through the primary coil in a reverse direction to a threshold level; and 
 a controller coupled to the first, the second and the third switches, the controller configured to generate the first, the second and the third control signals. 
 
     
     
       2. The power conversion apparatus of  claim 1 , wherein the snubber network of the active clamp circuit further comprises first and second capacitors coupled in series with the second switch. 
     
     
       3. The power conversion apparatus of  claim 2 , wherein the snubber network of the active clamp circuit further comprises a first diode coupled in series with the second switch, whereby the first diode is configured to retain the absorbed leakage energy in the first and second capacitors by disconnecting the first and second capacitors from the primary coil. 
     
     
       4. The power conversion apparatus of  claim 3 , wherein the snubber network of the active clamp circuit further comprises a second diode coupled in parallel with the first capacitor and the first diode, whereby the second diode is configured to conduct a discharging current of the second capacitor. 
     
     
       5. The power conversion apparatus of  claim 3 , wherein the snubber network of the active clamp circuit further comprises a third diode coupled in parallel with the second capacitor and the first diode, whereby the third diode is configured to conduct a discharging current of the first capacitor. 
     
     
       6. The power conversion apparatus of  claim 1 , wherein the reverse circulating network of the active clamp circuit further comprises a fourth diode coupled in series with the third switch. 
     
     
       7. The power conversion apparatus of  claim 1 , wherein the first control signal is generated to achieve zero voltage switching of the first switch. 
     
     
       8. The power conversion apparatus of  claim 1 , wherein the second control signal is generated to achieve zero voltage switching of the second switch. 
     
     
       9. The power conversion apparatus of  claim 1 , further comprising a diode configured to achieve zero voltage switching of the third switch, the diode coupled between the snubber network and the reverse circulating network. 
     
     
       10. A method for operating a power conversion apparatus, the power conversion apparatus comprising primary and secondary coils, the primary coil configured to receive an input voltage, the secondary coil electromagnetically coupled with the primary coil and configured to provide an output voltage, the method comprising:
 generating, with a controller, first, second and third control signals; 
 controlling, with a first switch and based on the first control signal, a flow of current through the primary coil in a forward direction; 
 controlling, with a second switch and based on the second control signal, a flow of current through the primary coil and a snubber network of an active clamp circuit, the snubber network comprising two or more capacitors configured to charge in series and discharge in parallel, so as to recover an absorbed leakage energy from the primary coil until the flow of current in the reverse direction reaches a threshold level; and 
 controlling, with a third switch and based on the third control signal, a flow of current through a reverse circulating network of the active clamp circuit coupled in parallel with the primary coil so as to clamp the current through the primary coil in the reverse direction to the threshold level. 
 
     
     
       11. The method of  claim 10 , wherein the snubber network of the active clamp circuit further comprises first and second capacitors coupled in series with the second switch. 
     
     
       12. The method of  claim 11 , wherein the snubber network of the active clamp circuit further comprises a first diode coupled in series with the second switch, whereby the first diode is configured to retain the absorbed leakage energy in the first and second capacitors by disconnecting the first and second capacitors from the primary coil. 
     
     
       13. The method of  claim 12 , wherein the snubber network of the active clamp circuit further comprises a second diode coupled in parallel with the first capacitor and the first diode, whereby the second diode is configured to conduct a discharging current of the second capacitor. 
     
     
       14. The method of  claim 12 , wherein the snubber network of the active clamp circuit further comprises a third diode coupled in parallel with the second capacitor and the first diode, whereby the third diode is configured to conduct a discharging current of the first capacitor. 
     
     
       15. The method of  claim 10 , wherein the reverse circulating network of the active clamp circuit further comprises a fourth diode coupled in series with the third switch. 
     
     
       16. The method of  claim 10 , wherein the first control signal is generated to achieve zero voltage switching of the first switch. 
     
     
       17. The method of  claim 10 , wherein the second control signal is generated to achieve zero voltage switching of the second switch. 
     
     
       18. The method of  claim 10 , wherein the power conversion apparatus further comprises a diode configured to achieve zero voltage switching of the third switch, the diode coupled between the snubber network and the reverse circulating network. 
     
     
       19. A method of actively clamping a power conversion circuit, the power conversion circuit comprising primary and secondary coils, the primary coil configured to receive an input voltage from a power source, the secondary coil electromagnetically coupled with the primary coil and configured to provide an output voltage, the method comprising:
 turning on a first switch so as to store energy in a primary coil; 
 turning off the first switch and turning on second and third switches so as to transfer a leakage energy associated with the primary and secondary coils to a first and second capacitors by charging the first and second capacitors in series; 
 retaining the leakage energy in the first and second capacitors by disconnecting the first and second capacitors from the primary coil by using a first diode; 
 transferring the leakage energy from the first and second capacitors to the primary coil by discharging the first and second capacitors in parallel through second and third diodes; 
 turning off the second switch so as to clamp a flow of current through the primary coil in a reverse direction by using the third switch and a fourth diode; and 
 turning off the third switch so as to transfer the leakage energy from the primary coil to the power source. 
 
     
     
       20. The method of  claim 19 , wherein the second switch is turned on with a first delay after the turning-off of the first switch, so as to achieve zero voltage switching of the first switch. 
     
     
       21. The method of  claim 19 , wherein the third switch is switched off with a second delay before the next turning-on of the first switch, so as to achieve zero voltage switching of the first switch. 
     
     
       22. The method of  claim 19 , wherein while being charged, the first and second capacitors are configured to operate in a series resonant manner with the primary coil and a leakage inductance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 62/384,244, entitled “Fixed Frequency Series-Parallel Mode (SPM) Active Clamp Flyback Converter”, filed on Sep. 7, 2016, the contents of which is herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to the field of power converters, and in particular, to flyback converters with a series-parallel mode (SPM) active clamp circuit. 
     BACKGROUND 
     Flyback converter is one of the most common types of power converters in low-power, switched-mode power supplies. They are widely used in, e.g., electronic gadgets, cell phones, notebook computers, and/or other types of consumer electronics, especially when galvanic isolation is needed between input power source(s) and output load(s). A flyback converter may comprise a primary coil and a secondary coil, which are electromagnetically coupled with each other. By controlling the flow of current through the primary coil using a switch (a metal oxide semiconductor field-effect transistor (MOSFET), for example), energy may be transferred from the power source (coupled to the primary coil) to the load (coupled to the secondary coil). 
     In practice, the primary and secondary coils may have an associated parasitic leakage inductance that also captures energy. The leakage inductance may cause additional losses unless its leakage energy is recovered. For consumer electronics, especially battery operated electronics, the energy recovery may become even more important because of limited power availability and thermal management challenges in miniaturized electronics. Another practical challenge is the mitigation of electromagnetic interference (EMI). Flyback converters, being operated by turning on and off switches, may generate EMI. Specific features in consumer electronics may place EMI constraints on a power supply of such device. For example, the “Multi-Touch” technology used with touch screens may demand low common-mode noise within specific frequency band(s). Such requirements may require flyback converters to be operated at appropriate fixed switching frequencies. Therefore, the inventor has recognized the need for a flyback converter capable of recovering the leakage energy in high efficiency and operable with fixed switching frequencies. 
     SUMMARY 
     Disclosed herein is a flyback converter with a series-parallel mode (SPM) active clamp circuit. In some embodiments, the flyback converter may comprise electromagnetically coupled primary and secondary coils. By using a first switch (for example, a MOSFET) the flyback converter may transfer energy from a power source (coupled to the primary coil) to a load (coupled to the secondary coil). In some embodiments, the flyback converter may also comprise an active clamp circuit coupled in parallel with the primary coil. The active clamp circuit may include multiple networks in parallel. The first network of the active clamp circuit may include a second switch (for example, a second MOSFET), one or more snubber capacitors, and one or more diodes, which may be configured to absorb and retain the leakage energy from the leakage inductance of the flyback converter. The second network of the active clamp circuit may include a third switch (for example, a third MOSFET) and a diode, which may be configured to (1) create a circulating circuit for the flow of current through the primary coil in a reverse direction and (2) clamp the current to a threshold level. With the active clamp circuit, the flyback converter may first re-capture the leakage energy in the active clamp circuit and then recover it back to the power source. Additionally, in some embodiments, the active clamp circuit may also facilitate zero-voltage switching (ZVS), for example, for the first and second switches at turning-on, which may further improve the flyback converter&#39;s efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the disclosed concepts are illustrated by way of example and not by way of limitation in the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an”, “one” or “another” embodiment in this disclosure are not necessarily to the same or different embodiment, and they mean at least one. In order to be concise, a given figure may be used to illustrate the features of more than one embodiment, or more than one species of the disclosure, and not all elements in the figure may be required for a given embodiment or species. 
         FIG. 1  is schematic diagram illustrating exemplary SPM active clamp flyback converter  100  in accordance with one embodiment. 
         FIG. 2  illustrates the flow of current through the primary coil of exemplary flyback converter  100  in Operational Stage I in accordance with one embodiment. 
         FIG. 3  illustrates the flow of current through the primary coil of exemplary flyback converter  100  in Operational Stage II in accordance with one embodiment. 
         FIG. 4  illustrates the flow of current through the primary coil and a first snubber capacitor of exemplary flyback converter  100  in Operational Stage III in accordance with one embodiment. 
         FIG. 5  illustrates the flow of current through the primary coil and a second snubber capacitor of exemplary flyback converter  100  in Operational Stage III in accordance with one embodiment. 
         FIG. 6  illustrates the flow of current through the primary coil of exemplary flyback converter  100  in Operational Stage IV in accordance with one embodiment. 
         FIG. 7  illustrates the flow of current through the primary coil of exemplary flyback converter  100  in Operational Stage V in accordance with one embodiment. 
         FIG. 8  illustrates waveforms of the voltage across the first switch, the current through the primary coil, and control signals of exemplary flyback converter  100  in Operational Stages I-V in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed concepts. As part of this description, some of this disclosure&#39;s drawings represent structures and devices in block diagram form in order to avoid obscuring the disclosure. In the interest of clarity, not all features of an actual implementation are described in this disclosure. Moreover, the language used in this disclosure has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the disclosed subject matter, resort to the claims being necessary to determine such disclosed subject matter. 
       FIG. 1  is a schematic diagram illustrating exemplary series-parallel mode (“SPM”) active clamp flyback converter  100  in accordance with one embodiment. Flyback converter  100  may include primary coil P 1   105  and secondary coil S 1   110 , which may be electromagnetically coupled with each other. Primary coil P 1   105  may receive an input voltage V IN  from power source  115 , while secondary coil S 1   110  may supply an output voltage V OUT  to load  120  (for example, resistor R LOAD ) through diode D 4   130 , as shown in  FIG. 1 . To simplify explanations, it may be assumed that primary coil P 1   105  and secondary coil S 1   110  possess an ideal electromagnetic coupling, meaning a flow of current through primary coil P 1   105  may transfer the complete energy stored in primary coil P 1   105  to secondary coil S 1   110 . Leakage inductance of flyback converter  100 , including leakage inductance of primary coil P 1   105  and/or secondary coil S 1   110 , may be represented by leakage inductance L 1   135 , as shown in  FIG. 1 . Leakage inductance L 1   135  may capture a leakage energy that is not transferred from the primary coil to the secondary coil. 
     Flyback converter  100  may further comprise switch Q 1   125 , for example, a first MOSFET, coupled in series between power source  115  and primary coil P 1   105 . Also, flyback converter  100  may include snubber network  180  and reverse circulating network  185 , each of which may be coupled in parallel with primary coil P 1   105  (and leakage inductance L 1   135 ) and two of which may form an active clamp circuit. In one embodiment, snubber network  180  may comprise switch Q 3   140 , for example, a second MOSFET, in series with snubber capacitors C 1   145  and C 2   150 , and diode D 1   155 . A second diode D 2   160  may be coupled in parallel across snubber capacitor C 2   150  and diode D 1   155 , and a third diode D 3   165  may be coupled in parallel across snubber capacitor C 1   145  and diode D 1   155 . Reverse circulating network  185  may include switch Q 2   170 , for example, a third MOSFET, in series with diode D 5   175 . Operation of snubber network  180  and reverse circulating network  185  as an active clamp circuit will be described in further detail below. Flyback converter  100  may also include controller  190 , which may be coupled to switches Q 1   125 , Q 2   170  and Q 3   140  and generate controls signals for the switches, respectively. Flyback converter  100  may further include an optional diode D 6   190 , which couples snubber network  180  with reverse circulating network  185 . Referring to  FIG. 1 , diode D 6   190  may be coupled between the drains of switches Q 3   140  (of snubber network  180 ) and Q 2   170  (of reverse circulating network  185 ). The purpose and functionality of optional diode D 6   190  will be described in the following sections. 
       FIG. 1  also depicts gate drive circuits for switch Q 1   125  (comprising voltage source V 1  and resistor R 1 ), switch Q 2   170  (comprising voltage source V 2  and resistor R 2 ), and switch Q 3   140  (comprising voltage source V 3  and resistor R 3 ), respectively. Note that switches Q 1   125 , Q 2   170  and Q 3   140  may each comprise an intrinsic anti-parallel body diode and a parallel effective capacitance, which are not shown in  FIG. 1 . As an alternative to MOSFETs, flyback converter  100  may use other types of switches, for example, insulated gate bipolar transistors (IGBTs), junction gate field-effect transistors (JFETs), and silicon carbine and/or gallium nitride devices. Note that instead of using diode D 4   130 , flyback converter  100  may use other types of switches, for example, a MOSFET, to provide synchronous rectification. 
     Operations of flyback converter  100  may be explained in sequential Operational Stages I-V, which are schematically illustrated in  FIGS. 2-7 .  FIG. 2  illustrates operations of flyback converter  100  in Operational Stage I. In Operational Stage I, flyback converter  100  may turn on (or close) switch Q 1   125 , for example, by a first control signal with a fixed frequency, and maintain switches Q 2   170  and Q 3   140  off, by using controller  190  of flyback converter  100 . When switch Q 1   125  is closed, current will flow from power source  115  through primary coil P 1   105 , through switch Q 1   125 , as illustrated by lines  205 - 230 . The current flowing through primary coil P 1   105  is hereinafter referred to as the primary current I P1 . As the primary current I P1  in primary coil P 1   105  builds up, the induced voltage across the secondary coil S 1   110  may place diode D 4   130  into reverse bias, thus blocking a flow of current from secondary coil S 1   110  to load  120 . Because energy is not being transferred to the secondary coil, the energy delivered by power source  115  may be accumulated (or stored) in primary coil P 1   105 . Further, as the primary current I P1  also flows through leakage inductance L 1   135 , leakage inductance L 1   135  may store a leakage energy. 
       FIG. 3  illustrates operations of flyback converter  100  in Operational Stage II. In Operational Stage II, by using controller  190 , flyback converter  100  may turn off (or open) switch Q 1   125 , and after a first delay, turn on switches Q 3   140  and Q 2   170 , for example, by a second and third control signals with respective fixed frequencies. The first delay may be inserted to facilitate zero voltage switching (“ZVS”), for example, turning-on under zero voltage, for switch Q 3   140 , as well as prevent occurrences of short-circuit faults through the switches. In Operational Stage II, the primary current I P1  through primary coil P 1   105  may decline as the primary coil has been disconnected from power source  115  by turning off switch Q 1   125 . This may generate an induced voltage across secondary coil S 1   110  that causes diode D 3   130  to become forward biased and begin to conduct, thus delivering the stored energy from primary coil P 1   105  to secondary coil S 1   110  and load  120 . Meanwhile, after switch Q 1   125  is turned off but before switch Q 3   140  is turned on (i.e., during the first delay), the primary current I P1  may be forced to flow through the body diode (not shown) of switch Q 3   140  and diode D 1   155  into snubber capacitors C 1   145  and C 2   150 , as indicated by lines  305 - 320 . Snubber capacitors C 1   145  and C 2   150  may be charged in series, and the leakage energy may be transferred from leakage inductance L 1   135  to the two snubber capacitors. Note that flyback converter  100  may further include optional diode D 6   190 , and first turn on switch Q 3   140  and then switch Q 2   170  to achieve ZVS for switch Q 2   170 . For example, after switch Q 3   140  is closed, it may create an approximately zero voltage across switch Q 3   140 . Through diode D 6   190 , it may clamp the voltage across switch Q 2   170  (between switch Q 2   170 &#39;s drain and source) to approximately zero as well. Therefore, flyback converter  100  may turn on switch Q 2   170  at ZVS. When Q 2   170  turns on, the energy stored in the parasitic capacitance of its output capacitance (not shown) may dissipate in switch Q 2   170  as turn-on power loss. Optional diode D 6   190  may ensure that the energy stored in this capacitance is recovered when leakage energy is transferred to snubber capacitors C 1   145  and C 2   150 . 
     During the first delay, the body diode of switch Q 3   140  may conduct, which may bring down the voltage across switch Q 3   140  to approximately zero. Thus, flyback converter  100  may turn on switch Q 3   140  under zero voltage, e.g., after the first delay. Meanwhile, flyback converter  100  may also turn on switch Q 2   170 . The ZVS may further reduce losses and improve flyback converter  100 &#39;s efficiency. Once switches Q 3   140  and Q 2   170  are closed, the primary current I P1  may change from flowing through the body diode of switch Q 3   140  to flowing through switch Q 3   140  itself, thus continuously charging snubber capacitors C 1   145  and C 2   150 . The total stored energy in snubber capacitors C 1   145  and C 2   150  may depend on the snubber capacitors&#39; respective voltages. In particular, the energy may be determined according to equation (1):
 
 E   snubber =0.5×[ Cap   1   ×V   C1   2   +Cap   2   ×V   C2   2 ]  (1)
 
where Cap 1  and Cap 2  are the respective capacitances of snubber capacitors C 1   145  and C 2   150 , and V C1  and V C2  are the respective voltages across snubber capacitors C 1   145  and C 2   150 .
 
     The voltages V C1  and V C2  of snubber capacitors C 1   145  and C 2   150  may be determined by the reflected output voltage V OR  across primary coil P 1   105  and an induced leakage voltage V L1  across leakage inductance L 1   135 . To simplify explanations, snubber capacitors C 1   145  and C 2   150  may be assumed to have equal capacitances. Thereby, each snubber capacitors C 1   145  and C 2   150  may be charged to half of the reflected output voltage V OR , plus half of the induced leakage voltage V L1  (e.g., V C1 =V C2 =0.5×(V OR +V L1 )). The reflected output voltage V OR  across primary coil P 1   105  may be determined by equation (2):
 
 V   OR =( V   OUT   +V   f   _   D4 )×( Np/Ns )  (2)
 
where V OUT  is the output voltage of flyback converter  100 , V f   _   D4  is the forward voltage drop across diode D 4   130 , and Np/Ns represents the turns-ratio between primary coil P 1   105  and secondary coil S 1   110 . For example, if flyback converter  100  converts 300V input voltage (e.g., V IN =300V) to 5V voltage (e.g., V OUT =5V), the turns-ratio Np/Ns between the primary and secondary coils is 14:1, and the forward voltage drop across diode D 4   130  is 0.7V, the reflected output voltage V OR  may be (5+0.7)×14=79.8V. Each snubber capacitor C 1   145  and C 2   150  may be charged, for example, at least to a voltage equal to half of the reflected voltage V OR , and may be further charged higher by absorbing the leakage energy from leakage inductance L 1   135 . In the above example, if each snubber capacitor C 1   145  and C 2   150  is charged to 50V, then the charge corresponding to 40V (i.e., 79.8/2≅40V) is due to reflected output voltage V OR  and the charge of additional 10V (to reach 50V level) is due to half of the leakage energy. The reflected output voltage V OR  may remain substantially constant because flyback converter  100 &#39;s output voltage V OUT  may be regulated to a constant level under different loading conditions, while the induced leakage voltage V L1  may increase and/or decrease with the loading condition of flyback converter  100 .
 
     Once the leakage energy of leakage inductance L 1   135  is transferred to the series combination of snubber capacitors C 1   145  and C 2   150 , diode D 1   155  may block a flow of current in the reverse direction. Further, since the respective voltages V C1  and V C2  of snubber capacitors C 1   145  and C 2   150  are less than the total voltage V TOTAL  across primary coil P 1   105  and leakage inductance L 1   135  (e.g., V TOTAL =V OR +V L1 ), diodes D 2   160  and D 3   165  may become reverse biased. Snubber capacitors C 1   145  and C 2   150  may thus be disconnected from primary coil P 1   105 , which causes C 1   145  and C 2   150  to retain the leakage energy that they absorbed from leakage inductance L 1   135 . Note that the positive polarity of the voltages V C1  and V C2  is from bottom end to top end of each snubber capacitor C 1   145  and C 2   150 , which aligns with the direction of primary current I P1  that charges the two snubber capacitors in series. Accordingly, the voltage V Q1  across switch Q 1   125  may become the sum of the voltages V C1 , V C2 , and input voltage V IN , for example, V Q1 =V C1 +V C2 +V IN ≅400V by using the exemplary numerical values given above. 
     When the energy of primary coil P 1   105  is depleted and transferred to the secondary coil, primary coil P 1   105  may resonate with the effective capacitance (not shown) of switch Q 1   125  with minimum losses, which may cause the voltage potential at switch Q 1   125 &#39;s drain node  350  to fall. When the total voltage V TOTAL  across primary coil P 1   105  and leakage inductance L 1   135  falls below the respective voltages V C1  and V C2  of snubber capacitors C 1   145  and C 2   150 , diodes D 2   160  and D 3   165  may become forward biased and begin to conduct. Further, if the voltages V C1  and V C2  are equal, diodes D 2   160  and D 3   165  may begin to conduct at about the same time. Flyback converter  100  may thus move into Operational Stage III, as illustrated in  FIGS. 4 and 5 . 
     Referring to  FIGS. 4 and 5 , because switch Q 3   140  remains closed, snubber capacitors C 1   145  and C 2   150  may reconnect with primary coil P 1   105  in parallel. The two snubber capacitors may transfer their stored leakage energy to primary P 1   105  (in a resonant manner with minimum losses), thereby generating a reverse primary current I P1 , as indicated by lines  405 - 440  in  FIGS. 4 and 505-530  in  FIG. 5 . Because snubber capacitors C 1   145  and C 2   150  are in parallel, the voltage V Q1  across switch Q 1   125  may become the sum of voltage V C1  (or V C2  if V C2  equals to V C1 ) and input voltage V IN , for example, V Q1 =V C1 +V IN ≅350V by using the exemplary numerical values given above. Note that optional diode D 6   190  may block the reverse primary current I P1  flowing from snubber network  180  into reverser circulating network  185 . 
     When the reverse primary current I P1  reaches a predetermined threshold level, flyback converter  100  may turn off switch Q 3   140  and move into Operational Stage VI, as shown in  FIG. 6 . As switch Q 2   170  still remains closed, switch Q 2   170  and diode D 5   175  may create a circulating circuit for the reverse primary current I P1  as shown by lines  605 - 620 . The resistivity of the circulating circuit of switch Q 2   170 , D 5   175 , primary coil P 1   105  and leakage inductance L 1   135  may be configured to be small. Thus, the amplitude of the reverse primary current I P1  may be clamped at or marginally below the predetermined threshold level. Additionally, as switch Q 2   170  and diode D 5   175  may generate only small voltage drops, the voltage V Q1  across switch Q 1   125  may approximately equal to input voltage V IN , for example, V Q1 ≅V IN =300V by using the exemplary numerical values given above. 
       FIG. 7  illustrates the Operational Stage V of flyback converter  100 . In Operational Stage V, flyback converter  100  may turn off switch Q 2   170 , and after a second delay, turn on switch Q 1   125 . The second delay may be inserted to facilitate ZVS, for example, turning-on under zero voltage, for switch Q 1   125 , as well as prevent occurrences of short-circuit faults through the switches. In Operational Stage V, the reverse primary current I P1  may be directed to flow through the effective capacitance of switch Q 1   125  (not shown), because switch Q 2   170  is turned off. Primary coil P 1   105 , leakage inductance L 1   135 , and the effective capacitance of switch Q 1   125  may operate in a resonant manner with minimum losses, causing the voltage potential at switch Q 1   125 &#39;s drain node  350  to fall towards zero. As the voltage potential of drain node  350  reduces, the body diode of switch Q 1   125  (not shown) may begin to conduct. The conduction of the body diode may cause the voltage V Q1  across switch Q 1   125  to fall to approximately zero. Therefore, flyback converter  100  may turn on switch Q 1   125  under a zero-voltage condition, e.g., after the second delay. The ZVS may further reduce losses and improve flyback converter  100 &#39;s efficiency. Once switch Q 1   125  is closed, the reverse primary current I P1  of primary coil P 1   105  may flow into power source  105 , as illustrated by lines  705 - 730  in  FIG. 7 , thus recovering the leakage energy stored in primary coil P 1   105  back to power source  115 . 
     As may now be better understood with reference to  FIGS. 1-7 , snubber network  180  may use snubber capacitors C 1   145  and C 2   150  to absorb and retain the leakage energy released from leakage inductance L 1   135 , while reverse circulating network  185  may create a circulating circuit to clamp the reverse primary current of primary coil P 1   105  to a threshold level. Next, when both of the networks are disconnected from primary coil P 1   105  by using switches Q 3   140  and Q 2   170 , flyback converter  100  may recover the leakage energy back to power source  115 . 
       FIG. 8  depicts, from top to bottom, time-domain waveforms of voltage V Q1  of switch Q 1   125 , primary current I P1  through primary coil P 1   105 , the second control signal V gs   _   Q3  of switch Q 3   140 , the third control signal V gs   _   Q2  of switch Q 2   170 , and the first control signal V gs   _   Q1  of switch Q 1   125 , in correspondence with the above-described Operational Stages I-V:
         In Operational Stage I, the first control signal V gs   _   Q1  may be asserted (e.g., to a valid high level), as shown by waveform  825 , which may turn on switch Q 1   125 . In reference to the descriptions in  FIG. 2 , the primary current I P1  may increase as illustrated by waveform  810 , and the voltage V Q1  of switch Q 1   125  may become a low voltage drop during conduction as depicted by waveform  805 . Energy is transferred from power source  115 , and stored in primary coil P 1   105  and leakage inductance L 1   135 .   In Operational Stage II, the first control signal V gs   _   Q1  may be de-asserted (e.g., to a valid low level), as shown by waveform  825 , which may turn off switch Q 1   125 . Accordingly, the primary current I P1  may start to decline as illustrated by waveform  810 . After a first delay, the second and third control signals V gs   _   Q3  and V gs   _   Q2  may be asserted, as shown by waveforms  815  and  820 , respectively, thus turning on switches Q 3   140  at ZVS and Q 2   170 . In reference to the descriptions in  FIG. 3 , stored energy of primary coil P 1   105  may be delivered to the secondary coil S 1   110  and load  120 , while leakage energy of leakage inductance L 1   135  may be transferred to and retained by snubber capacitors C 1   145  and C 2   150  in series mode. Accordingly, the voltage V Q1  across switch Q 1   125  may reach the sum of input voltage V IN  and voltages across snubber capacitors C 1   145  and C 2   150 , for example, V Q1 =V C1 +V C2 +V IN ≅400V by using the exemplary numerical values given above, as indicated by the sloping, flat area  830  in waveform  805 .   In Operational Stage III, when the energy of primary coil P 1   105  is depleted, diodes D 2   160  and D 3   165  may begin to conduct, thus allowing snubber capacitors C 1   145  and C 2   150  to transfer the leakage energy that they absorbed in Operational Stage II to primary coil P 1   105 . In reference to the descriptions in  FIGS. 4 and 5 , snubber capacitors C 1   145  and C 2   150  may, in parallel, charge primary P 1   105  in a resonant manner with minimum losses and generate a reverse primary current I P1 , as indicated by waveform  810 , for example, primary current I P1  may become negative. Accordingly, the voltage V Q1  across switch Q 1   125  may become the sum of voltage V C1  (or V C2  if V C2  equals to V C1 ) and input voltage V IN , for example, V Q1 =V C1 +V IN ≅350V by using the exemplary numerical values given above, as indicated by the sloping, flat area  835  in waveform  805 .   In Operational Stage IV, when the stored leakage energy in snubber capacitors C 1   145  and C 2   150  have been transferred to primary coil P 1   105 , the second control signal V gs   _   Q3  may be de-asserted, as shown by waveform  815 , which may turn off switch Q 3   140 . In reference to the descriptions in  FIG. 6 , the reverse primary current I P1  may be directed to circulate through switch Q 2   170  and diode  175 . As the resistivity of the circulating circuit may be configured to be small, the reverse primary current I P1  may be held nearly flat, as shown in waveform  810 . Accordingly, as switch Q 2   170  and diode D 5   175  may generate small voltage drops during conduction, the voltage V Q1  across switch Q 1   125  may drop to input voltage V IN , for example, V Q1 ≅V IN =300V by using the exemplary numerical values given above, as indicated by the sloping, flat area  840 .   In Operational Stage V, the third control signal V gs   _   Q2  may be de-asserted, as shown by waveform  820 , which may turn off switch Q 2   170 . In reference to the descriptions in  FIG. 7 , the reverse primary current I P1  may be directed to flow through the effective capacitance of switch Q 1   125 , in a resonant manner with minimum losses, causing the voltage potential at switch Q 1   125 &#39;s drain node  350  to fall towards zero. The reverse primary current I P1  of primary coil P 1   105  may flow into power source  105 , thus recovering the leakage energy from primary coil P 1   105  back to power source  115 . Note that the exemplary numerical values used in above descriptions are for purposes of illustration only, and that other flyback converter configurations, input voltages, output voltages, turns-ratio and/or diode forward voltage drop may be used.       

     In reference to the waveforms in  FIG. 8 , it may now be better understood that flyback converter  100  may operate switches Q 1   125  and Q 2   170  at fixed frequencies complementarily with each other with the first and second delays, as shown by waveforms  820  and  825 . Flyback converter  100  may turn on switch Q 3   140  with switch Q 2   170  and turn off switch Q 3   140  when the reverse primary current I P1  reaches the threshold level, as shown by waveforms  815  and  810 . The increasing rate of reverse primary current I P1  may depend on the leakage energy stored in and delivered by snubber capacitors C 1   145  and C 2   150 , which is further based on the loading condition of flyback converter  100 . Thereby, switch Q 3   140 &#39;s turn off time may vary. If flyback converter  100  does not include reverse circulating network  185 , which comprises switch Q 2   170  and D 5   175 , flyback converter  100  may have to turn on switch Q 1   125  sooner, for example, after a delay once switch Q 3   140  is opened, thus causing both switches Q 1   125  and Q 3   140  to be operated at variable frequencies. Alternatively, flyback converter  100  may still control switch Q 1   125  at a fixed frequency but may have to maintain switch Q 3   140  on until the next switching cycle of switch Q 1   125  in order to provide conductions for the reverse primary current I P1 . This may cause continuous rising of reverse primary current I P1  and end up with a much larger reverse current. Hence, with reverse circulating network  185  formed by switch Q 2   170  and D 5   175 , flyback converter  100  may sustain the fixed frequency operation of switch Q 1   125  and meanwhile clamp the reverse primary current I P1  to a threshold level. 
     The various embodiments described above are provided by way of illustration only and should not be constructed to limit the scope of the disclosure. Various modifications and changes can be made to the principles and embodiments herein without departing from the scope of the disclosure and without departing from the scope of the claims.

Metadata:
Filing Date: 20170113
Publication Date: 20180320
Grant Date: 20180320
Priority Date: 20160907
Inventors: PHADKE VIJAY G.
Assignee: APPLE INC
CPC Classifications: [{"code": "H02M2001/342", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M3/33507", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02M1/34", "inventive": true, "first": false, "tree": "[]"}, {"code": "H02M1/34", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02M1/342", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/342", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/0058", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M1/0058", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y02B70/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y02B70/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "H02M3/33507", "inventive": true, "first": true, "tree": "[]"}, {"code": "H02M1/34", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 61281691