Control method, control circuit and control chip of switching power supply

Abstract

The application provides a control method, a control circuit and a control chip of a switching power supply. Different working modes are switched according to different output voltages. When the output voltage is lower than a first reference value and the difference between the output voltage and the first reference value exceeds a threshold value, the working frequency of a power tube and the output voltage are in proportional relationship according to corresponding slopes. When the output voltage is lower than the first reference value and the difference between the output voltage and the first reference value is lower than the threshold value, the power tube works at a fixed frequency. When the output voltage is higher than the first reference value, the power tube works at a variable frequency. Thus, the peak current is not accumulated during the output short circuit or the starting process, so that the phenomenon that the peak current exceeds the expectation during the starting or the output short circuit process is avoided, the power tube and other devices such as inductors are protected, and the starting ability of the switching power supply during the starting process is ensured.

Description

Technical Field

[0001] This invention relates to the field of switching power supply control technology, and in particular to a switching power supply control method, control circuit, and control chip. Background Technology

[0002] Figure 1 This is a schematic diagram of the system structure of a high-side buck AC / DC switching power supply without a VCC capacitor. When the power transistor Q in the control chip IC is turned on, the path between its PIN1 (drain DRAIN pin) and PIN3 (ground GND pin) is open, the bus voltage Vindc is connected to one end of the inductor L, and the other end of the inductor L provides the output voltage Vo. The inductor current IL increases at a rate of (Vindc-Vo) / L. The time corresponding to this process is the on-time Ton. When the power transistor Q is turned off, because the inductor current IL cannot change abruptly, the freewheeling diode D0 is turned on, and the voltage difference across the inductor L is -Vo. The inductor current IL decreases at a rate of Vo / L. The time corresponding to this process is the demagnetization time Toff.

[0003] As shown in the formula for the inductor current IL above, when the output voltage Vo is very low, the inductor current IL decreases at a very low slope during the Toff time. Furthermore, in typical switching power supply systems, there is a minimum on-time limit, Tonmin. Therefore, during output short circuits and startup, the switching power supply often operates at its maximum frequency, with a short Toff time and a very low output voltage Vo. The demagnetization speed of the inductor current IL is slow, and due to the minimum on-time limit Tonmin, the increase in inductor current IL within the on-time Tonmin of the power transistor Q's switching cycle is greater than the decrease in current within the demagnetization time Toff. This can easily lead to a gradual increase in the peak inductor current, especially at high input voltages, which can cause peak current accumulation and pose a risk of uncontrolled peak current.

[0004] The peak current IPK is sensed by a corresponding peak current sensing resistor Rcs (not shown). When the voltage drop across the sensing resistor Rcs is Vfb, the peak current IPK can be expressed as Vfb / Rcs. Figure 2 This is a common method to avoid excessive accumulation of peak current. Figure 2It is known that during startup or output short circuit, if the peak current IPK (i.e., Vfb / Rcs) accumulates, and after the accumulated peak current IPK reaches a certain value (i.e., V_CLK_DOWN / Rcs, often set to be greater than the maximum peak current ILIM required for normal system operation, i.e., V_ILIM / Rcs), the system operating frequency is reduced (i.e., frequency reduction) to avoid further accumulation of the peak current IPK. This method can solve certain problems, but the following issues still exist: 1. The maximum peak current V_CLK_DOWN / Rcs generated after frequency reduction will still be greater than the maximum peak current ILIM required for normal system operation. This requires inductors, transformers, power transistors, etc., to have larger saturation currents, resulting in wasted area of ​​power transistors, etc.; 2. The required reduction in operating frequency generally varies under different output voltages. If the design is unreasonable, there may be heavy load startup problems, or peak current accumulation problems in a certain output voltage range. Summary of the Invention

[0005] The purpose of this invention is to provide a control method, control circuit, and control chip for a switching power supply, which can avoid the problem of peak current accumulation during the startup or output short circuit of the switching power supply, while ensuring the startup capability during the startup process.

[0006] To achieve the above objectives, the present invention provides a control method for a switching power supply, the switching power supply including a coupled power transistor and an inductor, receiving an input voltage and generating an output voltage, the control method comprising:

[0007] The output voltage is detected, and the output voltage is compared with a first reference value;

[0008] When the output voltage is lower than the first reference value and the difference between the output voltage and the first reference value exceeds a threshold, the switching power supply is controlled to enter the first working state.

[0009] When the output voltage is lower than the first reference value, but the difference is lower than the threshold, the switching power supply is controlled to enter the second operating state;

[0010] When the output voltage is higher than the first reference value, the switching power supply is controlled to enter a third operating state; wherein...

[0011] In the first operating state, a slope is obtained based on multiple system parameters of the switching power supply, and the operating frequency and output voltage of the power transistor are controlled to be proportional to the slope; in the second operating state, the power transistor is controlled to operate at a fixed frequency; in the third operating state, the power transistor is controlled to operate at a variable frequency.

[0012] Optionally, the control method of the switching power supply further includes: obtaining a relationship curve between the operating frequency and the output voltage when the output current provided by the switching power supply is constant, based on a first set of system parameters of the switching power supply, wherein the relationship curve has a first slope; the first set of system parameters includes the current change value of the power transistor, the input voltage, and the inductance value of the inductor.

[0013] Optionally, the control method of the switching power supply further includes: obtaining a relationship curve between the operating frequency and the output voltage required to prevent the accumulation of peak current of the switching power supply, based on a second set of system parameters of the switching power supply, wherein the relationship curve has a second slope; the second set of system parameters includes the minimum on-time of the power transistor, the input voltage, and the inductance value of the inductor.

[0014] Optionally, the control method of the switching power supply further includes: controlling the operating frequency and output voltage of the power transistor to be directly proportional to the slope, wherein the slope is greater than the first slope.

[0015] Optionally, the control method of the switching power supply further includes: controlling the operating frequency and output voltage of the power transistor to be directly proportional to the slope, wherein the slope is less than the second slope.

[0016] Optionally, the control method of the switching power supply further includes: in the second operating state, controlling the power transistor to operate at a fixed frequency of the maximum operating frequency.

[0017] Based on the same inventive concept, the present invention also provides a control circuit for a switching power supply, wherein the switching power supply includes a coupled power transistor and an inductor, receives an input voltage, and generates an output voltage, wherein...

[0018] The control circuit includes a first control branch and a second control branch. The first control branch receives a feedback signal of the output voltage and a first reference value, and the second control branch receives the feedback signal and the second reference value.

[0019] When the value of the feedback signal is higher than the first reference value, the first control branch controls the power transistor to operate at a higher frequency.

[0020] When the value of the feedback signal is lower than the first reference value and the difference between the value of the feedback signal and the first reference value is lower than a threshold, the first control branch and the second control branch control the power transistor to operate at a fixed frequency.

[0021] When the value of the feedback signal is lower than the first reference value, but the difference exceeds the threshold, the second control branch controls the operating frequency and output voltage of the power transistor to be proportional.

[0022] The proportional relationship is obtained based on multiple system parameters of the switching power supply.

[0023] Optionally, the first control branch includes an error amplifier, the positive input terminal of which receives the first reference value, the negative input terminal of which receives the feedback signal, and the error amplifier outputs a compensation signal.

[0024] Optionally, the first control branch further includes a first transconductance amplifier, which is coupled to the error amplifier and receives the compensation signal. The first transconductance amplifier also receives a third reference value and generates a first current based on the compensation signal and the third reference value.

[0025] Optionally, the second control branch includes a second transconductance amplifier, which receives the feedback signal and the second reference value, and generates a second current based on the feedback signal and the second reference value.

[0026] Optionally, the control circuit further includes a current source and a current mirror. The current mirror is coupled to the current source and receives the first current or the second current. The current mirror outputs a control current, which determines the operating frequency of the switching power supply power transistor.

[0027] Optionally, the control circuit controls the clock frequency according to the control current and generates a control signal according to the clock frequency. The control signal instructs the operation of the power transistor and determines the operating frequency of the power transistor.

[0028] Based on the same inventive concept, the present invention also provides a control chip, which includes the control circuit as described in the present invention.

[0029] Optionally, the control chip further includes:

[0030] The feedback pin is coupled to the output voltage terminal of the switching power supply;

[0031] The input pin is coupled to one end of the power transistor;

[0032] The grounding pin is coupled to the other end of the power transistor;

[0033] The power transistor is built into the control chip and is coupled to and controlled by the control circuit.

[0034] Optionally, the control chip further includes a sampling circuit, which is coupled to the feedback pin and built into the control chip.

[0035] Optionally, the control chip is connected to an external sampling circuit, which is coupled between the output voltage terminal of the switching power supply and the feedback pin.

[0036] Compared with the prior art, the technical solution of the present invention has at least one of the following beneficial effects:

[0037] 1. The switching power supply switches between different operating modes according to different output voltages. When the output voltage is lower than a first reference value and the difference between the output voltage and the first reference value exceeds a threshold, the switching power supply is controlled to enter the first operating state, and the operating frequency of the power transistor and the output voltage are proportional to the corresponding slope. When the output voltage is lower than the first reference value and the difference between the output voltage and the first reference value is lower than the corresponding threshold, the switching power supply is controlled to enter the second operating state, and the power transistor is controlled to operate at a fixed frequency. When the output voltage is higher than the first reference value, the switching power supply is controlled to enter the third operating state, and the power transistor is controlled to operate at a variable frequency. In this way, during the output short circuit or startup process, the peak current is controlled to prevent accumulation, thereby avoiding the phenomenon that the peak current exceeds the expectation during startup or output short circuit, thus protecting the power transistor and inductor and other devices, while ensuring the startup capability of the switching power supply during startup.

[0038] 2. Under the same peak current application, the technical solution of the present invention can use power transistors and inductors with smaller saturation current, saving system cost and control circuit area. Attached Figure Description

[0039] Those skilled in the art will understand that the accompanying drawings are provided to better understand the invention and do not constitute any limitation on the scope of the invention. Wherein:

[0040] Figure 1 This is a schematic diagram of the system structure of a high-side buck AC / DC switching power supply without a VCC capacitor.

[0041] Figure 2 This is a schematic diagram of the peak current detection signal in a method to avoid excessive accumulation of peak current.

[0042] Figure 3 This is a schematic diagram of the f (operating frequency) - Vo (output voltage) relationship curve in the first operating state of the control method of the switching power supply in a specific embodiment of the present invention.

[0043] Figure 4 This is a schematic diagram of the control circuit of the switching power supply according to a specific embodiment of the present invention. Detailed Implementation

[0044] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without one or more of these details. In other instances, certain technical features well-known in the art have not been described in order to avoid confusion with the invention. It should be understood that the invention can be embodied in various forms and should not be construed as limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The same reference numerals denote the same elements throughout. It should be understood that when an element is referred to as "connected to" or "coupled to" other elements, it may be directly connected to other elements, or there may be intervening elements. Conversely, when an element is referred to as "directly connected to" other elements, there are no intervening elements. As used herein, the singular forms "a," "an," and "the" are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term "comprising" is used to identify the presence of features, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups. When used herein, the term "and / or" includes any and all combinations of the associated listed items.

[0045] The technical solution proposed by the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer from the following description. It should be noted that the drawings are all in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of the present invention.

[0046] As described in the background section, for the control of AC / DC switching power supplies or DC / DC switching power supplies with various topologies such as buck, boost, and buck-boost, the output voltage Vo is very low during output short circuit and startup, and the inductor current IL demagnetizes slowly. However, once the switching power supply operates at its maximum frequency, the Toff time is short, and due to the limitation of the minimum on-time Tonmin, the amount of current increase in the inductor current IL within one switching cycle of the power transistor Q during the on-time Tonmin is greater than the amount of current decrease during the demagnetization time Toff. This can easily cause the peak current to gradually increase, especially at high input voltages, which can lead to peak current accumulation and the risk of uncontrolled peak current.

[0047] Therefore, in order to solve the above problems and ensure the normal startup of the switching power supply system, this invention provides a technical method that switches the switching power supply to different operating modes according to different output voltages during the entire startup process from the moment of startup to constant voltage output. This ensures that the output current capability meets the requirements, while ensuring that the inductor current does not accumulate when operating during the shortest conduction time. As a result, during output short circuit or startup, the peak current is controlled to prevent accumulation, thereby avoiding the phenomenon that the peak current exceeds the expectation during startup or output short circuit. This protects the power transistors and inductors, while ensuring the startup capability of the switching power supply during startup.

[0048] Specifically, one embodiment of the present invention provides a control method for a switching power supply, the switching power supply including a coupled power transistor and an inductor, receiving an input voltage and generating an output voltage. As an example, the switching power supply is a high-side buck switching power supply without a VCC capacitor; please refer to [reference needed]. Figure 1 and Figure 4 When the power transistor Q in the control chip IC of this switching power supply is turned on, the path between its PIN1 (drain DRAIN pin) and PIN3 (ground GND pin) is turned on. The input voltage Vindc is connected to one end of the inductor L, and the other end of the inductor L provides the output voltage Vo. In this switching power supply system, the power transistor Q has a minimum on-time Tonmin, the maximum peak current required for normal system operation is ILIM, and the switching period of the power transistor Q is T = Ton + Toff.

[0049] Furthermore, this switching power supply can operate in two modes: Continuous On-Mode (CCM) and Discontinuous On-Mode (DCM). The specific mode used depends on the minimum value of the inductor current IL in each switching cycle. If the power transistor Q switches from Toff to Ton before the inductor current IL drops to zero, the current IL in the inductor L remains above zero; this operating mode is called Continuous On-Mode (CCM). If Toff lasts long enough for the inductor current IL to drop to zero, the inductor current IL will be zero for a period of time. During this time, both diode D0 and power transistor Q are in the off state (i.e., cutoff state); this operating mode is called Discontinuous On-Mode (DCM).

[0050] Please refer to Figure 4 The control method for the switching power supply in this embodiment specifically includes the following steps:

[0051] S1. Detect the output voltage Vo of the switching power supply to obtain a voltage signal Vfb reflecting the output voltage. Compare the voltage signal Vfb with the first reference value CVREF. It can be understood that the voltage signal Vfb can reflect the output voltage Vo in a 1:1 ratio, or it can be proportional to the output voltage Vo. In this case, the first reference value CVREF also needs to be adjusted accordingly. Furthermore, the voltage signal Vfb can also be presented in the form of a current signal, etc., and different detection schemes are selected according to the design requirements of different application circuits. For simplicity, the output voltage Vo is used below to represent the detection signal value obtained by detecting the output voltage Vo in step S1.

[0052] S2, when the output voltage Vo is lower than the first reference value CVREF and the difference between the output voltage Vo and the first reference value CVREF exceeds a threshold, the switching power supply is controlled to enter the first working state. In the first working state, a slope K is obtained according to multiple system parameters of the switching power supply, and the operating frequency f of the power transistor Q and the output voltage Vo are controlled to be proportional to the slope K.

[0053] S3, when the output voltage Vo is lower than the first reference value CVREF, but the difference between the output voltage Vo and the first reference value CVREF is lower than the above-mentioned threshold, the switching power supply is controlled to enter the second working state, and in the second working state, the power transistor Q is controlled to operate at a fixed frequency.

[0054] S4, when the output voltage Vo is higher than the first reference value CVREF, control the switching power supply to enter the third working state, and in the third working state, control the power transistor Q to work in frequency conversion.

[0055] Please refer to Figure 1 and Figure 4 In step S1, a corresponding sampling circuit (e.g.) can be used. Figure 1 The output voltage Vo is detected by the branch where the diode is located, so as to obtain the sampling voltage Vfb that can provide feedback on the change of the output voltage Vo of the switching power supply, and then the sampling voltage Vfb is compared with the first reference value CVREF.

[0056] In step S2, when the output voltage Vo is lower than the first reference value CVREF and the difference between the output voltage Vo and the first reference value CVREF exceeds a threshold, the error amplifier EA outputs the compensation signal COMP, which reaches saturation. At this time, the switching frequency is controlled by the transconductance amplifier OTA1. Specifically, based on the first set of system parameters of the switching power supply, the relationship curve between the operating frequency f and the output voltage Vo when the output current provided by the switching power supply is constant can be obtained (defined as the first reference curve): f = K1 * Vo. Figure 3As shown, the relationship curve has a first slope K1. The first set of system parameters may include the current change of the power transistor Q (or the change of the inductor current IL), the input voltage Vindc, and the inductance value of the inductor L. Further, based on the second set of system parameters of the switching power supply, the relationship curve between the operating frequency f and the output voltage Vo (defined as the second reference curve) required for the peak current of the switching power supply to not accumulate can be obtained, such as f = K2 * Vo. Figure 3 As shown, the relationship curve has a second slope K2. The second set of system parameters may include system parameters of the switching power supply such as the minimum on-time Tonmin of the power transistor Q, the input voltage Vindc, and the inductance value of the inductor L.

[0057] In constant voltage mode, the output current needs to remain constant. In CCM mode, the initial value of the inductor current (IPK_Start) also needs to remain constant (while in DCM mode, the initial value of the inductor current is 0, which is inherently constant). Therefore, the relationship between the operating frequency f and the output voltage Vo when the output current provided by this switching power supply is constant can be obtained by the following formula:

[0058] Toff=(ILIM-IPK_Start)*L / Vo;

[0059] Ton=(ILIM-IPK_Start)*L / (Vindc-Vo);

[0060] Therefore, T = Ton + Toff = (ILIM - IPK_Start) * L[1 / (Vindc - Vo) + 1 / Vo];

[0061] Thus, the first reference curve is obtained as f = 1 / T = K1 * Vo * (Vindc - Vo) / Vindc. Wherein, K1 is the first slope and K1 = 1 / (ILIM - IPK_Start), and (ILIM - IPK_Start) is the change in current of power transistor Q (or the change in inductor current IL), the average value of which can be expressed as the output current value.

[0062] Optionally, when Vindc (e.g., above 100V) differs from Vo (e.g., below a few tens of V) by more than an order of magnitude, the formula for the first reference curve can be approximated as f = K1 * Vo. That is, the first reference curve obtained when the output current is constant indicates that the operating frequency of the switching power supply is proportional to the output voltage. Based on the load capacity required for the switching power supply to start up, i.e. the minimum output current value, the corresponding K1 can be calculated.

[0063] In addition, during the starting process of the switching power supply, the condition for no peak current accumulation of the switching power supply is: Tonmin*Vindc / L≤(T-Tonmin)*Vo / L. When the output voltage Vo does not reach the required constant voltage value, the peak current IPK is maintained at the maximum peak value ILIM. Considering the harshest environment, under the maximum input voltage and fixed minimum on-time, the relationship curve between the operating frequency f and the output voltage Vo required when no peak current accumulation occurs in the switching power supply can be obtained, and the corresponding K2 is calculated.

[0064] Thereby, when the switching power supply is controlled to operate in the first operating state in step S2, the operating frequency f and the output voltage Vo are further controlled to be in a proportional relationship according to the corresponding frequency K, and K1<K<K2. That is, when the switching power supply is controlled to operate in the first operating state in step S2, as long as the relationship curve between the operating frequency f of the power tube Q and the output voltage Vo is controlled to be between the above-mentioned first reference curve and the second reference curve, it can be ensured that the output current capability is sufficient during the starting process of the switching power supply, and the phenomenon of peak current accumulation will not occur during starting or output short circuit.

[0065] In step S3, as Vo increases, although Vfb is lower than CVREF, the difference between the two will be less than the above threshold, and the switching power supply enters the second operating state, and the power tube Q is controlled to operate at a fixed frequency. Preferably, when the switching power supply enters the second operating state, the power tube Q is controlled to operate at a fixed frequency with the maximum operating frequency, so as to reach the maximum peak current required by the system as soon as possible.

[0066] In step S4, as Vo increases, Vfb will be higher than the first reference value CVREF, thereby controlling the switching power supply to enter the third operating state, and in the third operating state, the power tube Q is controlled to operate in a frequency-converting manner, thereby achieving the effect of constant voltage or constant current control, and avoiding further accumulation of peak current. The adjustment amount of the operating frequency of the power tube Q in step S3 needs to be designed according to the finally required constant voltage value, so as to avoid the problem of heavy-load starting and the problem of peak current accumulation in a certain output voltage section.

[0067] Please refer to Figure 4To implement the control method of the switching power supply of the present invention, an embodiment of the present invention also provides a control circuit U0 for the switching power supply. The switching power supply includes a coupled power transistor Q and an inductor L, receives an input voltage Vbus, and generates an output voltage Vo. The control circuit U0 includes a first control branch U1 and a second control branch U2. The first control branch U1 receives a feedback signal of the output voltage Vo and a first reference value CVREF. The value of the feedback signal is the sampling voltage Vfb obtained by sampling the output voltage Vo, which can provide feedback on the change of the output voltage Vo. The second control branch U2 receives the feedback signal and the second reference value REF1. Specifically, when the value of the feedback signal Vfb is higher than the first reference value CVREF, the first control branch U1 controls the power transistor Q to operate at a variable frequency; when the value of the feedback signal Vfb is lower than the first reference value CVREF and the difference between the value of the feedback signal Vfb and the first reference value CVREF is lower than a threshold, the first control branch U1 and the second control branch U2 control the power transistor Q to operate at a fixed frequency; when the value of the feedback signal Vfb is lower than the first reference value CVREF, but the difference exceeds the threshold, the second control branch U2 controls the operating frequency f of the power transistor Q to be proportional to the output voltage Vo, and this proportional relationship is obtained based on multiple system parameters of the switching power supply.

[0068] As an example, the first control branch U1 includes an error amplifier EA and a first transconductance amplifier OTA1, the second control branch U2 includes a second transconductance amplifier OTA2, and the control circuit U0 in this embodiment also includes a current source I0, a current mirror U3, an oscillator OSC, and a logic and drive circuit U4.

[0069] The positive input terminal "+" of the error amplifier EA receives the first reference value CVREF, and the negative input terminal "-" receives the feedback signal Vfb. The error amplifier EA outputs a compensation signal COMP. The first input terminal of the first transconductance amplifier OTA1 is coupled to the output terminal of the error amplifier EA and receives the compensation signal COMP. The second input terminal of the first transconductance amplifier OTA1 receives the second reference value REF1. The first input terminal of the second transconductance amplifier OTA2 receives the feedback signal Vfb, and the second input terminal receives the third reference value REF2. The output terminal of the current source I0 and the input terminal of the current mirror U3 form a connection node a between the current source I0 and the current mirror U3. The output terminals of the first transconductance amplifier OTA1 and the second transconductance amplifier OTA2 are both coupled to the connection node a between the current source I0 and the current mirror U3. The output terminal of the current mirror U3 is coupled to one end of the oscillator OSC, and the other end of the oscillator OSC is coupled to the logic AND drive circuit U4. The logic AND drive circuit U4 is coupled to the gate of the power transistor Q. The first transconductance amplifier OTA1 generates a second current I2 based on the compensation signal COMP and the second reference value REF1. The second transconductance amplifier OTA2 generates a first current I1 based on the compensation signal Vfb and the third reference value REF2. The current mirror U3 outputs a control current I_OSC to determine the operating frequency f of the switching power supply based on the current output by the current source I0 and the first current I1 or the second current I2 (for example, the outputs of transconductance amplifiers OTA1 and OTA2 are pull-down currents, and the control current I_OSC is the difference between the current output by the current source I0 and the first current I1 or the second current I2). The oscillator OSC outputs a corresponding oscillation signal (not shown) with a corresponding clock frequency to the logic and drive circuit U4 based on the control current I_OSC. The logic and drive circuit U4 generates a control signal (not shown) based on the oscillation signal with the corresponding clock frequency to indicate and drive the operation (i.e., turn on or off) of the power transistor Q and determine the operating frequency f of the power transistor Q. In other words, the control current I_OSC output by the current mirror U3 determines the clock frequency of the oscillation signal output by the oscillator OSC, which in turn determines the period of the control signal output by the logic and drive circuit U4, and thus determines the switching period of the power transistor Q.

[0070] It should be understood that the function of the current mirror U3 is to mirror the current at connection node a to provide the control current I_OSC required by the oscillator OSC. The mirror ratio can be 1:1, 1:n, or n:1, etc., and this invention does not specifically limit this. That is, the specific circuit structure of the current mirror U3 in this embodiment can be implemented using any suitable circuit structure. For example, the current mirror U3 includes mirrored MOS transistors M1 and M2. The drain of M1 serves as the input terminal of the current mirror U3, connecting to the gate of M1, the gate of M2, and connection node a. The drain of M2 serves as the output terminal of the current mirror U3, connected to the oscillator OSC. The parameters of M1 and M2 determine the mirror ratio of the current mirror U3.

[0071] In this embodiment, please refer to Figure 1 and Figure 4 The first input terminal "-" of the error amplifier EA is coupled to the corresponding sampling circuit (e.g., Figure 1 The output terminal of the branch containing diode D0 receives the sampling voltage Vfb (i.e., the feedback signal) obtained by sampling the output voltage Vo of the switching power supply from the sampling circuit. The second input terminal "+" of the error amplifier EA receives the constant voltage reference voltage CVREF. The error amplifier EA is used to amplify the difference between the sampling voltage Vfb and the constant voltage reference voltage CVREF and output it as the compensation signal COMP. The lower Vfb is, the higher COMP is. Because the amplification factor of the error amplifier EA is relatively high, the output voltage Vo is low at the beginning of the startup process (increasing from 0). At this time, the compensation signal COMP will be clamped at the highest value, and the output signal of the first control branch remains unchanged. The switching power supply system is frequency controlled by the second control branch U2. As the startup process progresses, Vfb gradually increases. The working principle of the control circuit U0 in this embodiment during the startup process is as follows:

[0072] Initially, the sampling voltage Vfb is lower than the first reference value CVREF and Vfb is far from CVREF (i.e., the difference between the two is at its maximum at the beginning). The compensation signal COMP output by the error amplifier EA is set to its highest value. At this time, the compensation signal COMP output by the error amplifier EA is higher than the third reference value REF2. The switching power supply operates in the first operating state. The first transconductance amplifier OTA1 draws current, while the second transconductance amplifier OTA2 does not draw current (i.e., the second current I2 = 0). I_OSC = I0 - I1, and I1 is proportional to the difference between Vfb and REF1. Therefore, when the switching power supply is actually operating in the first operating state, the operating frequency f of the power transistor Q increases with the increase of the output voltage Vo. When the output voltage Vo is low, the operating frequency f is low; when the output voltage Vo is high, the operating frequency f is high. By designing the gain of the transconductance amplifier OTA1, the curve of the operating frequency f of the power transistor Q changing with the output voltage Vo in the first operating state can meet the above-mentioned slope requirements. By limiting the relationship curve between the operating frequency f and the output voltage Vo of the switching power supply to above the first reference curve (i.e., the relationship curve between the operating frequency f and the output voltage Vo that meets the minimum current output capability of the switching power supply) and below the second reference curve (i.e., the relationship curve between the operating frequency f and the output voltage Vo that prevents peak current accumulation), the system can meet the load-carrying capacity at startup without the phenomenon of peak current accumulation.

[0073] When the output voltage Vo gradually increases to near or reach the desired constant value, the sampling voltage Vfb is lower than the first reference value CVREF, and the compensation signal COMP output by the error amplifier EA is lower than the second reference voltage REF2. At this time, Vfb is slightly higher than REF1, and the switching power supply operates in the second operating state. Neither the first transconductance amplifier OTA1 nor the second transconductance amplifier OTA2 draws current, I_OSC = I0, and the power transistor Q operates at a fixed frequency, for example, at the highest frequency. The second reference value REF1 is slightly lower than the first reference value CVREF, thus ensuring that the first transconductance amplifier OTA1 and the second transconductance amplifier OTA2 function in different operating states.

[0074] As the output voltage Vo continues to increase, Vfb exceeds the first reference value CVREF, causing COMP to decrease further. The switching power supply operates in the third operating state. The second transconductance amplifier OTA2 draws current from the connection node a, reducing the current entering the current mirror and thus reducing the current I_OSC. This lowers the actual operating frequency f and power of the switching power supply, keeping the output voltage constant. At this time, the output current of the switching power supply is also constant, and the system operates in the PFM (Pulse Frequency Modulation) stage. At this point, I_OSC = I0 - I2, and I2 is proportional to the difference between COMP and REF2.

[0075] The frequency control circuit in the third operating state of this embodiment actually adjusts the operating frequency f of the power transistor Q of the switching power supply based on the sampled voltage Vfb (a variable) of the output voltage Vo and the difference between the sampled voltage Vfb and multiple reference values. Thus, during system startup, although the value of COMP is at its maximum and the first control branch operates in an open-loop state, the operating frequency f of the switching power supply can increase with the increase of the output voltage Vo, thereby preventing excessive accumulation of peak current during startup. At this time, the relationship curve between the actual operating frequency f of the switching power supply and the output voltage Vo can be limited to the first reference curve (i.e., the relationship curve between the operating frequency f and the output voltage Vo in the worst case where the peak current does not accumulate during startup). At the same time, during constant current and constant voltage operation, the system operating frequency f is only modulated by the amplified output COMP of the difference between the sampled voltage Vfb and the constant voltage reference voltage CVREF.

[0076] Based on the same inventive concept, please refer to Figure 1 and Figure 4 An embodiment of the present invention also provides a control chip IC, which includes the control circuit U0 of this embodiment.

[0077] In this embodiment, the control chip IC has PIN1 to PIN3 and a built-in power transistor Q. PIN1 is the input pin, coupled to the bus voltage Vbus and the drain DRAIN of the power transistor Q. PIN2 is the feedback pin FB, coupled to the output voltage terminal of the switching power supply through a corresponding sampling circuit. Specifically, the sampling circuit can be as follows: Figure 1 The diode D0 shown has its anode connected to the output voltage terminal and its cathode connected to the feedback pin FB. PIN3 is the ground pin GND, coupled to the source of the power transistor Q and the other end of the inductor L. In another embodiment, the control chip IC also has a built-in sampling circuit. One end of the sampling circuit is coupled to the output voltage terminal, and the other end is coupled to the feedback pin FB, providing the required sampling voltage Vfb to the control circuit U0 inside the control chip IC. This sampling voltage Vfb can provide feedback on changes in the output voltage Vo of the switching power supply, thereby enabling the control circuit U0 to adjust the operating frequency of the power transistor Q (i.e., the operating frequency f of the switching power supply), and thus adjust the output voltage and output current of the switching power supply.

[0078] This sampling circuit can be implemented using any suitable circuit structure, for example... Figure 1 The diodes shown, or the voltage divider circuit formed by resistor strings, etc.

[0079] It should be understood that the internal and external circuits of the control chip IC described above are not limited to the examples given, and can also include any suitable related circuits. For example, a minimum on-time control circuit (not shown) may be provided inside the control chip IC to control the minimum on-time Tonmin of the power transistor Q to increase with the increase of the output voltage Vo during startup. This allows for the selection of a relatively low Tomin when the output voltage Vo is low, while controlling the relationship curve between the actual operating frequency f of the switching power supply and the output voltage Vo between the first and second reference curves mentioned above, thus making it easier to avoid the problem of peak current accumulation.

[0080] In this embodiment, the control chip IC can be applied to switching power supplies with any suitable topology, such as buck or boost.

[0081] In summary, the technical solution of this invention can be combined with the actual application requirements of switching power supplies to realize a control method for switching the working mode of a switching power supply according to different output voltages. This allows the operating frequency and conduction time of the switching power supply to change with the output voltage, thereby avoiding the phenomenon of peak current exceeding expectations during the startup or output short circuit of the switching power supply. This achieves protection for the power transistors and inductors of the switching power supply. Moreover, under the same peak current application, power transistors and inductors with smaller saturation current can be used, saving system costs.

[0082] The above description is only a description of preferred embodiments of the present invention and is not intended to limit the scope of the present invention in any way. Any changes or modifications made by those skilled in the art based on the above disclosure shall fall within the protection scope of the present invention.

Claims

1. A control method for a switching power supply, the switching power supply comprising a coupled power transistor and an inductor, receiving an input voltage and generating an output voltage, characterized in that, include: The output voltage is detected, and the output voltage is compared with a first reference value; When the output voltage is lower than the first reference value and the difference between the output voltage and the first reference value exceeds a threshold, the switching power supply is controlled to enter the first working state. When the output voltage is lower than the first reference value, but the difference is lower than the threshold, the switching power supply is controlled to enter the second operating state; When the output voltage is higher than the first reference value, the switching power supply is controlled to enter a third operating state; wherein... In the first operating state, based on the first set of system parameters of the switching power supply, a relationship curve between the operating frequency and the output voltage when the output current provided by the switching power supply is constant is obtained. The relationship curve has a first slope. The first set of system parameters includes the current change value of the power transistor, the input voltage, and the inductance value of the inductor. A third slope greater than the first slope is then obtained, and the operating frequency and output voltage of the power transistor are controlled to be proportional to the third slope. In the second operating state, the power transistor is controlled to operate at a fixed frequency. In the third operating state, the power transistor is controlled to operate at a variable frequency.

2. The control method for a switching power supply as described in claim 1, characterized in that, Also includes: Based on the second set of system parameters of the switching power supply, a relationship curve between the operating frequency and the output voltage required to prevent the accumulation of peak current in the switching power supply is obtained, and the relationship curve has a second slope; the second set of system parameters includes the minimum on-time of the power transistor, the input voltage, and the inductance value of the inductor.

3. The control method for a switching power supply as described in claim 2, characterized in that, Also includes: The operating frequency and output voltage of the power transistor are controlled to be proportional to the third slope, which is less than the second slope.

4. The control method for a switching power supply as described in claim 1, characterized in that, Also includes: In the second operating state, the power transistor is controlled to operate at a fixed frequency of the maximum operating frequency.

5. A control circuit for a switching power supply, the switching power supply comprising a coupled power transistor and an inductor, receiving an input voltage and generating an output voltage, characterized in that, The control circuit includes a first control branch and a second control branch. The first control branch receives a feedback signal reflecting the output voltage and a first reference value, and the second control branch receives the feedback signal and the second reference value. When the value of the feedback signal is higher than the first reference value, the first control branch controls the power transistor to operate at a higher frequency. When the value of the feedback signal is lower than the first reference value and the difference between the value of the feedback signal and the first reference value is lower than a threshold, the first control branch and the second control branch control the power transistor to operate at a fixed frequency. When the value of the feedback signal is lower than the first reference value and the difference exceeds the threshold, the relationship curve between the operating frequency and the output voltage when the output current provided by the switching power supply is constant is obtained according to the first set of system parameters of the switching power supply. The relationship curve has a first slope. The first set of system parameters includes the current change value of the power transistor, the input voltage and the inductance value of the inductor. A third slope greater than the first slope is then obtained. The second control branch controls the operating frequency and output voltage of the power transistor to be proportional to the third slope.

6. The control circuit as described in claim 5, characterized in that, The first control branch includes an error amplifier, the positive input terminal of which receives the first reference value, the negative input terminal of which receives the feedback signal, and the error amplifier outputs a compensation signal.

7. The control circuit as described in claim 6, characterized in that, The first control branch further includes a first transconductance amplifier, which is coupled to the error amplifier and receives the compensation signal. The first transconductance amplifier also receives a third reference value and generates a first current based on the compensation signal and the third reference value.

8. The control circuit as described in claim 5, characterized in that, The second control branch includes a second transconductance amplifier, which receives the feedback signal and the second reference value, and generates a second current based on the feedback signal and the second reference value.

9. The control circuit as described in claim 7 or 8, characterized in that, The control circuit also includes a current source and a current mirror. The current mirror is coupled to the current source and receives a first current or a second current. The current mirror outputs a control current, which determines the operating frequency of the power transistor of the switching power supply.

10. The control circuit as described in claim 9, characterized in that, The control circuit controls the clock frequency according to the control current and generates a control signal according to the clock frequency. The control signal instructs the operation of the power transistor and determines the operating frequency of the power transistor.

11. A control chip, characterized in that, Includes the control circuit as described in any one of claims 5-10.

12. The control chip as described in claim 11, characterized in that, Also includes: The feedback pin is coupled to the output voltage terminal of the switching power supply; The input pin is coupled to one end of the power transistor; The grounding pin is coupled to the other end of the power transistor; The power transistor is built into the control chip and is coupled to and controlled by the control circuit.

13. The control chip as described in claim 12, characterized in that, Also includes: A sampling circuit is coupled to a feedback pin and is built into the control chip.

14. The control chip as described in claim 12, characterized in that, The control chip is connected to an external sampling circuit, which is coupled between the output voltage terminal of the switching power supply and the feedback pin.

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