Abstract:
This disclosure relates to flyback transformer-based power converters that are capable of providing multiple output voltage levels. With respect to USB-PD adapter design, the flyback converter&#39;s output may be changed from 12V to 20V—based on the charging device&#39;s request. By using a bias circuit that monitors an output voltage level of the flyback converter, a bias voltage for the bias circuit may be determined to improve efficiency of the flyback converter. Embodiments include a comparator, microcontroller or switches to compare output voltage levels and provide bias voltages to the bias circuit.

Description:
BACKGROUND 
       [0001]    This disclosure relates generally to flyback converter-based power converters that are capable of providing multiple output voltage levels in an efficient manner. Other embodiments are also described herein. 
         [0002]    Alternating current (AC) power is typically supplied from wall outlets and is sometimes referred to as line power. Electronic devices often include circuitry that runs from direct current (DC) power. AC to DC power converter circuitry can be used to convert AC power to DC power. The DC power may be used to power an electronic device that runs on DC power. The DC power may also be used to charge a battery in an electronic device. 
         [0003]    AC to DC power converters often include transformers. A transformer in an AC to DC power converter may have primary and secondary windings. A pulse width modulation (PWM) circuit on the primary side of a transformer may generate pulses of current that pass through the primary winding of the transformer. On the secondary side of the transformer, a diode may be used to rectify the output of the secondary winding. 
         [0004]    Some AC to DC power converter circuits use synchronous rectifier (SR) output stages. SR output stages may include a metal-oxide-semiconductor field-effect transistor (MOSFET). The MOSFET is driven so as to rectify the output waveform from the transformer in the same way that the diode is used in other power converter designs, while avoiding high diode voltage drops when conducting current (e.g., ˜0.7V). 
         [0005]    Certain power converter designs may have potential drawbacks. One drawback is that different electronic devices may have different voltage requirements. Bias circuits that supply bias voltages are designed at a particular operating voltage. Efficient operation at one level may result in inefficient operation at another level since the topology of the bias circuit cannot change. 
         [0006]    To attempt to deal with some of these drawbacks, some power converters may use bias circuits to control primary side and secondary side circuits. These bias circuits use voltages from one or more auxiliary transformer windings to generate control voltages to the primary and secondary side circuits. However, such configurations can result in inefficient operation in power converters that produce multiple outputvoltages. 
       SUMMARY 
       [0007]    Described herein are various devices and methods for operating improved flyback converters in which a transformer with a tapped secondary winding is used, along with one or more switching devices, to control the duty cycle and perform synchronous rectification. 
         [0008]    Flyback converters with a wide range of potential output voltages may be applicable in a number of power conversion contexts. For example, they may be particularly applicable in the context of the new Universal Serial Bus-Power Delivery (“USB-PD”) standard. The USB-PD industry standard is designed to be adaptable enough to be used for charging and data transfer to and from any device over a single cable. Because a wider range of devices will soon support the USB-PD standard, users will desire to use the same power adapter to charge all of their USB-PD compatible devices. For power adapters designed to work with the USB-PD standard, the flyback output of the adapter may need to be changed over a wide range of output voltages, based on the charging device&#39;s request. 
         [0009]    The dual output voltage requirement (e.g., 12V and 20V) from a 120 VAC voltage makes it very difficult to provide efficient operation of the flyback converter at both output voltage levels for USB-PD compatible devices. According to some embodiments, using a secondary bias circuit that has a comparator to monitor an output voltage of 12V or 20V of the flyback converter, the secondary bias voltage can be selected at a level that improves the overall efficiency of design. In one embodiment, a comparator compares a reference voltage at a non-inverting input with an inverting input that senses the output voltage so as to provide either a high output voltage level at its output or a low output voltage level at the output. The output voltage of the comparator can be used to either turn ON or turn OFF a switch connected to the output of the comparator. According to some embodiments disclosed herein, comparison of the output voltage with a reference voltage can be performed with a microcontroller or other switch(es) in lieu of a comparator. Such embodiments may include an auxiliary winding on the transformer that can be used as a primary bias voltage when the voltage level of the output is at 20V. 
         [0010]    The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above Summary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. Also, in the interest of conciseness, a given figure may be used to illustrate the features of more than one embodiment of the invention, or more than one species of the invention, and not all elements in the figure may be required for a given embodiment or species. 
           [0012]      FIG. 1  illustrates a conventional flyback converter circuit. 
           [0013]      FIG. 2  illustrates a dual-output flyback converter circuit with a secondary bias circuit in accordance with one embodiment. 
           [0014]      FIG. 3A  illustrates a timing diagram for the flyback converter circuit of  FIG. 2  and shows a transition time from one voltage level to another voltage level. 
           [0015]      FIG. 3B  illustrates a timing diagram showing bias voltages for a secondary bias circuit of the flyback converter of  FIG. 2 . 
           [0016]      FIG. 3C  illustrates a timing diagram showing inverting and non-inverting inputs to a comparator of the flyback converter of  FIG. 2 . 
           [0017]      FIG. 3D  illustrates a timing diagram showing gate to source voltages for a switch in a secondary bias circuit of  FIG. 2  in accordance with one embodiment. 
           [0018]      FIG. 3E  illustrates a timing diagram showing auxiliary winding voltages for voltage output levels of the a secondary bias circuit of the flyback converter of  FIG. 2 . 
           [0019]      FIG. 4  illustrates a embodiment of a dual-output flyback converter circuit with a secondary microcontroller capable of monitoring an output voltage level of a flyback converter. 
           [0020]      FIG. 5  illustrates a dual-output flyback converter circuit with a primary bias circuit. 
           [0021]      FIG. 6  illustrates an embodiment of a dual-output flyback converter circuit having a secondary bias circuit with back-to-back transistors to monitor output voltage levels of the flyback converter circuit. 
           [0022]      FIG. 7  illustrates an embodiment of a dual-output flyback converter circuit having a secondary bias circuit with the gate (base) and source (emitter) of switches to monitor output voltage levels of the flyback converter circuit. 
           [0023]      FIG. 8  is a block diagram of a portable device in which a flyback converter circuit can be used in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the inventive concept. 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 invention. In the interest of clarity, not all features of an actual implementation are described in this specification. 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 inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. Reference in this disclosure to “one embodiment” or to “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation of the invention, and multiple references to “one embodiment” or “an embodiment” should not be understood as necessarily all referring to the same embodiment. 
         [0025]    Turning now to  FIG. 1 , a conventional AC/DC flyback converter circuit  100  is shown. Flyback converter circuit  100  is a single stage dual-output flyback converter for providing dual Direct Current (“DC”) outputs such as, for example, 12 Volts and 20 Volts to an output load. Flyback converter circuit  100  has a transformer  105  with a primary winding P 1  (i.e., to the left of transformer  105 ) and a secondary winding S 1  (i.e., to the right of transformer  105 ). Primary winding P 1  is electrically connected to a primary side circuit  110  and secondary winding S 1  is electrically connected to secondary side circuit  115 . 
         [0026]    Primary side circuit  110  includes rectifier network  120 , a primary bias circuit  140 , a controller  150  and a primary switching mechanism Q 1  or (switch  155 ). An input power source  125 , for example, power mains that provides Alternating Current (‘AC”) power (e.g., a 120 Volt-AC or “120 VAC”) is electrically connected to rectifier network  120  to generate a rectified voltage. Rectifier network  120  can be a full-bridge diode rectifier. The rectified voltage from rectifier  120  is applied to the primary winding P 1  of the transformer  105  via smoothing capacitor  122  using switching mechanism  155 . Switching mechanism  155 , which can be a P-FET, N-FET, or any other suitable power switch, may be controlled at node A from controller  150  in order to control the duty cycle of the flyback converter  100  that is the amount of time during which the primary winding P 1  of transformer  105  (i.e., the left side, as shown in  FIG. 1 ) is connected to the primary side input power source  125  (during which time energy is stored in the primary winding P 1  of the transformer  105 ). 
         [0027]    Transformer  105  also includes a primary auxiliary winding  130  and a secondary auxiliary winding  135 . Secondary auxiliary winding  135  and primary auxiliary winding  130  are designed with a turns ratio that provides a predetermined or calculated voltage on secondary auxiliary winding  135  using energy stored in primary auxiliary winding  130 . A primary bias circuit  140  includes a diode and linear regulator (e.g., switch  142  and Zener diode D 1   144 ) that is connected to the auxiliary winding  130 . The primary bias circuit  140  receives pulsating energy stored in the primary auxiliary winding  130 . The energy, in the form of a voltage is rectified through the diode  144  and regulated through a linear regulator circuit to provide a fixed DC bias voltage (e.g., primary VCC) to controller  150  on line  152 . Bias voltage VCC provided to controller  150  may be used to control the gate contact of switch Q 1  ( 155 ). 
         [0028]    Also shown in  FIG. 1 , a secondary side circuit  115  includes a secondary bias circuit  145 , switching mechanism  160  (or switch  160 ), FET driver  165  and Housing and communication circuits  170 . Secondary bias circuit  145  is substantially similar to primary bias circuit  140  and a diode and linear regulator (e.g., switch  146  and Zener diode D 8   148 ) that is connected to the secondary auxiliary winding  135 . The secondary bias circuit  145  receives energy stored in the secondary auxiliary winding  135 . The energy, in the form of a voltage is rectified through the diode and regulated through a linear regulator circuit to provide a fixed DC bias voltage (e.g., secondary VCC) on line  154 . The DC voltage on line  154  is provided to FET Driver  165  and Communication Circuit  170 . Bias voltage VCC provided to Driver  165  may be used to control the gate contact of switch Q 3  ( 160 ). Communication circuit  170  includes circuits for handshaking and/or communicating with devices connected to converter circuit  100  as load devices. The circuit  170  transmits output voltage and signals to devices connected to load. Communication circuits  170  also include overcurrent and overvoltage protection circuits. 
         [0029]    During operation of converter circuit  100 , when the switch Q 1   155  is closed, the primary winding P 1  of the transformer  105  is directly connected to the input voltage source  125 . The primary current and magnetic flux in the transformer  105  increases, thereby storing energy in the transformer  105 . A voltage may also be induced in the secondary winding S 1  of the transformer  105 . Switch  160 , which may, e.g., be a N-FET, may be used to connect the secondary winding S 1  of transformer  105  to secondary side circuit  115  allowing current to flow from the transformer  105  to the output load and output capacitor  175 . Turning ON switch  160  closes the secondary circuit and causes the energy of the secondary winding to charge the output capacitor  175  and supply energy to the output load. The output voltage, Vout, may be measured at point  180  on Vbus. The energy from the transformer core thus recharges the capacitor and supplies power to the secondary system load. Further, auxiliary windings  130  and  135  are also energized to supply energy to primary and secondary bias circuits  140 ,  145 . Primary and secondary bias circuit  140 ,  145  may convert the energy to provide a fixed DC bias voltage to a controller  150 , on primary side circuit  110  and driver  165 , on secondary side circuit  115  in order to drive respective switches  155 ,  160 . 
         [0030]    Some of the challenges associated with the design of the flyback converter circuit  100  of  FIG. 1  include limiting power losses in the circuit  100  when output voltage on Vbus varies between the dual outputs of 12 V and 20 V. As output voltage is switched between 12 Volt (hereinafter “V”) and 20 V, the auxiliary winding voltages also change due to the turns ratio. Changing the auxiliary winding voltages results in increased power dissipation losses in the bias circuits and degradation in the efficiency of the flyback converter circuit  100 . 
         [0031]    Turning now to  FIG. 2 , the conventional flyback converter circuit  100  of  FIG. 1  has been modified to overcome some of its drawbacks in the form of an AC/DC flyback power converter circuit  200 . The flyback power converter circuit  200  includes a flyback converter  205  that can provide an output load voltage of either 12V DC or 20 V DC voltage while improving efficiency by at least 5% at light load. The power converter circuit  200  includes a comparator  254  in a secondary bias circuit that monitors output voltage and controls a bias on an auxiliary winding of a flyback transformer  215  using the monitored output voltage, while other circuits depicted in  FIG. 1  are not shown for clarity such as, for example, a communication circuit  170 . Flyback circuit  200  may also include the communication circuit  170  of  FIG. 1  for providing handshaking between the circuit  200  and an external device such as, for example, external or output load  252 . 
         [0032]    As shown in  FIG. 2 , flyback converter circuit  200  includes a primary side circuit including a rectifier  220  (e.g., a full-bridge diode rectifier), smoothing capacitors  218 , switch  230  and controller  225 . In the illustrated embodiment, rectifier  220  rectifies AC voltage (e.g., 120 VAC) using full-bridge rectification. In another embodiment, a half-bridge rectifier may be used as rectifier  220 . The rectified voltage may be provided to a primary winding (i.e., the left side winding of transformer  215 , as shown in  FIG. 2 ) through one or more smoothing capacitors  218 . A switch  230  connected to the primary winding may receive a controlling voltage from controller  225  to close the primary side circuit and directly connect the primary winding to the rectifier  220 . Switch  230 , which may comprise a P-FET, N-FET, or any other suitable power switch, and may control the duty cycle of the flyback converter  200 ; that is, the amount of time during which the primary winding P 1  of transformer  215  (i.e., the left side, as shown in  FIG. 2 ) is connected to the primary side input rectifier  220  using, for example, a pulse width modulated signal, e.g., a PWM signal, thus storing energy in the primary winding P 1  of the transformer  215 . 
         [0033]    Illustrative flyback transformer  215  includes primary and secondary windings and an auxiliary winding. The turns ratio between the windings can be selected to generate winding voltages on each of the secondary winding and the auxiliary winding of transformer  215 . In an embodiment, the turns ratio of the auxiliary winding may be selected to generate 7.5 Volts at point  280  when a 12 Volt output voltage at the load is generated and to generate 13 Volts at point  280  when a 20V output voltage at the load is generated. The secondary winding (i.e., the right side winding shown in  FIG. 2 ) of transformer  215  may be connected to a switch  216  and one or more ripple filtering capacitors  250 . In addition, the circuit  250  may be connected to an output load  252  through the secondary winding. Switch  216  may receive a controlling voltage from a controller (not shown) in order to close the secondary side circuit and directly connect the secondary winding to the output load  252 . Closing the secondary switch  216  with a control signal at node B provides an output voltage to a load  252  at line  256  that may be connected to the circuit  200 . In an embodiment, the primary winding may be controlled to provide dual-output DC voltages at line  256 . In one non-limiting example, the output voltage provided at line  256  can be either a 12V DC output or a 20V DC output. 
         [0034]    Secondary side circuit also includes a secondary bias circuit  210  having a comparator  254 , switch  258  electrically connected to a diode D 3   240  and capacitor  245 . The comparator  254  includes a non-inverting input  262  that may be connected to a Zener diode D 9 , measured at line  268 , which is the reference voltage of the comparator  254 . An inverting input  264  is connected to Vout at line  256  through a resistor divider of resistors R 22  and R 24 . An output  266  of comparator  254  may be connected to the gate of switch Q 3  ( 258 ) through resistor R 18 . Further, drain of switch  258  can be connected to Vout at line  256  and source can be connected to diode D 8  ( 260 ). The comparator  264  may be configured to monitor an output voltage Vout measured at line  256  and provide a bias voltage (Secondary VCC). A diode  260  may act as a switch between the source terminal of switch  258  and line  282 . Line  282  may provide a bias voltage  282  (i.e., secondary voltage VCC) from the secondary auxiliary winding  235  of the flyback transformer  215 . Based on a comparison of the non-inverting and inverting inputs  262  and  264 , respectively, an output  266  of comparator  254  may provide a driving voltage to the gate (Vgs) of switch  258  (See  FIG. 3D ) to either turn ON or turn OFF the switch  258 . 
         [0035]      FIG. 3A  depicts a timing diagram  300  for flyback converter circuit  200  during generation of dual output voltages, for example, during generation of a 12V output  305  or  315  or generation of a 20V output  310 . Also shown in the diagram, the flyback converter illustrates a relatively short transition period  320  or  325  while the output voltage is transitioning from 12V to 20V or from 20V to 12V such as, for example, when a 12V device, for example, a tablet computer such as an iPad® from Apple is unplugged from flyback converter  200  and a 20V device, for example, a laptop computer such as a MacBook® from Apple is plugged into converter  200  (IPAD and MACBOOK are registered trademarks of Apple Inc.). The transition period  320  or  325  represents the time period for handshaking that occurs between the communications circuits in the converter and an external device (e.g., represented by load device  252  in  FIG. 2 ). 
         [0036]    With reference to  FIGS. 2 and 3B-3D , comparator  254  may monitor output voltage of flyback converter circuit  200  and provide control voltages that control switching of the secondary bias circuit  210 . In an embodiment, during 12V operation where flyback converter circuit  200  is supplying 12 V DC to line  256  (i.e., Vout=12V), 12V DC may also be supplied to R 26 , drain of Q 3   258 , and resistor R 21 . Voltage at non-inverting input  262  can be, for example, 3.6V from the Zener diode D 9  and voltage of inverting input  264  may be given as (Vout×R 24 )/(R 22 +R 24 ) (See  FIG. 3C ). The Resistors R 22  and R 24  may be selected so that the Voltage at inverting input  264  is approximately 2.6V (See  FIG. 3C ) for Vout=12V. Since, in such an embodiment, the voltage of non-inverting input  262  (3.6V) is greater than voltage of inverting input  264  (2.6V), comparator output  266  is high, which provides a voltage to gate  284  to turn ON switch  258  and provide approximately 11V to the secondary VCC  282 . As VCC  282  of 11V is greater than auxiliary winding voltage  280  of 7.8 V (shown in  FIG. 3C ), diode D 3   240  is reversed biased and does not allow auxiliary winding voltage to be transmitted to secondary VCC  282 . 
         [0037]    In an embodiment, during 20V operation where flyback converter circuit  200  is supplying a 20V DC to line  256  (i.e., Vout=20V), 20V at line  256  is supplied to R 26 , Drain of Q 3   258 , and R 21 . Voltage at Non-inverting input  262  of comparator  254  is 3.6V from the Zener diode D 9 . Voltage of inverting input  264  of comparator  254  may be given by (Vout×R 24 )/(R 22 +R 24 ) (See  FIG. 3C ). As shown in  FIG. 3C , the voltage at inverting input  264  of comparator  254  is 4.6V for Vout=20V. Since the voltage at non-inverting input  262  (3.6V) is less than the voltage at inverting input  264  (4.6V), the output voltage of comparator  266  is low, which turns OFF the switch  258 , causing diode D 3   240  to be forward biased (i.e., ON). This, in turn, can cause the auxiliary winding voltage  280 , which is a PWM waveform with a peak voltage of 12.5V to be rectified through diode D 3   240  and to be applied to the secondary VCC  282 . 
         [0038]      FIG. 4  illustrates an embodiment of flyback converter circuit  200  of  FIG. 2  where comparator  254  and its associated components such as Zener diode D 9 , resistors R 21 , R 22  are replaced with a secondary microcontroller  405 , while all other features and functions of flyback converter circuit  400  remain substantially the same as flyback converter circuit  200  of  FIG. 2 . Controller  415  controls switch Q with a driving signal at node C. Microcontroller  405  includes circuits that perform handshaking, overvoltage protection, overcurrent protection, over power protection. Microcontroller  405  also includes a reference that compares the output voltage  410  at the load with a reference voltage. Using a microcontroller  405  saves space in the housing of the flyback converter circuit  400  as well as reduces component count. In operation, when Vout at line  410  is 12V, switch Q 3  is turned OFF and when Vout at line  410  is 20V, switch Q 3  is turned ON. 
         [0039]      FIG. 5  illustrates another embodiment of flyback converter circuit  200  of  FIG. 2  where comparator  254  and its associated components such as Zener diode D 9 , resistors R 22 , R 21 , R 24  and capacitor C 12  in a secondary bias circuit of  FIG. 2  are provided by a primary bias circuit  505 . The primary bias circuit  505  operates to the flyback converter circuit&#39;s  500  efficiency at light loads when compared to conventional flyback converter circuits (e.g., see  FIG. 1 ). 
         [0040]    Primary bias circuit  505  can include primary auxiliary windings P 2  and P 3 . The primary auxiliary winding P 2  may be used as the primary bias voltage when the output voltage at line  530  is 12V. The second auxiliary winding P 3  of the transformer may be used as the primary bias voltage when the flyback converter is producing the second output voltage 20V. In an embodiment, the second auxiliary winding P 3  has a smaller number of turns than the first auxiliary winding P 2 . The amplitude of the voltage provided by P 2  is equal to 20V and the amplitude of the voltage provided by P 3  is equal to 12V. Similar to the embodiment shown and described in  FIG. 2 , comparator  510  may monitor voltage on line  515  using resistor divider network formed by resistors R 8  and R 13  and Zener diode D 2 . The comparator  510  may compare the non-inverting voltage  520  with the inverting voltage  525  to determine which winding, P 2  or P 3 , to use to provide the primary bias voltage VCC. The primary bias voltage VCC can be approximately 11V for output voltages of 12V and 20V. 
         [0041]      FIG. 6  illustrates an embodiment of flyback converter circuit  600  that is similar to the embodiment of flyback converter circuit  200  described in  FIG. 2  but with the comparator circuit of  FIG. 2  replaced with switching mechanism or switches Q 4  and Q 5  in the secondary bias circuit  605 . Switching mechanism Q 4  and Q 5 , can be a NPN transistor, PNP transistor, or any other suitable power switch. As shown, the base  620  of switch Q 5  has a voltage that is set by the Zener diode D 9 . In an example, the Zener diode has a voltage of approximately 3.6V. 
         [0042]    In operation, when the flyback converter output voltage is 12V at line  610 , the voltage at the base  615  of the switch Q 4  is lower than the voltage at the base  620  of the switch Q 5  because of a back-to-back connection of switches Q 4  and Q 5 . As a result, switch Q 4  is turned OFF and the collector voltage of switch Q 4  at line  625  is high, which provides a gate voltage so as to turn switch Q 3  ON. Turning switch Q 3  ON causes Vout at line  610  to forward bias diode D 8  and reverse block the diode D 3  and provide a secondary VCC of approximately 11.3V at line  630 . 
         [0043]    In addition, when the flyback converter output voltage is 20V at line  610 , the voltage of the base  615  of switch Q 4  is higher than the voltage of the base  620  of switch Q 5  due to the back-to-back connection of switches Q 4  and Q 5 . As a result, switch Q 4  is turned ON and the collector voltage at line  625  of switch Q 4  is low, turning OFF the switch Q 3  and breaking the connection between line  610  and line  630 . The secondary VCC is then provided by the voltage of the Auxiliary winding S 2  rectified by diode D 3  and filtered by the capacitor C 6 . 
         [0044]      FIG. 7  illustrates an embodiment of flyback converter circuit  700  that is also similar to the embodiment of flyback converter circuit  200  described in  FIG. 2  but with the comparator circuit of  FIG. 2  replaced with switching mechanism or switches Q 3  and Q 5  in the secondary bias circuit  705 . Switching mechanism Q 3  and Q 5 , can be a P-FET, N-FET, JFET, or any other suitable power switch. 
         [0045]    In operation, when the Flyback converter circuit  700  output voltage at line  710  is 12V, the voltage at the gate  715  of the N-channel MOSFET Q 5  is lower than that of the source  720  of the N-channel MOSFET Q 5  which is the fixed reference voltage set the by the Zener diode D 9 . As a result, the switch Q 5  is turned OFF and the switch Q 3  is fully ON. The diode D 3  is reverse blocked. The secondary VCC  725  is provided by the 12V output on line  710 . 
         [0046]    When the Flyback converter output voltage is 20V at line  710 , the voltage of the gate  715  of the N-channel MOSFET Q 5  is higher than that of the source  720  of the N-channel MOSFET Q 5 . As a result, the switch Q 5  is turned ON pulling the gate  730  of the switch Q 3  to a lower voltage which turns OFF Q 3 . The secondary VCC  725  is provided by the voltage of the Auxiliary winding S 2  rectified by diode D 3  and filtered out by the capacitor C 6 . 
         [0047]    Turning now to  FIG. 8 , an example portable electronic device  800  in which an embodiment of the invention may be implemented is shown. While some of the benefits of the invention are more apparent in such power consumption-sensitive devices, an embodiment of the invention may also find use in non-portable electronic devices, such as desktop computers. The portable device  800  shown has an external or outer housing (shown in dotted lines) in which a number of its constituent sub-systems may be installed, including, in this example, an applications processor  802 , a cellular network RF interface  804 , a digital camera  806 , a touch screen  808  a proximity sensor  810  and an inertial sensor  812 . These sub-systems may be found in a typical smart phone or tablet computer that also contains a rechargeable battery  814  to power all of the sub-systems shown. In other portable devices, some of these sub-systems may be absent. One or more of the illustrated sub-systems may be powered by an output node of a power conversion circuit with a flyback converter as described above (e.g., one of nodes  256 ,  410 ,  530 ,  610  and  710 ). 
         [0048]    The power conversion circuit  816  may use a secondary bias circuit that has a comparator to monitor an output voltage of the flyback converter (e.g., 12V or 20V), the secondary bias voltage can be selected at a level that improves the overall efficiency of design at light load conditions. In an embodiment, a comparator compares a reference voltage at a non-inverting input and senses the output voltage at an inverting input to provide either a high output voltage level or a low output voltage level. The output voltage of the comparator can also be used to either turn ON or turn OFF a switch connected to the output of the comparator. According to some embodiments disclosed herein, comparison of the output voltage with a reference voltage can be performed with a microcontroller or other switches in lieu of a comparator. Such embodiments may include an auxiliary winding on the flyback transformer that can be used as a primary bias voltage when the voltage level of the output is set high (e.g., at 20V). 
         [0049]    It is to be understood that the above description is intended to be illustrative, and not restrictive. The material has been presented to enable any person skilled in the art to make and use the invention as claimed and is provided in the context of particular embodiments, variations of which will be readily apparent to those skilled in the art (e.g., some of the disclosed embodiments may be used in combination with each other). In addition, it will be understood that some of the operations identified herein may be performed in different orders. The scope of the invention therefore should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.