Frequency control system and method for switching power supplies

Abstract

This invention discloses a frequency control system and method for a switching power supply. The system includes: a voltage detection circuit, wherein the input signal of the voltage detection circuit is a feedback voltage, and the output signal is a plurality of control signals controlling a foldback circuit; a foldback circuit, wherein the foldback circuit receives the control signals and generates a switching voltage or current signal; an oscillator circuit, including a charging capacitor, and generating a signal with a switching frequency based on the voltage of the charging capacitor; and an RC filter circuit, which filters the switching voltage or current signal and generates a slowly varying current signal, which serves as the charging current of the charging capacitor. This invention, through the RC filter circuit, effectively solves the non-monotonic phenomena such as overshoot and undershoot of the output voltage at the frequency switching point during the soft-start process of a switching power supply converter with frequency foldback function.

Description

Technical Field

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

[0002] Switching power converters (DC / DC converters) utilize frequency foldback to better control inductor current during soft-start or overload conditions. During soft-start or overload, the DC / DC output voltage VOUT is significantly lower than the normal target value, resulting in a slow inductor current discharge rate. After several switching cycles, the inductor current can easily accumulate to a large value, potentially leading to runaway operation. Therefore, under these operating conditions, it is necessary to reduce the switching frequency Fsw, and a common method is frequency foldback.

[0003] The frequency foldback function is illustrated by reducing the switching frequency to 1 / 8, 1 / 4, and 1 / 2 of the normal switching frequency. Under normal operating conditions, the target output voltage VOUT0 is used, and the switching frequency Fsw0 is used. During soft start or overload, when the output voltage VOUT is lower than the target value VOUT0, the switching frequency Fsw is set according to the following conditions:

[0004] If VOUT <VOUT0*25%,Fsw=Fsw0 / 8;

[0005] If VOUT0*25% <VOUT<VOUT0*50%,Fsw=Fsw0 / 4;

[0006] If VOUT0*50% <VOUT<VOUT0*75%,Fsw=Fsw0 / 2;

[0007] If VOUT0*75% <VOUT,Fsw=Fsw0。

[0008] Commonly used technical solutions for implementing frequency foldback functionality include: Figure 1 As shown. A voltage divider resistor is used to generate the FB voltage from the VOUT voltage. Three comparators compare the FB voltage with corresponding reference voltages of 25%REF, 50%REF, and 75%REF, generating control signals F1, F2, and F3. These control signals control the charging current I of the capacitor in the oscillator. c The comparator and the reset circuit RESET generate the oscillation clock signal CLK, and the operating waveform is as follows: Figure 2 As shown.

[0009] The above technical solution can be expressed mathematically as follows:

[0010]

[0011]

[0012]

[0013]

[0014]

[0015]

[0016] In existing frequency foldback solutions, the switching frequency Fsw jumps significantly whenever the output voltage VOUT exceeds the threshold voltage set for the frequency foldback function. The switching frequency difference between two adjacent switching cycles before and after this jump is significant. The switching power converter adjusts its loop operating point to adapt to the sudden change in switching frequency. This adjustment process can lead to overshoot and undershoot in the output voltage, making the output voltage non-monotonic and uneven. Figure 3 The diagram illustrates a typical soft-start waveform of the output voltage of a switching power supply converter with frequency foldback function. It can be seen that there is a significant overshoot in the output voltage at the switching frequency switching point.

[0017] The overshoot and undershoot of the output voltage during the soft-start process caused by the frequency foldback function may lead to malfunctions in downstream load circuits, such as repeatedly triggering the undervoltage lockout function. Therefore, it is necessary to avoid overshoot and undershoot in the output voltage as much as possible.

[0018] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0019] The purpose of this invention is to provide a frequency control system and method for a switching power supply, which can solve the overshoot and undershoot phenomena of output voltage caused by the frequency foldback function in the prior art, so that the output voltage VOUT can start smoothly and monotonically during the soft start process.

[0020] To achieve the above objectives, embodiments of the present invention provide a frequency control system for a switching power supply, comprising:

[0021] A voltage detection circuit, wherein the input signal of the voltage detection circuit is the feedback voltage, and the output signal is multiple control signals for controlling the foldback circuit;

[0022] A foldback circuit receives the control signal and generates a switching voltage or current signal.

[0023] An oscillator circuit includes a charging capacitor and generates a signal with a switching frequency based on the voltage of the charging capacitor.

[0024] An RC filter circuit filters the abrupt voltage or current signal and generates a gradually changing current signal, which serves as the charging current for the charging capacitor.

[0025] In one or more embodiments of the present invention, the voltage detection circuit includes a plurality of first comparators, which respectively compare the feedback voltage with a corresponding reference voltage to generate a plurality of control signals.

[0026] In one or more embodiments of the present invention, the foldback circuit includes multiple current branches connected in parallel to the RC filter circuit, and at least one of the current branches is provided with a control switch. The multiple control signals output by the voltage detection circuit control the control switch in the corresponding current branch to turn on or off.

[0027] In one or more embodiments of the present invention, the foldback circuit further includes a current source, which generates a branch current in the current branch through a current mirror circuit, and a transistor connected in series with the control switch is disposed in the current branch.

[0028] In one or more embodiments of the present invention, the current signal filtered by the RC filter circuit passes through the current mirror circuit to generate the charging current of the charging capacitor.

[0029] In one or more embodiments of the present invention, the foldback circuit includes logic circuitry that connects the input of the RC filter circuit to different reference voltages based on the control signal.

[0030] In one or more embodiments of the present invention, the output signal of the RC filter circuit generates a slowly changing first current signal on the first resistor through the first buffer, and the first current signal generates the charging current of the charging capacitor through the current mirror circuit.

[0031] In one or more embodiments of the present invention, the foldback circuit includes a voltage divider circuit, which includes a plurality of resistors connected in series, at least one of the resistors being connected in parallel with a control switch, and the control signal output by the voltage detection circuit controls the corresponding control switch to turn on or off, so as to adjust the output voltage of the voltage divider circuit; the output voltage of the voltage divider circuit serves as the input of the RC filter circuit.

[0032] In one or more embodiments of the present invention, the foldback circuit further includes a second buffer, the second buffer generating a second current signal on the second resistor, the second current signal being mirrored to the voltage divider circuit via a current mirror circuit.

[0033] In one or more embodiments of the present invention, the output signal of the RC filter circuit generates a slowly varying third current signal on the third resistor through the third buffer, and the third current signal generates the charging current of the charging capacitor through the current mirror circuit.

[0034] To achieve the above objectives, embodiments of the present invention provide a frequency control method for a switching power supply, comprising:

[0035] The voltage detection circuit outputs multiple control signals to control the foldback circuit based on the input signal of the feedback voltage.

[0036] The foldback circuit receives the control signal and generates a switching voltage or current signal;

[0037] The RC filter circuit filters the abrupt voltage or current signal and generates a slowly changing current signal.

[0038] The slowly varying current signal serves as the charging current for the charging capacitor in the oscillator circuit.

[0039] The oscillator circuit generates a signal with a switching frequency based on the voltage of the charging capacitor.

[0040] Compared with existing technologies, this invention, through an RC filter circuit, effectively solves the non-monotonic phenomena such as overshoot and undershoot of the output voltage at the frequency switching point during the soft-start process of a switching power converter with frequency foldback function. By smoothly and gradually controlling the charging current that generates the switching frequency, the switching frequency switching is also smoothly transitioned, effectively avoiding abnormal waveforms such as overshoot and undershoot on the output voltage VOUT. Attached Figure Description

[0041] Figure 1 This is a circuit diagram of a switching power supply frequency control system in the prior art;

[0042] Figure 2 This is a schematic diagram of the working waveform of the comparator and the reset circuit RESET forming the oscillation clock signal CLK in the prior art;

[0043] Figure 3 It is the soft-start waveform of the output voltage of the frequency control system of the switching power supply in the existing technology;

[0044] Figure 4a This is a circuit diagram of a switching power supply frequency control system according to a first embodiment of the present invention.

[0045] Figure 4b This is the soft-start waveform of the output voltage of the switching power supply frequency control system according to the first embodiment of the present invention;

[0046] Figure 5a This is a circuit diagram of a switching power supply frequency control system according to a second embodiment of the present invention.

[0047] Figure 5b This is the soft-start waveform of the output voltage of the switching power supply frequency control system according to the second embodiment of the present invention;

[0048] Figure 6a This is a circuit diagram of a switching power supply frequency control system according to a third embodiment of the present invention.

[0049] Figure 6b This is the soft-start waveform of the output voltage of the switching power supply frequency control system according to the third embodiment of the present invention. Detailed Implementation

[0050] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0051] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0052] like Figure 4a As shown, the frequency control system 100 of the switching power supply according to the first embodiment of the present invention includes a voltage detection circuit 11, a foldback circuit 12, an oscillator circuit 13, and an RC filter circuit 14.

[0053] The voltage detection circuit 11 includes two series-connected voltage divider resistors 111 and 112, and three comparators 113, 114, and 115. One end of the series-connected voltage divider resistors 111 and 112 is connected to the feedback voltage VOUT (system output voltage), and the other end is grounded. The positive inputs of the three comparators 113, 114, and 115 are connected between the two voltage divider resistors 111 and 112, and the negative inputs of the three comparators 113, 114, and 115 are connected to reference voltages VOUT with different ratios. REF .

[0054] In one embodiment, the negative input of comparator 113 is connected to 75% REF and outputs control signal F3; the negative input of comparator 114 is connected to 50% REF and outputs control signal F2; and the negative input of comparator 115 is connected to 25% REF and outputs control signal F1.

[0055] It should be noted that the number of comparators 113 and the proportion of the negative phase input connected to the reference voltage can be adjusted as needed, and this case does not limit it. For example, the number of comparators 113 can also be set to 4, 5 or 6 as needed, and the negative phase input of the comparators can also be connected to 10%REF, 15%REF, 20%REF, 80%REF, etc.

[0056] In this embodiment, the voltage detection circuit 11 generates the FB voltage from the feedback voltage VOUT through the voltage divider resistors 111 and 112. The FB voltage serves as the non-inverting input of the comparators 113, 114, and 115 to generate logic control signals F3, F2, and F1, respectively. These logic control signals are used to control the switching on or off.

[0057] The foldback circuit 12 receives the control signals F3, F2, and F1, and generates a proportionally varying current signal I. k .

[0058] In one embodiment, the foldback circuit 12 includes a current source 121 and four current branches. The four current branches are connected in parallel and then connected to the RC filter circuit 14.

[0059] Of the four current branches, the first current branch has a transistor 123, the second current branch has a transistor 124 connected in series and a control switch 127, the third current branch has a transistor 125 connected in series and a control switch 128, and the fourth current branch has a transistor 126 connected in series and a control switch 129.

[0060] Current source 121 generates current I B0 The current mirror circuit formed by transistor 122 generates branch currents of different proportions in the four current branches. In one embodiment, based on the different aspect ratios of the transistors, 1 / 8 I can be generated in the first current branch. B0 One-eighth of I can be generated in the second current branch. B0 2 / 8 of I can be generated in the third current branch. B0 4 / 8 of I can be generated in the fourth current branch. B0 .

[0061] Control signals F1, F2, and F3 are used to control the on / off states of control switches 127, 128, and 129, respectively, thereby controlling current signals I at different ratios. k The generation of I. For example, when control switches 127, 128, and 129 are all on, I k =I B0 When control switches 127, 128, and 129 are all off, I k =1 / 8 of I B0When control switch 127 is turned on and control switches 128 and 129 are turned off, I k =2 / 8 of I B0 When control switches 127 and 128 are turned on and control switch 129 is turned off, I k =4 / 8 of I B0 .

[0062] In this technical solution, the aspect ratio of the transistors in the current branches can be set as needed, and this application does not impose any restrictions. Furthermore, the number of current branches with control switches is set according to the number of control signals. For example, when there are four groups of control signals, there are four corresponding current branches with control switches, with each group of control signals controlling the on / off state of the control switch in one branch.

[0063] Oscillator circuit 13 includes charging capacitor C OSC And based on the charging capacitor C OSC voltage V C Generate a signal CLK with a switching frequency.

[0064] In one embodiment, the oscillator circuit 13 further includes a comparator 131 and a reset circuit RESET, which together form the clock signal CLK for the oscillation. Specifically, the voltage V C The negative input of comparator 131 is connected to the reference voltage REF2. The reset circuit RESET is located between the positive input and output of comparator 131, and includes transistor 132 and pulse generator 133.

[0065] In other embodiments, the pulse generator 133 can be disabled in the RESET circuit, and the pulse generator 133 can also be connected to the output of the comparator 131.

[0066] RC filter circuit 14 consists of resistor R F and capacitor C F Configuration for processing the switching current signal I k The signal is filtered to generate a slowly varying current signal Ic, which serves as the charging capacitor C. OSC The charging current.

[0067] In one embodiment, the current signal filtered by the RC filter circuit 14 passes through current mirror circuits 15 and 16 to generate the charging current Ic of the charging capacitor. The current mirror circuit 15 is composed of transistors 151 and 152, and the current mirror circuit 16 is composed of transistors 161 and 162.

[0068] The control method of the frequency control system 100 of the switching power supply according to the first embodiment of the present invention includes: detecting the VOUT voltage through a group of comparators 113, 114, 115 to generate control signals F1, F2, F3, and the three control signals control the current mirror to generate a proportionally changing current I k . The current low-pass filter 14 composed of RF / CF filters the stepped I k current waveform, and then the current mirror mirrors to generate the charging current Ic of the capacitor Cosc of the oscillator 13.

[0069] As Figure 4b shown, whenever the switching of F1 / F2 / F3 causes a jump in the I k current, the current low-pass filter 14 composed of RF / CF will smooth the jump edge of the I k current and generate a slowly changing current Ic. Therefore, the oscillation frequency generated by the oscillator 13 also changes slowly. When the switching power supply converter responds to this slowly changing switching frequency, the output voltage VOUT will not have voltage overshoot or undershoot, and maintains the state of monotonically rising VOUT during the soft start process.

[0070] As Figure 5a shown, the frequency control system 200 of the switching power supply according to the second embodiment of the present invention includes a voltage detection circuit, a fold-back circuit 22, an oscillator circuit 23, and an RC filter circuit 24.

[0071] The voltage detection circuit uses the voltage detection circuit 11 in Figure 4a . The FB voltage is generated from the feedback voltage VOUT through the voltage dividing resistors 111, 112, and this FB voltage serves as the positive-phase input of the comparators 113, 114, 115 to respectively generate logic control signals F3, F2, F1.

[0072] The fold-back circuit 22 includes a logic circuit 221 and a 4-to-1 analog switch. The 4-to-1 analog switch includes 4 selectable reference voltages with different ratios. The control signals F3, F2, F1 generate the control signal of the 4-to-1 analog switch through the logic circuit 221, and the input end of the RC filter circuit 24 can be connected to different reference voltages through different control signals.

[0073] In one embodiment, the selectable voltages of the 4-to-1 analog switch include REF2, 1 / 2 of REF2, 1 / 4 of REF2, and 1 / 8 of REF2. Correspondingly, the selection method of its reference voltage is as follows:

[0074] When F1 = 0, select the REF2 / 8 reference voltage;

[0075] When F1 = 1 & F2 = 0, select the REF2 / 4 voltage;

[0076] When F1 = 1 & F2 = 1 & F3 = 0, select the REF2 / 2 voltage;

[0077] When F3 = 1, select voltage REF2.

[0078] It should be noted that when the number of control signals changes, for example, to 4 groups, more logic control combinations can be generated. At this time, the number of selectable reference voltages for the analog switch can also be increased accordingly, and the ratio of different reference voltages can be adjusted as needed. This embodiment does not limit this.

[0079] Oscillator circuit 23 includes charging capacitor C OSC And based on the charging capacitor C OSC voltage V C Generate a signal CLK with a switching frequency.

[0080] In one embodiment, the oscillator circuit 23 further includes a comparator 231 and a reset circuit RESET, which together form the clock signal CLK for the oscillation. Specifically, the voltage V C The negative input of comparator 231 is connected to the reference voltage REF3. The reset circuit RESET is located between the positive input and output of comparator 231, and includes transistor 232 and pulse generator 233.

[0081] In other embodiments, the pulse generator 233 can be disabled in the RESET circuit, and the pulse generator 233 can also be connected to the output of the comparator 231.

[0082] RC filter circuit 24 consists of resistor R F and capacitor C F It is configured to filter the incoming abrupt voltage signal and output a gradually changing voltage signal REF2B.

[0083] The voltage signal REF2B generates a slowly changing current signal Iset across the resistor Rset through the buffer 25. Specifically, the voltage signal REF2B is connected to the positive input terminal of the buffer 25, and a transistor 27 is connected between the negative input terminal and the output terminal of the buffer 25. The transistor 27 is connected to the resistor Rset and then grounded.

[0084] The resistor Rset can be an external resistor connected to the chip. Different resistance values ​​can be selected to set the required switching frequency.

[0085] The current signal Iset passes through the current mirror circuit 26 to generate the charging capacitor C. OSC The charging current Ic. The current mirror circuit 26 is composed of transistors 261 and 262.

[0086] In this embodiment, a low-pass voltage filter 24 composed of RC is connected after the 4-to-1 analog switch, so that the reference voltage REF2B has a gradual change based on REF2, such as... Figure 5b As shown, REF2B generates a current Iset across resistor Rset via buffer 25, which also has a gradually changing edge, and is finally mirrored to the charging current Ic of oscillator capacitor Cosc. Therefore, the oscillation frequency generated by the oscillator also changes slowly, which can avoid voltage overshoot and undershoot of VOUT voltage.

[0087] like Figure 6a As shown, the frequency control system 300 of the switching power supply according to the third embodiment of the present invention includes a voltage detection circuit, a foldback circuit 32, an oscillator circuit 33, and an RC filter circuit 34.

[0088] The voltage detection circuit adopts Figure 4a The voltage detection circuit 11 in the circuit generates the FB voltage from the feedback voltage VOUT through the voltage divider resistors 111 and 112. The FB voltage is used as the non-inverting input of the comparators 113, 114 and 115 to generate logic control signals F3, F2 and F1 respectively.

[0089] The foldback circuit 32 includes a voltage divider circuit comprising four resistors connected in series, with resistance values ​​of R1 / 8, R1 / 8, R1 / 4, and R1 / 2, respectively. A switch is connected in parallel with each of the three resistors: one with resistance R1 / 8, one with resistance R1 / 4, and one with resistance R1 / 2. Control signals F3, F2, and F1 control the on / off state of these three switches to adjust the output voltage of the voltage divider circuit. The output voltage of the voltage divider circuit serves as the input to the RC filter circuit 34.

[0090] One end of the four resistors connected in series is grounded, and the other end is connected to the RC filter circuit 34.

[0091] It should be noted that when the number of control signals changes, for example, to 4 groups, more logic control combinations can be generated. At this time, the number of resistors can also be increased to 5, and the resistance ratio of different resistors can be adjusted as needed. This embodiment does not limit this.

[0092] The voltage signal REF2 generates a transitional current signal Iset across resistor Rset via buffer 322. The current signal Iset further generates a mirrored current in the voltage divider circuit via current mirror circuit 323.

[0093] Specifically, the voltage signal REF2 is connected to the positive input terminal of buffer 322. A transistor 323 is connected between the negative input terminal and the output terminal of buffer 322. Transistor 323 is connected to ground after being connected to resistor Rset. The current mirror circuit 323 is composed of transistors 3231 and 3232.

[0094] Oscillator circuit 33 includes charging capacitor C OSC And based on the charging capacitor C OSC voltage V C Generate a signal CLK with a switching frequency.

[0095] In one embodiment, the oscillator circuit 33 further includes a comparator 331 and a reset circuit RESET, which together form the clock signal CLK for the oscillation. Specifically, the voltage V C The negative input of comparator 331 is connected to the reference voltage REF3. The reset circuit RESET is located between the positive input and output of comparator 331, and includes transistor 332 and pulse generator 333.

[0096] RC filter circuit 34 consists of resistor R F and capacitor C F It is configured to filter the incoming abrupt voltage signal and output a gradually changing voltage signal REF2B.

[0097] The voltage signal REF2B generates a slowly changing current signal I2 across resistor R2 through buffer 35. Specifically, the voltage signal REF2B is connected to the positive input terminal of buffer 35, and a transistor 37 is connected between the negative input terminal and the output terminal of buffer 35. Transistor 37 is connected to resistor R2 and then grounded.

[0098] Resistor R2 can be an external resistor connected to the chip. Different resistance values ​​can be selected to set the required switching frequency.

[0099] The current signal I2 passes through the current mirror circuit 36, generating the charging capacitor C. OSC The charging current Ic. The current mirror circuit 36 ​​is composed of transistors 361 and 362.

[0100] In this embodiment, the control method of the frequency control system 300 of the switching power supply includes: the REF2 voltage is restored to a voltage by the current Iset generated by the resistor Rset through the buffer 322, which is then restored to a voltage across the resistor series R1 / 8, R1 / 8, R1 / 4, R1 / 4. This voltage is controlled by three signals F1, F2, and F3, and a voltage jump occurs when F1 / F2 / F3 changes. A voltage low-pass filter 34 composed of RF / CF filters the voltage jump, generating a smoothly changing voltage REF2B. Figure 6b This is an illustration. The REF2B voltage is then converted into current across resistor R2 via buffer 35, and finally mirrored to obtain the charging current Ic of the oscillator capacitor Cosc. Therefore, the oscillation frequency generated by the oscillator also changes slowly, which can avoid voltage overshoot and undershoot of VOUT. Compared with the second technical implementation scheme, this scheme is more suitable for situations where Rset is an external resistor connecting to the chip to set the switching frequency, avoiding the problem of interference caused by too low a voltage on Rset.

[0101] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A frequency control system for a switching power supply, characterized in that, include: A voltage detection circuit, wherein the input signal of the voltage detection circuit is the feedback voltage, and the output signal is multiple control signals for controlling the foldback circuit; A foldback circuit receives the control signal and generates a switching voltage or current signal. An oscillator circuit includes a charging capacitor and generates a signal with a switching frequency based on the voltage of the charging capacitor. An RC filter circuit filters the abrupt voltage or current signal and generates a gradually changing current signal, which serves as the charging current for the charging capacitor.

2. The frequency control system for the switching power supply as described in claim 1, characterized in that, The voltage detection circuit includes multiple first comparators, which compare the feedback voltage with the corresponding reference voltage to generate multiple control signals.

3. The frequency control system for the switching power supply as described in claim 2, characterized in that, The foldback circuit includes multiple current branches, which are connected in parallel to the RC filter circuit. At least one of the current branches is equipped with a control switch, and the multiple control signals output by the voltage detection circuit control the control switch in the corresponding current branch to turn on or off.

4. The frequency control system for the switching power supply as described in claim 3, characterized in that, The foldback circuit also includes a current source, which generates a branch current in the current branch through a current mirror circuit. A transistor connected in series with the control switch is provided on the current branch.

5. The frequency control system for the switching power supply as described in claim 3, characterized in that, The current signal filtered by the RC filter circuit passes through the current mirror circuit to generate the charging current of the charging capacitor.

6. The frequency control system for the switching power supply as described in claim 2, characterized in that, The foldback circuit includes logic circuitry that connects the input of the RC filter circuit to different reference voltages based on the control signal.

7. The frequency control system for the switching power supply as described in claim 6, characterized in that, The output signal of the RC filter circuit generates a slowly varying first current signal across the first resistor via the first buffer. The first current signal passes through the current mirror circuit to generate the charging current of the charging capacitor.

8. The frequency control system for the switching power supply as described in claim 2, characterized in that, The foldback circuit includes a voltage divider circuit, which includes multiple resistors connected in series. At least one of the resistors is connected in parallel with a control switch. The control signal output by the voltage detection circuit controls the corresponding control switch to turn on or off, so as to adjust the output voltage of the voltage divider circuit. The output voltage of the voltage divider circuit serves as the input to the RC filter circuit.

9. The frequency control system for the switching power supply as described in claim 8, characterized in that, The foldback circuit further includes a second buffer, which generates a second current signal across the second resistor. The second current signal is mirrored to the voltage divider circuit through the current mirror circuit.

10. The frequency control system for the switching power supply as described in claim 8 or 9, characterized in that, The output signal of the RC filter circuit generates a slowly varying third current signal across the third resistor via the third buffer. The third current signal passes through the current mirror circuit to generate the charging current of the charging capacitor.

11. A frequency control method for a switching power supply, characterized in that, include: The voltage detection circuit outputs multiple control signals to control the foldback circuit based on the input signal of the feedback voltage. The foldback circuit receives the control signal and generates a switching voltage or current signal; The RC filter circuit filters the abrupt voltage or current signal and generates a slowly changing current signal. The slowly varying current signal serves as the charging current for the charging capacitor in the oscillator circuit. The oscillator circuit generates a signal with a switching frequency based on the voltage of the charging capacitor.

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