Abstract:
An energy recovery snubber circuit for a power converter which includes a flyback transformer driven by a converter switch is disclosed. The snubber circuit includes two capacitors which are connected such that, when the snubber circuit is connected to a primary winding of the flyback transformer, the capacitors are charged in series by current flowing in a first direction in the primary winding when the converted switch is turned OFF, to recover energy stored in the leakage inductance of the transformer, and discharged in parallel to cause current flow in a second direction in the primary winding of the transformer, to thereby transfer the recovered energy to the transformer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to United Kingdom Patent Application No. 1403202.3 filed Feb. 24, 2014, the disclosure of which is hereby incorporated in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an energy recovery snubber for a power converter. 
     Description of Related Art 
       FIG. 1  shows a typical configuration of a flyback converter with a dissipative RCD snubber, which comprises components R 2 , R 3 , D 1  and C 1 . The snubber is designed to absorb and dissipate energy stored in the leakage inductance of the coupled inductor (flyback transformer) L 1 . In a typical design, a snubber of this type will dissipate in the region of 2-4% of the throughput power of the converter, and thus reduces the efficiency of the converter by the same amount. 
     The primary purpose of the snubber is to limit the voltage across switch M 1  so that this voltage stays within safe operating limits, and thereby prevents the energy stored in the leakage inductance of the coupled inductor causing the switch to avalanche. 
       FIG. 2  shows approximate operating waveforms in boundary conduction mode (BCM) for the circuit in  FIG. 1 . The drain-source voltage of switch M 1  is clamped at a value approximately equal to the voltage on the clamp capacitor C 1 , while current decays in the primary winding L 1   a  and rises in secondary winding(s) L 1   b . Once current has fallen to zero in the primary winding, the drain-source voltage of switch M 1  will exhibit a damped oscillation back to the reflected secondary voltage, Vreflected, and current will decay at a rate proportional to the output voltage. Once current in the secondary winding(s) falls to zero, the drain-source voltage on switch M 1  falls, initiating the next cycle. 
       FIG. 3  shows an active clamp flyback converter, which provides a known alternative to the use of RCD and other dissipative snubbers. In the active clamp flyback topology, switch M 2  is controlled to conduct (ie, to be ON) when M 1  is not conducting (ie, OFF) and to be OFF when switch M 1  is ON. Deadtime, when both switches are OFF is typically added to allow for reducing switching losses. 
     In the active clamp flyback, capacitor C 1  is charged to a voltage that is approximately equal to the reflected secondary voltage, Vreflected. When switch M 1  is turned OFF at the end of its ON-time, a resonance takes place between the leakage inductance of the coupled inductor L 1  and capacitor C 1 . Capacitor C 1  initially charges and then starts to discharge. In the steady-state, the ampere-seconds applied to C 1  over a complete switching cycle must be zero, and the resonant period is preferably longer than the converter OFF-time (ie, the OFF-time of switch M 1 ). 
       FIG. 4  shows approximate operating waveforms for the circuit in  FIG. 3 . However, it will be appreciated that the exact waveforms will depend on the damping in the system and the amount of leakage energy being handled by the active clamp. 
     Ideally, the primary current will exhibit one resonant cycle during the OFF-time of the converter, as shown in  FIG. 4 . However, in practice, this is difficult to achieve, especially when using a controller designed to operate in boundary conduction mode (BCM). Typically, the primary current exhibits multiple resonant cycles during the OFF-time of the converter, leading to jitter on the zero-current point in the secondary circuit. The resonant current in the primary winding leads to extra losses in the primary winding of the transformer, and the impact of jitter will vary depending on the OFF-time. 
     Although these issues might be resolved by not operating in BCM, BCM is a preferred mode of operation for low-power converters, since it results in relatively low switching losses over the entire load range when used with burst mode, valley counting, and other techniques employed in the latest controllers. Moreover, synchronous rectification of BCM flyback converters is also relatively easy to implement with a low parts count. 
     It is an object of the present invention to overcome the drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, there is provided an energy recovery snubber circuit for a power converter which comprises a flyback transformer driven by a converter switch M 1 , 
     the snubber circuit comprising two capacitors C 1 , C 2  which are connected such that, when the snubber circuit is connected to a primary winding L 1   a  of the flyback transformer, the capacitors are: 
     charged in series by current flowing in a first direction in the primary winding when switch M 1  is turned OFF, to recover energy stored in the leakage inductance of the transformer; and 
     discharged in parallel to cause current flow in a second direction in the primary winding of the transformer, to thereby transfer the recovered energy to the transformer. 
     Thus, energy stored in the leakage inductance of the transformer is, at least in part, recovered by the capacitors and then delivered to the output via the transformer. In this respect, when the capacitors discharge, this causes a current to flow in the primary winding which induces an additional current in the secondary winding whereby the recovered energy is delivered to the output. 
     With this configuration, once the snubber circuit delivers the energy stored in the leakage inductance of the transformer to the output, it is then inactive until the next switching cycle of the converter. This reduces losses in the primary winding of the inductor and overcomes the problem of jitter seen with the known active clamp topology. Further, unlike the active-clamp snubber discussed above, small signal variations in duty-cycle will not give rise to variations in clamp voltage as the circuit tries to re-establish an amps-seconds balance. Moreover, transformer design becomes simple from the point of view of leakage inductance, such that sophisticated winding techniques are not required in order to have tight control over this parameter. 
     Typical operating waveforms of a flyback converter utilising the snubber circuit of the present invention are very similar to those achieved with an RCD snubber, except that a single half-cycle resonance takes place to deliver the recovered energy to the load. 
     Preferably, the snubber circuit comprises a snubber switch M 2 , connected between the primary winding of the transformer and the capacitors C 1 , C 2 , such that the capacitors are discharged in parallel when switch M 2  is ON. 
     According to another embodiment of the present invention, there is provided a power converter which comprises: 
     a flyback transformer driven by a converter switch M 1 ; and 
     an energy recovery snubber circuit, the snubber circuit comprising: 
     two capacitors C 1 , C 2  connected in series to a primary winding L 1   a  of the transformer via a first rectifier D 1 , and connectable in parallel to the primary winding L 1   a  via second and third rectifiers D 2 , D 3  and a snubber switch M 2 , 
     such that, when the converter switch M 1  is turned OFF, current flowing in a first direction in the primary winding L 1   a  of the transformer flows through the snubber circuit via rectifier D 1 , to charge the capacitors C 1 , C 2  in series, and thereby recover energy stored in the leakage inductance of the transformer, 
     and such that, when the snubber switch M 2  is ON, the charged capacitors discharge in parallel via rectifiers D 2 , D 3  and snubber switch M 2 , to cause current flow in a second direction in the primary winding of the transformer, and thereby transfer the recovered energy to the transformer. 
     Preferably, capacitors C 1 , C 2  are connected in series to the primary winding of the transformer via a fourth rectifier. Thus, the fourth rectifier allows current flow in the first direction. 
     Preferably, the fourth rectifier is a parasitic diode of the snubber switch M 2 . For example, as exists in the case of a MOSFET switch. Alternatively, the fourth rectifier may be a separate diode connected in parallel (anti-parallel) with switch M 2 . Any other suitable configuration, which allows current flow in the first direction, but blocks current flow in the second direction while the snubber switch M 2  is OFF, may also be employed. 
     Preferably, the primary winding L 1   a  has two winding sections L 1   a   1  and L 1   a   2 , and wherein the third rectifier D 3  connects capacitor C 2  to a node or tapping point f located between said winding sections. 
     With this configuration, capacitor C 1  charges to a voltage approximately equal to the reflected secondary voltage, Vreflected, while capacitor C 2  charges to a voltage in the range 0-Vreflected, the actual value depending on the turns ratio between the winding sections. More specifically, C 2  charges to a voltage given by
 
 V ( C 2)= V reflected* N ( L 1 a 2)/( N ( L 1 a 1)+ N ( L 1 a 2))  (1)
 
where N(L 1   a   1 ) is the number of turns for inductance L 1   a   1  and N(L 2   a   2 ) is the number of turns for inductance L 1   a   2 .
 
     The snubber circuit preferably comprises a controller for controlling the snubber switch M 2  to switch periodically between an on-state (ON) and an off-state (OFF). Thus, in each switching cycle, switch M 2  has an on-time when it is in the on-state and an off-time when it is in the off-state. 
     It will be appreciated that the main switch of the converter M 1  also switches periodically between an on-state and an off-state under the control of a main converter controller. Thus, in each switching cycle, switch M 1  has an on-time when it is in the on-state and an off-time when it is in the off-state. 
     Preferably, the controller controls switch M 2  to switch ON whilst switch M 1  is in said off-state. Thus, switch M 2  switches ON with zero-voltage switching. Preferably, the controller controls switch M 2  to switch off whilst switch M 1  is in said off-state, ie, before the main converter controller initiates the next on-time for the main switch M 1 . 
     In particular, in each switching cycle of the main switch M 1 , switch M 2  is preferably controlled to switch ON and then OFF during the off-time of switch M 1 . Preferably, switch M 2  remains OFF for the duration of the on-time of switch M 1 . 
     Preferably, the controller controls switch M 2  to have a fixed on-time in each switching cycle. 
     Alternatively, the controller implements a minimum on-time control scheme. In particular, switch M 2  is preferably switched ON with an on-time which is greater than the half-period of the resonance between the leakage inductance and capacitors C 1  and C 2 . 
     In either case, the resonant current that flows in the primary winding will fall to zero with the sum of the voltages on capacitors C 1  and C 2  being greater than 
     Vreflected, and a positive half cycle cannot occur since rectifier D 1  will not become forward biased until the next off-transition of switch M 1 . That is to say, the sum of the voltages on capacitors C 1  and C 2  is greater than Vreflected at all times. 
     The values of C 1  and C 2  are preferably selected such that, when switch M 1  is switched OFF, each capacitor is charged to less than 1.5*Vreflected, where Vreflected is the reflected secondary voltage; and such that, after the capacitors have discharged, the sum of the voltages on the two capacitors is greater than Vreflected. 
     The controller for switch M 2  is preferably a synchronous rectifier controller. In particular, it may comprise a FAN6224 synchronous rectifier controller. If the FAN6224 is used, at light load the snubber switch M 2  is not gated and a transorb or other dissipative snubber becomes active. 
     The features and advantages described above in relation to either embodiment of the invention apply equally to all embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described with reference to the accompanying drawings in which:— 
         FIG. 1  shows a flyback converter with a known RCD dissipative snubber; 
         FIG. 2  shows approximate waveforms for operation in BCM of the circuit shown in  FIG. 1 ; 
         FIG. 3  shows a known active clamp flyback topology; 
         FIG. 4  shows ideal waveforms for the circuit shown in  FIG. 3 ; 
         FIG. 5  shows a flyback converter with an energy recovery snubber which embodies the present invention; 
         FIG. 6  shows typical waveforms for the converter shown in  FIG. 5 ; 
         FIG. 7  shows another flyback converter with an energy recovery snubber which embodies the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The converter illustrated in  FIG. 5  comprises a high voltage rail  10  and a low voltage rail  12  which are supplied by a voltage supply (not shown). One side of a bulk capacitor C is connected to the high voltage rail  10  and its other side is connected to the low voltage rail  12 . 
     A transformer (coupled inductor) L 1  has a primary winding L 1   a  which comprises two windings L 1   a   1  and L 1   a   2  which are connected together in series at a node f. In this respect, among other suitable configurations, windings L 1   a   1  and L 1   a   2  may respectively comprise sections of a single primary winding component separated by a transformer tap at node f, or they may be two separate winding components connected to one another at node f. Windings L 1   a   1  and L 1   a   2  are inductively coupled to a secondary winding L 1   b . One end of the primary winding is connected to the high voltage rail  10  at a node a in the circuit. The other end of the primary winding is connected to a node b in the circuit. Node f is located at the interconnection of windings L 1   a   1  and L 1   a   2 . Preferably, the turns ratio of these windings, L 1   a   1 :L 1   a   2 , is in the range 0-1. 
     The drain terminal of a switch M 1  is connected to node b, and the source terminal of switch M 1  is connected to one end of a resistor R 1 , the other end of which is connected to the low voltage rail  12 . The gate terminal of switch M 1  is connected to a controller, not shown. Switch M 1  is typically a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Resistor R 1  is used by a controller (not shown) for sensing primary current when M 1  is on. R 1  may be omitted if alternative current sensing means is used. 
     One end of secondary winding L 1   b  is connected to the anode of a diode D. The cathode of diode D 2  is connected to one side of an output capacitor C 3 . The other side of capacitor C 3  is connected to the other end of secondary winding L 1   b . Secondary winding L 1   b  is oriented relative to the primary winding L 1   a  and diode D 2  such that the voltage induced by current flow in L 1   a  when switch M 1  is conducting reverse biases diode D, and such that the voltage induced when switch M 1  is not conducting forward biases diode D. It will be appreciated by someone skilled in the art that the output rectifier diode D could be replaced by any device capable of being used as a rectifier. 
     Output capacitor C 3  supplies an output voltage Vout to a load (not shown). 
     In use, the bulk storage capacitor C draws and stores energy from the voltage supply. When switch M 1  is ON, the primary winding of the transformer is connected to capacitor C which supplies an input voltage to the transformer. In this state, the primary current and magnetic flux in the transformer increases, inducing a negative voltage in the secondary winding, such that diode D is reverse biassed. In this state, current cannot flow in the secondary winding and energy is stored in the transformer. When switch M 1  is subsequently switched OFF, the primary current and magnetic flux in the transformer drops, inducing a positive voltage in the secondary winding, such that diode D is forward biased. In this state current can flow in the secondary winding, and energy stored in the transformer is transferred to the output capacitor C 3  to be transferred to the load. 
     To this extent, the power converter is configured to operate as a flyback converter, and the transformer may be termed a flyback transformer. 
     Energy recovery snubber  14  comprises capacitors C 1  and C 2 , diodes D 1 , D 2  and D 3 , and a second MOSFET switch M 2 . One side of capacitor C 1  is connected to node a. The other side of capacitor C 1  is connected to the cathode of diode D 1 . The anode of diode D 1  is connected to one side of capacitor C 2 . The other side of capacitor C 2  is connected to the drain terminal of switch M 2 . The source terminal of switch M 2  is connected to node b. The anode of diode D 2  is connected to a node c located between capacitor C 1  and diode D 1 . The cathode of diode D 2  is connected to a node d located between capacitor C 2  and switch M 2 . The anode of diode D 3  is connected to node f located between windings L 1   a   1  and L 1   a   2  and the cathode of diode D 3  is connected to a node e located between diode D 1  and capacitor C 2 . As shown in  FIG. 5 , switch M 2  has a parasitic anti-parallel diode. Thus, capacitors C 1  and C 2  are connected by switch M 2  and diodes D 1 , D 2 , D 3  to be charged in series and to be discharged in parallel. In particular, capacitors C 1  and C 2  are charged through the path L 1   a   1 , L 1   a   2 , M 2  (ie its parasitic diode), C 2 , D 1 , C 1 , while capacitor C 1  discharges through the path D 2 , M 2 , L 1   a   1  and L 1   a   2 , and capacitor C 2  discharges through the path M 2 , L 1   a   2  and D 3 . The gate terminal of switch M 2  is connected to a controller (not shown). This describes the circuit in the case where M 2  is an N-channel MOSFET. However, it will be appreciated that M 2  may alternatively be a P-channel MOSFET, or another switch type. 
     When switch M 1  is turned OFF, capacitors C 1  and C 2  are charged due to the energy stored in the leakage inductance of the coupled inductor L 1 . 
     Capacitors C 1 , C 2  are assumed to have a sufficiently high value that, in normal operation, each is charged to less than 1.5 times the reflected secondary voltage Vreflected when M 1  is turned OFF. Capacitors C 1 , C 2  are also assumed to have a sufficiently high value that the sum of the voltages on the two capacitors is greater than Vreflected at all times, in normal operation. 
     M 2  is gated ON, preferably with a fixed ON-time, or with an ON-time that is greater than the half-period of the resonance between the leakage inductance and the capacitors C 1  and C 2 . 
     Accordingly, the resonant current that flows will fall to zero with the sum of the voltages on capacitors C 1  and C 2  charged to greater than Vreflected. Thus, the sum of the voltages on capacitors C 1 , C 2  is greater than Vreflected at all times, and a positive resonant half-cycle cannot occur because diode D 1  will not become forward biased until the next turn-off transition of switch M 1 . 
     C 1  will be charged to the reflected voltage Vreflected and C 2  will be charged to approximately
 
 V ( C 2)= V reflected* N ( L 1 a 2)/( N ( L 1 a 1)+ N ( L 1 a 2))  (1)
 
where N(L 1   a   1 ) is the number of turns on winding section L 1   a   1  and N(L 2   a   2 ) is the number of turns on winding section L 1   a   2 .
 
     Preferably, switch M 2  is turned ON after switch M 1  is switched OFF so that it switches ON with Zero-Voltage switching. 
     Switch M 2  can then be switched OFF with zero current. Unlike the active clamp flyback circuit, small signal variations in duty cycle will not give rise to variations in clamp voltage as the circuit tries to re-establish an amps-second balance. 
     Typical operating waveforms for the circuit of  FIG. 5  are shown in  FIG. 6 . These waveforms are very similar to the operating waveforms achieved with an RCD snubber, except that a single half-cycle resonance takes place to deliver the energy recovered by the snubber back to the load. 
     It is also possible to gate switch M 2  ON towards the end of the OFF-time of switch M 1  to reduce output filter ripple current and provide zero-voltage turn-ON for switch M 1 . 
     The controller preferably implements a fixed ON-time control for M 2 . 
     Alternatively, a volt-second balancing technique is used such as that implemented in the FAN6224 synchronous rectifier controller. In this case, at light load, the energy recovery snubber may be disabled and a transorb or other dissipative snubber activated. 
     It is also possible to use other synchronous rectifier controllers that implement a minimum ON-time control scheme, or any other controller that allows switch M 2  to conduct when switch M 1  is not conducting, ensuring that switch M 2  turns OFF before switch M 1  initiates the next ON-time. 
     An advantage of the energy recovery snubber of the present invention is that transformer design becomes simple from the point of view of leakage inductance. Thus, it is not necessary to use sophisticated winding techniques to have control over this parameter. 
     With the energy recovery snubber of the present invention, the peak drain-source voltage of switch M 1  Vdspk is given by
 
 Vdspk=V in+(1+ N ( L 1 a 2))/( N ( L 1 a 1)+ N ( L 1 a 2))× V reflected  (2)
 
where Vin is the input voltage, N(L 1   a   1 ) is the number of turns for inductance L 1   a   1 , N(L 1   a   2 ) is the number of turns for inductance L 1   a   2 , and Vreflected is the reflected secondary voltage. In contrast, with the active clamp circuit of  FIG. 3 ,
 
 Vdspk=V in+ V reflected  (3)
 
     Thus, Vdspk is higher with the present invention than with the active clamp topology. Nevertheless, the advantages of the present invention outweigh this one disadvantage. In particular, the circuit operates with approximately the same waveforms as a conventional BCM flyback converter, and can utilise the low cost and low parts-count controllers optimised for this zero-switching topology. 
       FIG. 7  shows a another flyback converter with an energy recovery snubber which embodies the present invention. The circuit is identical to the circuit shown in  FIG. 5  except that windings L 1   a   1  and L 1   a   2  are replaced by a single winding L 1   a ; and the anode of diode D 3  is connected to node a. 
     Operation of the converter of  FIG. 7  is similar to that of  FIG. 5 , except that capacitor C 1  discharges through the path D 2 , M 2 , L 1   a  and capacitor C 2  discharges through the path M 2 , L 1   a , D 3 . 
     The present invention has been described in the context of a flyback power converter topology. However, it will be appreciated that the principles of the invention may be applied to any converter topologies which contain one or more transformers whose primary and secondary windings are connected for operation as a flyback transformer. 
     The embodiments described above employ a MOSFET switch as snubber switch M 2 , which incorporates an antiparallel diode which allows current flow to charge the capacitors C 1 , C 2  in series. However, it will be appreciated that the MOSFET switch could be replaced by a different switching device used in combination with a separate anti-parallel diode. 
     It will be understood that the embodiments illustrated above show an application of the invention only for the purposes of illustration. In practice the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.