Isolated switching converter, controller and control method thereof

Abstract

The present invention discloses an isolated switching converter, a controller, and a control method thereof. The switching converter includes a primary switching tube coupled to a primary winding of a transformer, and a secondary switching tube coupled to a secondary winding of the transformer. The control method includes: controlling the first switching of the secondary switching tube based on a primary shutdown detection signal and a current zero-crossing detection signal; generating a second conduction enable signal; controlling the second shutdown of the secondary switching tube based on the drain voltage, the output voltage, and the resistance value of a resistor; providing a primary turn-on enable signal when the second shutdown of the secondary switching tube is detected; sending the primary turn-on enable signal to an isolation circuit to generate a synchronization signal electrically isolated from the primary turn-on enable signal; and generating a primary control signal based on the synchronization signal and zero-crossing detection of the voltage across the primary switching tube to control the primary switching tube.

Description

Technical Field

[0001] The present invention relates to electronic circuits, and in particular to an isolated switching converter using a soft switching technology, a controller thereof and a control method thereof. Background Art

[0002] The Universal Serial Bus (USB) Power Delivery (PD) standard has begun to gain popularity among smart device and laptop manufacturers. The USB PD standard allows for higher power levels (up to 100W) and adaptive output voltages (e.g., 5V to 28V). This trend requires higher power, faster speeds, and smaller isolated switching power supplies.

[0003] However, as silicon-based devices approach their theoretical performance limits, further performance improvements of existing isolated power supplies become more difficult to achieve, making it difficult to meet the higher power transmission requirements of PD standards while maintaining high efficiency and low cost. Summary of the Invention

[0004] In view of one or more problems existing in the prior art, the present invention aims to provide an isolated switching converter and its controller and control method that can meet the higher power transmission requirements of PD standards while maintaining high efficiency and low cost.

[0005] According to an embodiment of the present invention, a controller for an isolated switching converter includes a transformer having a primary winding and a secondary winding, a primary switch tube coupled to the primary winding, and a secondary switch tube coupled to the secondary winding. The controller includes: a conduction time control circuit having a first input terminal, a second input terminal, a third input terminal, and an output terminal, wherein the first input terminal is coupled to the drain terminal of the secondary switch tube to receive the drain terminal voltage of the secondary switch tube, the second input terminal receives the output voltage of the switching converter, and the third input terminal is coupled to a resistor outside the controller. The conduction time control circuit is based on the drain terminal voltage of the secondary switch tube, the output voltage, and the resistance value of the resistor. Adjust the conduction time of the secondary switch tube for the second conduction; the primary turn-on enable circuit provides a primary turn-on enable signal when detecting the second turn-off of the secondary switch tube; the isolation circuit has an input end for receiving the primary turn-on enable signal and generates a synchronization signal electrically isolated from the primary turn-on enable signal at the output end; the voltage zero-crossing detection circuit detects whether the voltage at both ends of the primary switch tube passes through zero and generates a voltage zero-crossing detection signal; and the primary logic circuit is coupled to the output end of the isolation circuit to receive the synchronization signal, coupled to the voltage zero-crossing detection circuit to receive the voltage zero detection signal, and generates a primary control signal based on the synchronization signal and the voltage zero-crossing detection signal to control the primary switch tube.

[0006] An isolated switching converter according to an embodiment of the present invention includes the controller as described above.

[0007] According to an embodiment of the present invention, a control method for an isolated switching converter is provided. The switching converter includes a transformer having a primary winding and a secondary winding, a primary switch tube coupled to the primary winding, and a secondary switch tube coupled to the secondary winding. The control method includes: detecting whether the primary switch tube is turned off and generating a primary turn-off detection signal; detecting whether the current flowing through the secondary switch tube crosses zero and generating a current zero-crossing detection signal; controlling the first turn-on and turn-off of the secondary switch tube based on the primary turn-off detection signal and the current zero-crossing detection signal; generating a second turn-on enable signal to control the secondary switch tube. the second turn-on of the secondary switch tube; based on the drain voltage of the secondary switch tube, the output voltage of the switching converter and the resistance value of the external resistor, generating a turn-on time control signal to control the second turn-off of the secondary switch tube; when the second turn-off of the secondary switch tube is detected, providing a primary turn-on enable signal; sending the primary turn-on enable signal to the isolation circuit to generate a synchronization signal electrically isolated from the primary turn-on enable signal; performing zero-crossing detection on the voltage across the primary switch tube to generate a voltage zero-crossing detection signal; and generating a primary control signal based on the synchronization signal and the voltage zero-crossing detection signal to control the primary switch tube.

[0008] In an embodiment of the present invention, the drain voltage of the secondary switch and the output voltage of the switching converter are detected in real time. Furthermore, the resistance of the external resistor is adjustable, thereby adaptively controlling the duration of the secondary switch's second conduction. This minimizes the energy consumption of the primary switch during zero-voltage turn-on, enabling the primary switch to achieve zero-voltage turn-on close to the theoretical limit. Furthermore, the use of wide-bandgap devices, which have smaller output capacitance and higher operating frequencies than silicon-based devices, further reduces the voltage at zero-voltage turn-on of the primary switch, reducing the size of the isolated switching converter and thus achieving higher power density and improved efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a block diagram of an isolated switching converter 100 according to an embodiment of the present invention;

[0010] Figure 2 is a flow chart of a control method 200 for an isolated switching converter according to an embodiment of the present invention;

[0011] Figure 3 is a block diagram of an isolated switching converter 100A according to an embodiment of the present invention;

[0012] Figure 4 is a flow chart of a control method 200A for an isolated switching converter according to an embodiment of the present invention;

[0013] Figure 5 According to an embodiment of the present invention Figure 2 The circuit principle diagram of the conduction time control circuit 301A shown;

[0014] Figure 6 Flowchart of the method for step 205 of generating the on-time control signal according to an embodiment of the present invention;

[0015] Figure 7 1 is a working waveform diagram of the isolated switching converter 100A according to an embodiment of the present invention;

[0016] Figure 8 is a circuit schematic diagram of an isolated switching converter 100B according to an embodiment of the present invention;

[0017] Figure 9 According to an embodiment of the present invention Figure 8 FIG. 1 is a working waveform diagram of the isolated switching converter 100B. DETAILED DESCRIPTION

[0018] Specific embodiments of the present invention will be described in detail below. It should be noted that the embodiments described herein are for illustrative purposes only and are not intended to limit the present invention. In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that these specific details are not necessarily required to practice the present invention. In other instances, well-known circuits, materials, or methods are not specifically described to avoid obscuring the present invention.

[0019] Throughout this specification, references to "one embodiment," "an embodiment," "an example," or "an example" mean that a particular feature, structure, or characteristic described in connection with that embodiment or example is included in at least one embodiment of the present invention. Therefore, the phrases "in one embodiment," "in an embodiment," "an example," or "an example" appearing in various places throughout this specification do not necessarily all refer to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable combinations and / or subcombinations in one or more embodiments or examples. Furthermore, those of ordinary skill in the art will appreciate that the figures provided herein are for illustrative purposes and are not necessarily drawn to scale. It should be understood that when an "element" is referred to as being "connected to" or "coupled to" another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected to" or "directly coupled to" another element, there are no intervening elements. Identical reference numerals indicate identical elements. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0020] The present invention can be applied to any isolated switching converter. In the following detailed description, for the sake of brevity, only a flyback converter is used as an example to explain the specific working principle of the present invention.

[0021] Figure 1 FIG. 1 is a block diagram of an isolated switching converter 100 according to an embodiment of the present invention. Figure 1 As shown, the isolated switching converter 100 includes a transformer T, a primary switch tube 10, a secondary switch tube 20, and a controller 30. The transformer T has a primary winding and a secondary winding to provide isolation. The primary winding and the secondary winding each have a first end and a second end. The first end of the primary winding receives the input voltage Vin, the first end of the secondary winding provides a DC output voltage Vo, and the second end is coupled to the secondary reference ground (SGND). The primary switch tube 10 is coupled between the second end of the primary winding and the primary reference ground (PGND). The secondary switch tube 20 is coupled between the second end of the secondary winding and the load. However, those skilled in the art will appreciate that the secondary switch tube 20 can also be coupled between the first end of the secondary winding and the load.

[0022] Primary switch 10 is coupled to the second end of the primary winding and controls the transfer of energy stored in the primary winding to the secondary winding. Secondary switch 20, coupled to the secondary winding, acts as a synchronous rectifier, replacing traditional rectifier diodes to reduce losses and improve the efficiency of isolated switching converter 100. Furthermore, by utilizing circuit parasitic elements (e.g., the leakage inductance of transformer T and the output capacitance of primary switch 10) to turn on primary switch 10 at zero voltage, switching losses can be further reduced.

[0023] exist Figure 1 In the illustrated embodiment, the controller 30 includes an on-time control circuit 301, a secondary logic circuit 302, a primary turn-on enabling circuit 303, an isolation circuit 304, a voltage zero-crossing detection circuit 305, and a primary logic circuit 306. In some embodiments, the controller 30 is integrated into an integrated circuit chip and includes a plurality of pins.

[0024] exist Figure 1 In the embodiment shown, the switching converter 100 operates in discontinuous current mode, and the primary switch 10 is turned on at zero voltage. Before the primary switch 10 achieves zero voltage turn-on, the secondary switch 20 is turned on twice. Specifically, after the current flowing through the secondary switch 20 passes through zero, the secondary switch 20 is turned on again to generate a negative current flowing through the magnetizing inductance of the transformer T. This negative current is used to discharge the output capacitance of the primary switch 10. In the embodiment of the present invention, the second turn-on duration of the secondary switch 20 is T ON_ZVS It is variable to completely discharge the output capacitance of the primary switch tube 10 and achieve full zero voltage conduction (Full ZVS) of the primary switch tube 10.

[0025] like Figure 1As shown, the on-time control circuit 301 has a first input terminal, a second input terminal, a third input terminal and an output terminal, wherein the first input terminal is coupled to the drain terminal of the secondary switch tube 20 via the pin SRD of the controller 30 to receive the drain terminal voltage V Sec_DS The second input terminal receives the output voltage Vo via the pin VO of the controller 30, and the third input terminal is coupled to the pin ZVS. The pin ZVS is used to connect an external resistor R TZVS , to adjust the conduction time T of the secondary switch tube 20 for the second conduction ON_ZVS In one embodiment, a current source coupled to the output voltage Vo is further coupled to the pin ZVS to provide a current to the resistor R TZVS Provides a control current Is.

[0026] The on-time control circuit 301 is based on the drain voltage V Sec_DS , output voltage Vo and resistor R TZVS The resistance value is used to adjust the conduction time T of the secondary switch tube for the second conduction ON_ZVS The secondary logic circuit 302 generates a secondary control signal CTRLS to control the secondary switch 20 based on the on-time control signal ZOFF.

[0027] exist Figure 1 In the illustrated embodiment, when the primary turn-on enable circuit 303 detects the second shutdown of the secondary switch 20, it provides a primary turn-on enable signal PRON to the input of the isolation circuit 304. The isolation circuit 304 generates a synchronization signal SYNC at its output that is electrically isolated from the primary turn-on enable signal PRON, thereby achieving electrical isolation between the primary and secondary sides. The isolation circuit 304 may include an optocoupler, a transformer, a capacitive isolation device, or any other suitable electrical isolation device. In other embodiments, the isolation circuit 304 may be provided external to the controller integrated circuit.

[0028] The voltage zero-crossing detection circuit 305 detects the voltage V across the primary switch 10. Pri_DS In one embodiment, the voltage zero-crossing detection circuit 305 is coupled to the auxiliary winding of the transformer T and receives the voltage V across the primary switch tube via the ZCD pin of the controller 30. Pri_DS The detection signal V ZCD and the voltage detection signal V ZCD With zero-crossing threshold V ZCD_TH Compare and provide voltage zero-crossing detection signal CP1 at the output terminal. In one embodiment, the zero-crossing threshold V ZCD_TH is 20mV.

[0029] The primary logic circuit 306 is coupled to the output terminal of the isolation circuit 304 to receive the synchronization signal SYNC, and is coupled to the voltage zero-crossing detection circuit 305 to receive the voltage zero-crossing detection signal CP1. The primary logic circuit 306 generates a primary control signal CTRLP based on the synchronization signal SYNC and the voltage zero-crossing detection signal CP1, and provides the primary control signal CTRLP to the control terminal of the primary switch tube 10 via the pin PDrv to control the primary switch tube 10.

[0030] In some embodiments, a delay circuit is coupled between the voltage zero-crossing detection circuit 305 and the primary logic circuit 306. In one embodiment, when the synchronization signal SYNC is valid and the voltage across the primary switch tube crosses zero, the primary switch tube 10 undergoes a delay t of the delay circuit. Delay In one embodiment, the delay time of the delay circuit is t Delay It can be adjusted by a second resistor externally connected to the controller 30 .

[0031] Generally, silicon-based devices (such as MOSFETs) require a large amount of energy to fully discharge their output capacitance to achieve zero-voltage turn-on due to their large output capacitance. In practical applications, due to a combination of cost and loss considerations, the output capacitance of silicon-based devices is often not fully discharged. Therefore, when implementing zero-voltage turn-on technology, only partial zero-voltage turn-on can be achieved, and the voltage across the silicon-based device is often not 0V but 15-25V. This partial zero-voltage turn-on not only increases turn-on losses, but also causes large spikes in the drain voltage of the secondary switch tube, generating electromagnetic interference.

[0032] In one embodiment of the present invention, the primary switch tube 10 includes an emerging wide bandgap device, such as a device based on gallium nitride (GaN) and silicon carbide (SiC), instead of a traditional silicon-based device. Wide bandgap devices can operate at higher switching frequencies without reducing efficiency, while having output capacitance much lower than that of silicon-based devices. Therefore, such devices will further reduce the size of the isolated converter while bringing significant efficiency improvements. Moreover, the on-time control circuit 301 of the present invention can dynamically adjust the on-time T ON_ZVS , in order to adaptively provide the minimum energy required to fully discharge the output capacitor according to the actual circuit needs. In one embodiment of the present invention, the zero-crossing threshold V ZCD_TH It is tens of millivolts, which is much smaller than the voltage when silicon-based devices are turned on at zero voltage.

[0033] According to an embodiment of the present invention, the external resistor R TZVSThe resistance value can be modified and adjusted as needed, the output voltage Vo can be easily and directly detected by pin VO of controller 30, and the maximum drain voltage of the secondary switch can also be easily detected by pin SRD of controller 30. Using the embodiments of the present invention, isolated switching converter 100 can meet the high power density, high switching frequency, high efficiency, and electromagnetic interference standards required by USB PD applications while maintaining a low cost for the entire isolated switching converter.

[0034] Figure 2 FIG2 is a flow chart of a control method 200 for an isolated switching converter according to an embodiment of the present invention. The control method 200 includes steps 205 to 209. In step 205, based on the drain voltage V Sec_DS , the output voltage Vo of the switching converter and the resistor R TZVS The resistance value generates a conduction time control signal to adjust the conduction time T of the secondary switch tube 20 for the second conduction. ON_ZVS In step 206 , it is detected whether the secondary switch tube 20 is turned off for the second time, and when the second time is detected, a primary turn-on enable signal PRON is provided to the input terminal of the isolation circuit.

[0035] In step 207 , a signal is coupled to the output terminal of the isolation circuit to receive a synchronization signal SYNC electrically isolated from the primary turn-on enable signal PRON.

[0036] In step 208 , it is detected whether the voltage across the primary switch tube is zero-crossing, that is, whether the voltage across the primary switch tube is less than a zero-crossing threshold, and a voltage zero-crossing detection signal is generated.

[0037] In step 209, based on the synchronization signal and the voltage zero-crossing detection signal, a primary control signal is provided to control the primary switch 10. In one embodiment, when the synchronization signal arrives and the voltage across the primary switch 10 is detected to be zero-crossing, the primary switch 10 is switched on after a delay of t Delay Then conduction.

[0038] Figure 3 FIG. 1 is a block diagram of an isolated switching converter 100A according to an embodiment of the present invention. Figure 3 The controller 30A shown is connected to Figure 1 The controller 30 shown is substantially similar except that Figure 3 The controller 30A shown further includes a primary shutdown detection circuit 307 located on the secondary side, a current zero-crossing detection circuit 308 , a quasi-resonant control circuit 309 , and a second comparison circuit 310 located on the primary side.

[0039] exist Figure 3In the illustrated embodiment, the primary shutdown detection circuit 307 detects whether the primary switch 10 is turned off and generates a primary shutdown detection signal PROFF. The primary shutdown detection circuit 307 can determine whether the primary switch 10 is turned off based on electrical parameters such as the drain voltage of the secondary switch 20, the current flowing through the secondary switch 20, and the voltage across the secondary winding. The primary shutdown detection circuit 307 can also obtain a signal indicating whether the primary switch 10 is turned off from the primary side through other means.

[0040] The current zero-crossing detection circuit 308 detects whether the current flowing through the secondary switch 20 crosses zero and generates a zero-crossing detection signal ZCD1. The quasi-resonant control circuit 309 is coupled to the secondary switch 20 to detect the resonant voltage of the switching converter and generate a second turn-on enable signal ZON at a target valley of the resonant voltage.

[0041] It should be understood by those skilled in the art that the present invention can be applied to any isolated switching converter operating in a discontinuous current mode. Figure 3 The quasi-resonant control provided is merely exemplary, and isolated flyback converters in a discontinuous current mode controlled in other ways also fall within the spirit and scope of protection of the present invention.

[0042] As mentioned above, the on-time control circuit 301A is based on the drain voltage V Sec_DS , output voltage Vo and resistor R TZVS The resistance value is used to adjust the conduction time T of the secondary switch tube for the second conduction ON_ZVS , and when the conduction time of the secondary switch tube 20 reaches T ON_ZVS The on-time control signal ZOFF is generated.

[0043] Secondary logic circuit 302A has a first input, a second input, a third input, a fourth input, and an output. The first input is coupled to primary shutdown detection circuit 307 to receive primary shutdown detection signal PROFF. The second input is coupled to the output of current zero-crossing detection circuit 308 to receive current zero-crossing detection signal ZCD1. The third input is coupled to quasi-resonant control circuit 309 to receive second turn-on enable signal ZON. The fourth input is coupled to on-time control circuit 301A to receive on-time control signal ZOFF. Secondary logic circuit 302A generates secondary control signal CTRLS based on primary shutdown detection signal PROFF and current zero-crossing detection signal ZCD1 to control the first switching of secondary switch 20. Furthermore, secondary logic circuit 302A also generates secondary control signal CTRLS based on second turn-on enable signal ZON and on-time control signal ZOFF to control the second switching of secondary switch 20. When the primary turn-on enabling circuit 303 detects the second turn-off of the secondary switch 20 , it provides a primary turn-on enabling signal PRON.

[0044] Switching converter 100A also includes a second comparison circuit 310. Second comparison circuit 310 has a first input, a second input, and an output. The first input receives a primary current sampling signal ISENP representing the current flowing through primary switch 10, and the second input receives a first threshold voltage VTH1. Second comparison circuit 310 compares primary current sampling signal ISENP with the first threshold voltage VTH1 and generates a second comparison signal CP2 at its output. Primary logic circuit 306A is coupled to the output of second comparison circuit 310 to receive second comparison signal CP2. Based on second comparison signal CP2, voltage zero-crossing detection signal CP1, and synchronization signal SYNC, it generates a primary control signal CTRLP to control primary switch 10. When current ISENP flowing through primary switch 10 reaches the first threshold voltage VTH1, primary switch 10 is turned off. First threshold voltage VTH1 can be a constant value or vary with synchronization signal SYNC.

[0045] Figure 4 FIG. 1 is a flow chart of a control method 200A for an isolated switching converter according to an embodiment of the present invention. Figure 4 In the embodiment shown, the control method 200A includes Figure 2 In addition to steps 205 to 209 shown, the process further includes steps 201 to 204.

[0046] Specifically, in step 201, it is detected whether the primary switch is turned off, and a primary turn-off detection signal is generated. In step 202, it is detected whether the current flowing through the secondary switch crosses zero, and a current zero-crossing detection signal is generated. In step 203, based on the primary turn-off detection signal and the current zero-crossing detection signal, a secondary control signal is generated to control the initial switching of the secondary switch. When it is detected that the primary switch is turned off, the secondary switch is turned on for the first time. When the current flowing through the secondary switch reaches zero, the secondary switch is turned off for the first time.

[0047] Next, at step 204, the circuit couples to the secondary switch to detect the resonant voltage waveform of the switching converter and generates a second turn-on enable signal at a target valley of the resonant voltage. In one embodiment, when the resonant voltage across the secondary switch reaches the target valley, the secondary switch is turned on a second time. The circuit then proceeds to step 205, providing an on-time control signal to control the second turn-off of the secondary switch.

[0048] Figure 5 According to an embodiment of the present invention Figure 2 The circuit principle diagram of the conduction time control circuit 301A is shown in FIG. Figure 5 In the embodiment shown, the on-time control circuit 301A includes a first detection circuit 3011, a second detection circuit 3012, a capacitor Cs, a current setting unit 3013, a switch control unit 3014, and a comparison circuit 3015. The first detection circuit 3011 is coupled to the drain terminal of the secondary switch tube 20 via the pin SRD, and detects the maximum value of the drain terminal voltage of the secondary switch tube V SRD Perform sampling and holding, and provide the maximum value of the secondary switch tube drain voltage V at the output SRD The first voltage signal V1 is proportional to the maximum value V SRD The proportional coefficient is K1, that is, V1=V SRD The second detection circuit 3012 has an input terminal and an output terminal. The input terminal is coupled to receive the output voltage Vo of the switching converter 100A via the pin VO. The output terminal provides a second voltage signal V2 proportional to the output voltage Vo. The proportionality coefficient between the second voltage signal V2 and the output voltage is K2, i.e., V2 = Vo*K2.

[0049] The current setting unit 3013 is coupled to the output terminal of the second detection circuit 3012 and is coupled to the external resistor R through the pin ZVS. TZVS , based on the second voltage signal V2 and the resistor R TZVS Generates a control current Is at the output. Figure 5 In the embodiment shown, the current setting unit 3013 includes a 1:1 current mirror. TZVSThe current mirror determines the size of the control current Is, and provides the control current Is to the output terminal.

[0050] like Figure 5 As shown, capacitor Cs has a first terminal and a second terminal, wherein the first terminal is coupled to the output terminal of the current setting unit 3013 to receive the control current Is, and the second terminal is coupled to the secondary reference ground. The switch control unit 3014 includes switches S1 and S2 and a trigger FF1. Switches S1 and S2 are normally closed switches, wherein switch S1 is coupled in parallel with the current setting unit 3013 and is coupled between the power supply VS and the first terminal of capacitor Cs. Switch S2 is coupled between the output terminal of the second detection circuit 3012 and the first terminal of capacitor Cs. The trigger FF1 has a set terminal, a reset terminal, and an output terminal. The set terminal receives the second conduction enable signal ZON, and the reset terminal receives the conduction duration control signal ZOFF via a one-shot circuit. The output terminal provides a control signal to switches S1 and S2.

[0051] The comparison circuit 3015 includes a comparator CMP. The comparator CMP has a non-inverting input terminal coupled to the first terminal of the capacitor Cs to receive the voltage VCs across the capacitor Cs, an inverting input terminal coupled to the first detection circuit 3011 to receive the first voltage signal V1, and an output terminal providing the on-time control signal ZOFF.

[0052] like Figure 5 As shown, when the secondary switch 20 begins to conduct for the second time, that is, when the second conduction enable signal ZON changes from a low level to a high level, the voltage VCs across the capacitor Cs begins to increase from the second voltage signal V2. When the voltage across the capacitor Cs reaches the first voltage signal V1, the output of the comparison circuit 3015 flips, and the on-time control signal ZOFF at its output changes from a low level to a high level, turning off the secondary diode 20 for the second time. Subsequently, the voltage VCs across the capacitor Cs is reset to the second voltage signal V2. In one embodiment, the on-time control circuit 301A further includes a voltage follower. This voltage follower is further coupled between the output of the second detection circuit 3012 and the switch S2 to provide signal interference protection.

[0053] according to Figure 5 In the embodiment shown, the second conduction duration of the secondary switch tube 20 is T ON_ZVS It can be deduced according to the following formula:

[0054]

[0055] As shown in the above formula (1), the conduction time T ON_ZVS By changing the external resistor R TZVS At the same time, the conduction time T ON_ZVSAdaptively adjusts to the output voltage Vo and the maximum drain voltage V of the secondary switch tube SRD The isolating switch converter 100A is adapted to a wide range of input voltage and output voltage conditions and provides the minimum energy required to achieve complete zero voltage conduction. In one embodiment, K in formula (1) is a value greater than 1.

[0056] It should be understood by those skilled in the art that the conduction time T that satisfies formula (1) ON_ZVS Not only can Figure 5 The embodiment shown can also be obtained by adopting other circuit designs with the same function. Figure 5 The advantage of the embodiment shown is that, in actual mass production, the conduction time T ON_ZVS The accuracy of the on-time T depends mainly on the variation of the capacitor Cs. In other words, according to the embodiment of the present invention, only the accuracy of the capacitor Cs needs to be controlled to control the on-time T in mass production. ON_ZVS precision.

[0057] Figure 6 This is a flow chart of the method for generating the conduction duration control signal step 205 according to an embodiment of the present invention. Figure 6 In the illustrated embodiment, step 205 of generating the on-time control signal further includes steps 2051 - 2055 .

[0058] In step 2051, the maximum value of the drain terminal voltage of the secondary switch tube is sampled and held, and a first voltage signal proportional to the maximum value of the drain terminal voltage of the secondary switch tube is provided.

[0059] At step 2052, a second voltage signal proportional to the output voltage is provided.

[0060] In step 2053 , a control current is generated based on the second voltage signal and the resistance value of the resistor.

[0061] In step 2054 , in response to the second conduction enable signal, a capacitor is charged with a control current so that the voltage across the capacitor increases starting from the second voltage signal.

[0062] In step 2055 , when the voltage across the capacitor increases to the first voltage signal, an on-time control signal is generated to stop charging the capacitor, and the voltage across the capacitor is reset to the second voltage signal.

[0063] Figure 7 FIG. 1 is a working waveform diagram of an isolated switching converter 100A according to an embodiment of the present invention. Figure 7As shown, in one switching cycle of the switching converter 100A, at time t1, the primary control signal CTRLP changes from high level to low level, and the primary switch tube 10 is turned off. After the primary switch tube 10 is turned off, the drain voltage V Sec_DS The voltage changes from positive to negative, causing the secondary control signal CTRLS to change from a low level to a high level, and the secondary switch tube 20 is then turned on for the first time.

[0064] Subsequently, at time t2, the secondary current ISENS flowing through the secondary switch tube 20 decreases to zero, and the secondary control signal CTRLS changes from high to low, and the secondary switch tube 20 is turned off for the first time. Next, when the currents flowing through the primary and secondary are both zero, the parasitic capacitance of the energy storage element and the switch tube begins to resonate, generating a resonant voltage. The waveform of this resonant voltage is detected by the quasi-resonant circuit on the secondary side. At time t3, due to the use of quasi-resonant control, the drain terminal voltage V Sec_DS When the resonant voltage reaches the target valley (e.g., the second valley) of the current working cycle, the second conduction enable signal ZON changes from low level to high level, and the secondary control signal CTRLS also becomes high, and the secondary switch tube 20 is turned on for the second time. At the same time, the conduction time control circuit 301A starts to work and starts to charge the capacitor C S The voltage VCs across the capacitor Cs increases from the second voltage signal V2 (ie, K2*Vo) until time t4, when the voltage VCs across the capacitor increases to the first voltage signal V1, ie, V SRD * K1, the on-time control signal ZOFF changes from low level to high level, and the secondary control signal CTRLS changes from high level to low level accordingly, and the secondary switch tube 20 is turned off for the second time.

[0065] In addition, the primary enable circuit 307 detects the second turn-off of the secondary switch 20 and provides a primary enable signal PRON. When the primary enable signal PRON rises, the synchronization signal SYNC output by the isolation circuit 304 also changes from low to high.

[0066] At time t5, the voltage zero-crossing detection circuit detects the voltage V on the auxiliary winding. ZCD Zero crossing. After delay t Delay After that, at time t6, the primary switch tube 10 is turned on by zero voltage. Figure 7 As shown, the voltage V across the primary switch tube 10 Pri_DS The voltage at the turn-on moment is very small, and zero voltage conduction close to the theoretical limit can be achieved.

[0067] When the primary current sampling signal ISENP increases to the first threshold voltage VTH1, the primary switch tube 10 is turned off. After the primary switch tube 10 is turned off, the drain terminal voltage V Sec_DS When the positive voltage changes to a negative voltage, the secondary switch tube 20 is turned on.

[0068] The above steps are repeated continuously, and the second conduction time of the secondary switch tube 20 is T ON_ZVS It is adjusted in real time so that the primary switch tube 10 can achieve zero voltage conduction.

[0069] Figure 8 FIG is a circuit diagram of an isolated switching converter 100B according to an embodiment of the present invention. Figure 3 Compared to the isolated converter 100A shown, Figure 8 The illustrated isolated switching converter 100B further includes a conduction window setting circuit 311 coupled between the output of the isolation circuit 304 and the primary logic circuit 306B. The conduction window setting circuit 311 has an input and an output. Its input is coupled to the output of the isolation circuit 304 to receive a synchronization signal SYNC. Based on the synchronization signal SYNC, the conduction window setting circuit 311 establishes a primary turn-on window and provides a window enable signal ENW having the primary turn-on enable window at its output. The primary logic circuit 306B generates a primary control signal CTRLP based on the window enable signal ENW, the voltage zero-crossing detection signal CP1, and the second comparison signal CP2.

[0070] In one embodiment, the length of the primary turn-on window is a small portion of the switching period, such as 5% to 15% of the switching period, but is always greater than the delay t Delay In one embodiment, primary logic circuit 306B is configured to determine whether the end of the delay after the voltage across the primary switch crosses zero occurs within the primary turn-on window. If the end of the delay occurs within the primary turn-on window, primary logic circuit 306B provides primary control signal CTRLP at the end of the delay to turn on primary switch 10. Otherwise, primary logic circuit 306B provides primary control signal CTRLP to turn on primary switch 10 at the end of the primary turn-on window.

[0071] Figure 9 According to an embodiment of the present invention Figure 8 The operating waveform diagram of the isolated switching converter 100B is shown in FIG. Figure 9 As shown, when the rising edge of the synchronization signal SYNC comes, a window enable signal ENW with a primary turn-on window is established. In the primary turn-on window, the zero crossing of the voltage across the primary switch tube and the delay t Delay .like Figure 9As shown, in response to the zero-crossing detection of the primary switch tube within the primary window, the primary switch tube 10 is switched on after a delay of t Delay In other embodiments, the primary switch 10 is turned on in response to the end point of the primary turn-on window.

[0072] In the specification, relative terms such as first and second, etc. may be used only to distinguish one entity or action from another entity or action, and do not necessarily or imply any entity such relationship or order between these entities or actions. Numerical sequences such as "first", "second", "third", etc. refer only to different individuals in a plurality and do not imply any order or sequence unless specifically defined by the claim language. The order of the text in any claim does not mean that the processing steps must be performed in a temporary or logical order according to such order, unless specifically provided by the claim language. These processing steps can be interchanged in any order without departing from the scope of the invention, as long as such interchange does not contradict the claim language and does not appear logically absurd.

[0073] The above description and embodiments are merely exemplary and are not intended to limit the scope of the present invention. Variations and modifications to the disclosed embodiments are possible, and other feasible alternative embodiments and equivalent variations of the elements in the embodiments will be apparent to those skilled in the art. Other variations and modifications to the disclosed embodiments do not exceed the spirit and scope of the present invention.

Claims

1. A controller for an isolated switching converter, the switching converter comprising a transformer having a primary winding and a secondary winding, a primary switch coupled to the primary winding, and a secondary switch coupled to the secondary winding, the controller comprising: A conduction duration control circuit having a first input terminal, a second input terminal, a third input terminal, and an output terminal, wherein the first input terminal is coupled to the drain terminal of the secondary switch tube to receive the drain terminal voltage of the secondary switch tube, the second input terminal receives the output voltage of the switching converter, and the third input terminal is coupled to a resistor external to the controller, wherein after the current flowing through the secondary switch tube passes through zero, the secondary switch tube is turned on a second time, and the conduction duration control circuit adjusts the conduction duration of the secondary switch tube during the second conduction based on the drain terminal voltage of the secondary switch tube, the output voltage, and the resistance value of the resistor; The primary turn-on enabling circuit provides a primary turn-on enabling signal when detecting the second turn-off of the secondary switch tube; an isolation circuit having an input terminal for receiving a primary turn-on enable signal and generating a synchronization signal electrically isolated from the primary turn-on enable signal at an output terminal; A voltage zero-crossing detection circuit detects whether the voltage across the primary switch tube has crossed zero and generates a voltage zero-crossing detection signal; as well as The primary logic circuit is coupled to the output end of the isolation circuit to receive the synchronization signal, coupled to the voltage zero-crossing detection circuit to receive the voltage zero detection signal, and generates a primary control signal based on the synchronization signal and the voltage zero-crossing detection signal to control the primary switch tube.

2. The controller of claim 1, further comprising: A primary shutdown detection circuit detects whether the primary switch tube is turned off and generates a primary shutdown detection signal; A current zero-crossing detection circuit detects whether the current flowing through the secondary switch tube crosses zero and generates a current zero-crossing detection signal; A quasi-resonant control circuit is coupled to the secondary switch tube to detect the resonant voltage of the switching converter and generate a second turn-on enable signal at a target valley of the resonant voltage; as well as The secondary logic circuit generates a secondary control signal based on the primary shutdown detection signal and the current zero-crossing detection signal to control the first switching of the secondary switch tube, and generates a secondary control signal based on the second conduction enable signal and the conduction time control signal to control the second switching of the secondary switch tube.

3. The controller according to claim 2, wherein the on-time control circuit comprises: a first detection circuit coupled to the drain terminal of the secondary switch tube to sample and hold a maximum value of the drain terminal voltage of the secondary switch tube, and provide a first voltage signal proportional to the maximum value of the drain terminal voltage of the secondary switch tube at an output terminal, wherein a proportionality coefficient between the first voltage signal and the maximum value is K1; a second detection circuit providing a second voltage signal proportional to the output voltage at an output terminal, wherein a proportionality coefficient between the second voltage signal and the output voltage is K2; a current setting unit coupled to the output terminal of the second detection circuit and the external resistor, and generating a control current based on the second voltage signal and the resistor; as well as a capacitor alternately coupled to the current setting unit and the output terminal of the second voltage detection circuit to charge and reset the capacitor, respectively; as well as The switch control unit charges the capacitor with a controlled current starting from the second conduction of the secondary switch tube, so that the voltage across the capacitor increases from the second voltage signal until the voltage across the capacitor reaches the first voltage signal, generating a conduction time control signal.

4. The controller according to claim 3, wherein the on-time of the on-time control circuit is: Among them is C S Capacitor value, R TZVS is the resistance of the resistor, V SRD is the maximum value of the drain voltage of the secondary switch tube, Vo is the output voltage, and K is the ratio of the second proportional coefficient K2 to the first proportional coefficient K1. The controller according to claim 4 , wherein K is a number greater than 1 and less than 2.

6. The controller of claim 1 , further comprising: A conduction window setting circuit receives a synchronization signal and sets a window enable signal having a primary opening window based on the synchronization signal; A second comparison circuit compares the primary current detection signal flowing through the primary switch tube with the first threshold voltage and generates a second comparison signal at the output end; as well as The primary logic circuit further generates a primary control signal based on the window enable signal, the voltage zero-crossing detection signal and the second comparison signal. 7 . The controller of claim 1 , wherein at least one of the primary switch tube and the secondary switch tube comprises a gallium nitride device or a silicon carbide device.

8. An isolated switching converter, comprising the controller according to any one of claims 1 to 7.

9. A control method for an isolated switching converter, the switching converter comprising a transformer having a primary winding and a secondary winding, a primary switch coupled to the primary winding, and a secondary switch coupled to the secondary winding, the control method comprising: Detect whether the primary switch tube is turned off and generate a primary turn-off detection signal; Controlling the first conduction of the secondary switch tube based on the primary shutdown detection signal; Detect whether the current flowing through the secondary switch tube crosses zero, and generate a current zero-crossing detection signal; Based on the current zero-crossing detection signal, controlling the first turn-off of the secondary switch tube; Generate a second conduction enable signal to control the second conduction of the secondary switch tube; Based on the drain voltage of the secondary switch tube, the output voltage of the switching converter and the resistance value of the external resistor, a conduction time control signal is generated to control the second turn-off of the secondary switch tube; When the second turn-off of the secondary switch tube is detected, a primary turn-on enable signal is provided; Sending the primary turn-on enable signal to the isolation circuit to generate a synchronization signal electrically isolated from the primary turn-on enable signal; Perform zero-crossing detection on the voltage across the primary switch tube to generate a voltage zero-crossing detection signal; as well as Based on the synchronization signal and the voltage zero-crossing detection signal, a primary control signal is generated to control the primary switch tube.

10. The control method according to claim 9, wherein the method of generating the on-time control signal comprises: Sampling and holding the maximum value of the drain terminal voltage of the secondary switch tube, and providing a first voltage signal proportional to the maximum value of the drain terminal voltage of the secondary switch tube, wherein the proportional coefficient of the first voltage signal to the maximum value is K1; Providing a second voltage signal proportional to the output voltage, wherein a proportionality coefficient between the second voltage signal and the output voltage is K2; generating a control current based on the second divided voltage signal and the resistance value of the resistor; as well as In response to the second conduction enable signal, charging a capacitor using a control current so that the voltage across the capacitor increases starting from the second voltage signal; as well as When the voltage across the capacitor increases to a first voltage signal, a conduction time control signal is generated.

11. The control method according to claim 10, wherein the second conduction duration of the secondary switch is: Among them C S is the capacitance of the capacitor, R TZVS is the resistance of the resistor, V SRD is the maximum value of the drain voltage of the secondary switch tube, Vo is the output voltage, and K is the ratio of the second proportional coefficient K2 to the first proportional coefficient K1.

12. The control method according to claim 9, further comprising: Establishing a primary activation window based on a synchronization signal; Determine whether the delay end point after the voltage across the primary switch tube passes through zero is within the primary turn-on window; If the delay end point occurs within the primary turn-on window, a primary control signal is provided to turn on the primary switch tube; as well as Otherwise, a primary control signal is provided to turn on the primary switch tube at the end of the primary turn-on window.

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