BACKGROUND

[0001] Power converters can be implemented in a variety of ways. One type of power converter is a flyback converter. Like most switching converters, a flyback converter includes a controller that implements pulse width modulation (PWM) of a transistor coupled to the primary' winding of a transformer. The output voltage from a flyback converter is derived from the secondary winding of the transformer. An auxiliary voltage also derived from the secondary side of the transformer (via an auxiliary winding) provides the operating voltage to the controller. Most, if not all, flyback converters include a start-up circuit to provide the operating voltage to the controller from the input voltage of the flyback converter until the controller is then able to begin the PWM switching cycles of the transistor. At that point, the start-up circuit is deactivated, and the controller continues operating based on the auxiliary voltage derived from the secondary side of the transformer. Some flyback converters also implement switch node sensing via a switch node sensing circuit to help reduce switching losses associated with the transistor that is controlled by the controller. The startup circuit and switch node sensing circuits are separate circuits. Another trend for flyback converters is that the input voltage to the converter is being pushed to higher and higher levels. In some examples, the input voltage may be in the range of 1000 V, or even higher. Such high input voltages can represent a design challenge for flyback converters.

SUMMARY

[0002] In one example, a power converter includes a power stage to provide a current through a primary winding of a transformer in response to a PWM signal and to induce a current in a secondary winding of the transformer to generate an output voltage. The power stage has a switching terminal. The power converter also includes a controller, a clamp circuit, and an impedance device. The controller includes a first transistor coupled with a second transistor to initiate an operational voltage during a startup mode and to provide a control voltage based on an amplitude of a switching voltage at the switching terminal during a switching mode. The clamp circuit couples between the control input of the first transistor and a reference terminal and clamps a voltage at the first control input responsive to the switching voltage exceeding a clamp voltage. The impedance device couples between the switching terminal and the clamp circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0003] FIG. l is a schematic diagram of an example power converter having separate startup and switching terminal sense circuits.

[0004] FIG. 2 is a schematic diagram of an example power converter having a circuit that implements both the startup and switching terminal sense functions with multiple transistors to accommodate higher levels of an input voltage.

[0005] FIG. 3 is a schematic diagram of a switching controller usable within the power converter of FIG. 2, in an example.

[0006] FIG. 4 is a schematic diagram of a switching controller usable within the power converter of FIG. 2, in another example.

[0007] FIG. 5 is a graph illustrating various voltages within the switching controller of FIG. 4, in an example.

[0008] FIG. 6 is a schematic diagram of a switching controller that implements a startup function, but not a switching terminal sense function, in an example.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0009] The same reference numbers or other reference designators are used in the drawings to designate the same or similar (either by function and/or structure) features.

[0010] T 'his disclosure generally relates to a power converter with switching control. The power converter generates an output voltage in response to an input voltage. As an example, the power converter can be configured as a flyback power converter. The power converter includes a gate driver stage that generates one or more switching signals in response to a respective one or more pulsewidth modulation (PWM) signals In one example, the gate driver generates a first switching signal in response to a first PWM signal and a second switching signal in response to a second PWM signal . The power converter also may include a power stage. The power stage includes a transformer and a first switch (e.g., transistor) controlled by the switching signal (s) (e.g., first and second switching signals). As an example, the first switch can be activated via the first switching signal to provide a primary current through a primary winding of the transformer, such as from an input voltage to a reference terminal (e.g., ground). The primary current in the primary winding may induce a secondary current in a secondary winding of the transformer. A rectifier may be coupled to the secondary winding to rectify the voltage of the secondary winding to thereby generate an output voltage A second switch in the power stage may be activated via the second switching signal to circulate a magnetizing current associated with the transformer in a circuit path that includes an LC resonator.

[0011] In addition, the power converter may include a controller that generates the PWM signal(s) based on a feedback signal from the power stage. In one example, the controller includes a comparator that is configured to generate the PWM signal in response to a comparison of the amplitude of a control voltage and a predetermined switching threshold voltage. Furthermore, the power converter may include multiple control transistors that are coupled in series and to a switching terminal within the power stage. The multiple control transistors are usable to initiate an operational voltage for the switching controller during a startup mode. A startup mode is the state of the power converter in which the input voltage is increasing from 0V towards its steady state level (e.g., 800V, 1000V, etc.) and before the switching controller begins implementing PWM switching cycles within the power stage. In some examples, the multiple control transistors are also used to implement switching terminal voltage sensing. The controller provides the control voltage based on an amplitude of the switching voltage that is associated with the switching terminal. In one example, the predetermined switching threshold voltage has a low amplitude (e.g., less than 1 volt), such that the comparison provides the PWM signal to implement an approximately zero-volt switching (ZVS) of the first switch to reduce switching losses associated with the first switch. Accordingly, in some examples the multiple control transistors are usable io provide (a) a startup operational voltage tor the switching controller and, for flyback converters that need switching terminal sensing, (b) a switching terminal sensing function (e.g., zero-voltage switching of the first switch)

[0012] FIG. 1 is a schematic diagram of an example power converter 100. In this example, power converter 100 is a flyback converter. An input voltage VIN is provided at the input terminal 101 of the power converter, and the power converter produces an output voltage VOUT at the output terminal 155. Power converter 100 includes a power stage 154, a switching controller 110, a switching terminal sense circuit 120, and a startup circuit 130. The power stage includes a transformer 56, a transistor Qi, diodes Di, Zi (Zi is a Zener diode), DOUT, and DAUX, capacitors CF, and CVDD. Transformer 56 includes a primary winding Wp, a secondary winding Ws, and an auxiliary winding WA. In this example, transistor Qi is an n-channel field effect transistor (NFET) and is coupled between one terminal of the primary winding Wp and a reference terminal 103 (e.g., ground). The switching controller 110 has an output 111 coupled to the gate of transistor Qi. The switching controller 110 produces a control signal, PWM, at its output 111 to control the ON and OFF state of transistor Qi in accordance with the target duty cycle of the PWM control signal.

[00131 The drain of transistor Qi is coupled to the primary winding Wp at a switching terminal 58. The voltage on the switching terminal 58 is switching voltage Vsw. The switching terminal 58 is also coupled to the switching terminal sense circuit 120. Based on the switching voltage Vsw on the switching terminal 58, the switching terminal sense circuit 120 generates a control signal 121 to the switching controller 110 to indicate when the voltage Vsw is close to or equal to 0V. The switching controller 110 thus can determine when the switching voltage Vsw is below a threshold (e.g., close to or equal to the voltage of the reference terminal 103) so that the controller can activate (turn ON) transistor Qi when the drain-to-source voltage (Vds) of transistor Qi is close to or equal to 0V thereby reducing switching losses associated with transistor Qi.

[0014] The anode of diode DOUT is coupled to the secondary winding Ws. Capacitor CF is coupled between the cathode of diode DOUT and the secondary side’s reference terminal 133. Diode DOUT rectifies the voltage from the secondary winding Ws to produce an output voltage VOUT on the power converter’s output terminal 155. The anode of diode DAUX is coupled to the auxiliary winding WA. Capacitor CVDD is coupled between the cathode of diode DAUX and the secondary side’s reference terminal 133. Diode DAUX rectifies the voltage from the auxiliary winding WA to produce a voltage VDD, which is provided to the switching controller 110 as its operating voltage during the normal operating mode. The normal operating mode refers to the mode in which the switching controller 110 is powered on controlling the ON and OFF state of transistor QI in accordance with the PWM switching cycles of the PWM signal.

[0015] Voltages VOUT and VDD are only available if the switching controller 110 is powered on and thus able to produce the PWM control signal to the gate of transistor Qi to thereby generate the switching voltage Vsw to transfer energy through the transformer 56. The startup circuit 130 is coupled between the input terminal 101 and switching controller 110. During startup of the power converter 100, the startup circuit provides a voltage VDD_SU to power on the controller 110. Once the switching controller 110 is powered on with the help of the startup circuit 130, the switching controller 110 can begin providing the PWM control signal to the gate of transistor Qi to cause energy transfer through the transformer 56. The energy transferred through the transformer 56 can be used to produce the output voltage VOUT and the operating voltage VDD for the switching controller 110. The startup circuit 130 then can be deactivated. [0016] The startup circuit 130 is a separate circuit from the switching terminal sense circuit 120. These two separate circuits unfortunately occupy considerable area on a semiconductor die on which the power converter 100 is fabricated.

[0017] FIG. 2 illustrates an example of a power converter circuit 250, which addresses one or more of the problems described above. The power converter circuit 250 converts a DC input voltage VIN (e.g., a rectified AC input voltage) at an input terminal 201 to a DC output voltage VOUT at an output terminal 255. Some or all the components of the power converter circuit 250 can be implemented in any of a variety of ways, such as on an integrated circuit ((C). In the example of FIG. 2, the power converter circuit 250 includes a gate driver 252, a power stage 254, and a switching controller 260. The power stage 254 includes a transformer 256 having a primary winding Wp, a secondary winding Ws, and an auxiliary winding WA. The power stage 254 also includes an input inductor LK corresponding to a leakage inductance of the transformer 256, a resonant capacitor Ci, and a pair of switches shown in the example of FIG. 2 as transistors Qj and Q?„ In this example, transistors Qi and Qz are NFETs. The source of transistor Qz is coupled to the drain of transistor Qi and to the primary wdnding WP at a switching terminal 258. The voltage on the switching terminal 258 is switching voltage Vsw. The anode of diode Dour is coupled to the secondary winding. Capacitor CF is coupled between the cathode of diode Dour and the reference terminal 133. The voltage across capacitor CF is the output voltage VOUT from the power converter circuit 250. The anode of the diode DAUX is coupled to the auxiliary winding WA, and capacitor CVDD is coupled between the cathode of diode DAUX and the reference terminal 133 The voltage across capacitor CVDD is voltage VDD and is provided to the switching controller 260 for its operating voltage during the normal operating mode.

[0018] The switching controller 2.60 includes a comparator 262 and multiple control transistors 264. The switching controller 260 is coupled to the output terminal 255, the capacitor CVDD, and the switching terminal 258. The switching controller produces the signals PWMi and PWM2 to inputs of the gate driver 252. Outputs of the gate driver 252 are coupled to the gates of transistors Qi and Qz and provide switching signals SSi and SSz to the respective gates of transistors Qi and Qz, as shown. Accordingly, transistor Qi is controlled via the switching signal SSi, and transistor Qz is controlled via the switching signal SS?..

[0019] The power converter circuit 250 implements repeating PWM switching cycles. Each switching cycle includes at least a first switching phase and a second switching phase During the first switching phase, the switching controller 260 causes transistor Qito activate (turn ON) into the saturation region via the first switching signal SSi to cause a primary current Ip to flow from the input terminal 201 (Vin) through the input inductor LK, the primary winding WP, and transistor Qi to the reference terminal 103. In response to the primary current Ip in the primary winding Wp, a secondary current Is is induced in the secondary winding Wsand flows through diode Dour to generate the output voltage VOUT across a load (not shown) and capacitor CF.

[0020] L Jpon deactivation of transistor Qi via the first switching signal SSi, and after a short predetermined dead-time, during a second switching phase, the switching controller 260 causes transistor Qato activate via the gate driver’s switching signal SSz to, along with the secondary winding Ws, output voltage VOUT, and diode Dour, cause a magnetizing current, which is a component of the primary winding current Ip, to decay to zero amperes, and then to reverse direction by the voltage of die resonant capacitor Ci, The negative magnetizing current , following the deactivation of transistor Q?, discharges the junction capacitance of transistor Qi and charges the junction capacitance of transi sior Qt As the charge of transi stor Qi is removed, the Vds of transistor Qi reduces to approximately zero volts before the switching controller 260 again activates transistor Qi. As described above, activating transistor Qi when its drain-to-source voltage is approximately zero volts is referred to as zero voltage switching (ZVS). As a result of the switching controller 260 implementing ZVS, which is useful by, for example, active clamp flyback and ZVS flyback converters, the power converter circuit 250 has relatively low switching losses associated with transistor Qp During the activation time of transistor Qz, the leakage inductance Lxand the capacitor Ct form the aforementioned LC resonant tank to circulate the leakage inductance LK energy to the output, such that clamping losses can also be efficiently reduced.

[0021] The magnitude of voltage VOUT is, in part, a function of the tums-ratio between the primary winding Wp and the secondary' winding Ws. Similarly, the amplitude of voltage VAUX across the auxiliary winding WA. is, in part, a function of the tums-ratio between the primary' winding Wp and the auxiliary winding WA. The controller’s operational voltage VDD corresponds to the auxiliary voltage VAUX associated with the auxiliary winding WA (VDD is VAUX less die forward bias voltage drop associated with diode DAUX)

[0022] In some examples, the multiple control transistors 264 associated with the switching controller 260 include two transistors. In other examples, the multiple control transistors 264 include three (or more) transistors. The multiple control transistors 264 may be depletion-mode transistors that generate a switching control voltage in response to the switching voltage Vsw. The comparator 262 asserts the signal PWM2 based on a comparison of the switching control voltage Vsw and a predetermined switching threshold voltage. As an example, the power converter circuit 250 implements zero-voltage switching of transistor Q to thereby activate transistor Qi at approximately zero volts amplitude of the switching voltage Vsw at the switching node 258 In addition, the multiple control transistors 264 may provide a startup voltage for the switching controller 260 at initiation of the power converter circuit 250.

[0023] P IG. 3 is a schematic diagram of an example switching controller 350 usable to implement the switching controller 260 of FIG. 2. In the example of FIG. 3, switching controller 350 includes a voltage regulator 356 (e.g., a low-dropout (LDO) voltage regulator), a mode controller 358, comparator 262, a voltage source circuit 362 (which may be programmable or non-programmable), a timing generator 364, the multiple control transistors 264, a clamp circuit 320, an impedance device 330, capacitor CHVG, resistors Ri, R2, R3, and Rz, and switches SWi, SW2, SW3, and SVV4. Each switch SWi-SWr may be implemented as a separate transistor. In this example, the multiple control transistors 264 include two transistors Qs and Qr coupled in series. Each of the transistors Q? and Qi may be a depletion-mode transistor (e.g., an n-channel depletion-mode transistor). A depletion-mode transistor is normally ON and is turned OFF by application of a gate voltage that is at least the threshold voltage below its source voltage. Accordingly, a depletion-mode transistor is ON with a Vgs equal to 0V. The drain of the transistor Q4 is coupled to the switching terminal 258. The impedance device is coupled between the switching terminal 258 and the gate of the control transistor Q4. The clamp circuit 320 is coupled between the gate of transistor Q4 and the reference terminal 103. Resistor Rz is coupled between the source of transistor Qr (and drain of transistor Qs) and the gate of control transistor Q4

[0024] Capacitor CHVG is coupled between the gate of transistor Q3 and the reference terminal 103. Resistor Ri is coupled between the source of transistor Q3 and capacitor CHVG. One terminal of switch SW4 is coupled to resistor Ri and to capacitor CHVG. The other terminal of switch SW4 is coupled to an output terminal 356a of the voltage regulator 356. The cathode of diode DAUX (FIG. 2) is coupled to an input terminal 356b of the voltage regulator 356 and to a power input terminal 358a of the mode controller 358. Accordingly, the power input terminal 358a of the mode controller 358 and the input terminal 356b of the voltage regulator 356 receives operational voltage VDD from the auxiliary winding WA of transformer 256 during the normal operating mode of the power converter 250. Switch SW3 is coupled between the upper terminal of capacitor CHVG (coupled to the gate of transistor Q3) and the power input terminal 358 of the mode controller 358.

[00251 Resistor R3 is coupled between the source of transistor Q3 and a terminal of switch SW2, with the other terminal of switch SW2 coupled to the negative (-) input of comparator 262. The voltage on the source of transistor Q3 is voltage Vsws, and the voltage on the negative input of comparator 262 is a voltage Vswc. Resistor R4 is coupled between the negative input of comparator 262 and the reference terminal 103. Resistor R2 is coupled between the source of transistor Q3 and a terminal of switch SWi, with the other terminal of switch SWi coupled to the power input terminal 358a of the mode controller 358.

[0026] The voltage source circuit 362 is coupled to the positive (+) input of comparator 262. The voltage source circuit 362 may be programmable (as indicated) or non-programmable and produces a voltage VTH ZVS. The output of comparator 262 is coupled to an input 364a of the timing generator 364. The output 364b of the timing generator 364 provides the signal PWM2 and is coupled to an input 358b of the mode controller 358.

[0027] The mode controller 358 may be implemented as a finite state machine including logic gates, flip-flops, registers, etc. The mode controller 358 produces output signals on outputs 358c, 358d, 358e, and 358f, which are coupled to control inputs of switches SWi, SW2, SW3, and SW4, respectively. The mode controller 358 produces signal SCi, SC2, SC3, and SC4 to the respective control inputs of switches SWi, SW2, SW3, and SW4. Through each signal SCi - SC4, the mode controller 358 can open (disable) or close (enable) each respective switch.

[0028] The clamp circuit 320 is coupled between the gate of transistor Q4 at a clamp terminal 321 and the reference terminal 103. In the example of FIG. 3, the clamp circuit 320 includes multiple Zener diodes 320a coupled in series between the clamp terminal 321 and the reference terminal 103. The number of Zener diodes and their breakdown voltages define the clamp voltage of the clamp circuit 320. The target clamp voltage of the clamp circuit 320 is application-specific and depends on, for example, the maximum possible magnitude of voltage Vsw. When the clamp circuit 320 is activated, the voltage on the clamp terminal 321 is a fixed voltage (the clamp voltage), which is the sum of the breakdown voltages of the multiple Zener diodes 320a of the clamp circuit 320. In one example, the clamp voltage on the clamp terminal 321 is approximately one-half of the maximum magnitude of voltage Vsw during the normal operating mode. For example, if the maximum magnitude of voltage Vsw during the normal operating mode is 1000V, then the clamp circuit 320 activates when the voltage Vsw reaches approximately 500V. For a voltage Vsw below the clamp voltage of the clamp circuit 320, the clamp circuit 320 is not activated, and the Vgs of transistor Q4 is approximately 0V due to the resistor coupled between the gate and the source of transistor Q4.

[0029] In the example of FIG. 4, the impedance device 330 is a capacitor CFF. The impedance device 330 may be implemented as multiple capacitors, for example, coupled in series and/or in parallel. In another example, the impedance device 330 includes a capacitor coupled in series with a resistor.

[0030] During the startup mode of the power converter, the switching controller 350 provides an operational voltage to the mode controller 358. During the power converter’s normal mode of operation, the auxiliary winding Wp provides the operating voltage VDD to the switching controller 350, and the switching controller 350 senses the voltage on the switching terminal 258 to implement ZVS of transistor QI (FIG. 2). That is, the switching controller 350 provides both startup and switching terminal sensing functionality.

[0031] During the startup mode of the power converter circuit 250, die operating voltage VDD from the auxiliary winding WA has an amplitude that is less than an under-voltage lock-out (UVLO) amplitude, which may have a predetermined amplitude that is stored in the mode controller 358. The mode controller 358 may compare the operational voltage VDD with the UVLO amplitude to determine whether the power converter circuit 250 is in the startup mode (e.g., VDD less than UVLO voltage) or the normal operating mode (e.g., VDD equal to or greater than the UVLO voltage). In response to determining that the operational voltage VDD is less than the UVLO amplitude (e g., a first UVLO amplitude), the mode controller 358 determines that the power converter circuit 250 is in the startup mode, and thus, as further described below ⁷ , circuitry within the switching controller 350 responds by building up a sufficient operating voltage for the mode controller 358 in order for the controller to be able to provide pulse-width modulated switching of the transistors Qi and Q?. In response to determining that the power converter circuit 250 is in the startup mode, the mode controller 358 causes switches SWr and SW-i to be deactivated (open) via the respective switch control signals SC? and SCr and causes switches SWi and SW? to be activated (e.g., closed) via the respective switch control signals SCt and SC?.

[0032] In the startup mode, the power stage 254 receives the input voltage VIN which is increasing towards its steady state level (e.g., 1000V). With no PWM switching of transistors Qi and Q? (because mode controller 358 is not yet powered on), voltage Vsw approximately equals the input voltage VLM minus a possible voltage drop across the primary winding Wp. As described previously, transistors Qj and Q4 may be depletion-mode transistors and are in an activated state (ON) in response to approximately a zero amplitude Vgs. Accordingly, during the startup mode, transistors Qs and Qr are in the activated state. With transistors Qs and Q4 activated, transistors Qs and Q4 collectively act as a current source during the startup mode to begin charging capacitors Cnvc and CVDD via currents through resistor Ri and through the closed switches SWi and SW3.

[0033] As a result, the operational voltage VDD and the voltage VHVG begin to increase during the startup mode Eventually, the operational voltage VDD reaches an amplitude that is approximately equal to the UVLO amplitude (e.g., a first UVLO amplitude). Up until that point, current through activated transistors Q3 and Q-t charged capacitors CHVG and CVDD to provide a. bias voltage to power on the mode controller 358. In this way, the switching converter 350 provides a voltage startup functionality for the power converter 250.

[0034] In response to the operational voltage VDD reaching an amplitude that is approximately equal to the UVLO amplitude, the mode controller 358 switches to the normal operating mode. Upon entry’ into the normal operating mode, via the switch control signals SCi and SCfi, the mode controller 358 deactivates (opens) switches SWi and SW3 and maintains the switches SWi and SWs in the deactivated state. Additionally, the mode controller 358 activates (closes) switch SWt to permit the regulated output voltage VHVG from voltage regulator 356 to be provided to the gate of transistor Qs. With its gate voltage ( VHVG) at a fixed voltage controlled by the voltage regulator 356, transistor Q3 deactivates when the source voltage at Qs is at a threshold voltage of Qs that is above the output voltage of the voltage regulator; otherwise, transistor Q?, is activated. When the Vsw voltage becomes large enough to activate the clamp circuit 320, the voltage on the gate of transistor Q4 becomes fixed, and as its source voltage rises further, transistor Q4 deactivates.

(0035] The mode controller 358 monitors the operational voltage VDD during the normal operating mode. Upon detecting that the operational voltage VDD becomes less than the UVLO amplitude (e.g., the controller 260 detects a fault condition of the power converter, or the VDD voltage from the rectified voltage of the auxiliary winding (WA) falls below UVLO due to abnormal output voltage drop), the mode controller 358 transitions the power converter circuit 250 back to the startup mode (e.g., by causing switches SW2 and SW4 to open and switches SWi and SW3 to close). In one example, the mode controller 158 returns the power converter circuit 250 to the startup mode in response to the operational voltage VDD decreasing below a second UVLO amplitude that is less than the first UVLO amplitude corresponding to the threshold for the normal operating mode.

[0036] In the normal operating mode, as described previously, the switching controller 350 asserts the signal PWMs based on the amplitude of the voltage Vswc compared to the voltage VTH ZVS from the voltage source circuit 362. Following the deactivation of the transistor Qi, the switching voltage Vsw may have a relatively high amplitude. For example, when transistor QI deactivates, the amplitude of the voltage Vsw may become approximately equal to the sum of Vm (e g., 1000 V) and the voltage across the primary winding Wp. In response to the transistor Q2 being deactivated, the switching terminal 258 can begin to resonantly transition downward, thus causing the switching voltage Vsw, and thus Vswc, to decrease. Some power converters may implement. ZVS of transistor Qi in which transistor Qi is activated (turned ON) at approximately a zero-volt amplitude of the switching voltage Vsw.

[0037] During the normal operating mode, the voltage Vsw transitions from a lower voltage (e.g., the voltage of the reference terminal 103) to a higher voltage (e.g., input voltage ViNplus the voltage of the primary winding W?). As described below, transistors Q3 and Q4 are disabled (OFF) at higher voltage levels of Vsw and are enabled (ON) at lower voltage levels of Vsw. When transistors Q3 and Q4 are OFF, transistors Q3 and Q4 function as a voltage divider between voltage Vsw and the voltage of the reference terminal 103. Accordingly, each transistor Q? and Qr advantageously need not be rated (e.g , the maximum permited Vds) for the full voltage amplitude of VIN (or input voltage Vix plus the voltage of the primary winding Wp), and thus can be smaller than transistors Qr or Q4 would have been if they were rated for a Vds of 1000V. For example, in an application in which VIN is 1000V, each of transistors Qs and Q4 may have a Vds rating of only 600V, and advantageously not be rated for 1000V Vds.

[0038] The following discussion describes the operation of the switching controller 350 as the voltage V sw transitions from a lower voltage (when transistor Qi activates) to a higher voltage (when transistor Qi deactivates) and then back to the lower voltage. Starting from a lower level of voltage Vsw, the voltage Vsw is below the clamp voltage (e.g., 500V) of the clamp circuit 320, and thus the clamp circuit 320 is not activated. Accordingly, current does not flow through the clamp circuit 320, and thus the gate and source of transistor Q4 are at the same voltage (the Vgs of transistor Qr is 0V). With the Vgs of transistor at 0V, transistor Qa is in the activated state (ON). With transistor Qr activated, the voltage on the drain of transistor Q3 is at the lower voltage, which is less than the threshold voltage of transistor Qs below voltage VHVG, and thus transistor Qs also is activated.

[0039] The mode controller 358 has activated switch SW2 to permit the comparator 262 to compare voltage Vswc (w'hich is approximately equal to voltage Vsw) to the threshold voltage VTH ZVS from the voltage source circuit 362 When voltage Vswc falls to voltage VIH. die comparator forces its output signal CMP to a logic high state. A signal CMP logic high occurring within a defined dead time (from PWM2 falling edge to PWM1 rising edge) indicates that the current switching cycle can achieve zero voltage switching albeit with a relatively high negative magnetizing current. In response, the timing generator will slightly reduce the on-time of PWM2 at the next switching cycle to incrementally reduce the negative magnetizing current. On the other hand, the signal CMP not becoming logic high within the defined dead time indicates that the current switching cycle cannot achieve zero voltage switching. In response, the timing generator slightly increases die on-time of PWM2 at the next switching cycle to increase the negative magnetizing current for discharging the switching node voltage slightly more. The timing generator 364 may be or include a one-shot circuit with adjustable on-time based on the logic level status of the signal CMP. When the PWMi signal becomes logic low, transistor Qi turns off and the positive magnetizing current will charge the switch node 258, and switching voltage Vsw rises. The signal PWM2 becomes logic high after the dead time from PWMi falling edge. When the gate driver 252 (FIG. 2) responds to PWMj by activating transistor Qz, the voltage Vsw reaches its highest level The mode controller 358 responds to the positive assertion of signal PWMzby deactivating switch SW2 to effectively disable comparator 262 (which is not needed at this point during the switching cycle).

[0040] Eventually, voltage Vsw rises to the clamp voltage of the voltage clamp circuit 320, and die clamp circuit activates. During Vsw rising period, the fast dv/dt will create a charge current through the impedance device 330 (e.g., a capacitor), so the capacitor Ctr allows the voltage on the clamp terminal 321 to simultaneously rise with Vsw and activate the clamp circuit quickly. The activation of the clamp circuit 320 fixes (clamps) the voltage on the gate of transistor Qv Resistor Rz provides a current path from die source of transistor Q4 to the Zener diode slack of the clamp circuit 320. Resistor Rz is sized to control the magnitude of current that flows in the Zener stack from the resistor. The magnitude of the current from resistor Rz is approximately equal to the leakage current within the Zener diodes 320a. By balancing the leakage cun-ent from the Zener diodes, resistor Rz helps to maintain the clamp circuit 320 in its activated state when the voltage Vsw is above the clamp voltage of the clamp circuit Because power is dissipated in the resistor Rz, the impedance device 330 allows resistor Rz to have a relatively high resistance to overcome the Zener diode leakage current with negligible impact on converter efficiency. On the other hand, in an example that lacks the impedance device 330. resistor Rz should have a relatively low ⁷ resistance value to activate the clamp circuit 320 quickly enough during a switch node rising edge, and the low resistance will consume more power which wall reduce converter efficiency.

[0041] With the gate voltage of transistor Qa fixed due to the clamp circuit 320 being activated, as the voltage VSW continues to rise, the source voltage becomes more than the threshold voltage above the gate voltage of transistor Qu At this point, transistor Q4 becomes deactivated. Meat iw hile, transistor Qa also deactivates when its source voltage (Vsws) becomes more than the threshold voltage above the gate voltage of transistor Qs, which is fixed by the voltage regulator 356. At this point, both transistors Qs and Qr thereby protect the rest of the circuitry of the switching controller 350 as the voltage Vsw continues to rise (e.g., up to 1000V). The series coupling of the drain-to- source capacitances of transistors Qj and Q-; function as a voltage divider so that the Vds of each transistor is approximately one-half of the voltage Vsw.

[0042] When the one-shot pulse from the timing generator 364 expires, signal PWM2 becomes logic low followed (after the dead time) by signal PWM1 becoming logic high. During the dead time, the negative magnetizing current discharges the switch node voltage, so the voltage VSW becomes below the clamp voltage of the voltage clamp circuit 320, and the voltage clamp circuit deactivates. In response, transistor Qr activates With the gate of transistor Q4 coupled to its source via resistor R1 and no current flowing through transistor Qr, transistor Qj also activates. The timing generator implements a pulse width of signal PWM?.. The falling edge of signal PWM2 causes the mode controller 358 to close switch SW2. With die transistors and Q4 activated and switch SW2 closed, comparator 262 again compares voltages Vswc and VTH SO that the switching controller 350 can detect if ZVS has occurred within the dead time between the falling edge of the signal PWM2 and the rising edge of the signal PWM1. The foregoing process repeats.

[0043] In the example of FIG. 3, die drain of Q4 is coupled to the switching terminal 258 so that the circuitry including transistors Qs and Q-g the clamp circuit 320, the impedance device 330, and resistor Rz can function to power on the mode controller 358 during the startup mode and to provide zero-voltage switching of transistor Qi. In some power converter applications (e.g., flyback converters implementing continuous conduction mode (CCM) and/or discontinuous conduction mode (DCM)), the startup function is needed hut not the zero-voltage switching of transistor Qi. In such applications, the drain of transistor Q4 may be coupled to the input voltage terminal 101 instead of to the switching terminal 258,

[0044] The example of FIG. 3, the multiple control transistors 264 includes two, serially -coupled depletion-mode transistors Qs and Q4 to operate as a voltage divider for higher levels of voltage VIN (e.g., 1000V). Some applications may have even higher levels of voltage VIN (1200V, 1500V, etc.). Accordingly, in some examples, the multiple control transistors 264 may include three (or more) depletion-mode transistors.

[0045] FIG. 4 is a schematic diagram of an illustrative switching controller 550 in which the multiple control transistors 264 includes three transistors — transistors Q3 and Q4 as described above and also transistor Q5. Most of the switching controller 550 is largely the same as the switching controller 350 of FIG. 3, and reference can thus be made to the preceding description of switching controller 350. Transistors Q3, Q4, and Qs are depletion-mode transistors (e.g., NFETs). Transistor Q5 is coupled between transistors Q3 and Q4.

[0046] The impedance device 330 (e.g., capacitor CFF) is coupled between the drain and gate of transistor Q4, as described above. Similarly, resistor Rz is coupled between the source and gate of transistor Q4. Switching controller 550 includes a clamp circuit 520 which includes clamp terminals 521 and 522. Clamp terminal 521 is coupled to the gate of transistor Q4, and clamp terminal 522 is coupled to the gate of transistor Qs. The clamp circuit 520 includes serially-coupled Zener diodes Zsi, Z52, Z53, Z54, Z55, and Z56. Although the clamp circuit 530 in the example of FIG. 4 includes six Zener diodes, the clamp circuit 520 may include fewer than six Zener diodes or more than six Zener diodes in other examples. The cathode of Zener diode Z+ is the clamp terminal 521, which is coupled to the gate of transistor Q4. The cathode of Zener diode Z54 is the clamp terminal 522, which is coupled to the gate of transistor Qs.

[0047] The clamp voltage of the clamp circuit 520 is sum of the breakdown voltages of the series combination of the Zener diodes. In one example, the number of Zener diodes and their breakdown voltages are set such that the clamp voltage of the clamp circuit 520 is approximately equal to two- thirds of the maximum level of voltage Vsw. When voltage Vsw is above the clamp voltage of the clamp circuit 520, the clamp circuit 520 activates and the voltage on the gate of transistor Q4 is clamped at the clamp voltage, which is approximately two-thirds of the maximum level of voltage Vsw. In the example of FIG 4, when the clamp circuit 520 activates, the voltage on the cathode of Zener diode Z54 will be approximately one-half of the clamp voltage of the clamp circuit 520, and thus the voltage on the gate of transistor Q5 will be approximately one-third of the clamp voltage. When the clamp circuit activates, the gate voltages of transistors Q4 and Qs are fixed by the clamp circuit 520 and transistors Q4 and Q5 are in the deactivated state (OFF). As described above, the voltage regulator 356 fixes the voltage on the gate of transistor Q3. Transistor Q3 also will be in the deactivated state when the voltage Vsw is large enough to activate the clamp circuit 520. With all three transistors Q3, Q4, and Qs deactivated, the series-combination of transistors Q3-Q5 operate as a voltage divider with approximately one-third of the voltage Vsw divided across each of the transistors. For example, if voltage Vsw is 1200V, the Vds of each of transistors Q3-Q5 will be approximately 400V. Accordingly, each transistor Q3-Q5 need not be sized to accommodate the entire voltage Vsw.

[0048] FIG. 5 is an example of a graph of the voltages Vsw, the voltage on the drain of transistor Q5 (VD Q5), and the voltage on the drain of transistor Q3 (VD Q3) with respect to time as the voltage Vsw increases from approximately 0V to 1400V and back to 0V. When voltage Vsw is at 1400V, the voltage on the drain of transistor Qs is approximately 933V, and the voltage on the drain of transistor Q3 is approximately 466V. The Vds of each transistor Q3-Q5 is approximately one-third of the voltage Vsw and thus substantially less than the voltage Vsw.

[0049] As described above, some flyback converters (e.g., flyback converters that implement continuous conduction mode (CCM) or discontinuous conduction mode (DCM)) do not need to perform ZVS of transistor Qi, and thus only the startup functionality to bias on the switching converter is needed. FIG 6 is a schematic diagram of an example power converter including a switching controller 650 coupled to power stage 154. The switching controller 650 includes control logic circuit 655, transistors Q3 and Q4 (e.g., depletion-mode NFETs as described above), transistor QDISABLE (e.g., an NFET), clamp circuit 320, impedance device 330, resistors Rz, RLIM, and Rzs2, Zener diode Zs2, and capacitors CVDD (described above as part of the power stage) and CREF.

[0050] The control logic circuit 655 includes terminals including a VDD terminal 655a, a voltage reference (REF) terminal 655b, and an output terminal 655c. Resistor RLIM is coupled between the source of transistor Q3 and VDD terminal 655a. Capacitor CVDD is coupled between the VDD terminal 655a and the reference terminal 103. The REF terminal 655b is coupled to the gate of transistor QDISABLE. Capacitor CREF is coupled between the REF terminal 655b and the reference terminal 103. The output terminal 655c is coupled to the gate of the switching transistor within the power stage 154 (e g., transistor Qi, FIGS. 1 and 2). The control logic 655 may include switches, logic gates, flipflops, comparators, etc. that is operative to generate a pulse width modulation signal PWM at its output terminal 655c as a control signal for the gate of transistor Qi.

[0051] Transistors Q3 and Q4 are coupled in series between the input voltage terminal 101 and resistor RLIM. AS described above, the clamp circuit 320 is coupled between the gate of transistor Q4 and the reference terminal 103, the impedance device 330 is coupled between the drain and gate of transistor Q4, and resistor Rz is coupled between the source and gate of transistor Q4. The clamp circuit 320, impedance device 330, and resistor Rz operate in much the same manner as described above to clamp the voltage on the gate of transistor Q4 at levels of voltage VIN above the clamp voltage implemented by the clamp circuit 320. The control logic circuit 655 may include a voltage regulator to provide a reference voltage at its REF terminal 655b based on the voltage VDD at its VDD terminal 655a. The reference voltage at the REF terminal 655b is the gate voltage for transistor QDISABLE. Transistor QDISABLE is deactivated when the reference voltage at the REF terminal 655b is logic low, which is the case when the voltage on the control logic’s VDD terminal is below its UVLO level and is activated when the reference voltage at the REF terminal is logic high (VDD above UVLO). The control logic 655 causes transistor QDISABLE to be activated during the startup mode and causes transistor QDISABLE to be deactivated during the normal operating mode.

[0052] In the example of FIG. 6, the drain of transistor Q4 is coupled to the input voltage terminal 101 instead of the switching terminal 258 (as was the case for the switching controller 350 and 550 of FIGS. 3 and 5, respectively). During the startup mode, VDD is below UVLO, and the voltage on the REF terminal 655b is 0V, which deactivates transistor QDISABLE. The diode drop across diode Zs2 causes transistor Q3 to be activates, and the voltage on the drain of transistor Q3 is approximately equal to VDD (which is less than the UVLO voltage level). In this state (Q3 activated, QDISABLE deactivated, and voltage VDD less than UVLO), the transistor Q4 also is activated because its gate voltage (which is increasing as VIN increases during startup) is higher than its source voltage.

[0053] Eventually VIN exceeds the UVLO voltage level at which time the control logic 655 enables an internal reference voltage circuit to set the voltage REF at a level high enough (e.g., 5V) to activate transistor QDISABLE. With transistor QDISABLE activated, the gate voltage of transistor Q3 is pulled lower than its source voltage, thereby deactivating transistor Q3. With transistor Q3 deactivated, the voltage on the source of transistor Q3 and the drain of transistor Q4 increases as VIN increases. When the source voltage of transistor Q4 is above the gate voltage of transistor Q4, transistor Q4 deactivates. Also, when VIN reaches the clamp voltage of the clamp circuit 320, the clamp circuit 320 activates and fixes the gate voltage for transistor Q4. With both transistors Q3 and Q4 in their deactivated state, the drain-to-source capacitances of transistors Q3 and Q4 function as a voltage divider dividing the input voltage VIN approximately evenly between transistors Q3 and Q4. Accordingly, neither transistor Q3 nor transistor Q4 need be sized to accommodate the full magnitude of the input voltage VIN.

[0054] In this description, the term “couple” may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action: (a) in a first example, device A is coupled to device B by direct connection; or (b) in a second example, device A is coupled to device B through intervening component C if intervening component C does not alter the functional relationship between device A and device B, such that device B is controlled by device A via the control signal generated by device A.

[0055] Also, in this description, the recitation “based on” means “based at least in part on.” Therefore, if X is based on Y, then X may be a function of Y and any number of other factors. [0056] A device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or reconfigurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof.

[0057] As used herein, the terms “terminal”, “node”, “interconnection”, “pin” and “lead” are used interchangeably. Unless specifically stated to the contrary, these terms are generally used to mean an interconnection between or a terminus of a device element, a circuit element, an integrated circuit, a device or other electronics or semiconductor component.

[0058] A circuit or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described circuitry or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors, and/or inductors), and/or one or more sources (such as voltage and/or current sources) may instead include only the semiconductor elements within a single physical device (e g., a semiconductor die and/or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and/or the sources to form the described structure either at a time of manufacture or after a time of manufacture, for example, by an end-user and/or a third-party.

[0059] While the use of particular transistors is described herein, other transistors (or equivalent devices) may be used instead with little or no change to the remaining circuitry. For example, a field effect transistor (“FET”) (such as an n-channel FET (NFET) or a p-channel FET (PFET)), a bipolar junction transistor (BJT - e.g., NPN transistor or PNP transistor), an insulated gate bipolar transistor (IGBT), and/or a junction field effect transistor (JFET) may be used in place of or in conjunction with one or more of the devices described herein. The transistors may be depletion mode devices, drain-extended devices, enhancement mode devices, natural transistors or other types of device structure transistors. Furthermore, the devices may be implemented in/over a silicon substrate (Si), a silicon carbide substrate (SiC), a gallium nitride substrate (GaN) or a gallium arsenide substrate (GaAs).

[0060] References may be made in the claims to a transistor’s control input and its current terminals. In the context of a FET, the control input is the gate, and the current terminals are the drain and source. In the context of a BJT, the control input is the base, and the current terminals are the collector and emitter.

[0061] References herein to a transistor being “activated” or “ON” means that the conduction channel of the transistor is present and drain current may flow through the transistor. References herein to a transistor being “deactivated or “OFF” means that the conduction channel is not present so drain current does not flow through the transistor. A “deactivated” transistor, however, may have current flowing through the transistor’s body-diode.

[0062] Circuits described herein are reconfigurable to include additional or different components to provide functionality at least partially similar to functionality available prior to the component replacement. Components shown as resistors, unless otherwise stated, are generally representative of any one or more elements coupled in series and/or parallel to provide an amount of impedance represented by the resistor shown. For example, a resistor or capacitor shown and described herein as a single component may instead be multiple resistors or capacitors, respectively, coupled in parallel between the same nodes. For example, a resistor or capacitor shown and described herein as a single component may instead be multiple resistors or capacitors, respectively, coupled in series between the same two nodes as the single resistor or capacitor. [0063] While certain elements of the described examples are included in an integrated circuit and other elements are external to the integrated circuit, in other example embodiments, additional or fewer features may be incorporated into the integrated circuit. In addition, some or all of the features illustrated as being external to the integrated circuit may be included in the integrated circuit and/or some features illustrated as being internal to the integrated circuit may be incorporated outside of the integrated. As used herein, the term “integrated circuit” means one or more circuits that are: (i) incorporated in/over a semiconductor substrate; (ii) incorporated in a single semiconductor package; (iii) incorporated into the same module; and/or (iv) incorporated in/on the same printed circuit board. [0064] Uses of the phrase “ground” in the foregoing description include a chassis ground, an Earth ground, a floating ground, a virtual ground, a digital ground, a common ground, and/or any other form of ground connection applicable to, or suitable for, the teachings of this description. In this description, unless otherwise stated, “about,” “approximately” or “substantially” preceding a parameter means being within +/- 10 percent of that parameter or, if the parameter is zero, a reasonable range of values around zero.

[0065] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.