A slope detection circuit and method for a flyback switching power supply

Abstract

The application discloses a slope detection circuit for a flyback switching power supply, comprising a voltage follower unit, an input end of the voltage follower unit receiving an auxiliary winding induced voltage VS of the flyback switching power supply and outputting a follower voltage V1 corresponding to a change of the auxiliary winding induced voltage; a slope detection unit connected with the voltage follower unit, for generating a slope detection voltage according to a change rate of the follower voltage V1; a comparison unit connected with the slope detection unit, for comparing the slope detection voltage with a reference voltage and outputting an indication signal when the slope detection voltage drops below the reference voltage; wherein the indication signal represents that the flyback switching power supply enters a free resonance stage.

Description

Technical Field

[0001] This invention relates to the field of circuit design, and more specifically to a slope detection circuit and method for flyback switching power supplies. Background Technology

[0002] With the continuous development of switching power supply technology, quasi-resonant flyback switching power supplies have been widely used in consumer electronics, power adapters, and industrial power supplies due to their advantages such as small size, high efficiency, and relatively low electromagnetic interference. In quasi-resonant control mode, the turn-on timing of the MOSFET typically needs to match the resonance process formed by the transformer's magnetizing inductance and parasitic capacitance to reduce switching losses and improve overall system efficiency. Therefore, accurately determining whether the flyback switching power supply enters the free resonance stage after energy transfer is a crucial prerequisite for achieving stable quasi-resonant control.

[0003] To determine whether a system has entered a quasi-resonant state, it is typically necessary to detect the changing characteristics of the induced voltage or related voltage signals in the auxiliary winding. Slope detection circuits are a common implementation method. Existing technologies employ various circuit implementations for slope detection, ranging from analog circuit-based schemes to those combining digital logic or mixed-signal processing. These schemes vary significantly in structural form and implementation complexity. However, in practical integrated circuit applications, these schemes generally suffer from relatively complex circuit structures, a large number of components, and strong dependence on process conditions and parameter matching. This results in limitations in chip area, power consumption control, and system reliability, making it difficult to achieve an effective balance between performance and cost. Summary of the Invention

[0004] The purpose of this invention is to provide a slope detection circuit and method for flyback switching power supplies, so as to improve the detection accuracy and operational reliability of flyback switching power supplies in the quasi-resonant control process.

[0005] To achieve the above objectives, the present invention discloses the following technical solution: A first aspect of the present invention provides a slope detection circuit for a flyback switching power supply, comprising: The voltage follower unit receives the induced voltage VS of the auxiliary winding of the flyback switching power supply at its input terminal and outputs a follower voltage V1 that changes in accordance with the induced voltage of the auxiliary winding. A slope detection unit, connected to the voltage follower unit, is used to generate a slope detection voltage based on the rate of change of the follower voltage V1. A comparison unit, connected to the slope detection unit, is used to compare the slope detection voltage with a reference voltage, and outputs an indication signal when the slope detection voltage drops below the reference voltage; The indicator signal indicates that the flyback switching power supply has entered the free resonance stage.

[0006] Optionally, the voltage follower unit includes a first constant current source I1 and a first MOSFET M1; The input terminal of the first constant current source I1 is connected to the power supply voltage, the output terminal is connected to the source of the first MOS transistor M1, the drain of the first MOS transistor M1 is grounded, and the gate of the first MOS transistor M1 is connected to the auxiliary winding induced voltage VS. The source voltage of the first MOS transistor M1 is used as the follower voltage V1.

[0007] Optionally, the slope detection unit includes a first capacitor C1 and a second constant current source I2; The input terminal of the second constant current source I2 is connected to the power supply voltage, the first terminal of the first capacitor C1 is connected to the source of the first MOSFET M1, and the second terminal of the first capacitor C1 is connected to the output terminal of the second constant current source I2.

[0008] Optionally, the slope detection unit further includes: The first resistor R1 has its first end connected to the output terminal of the second constant current source I2 and its second end grounded. The voltage at the first end of the first resistor R1 is used as the slope detection voltage.

[0009] Optionally, the comparison unit includes a third constant current source I3 and a second resistor R2; The input terminal of the third constant current source I3 is connected to the power supply voltage, the output terminal of the third constant current source I3 is connected to the first terminal of the second resistor R2, the second terminal of the second resistor R2 is grounded, and the voltage at the first terminal of the second resistor R2 is used as the reference voltage.

[0010] Optionally, the slope detection unit further includes a third resistor R3 and a second MOS transistor M2; The first end of the third resistor R3 is connected to the output end of the second constant current source I2, and the second end of the third resistor R3 is connected to the second end of the first capacitor C1. The gate of the second MOS transistor M2 is connected to the first terminal of the third resistor R3, the drain is connected to the second terminal of the third resistor R3, and the source is grounded; The drain voltage of the second MOS transistor M2 is used as the slope detection voltage.

[0011] Optionally, the comparison unit includes a fourth constant current source I4, a fourth resistor R4, and a third MOSFET M3; The input terminal of the fourth constant current source I4 is connected to the power supply voltage, the output terminal is connected to the first terminal of the fourth resistor R4, and the second terminal of the fourth resistor R4 is connected to the drain of the third MOS transistor M3. The gate of the third MOS transistor M3 is connected to the first terminal of the fourth resistor R4, and the source of the third MOS transistor M3 is grounded. The drain voltage of the third MOS transistor M3 is used as the reference voltage.

[0012] Optionally, the comparison unit further includes: The comparator CMP has its non-inverting input connected to the reference voltage, its inverting input connected to the slope detection voltage, and its output serving as the output of the comparator unit.

[0013] Optionally, the first MOS transistor M1 is a PMOS transistor, and the second MOS transistor M2 and the third MOS transistor M3 are both NMOS transistors.

[0014] Secondly, the present invention provides a slope detection method for a flyback switching power supply, comprising the following steps: Obtain the induced voltage VS of the auxiliary winding of the flyback switching power supply, and generate a following voltage V1 that varies with the induced voltage VS of the auxiliary winding. A slope detection voltage is generated based on the rate of change of the following voltage V1; The slope detection voltage is compared with a reference voltage. When the slope detection voltage drops below the reference voltage, an indication signal is output. The indication signal is used to characterize that the flyback switching power supply has entered the free resonance stage.

[0015] The effects described in the invention are merely those of the embodiments, and not all the effects of the invention. One of the above technical solutions has the following advantages or beneficial effects: The slope detection circuit for a flyback switching power supply provided in this application detects the slope of the induced voltage change in the auxiliary winding of the flyback switching power supply. When the voltage drop slope reaches a preset condition, an indication signal is output. This allows for reliable characterization of the flyback switching power supply's state transition from the energy transfer stage to the free resonance stage without directly participating in power switching control. This approach enables the system to clearly distinguish between periods of slight voltage disturbances or atypical oscillations and the true free resonance stage, improving the accuracy and stability of quasi-resonance state determination and providing clear and reliable state indications for subsequent control or detection modules.

[0016] Furthermore, in the specific implementation of the slope detection circuit, a combined structure of a constant current source, an RC network, and switching devices is introduced. By processing the slope detection voltage change rate through current-based and voltage-based methods, stable extraction and comparison of slope characteristics are quickly achieved. This structure reduces the influence of parasitic parameters of semiconductor devices, lowers sensitivity to process deviations and parameter fluctuations, has a relatively fast response speed, and is easy to integrate into power management chips. While ensuring detection consistency, it helps to reduce circuit complexity, chip area, and power consumption, thereby improving the overall system reliability and engineering applicability. Attached Figure Description

[0017] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments consistent with this application and, together with the specification, serve to explain the principles of this application.

[0018] Figure 1 The circuit topology of the flyback switching power supply is shown. Figure 2 A structural block diagram of a slope detection circuit for a flyback switching power supply according to an embodiment of the present invention is shown. Figure 3 A circuit topology diagram of a slope detection circuit according to an embodiment of the present invention is shown; Figure 4 The relevant signal waveforms of the slope detection circuit of the present invention are shown. Figure 5 A circuit topology diagram of a slope detection circuit according to another embodiment of the present invention is shown; Figure 6 A schematic flowchart of a slope detection method for a flyback switching power supply according to an embodiment of the present invention is shown. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0020] It should be noted that references to "an embodiment," "embodiment," "example embodiment," etc., in this specification refer to the described embodiment including specific features, structures, or characteristics; however, not every embodiment must include these specific features, structures, or characteristics. Furthermore, such expressions do not refer to the same embodiment. Moreover, when describing specific features, structures, or characteristics in conjunction with embodiments, whether or not explicitly described, it is indicated that incorporating such features, structures, or characteristics into other embodiments is within the knowledge of those skilled in the art.

[0021] Furthermore, certain terms are used in the specification and subsequent claims to refer to specific components or parts. Those skilled in the art will understand that manufacturers may use different names or terms to refer to the same component or part. This specification and subsequent claims do not distinguish components or parts by differences in name, but rather by differences in function. The terms "comprising" and "including" used throughout the specification and subsequent claims are open-ended and should be interpreted as "including but not limited to." Additionally, the term "connection" here includes any direct and indirect electrical connection means. Indirect electrical connection means include connections made through other means.

[0022] Figure 1 The circuit topology of a flyback switching power supply is shown. (For example...) Figure 1 As shown, this flyback switching power supply includes a primary winding NP, a secondary winding NS, and an auxiliary winding NA. One end of the primary winding NP is connected to the input power supply, and the other end is grounded through a primary MOSFET Q and a resistor R14. The control terminal of the primary MOSFET Q is connected to the power supply chip to control the on / off state of the primary winding. The secondary winding NS is used to output energy to the load side. The first end of the auxiliary winding NA is connected to the VCC pin of the power supply chip through a diode D13 to provide the operating voltage to the power supply chip, and the second end of the auxiliary winding NA is grounded. Simultaneously, the first end of the auxiliary winding NA is also grounded through resistors R12 and R13 in sequence. The node voltage between resistors R12 and R13 serves as the auxiliary winding induced voltage VS of the flyback switching power supply and is output to the power supply chip.

[0023] During the operation of a flyback switching power supply, after the energy of the secondary coil NS is released, a free resonance occurs between the inductance of the primary coil NP and the parasitic capacitance Cds of the primary MOSFET Q. This free resonance is induced by the auxiliary coil NA and reflected in the induced voltage VS of the auxiliary winding, causing VS to exhibit obvious high-frequency oscillation characteristics. The slope detection circuit provided by this invention is based on detecting the change characteristics of the induced voltage VS of the auxiliary winding. By monitoring the falling slope of VS, it determines the moment when the flyback switching power supply enters the free resonance stage from the energy transfer stage. This detection result can be used as the starting condition for subsequent zero-crossing detection, timing control, or related logic modules. Therefore, the reliability of the output signal of the slope detection circuit is directly related to the accuracy of subsequent control signals, thereby affecting the overall efficiency and operating performance of the flyback switching power supply system.

[0024] Figure 2 A structural block diagram of a slope detection circuit for a flyback switching power supply according to an embodiment of the present invention is shown. Figure 2 As shown, the slope detection circuit includes a voltage follower unit 10, a slope detection unit 20, and a comparison unit 30. The input of the voltage follower unit 10 receives the induced voltage VS from the auxiliary winding of the flyback switching power supply and outputs a follower voltage V1 that changes in accordance with the induced voltage of the auxiliary winding. The slope detection unit 20 is connected to the voltage follower unit 10 and generates a slope detection voltage based on the rate of change of the follower voltage V1. The comparison unit 30 is connected to the slope detection unit 20 and compares the slope detection voltage with a reference voltage. When the slope detection voltage drops below the reference voltage, it outputs an indication signal. This indication signal indicates that the flyback switching power supply has entered the free resonance stage.

[0025] According to the above embodiments, by detecting the slope of the change characteristics of the induced voltage in the auxiliary winding of the flyback switching power supply, and outputting an indication signal when the voltage drop slope reaches a preset condition, the state transition of the flyback switching power supply from the energy transfer stage to the free resonance stage can be reliably characterized without directly participating in power switch control. This method enables the system to clearly distinguish between the stage of slight voltage disturbance or atypical oscillation and the stage of true free resonance, which is beneficial to improving the accuracy and stability of quasi-resonance state determination and providing clear and reliable state indications for subsequent control or detection modules.

[0026] To facilitate understanding of the implementation of the slope detection circuit of the present invention, the internal circuit structure and signal relationships are described in detail below with reference to specific embodiments.

[0027] Example 1: Figure 3 A circuit topology diagram of a slope detection circuit according to an embodiment of the present invention is shown. Figure 3 As shown, the voltage follower unit 10 includes a first constant current source I1 and a first MOSFET M1. The input terminal of the first constant current source I1 is connected to the power supply voltage, and the output terminal is connected to the source of the first MOSFET M1. The drain of the first MOSFET M1 is grounded, and the gate of the first MOSFET M1 is connected to the auxiliary winding induced voltage VS. The source voltage of the first MOSFET M1 serves as the follower voltage V1.

[0028] In this embodiment, the first MOS transistor M1 operates under the bias of the first constant current source I1. This first MOS transistor M1 is a PMOS transistor, and its gate is connected to the auxiliary winding induced voltage VS. This allows the source voltage of the first MOS transistor M1 to change accordingly with the change of VS, thereby forming a follower voltage V1. In this way, the change of the auxiliary winding induced voltage VS is introduced into the chip and provided to subsequent circuits in the form of the follower voltage V1, providing a basic signal for the slope detection process. Based on this, a voltage follower structure composed of a constant current source and a MOS transistor is used to ensure that the follower voltage V1 and the auxiliary winding induced voltage VS maintain the same trend in change, avoiding direct application of the auxiliary winding voltage to subsequent circuits, and isolating the reverse influence of current changes in the subsequent slope detection circuit on the VS signal. This structure helps reduce interference between the auxiliary winding voltage signal and the detection comparison process, allowing subsequent circuits to process the voltage change rate under relatively stable and controllable voltage conditions, thus ensuring the accuracy of the slope detection results.

[0029] refer to Figure 3 The slope detection unit 20 includes a first capacitor C1 and a second constant current source I2. The input terminal of the second constant current source I2 is connected to the power supply voltage, the first terminal of the first capacitor C1 is connected to the source of the first MOSFET M1, and the second terminal of the first capacitor C1 is connected to the output terminal of the second constant current source I2.

[0030] In this embodiment, the first capacitor C1 is a slope detection capacitor, a key component responding to the rate of change of the following voltage V1. When the following voltage V1 changes with the induced voltage VS of the auxiliary winding, a current component I5 related to the rate of voltage change is generated across the first capacitor C1. Simultaneously, under the action of the second constant current source I2, this current component I5 works together with the constant current to form a node voltage VN1 at the second terminal of the first capacitor C1 that varies with the rate of voltage change. Thus, the rate of change of the following voltage V1 is converted into a corresponding voltage change characteristic and output to the subsequent circuit.

[0031] Furthermore, during the slope detection process, the current component I5 generated by the first capacitor C1 changes through the branch containing the second constant current source. This current does not return to the node where the auxiliary winding induced voltage VS is located via the gate path of the MOS transistor. Therefore, the current change in the slope detection path does not directly affect the VS signal, thereby reducing its reverse disturbance to the VS signal and enabling the following voltage V1 to more stably and accurately reflect the change process of VS.

[0032] In this way, the embodiment transforms the change process of the induced voltage in the auxiliary winding from a change in voltage amplitude into an electrical signal related to the rate of change, allowing the slope information to be extracted in an intuitive and controllable form. Compared to directly judging the voltage amplitude, this structure is more conducive to accurately reflecting the characteristics of the rapid voltage drop phase, providing a clear and reliable slope criterion for the subsequent comparison unit 30 to determine whether the system has entered the free resonance phase.

[0033] refer to Figure 3 The slope detection unit 20 also includes a first resistor R1. The first end of the first resistor R1 is connected to the output terminal of the second constant current source I2, and the second end is grounded. The voltage at the first end of the first resistor R1 serves as the slope detection voltage VN1.

[0034] In this embodiment, the first resistor R1 is used to convert the current change formed by the combined action of the second constant current source I2 and the first capacitor C1 into a voltage, so that the result of the slope detection process is reflected in voltage form. When the following voltage V1 changes and a current component related to the rate of change is introduced under the action of the first capacitor C1, this current forms a corresponding voltage change across the first resistor, thereby obtaining the slope detection voltage VN1. Thus, the voltage change rate information is further stably mapped into a voltage signal that can be directly used for comparison.

[0035] refer to Figure 3 The comparison unit 30 includes a third constant current source I3 and a second resistor R2. The input terminal of the third constant current source I3 is connected to the power supply voltage, and the output terminal of the third constant current source I3 is connected to the first terminal of the second resistor R2. The second terminal of the second resistor R2 is grounded, and the voltage at the first terminal of the second resistor R2 serves as the reference voltage VP1.

[0036] In this embodiment, the third constant current source I3 and the second resistor R2 cooperate to form a stable reference voltage generation structure. Since the output current of the third constant current source I3 remains constant, the current flowing through the second resistor R2 does not change significantly with the system's operating state, and the voltage formed at its terminals remains at a relatively stable level, serving as the reference voltage VP1 for the subsequent comparison process. This reference voltage VP1 is independent of the transient changes in the auxiliary winding induced voltage VS, providing a fixed reference for determining the slope detection voltage. Through the above setup, the comparison unit 30 can determine the slope detection voltage under stable reference conditions, making the comparison result primarily determined by the voltage change rate, and less susceptible to power fluctuations or changes in the amplitude of the auxiliary winding voltage. This helps improve the consistency and reliability of the indicator signal generation time.

[0037] refer to Figure 3The comparison unit 30 also includes a comparator CMP. The non-inverting input of the comparator CMP is connected to a reference voltage, the inverting input is connected to a slope detection voltage, and the output is used as the output of the comparison unit 30 to output an indication signal when the slope detection voltage drops below the reference voltage.

[0038] In this embodiment, the comparator CMP receives a reference voltage and a follower voltage V1 from the slope detection unit 20, and determines the state of the system slope change by comparing the magnitudes of the two. When the follower voltage V1 drops below the reference voltage, the comparator output generates a high-level indication signal, indicating that the flyback switching power supply has entered the free resonance stage. Through this comparison method, the indication signal can accurately reflect the quasi-resonance start time of the system, providing a reliable trigger condition for subsequent zero-crossing detection or timing logic, helping to ensure the timing accuracy of MOSFET control, and improving the overall efficiency and stability of the switching power supply.

[0039] Figure 4 The relevant signal waveforms of the slope detection circuit of the present invention are shown. (Reference) Figure 3 and Figure 4 The slope detection circuit in this embodiment works as follows: the voltage VP1 at the non-inverting input of the comparator CMP is the voltage of the second resistor R2. Since the current of the third constant current source I3 is constant, the reference voltage VP1 is also a fixed value. The following voltage V1 is introduced by the induced voltage VS of the auxiliary winding, and its level changes with the change of VS, thereby causing the slope detection voltage VN1 formed by the first resistor R1 to fluctuate.

[0040] During the phase when the secondary coil NS is normally conducting and transferring energy, the induced voltage VS of the auxiliary winding remains stable, and the follower voltage V1 is correspondingly stable. At this time, the slope detection voltage... And VN1 > VP1, the output signal Sout of comparator CMP is low. At this time, the output signal Sout is in a low-level stable state and does not constitute an indication signal output. After the energy in the secondary coil NS is released, the induced voltage VS of the auxiliary winding begins to decrease and enters the resonance process. The following voltage V1 decreases rapidly. Due to the effect of the first capacitor C1, a current related to the voltage change rate is generated during the voltage change process. Here, ∆u / ∆t reflects the slope of the voltage change across the first capacitor C1, thus indirectly reflecting the slope of the induced voltage VS of the auxiliary winding. The first capacitor C1 isolates the DC component and transmits only the voltage change signal, enabling the slope detection unit 20 to accurately respond to changes in VS. A sudden drop in voltage V1 causes the slope detection voltage VN1 to drop rapidly accordingly. At this time, the output signal Sout of the comparator CMP flips from low level to high level. This high-level signal corresponds to the output form of an indicator signal, which is used to indicate that the system has entered the resonance process.

[0041] As the induced voltage VS of the auxiliary winding continues to decrease, the slope detection voltage VN1 further decreases, and the output signal Sout remains high. When the slope of the induced voltage VS reaches its maximum, the current I5 generated by the first capacitor C1 reaches its maximum value, and the corresponding slope detection voltage VN1 drops to its minimum point. Subsequently, as the rate of decrease of the induced voltage VS gradually slows down, the current I5 decreases accordingly, and the slope detection voltage VN1 begins to rise. When the slope detection voltage VN1 again satisfies VN1 > VP1, the output signal Sout of the comparator CMP flips to a low level, which corresponds to the end of the indication signal. When the slope of the induced voltage VS decreases to 0, the current I5 also decreases to 0. During the process of the induced voltage VS crossing the minimum point and entering the rising phase, the slope detection voltage VN1 is always greater than the reference voltage VP1, and the output signal Sout remains low.

[0042] Therefore, during the resonant cycle of the first auxiliary winding induced voltage VS, the output signal Sout outputs a high-level pulse, which constitutes one indication signal output process. In the subsequent second resonant cycle, a second high-level pulse will be output in the same manner. When the system detects a high-level pulse in the output signal Sout, i.e., when the indication signal is detected, it can determine that the flyback switching power supply has entered the resonant state.

[0043] In this invention, to accurately detect the falling slope of the induced voltage VS in the auxiliary winding, the target slope is first determined. Based on this, a suitable first capacitor C1 is selected so that the current I5 generated through the first capacitor C1 meets the design requirements. Subsequently, according to the required parameters of I2, I3, and resistors R1 and R2, the input of comparator CMP is configured so that its output signal Sout can drop to the set slope when the induced voltage VS of the auxiliary winding falls to the set slope. The output signal Sout reliably flips at the designated time. Therefore, the flip point of Sout can serve as an indicator signal for the system entering a quasi-resonant state, providing accurate triggering conditions for subsequent zero-crossing detection or control logic. This invention not only ensures the controllability and reliability of quasi-resonant state judgment but also provides a clear basis for circuit parameter design, thereby improving the response accuracy and operational stability of the switching power supply control system.

[0044] In the first embodiment described above, the comparator CMP is composed of semiconductor devices, specifically MOS transistors, and its input terminal has a parasitic capacitance (not shown in the figure). When the current of the second constant current source I2 is small, and the first resistor R1 and the parasitic capacitance are large, the parasitic capacitance will cause the response speed of the slope detection voltage VN1 to decrease, resulting in a lag phenomenon, which will affect the accuracy of subsequent sampling. To solve the above problem, the present invention provides a second embodiment, which improves the response speed of the slope detection voltage by optimizing the circuit structure or parameter configuration, thereby ensuring the accuracy and reliability of the indication signal.

[0045] Example 2: Figure 5 A circuit topology diagram of a slope detection circuit according to another embodiment of the present invention is shown. Figure 5 As shown, the difference between this embodiment and the first embodiment above lies in the specific structural settings of the slope detection unit 20 and the comparison unit 30.

[0046] Specifically, in this embodiment, reference Figure 5 The slope detection unit 20, in addition to including the second constant current source I2 and the first capacitor C1, further includes a third resistor R3 and a second MOSFET M2. The first terminal of the third resistor R3 is connected to the output terminal of the second constant current source I2, and the second terminal of the third resistor R3 is connected to the second terminal of the first capacitor C1. The gate of the second MOSFET M2 is connected to the first terminal of the third resistor R3, the drain is connected to the second terminal of the third resistor R3, and the source is grounded. The drain voltage of the second MOSFET M2 serves as the slope detection voltage VN2.

[0047] In this embodiment, a third resistor R3 and a second MOSFET M2 are introduced into the slope detection unit 20, changing the method of obtaining the slope detection voltage compared to the previous embodiment. The slope detection voltage VN2 is no longer directly taken from the connection node between the constant current source and the resistor, but is indirectly formed through the conduction state of the second MOSFET M2. The second MOSFET M2 is a NOMS transistor, and its conduction is controlled by the node potential where the third resistor R3 is located. Its drain node voltage is used to reflect the slope information related to the rate of change of the auxiliary winding induced voltage VS. In this structure, a controlled current and voltage relationship is formed between the second constant current source I2, the third resistor R3, and the second MOSFET M2, so that the second MOSFET M2 acts as a buffer for the inverting input terminal of the comparator CMP during the slope detection process. Specifically, through the conduction characteristics of the second MOSFET M2, the slope detection voltage VN2 is soft-clamped, limiting its voltage rise trend, thereby avoiding the parasitic capacitance at the input terminal of the subsequent comparator from having a significant impact on the slope detection node.

[0048] By setting up as described above, without changing the basic function of slope detection, it is beneficial to improve the response speed of the subsequent circuit to the slope detection path, so that the slope detection voltage can maintain a good response speed and stability during rapid changes, thereby improving the accuracy and reliability of subsequent comparison and judgment.

[0049] refer to Figure 5 In this embodiment, the comparison unit 30, in addition to including the comparator CMP, further includes a fourth constant current source I4, a fourth resistor R4, and a third MOSFET M3. The input terminal of the fourth constant current source I4 is connected to the power supply voltage, the output terminal is connected to the first terminal of the fourth resistor R4, and the second terminal of the fourth resistor R4 is connected to the drain of the third MOSFET M3. The gate of the third MOSFET M3 is connected to the first terminal of the fourth resistor R4, and the source of the third MOSFET M3 is grounded. The drain voltage of the third MOSFET M3 serves as the reference voltage VP2.

[0050] In this embodiment, the reference voltage VP2 is no longer directly provided by the compared node, but is achieved through an independent branch consisting of a constant current source, resistive components, and a MOSFET. The third MOSFET M3 is controlled to conduct based on the potential change of the node containing the fourth resistor R4. This third MOSFET M3 is an NMOS transistor, and its drain node forms the reference voltage VP2 for comparison, thus isolating the establishment process of the reference voltage VP2 from the slope detection path. Through this configuration, the stability and controllability of the reference voltage VP2 are improved, reducing the impact of parasitic effects at the comparator CMP input and disturbances in the preceding signal on the comparison result. This is beneficial for improving the consistency and reliability of the comparison unit 30 under different operating conditions.

[0051] Specifically, in this embodiment, the parameters of each device can be calculated based on the circuit's operating relationship. Taking the second MOS transistor M2 as an NMOS transistor as an example, its drain-source current can be described by the current relationship formula of an NMOS transistor: .

[0052] Where K is a process-dependent constant, which is fixed after the NMOS transistor process is determined; W / L is the width-to-length ratio of the NMOS transistor; Vgs is the gate-source voltage, and Vth is the turn-on voltage. Simplifying the above relationships, we can obtain the simplified expression for the gate-source voltage: .

[0053] At the critical state of comparator CMP switching, the slope detection voltage VN2 is equal to the reference voltage VP2. At this time, the gate-source voltage relationship between the slope detection unit 20 and the corresponding NMOS transistor in the reference voltage generation branch satisfies a certain balance condition, which can be expressed as: Vgs2 - I2·R3 = Vgs3 - I4·R4, Further analysis yields: I2·R3 - I4·R4 = Vgs2 - Vgs3.

[0054] Substituting Vgs into the above equation, we can obtain the constraint relationship between the parameters: As long as this equation is satisfied, the corresponding resistor and constant current source values ​​can be reasonably selected during the design process, thereby realizing the correspondence between the comparator flip point and the expected slope detection threshold.

[0055] According to the current relationship formula for MOSFETs, the drain-source current of a MOSFET has a square relationship with its gate-source voltage. In contrast, the voltage and current across a resistor satisfy a linear relationship, V = I·R. Therefore, under the same current change, the change in the gate-source voltage of a MOSFET is relatively small, effectively demonstrating a buffering or soft-clamping effect on voltage changes.

[0056] In this embodiment, the slope detection voltage VN2 can be expressed as VN2 = Vgs2 - I2·R3, where I2·R3 is a fixed voltage determined by the constant current source and the resistor. Therefore, the change in VN2 is mainly determined by Vgs2, which is expressed as: .

[0057] On the other hand, the current I5 generated in the slope detection path is the current on the first capacitor C1. This current is generated by the rate of change of the voltage across the capacitor, thus reflecting the slope of the induced voltage VS of the auxiliary winding. In this way, the slope information is reflected in VN2 in a controlled and smooth form, which helps to improve the stability and reliability of slope detection and subsequent comparison and judgment.

[0058] refer to Figure 4 and Figure 5 When the induced voltage VS of the auxiliary winding begins to decrease, the slope detection voltage VN2 decreases synchronously. Because a soft-clamp structure formed by a MOSFET is introduced into the slope detection path in this embodiment, the voltage change of the slope detection voltage VN2 no longer exhibits a linear relationship with the current. Instead, it is constrained by the gate-source voltage characteristics, resulting in an overall voltage level lower than the slope detection voltage VN1 in Embodiment 1, and a more gradual and controlled change.

[0059] Under the same reference voltage conditions, because the slope detection voltage VN2 starts to decrease from a lower initial level, the comparator's switching timing is earlier than in Embodiment 1 when it detects that the slope detection voltage VN2 has decreased to the reference voltage VP2. Figure 4 As shown, there is a time difference Δt between the rising edges of the output signal Sout in Embodiment 1 and the output signal Sout2 in this embodiment. That is, there is a time difference Δt between the generation time of the indicator signal in Embodiment 1 and the generation time of the indicator signal in this embodiment. This advance can compensate for the response lag introduced by the parasitic capacitance of the comparator input stage to a certain extent, so that the flipping time of the comparator is closer to the time when the auxiliary winding induced voltage VS actually enters the resonance stage.

[0060] Through the above structure and working method, this embodiment improves the accuracy of the slope detection signal triggering time without increasing the complexity of control logic, thereby helping to improve the reliability of subsequent resonance determination and related control signals.

[0061] According to the above embodiments, in the specific implementation of the slope detection circuit, a combined structure of a constant current source, an RC network, and switching devices is introduced. By processing the slope detection voltage change rate through currentization and voltageization, stable extraction and comparison of slope characteristics can be quickly achieved. This structure avoids the influence of parasitic parameters of semiconductor devices, reduces sensitivity to process deviations and parameter fluctuations, has a relatively fast response speed, and is easy to integrate into power management chips. While ensuring detection consistency, it helps to reduce circuit complexity, chip area, and power consumption, thereby improving the overall system reliability and engineering applicability.

[0062] Figure 6 A schematic flowchart of a slope detection method for a flyback switching power supply according to an embodiment of the present invention is shown. Figure 6 As shown, the method includes the following steps: Step S100: Obtain the auxiliary winding induced voltage VS of the flyback switching power supply, and generate a following voltage V1 that changes accordingly with the auxiliary winding induced voltage VS.

[0063] Step S200: Generate a slope detection voltage based on the rate of change of the following voltage V1.

[0064] Step S300: The slope detection voltage is compared with the reference voltage. When the slope detection voltage drops below the reference voltage, an indication signal is output. The indication signal is used to characterize that the flyback switching power supply has entered the free resonance stage.

[0065] The specific implementation methods of each step in the above-described slope detection method for flyback switching power supplies refer to the relevant content of the embodiment in the slope detection circuit for flyback switching power supplies described above, and will not be repeated here.

[0066] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0067] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A slope detection circuit for a flyback switching power supply, characterized in that, include: The voltage follower unit receives the induced voltage VS of the auxiliary winding of the flyback switching power supply at its input terminal and outputs a follower voltage V1 that changes in accordance with the induced voltage of the auxiliary winding. A slope detection unit, connected to the voltage follower unit, is used to generate a slope detection voltage based on the rate of change of the follower voltage V1. A comparison unit, connected to the slope detection unit, is used to compare the slope detection voltage with a reference voltage, and outputs an indication signal when the slope detection voltage drops below the reference voltage; The indicator signal indicates that the flyback switching power supply has entered the free resonance stage.

2. The slope detection circuit according to claim 1, characterized in that, The voltage follower unit includes a first constant current source I1 and a first MOS transistor M1; The input terminal of the first constant current source I1 is connected to the power supply voltage, the output terminal is connected to the source of the first MOS transistor M1, the drain of the first MOS transistor M1 is grounded, and the gate of the first MOS transistor M1 is connected to the auxiliary winding induced voltage VS. The source voltage of the first MOS transistor M1 is used as the follower voltage V1.

3. The slope detection circuit according to claim 2, characterized in that, The slope detection unit includes a first capacitor C1 and a second constant current source I2; The input terminal of the second constant current source I2 is connected to the power supply voltage, the first terminal of the first capacitor C1 is connected to the source of the first MOSFET M1, and the second terminal of the first capacitor C1 is connected to the output terminal of the second constant current source I2.

4. The slope detection circuit according to claim 3, characterized in that, The slope detection unit further includes: The first resistor R1 has its first end connected to the output terminal of the second constant current source I2 and its second end grounded. The voltage at the first end of the first resistor R1 is used as the slope detection voltage.

5. The slope detection circuit according to claim 4, characterized in that, The comparison unit includes a third constant current source I3 and a second resistor R2; The input terminal of the third constant current source I3 is connected to the power supply voltage, the output terminal of the third constant current source I3 is connected to the first terminal of the second resistor R2, the second terminal of the second resistor R2 is grounded, and the voltage at the first terminal of the second resistor R2 is used as the reference voltage.

6. The slope detection circuit according to claim 3, characterized in that, The slope detection unit also includes a third resistor R3 and a second MOS transistor M2; The first end of the third resistor R3 is connected to the output end of the second constant current source I2, and the second end of the third resistor R3 is connected to the second end of the first capacitor C1. The gate of the second MOS transistor M2 is connected to the first terminal of the third resistor R3, the drain is connected to the second terminal of the third resistor R3, and the source is grounded; The drain voltage of the second MOS transistor M2 is used as the slope detection voltage.

7. The slope detection circuit according to claim 6, characterized in that, The comparison unit includes a fourth constant current source I4, a fourth resistor R4, and a third MOSFET M3; The input terminal of the fourth constant current source I4 is connected to the power supply voltage, the output terminal is connected to the first terminal of the fourth resistor R4, and the second terminal of the fourth resistor R4 is connected to the drain of the third MOS transistor M3. The gate of the third MOS transistor M3 is connected to the first terminal of the fourth resistor R4, and the source of the third MOS transistor M3 is grounded. The drain voltage of the third MOS transistor M3 is used as the reference voltage.

8. The slope detection circuit according to claim 5 or 7, characterized in that, The comparison unit further includes: The comparator CMP has its non-inverting input connected to the reference voltage, its inverting input connected to the slope detection voltage, and its output serving as the output of the comparator unit.

9. The slope detection circuit according to claim 7, characterized in that, The first MOS transistor M1 is a PMOS transistor, and the second MOS transistor M2 and the third MOS transistor M3 are both NMOS transistors.

10. A slope detection method for a flyback switching power supply, characterized in that, Includes the following steps: Obtain the induced voltage VS of the auxiliary winding of the flyback switching power supply, and generate a following voltage V1 that varies with the induced voltage VS of the auxiliary winding. A slope detection voltage is generated based on the rate of change of the following voltage V1; The slope detection voltage is compared with a reference voltage. When the slope detection voltage drops below the reference voltage, an indication signal is output. The indication signal is used to characterize that the flyback switching power supply has entered the free resonance stage.

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