A soft-start circuit and flyback converter

Abstract

This invention discloses a soft-start circuit and a flyback converter. The soft-start circuit includes a discharge circuit, an energy storage circuit, and a clamping circuit. The discharge circuit controls the energy storage circuit to discharge when the supply voltage is less than a preset threshold, and controls the energy storage circuit to stop discharging when the supply voltage is greater than or equal to the preset threshold. The energy storage circuit is used to charge the flyback converter through the power control circuit when the supply voltage is greater than or equal to the preset threshold. The clamping circuit clamps the voltage of the energy storage circuit during charging and outputs the clamped voltage to the control terminal of the power control circuit to control the start-up of the flyback converter. Because the discharge circuit controls the energy storage circuit to discharge when the supply voltage is less than the preset threshold, the energy storage circuit discharges voltage before the power control circuit charges it, preventing the soft-start circuit from failing due to residual voltage in the energy storage circuit, thereby solving the problem of large start-up current in the flyback converter.

Description

Technical Field

[0001] This invention relates to the field of power supply technology, and in particular to a soft-start circuit and a flyback converter. Background Technology

[0002] In power supply system design, a soft-start circuit is typically incorporated to reduce the inrush current generated when charging the output filter capacitor during startup. This circuit usually controls the power supply's output power to gradually increase during startup via a soft-start capacitor. The soft-start circuit controls the power supply to charge the output capacitor with a small current when the output voltage is relatively low, and then resumes a larger current supply once the output voltage reaches or approaches the set output voltage, thus meeting the power supply's normal output current requirements. The soft-start circuit prevents the power supply from charging the output capacitor with a large current during startup, which could lead to unnecessary current stress damage.

[0003] Since the reset time of the soft-start capacitor is generally relatively long, if the flyback converter is restarted shortly after being shut down, the soft-start capacitor may not have completed its reset, causing the soft-start circuit to fail and resulting in a large starting current for the flyback converter. Summary of the Invention

[0004] This invention provides a soft-start circuit and a flyback converter to solve the problem of large starting current in existing soft-start circuits.

[0005] In a first aspect, embodiments of the present invention provide a soft-start circuit applied to a flyback converter, the circuit comprising a discharge circuit, an energy storage circuit, and a clamping circuit:

[0006] The discharge circuit is used to control the energy storage circuit to discharge when the power supply voltage is less than a preset threshold, and to control the energy storage circuit to stop discharging when the power supply voltage is greater than or equal to the preset threshold.

[0007] The energy storage circuit is used to charge the flyback converter through the power control circuit when the voltage at the power supply terminal is greater than or equal to the preset threshold.

[0008] The clamping circuit is used to clamp the voltage of the energy storage circuit during the charging process of the energy storage circuit, and output the clamped voltage to the control terminal of the power control circuit to control the start of the flyback converter.

[0009] In one possible implementation, the discharge circuit includes a first control unit and a second control unit:

[0010] The first control unit is configured to, when the voltage at the power supply terminal is less than the preset threshold, open the path between the first terminal of the energy storage circuit and ground, so as to discharge the energy storage circuit;

[0011] The second control unit is used to disconnect the path between the energy storage circuit and ground, and to connect the path between the power supply terminal and ground, when the voltage at the power supply terminal is greater than or equal to the preset threshold, so as to stop the energy storage circuit from discharging.

[0012] In one possible implementation, the first control unit includes a first resistor and a first switching transistor;

[0013] One end of the first resistor is connected to the power supply terminal, and the second end of the first resistor is connected to the control terminal of the first switching transistor.

[0014] The first terminal of the first switching transistor is connected to the first terminal of the energy storage circuit, and the second terminal of the first switching transistor is connected to the second terminal of the energy storage circuit and grounded.

[0015] In one possible implementation, the second control unit includes: a first Zener diode, a second Zener diode, a second resistor, and a second switching transistor;

[0016] The first end of the second resistor is connected to the power supply terminal, and the second end of the second resistor is connected to the cathode of the first Zener diode.

[0017] The anode of the first Zener diode is connected to the cathode of the second Zener diode;

[0018] The anode of the second Zener diode is connected to the control terminal of the second switching transistor;

[0019] The first terminal of the second switch is connected to the second terminal of the first resistor, and the second terminal of the second switch is grounded.

[0020] In one possible implementation, the second control unit further includes a filtering module;

[0021] The filtering module is used to filter the voltage input to the control terminal of the second switching transistor.

[0022] In one possible implementation, the filtering module includes a first capacitor and a third resistor;

[0023] The first capacitor and the third resistor are connected in parallel, and the first end of the parallel connection is connected to the control terminal of the second switch, while the second end of the parallel connection is grounded.

[0024] In one possible implementation, the energy storage circuit includes a second capacitor;

[0025] The first terminal of the second capacitor is connected to the clamping circuit at the voltage output terminal of the power control circuit, and the second terminal of the second capacitor is grounded.

[0026] In one possible implementation, the clamping circuit includes a first diode and a second diode;

[0027] The first diode and the second diode are connected in parallel, and the cathode of the parallel diode is connected to the first terminal of the second capacitor, while the anode of the parallel diode is connected to the control terminal of the power control circuit.

[0028] In one possible implementation, the circuit as described in any of the above descriptions further includes a fourth resistor;

[0029] The fourth resistor is connected between the energy storage circuit and the voltage output terminal of the power control circuit.

[0030] In a second aspect, embodiments of the present invention provide a flyback converter, including a power control circuit and a soft-start circuit as described in any of the first aspects.

[0031] The beneficial effects of this invention are as follows:

[0032] This invention discloses a soft-start circuit and a flyback converter. The soft-start circuit is applied to a flyback converter and includes a discharge circuit, an energy storage circuit, and a clamping circuit. The discharge circuit controls the energy storage circuit to discharge when the supply voltage is less than a preset threshold and to stop discharging when the supply voltage is greater than or equal to the preset threshold. The energy storage circuit is used to charge the flyback converter through the power control circuit when the supply voltage is greater than or equal to the preset threshold. The clamping circuit clamps the voltage of the energy storage circuit during charging and outputs the clamped voltage to the control terminal of the power control circuit to control the start-up of the flyback converter. Since the discharge circuit can control the energy storage circuit to discharge when the supply voltage is less than the preset threshold, it discharges the voltage of the energy storage circuit before the power control circuit charges it, preventing the soft-start circuit from failing due to residual voltage in the energy storage circuit, thereby solving the problem of large start-up current in the flyback converter. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of a soft-start circuit in related technologies;

[0035] Figure 2 This is a schematic diagram of a soft-start circuit provided in an embodiment of the present invention;

[0036] Figure 3 This is a schematic diagram of another soft-start circuit provided in an embodiment of the present invention;

[0037] Figure 4 A schematic diagram of another soft-start circuit is provided for an embodiment of the present invention;

[0038] Figure 5 A schematic diagram of another soft-start circuit is provided for an embodiment of the present invention;

[0039] Figure 6 This is a schematic diagram of a flyback converter provided in an embodiment of the present invention. Detailed Implementation

[0040] 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. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0041] In power supply system design, to reduce the inrush current generated when the power supply output filter capacitor is charged during startup, a soft-start circuit is often designed for the power supply, such as... Figure 1 The diagram shows a schematic of a soft-start circuit in the related art. The soft-start circuit 101 is applicable to the power control circuit 102. The soft-start circuit 101 includes a soft-start capacitor C, a transistor MPSA63, a diode D, a resistor R, and a resistor R'. The power control circuit 102 is the control circuit of a flyback converter, such as the UC3842 chip.

[0042] In a specific implementation, pin 6 of the power control circuit 102 outputs a drive signal. When the duty cycle of pin 6 increases, the output current of the flyback converter rises. Since the duty cycle of pin 6 is controlled by pin 1 of the power control circuit 102, when the voltage at pin 1 rises, the duty cycle of pin 6 increases.

[0043] When the flyback converter starts up, the internal feedback circuit detects a low output voltage and outputs a 1mA current through pin 1, causing the voltage at pin 1 to continuously increase, thereby increasing the output current of the flyback converter. Simultaneously, pin 1 of the power supply control circuit 102 is connected to the soft-start capacitor C through transistor MPSA63. When the voltage at pin 1 is higher than the voltage of the soft-start capacitor C, transistor MPSA63 conducts, clamping the voltage at pin 1 to a potential no higher than that of the soft-start capacitor C. At the same time, when the flyback converter starts up, the power supply terminal Vin supplies power to Vcc through the soft-start circuit 101. When Vcc reaches the startup voltage, an internal 5V voltage is generated and charges the soft-start capacitor through pin 8 and resistor R'. Since the soft-start capacitor C discharges through resistor R before power-on, its voltage is 0V at power-on. The 5V power supply at pin 8 charges the soft-start capacitor C through resistor R', gradually increasing the voltage of the soft-start capacitor C to the voltage divided by R and R'.

[0044] During the charging process, the voltage of the soft-start capacitor C gradually changes from low to high. Therefore, the voltage at pin 1 of the power control circuit 102 also changes from low to high. Since the duty cycle of pin 6 is controlled by pin 1, the duty cycle of pin 6 also changes from low to high. Because the output current of the flyback converter is controlled by the duty cycle, it also changes from small to large. The gradual change of the flyback converter's output current from small to large achieves soft start.

[0045] After the flyback converter is powered off, the soft-start capacitor C discharges to zero through resistors R and R'. To limit power consumption, the resistance values ​​of resistors R and R' are generally chosen to be relatively large, so the reset time of the soft-start capacitor C is generally long. If the flyback converter is powered off and then quickly restarted, the voltage of the soft-start capacitor C is not zero, resulting in a large starting current of the flyback converter, which in turn affects the safety of other loads in the circuit.

[0046] To address the aforementioned problems, this invention provides a soft-start circuit and a flyback converter to solve the problem of high startup current in existing flyback converters.

[0047] The following describes the soft-start circuit provided by an exemplary embodiment of this application in conjunction with the application scenarios described above and with reference to the accompanying drawings. It should be noted that the above application scenarios are only shown to facilitate understanding of the spirit and principles of this application, and the embodiments of this application are not limited in any way in this respect.

[0048] like Figure 2 The diagram shown is a schematic representation of a soft-start circuit provided in an embodiment of the present invention, applied to a flyback converter. The circuit includes a discharge circuit 201, an energy storage circuit 202, and a clamping circuit 203.

[0049] The discharge circuit 201 is used to control the energy storage circuit 202 to discharge when the power supply terminal Vin voltage is less than a preset threshold, and to control the energy storage circuit 202 to stop discharging when the power supply terminal Vin voltage is greater than or equal to the preset threshold.

[0050] The energy storage circuit 202 is used to charge the power control circuit 102 in the flyback converter when the supply terminal Vin voltage is greater than or equal to a preset threshold.

[0051] The clamping circuit 203 is used to clamp the voltage of the energy storage circuit 202 during the charging process of the energy storage circuit 202, and output the clamped voltage to the control terminal of the power control circuit 102 to control the start of the flyback converter.

[0052] This invention provides a soft-start circuit for a flyback converter. The circuit includes a discharge circuit, an energy storage circuit, and a clamping circuit. The discharge circuit controls the energy storage circuit to discharge when the supply voltage is less than a preset threshold, and controls the energy storage circuit to stop discharging when the supply voltage is greater than or equal to the preset threshold. The energy storage circuit is used to charge the flyback converter via the power control circuit when the supply voltage is greater than or equal to the preset threshold. The clamping circuit clamps the voltage of the energy storage circuit during charging and outputs the clamped voltage to the control terminal of the power control circuit to control the start-up of the flyback converter. Because the discharge circuit can control the energy storage circuit to discharge when the supply voltage is less than the preset threshold, it discharges the voltage of the energy storage circuit before the power control circuit charges it, preventing the soft-start circuit from failing due to residual voltage in the energy storage circuit, thereby solving the problem of high start-up current in the flyback converter.

[0053] It should be noted that the energy storage circuit 202 in this embodiment of the invention is a soft-start capacitor.

[0054] The soft-start circuit in this embodiment of the invention will be described in detail below with reference to the power control circuit 102 of the aforementioned flyback converter:

[0055] like Figure 3 The diagram shown is a schematic representation of a soft-start circuit according to an embodiment of the present invention. The discharge circuit 201 includes a first control unit 301 and a second control unit 302.

[0056] The first control unit 301 is used to connect the first terminal of the energy storage circuit 202 to ground when the power supply terminal Vin voltage is less than a preset threshold, so as to discharge the energy storage circuit 202.

[0057] The second control unit 302 is used to disconnect the path between the energy storage circuit 202 and the ground, and to connect the path between the power supply terminal Vin and the ground when the voltage at the power supply terminal Vin is greater than or equal to a preset threshold, so that the energy storage circuit 202 stops discharging.

[0058] Specifically, such as Figure 3 As shown, the first control unit 301 may include a first resistor R1 and a first switching transistor M1; wherein, one end of the first resistor R1 is connected to the power supply terminal Vin, and the second end of the first resistor R1 is connected to the control terminal G1 of the first switching transistor M1; the first end of the first switching transistor M1 is connected to the first end of the energy storage circuit 202, and the second end of the first switching transistor M1 is connected to the second end of the energy storage circuit 202 and grounded.

[0059] The second control unit 302 may include a first Zener diode D1, a second Zener diode D2, a second resistor R2, and a second switch M2; wherein, the first end of the second resistor R2 is connected to the power supply terminal Vin, and the second end of the second resistor R2 is connected to the cathode of the first Zener diode D1; the anode of the first Zener diode D1 is connected to the cathode of the second Zener diode D2; the anode of the second Zener diode D2 is connected to the control terminal G2 of the second switch M2; the first end of the second switch M2 is connected to the second end of the first resistor R1, and the second end of the second switch M2 is grounded.

[0060] In one possible implementation, such as Figure 4 As shown, the second control unit 302 also includes a filtering module 303, which is used to filter the voltage input to the control terminal G1 of the second switching transistor M2.

[0061] Specifically, such as Figure 3 As shown, the filter module 303 may include a first capacitor C1 and a third resistor R3;

[0062] The first capacitor C1 and the third resistor R3 are connected in parallel, and the first end of the parallel connection is connected to the control terminal G2 of the second switch M2, while the second end of the parallel connection is grounded.

[0063] like Figure 3 and Figure 4 As shown, the energy storage circuit 202 is the second capacitor C2;

[0064] Specifically, the first terminal of the second capacitor C2 is connected to the voltage output terminal of the power control circuit 102 and the clamping circuit 203, respectively, and the second terminal of the second capacitor C2 is grounded.

[0065] The clamping circuit 203 may include a first diode D3 and a second diode D4; wherein the first diode D3 and the second diode D4 are connected in parallel, and the cathode of the parallel connection is connected to the first terminal of the second capacitor C2, and the anode of the parallel connection is connected to pin 1 of the power control circuit 102.

[0066] like Figure 5 As shown, the soft-start circuit also includes a fourth resistor R4;

[0067] Specifically, the fourth resistor R4 is connected between the energy storage circuit 202 and the voltage output terminal of the power control circuit 102.

[0068] The above describes the structure of the soft-start circuit provided in the embodiments of the present invention. The working principle of the soft-start circuit will be explained below based on the above soft-start circuit.

[0069] In practical implementation, when the flyback converter is turned on, the input voltage Vin at the power supply terminal rises. When the Vin voltage is less than a preset threshold, the Vin output voltage through the first resistor R1, and the first switch M1 is turned on. At this time, the second capacitor C2 discharges through the first switch M1, and the voltage of the second capacitor C2 remains at 0V after the discharge is complete. Meanwhile, the voltage at pin 1 of the power control circuit 102 is clamped low by the clamping circuit 203, so pin 6 of the power control circuit 102 outputs the drive signal and remains at a low level.

[0070] When the input voltage at the power supply terminal Vin is greater than or equal to a preset threshold, the voltage output from Vin through the second resistor R2 causes the first Zener diode D1 and the second Zener diode D2 to conduct. At this time, Vin, through the second resistor R2, the first Zener diode D1, and the second Zener diode D2, causes the second switching transistor M2 to conduct. The conduction of the second switching transistor M2 causes the voltage at the control terminal G1 of the first switching transistor M1 to drop to 0V, and the first switching transistor M1 is turned off. At this time, pin 8 of the power control circuit 102, i.e., the 5V power output pin, charges the second capacitor C2 through the fourth resistor R4, causing the capacitor voltage to rise slowly. Meanwhile, the voltage at pin 1 of the power control circuit 102 also rises slowly due to the increase in the clamping voltage. According to the control characteristics of the UCC28C45, the duty cycle of the drive output (pin 6) will rise slowly, thus achieving a soft start for the flyback converter.

[0071] It should be noted that the preset threshold here can be the forward voltage of the first Zener diode D1 and the second Zener diode D2.

[0072] When the flyback converter is powered off, the voltage Vin drops below a preset threshold and gradually decreases to a safe voltage. When Vin drops below the preset threshold, the first Zener diode D1 and the second Zener diode D2 are turned off. The second switch M2 is also turned off because its control terminal voltage is pulled low by the third resistor R3. Before the supply voltage Vin drops to 0V, current will still flow through the first resistor R1 to turn on the first switch M1. Therefore, the voltage across the second capacitor C2 will be pulled low to 0V, thus achieving a rapid reset.

[0073] If a residual voltage appears on the second capacitor C2 after the flyback converter is shut down for various reasons, when the flyback converter restarts, since the output voltage at the power supply terminal Vin rises from 0V to the rated input voltage, there will inevitably be a situation where the output voltage at the power supply terminal Vin is less than the preset threshold.

[0074] Similarly, when the output voltage of the power supply terminal Vin is less than the preset threshold, the first switch M1 is turned on and the second capacitor C2 is discharged to 0V, thereby achieving a fast reset and ensuring that the flyback converter can enter a reliable soft-start state every time it starts.

[0075] It should be noted that, Figures 2-5 The power control circuit 102 in the middle is Figure 1 The power control circuit 102 in the present invention is a power control circuit in the related art. The specific structure of the power control circuit is not described in the embodiments of the present invention.

[0076] Based on the same inventive concept, this embodiment of the invention also provides a flyback converter. The implementation of the flyback converter can refer to the implementation of the soft-start circuit described above, and the repeated parts will not be described again.

[0077] like Figure 6 The diagram shown is a schematic diagram of a flyback converter provided in an embodiment of the present invention. The flyback converter includes any of the above-mentioned soft-start circuits 601 and power control circuits 102.

[0078] This invention discloses a soft-start circuit and a flyback converter. The soft-start circuit, applied to a flyback converter, includes a discharge circuit, an energy storage circuit, and a clamping circuit. The discharge circuit comprises a first control unit and a second control unit. When the power supply voltage is less than a preset threshold, the first control unit controls the energy storage circuit to discharge; when the power supply voltage is greater than or equal to the preset threshold, the second control unit controls the energy storage circuit to stop discharging. The energy storage circuit is used to charge the flyback converter through the power control circuit when the power supply voltage is greater than or equal to the preset threshold. The clamping circuit clamps the voltage of the energy storage circuit during charging and outputs the clamped voltage to the control terminal of the power control circuit to control the start-up of the flyback converter. Additionally, the soft-start circuit includes a filtering module to filter the voltage input to the control terminal of the second switching transistor. Because the discharge circuit can control the energy storage circuit to discharge when the power supply voltage is less than the preset threshold, it discharges the voltage of the energy storage circuit before the power control circuit charges it, preventing the soft-start circuit from failing due to residual voltage in the energy storage circuit, thereby solving the problem of high start-up current in the flyback converter.

[0079] Those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims and their equivalents, this invention is also intended to include these modifications and variations.

Claims

1. A soft-start circuit, characterized in that, Applied to flyback converters, this circuit includes a discharge circuit, an energy storage circuit, and a clamping circuit. The discharge circuit is used to control the energy storage circuit to discharge when the power supply voltage is less than a preset threshold, and to control the energy storage circuit to stop discharging when the power supply voltage is greater than or equal to the preset threshold. The energy storage circuit is used to charge the flyback converter through the power control circuit when the voltage at the power supply terminal is greater than or equal to the preset threshold. The clamping circuit is used to clamp the voltage of the energy storage circuit during the charging process of the energy storage circuit, and output the clamped voltage to the control terminal of the power control circuit to control the start of the flyback converter. The discharge circuit includes a first control unit and a second control unit: The first control unit is configured to, when the voltage at the power supply terminal is less than the preset threshold, open the path between the first terminal of the energy storage circuit and ground, so as to discharge the energy storage circuit; The second control unit is used to connect the power supply terminal and ground when the voltage of the power supply terminal is greater than or equal to the preset threshold, so as to disconnect the first terminal of the energy storage circuit and ground through the first control unit, so as to stop the energy storage circuit from discharging.

2. The circuit as described in claim 1, characterized in that, The first control unit includes a first resistor and a first switching transistor; One end of the first resistor is connected to the power supply terminal, and the second end of the first resistor is connected to the control terminal of the first switching transistor. The first terminal of the first switching transistor is connected to the first terminal of the energy storage circuit, and the second terminal of the first switching transistor is connected to the second terminal of the energy storage circuit and grounded.

3. The circuit as described in claim 2, characterized in that, The second control unit includes: a first Zener diode, a second Zener diode, a second resistor, and a second switching transistor; The first end of the second resistor is connected to the power supply terminal, and the second end of the second resistor is connected to the cathode of the first Zener diode. The anode of the first Zener diode is connected to the cathode of the second Zener diode; The anode of the second Zener diode is connected to the control terminal of the second switching transistor; The first terminal of the second switch is connected to the second terminal of the first resistor, and the second terminal of the second switch is grounded.

4. The circuit as described in claim 3, characterized in that, The second control unit also includes a filtering module; The filtering module is used to filter the voltage input to the control terminal of the second switching transistor.

5. The circuit as described in claim 4, characterized in that, The filtering module includes a first capacitor and a third resistor; The first capacitor and the third resistor are connected in parallel, and the first end of the parallel connection is connected to the control terminal of the second switch, while the second end of the parallel connection is grounded.

6. The circuit as described in claim 1, characterized in that, The energy storage circuit includes a second capacitor; The first terminal of the second capacitor is connected to the voltage output terminal of the power control circuit and the clamping circuit, respectively, and the second terminal of the second capacitor is grounded.

7. The circuit as described in claim 6, characterized in that, The clamping circuit includes a first diode and a second diode; The first diode and the second diode are connected in parallel, and the cathode of the parallel diode is connected to the first terminal of the second capacitor, while the anode of the parallel diode is connected to the control terminal of the power control circuit.

8. The circuit as described in any one of claims 1-7, characterized in that, It also includes a fourth resistor; The fourth resistor is connected between the energy storage circuit and the voltage output terminal of the power control circuit.

9. A flyback converter, characterized in that, It includes a power control circuit and a soft-start circuit as described in any one of claims 1-8.

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