POWER CONVERTER CONTROLLER WITH BIAS DRIVE CIRCUIT FOR BIAS SUPPLY

A power converter controller with a bias drive circuit for bias supply is provided herein. The controller includes a primary drive circuit configured to control operation of a primary switch coupled to a primary winding associated with the energy transfer element. The primary drive circuit can cause the primary switch to transition between a conducting state during a first portion of a switching cycle and a nonconducting state during a second portion of the switching cycle. The controller also includes a bias drive circuit configured to control operation of a bias switch coupled to an auxiliary winding associated with the energy transfer element to drive a bias current to a bypass capacitor coupled to the bias drive circuit for providing a bias supply to the controller.

BACKGROUND

Field of the Disclosure

The present disclosure relates generally to power converters, and more specifically to controllers for power converters.

Discussion of the Related Art

Electronic devices use power to operate. Switched mode power converters are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power converter, a high voltage alternating current (ac) input is converted to provide a well-regulated direct current (dc) output through an energy transfer element. The switched mode power converter controller usually provides output regulation by sensing one or more signals representative of one or more output quantities and controlling the output in a closed loop. In operation, a switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period), varying the switching frequency, or varying the number of pulses per unit time of the switch in a switched mode power converter.

Power converters generally include one or more controllers that sense and regulate the output of the power converter. These controllers generally require a regulated or unregulated voltage source to power the circuit components of the controller. A capacitor, sometimes referred to as a bypass capacitor, is coupled to a controller to provide bias supply to the circuits of the controller, such that the circuits may have the appropriate voltage and/or current to operate.

DETAILED DESCRIPTION

Power converters generally include one or more controllers, which control the turn ON and turn OFF of one or more switches to regulate the output of the power converter. These controllers generally require a regulated or unregulated voltage source to power the circuit components of the controller. A bypass capacitor is one example of a voltage source that may be coupled to a controller and provide a bias supply to circuits of the controller such that the circuits may have the appropriate voltage and/or current to operate. The bypass capacitor is generally regulated to provide sufficient operating power for the controller.

An isolated power converter may include a primary controller, also referred to as a first controller or an input controller, and a secondary controller, also referred to as a second controller or an output controller, which are galvanically isolated from one another by an energy transfer element (e.g., a transformer). In other words, a direct current (dc) voltage applied between input side and output side of the power converter will produce substantially zero current.

The primary controller is configured to control a power switch on the primary side of the power converter to control the transfer of energy from the primary winding of the energy transfer element to the secondary winding of the energy transfer element. The secondary controller is coupled to circuit components on the secondary side of the isolated power converter. It should be appreciated that the primary side may also be referred to as the input side while the secondary side may be referred to as the output side. The secondary controller may also be configured to control a secondary switch coupled to the secondary winding of the energy transfer element, such as a transistor used as a synchronous rectifier for the power converter.

The primary controller may receive a signal, such as a feedback signal, representative of the output of the power converter. In response to the feedback signal, the primary controller controls the switching of the power switch to transfer energy to the secondary side. In another example, the secondary controller may transmit a signal to the primary controller, which controls how the primary controller switches the power switch to transfer energy to the secondary side.

In general, both the primary side and the secondary side of the power converter each includes a bypass capacitor to provide operating power to circuits of the primary controller or the secondary controller, respectively. The bypass capacitor for the primary controller is generally coupled to an auxiliary winding of an energy transfer element, such as a transformer or coupled inductor, and the bypass capacitor is charged from the auxiliary winding. The bias voltage (VBIAS) across the bypass capacitor is generally regulated to a sufficient level to operate circuits of the primary controller. For example, the bias voltage may be regulated to a reference voltage, such as 12 volts (V).

The auxiliary winding voltage (VAUX) is a function of the input voltage (VIN) of the power converter during the on-time of the power switch and is a function of the output voltage (VOUT) of power converter during the off-time of the power switch. For certain applications, the output voltage VOUTmay vary between a wide range of values. For example, universal serial bus (USB) Power Delivery (USB-PD) standards may require output voltages between the range of 5V to 48V or greater. The wide range of output voltages introduces a challenge for generating a low voltage bias supply for the primary and secondary controllers in a flyback power converter. The auxiliary winding of a flyback transformer is used to provide the primary controller voltage supply. However, the auxiliary winding voltage VAUXmay be proportional to the output voltage Voutwhen wound with a flyback polarity. As such, the auxiliary winding may be designed to provide enough power to the primary controller when the output voltage VOUTis at its minimum value. Consequently, when the output voltage VOUTis at its maximum voltage, the auxiliary winding voltage VAUXcan be significantly higher. As such, the auxiliary winding voltage VAUXmay vary greatly due to the wide range for the output voltage VOUT, but the bias voltage VBIASis regulated to the reference voltage. In some cases, primary controllers may require a minimum supply voltage of 12V or in some cases approximately 5V (which may be the auxiliary winding voltage VAUX) when the output voltage VOUTis 5V. In this regard, when the output voltage VOUTis at 48V, the auxiliary winding voltage VAUXmay be calculated to be about 115V (e.g., (48/5)×12=115V). This 115V potential may need to be reduced to the 12V potential required by the primary controller.

Previous approaches may have utilized a linear regulator to regulate the bias voltage VBIASto a fixed value. However, at higher output voltages, the current consumption and voltage drop across the linear regulator may lead to significant power dissipation, increasing temperature and reducing the overall efficiency of the power converter. As such, there is a need for more efficient techniques in deriving a primary controller voltage supply.

The subject technology of the present disclosure employs a technique where the auxiliary winding is designed such that it provides the minimum supply voltage required by the primary controller when VOUTis at its minimum value. For all higher values of VOUT, the primary controller employs a switch that is turned on when the primary controller requires power during the flyback period of a switching cycle. The switch may be integrated with the primary controller in some implementations, or may be external to the primary controller in other implementations. During the switch on time current is supplied to a storage capacitor. In such a scheme, for the time that the switch is on, the reflected voltage of the transformer is clamped to a voltage below that which would be generated by the output voltage. As such, substantially no energy is delivered to the output of the power converter while the switch is on. When the storage capacitor has enough charge to operate the primary controller, the switch is turned off and the reflected voltage rises to that generated by VOUTand the energy stored in the transformer is then delivered to the power converter output. The switch can be turned on and off at any time during the flyback period to deliver energy to the storage capacitor. By doing this, the auxiliary winding voltage is clamped to that of the storage capacitor for the duration of the switch on time. As such, there is substantially no voltage drop across the switch, which is in contrast with the previously-described approaches of supplying current to a primary controller involving a linear regulator.

Embodiments of the present disclosure include a bias switch coupled to the auxiliary winding and the bypass capacitor, which provides a bias supply to a controller. In another embodiment of the present disclosure, the bias switch is coupled to the output winding of the power converter and the bypass capacitor, which provides a bias supply to the output or secondary controller. The controller includes a bias drive circuit, which controls the turn ON and turn OFF of the bias switch. In example embodiments, the bias switch is turned ON in response to the bias voltage VBIASacross the bypass capacitor being below a reference. Further, the bias switch is turned ON during the off-time of the power switch. In one example, the bias switch is turned ON until the bias voltage VBIASreaches the reference. The bias switch may also be turned ON for a threshold duration of time during the off-time of the power switch.

FIG.1Aillustrates a power converter100including a first controller132(e.g. primary controller) including a bias drive circuit152to control a bias switch SB140, in accordance with embodiments of the present disclosure. The power converter100further includes a clamp circuit104, energy transfer element T1106, an input winding108of the energy transfer element T1106, an output winding110of the energy transfer element T1106, an auxiliary winding112of the energy transfer element T1106, a power switch S1114, an input return111, an output rectifier DO120, an output capacitor CO124, an output return119, an output sense circuit128, the first controller132(e.g. primary controller), a bypass capacitor CBP144(e.g. supply capacitor for the first controller132), and a diode D1138. A communication link131between the output sense circuit128and the first controller132is also illustrated. The first controller132is shown as including a primary drive circuit150and a bias drive circuit152.

Further shown inFIG.1Aare an input voltage VIN102, a switch current Ip116, a switch voltage VDS118, a secondary current Is122, an output voltage VOUT123, an output current IO125, an output quantity Uo126, a feedback signal FB130, a drive signal DR134, a current sense signal ISNS136, a bias voltage VBIAS144, a bias current IBIAS146, an auxiliary winding voltage VAUX147, and a bias switch drive signal BDR148.

In the illustrated example, the power converter100is shown as having a flyback topology, but it should be appreciated that other known topologies and configurations of power converters may also benefit from the teachings of the present disclosure. Further, the input side of power converter100is galvanically isolated from the output side of the power converter100, such that input return111is galvanically isolated from output return119. Since the input side and output side of power converter100are galvanically isolated, there is no direct current (dc) path across the isolation barrier of energy transfer element T1106, or between input winding108and output winding110, or between auxiliary winding112and output winding110, or between input return111and output return119.

The power converter100provides output power to a load127from an unregulated input voltage VIN102. In one embodiment, the input voltage VIN102is a rectified and filtered ac line voltage. In another embodiment, the input voltage VIN102is a de input voltage. The input voltage VIN102is coupled to the energy transfer element T1106. In some examples, the energy transfer element T1106may be a coupled inductor, transformer, or an inductor. The energy transfer element T1106is shown as including three windings, input winding108(also referred to as a primary winding), output winding110(also referred to as a secondary winding), and an auxiliary winding112(also referred to as a bias winding or a tertiary winding). The energy transfer element T1106is shown as having an input winding108with Npnumber of turns, the output winding110with Nsnumber of turns, and the auxiliary winding112with NAuxnumber of turns. However, the energy transfer element T1106may have more than three windings.

Coupled across the input winding108is the clamp circuit104. The clamp circuit104limits the maximum voltage on the power switch S1114. The power switch S1114is shown as coupled to the input winding108and input return111. In one example, the power switch S1114may be a transistor such as a metal-oxide-semiconductor field-effect transistor (MOSFET), bipolar junction transistor (BJT), an insulated-gate bipolar transistor (IGBT), a gallium nitride (GaN) based transistor or a silicon carbide (SiC) based transistor. In another example the power switch S1114may be a cascode switch including a normally-on first switch and a normally-off second switch coupled together in a cascode configuration. The first switch may generally be a GaN or SiC based transistor while the second switch may be a MOSFET, BJT, or IGBT.

Output winding110is coupled to the output rectifier DO120, which is exemplified as a diode. However, the output rectifier may be exemplified as a transistor used as a synchronous rectifier. Output capacitor CO124is shown as being coupled to the output rectifier DO120and the output return119. The output current IO125and output voltage VOUT123are provided to the load127. The power converter100further includes circuitry to regulate the output quantity Uo126, which in one example may be the output voltage VOUT123, output current IO125, or a combination of the two. For the example shown, the output sense circuit128is configured to sense the output quantity Uo126to provide the feedback signal FB130, representative of the output (e.g. the output quantity Uo126) of the power converter100, to the first controller132.

The first controller132receives the feedback signal FB130via a communication link131which provides galvanic isolation. The communication link131may provide galvanic isolation utilizing an inductive coupling (such as a transformer or a coupled inductor, an optocoupler), capacitive coupling, or other device could be utilized for the communication link131and maintain the galvanic isolation.

The first controller132controls the turn ON and turn OFF of the power switch S1114in response to the feedback signal FB130. As used herein, the power switch S1114that is turned ON may conduct current, and can be referred to as being transitioned into a conducting state. Accordingly, the power switch S1114that is ON can be referred to as being in the conducting state. The power switch S1114that is turned OFF may not conduct current, and can be referred to as being transitioned into a non-conducting state. Accordingly, the power switch S1114that is OFF can be referred to as being in the non-conducting state. In one example, the first controller132may be formed as part of an integrated circuit die that is manufactured as either a hybrid or monolithic integrated circuit. A portion of the power switch S1114may also be integrated in the same integrated circuit die as the first controller132or could be formed on its own integrated circuit die or dies. Further, it should be appreciated that both the first controller132and power switch S1114need not be included in a single package and may be implemented in separate controller packages or a combination of combined/separate package.

As illustrated inFIG.1A, the first controller132includes the primary drive circuit150and the bias drive circuit152. The primary drive circuit150is coupled to receive the feedback signal FB130and outputs the drive signal DR134to control the turn ON and turn OFF of the power switch S1114. For example, the primary drive circuit150may cause the primary switch S1114to transition between a conducting state during a first portion of a switching cycle and a nonconducting state during a second portion of the switching cycle. The primary drive circuit150may also receive the current sense signal ISNS136representative of the switch current ID116of the power switch S1114. The primary drive circuit150provides the primary drive signal DR134to the power switch S1114to control various switching parameters of the power switch S1114to control the transfer of energy from the input of to the output of the power converter100through the energy transfer element T1106to regulate the output of power converter100, such as the output quantity Uo126. Example of such parameters include switching frequency fSW(or switching period TSW), duty cycle, on-time and off-times, or varying the number of pulses per unit time of the power switch S1114. In addition, the power switch S1114may be controlled such that it has a fixed switching frequency or a variable switching frequency.

In one embodiment, the primary drive circuit150of the first controller132outputs the drive signal DR134to control the conduction of power switch S1114. In particular, the drive signal DR134is provided to control the turn ON of the power switch S1114in response to the feedback signal FB130. In one example, the drive signal DR134is a rectangular pulse waveform with high and low sections. High sections may correspond to the power switch S1114being ON while low sections correspond to the power switch S1114being OFF, or vice versa. While the power switch S1114is conducting, energy is stored in the energy transfer element T1106. The primary drive circuit150may control the turn OFF of the power switch S1114in response to the feedback signal FB130. In another embodiment, the primary drive circuit150may control the turn OFF of the power switch S1114in response to the switch current ID116provided by the current sense signal ISNS136reaching a current limit. It should be appreciated that other control methods may be used. For the power converter100shown inFIG.1A, when the power switch S1114is not conducting, energy stored in the energy transfer element T1106is transferred to the output winding110or to the auxiliary winding112.

Energy transfer element T1106includes the auxiliary winding112referenced to input return111. In one embodiment, the bias switch SB140is coupled to the auxiliary winding112having a same input return as the input winding108. The auxiliary winding112is shown as coupled to the diode D1138and the bypass capacitor CBP142. For the power converter100shown inFIG.1A, the bias voltage VBIAS144of the bypass capacitor CBP142can be derived from the auxiliary winding voltage VAUX147across the auxiliary winding112. Bypass capacitor CBP142is coupled to the first controller132to provide bias power for the circuits of the first controller132. In other words, the bias voltage VBIAS144is generally regulated to a sufficient level to operate circuits of the first controller132.

Bias switch SB140is shown as coupled to the bypass capacitor CBP142and the first controller132controls the turn ON and OFF of the bias switch SB140. Bypass capacitor CBP142is the voltage source for the first controller132which provides bias supply to the internal circuits of the first controller132such that the internal circuits have the appropriate voltage and/or currents to operate. As used herein, the bias switch SB140that is turned ON may conduct current, and can be referred to as being transitioned into a conducting state. Accordingly, the bias switch SB140that is ON can be referred to as being in the conducting state. The bias switch SB140that is turned OFF may not conduct current, and can be referred to as being transitioned into a non-conducting state. Accordingly, the bias switch SB140that is OFF can be referred to as being in the non-conducting state.

When the bias switch SB140is conducting, energy is transferred to the auxiliary winding112and to the bypass capacitor CBP142. Bias current IBIAS146is produced and the bypass capacitor CBP142is charged. As such, the bias voltage VBIAS144increases. The turning ON and OFF of the bias switch SB140regulates the voltage VBIAS144of the bypass capacitor CBP142such that the bypass capacitor CBP142may provide sufficient operating power for the first controller132.

The bias drive circuit152receives the bias voltage VBIAS144and outputs the bias drive signal BDR148to control the turn ON and turn OFF of the bias switch SB140. In one example, the bias drive circuit152may also receive a signal representative of the off-time of power switch S1114and outputs the bias drive signal BDR148to control the turn ON and turn OFF of the bias switch SB140. In one example, because diode D1138blocks the flow of current IBIAS146during the on-time of power switch S1114, the bias drive circuit152may output the bias drive signal BDR148to control the turn ON of the bias switch SB140if VBIAS144falls below the reference prior to the start of the off-time of power switch S1114. In other words, the bias drive circuit152may control operation of the bias switch SB140during a part of the first portion of the switching cycle. However, the bias current IBIAS146is provided to the bypass capacitor CBP142for providing a bias supply during at least part of the second portion of the switching cycle. In a further example, the bias drive circuit152may control operation of the bias switch SB140during at least part of the second portion of the switching cycle to drive the bias current IBIAS146to the bypass capacitor CBP142for providing a bias supply to the first controller132. In one example, the bias drive signal BDR148is a rectangular pulse waveform of high and low sections. High sections may correspond to the bias switch SB140being ON while low sections may correspond to the bias switch SB140being OFF, or vice versa. The bias drive circuit152may be coupled to receive the drive signal DR134as the signal representative of the off-time of power switch S1114, as shown by the dashed line. In some implementations, the bias drive circuit152can cause the bias switch SB140to transition into a conducting state during at least part of the second portion of the switching cycle based on the signal representative of the off-time of power switch S1114from the primary drive circuit150. It should be appreciated, however, that other signals may be utilized to represent the off-time of the power switch S1114. For example, the switch voltage VDs118may also be utilized to extract information regarding the off-time of the power switch S1114.

The bias drive circuit152controls the turn ON and OFF of the bias switch SB140to regulate the bias voltage VBIAS144across the bypass capacitor CBP142. For example, the bias drive circuit152can cause conduction of the bias current IBIAS146in the auxiliary winding112during at least part of the second portion of the switching cycle with the bias switch SB140in the conducting state and substantially no current is conducted in the auxiliary winding112during the first portion of the switching cycle with the bias switch SB140in the nonconducting state. In some embodiments, the bias drive circuit152can cause the bias switch SB140to transition into a conducting state during at least part of the second portion of the switching cycle based on a comparison between the bias voltage VBIAS144across the bypass capacitor CBP142and a reference. For example, the bias drive circuit152turns ON the bias switch SB140in response to the bias voltage VBIAS144falling below the reference. In some embodiments, the bias switch SB140is turned ON during the off-time of the power switch S1114. As such, at least a portion of the energy stored in the energy transfer element T1106during the on-time of power switch S1114is transferred to the bias capacitor CBP142instead of the output of the power converter100if the bias voltage VBIAS144is below the reference during the off-time of the power converter S1114. In other words, the bias drive circuit152turns ON the bias switch SB140such that the current (e.g. bias current IBIAS146) flows through the auxiliary winding112rather than through the output winding110(e.g. secondary current Is122). In some implementations, the bias switch SB140in the nonconducting state allows the secondary current Is122to flow through the output winding110. As used herein, the “on-time of power switch S1114” can refer to the “first portion of the switching cycle” and the “off-time of power switch S1114” can refer to the “second portion of the switching cycle.”

However, it should be appreciated that energy may be transferred to either the auxiliary winding112or the output winding110depending on the number of turns NAUX, Nsand the respective voltages across these windings. The turns ratio of the windings should be chosen such that the energy will be transferred to the auxiliary winding112and bypass capacitor CBP142in response to the first controller132determining that the bias voltage VBIAS144is out of regulation. Or in other words, the turns ratio of the windings should be chosen such that the energy will be transferred to the auxiliary winding112and bypass capacitor CBP142in response to the bias drive circuit152controlling the turn ON and conduction of the bias switch SB140. As such, the turns ratio (Ns, NAUX, Np) may be chosen such that the reflected voltage across the input winding108when energy is being transferred to the auxiliary winding112is lower than the reflected voltage across the input winding108when energy is being transferred to the output winding110.

For example, the reflected voltage across the input winding108when energy is being transferred to the output winding110, e.g. voltage VOR, is substantially the product of the output voltage VOUT123and the turns ratio between the input winding Npand the output winding Ns, or mathematically:

The reflected voltage across the input winding108when the energy is being transferred to the auxiliary winding112, e.g. voltage VBR, is substantially the product of the auxiliary winding voltage VAUX147and the turns ratio between the input winding Npand the auxiliary winding NAUX, or mathematically:

As such, the selection for turns Nsand NAuxmay be selected such that the ratio of the output voltage VOUT123to turns Nsis greater than the ratio of the auxiliary winding voltage VAUX147to turns NAUX, or mathematically:

In one embodiment, the bias drive circuit152is configured to cause the bias switch SB140to transition between a conducting state and a nonconducting state based on the bias voltage VBIAS144across the bypass capacitor CBP142. For example, the bias drive circuit152turns ON the bias switch SB140if the bias voltage VBIAS144falls below the reference. If the bias voltage VBIAS144falls below the reference prior to the start of the off-time of power switch S1114, the bias drive circuit152may turn ON the bias switch SB140at the beginning of the off-time of power switch S1114. In another example, the bias drive circuit152may turn ON the bias switch SB140a delay period after the beginning of the off-time of power switch S1114. In another example, because diode D1138blocks the flow of current IBIAS146during the on-time of power switch S1114, if VBIAS144falls below the reference prior to the start of the off-time of power switch S1114, the bias drive circuit152may turn ON the bias switch SB140before the beginning of the off-time of power switch S1114. However, the bias current IBIAS146flows once the power switch S1114is turned OFF. In a further embodiment, if the bias voltage VBIAS144falls below the reference during the off-time of the power switch S1114, the bias drive circuit152turns ON the bias switch SB140such that current (e.g. bias current IBIAS146) flows through the auxiliary winding112rather than the output winding110(e.g. secondary current Is122).

The bias drive circuit152turns OFF the bias switch SB140if the bias voltage VBIAS144exceeds the reference. The bias drive circuit152may also turn OFF the bias switch SB140if the bias switch SB140is ON for a threshold duration TTH. In a further example, the bias drive circuit152turns OFF the bias switch SB140if the bias current IBIAS146reaches zero. As will be further discussed, the bias drive circuit152may sense that the bias current IBIAS146has reached zero from directly sensing the bias current IBIAS146. Alternatively, the bias drive circuit152may sense that the bias current IBIAS146has reached zero by sensing the auxiliary winding voltage VAUX147. In this regard, the bias drive circuit152can cause the bias switch SB140to transition into the conducting state based on the auxiliary winding voltage VAUX147.

As such, the first controller132may regulate the bias voltage VBIAS144across the bypass capacitor CBP142to operate internal circuits of the first controller132.

FIG.1Billustrates a power converter101which is substantially similar to power converter100shown inFIG.1Aand similarly named and numbered elements couple and function as described above. At least one difference, however, is the first controller132is shown as also including the bias switch SB140. The bias switch SB140may be integrated into the same integrated circuit as the first controller132.

FIG.2Aillustrates bias drive circuit252A including a comparator254, and logic gate256shown as an AND gate. It should be appreciated that similarly named and numbered elements couple and function as described above. Bias drive circuit252A is coupled to receive the drive signal DR134, bias voltage VBIAS144, and outputs the bias drive signal BDR148.

Comparator254is coupled to receive the bias voltage VBIAS144and upper reference REF+258and lower reference REF−259. As shown, the bias voltage VBIAS144is received at the inverting input of comparator254while the upper reference REF+258and lower reference REF−259are received at the non-inverting input of comparator254. The value of upper reference REF+258is greater than the value of lower reference REF−259. The comparator254is shown as receiving two values at its non-inverting input to indicate that comparator254utilizes hysteresis. In operation, the output of comparator254is high in response to the bias voltage VBIAS144reaching or being less than the lower reference REF−259. The output of comparator254is low in response to the bias voltage VBIAS144reaching or being greater than the upper reference REF+258. In other words, the output of comparator254does not transition to a logic low value from a logic high value until the bias voltage VBIAS144has reached or is greater than the upper reference REF+258. Similarly, the output of comparator254does not transition to a logic high value from a logic low value until the bias voltage VBIAS144has fallen below the lower reference REF−258.

Logic gate256is coupled to receive the output of comparator254and the inverted drive signal DR134, as indicated by the small circle at the input of logic gate256. The output of logic gate256is the bias drive signal BDR148. For the example shown, the high sections for drive signal DR134correspond to the on-time of the power switch S1114while low sections for drive signal DR134correspond to the off-time of the power switch S1114. As such, high sections of the inverted drive signal DR134correspond to the off-time of power switch S1114while low sections correspond with the on-time of the power switch S1114.

In operation, the bias drive circuit252A outputs a high value for the bias drive signal BDR148, indicating to control the bias switch SB140ON, and outputs a low value for the bias drive signal BDR148, indicating to control bias switch SB140OFF. In some implementations, the bias drive circuit252A drives the bias drive signal BDR148to a first value that causes the bias switch SB140to transition into the conducting state when the bias voltage VBIAS144is lower than a first reference (e.g., lower reference REF−259). In this regard, the bias drive signal BDR148can be driven to the first value for a duration during which the bias voltage VBIAS144is increased towards a second reference (e.g., upper reference REF+258). In one example, the first value may represent a turn-on voltage or a turn-on current for the bias switch SB140. For example, the output of logic gate256, e.g. bias drive signal BDR148, is high to control the bias switch SB140ON if the drive signal DR134indicates the power switch S1114is OFF and the bias voltage VBIAS144has fallen below the lower reference REF−259. In some implementations, the bias drive circuit252A drives the bias drive signal BDR148to a second value smaller than the first value that causes the bias switch SB140to transition into the nonconducting state when the bias voltage VBIAS144reaches the second reference (e.g., upper reference REF+258) or the signal representative of the conducting state of the primary switch S1114from the primary drive circuit150indicates that the primary switch S1114is in the conducting state. In one example, the second value may represent a turn-off voltage or a turn-off current for the bias switch SB140. For example, the output of logic gate256, e.g. bias drive signal BDR148, is low to control the bias switch SB140OFF if the bias voltage VBIAS144has reached or is greater than the upper reference REF+258or if the drive signal DR134indicates that the power switch S1114is ON.

FIG.2Billustrates bias drive circuit252B including comparator254. It should be appreciated that similarly named and numbered elements couple and function as described above with respect toFIG.2A. Bias drive circuit252B is coupled to receive the bias voltage VBIAS144, and outputs the bias drive signal BDR148. Comparator254is coupled to receive the bias voltage VBIAS144and upper reference REF+258and lower reference REF−259. As shown, the bias voltage VBIAS144is received at the inverting input of comparator254while the upper reference REF+258and lower reference REF−259are received at the non-inverting input of comparator254. The value of upper reference REF+258is greater than the value of lower reference REF−259. The comparator254is shown as receiving two values at its non-inverting input to indicate that comparator254utilizes hysteresis. In operation, the output of comparator254(e.g. bias drive signal BDR148) is high in response to the bias voltage VBIAS144reaching or being less than the lower reference REF−259. The output of comparator254(e.g. bias drive signal BDR148) is low in response to the bias voltage VBIAS144reaching or being greater than the upper reference REF+258. In other words, the output of comparator254does not transition to a logic low value from a logic high value until the bias voltage VBIAS144has reached or is greater than the upper reference REF+258. Similarly, the output of comparator254does not transition to a logic high value from a logic low value until the bias voltage VBIAS144has fallen below the lower reference REF−258. As such, for the example shown inFIG.2A, the bias drive circuit252B outputs the bias drive signal BDR148to control the bias switch SB140ON when the bias voltage VBIAS144is less than the lower reference REF−259and outputs the bias drive signal BDR148to control the bias switch SB140OFF when the bias voltage VBIAS144reaches the upper reference REF+258.

FIG.3illustrates bias drive circuit352including a comparator254, logic gate256shown as an AND gate, and logic gate360shown as AND gate. It should be appreciated that similarly named and numbered elements couple and function as described above. Bias drive circuit352is coupled to receive the drive signal DR134, bias voltage VBIAS144, threshold duration signal TTH362, and outputs the bias drive signal BDR148.

Bias drive circuit352shares many similarities as bias drive circuits252A,252B ofFIGS.2A and2B, and similarly named and numbered elements couple and function as described above. At least one difference, however, is the logic gate360is coupled to receive the output of logic gate256and the threshold duration signal TTH362and outputs the bias drive signal BDR148. The bias drive circuit352controls the turn ON and OFF of bias switch SB140and limits the on-time of the bias switch SB140to less than or equal to a threshold period TTH. For example, the bias drive circuit352can cause the bias switch SB140to transition into the conducting state during at least part of the second portion of the switching cycle based on the threshold duration TTH corresponding to at least part of the second portion of the switching cycle. In some aspects, the threshold duration signal TTH362is representative of the on-time limit for the bias switch SB140. In other words, the threshold duration signal TTH362is representative of the threshold period TTH. For the example shown, the threshold duration signal TTH362is a rectangular pulse waveform of high and low sections. The threshold duration signal TTH362may transition to a high value coincident with the bias switch SB140turning ON. The duration of the high section may be substantially equal to a threshold period TTH. As used herein, the “on-time of bias switch SB140” can refer to the “second portion of the switching cycle” and the “off-time of bias switch SB140” can refer to the “first portion of the switching cycle.”

Logic gate360acts as a gating element which allows the output of logic gate356to pass as the bias drive signal BDR148in response the threshold duration signal TTH362. In operation, the output of logic gate360, e.g. bias drive signal BDR148, is high to control the bias switch SB140ON if the power switch S1114is turned OFF, the bias voltage VBIAS144has fallen below the lower reference REF−259, and the on-time of the bias switch SB140is less than the threshold duration TTH. The output of logic gate360, e.g. bias drive signal BDR148, is low to control the turn OFF of the bias switch SB140if the bias voltage VBIAS144has reached or is greater than the upper reference REF+258, if the drive signal DR134indicates that the power switch S1114is ON, or if the on-time of the bias switch SB140has reached the threshold duration TTH.

FIG.4illustrates bias drive circuit452including a comparator254, logic gate256shown as an AND gate, logic gate464shown as AND gate, and zero current sense circuit465. It should be appreciated that similarly named and numbered elements couple and function as described above. Bias drive circuit452is coupled to receive the drive signal DR134, bias voltage VBIAS144, and a sense signal representative of bias current IBIAS146. As shown, the sense signal representative of the bias current IBIAS146may be the bias current IBIAS146or the auxiliary winding voltage VAUX147.

Bias drive circuit452shares many similarities as bias drive circuits252A,252B ofFIGS.2A and2B, and similarly named and numbered elements couple and function as described above. At least one difference, however, is the logic gate464and the zero current sense circuit465. The bias drive circuit452controls the turn ON and OFF of the bias switch SB140and can also determine to turn OFF the bias switch SB140if the bias current IBIAS146reaches substantially zero. When the power switch S1114is OFF, a non-zero current in the auxiliary winding112or the output winding110(e.g. bias current IBIAS146or secondary current Is122) is an indication that energy is being transferred. Once the current in the auxiliary winding112or the output winding110falls to zero, there is no energy stored in the energy transfer element T1106. The bias drive circuit452senses that the energy has been transferred by sensing that the bias current IBIAS146has reached zero and turns off the bias switch SB140.

As shown, the zero current sense circuit465is coupled to receive the sense signal representative of the bias current IBIAS146, which may be the bias current IBIAS146or the auxiliary winding voltage VAUX147. The zero current sense circuit465determines if the bias current IBIAS146has reached zero and asserts an output to logic gate464in response to sensing the bias current IBIAS146.

Logic gate464is coupled to receive the output of logic gate256and the output of the zero current sense circuit465and outputs the bias drive signal BDR148. Logic gate464acts as a gating element which allows the output of logic gate256to pass as the bias drive signal BDR148in response to the output of the zero current sense circuit465. In some implementations, the bias drive circuit452drives the bias drive signal BDR148to a first value that causes the bias switch SB140to transition into the conducting state when the bias voltage VBIAS144is lower than a first reference (e.g., lower reference REF−259). In this regard, the bias drive signal BDR148can be driven to the first value for a duration during which the bias voltage VBIAS144is increased towards a second reference (e.g., upper reference REF+258) and the bias current IBIAS146has not reached zero current. For example, the output of logic gate464, e.g. bias drive signal BDR148, is high to control the bias switch SB140ON if the power switch S1114is turned OFF, the bias voltage VBIAS144has fallen below the lower reference REF−259, and the bias current IBIAS146is non-zero. In some implementations, the bias drive circuit452drives the bias drive signal BDR148to a second value smaller than the first value that causes the bias switch SB140to transition into the nonconducting state when the bias voltage VBIAS144reaches the second reference (e.g., upper reference REF+258), the signal representative of the conducting state of the primary switch S1114from the primary drive circuit150indicates that the primary switch S1114is in the conducting state, or the bias current IBIAS146has reached zero current. The output of logic gate464, e.g. bias drive signal BDR148, is low to control the turn OFF of the bias switch SB140if the bias voltage VBIAS144has reached or is greater than the upper reference REF+258, if the drive signal DR134indicates that the power switch S1114is ON, or the bias current IBIAS146has reached zero.

FIG.5illustrates bias drive circuit552including a comparator254, logic gate256shown as an AND gate, logic gate464shown as AND gate, and delay circuit578. It should be appreciated that similarly named and numbered elements couple and function as described above. Bias drive circuit552is coupled to receive the drive signal DR134and bias voltage VBIAS144.

Bias drive circuit552shares many similarities as bias drive circuits252A,252B ofFIGS.2A and2Band similarly named and numbered elements couple and function as described above. At least one difference, however, is the delay circuit578is configured to receive the drive signal DR134. The bias drive circuit552controls the turn ON and OFF of the bias switch SB140and delays the turn ON of the bias switch SB140. In particular, the bias drive circuit552controls the turn ON of the bias switch SB140such that the bias switch SB140cannot be turned on until a delay period TDELAY after the power switch S1114turns OFF. In some implementations, the bias drive circuit552can cause the bias switch SB140to transition into the conducting state during at least part of the second portion of the switching cycle based on a delayed version of the signal representative of the conducting state of the primary switch S1114from the primary drive circuit150.

As shown, the delay circuit578is coupled to receive the drive signal DR134. The inverted and delayed drive signal DR134is received by the logic gate256. Logic gate256also receives the output of comparator254. The output of logic gate256is the bias drive signal BDR148. It should be appreciated that the delay circuit578may be a leading edge delay circuit and delays leading edges in the drive signal DR134by the delay period TDELAY.

In operation, the bias drive circuit552outputs a high value for the bias drive signal BDR148, indicating to control the bias switch SB140ON, and outputs a low value for the bias drive signal BDR148, indicating to control bias switch SB140OFF. The output of logic gate256, e.g. bias drive signal BDR148, is high to control the bias switch SB140ON if the drive signal DR134indicates the power switch S1114is OFF and the bias voltage VBIAS144has fallen below the lower reference REF−259. However, the bias drive signal BDR148does not control the turn ON of the bias switch SB140until at least a delay period TDELAY after the power switch S1114turns OFF. The output of logic gate256, e.g. bias drive signal BDR148, is low to control the bias switch SB OFF if the bias voltage VBIAS144has reached or is greater than the upper reference REF+258or if the drive signal DR134indicates that the power switch S1114is ON.

It should be appreciated that the features of bias drive circuits252A,252B,352,452, and552may be used wholly or in part together.

FIG.6illustrates timing diagram600of example waveforms for the drive signal DR134, switch voltage VDS118of power switch S1114, bias voltage VBIAS144across bypass capacitor CBP142, bias drive signal BDR148, bias current IBIAS146, and secondary current Is122.FIG.6illustrates controlling the turn ON and turn OFF of the bias switch SB140in response to the bias voltage VBIAS144falling below the lower reference REF−258and reaching the upper reference REF+259. It should be appreciated thatFIG.6illustrates one example of controlling the bias switch SB140ON when the bias voltage VBIAS144falls below the lower reference REF−258and another example of the controlling the bias switch SB140ON when the bias voltage VBIAS144falls below the lower reference REF−258and the power switch S1114is controlled OFF.

For the examples shown, drive signal DR134and the bias drive signal BDR148are rectangular pulse waveforms of high and low sections. High sections correspond with the power switch S1114or bias switch SB140being ON, respectively, while low sections correspond with the power switch S1114or bias switch SB140being OFF, respectively.

At time to, the drive signal DR134transitions to a high value and the power switch S1114is controlled ON. The transition indicates the beginning of the on-time of power switch S1114and energy is stored in the energy transfer element. For the example shown, the duration between times t0and t1is the on-time of the power switch S1114and the switch voltage VDS118falls to substantially zero. Energy is stored as a current in the input winding108of the energy transfer element T1106. The bias drive signal BDR148is low and bias switch SB114is controlled OFF. Bias current IBIAS146and secondary current Is122are substantially zero, indicating no current flow in either the auxiliary winding112or the output winding110.

Between times t0and t1, the bias voltage VBIAS144is decreasing. As shown, the bias voltage VBIAS144has fallen below the lower reference REF−258.

At time t1, the drive signal DR134transitions to a low value and power switch S1114is controlled OFF. The transition indicates the beginning of the off-time of the power switch S1114and energy is transferred to either the output winding110or the auxiliary winding112. Since the bias voltage VBIAS144has fallen below the lower reference REF−258, the bias drive signal BDR148transitions to a high value to control the turn ON of bias switch SB140. The switch voltage VDs118increases and is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the auxiliary winding, e.g. voltage VBRof equation (2) above, or mathematically: VDS=VIN+VBR=VIN+NP/NAUXVAUX.

Alternatively, as shown by the dashed lines for the bias drive signal BDR148inFIG.6, the bias drive signal BDR148transitions to a logic high value to control the turn ON of bias switch SB140when the bias voltage VBIAS144has fallen below the lower reference REF−258between time t0and time t1. Although the bias switch SB140may be ON during a portion of the on-time of the power switch S1114, the diode D1138blocks the flow of current IBIAS146during the on-time of power switch S1114. The bias current IBIAS146does not flow until the power switch S1114is turned OFF. It should be appreciated that the bias drive signal BDR148may transition to a logic high value to control the turn ON of bias switch SB140any time after the bias voltage VBIAS144has fallen below the lower reference REF−258during the on-time of the power switch S1114. Further, the bias drive signal BDR148may transition to a logic high value to control the turn ON of bias switch SB140prior to the bias voltage VBIAS144falling below the lower reference REF−258during the on-time of the power switch S1114. The small bidirectional arrow shown illustrates that the transition to the logic high value may vary for the bias drive signal BDR148during the on-time of the power switch S1114for the switching cycle shown.

While the bias switch SB140is controlled ON and conducting, the energy is transferred from the energy transfer element T1106in the form of a current. A non-zero current (e.g., bias current IBIAS146) flows through auxiliary winding112, the bypass capacitor CBP142is charged, and the bias voltage VBIAS144increases. Energy is transferred to the auxiliary winding112and not the output winding110, as such the secondary current is substantially zero.

At time t2, the bias voltage VBIAS144reaches the upper reference REF+. The bias drive signal BDR148transitions to a low value and controls the bias switch SB140OFF. Energy is transferred to the output winding110and a non-zero secondary current Is122flows through output winding110. As shown, the switch voltage VDs118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the output winding110, e.g. voltage VORof equation (1) above, or mathematically: VDS=VIN+VOR=VIN+NP/NSVOUT. It should be appreciated that the reflected voltage due to the output winding110, e.g. voltage VOR, is greater than the reflected voltage due to the auxiliary winding112, e.g. voltage VBR. The bias voltage VBIAS144begins to decrease since there is no current charging the bypass capacitor CBP142.

At time t3, the secondary current Is122reaches zero, indicating that the energy previously stored in the energy transfer element T1106has been transferred. As such, ringing occurs (also referred to as a relaxation ring) on waveform VDs118due to the parasitic inductances and capacitances. For the example shown, the relaxation ring oscillates around the input voltage VIN102. At time t4, the drive signal DR134transitions to a high value to control the power switch S1114ON. The duration between times t1and t2is the off-time of the power switch S1114.

FIG.7illustrates timing diagram700of example waveforms for the drive signal DR134, switch voltage VDs118of power switch S1114, bias voltage VBIAS144across bypass capacitor CBP142, bias drive signal BDR148, bias current IBIAS146, and secondary current Is122.FIG.7illustrates controlling the turn ON of the bias switch SB140in response to the bias voltage VBIAS144falling below the lower reference REF−258and controlling the turn OFF of the bias switch SB140in response to the elapse of the threshold duration TTH after the turn ON of the bias switch SB140.

Prior to time t5, the bias voltage VBIAS114has fallen below the lower reference REF−258. At time t5, the drive signal DR134transitions low and controls the power switch S1114OFF. Since the power switch S1114is OFF and the bias voltage VBIAS114has fallen below the lower reference REF−258, the bias drive signal BDR148transitions to a high value and controls the turn ON of bias switch SB140. When the bias switch SB140is conducting, the switch voltage VDs118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the auxiliary winding. e.g. voltage VBRof equation (2) above, or mathematically: VDS=VIN+VBR.

At time t6, a threshold duration TTH662has elapsed after the turn ON of the bias switch SB140. Further, the threshold duration TTH662has elapsed before the bias voltage VBIAS114has reached the upper reference REF+. As such, the bias drive signal BDR148transitions low and controls the turn OFF of the bias switch SB140.

FIG.8illustrates timing diagram800of example waveforms for the drive signal DR134, switch voltage VDs118of power switch S1114, bias voltage VBIAS144across bypass capacitor CBP142, bias drive signal BDR148, bias current IBIAS146, and secondary current Is122.FIG.8illustrates controlling the turn ON of the bias switch SB140in response to the bias voltage VBIAS144falling below the lower reference REF−258and controlling the turn OFF of the bias switch SB140in response to a determination of no stored energy in the energy transfer element T1106. In one example, the bias current IBIAS146reaching zero indicates there is no stored energy in the energy transfer element.

Prior to time to, the bias voltage VBIAS144has fallen below the lower reference REF−258. At time to, the drive signal DR134transitions low and controls the power switch S1114OFF. Since the power switch S1114is OFF and the bias voltage VBIAS114has fallen below the lower reference REF−258, the bias drive signal BDR148transitions to a high value and controls the turn ON of bias switch SB140. When the bias switch SB140is conducting, the switch voltage VDs118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the auxiliary winding, e.g. voltage VBRof equation (2) above, or mathematically: VDS=VIN+VBR.

At time t10, the bias current IBIAS146has reached zero. As mentioned above, once the current in the auxiliary winding112(e.g. bias current IBIAS146) or the output winding110falls to zero, there is no energy stored in the energy transfer element T1106. In response to the bias current IBIAS146reaching substantially zero, indicating no energy stored in the energy transfer element T1106, the bias drive signal BDR148transitions low and controls the turn OFF of the bias switch SB140. Relaxation ringing also occurs due to the parasitic inductances and capacitances. For the example shown, no energy is transferred to the output winding110. In the example shown, the bias switch SB140is turned OFF prior to the bias voltage VBIAS144reaching the upper reference REF+259.

FIG.9illustrates timing diagram900of example waveforms for the drive signal DR134, switch voltage VDs118of power switch S1114, bias voltage VBIAS144across bypass capacitor CBP142, bias drive signal BDR148, bias current IBIAS146, and secondary current Is122.FIG.9illustrates controlling the turn ON of the bias switch SB140in response to the bias voltage VBIAS144falling below the lower reference REF−258and controlling the turn OFF of the bias switch SB140in response to a determination to turn ON the power switch S1114.

Prior to time t12, the bias voltage VBIAS144has fallen below the lower reference REF−258. At time t12, the drive signal DR134transitions low and controls the power switch S1114OFF. Since the power switch S1114is OFF and the bias voltage VBIAS114has fallen below the lower reference REF−258, the bias drive signal BDR148transitions to a high value and controls the turn ON of bias switch SB140. While the bias switch SB140is conducting, the switch voltage VDs118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the auxiliary winding, e.g. voltage VBRof equation (2) above, or mathematically: VDS=VIN+VBR.

At time t13, the drive signal DR134transitions high, indicating that the first controller has determined to turn ON the power switch S1114. In response to the determination to turn ON the power switch S1114, the bias drive signal BDR148transitions to a low value to control the turn OFF of the bias switch SB140. In the example shown, the bias switch SB140is turned OFF prior to the bias voltage VBIAS144reaching the upper reference REF+259. Since the current in the auxiliary winding112did not reach zero, there is a non-zero current present for the input winding108the next time the power switch S1114. As such, for the example shown inFIG.9, the power converter is operating in continuous conduction mode (CCM).

FIG.10illustrates timing diagram1000of example waveforms for the drive signal DR134, switch voltage VDs118of power switch S1114, bias voltage VBIAS144across bypass capacitor CBP142, bias drive signal BDR148, bias current IBIAS146, and secondary current Is122.FIG.10illustrates controlling the turn ON of the bias switch SB140in response to the bias voltage VBIAS144falling below the lower reference REF−258during the off-time of the power switch S1114and controlling the turn OFF of the bias switch SB140in response to the bias voltage VBIAS144reaching the upper reference REF+259.

At time t14, the drive signal DR134transitions to a low value, indicating the turn OFF of power switch S1114. The bias voltage VBIAS144is above the lower reference REF−258and bias drive signal BDR148remains low and controls the bias switch SB140OFF. Energy is transferred to output winding110and secondary current Is122is non-zero. The switch voltage VDS118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the output winding110as shown in equation (1), or mathematically: VDS=VIN+VOR.

At time t15, the bias voltage VBIAS144reaches the lower reference REF−258. The bias drive signal BDR148transitions high and controls the turn ON of bias switch SB140. While the bias switch SB140is conducting, the switch voltage VDs118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the auxiliary winding, e.g. voltage VBRof equation (2) above, or mathematically: VDS=VIN+VBR. As shown, the bias current IBIAS146is non-zero but the secondary current Is122is substantially zero, as the energy is now transferred to the auxiliary winding112rather than the output winding110.

At time t16, the bias voltage VBIAS144reaches the upper reference REF+259and the bias drive signal BDR148transitions to a low value to control the turn OFF of the bias switch SB140. However, the bias switch SB140was turned OFF while there is still stored energy in the energy transfer element T1106. As such, energy is delivered to output winding110and secondary current Is122is non-zero.

At time t17, secondary current Is122reaches zero and there is no stored energy in the energy transfer element T1106. As such, ringing occurs (also referred to as a relaxation ring) due to the parasitic inductances and capacitances. For the example shown, the relaxation ring oscillates around the input voltage VIN102. At time t18, the drive signal DR134transitions to a high value to control the power switch S1114ON.

FIG.11illustrates timing diagram1100of example waveforms for the drive signal DR134, switch voltage VDs118of power switch S1114, bias voltage VBIAS144across bypass capacitor CBP142, bias drive signal BDR148, bias current IBIAS146, and secondary current Is122.FIG.11illustrates controlling the turn ON of the bias switch SB140in response to the bias voltage VBIAS144falling below the lower reference REF−258during the off-time of the power switch S1114and controlling the turn OFF of the bias switch SB140in response to a determination of no stored energy in the energy transfer element T1106. In one example, the bias current IBIAS146reaching zero indicates there is no stored energy in the energy transfer element T1106.

At time t19, the drive signal DR134transitions to a low value, indicating the turn OFF of power switch S1114. The bias voltage VBIAS144is above the lower reference REF−258and bias drive signal BDR148remains low and controls the bias switch SB140OFF. Energy is transferred to output winding110and secondary current Is122is non-zero. The switch voltage VDS118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the output winding110as shown in equation (1), or mathematically: VDS=VIN+VOR.

At time t20, the bias voltage VBIAS144reaches the lower reference REF−258. The bias drive signal BDR148transitions high and controls the turn ON of bias switch SB140. While the bias switch SB140is conducting, the switch voltage VDs118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the auxiliary winding, e.g. voltage VBRof equation (2) above, or mathematically: VDS=VIN+VBR. As shown, the bias current IBIAS146is non-zero but the secondary current Is122is substantially zero, as the energy is now transferred to the auxiliary winding112rather than the output winding110.

At time t21, the bias current IBIAS146has reached zero. As mentioned above, once the current in the auxiliary winding112(e.g. bias current IBIAS146) or the output winding110falls to zero, there is no energy stored in the energy transfer element T1106. In response to the bias current IBIAS146reaching substantially zero, indicating no energy stored in the energy transfer element T1106, the bias drive signal BDR148transitions low and controls the turn OFF of the bias switch SB140. Relaxation ringing also occurs due to the parasitic inductances and capacitances. For the example shown, no energy is transferred to the output winding110. In the example shown, the bias switch SB140is turned OFF prior to the bias voltage VBIAS144reaching the upper reference REF+259.

FIG.12illustrates timing diagram1200of example waveforms for the drive signal DR134, switch voltage VDs118of power switch S1114, bias voltage VBIAS144across bypass capacitor CBP142, bias drive signal BDR148, bias current IBIAS146, and secondary current Is122.FIG.12illustrates controlling the turn ON of the bias switch SB140in response to the bias voltage VBIAS144falling below the lower reference REF−258and controlling the turn OFF of the bias switch SB140reaching the upper reference REF+259. Further,FIG.12illustrates the bias switch SB140is not turned ON until a delay period TDELAY1278has elapsed.

Prior to time t23, the bias voltage VBIAS144has fallen below the lower reference REF−258. At time t23, the drive signal DR134transitions low and controls the power switch S1114OFF. Since the power switch S1114is OFF and the bias voltage VBIAS114has fallen below the lower reference REF−258, the bias switch SB140should be controlled ON. However, the bias drive signal BDR148transitions to a high value to control the turn ON of bias switch SB140after the delay period TDELAY1278has elapsed. As such, during the delay period TDELAY1278, the energy is delivered to the output winding110and the secondary current Is122is non-zero. The switch voltage VDS118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the output winding110as shown in equation (1), or mathematically: VDS=VIN+VOR.

At time t24, the delay period TDELAY1278has elapsed and the bias drive signal BDR148transitions to a high value and controls the turn ON of bias switch SB140. Energy is transferred to auxiliary winding112rather than the output winding110. The bias current IBIAS146is non-zero while the secondary current Is122is substantially zero. When the bias switch SB140is conducting, the switch voltage VDS118is substantially the sum of the input voltage VIN102and the reflected voltage across the input winding108due to the auxiliary winding, e.g. voltage VBRof equation (2) above, or mathematically: VDS=VIN+VBR.

At time t15, the bias voltage VBIAS144has reached upper reference REF+259. Bias drive signal BDR148transitions low and controls the turn OFF of the bias switch SB140. At time t26, the drive signal DR134transitions to a high value to control the power switch S1114ON.

FIG.13illustrates a power converter1300, which is substantially similar to power converter100shown inFIG.1Aand power converter101shown inFIG.1B, and similarly named and numbered elements couple and function as described above. At least one difference, however, is the output rectifier DO1320is shown as a synchronous rectifier. Further, a second controller1367is coupled to receive the feedback signal FB130from output sense circuit128and communicates a request signal REQ1170to the first controller132. The second controller1367is configured to output a secondary drive signal SR1168to control the turn ON and turn OFF of the output rectifier DO1320.

The second controller1367is configured to output the request signal REQ1370in response to the feedback signal FB130. In another example, the second controller1367is configured to pass along the feedback signal FB130to the first controller132. For the example of a request signal REQ1370, the request signal REQ1370is representative of a request to turn ON the power switch S1114. The request signal REQ1370may include request events which are generated in response to the feedback signal FB130. In one example operation, the second controller1367is configured to compare the feedback signal FB130with a regulation reference. In response to the comparison, the second controller1367may output a request event in the request signal REQ1370to request the first controller132to turn ON the power switch S1114. The request signal REQ1370may be a rectangular pulse waveform which pulses to a logic high value and quickly returns to a logic low value. The logic high pulses may be referred to as request events. In other embodiments it is understood that request signal REQ1370could be an analog, continually varying signal, rather than a pulsed waveform, while still benefiting from the teachings of the present disclosure.

The second controller1367and the first controller132may communicate via the communication link131. For the example shown, the second controller1367is coupled to the secondary side of the power converter100and is referenced to the output return119while the first controller132is coupled to the primary side of the power converter1300and is referenced to the input return111. In some embodiments, the first controller132and the second controller1367are galvanically isolated from one another and communication link131provides galvanic isolation using an inductive coupling (such as a transformer or a coupled inductor, an optocoupler), capacitive coupling, or other device that maintains the isolation. However, it should be appreciated that in some embodiments, the second controller1367is not galvanically isolated from the first controller132. In one example, the communication link131may be an inductive coupling formed from a leadframe, which supports the first controller132and/or the second controller1367.

In one example, the first controller132and second controller1367may be formed as part of an integrated circuit that is manufactured as either a hybrid or monolithic integrated circuit. In one example, the power switch S1114may also be integrated in a single integrated circuit package with the first controller132and the second controller1367. In addition, in one example, first controller132and second controller1367may be formed as separate integrated circuit die. The power switch S1114or a portion of the power switch S1114may also be integrated in the same integrated circuit die as the first controller132or could be formed on its own integrated circuit die. Further, it should be appreciated that both the first controller132, the second controller1367and power switch S1114need not be included in a single package and may be implemented in separate controller packages or a combination of combined/separate packages.

FIG.14is a schematic diagram of an example isolated power converter1400including a second controller1467referenced to an output of the power converter1400with a bias drive circuit1452to control a bias switch1440, in accordance with embodiments of the present disclosure. The power converter1400ofFIG.14is substantially similar to power converter100shown inFIG.1Aand power converter101shown inFIG.1B, and similarly named and numbered elements couple and function as described above. At least one difference, however, is use of a winding sense signal WSNS1476that is representative of the voltage of the output winding110. The bias drive circuit1452utilizes the winding sense signal WSNS1476to determine when the power switch S1114has turned OFF. In other words, the winding sense signal WSNS1476may be utilized as a signal representative of the voltage of the output winding110.

Output winding110is coupled to output rectifier DO1420, which is exemplified as a diode. Output capacitor CO124is shown as being coupled to the output rectifier DO1420and output return119. The output current IO125and output voltage VOUT123are provided to the load127. The power converter1400further includes circuitry to regulate the output quantity Uo126, which in one example may be the output voltage VOUT123, output current IO125, or a combination of the two. For the example shown, the output sense circuit128is configured to sense the output quantity Uo126to provide the feedback signal FB130, representative of the output (e.g. the output quantity UO126) of the power converter1400, to the second controller1467. The second controller1467is coupled to receive the feedback signal FB130and communicates a request signal REQ1470to the first controller132. The second controller1467is configured to output the request signal REQ1470in response to the feedback signal FB130. In one example, the request signal REQ1470is representative of a request to turn ON the power switch S1114. The request signal REQ1470may include request events, which are generated in response to the feedback signal FB130.

The first controller132receives the request signal REQ1470via a communication link131, which provides galvanic isolation. The communication link131may provide galvanic isolation utilizing an inductive coupling (such as a transformer or a coupled inductor, an optocoupler), capacitive coupling, or other device could be utilized for the communication link131and maintain the galvanic isolation. The first controller132controls the turn ON and turn OFF of the power switch S1114in response to the request signal REQ1470. In one example, the first controller132controls the turn ON and turn OFF of the power switch S1114in response to request events in the request signal REQ1470.

In one embodiment, the first controller132outputs the drive signal DR134to control the conduction of the power switch S1114. In particular, the drive signal DR134is provided to control the turn ON of the power switch S1114in response to the request signal REQ1470. While the power switch S1114is conducting, energy is stored in the energy transfer element T1106. The first controller132may control the turn OFF of the power switch S1114in response to the feedback signal FB130. In another embodiment, the first controller132may control the turn OFF of the power switch S1114in response to the switch current ID116provided by the current sense signal ISNS136reaching a current limit. For the power converter1400shown inFIG.14, when the power switch S1114is not conducting, energy is transferred to the output winding110or to a bypass capacitor CBP1442of the second controller1467.

The second controller1467is coupled to receive the feedback signal FB130from output sense circuit128and a request circuit1472in the second controller1467communicates a request signal REQ1470to the first controller142via communication link131. The request circuit1472is configured to output the request signal REQ1470in response to the feedback signal FB130. In one embodiment, the request circuit1472compares the feedback signal FB130with a regulation reference. In response to the comparison, the request circuit1472may output a request event in the request signal REQ1470to request the first controller132to turn ON the power switch S1114.

Bias switch SB1440is shown as coupled to a bypass capacitor CBP1442. The second controller1467controls the turn ON and OFF of the bias switch SB1440. Bypass capacitor CBP1442is the voltage source for the second controller1467, which provides bias supply to the internal circuits of the second controller1467such that the internal circuits have the appropriate voltage and/or currents to operate. As used herein, the bias switch SB1440that is turned ON may conduct current, and can be referred to as being transitioned into a conducting state. Accordingly, the bias switch SB1440that is ON can be referred to as being in the conducting state. The bias switch SB1440that is turned OFF may not conduct current, and can be referred to as being transitioned into a non-conducting state. Accordingly, the bias switch SB1440that is OFF can be referred to as being in the non-conducting state.

When the bias switch SB1440is conducting, energy is redirected to the bypass capacitor CBP1442instead of to the output rectifier DO1420. In some aspects, the output rectifier DO1420is reversed biased when the bias switch SB1440is in the nonconducting state and the output rectifier DO1420is forward biased when the bias switch SB1440is in the conducting state. The turning ON and OFF of the bias switch SB1440regulates the voltage VBIAS1444of the bypass capacitor CBP1442such that the bypass capacitor CBP1442may provide sufficient operating power for the second controller1467.

The bias drive circuit1452receives the bias voltage VBIAS1444and the winding sense signal WSNS1476and outputs the bias drive signal BDR1448to control the turn ON and turn OFF of the bias switch SB1440. For example, the bias drive circuit1452may control operation of the bias switch SB1440during at least part of the second portion of the switching cycle to drive a secondary current Is122to the bypass capacitor CBP1442for providing a bias supply to the second controller1467. As shown, the secondary current Is122is the current flowing through the output winding110. When the bias switch SB1440is in the conducting state, the secondary current Is122flows to the bypass capacitor CBP1442instead of to diode DO1420and the output of the power converter1400. In some implementations, the bias drive circuit1452can cause the bias switch SB1440to transition into a conducting state during at least part of the second portion of the switching cycle based on the signal representative of the voltage of the output winding110. It should be appreciated, however, that other signals may be utilized to represent the voltage of the output winding110.

The bias drive circuit1452controls the turn ON and OFF of the bias switch SB1440to regulate the bias voltage VBIAS1444across the bypass capacitor CBP1442. For example, the bias drive circuit1452can control operation of the output rectifier DO1420coupled between the output winding110and the output capacitor CO124and/or a diode D21474coupled between the output winding110and the bias switch SB1440during at least part of the second portion of the switching cycle with the bias switch SB140in the conducting state and substantially no current is conducted through the bypass capacitor CBP1442during the first portion of the switching cycle with the bias switch SB140in the nonconducting state. In some embodiments, the bias drive circuit1452turns ON the bias switch SB1440such that the secondary current Is122flows through the diode D21474rather than to the output rectifier DO1420. In some implementations, the bias switch SB1440in the nonconducting state allows the secondary current Is122to flow to the output rectifier DO1420.

The above description of illustrated examples of the present disclosure, including what is described in the Abstract, are not intended to be exhaustive or to be limitation to the precise forms disclosed. While specific embodiments of, and examples for, the disclosure are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present disclosure. Indeed, it is appreciated that the specific example voltages, currents, frequencies, power range values, times, etc., are provided for explanation purposes and that other values may also be employed in other embodiments and examples in accordance with the teachings of the present disclosure.