INVENTOR:

ANDREY MALiNiN

BACKGROUND

1 . Field of Technology

[0001] Embodiments disclosed herein relate generally to switching power converters, and more specifically, to techniques for adaptive synchronous rectification control.

2. Description of the Related Arts

[0002] FIG. 1 is a circuit diagram of a conventional flyback type switching power converter 100 that uses a switch Q l such as a metal-oxide-semiconductor field-effect transistor (MOSFET). The switching power converter 100 includes a power stage 101 and a secondary output stage 103. Power stage 101 includes the switch Q l and a power transformer T 1. Power transformer Tl includes primary winding Np, secondary winding Ns, and auxiliary winding Na. Secondary output stage 103 includes diode Di and output capacitor Ci. A controller 105 controls the ON state and the OFF state of switch Q l using output drive signal 107 in the form of a pulse with on-times (TON) and off-times (TOFF).

[0003] AC power is received from an AC power source (not shown) and is rectified to provide the unregulated input voltage VDC. The input power is stored in transformer T 1 while the switch Q l is turned on, because the diode Di becomes reverse biased when the switch Q l is turned on. The rectified input power is then transferred to an electronic device across the capacitor Ci while the switch Q l is turned off, because the diode Di becomes forward biased when the switch Q l is turned off. Diode Di functions as an output rectifier and capacitor Ci functions as an output filter. The resulting regulated output voltage VOUT is delivered to the electronic device.

[0004] In high output current applications, the conduction loss of the diode Di operating as the output rectifier is significant. A MOSFET or other actively-controlled switch may replace the diode Di to minimize conduction loss in the power converter 100 during high output current applications. The MOSFET functions as a synchronous rectifier in the power converter 100. To achieve proper operation of the synchronous rectifier, a synchronous rectifier controller is added to the power converter 100 to control the operation of the synchronous rectifier. [0005] Because a MOSFET device is bi-directional, the on and off state of the MOSFET must be carefully controlled, essentially mimicking the functional operation of a diode device. When switch Ql is placed in the on state, primary current I _pr i flows through the primary winding Np of the transformer Tl . The synchronous rectifier MOSFET must be placed in the off state by the synchronous rectifier controller when switch Q 1 is on.

However, conventional synchronous rectifier controllers cannot reliably detect the optimum point at which to place the synchronous rectifier MOSFET from the on state to the off state. This results in variation of operational performance of synchronous rectification systems including increased power loss due to excessive body diode conduction or reverse current to occur.

SUMMARY

[0006] The embodiments herein disclose a method for adaptive synchronous rectification control of a synchronous rectifier in a switched mode power supply. In one embodiment, the switched mode power supply includes a synchronous rectifier controller that determines when to turn off the synchronous rectifier during each switching cycle based on a reference signal corresponding to when substantially all the energy stored in a transformer of the power supply has been delivered to an electronic load. The synchronous rectifier controller may adjust the length of time that the synchronous rectifier switch is turned on during subsequent switching cycles based on whether the synchronous rectifier switch was turned off before substantially all the energy stored in the transformer is delivered to the electronic load during a given switching cycle or based on whether the synchronous rectifier switch was turned off after a threshold amount of time from when all the energy stored in the transformer is delivered to the electronic load during the switching cycle.

[0007] The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings and specification. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The teachings of the embodiments of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. [0009] FIG. 1 is a circuit diagram of a conventional switching power converter.

[0010] FIG. 2 is a circuit diagram of a switching power converter including a synchronous rectifier circuit according to one embodiment.

[0011] FIG. 3 is a method flow diagram of a synchronous rectifier controller according to one embodiment.

[0012] FIG. 4 is a circuit diagram illustrating a detailed view of the synchronous rectifier circuit of FIG. 2 according to one embodiment.

[0013] FIG. 5 is a circuit diagram of a reference generator according to one embodiment.

[0014] FIG. 6 illustrates waveform diagrams of the switching power converter.

DETAILED DESCRIPTION OF EMBODIMENTS

[0015] The Figures (FIG.) and the following description relate to various embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles discussed herein.

[0016] Reference will now be made in detail to several embodiments, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict various embodiments for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein.

[0017] FIG. 2 is a circuit diagram of a flyback type switching power converter 200. As shown in FIG. 2, the power converter 200 includes a switch Q 1. In one embodiment, the switch Q l is a MOSFET. However, the switch Ql may be any type of switching device such as a bipolar junction transistor (BJT). The switching power converter 200 includes a power stage 201 and a secondary output stage 203. Power stage 201 includes the switch Ql and a power transformer Tl. Power transformer T 1 includes primary winding Np, secondary winding Ns, and auxiliary winding Na. Secondary output stage 203 includes an output capacitor Ci and a synchronous rectifier circuit. In one embodiment, the synchronous rectifier circuit includes a synchronous rectifier switch Q _S R and a synchronous rectifier (SR) controller 209 as will be further described below. As shown in FIG. 2, the synchronous rectifier circuit is coupled between an lower side of the secondary winding Ns of the transformer T 1 and the output ground terminal. A controller 205 controls the on state and the off state of switch Ql using output drive signal 207 in the form of a pulse with on-times (TON) and off-times (TO _FF ). That is, the controller 205 generates the output drive signal 207 that drives the switch Q 1.

[0018] AC power is received from an AC power source (not shown) and is rectified to provide the unregulated input voltage V _DC - When switch Ql is on, energy is stored in transformer T 1 via the primary winding magnetizing inductance. When the switch Q 1 is on, the synchronous rectifier switch Q _SR is disabled (i.e., off) and it's body diode is reverse biased thereby blocking a current pathway of the transformer secondary winding Ns. When switch Ql is turned off, energy stored in the magnetizing inductance of the transformer Tl will be transferred to capacitor CI through the body diode of the synchronous rectifier switch Q _SR . When the synchronous rectifier switch Q _SR is turned on, the additional pathway with the lower than forward biased diode voltage drop to deliver the stored energy to the capacitor CI is created. Generally, the synchronous rectifier switch Q _SR functions as an output rectifier and capacitor Ci functions as an output filter. The resulting regulated output voltage V _OUT is delivered to the electronic device with less loss than in case without the synchronous rectifier.

[0019] As mentioned previously, the controller 205 generates appropriate switch drive pulses 207 to control the on-times and off-times of switch Q l and regulate the output voltage V _OUT - The controller 205 controls switch Ql using a feedback loop based on the sensed output voltage V _SENSE and the sensed primary side current I _pr i in previous switching cycles of the switching power converter 200 to generate the timings to turn on or off the switch Q 1 in subsequent switching cycles, in a variety of operation modes including PWM (pulse width modulation) and/or PFM (pulse frequency modulation) modes. I _SENSE is the voltage across resistor R _SNS and is used by the controller 205 to sense the primary current I _pr i through the primary winding Np and switch Ql in the form of a voltage across the sense resistor R _SNS and ground.

[0020] FIG. 2 illustrates primary-side control, where the output voltage V _OUT is reflected across auxiliary winding Na of transformer Tl, which is input to controller 205 as the voltage V _SENSE via a resistive voltage divider comprised of resistors Ri and R ₂ . Based on the sensed output voltage, the controller 205 determines the times at which the switch Q 1 is turned on in PFM, PWM, or any other regulation mode employed by the controller 205. However, synchronous-rectification control as described herein may also be used in conjunction with secondary-side output voltage sensing. [0021] In one embodiment, the synchronous rectifier switch Q _S R is a MOSFET versus the diode D l in the conventional power converter 100. The synchronous rectifier switch QSR couples the lower side of the secondary winding Ns of the transformer Tl to the output ground of the converter 200 and operates as a pathway to transfer energy stored in the transformer Tl to the capacitor CI . As shown in FIG. 2, the source terminal of the synchronous rectifier switch Q _S R is coupled to the ground terminal of the output of power converter 200. The drain terminal of the synchronous rectifier switch QSR is coupled to the lowerside of the secondary winding Ns of the power transformer T 1. The gate terminal of the synchronous rectifier switch Q _S R is coupled to the SR controller 209 included in the synchronous rectifier circuit.

[0022] The SR controller 209 controls the on state and off state of the synchronous rectifier switch Q _S R using output drive signal 21 1. Generally, the SR controller 209 generates the output drive signal 21 1 to turn off the synchronous rectifier switch Q _S R when the switch Ql is on, and to turn on the synchronous rectifier switch Q _S R on when switch Ql is off.

[0023] During operation of the power converter 200 in the discontinuous conduction mode (DCM), the optimum time for the SR controller 209 to turn off the synchronous rectifier switch QSR is when substantially all of the energy stored in the transformer Tl has been delivered to the capacitor CI, which corresponds to the instant when the secondary current I _sec reaches 0 amps. In one embodiment, the SR controller 209 determines when to turn off the synchronous rectifier switch Q _S R during each switching cycle based on a reference value corresponding to when the secondary current I _sec reaches 0 amps. By observing the drain to source voltage of the synchronous rectifier switch Q _S R during each switching cycle of the power converter 200, the SR controller 209 makes necessary adjustments to the reference value for subsequent switching cycles thereby adjusting the length of time that the synchronous rectifier switch Q _S R is on during the subsequent switching cycles. This allows for operation of the power converter 200 that is not susceptible to variations in device and/or system level variations due to manufacturing and the operating environment.

[0024] FIG. 3 is a diagram describing how the SR controller 209 adjusts the reference value for turning off the synchronous rectifier switch QSR according to one embodiment. The SR controller 209 turns on 301 the synchronous rectifier switch Q _S R during a switching cycle N. Generally, the SR controller 209 turns on the synchronous rectifier switch Q _S R during switching cycle N when switch Ql is off during switching cycle N. The SR controller 209 turns off 303 the synchronous rectifier switch Q _S R during switching cycle N based on a reference value which was set by the SR controller 209 during the previous switching cycle N-l .

[0025] The SR controller 209 determines 305 if the body diode of the synchronous rectifier switch QSR is conducting after the synchronous rectifier switch QSR is turned off. Whether the body diode is conducting signifies if the SR controller 209 turned off the synchronous rectifier switch Q _S R too early or too late as will be further described below. The SR controller 209 determines if the body diode is conducting based on the drain-to-source voltage drop of the synchronous rectifier switch Q _S R. For example, a drain-to-source voltage drop of 0.7 V signifies that the body diode is conducting.

[0026] If the body diode of the synchronous rectifier switch Q _S R is conducting after the synchronous rectifier switch QSR is turned off, the SR controller 209 lowers 307 the reference value that will be used to turn off the synchronous rectifier switch Q _S R during the subsequent switching cycle N+1. By lowering the reference value, the SR controller 209 lengthens the duration in which the synchronous rectifier switch Q _S R is on during the switching cycle N+1. Conversely, if the body diode of the synchronous rectifier switch Q _S R is not conducting after the synchronous rectifier switch QSR is turned off, the SR controller 209 increases 309 the reference value that will be used to turn off the synchronous rectifier switch Q _S R during the subsequent switching cycle N+1. By raising the reference value, the SR controller 209 shortens the duration in which the synchronous rectifier switch Q _S R is on during the switching cycle N+1.

[0027] Note that in the embodiment where the synchronous rectifier switch QSR is coupled between the upper side of the secondary winding Ns and the capacitor CI, the SR controller 209 raises the reference value for the subsequent switching cycle N+1 in order to lengthen the time in which the synchronous rectifier switch QSR is on during the switching cycle N+1 and the SR controller 209 lowers the reference value for the subsequent switching cycle N+1 in order to shorten the time in which the synchronous rectifier switch QSR is on during the switching cycle N+1.

[0028] After the reference value is adjusted, the SR controller 209 repeats the method shown in FIG. 3 for the subsequent switching cycle N+1 to determine the reference value for switching cycle N+2. Thus, the SR controller 209 "hunts" for the optimal reference value that will be used to turn off the synchronous rectifier switch Q _S R during subsequent switching cycles.

[0029] FIG. 4 is a circuit diagram illustrating a detailed view of the synchronous rectifier switch QSR and the SR controller 209 according to one embodiment. In FIG. 4, an equivalent circuit model is shown of the synchronous rectifier switch Q _S R. The synchronous rectifier switch Q _S R is represented by an on-time resistor R _dS (ON ₎ , a switching element 401 coupled to the on-time resistor Rd _S (ON), and a body diode D _B ODY- The on-time resistor Rd _S (ON) is representative of the resistance of the MOSFET channel when the synchronous rectifier switch QSR is in the on-state.

[0030] In one embodiment, the SR controller 209 includes a comparator 403, a reference generator 405, and an off detector 407. The positive (+) input of the comparator 403 is coupled to the drain-to-source voltage VDS of the synchronous rectifier switch Q _S R and the input of the reference generator 304. When the synchronous rectifier switch QSR is on, the drain-to-source voltage VDS is representative of the voltage drop across the on-time resistor Rd _S (ON). Conversely, when the synchronous rectifier switch QSR is off, the drain-to- source voltage VDS is representative of the voltage drop across the body diode D _B ODY- The negative (-) input of the comparator 403 is coupled to the output of the reference generator 404. The output of the comparator 403 is coupled to the input of the off detector 407.

[0031] When the synchronous rectifier switch Q _S R is operated in the on state by the SR controller 209, the switching element 401 is closed thereby creating a pathway to transfer the energy stored in the transformer T 1. When the switching element 401 is closed, current I_Rds(ON) flows through the on-time resistor Rd _S (ON) to the electronic device causing the drain- to-source voltage drop VDS across the on-time resistor R _dS (ON ₎ . The on-resistor current

I_R _ds (ON ₎ is proportional to the secondary current I _S EC that is delivered to the electronic load. The magnitude of on-time resistor current I_R _dS (ON ₎ decreases as the energy stored in the transformer T 1 decreases.

[0032] The comparator 403 monitors the drain-to-source voltage VDS with respect to a reference signal 409 generated by the reference generator 405 during each switching cycle of the synchronous rectifier switch QSR. In one embodiment, the reference signal 409

corresponds to a voltage drop across the synchronous rectifier switch Q _S R indicative of an ideal scenario where the secondary current ISEC approaching 0 amps. When the drain-to- source voltage drop VDS is equivalent to the reference signal 409, the comparator 403 outputs a signal 41 1 that causes the off detector 407 to initiate the SR controller 209 to turn off the synchronous rectifier switch Q _S R. Accordingly, the SR controller 209 outputs an output drive signal 21 1 that operates the synchronous rectifier switch QSR in the off state. When the synchronous rectifier switch QSR is in the off state, the switching element 401 opens thereby blocking the pathway for transferring the energy stored in the transformer Tl to the capacitor CI . [0033] In one embodiment, the reference generator 405 also monitors the drain-to- source voltage VDS of the synchronous rectifier switch Q _S R during each switching cycle of the synchronous rectifier switch QSR. The reference generator 405 determines if the reference voltage 409 used to determine whether to turn off the synchronous rectifier switch QSR during a particular switching cycle resulted in the synchronous rectifier switch Q _S R being placed in the off state before substantially all of the energy stored in the transformer Tl was delivered to the output capacitor CI (i.e., too early) or if the reference voltage 409 resulted in the synchronous rectifier switch QSR being placed in the off state after a threshold amount of time from when substantially all of the energy stored in the transformer Tl is delivered to the output capacitor CI (i.e., too late). Based on the monitored voltage drop, the reference generator 405 adjusts the reference voltage 409 used to turn off the synchronous rectifier switch QSR during subsequent switching cycles thereby adjusting the length of time that the synchronous rectifier switch Q _S R is turned on during the subsequent switching cycles.

[0034] If the synchronous rectifier switch Q _S R is turned off before substantially all of the energy stored in the transformer Tl is delivered to the output capacitor CI, the remaining energy stored in the transformer T 1 is delivered to the electronic device via the body diode DBODY- In particular, body diode current I_D _B ODY starts to flow when the synchronous rectifier switch QSR is turned off in order to deliver the remaining energy stored in the transformer Tl to the output capacitor CI . As a result, power loss is increased due to the larger voltage drop across the body diode D _B ODY compared to the voltage drop across the on- time resistor R _< J _S (ON ₎ . Conversely, if the synchronous rectifier switch Q _S R is turned off after the threshold amount of time from when substantially all of the eenrgy stored in the transformer Tl is delivered to the output capacitor CI, the voltage across the transformer Tl reverses polarity and the secondary current I _S EC flows in the negative direction which results in increased noise in the power converter 200.

[0035] FIG. 5 is a circuit diagram illustrating a detailed view of the reference generator 405 according to one embodiment. The reference generator 405 includes a comparator 501 and a reference adjustment 503. The positive (+) input of the comparator 501 is coupled to the drain-to-source voltage VDS and the negative (-) input of the comparator 501 is coupled to a reference threshold voltage V_SET. In one embodiment, V_SET is the reference voltage for comparator 501 that is used to determine if the body diode D _B ODY conducts. If reference V_SET is reached, the synchronous rectifier switch QSR was turned off too early (prior to the secondary current ISEC reaching 0 AMPS) for that switching cycle. The reference voltage 409 is adjusted for the next switching cycle to ensure that the synchronous rectifier switch QSR is turned off "later" compared to the previous switching cycle. For example, the reference threshold voltage V SET is 0.1 V. The output of the comparator 501 is coupled to the reference adjust 503 which outputs the reference voltage 409 to the comparator 403.

[0036] In one embodiment, the comparator 501 monitors the drain-to-source voltage VDS during each switching cycle. The comparator 501 detects whether the synchronous rectifier switch QSR is turned off prior to the point where the secondary current reaches 0 Amps, or after the point where the secondary current reaches 0 Amps. If the comparator 401 output 505 goes "LOW" during the switching cycle, it indicates that the synchronous rectifier switch QSR was turned off too early, allowing for forward conduction through the body diode of synchronous rectifier switch Q _S R. Conversely, if the comparator 501 output 505 remains "HIGH" during the switching cycle, it indicates the synchronous rectifier switch Q _S R was turned off at the time or after the energy has been delivered to the output capacitor CI .

[0037] The reference adjust 503 determines whether the synchronous rectifier switch QSR is operated in the off state too early or too late based on the output 505 of the comparator 501. If the synchronous rectifier switch Q _S R is operated in the off state too early, the reference adjust 503 may increase the reference voltage 409 that will be used to turn off the synchronous rectifier switch Q _S R during the next switching cycle of the synchronous rectifier switch QSR. By increasing the reference voltage 409, the synchronous rectifier switch QSR is allowed to operate in the on state for a longer period of time thereby providing more time for the stored energy to be transferred to the output capacitor CI. If the synchronous rectifier switch QSR is operated in the off state after the threshold amount of time from when all the stored power is delivered to the electronic load, reference adjust 503 may decrease (i.e., lower) the reference voltage 409 that will be used to turn off the synchronous rectifier switch QSR during the next switching cycle of the synchronous rectifier switch Q _S R. By decreasing the reference voltage 409, the synchronous rectifier switch QSR operates in the on state for a shorter period of time thereby providing less time for the stored energy to be transferred to the output capacitor CI .

[0038] As mentioned previously, in embodiments where the synchronous rectifier switch QSR is coupled between the upper side of the secondary winding Ns and the output capacitor CI, the reference adjust 503 may decrease the reference voltage 409 if the synchronous rectifier switch QSR is operated in the off state too early thereby increasing the amount of time that the synchronous rectifier switch QSR is allowed to operate in the on state. Conversely, the reference adjust 503 may increase the reference voltage 409 if the synchronous rectifier switch QSR is operated in the off state too late thereby shortening the amount of time that the synchronous rectifier switch QSR is allowed to operate in the on state.

[0039] In one embodiment, the reference adjust 403 may adjust the reference voltage 409 by a fixed amount (e.g., (+/-) 0. 1 mV). Alternatively, the reference adjust 503 employs an algorithm that varies the sign and magnitude of the adjustment of the reference voltage 409.

[0040] FIG. 6 illustrates waveforms of the power converter 200 describing operation of the synchronous rectifier circuit 209 during a plurality of switching cycles of the power converter 200. In particular, FIG. 6 illustrates waveforms of the power converter 200 across multiple switching cycles when the synchronous rectifier switch Q _SR is operated in the off state before all the energy stored in the transformer Tl is delivered to the output capacitor CI (i.e., too early) and when the synchronous rectifier switch Q _S R is operated in the off state after a threshold amount of time from when substantially all the energy stored in the transformer Tl is delivered to the output capacitor CI (i.e., too late). The waveforms include an output drive signal 21 1 waveform, a reference voltage 409 waveform, a drain-to-source voltage (VDS) waveform of the synchronous rectifier switch QSR, and an on-resistor current I_Rd _S (ON) waveform.

[0041] At time Ti, the output drive signal 21 1 generated by SR controller 209 transitions high 601 thereby turning on the synchronous rectifier switch QSR. When the synchronous rectifier switch Q _S R is turned on at time Ti, the on-resistor current I_R _dS (ON ₎ instantaneously reaches a peak 603 on-resistor current I_R _dS (ON ₎ and begins to decline 607 as the stored energy in the transformer Ti is transferred to the output capacitor CI . Similarly, when the synchronous rectifier switch Q _S R is turned on at time Ti, the drain-to-source voltage VDS of the synchronous rectifier switch Q _S R instantaneously reaches a peak 609 drain-to- source voltage and begins to increase 51 1 as the energy stored in the transformer Ti is transferred to the output capacitor CI .

[0042] Comparator 403 compares the drain-to-source voltage VDS of the synchronous rectifier switch QSR with the reference voltage 409 which is at a first level 613 between time Ti and time T ₂ . The first level 613 of the reference voltage 409 indicates that the on-resistor current I_R _dS (ON ₎ (which is proportional to the secondary current I _S EC) is approaching 0 Amps. When the drain-to-source voltage VDS of the synchronous rectifier switch Q _S R is equivalent to the reference voltage 409 set by the first level 613, the off detector 407 initiates the SR controller 209 to turn off the synchronous rectifier switch QSR at time T ₂ represented by the output drive signal 21 1 transitioning low 615. However, as shown in FIG. 6, the on resistor current I_R _dS (ON ₎ at time T2 has not all been delivered to the electronic load. That is, current is still flowing through the on-time resistor R _dS (ON ₎ . Thus, the SR controller 209 turned off the synchronous rectifier switch QSR too early at time T ₂ . As a result, the remaining current is delivered to the electronic device via the body diode D _B ODY as shown by the increased drain- to-source voltage VDS 617 of the synchronous rectifier switch Q _S R between time T ₂ and time T ₃ .

[0043] As a result, the reference generator 405 increases the reference voltage 409 from the first level 613 to a second level 619 for the subsequent switching cycle defined by time T ₄ to time T5. At time T ₄ , the SR controller 209 outputs a high 601 output drive signal 21 1 thereby turning on the synchronous rectifier switch Q _S R. AS mentioned previously, when the synchronous rectifier switch QSR is turned on, the on resistor current I_Rd _S (ON) instantaneously reaches a peak 603 on resistor current I_Rd _S (ON) at time T ₄ and the on resistor current I_Rd _S (ON) begins to decline 607 as the stored energy in the transformer T 1 is transferred to the output capacitor CI . Similarly, when the synchronous rectifier switch Q _S R is turned on at time T ₄ , the drain-to-source voltage VDS of the synchronous rectifier switch Q _S R instantaneously reaches a peak 609 drain-to-source voltage VDS and begins to increase 61 1 as the stored energy in the transformer Tl is transferred to the output capacitor CI .

[0044] Comparator 403 compares the drain-to-source voltage VDS of the synchronous rectifier switch QSR with the reference voltage 409 which is at a second level 619. When the drain-to-source voltage VDS of the synchronous rectifier switch Q _S R is equivalent to the reference voltage 409 set by the second level 619, the off detector 407 initiates the SR controller 209 to turn off the synchronous rectifier switch QSR at time T5. However, as shown in FIG. 6, the on resistor current I_Rd _S (ON) reached substantially zero amps at time T6 rather than at time T5 indicating that substantially all the energy stored in the transformer T 1 was delivered to the output capacitor at time T ₆ . Thus, the SR controller 209 turned off the synchronous rectifier switch Q _S R too late. As a result, current through the on-time resistor R _ds (ON ₎ reverses polarity and energy from output capacitor CI will be stored in the transformer T 1. This energy will be transferred back to VDC after the synchronous rectifier switch QSR is turned off at the time T ₅ ..

[0045] In response to detecting that the synchronous rectifier switch Q _S R is turned off after substantially all the energy stored in the transformer Tl is delivered to the output capacitor CI (i.e., too late), the reference generator 405 lowers the reference voltage 409 from the second level 619 to a third level 625 for the subsequent switching cycle defined by time T7 to time T ₈ . Note that the third level 625 is equal to the first level 613. However, in other embodiments, the third level 625 may be lower than the second reference level 619, but greater than the first reference level 613. Thus, the SR controller 209 is hunting for the optimum reference level.

[0046] At time T ₇ , the SR controller 209 outputs a high 601 output drive signal 21 1 thereby turning on the synchronous rectifier switch Q _S R. AS mentioned previously, when the synchronous rectifier switch Q _S R is turned on, the on resistor current I_R _dS (ON ₎ instantaneously reaches a peak 603 on resistor current I_R _dS (ON ₎ at time T ₇ and begins to decline 607 as the stored energy in the transformer T 1 is transferred to the output capacitor CI . Similarly, when the synchronous rectifier switch Q _S R is turned on at time T ₇ , the drain-to-source voltage VDS of the synchronous rectifier switch Q _S R instantaneously reaches a peak 609 drain-to-source voltage and begins to increase 61 1 as the stored energy in the transformer Tl is transferred to the output capacitor CI .

[0047] When the drain-to-source voltage VDS of the synchronous rectifier switch Q _S R drops to the reference voltage 409 set by the third reference level 625, the OFF detector 407 initiates the SR controller 209 to turn off the synchronous rectifier switch QSR at time T ₈ . However, because the third level 625 is substantially equivalent to the first level 613, the SR controller 209 turned off the synchronous rectifier switch QSR too early at time T ₈ . As a result, the remaining current is delivered to the output capacitor CI via the body diode D _B ODY as shown by the increased drain-to-source voltage VDS 617 of the synchronous rectifier switch QSR between time Ts and time T9. Thus, in one embodiment the SR controller 209 may never identify the optimum reference level and may continually adjust the reference level across the plurality of switching cycles.

[0048] However, in embodiments where the third reference level 625 are less than the second reference level 619, but greater than the first reference level 613, the on resistor current I_R _dS (ON ₎ may be substantially zero amps when the synchronous rectifier switch Q _S R is turned off indicating that the energy stored in the transformer Tl has been delivered to the output capacitor CI when the synchronous rectifier switch Q _S R is turned off. Thus, the SR controller 209 turned off the synchronous rectifier switch QSR at the optimum time based on the adjusted reference voltage. Thus, in one embodiment, the SR controller 209 continually hunt across a plurality of switching cycles for the reference level that results in turning off the synchronous rectifier switch QSR at the precise time when substantially all the energy stored in the transformer Tl has been delivered to the output capacitor CI .

[0049] Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for operating the SR controller to control a synchronous rectifier switch Q _S R. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the embodiments discussed herein are not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope of the disclosure.