Flyback converter and control circuit and control method thereof

Abstract

The invention provides a flyback converter and a control circuit and a control method thereof, and the control circuit comprises a primary side control circuit which is used for controlling the on and off of a main power tube; the secondary side control circuit is used for controlling the switch-on and switch-off of the synchronous rectifier tube so as to control the first time of switch-on of the synchronous rectifier tube before the excitation current of the flyback converter reaches a zero crossing point for the first time and the second time of switch-on of the synchronous rectifier tube after zero crossing of the excitation current, and the secondary side control circuit determines second time according to a preset volt-second reference and the output voltage of the flyback converter. According to the invention, the extra turn-on pulse width of the synchronous rectifier tube is set in a volt-second integration mode, the magnitude of the negative excitation current can be controlled, negative current sampling during zero voltage turn-on is facilitated, and meanwhile, zero voltage turn-on in a full input voltage range can be realized to the greatest extent, so that the system efficiency is improved.

Description

Technical Field

[0001] The present application relates to the technical field of switching power supplies, and in particular to a flyback converter and a control circuit and a control method thereof. Background Art

[0002] The flyback converter is a common power converter topology used to convert the input voltage into the desired output voltage. In a traditional flyback converter, the conduction of the switch tube is usually achieved through voltage control or current control. However, the traditional flyback converter may generate switching losses and electromagnetic interference when the switch tube is switched.

[0003] In order to reduce these problems, a new flyback converter control technology, namely "zero voltage switching (ZVS)" technology, is proposed. That is, by performing additional reverse excitation on the exciting inductor of the flyback converter, the main power tube is turned on at zero voltage moment with the help of this energy after the synchronous rectifier tube is turned off, that is, the turn-on operation is performed near the zero crossing point of the voltage waveform. This can reduce the turn-on loss of the switch tube and help reduce electromagnetic interference.

[0004] By using zero voltage switching technology, the flyback converter can achieve higher efficiency and lower electromagnetic interference levels, thereby improving the performance and reliability of the entire power system. This technology has been widely used in many power applications and has important development potential in future power converter designs. Summary of the invention

[0005] In order to solve the above technical problems, the present application provides a flyback converter and its control circuit and control method, which are used to control the magnitude of the negative excitation current to facilitate the negative current sampling during zero voltage switching, and can also maximize the zero voltage switching under the full input voltage range to improve the system efficiency.

[0006] According to a first aspect of the present application, a control circuit of a flyback converter is provided, wherein the flyback converter comprises: a transformer, a main power tube, a synchronous rectifier tube, and a control circuit, wherein the control circuit comprises:

[0007] A primary control circuit, used to control the on and off of the main power tube;

[0008] The secondary side control circuit is used to control the on and off of the synchronous rectifier, so as to control the synchronous rectifier to be on for a first time before the exciting current of the flyback converter reaches the zero crossing point for the first time, and to be on for a second time after the exciting current passes through zero.

[0009] The secondary side control circuit determines the second time according to a preset volt-second reference and the output voltage of the flyback converter.

[0010] Optionally, the secondary side control circuit is configured as:

[0011] When the synchronous rectifier starts to conduct for the second time, the output voltage is used to perform volt-second integration on the integration capacitor to obtain an integration signal, and the second time is determined according to the integration signal and the volt-second reference.

[0012] Optionally, the volt-second reference is a preset value.

[0013] Optionally, the volt-second reference is set according to a reference resistor, and the volt-second reference is positively correlated with the resistance value of the reference resistor.

[0014] Optionally, the secondary side control circuit includes:

[0015] A volt-second reference setting circuit, which sets the volt-second reference according to the reference resistance and the reference current;

[0016] an integration circuit, which performs volt-second integration on the integration capacitor using the output voltage to obtain an integration signal when the synchronous rectifier starts to conduct for the second time;

[0017] A comparison circuit has a first input terminal receiving the volt-second reference and a second input terminal receiving the integration signal. When the integration signal reaches the volt-second reference, the comparison circuit triggers and controls the synchronous rectifier to turn off.

[0018] Optionally, the volt-second reference setting circuit comprises:

[0019] Reference resistor;

[0020] The volt-second reference conversion module provides the reference current and is used to convert the reference resistance according to the reference current to generate the volt-second reference.

[0021] Optionally, when the flyback converter operates in the BCM mode, the secondary side control circuit controls the synchronous rectifier to start conducting for the second time when the excitation current passes through zero for the first time;

[0022] When the flyback converter operates in the DCM mode, the secondary side control circuit controls the synchronous rectifier to start conducting for the second time at the valley moment of the drain-source voltage of the synchronous rectifier after the excitation current passes zero for the first time.

[0023] Optionally, the secondary-side control circuit is further used to sample the output voltage to obtain feedback information, and transmit the feedback information to the primary-side control circuit through a non-optical coupling isolation device;

[0024] The primary control circuit analyzes the feedback information to obtain at least one of the control frequency of the flyback converter and the peak current reference of the primary current, and controls the on and off of the main power tube.

[0025] According to a second aspect of the present application, a control circuit of a flyback converter is provided, comprising: a control circuit as described in any embodiment of the present application, wherein the control circuit is used to control the switching state of a switch tube in the flyback converter.

[0026] According to a third aspect of the present application, a control method for a flyback converter is provided. The flyback converter includes: a transformer, a main power tube, and a synchronous rectifier tube. The control method includes:

[0027] After the main power tube is turned off, controlling the synchronous rectifier tube to be turned on for a first time before the exciting current of the flyback converter reaches a zero crossing point for the first time;

[0028] After the excitation current passes through zero, the synchronous rectifier is controlled to be turned on for a second time, so as to achieve zero voltage turn-on of the main power tube in the next switching cycle.

[0029] The second time is determined according to a preset volt-second reference and the output voltage of the flyback converter.

[0030] Optionally, determining the second time according to a preset volt-second reference and the output voltage of the flyback converter includes:

[0031] When the synchronous rectifier starts to conduct for the second time, using the output voltage to perform volt-second integration on the integration capacitor to obtain an integration signal;

[0032] The second time is determined based on the integrated signal and the volt-second reference.

[0033] Optionally, the volt-second reference is a preset value.

[0034] Optionally, the volt-second reference is set according to a reference resistor, and the volt-second reference is positively correlated with the resistance value of the reference resistor.

[0035] The beneficial effects of this application include at least:

[0036] The embodiment of the present application controls the synchronous rectifier to conduct an additional pulse width (i.e., the second time) after the excitation current of the flyback converter passes through zero, so that additional reverse excitation of the transformer excitation inductance can be achieved, and zero voltage switching (ZVS) of the main power tube in the next switching cycle can be achieved. Compared with the traditional scheme, the implementation scheme of the present application sets the second conduction time of the synchronous rectifier by volt-second integration, i.e., the additional turn-on pulse width, so that the size of the negative excitation current generated when the transformer excitation inductance is reversely excited can be controlled, which is convenient for negative current sampling during zero voltage switching. At the same time, this volt-second setting method can also maximize the realization of zero voltage switching under the full input voltage range, optimize the ZVS effect under different input and output voltages, and improve system efficiency.

[0037] It should be noted that the above general description and the following detailed description are merely exemplary and explanatory and do not limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of an implementation of a flyback converter provided according to an embodiment of the present application is shown;

[0039] Figure 2 A schematic diagram showing an implementation of a secondary side control chip in a secondary side control circuit provided according to an embodiment of the present application is shown;

[0040] Figure 3 Show Figure 1 The signal waveform diagram of the flyback converter in DCM mode is shown;

[0041] Figure 4 Show Figure 1 The signal waveform diagram of the flyback converter in BCM mode is shown;

[0042] Figure 5 Show Figure 1 A schematic diagram of an implementation of the secondary side control circuit;

[0043] Figure 6 A schematic flow chart of a control method for a flyback converter provided according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0044] In order to facilitate the understanding of the present application, the present application will be described more comprehensively below with reference to the relevant drawings. The preferred embodiments of the present application are given in the drawings. However, the present application can be implemented in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

[0045] References to "one embodiment" or "some embodiments" etc. described in this specification mean that a particular feature, structure or characteristic described in conjunction with the embodiment is included in one or more embodiments of the present application. Thus, the phrases "in one embodiment", "in some embodiments", "in some other embodiments", "in some other embodiments", etc. that appear at different places in this specification do not necessarily refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized in other ways. The terms "including", "comprising", "having" and their variations all mean "including but not limited to", unless otherwise specifically emphasized in other ways.

[0046] In the description of the present application, the words "exemplary" or "for example" are used to indicate examples, illustrations or explanations. Any embodiment described as "exemplary" or "for example" in the present application should not be interpreted as being more preferred or more advantageous than other embodiments. "And / or" herein is a description of the association relationship of associated objects, indicating that three relationships may exist, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. "Multiple" refers to two or more than two. In addition, in order to facilitate the clear description of the technical solutions of the embodiments of the present application, the words "first", "second" and the like are used to distinguish between the same items or similar items with substantially the same functions and effects. Those skilled in the art will understand that the words "first", "second" and the like do not limit the quantity and execution order, and the words "first", "second" and the like do not limit them to be necessarily different.

[0047] In addition, the same figure marks in the figures represent the same or similar structures, and thus their repeated description will be omitted. That is, the various parts in this specification are described in a combination of parallel and progressive manners, and each part focuses on the differences from other parts. The same or similar parts between the various parts can be referenced to each other.

[0048] For wide-input and wide-output switching power supplies, in order to obtain a better ZVS effect for the main power tube, the relevant technical solutions mainly collect the input voltage and output voltage information of the system, first open-loop fit the additional open pulse width size curve of the synchronous rectifier tube, and then close-loop control the additional open pulse width of the synchronous rectifier tube to perform real-time compensation to confirm that the size of the additional open pulse width of the synchronous rectifier tube can achieve the optimal ZVS effect. However, the error of the additional open pulse width of the synchronous rectifier tube in the open-loop fitting is large, the adaptability is poor, the negative excitation is not uniform, and it is not convenient for the development of products without optocoupler isolation; and the real-time compensation method of closed-loop regulation has a complex control process, and there is a dynamic adjustment process for tracking, and there is an adaptation problem of the dynamic adjustment speed.

[0049] In response to the above problems, the embodiment of the present application discloses a new flyback converter control scheme, which mainly sets the size of the additional open pulse width of the synchronous rectifier tube by means of volt-second integration, does not require open-loop fitting and closed-loop adjustment operations, can not only control the size of the negative excitation current, but also maximize the zero voltage turn-on under the full input voltage range, has high adaptability, and can also improve system efficiency. In addition, the control scheme can be applied to topological structures without optocoupler isolation, which is convenient for the development of products without optocoupler isolation.

[0050] Figure 1 A schematic diagram of an implementation of a flyback converter provided in an embodiment of the present application is shown. Figure 2 A schematic diagram of an implementation of a secondary side control chip in a secondary side control circuit provided in an embodiment of the present application is shown. Figure 1 The isolated switching converter shown is a typical flyback converter topology. It should be noted that this is only an example, and the control scheme of the present application can also be applied to other isolated switching converters, such as forward, flyback, full-bridge or push-pull topologies.

[0051] exist Figure 1 In the example shown, the flyback converter 10 includes: a transformer TR including a primary winding Np and a secondary winding Ns, a voltage input circuit connected to the primary winding Np and a main power tube Q1, a voltage output circuit connected to the secondary winding Ns and a synchronous rectifier tube Q2, and a control circuit.

[0052] exist Figure 1 In the example shown, the voltage input circuit includes a rectifier circuit 12 connected between the power input port 11 of the flyback converter 10 and the primary winding Np, a capacitor EC1 connected between the rectifier circuit 12 and the primary winding Np, a capacitor EC2, and an inductor L DM , and an energy recovery circuit (for example, including a resistor R1, a capacitor C2, and a diode D1) connected between the same-name terminal and the opposite-name terminal of the primary winding Np, wherein the capacitor EC1, the capacitor EC2, the inductor L DM It is used to realize the functions of filtering the output signal of the rectifier circuit 12 and power factor calibration. Of course, other implementations are also possible. In some other embodiments, the above-mentioned filtering and power factor calibration functions can also be realized by other common filtering circuits and power factor correction circuits, which are no longer strictly limited in this article.

[0053] The voltage output circuit includes a capacitor Co connected between the secondary winding Ns and an output port 15 of the flyback converter 10, and the output port 15 is used to connect a load so that the load receives the electric energy (e.g., voltage and current) converted by the flyback converter 10. In some examples, the electric energy converted by the flyback converter 10 also passes through a filter before reaching the load.

[0054] The main power tube Q1 and the sampling resistor Rcs are connected in series between the primary winding Np and the reference ground, and the synchronous rectifier Q2 is connected between the secondary winding Ns and the voltage output circuit. In a possible embodiment, the main power tube Q1 and the synchronous rectifier Q2 are, for example, NMOS field effect transistors. In some other examples, the sampling resistor Rcs may not be provided in the flyback converter 10.

[0055] The control circuit includes a primary control circuit 13 and a secondary control circuit 14. In some examples, the primary control circuit 13 is respectively connected to the control end of the main power tube Q1 and the sampling resistor Rcs, and provides a drive signal Vgs1 to the control end of the main power tube Q1 according to the secondary feedback information and the sampled primary current information to control the on and off of the main power tube Q1. The secondary control circuit 14 is respectively connected to the control end, the drain and the output end of the flyback converter 10 of the synchronous rectifier Q2, and provides a drive signal Vgs2 to the control end of the synchronous rectifier Q2 according to the output voltage Vo of the flyback converter 10 and the drain-source voltage Vds2 of the synchronous rectifier Q2 to control the on and off of the synchronous rectifier Q2.

[0056] In this embodiment, the flyback converter 10 mainly adopts the secondary side master control mode. For example, the secondary side control circuit 14 samples the output voltage Vo to obtain the feedback information required for the primary side control, and transmits the feedback information to the primary side control circuit 13 in the form of pulses through non-optical coupling isolation devices such as transformer TR. The primary side control circuit 13 receives and analyzes the feedback information transmitted by the secondary side to obtain at least one of the control frequency of the flyback converter and the peak current reference of the primary side current (denoted as Ip), and controls the conduction and shutdown of the main power tube Q1 accordingly. Among them, the specific structure and working principle of the primary side control circuit 13 can be understood by referring to the existing technical solutions, and will not be described in detail in this article.

[0057] In some examples, the secondary control circuit 14 obtains an error compensation signal (referred to as a comp signal) according to the output voltage Vo and a preset voltage reference, and generates a pulse signal representing feedback information according to the comp signal. The specific principle can be understood by referring to the relevant prior art solutions, which will not be described in detail in this article.

[0058] The secondary control circuit 14 includes controlling the synchronous rectifier Q2 to be turned on for a first time before the exciting current of the flyback converter reaches the zero crossing point for the first time (including the zero crossing moment when the exciting current reaches the zero crossing point for the first time), and to be turned on for a second time (represented as t w In this embodiment, a small pulse opening method with controllable negative excitation is adopted to control the synchronous rectifier Q2 to be turned on for a second time t after the excitation current passes through zero. w , at the second time t when the synchronous rectifier Q2 is turned onw During this period, the magnetizing inductance of the flyback converter 10 (denoted as Lp, for example, provided by the primary winding Np in the transformer TR) is additionally reversely excited, that is, a negative magnetizing current is generated, and after the synchronous rectifier tube Q2 is turned off, the main power tube Q1 is turned on at zero voltage at zero voltage.

[0059] It should be noted that, in the present embodiment, the zero-crossing moment of the exciting current of the flyback converter (such as the first zero-crossing moment) can be regarded as the critical moment for the on-off switching of the synchronous rectifier Q2. The on-off state of the synchronous rectifier Q2 at this moment can be regarded as the on-state or the off-state, which can specifically assist in the reference judgment of the current working mode of the flyback converter.

[0060] refer to Figure 3 and Figure 4 , Figure 3 Shows Figure 1 The signal waveform diagram of the flyback converter in DCM mode is shown in FIG. Figure 4 Shows Figure 1 1 is a schematic diagram of signal waveforms of the flyback converter in the BCM mode. In these embodiments, the secondary control circuit 14 controls the synchronous rectifier Q2 to turn on at time t0. When the first zero-crossing moment of the excitation current is reached (such as time t1), it indicates that the synchronous rectifier Q2 is turned on for the first time (corresponding to the time period t0-t1). At the same time, the secondary control circuit 14 determines that the synchronous rectifier Q2 is turned on for an additional second time t according to the working mode of the flyback converter 10. w For example, when the flyback converter 10 is detected to be operating in the BCM mode (critical conduction mode), the secondary control circuit 14 controls the synchronous rectifier Q2 to start an additional conduction second time t when the excitation current first crosses zero (such as time t1). w ,like Figure 4 As shown, of course, in some embodiments, the synchronous rectifier Q2 may not be turned off at this moment, and the synchronous rectifier Q2 may be controlled to be directly turned off with a delay; when it is detected that the flyback converter 10 operates in the DCM mode (discontinuous conduction mode), the secondary control circuit 14 controls the synchronous rectifier Q2 to start an additional conduction second time t at the valley moment (such as moment t2) of the drain-source voltage Vds2 of the synchronous rectifier after the excitation current passes zero for the first time. w ,like Figure 3 It can be understood that since both time t1 and time t2 are the times when the magnetizing inductance current is zero, it can be ensured that the initial state of the magnetizing inductance is uniformly zero, which can improve the control accuracy of the flyback converter 10.

[0061] In this embodiment, the secondary control circuit 14 is based on a preset volt-second reference (denoted as V RTS) and the output voltage Vo of the flyback converter determine the second time t w In some embodiments, the secondary control circuit 14 is configured to start conducting the synchronous rectifier Q2 at the second time t w When the output voltage Vo is used to perform volt-second integration on the integrating capacitor, an integral signal is obtained, and then the integral signal and the volt-second reference V RTS Determine the second time t w .

[0062] In other words, the secondary side control circuit 14 determines the additional conduction time t of the synchronous rectifier Q2 according to the working mode of the flyback converter 10. w At the start time, the integrated signal and the volt-second reference V RTS Determine the second additional conduction time t of the synchronous rectifier Q2 w The end time of the second time t w .

[0063] In some preferred embodiments, the volt-second reference V RTS is a preset value. In specific implementation, the volt-second reference V RTS For example, we can use the reference resistance (denoted by R TS ) and the volt-second reference V RTS With reference resistor R TS The resistance value is positively correlated.

[0064] refer to Figure 2 In some embodiments, the secondary control circuit 14 includes a secondary control chip 21 and resistors and capacitors connected to corresponding pins of the secondary control chip 21. Figure 2 In the example shown, the secondary side control chip 21 includes but is not limited to: a VO pin, a COMP pin, an FB pin, an RTS pin, an RVT pin, a GR pin, a SW pin, a GT pin, a VCC pin and a GND pin, wherein the VO pin is coupled to the output end of the flyback converter 10; the COMP pin is connected to the output end of the flyback converter 10 through the resistor R C , capacitor C C The resistor R2 is coupled to the output terminal of the flyback converter 10; the FB pin is connected to the output terminal of the flyback converter 10 through the resistor R 2 The RTS pin is coupled to the output terminal of the flyback converter 10 and is coupled to the reference ground through the resistor R3. TS Coupled to the reference ground; RVT pin through the resistor R VT The SW pin is coupled to the gate of the synchronous rectifier Q2; the GT pin is coupled to the drain of the synchronous rectifier Q2; the VCC pin is connected to the reference ground through the capacitor C VCCS Coupled with the GND pin. It should be noted that Figure 2The pin layout of the secondary-side control chip 21 shown in the figure is only exemplary. In actual applications, the secondary-side control chip 21 may also use other pin layout schemes, and this embodiment does not impose a strict limitation on this.

[0065] Figure 5 Shows Figure 1 A schematic diagram of an implementation of the secondary side control circuit in Figure 5 In the example shown, the secondary control circuit 14 includes: a volt-second reference setting circuit 51, an integration circuit 52, a comparison circuit 53, a mode detection circuit 54 and an RS trigger 55. The volt-second reference setting circuit 51 is used to set the reference resistor R TS and the reference current (denoted as I RTS ) Set the volt-second reference V RTS The integration circuit 52 is used to use the output voltage Vo to integrate the capacitor C when the synchronous rectifier Q2 starts to conduct for the second time. VT Perform volt-second integration to obtain the integral signal V VT The first input terminal of the comparison circuit 53 receives the volt-second reference V RTS The second input terminal of the comparison circuit 53 receives the integral signal V VT , the comparison circuit 53 integrates the signal V VT Reach the volt-second reference V RTS The mode detection circuit 54 is used to detect the working mode of the flyback converter 10, and determine the additional conduction second time t of the synchronous rectifier Q2 according to different working modes. w The set end of the RS trigger 55 is connected to the output end of the mode detection circuit 54, the reset end of the RS trigger 55 is connected to the output end of the comparison circuit 53, and the output end of the RS trigger 55 is directly or through a buffer connected to the control end of the synchronous rectifier Q2.

[0066] In some embodiments, the volt-second reference setting circuit 51 further includes: a reference resistor R TS and a volt-second reference conversion module 511, wherein the volt-second reference conversion module 511 includes but is not limited to providing a reference current I RTS The volt-second reference conversion module 511 is used to convert the reference current I RTS For reference resistance R TS Convert to generate volt-second reference V RTS .

[0067] In some embodiments, the integration circuit 52 further includes: a voltage-to-current conversion module 521 and an integration capacitor C VTThe input end of the voltage-current conversion module 521 receives the output voltage Vo, and the output end of the voltage-current conversion module 521 outputs a current signal Ivo corresponding to the output voltage Vo; the integral capacitor C VT Connected between the output terminal of the voltage-to-current conversion module 521 and the reference ground, used to perform volt-second integration according to the current signal Ivo to obtain an integral signal V VT Of course, the integration circuit 52 also includes a switch tube (not shown) for controlling the start and end of the volt-second integration, and an integration capacitor C VT The voltage across both ends resets the switch tube (not shown).

[0068] refer to Figure 3 and Figure 4 , Figure 3 Shows Figure 1 The signal waveform diagram of the flyback converter in DCM mode is shown in FIG. Figure 4 Shows Figure 1 The signal waveform diagram of the flyback converter in BCM mode is shown in FIG. Figure 3 , Figure 4 and Figure 5 , the working principle of the secondary side control circuit 14 is as follows:

[0069] At time t0, the secondary control circuit 14 controls the synchronous rectifier Q2 to be turned on.

[0070] During the conduction period of the synchronous rectifier Q2, the mode detection circuit 54 detects the working mode of the flyback converter 10. When it is detected that the flyback converter 10 is working in the BCM mode, starting from the first zero crossing moment of the excitation current (moment t1), the current signal Ivo of the output voltage Vo is used to detect the integral capacitor C VT Perform volt-second integration and use the integrated signal V VT Reach the volt-second reference V RTS When the synchronous rectifier Q2 is turned off (such as time t5), it is equivalent to controlling the synchronous rectifier Q2 to delay the second time t in one switching cycle. w (corresponding to the time period t1-t5); when it is detected that the flyback converter 10 operates in the DCM mode, the synchronous rectifier Q2 is controlled to be turned off for the first time at the first zero-crossing moment of the excitation current (time t1), and the valley moment (such as time t2) of the drain-source voltage Vds2 of the synchronous rectifier after the first zero-crossing moment of the excitation current is detected, and starting from the valley moment (time t2), the current signal Ivo of the output voltage Vo is used to adjust the integral capacitor C VT Perform volt-second integration, and the integrated signal V VT Reach the volt-second reference V RTSAt the time (such as time t3), the synchronous rectifier Q2 is controlled to be turned off for the second time, which is equivalent to controlling the synchronous rectifier Q2 to be turned on twice in one switching cycle, and the second turn-on time is the second time t w (corresponding to the time period t2-t3).

[0071] In this embodiment, the volt-second reference V RTS The settings meet the following conditions:

[0072]

[0073] L p *I neg =N ps / V o *t w (2)

[0074] Wherein, Lp is the excitation inductance, Ineg is the average value of the negative excitation current, Ceq is the equivalent capacitance at the connection node between the main power tube Q1 and the primary winding Np, including but not limited to the parasitic capacitance between the drain and source terminals of the main power tube Q1, V DC is the DC bus voltage output by the rectifier circuit 11, Nps is the turns ratio of the primary and secondary windings of the transformer TR, Vo is the output voltage, t w It is the pulse width of the additional opening of the synchronous rectifier Q2, that is, the second time.

[0075] It can be concluded from formula (1) and formula (2) that, when the magnetizing inductance Lp of the flyback converter 10 is determined, when the volt-second reference V RTS When the preset value (such as a fixed value) is set, the negative excitation current corresponding to the excitation inductance Lp is consistent, that is, when the volt-second reference V RTS When set to the preset value, the corresponding negative excitation energy is fixed. At this time, at a certain DC bus V DC (or input voltage Vin), the ZVS of the primary main power tube Q1 can be achieved. Therefore, in some preferred examples, the highest input voltage Vin, i.e., the DC bus V DC Maximum case design volt-second reference V RTS , so that ZVS of the main power tube Q1 can be achieved under the full input voltage range.

[0076] In the embodiment of the present application, the size of the additional opening pulse width of the synchronous rectifier tube (ie, the second time t w ), which can not only control the magnitude of the negative excitation current, but also realize zero voltage switching under the full input voltage range to the maximum extent, which is beneficial to improving the system efficiency and realizing a simple circuit structure.

[0077] Furthermore, the embodiment of the present application also provides a control method for a flyback converter, and the control method can be applied to the flyback converter 10 shown in any of the above embodiments, such as Figure 6 As shown, the control method includes executing the following steps:

[0078] In step 61, after the main power tube is turned off, the synchronous rectifier tube is controlled to be turned on for a first time before the exciting current of the flyback converter reaches the zero crossing point for the first time.

[0079] In this step, the method of controlling the synchronous rectifier to be turned on for the first time before the excitation current of the flyback converter passes through zero can be implemented by referring to the relevant existing technical solutions. For example, the synchronous rectifier can be turned on after a predetermined dead time when the main power tube is turned off, and a fixed first time for the synchronous rectifier to be turned on can be set, or the synchronous rectifier can be controlled to be turned on to the first zero-crossing moment of the excitation current of the flyback converter, and this conduction time is used as the first time, etc.

[0080] In step 62, after the excitation current passes through zero, the synchronous rectifier is controlled to be turned on for a second time so as to achieve zero voltage turn-on of the main power tube in the next switching cycle, wherein the second time is determined according to a preset volt-second reference and the output voltage of the flyback converter.

[0081] In this step, the method for determining the second time according to the preset volt-second reference and the output voltage of the flyback converter includes: when the synchronous rectifier starts to conduct for the second time, using the output voltage to perform volt-second integration on the integration capacitor to obtain an integration signal; and determining the second time according to the integration signal and the volt-second reference. For example, the time required for the integration signal to reach the preset volt-second reference from the initial potential state (such as zero potential) is taken as the second time, which is equivalent to controlling the synchronous rectifier to turn off when the integration signal reaches the volt-second reference.

[0082] In some preferred examples, the preset volt-second reference is a preset value. In specific implementation, the volt-second reference is set according to a reference resistor, and the volt-second reference is positively correlated with the resistance value of the reference resistor.

[0083] It should be noted that, in specific implementation, the specific implementation of each step in the control method of the flyback converter described above and the technical effects that can be obtained can be found in the aforementioned flyback converter embodiment, and will not be repeated here.

[0084] In summary, the embodiment of the present application controls the synchronous rectifier to conduct an additional pulse width (i.e., the second time) after the excitation current of the flyback converter passes through zero, so that additional reverse excitation of the transformer excitation inductance can be achieved, and zero voltage switching (ZVS) of the main power tube in the next switching cycle can be achieved. Compared with the traditional scheme, the implementation scheme of the present application sets the second conduction time of the synchronous rectifier by volt-second integration, that is, the additional turn-on pulse width, so that the size of the negative excitation current generated when the transformer excitation inductance is reversely excited can be controlled, which is convenient for negative current sampling during zero voltage switching. At the same time, this volt-second setting method can also maximize the realization of zero voltage switching under the full input voltage range, optimize the ZVS effect under different input and output voltages, and improve system efficiency.

[0085] Finally, it should be noted that: Obviously, the above embodiments are only examples for clearly explaining the present application, and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the scope of protection of the present application.

Claims

1. A control circuit of a flyback converter, the flyback converter comprising: A transformer, a main power tube, a synchronous rectifier tube, and a control circuit, wherein the control circuit includes: A primary control circuit, used to control the on and off of the main power tube; The secondary side control circuit is used to control the on and off of the synchronous rectifier, so as to control the synchronous rectifier to be on for a first time before the exciting current of the flyback converter reaches the zero crossing point for the first time, and to be on for a second time after the exciting current passes through zero. The secondary side control circuit determines the second time according to a preset volt-second reference and the output voltage of the flyback converter.

2. The control circuit according to claim 1, wherein: The secondary side control circuit is configured as follows: When the synchronous rectifier starts to conduct for the second time, the output voltage is used to perform volt-second integration on the integration capacitor to obtain an integration signal, and the second time is determined according to the integration signal and the volt-second reference.

3. The control circuit according to claim 1 or 2, wherein: The volt-second reference is a preset value.

4. The control circuit according to claim 3, wherein: The volt-second reference is set according to a reference resistor, and the volt-second reference is positively correlated with the resistance value of the reference resistor.

5. The control circuit according to claim 4, wherein: The secondary side control circuit comprises: A volt-second reference setting circuit, which sets the volt-second reference according to the reference resistance and the reference current; an integration circuit, which performs volt-second integration on the integration capacitor using the output voltage to obtain an integration signal when the synchronous rectifier starts to conduct for the second time; A comparison circuit has a first input terminal receiving the volt-second reference and a second input terminal receiving the integration signal. When the integration signal reaches the volt-second reference, the comparison circuit triggers and controls the synchronous rectifier to turn off.

6. The control circuit according to claim 5, wherein: The volt-second reference setting circuit comprises: Reference resistor; The volt-second reference conversion module provides the reference current and is used to convert the reference resistance according to the reference current to generate the volt-second reference.

7. The control circuit according to claim 2, wherein: When the flyback converter operates in the BCM mode, the secondary side control circuit controls the synchronous rectifier to start conducting for the second time when the excitation current passes through zero for the first time; When the flyback converter operates in the DCM mode, the secondary side control circuit controls the synchronous rectifier to start conducting for the second time at the valley moment of the drain-source voltage of the synchronous rectifier after the excitation current passes zero for the first time.

8. The control circuit according to claim 1, wherein: The secondary side control circuit is also used to sample the output voltage to obtain feedback information, and transmit the feedback information to the primary side control circuit through a non-optical coupling isolation device; The primary control circuit analyzes the feedback information to obtain at least one of the control frequency of the flyback converter and the peak current reference of the primary current, and controls the on and off of the main power tube.

9. A control circuit for a flyback converter, comprising: The control circuit according to any one of claims 1 to 8, The control circuit is used to control the switching state of the switch tube in the flyback converter.

10. A control method for a flyback converter, the flyback converter comprising: Transformer, main power tube and synchronous rectifier tube, the control method includes: After the main power tube is turned off, controlling the synchronous rectifier tube to be turned on for a first time before the exciting current of the flyback converter reaches a zero crossing point for the first time; After the excitation current passes through zero, the synchronous rectifier is controlled to be turned on for a second time, so as to achieve zero voltage turn-on of the main power tube in the next switching cycle. The second time is determined according to a preset volt-second reference and the output voltage of the flyback converter.

11. The control method according to claim 10, wherein: Determining the second time according to a preset volt-second reference and the output voltage of the flyback converter includes: When the synchronous rectifier starts to conduct for the second time, using the output voltage to perform volt-second integration on the integration capacitor to obtain an integration signal; The second time is determined based on the integrated signal and the volt-second reference.

12. The control method according to claim 10 or 11, wherein: The volt-second reference is a preset value.

13. The control method according to claim 12, wherein: The volt-second reference is set according to a reference resistor, and the volt-second reference is positively correlated with the resistance value of the reference resistor.

Images