Soft start circuit and method for a switching power supply

Abstract

The present disclosure relates to the technical field of soft start control of switching power supply, in particular to a soft start circuit and method of switching power supply, the circuit comprising: a first comparison module connected with a power supply, used for sampling an input voltage of the switching power supply to determine whether the switching power supply is in a power-on state, and outputting a soft start signal when the switching power supply is in the power-on state and a voltage of the power supply is greater than a preset voltage threshold; a main control module connected with an output end of the first comparison module, used for performing soft start of the switching power supply according to the soft start signal; the circuit can match a soft start process of the switching power supply with a power-on state of a DC+ input end of the switching power supply.

Description

Technical Field

[0001] This disclosure relates to the field of soft-start technology for switching power supplies, and particularly to a soft-start circuit and method for a switching power supply. Background Technology

[0002] Switching power supplies are commonly used to perform DC / DC or DC / AC power conversion on the DC+ input voltage. Because switching power supplies can quickly and dynamically adjust their output performance under wide input voltage and output load conditions, and because they have high power conversion efficiency, they are widely used in electrical equipment. The principle of a switching power supply is to convert the error signal into a duty cycle control signal through a feedback loop including an error amplifier, and then use this duty cycle control signal to drive the switching transistor of the power supply to inject current into the capacitive load.

[0003] During the startup phase of a switching power supply, the error amplifier is in an unbalanced state, causing the feedback loop to operate at 100% duty cycle. This results in inrush current flowing into the capacitive load of the power supply, causing output voltage overshoot. The rapid changes in circuit state during startup also generate inrush current that can impact components such as switching transistors and rectifier diodes, affecting their lifespan and safe operation, and potentially causing damage to the switching transistors and / or other components.

[0004] In order to solve the technical problem of inrush current generated during the startup phase of a switching power supply when the DC+ input terminal is powered on, some technical solutions propose a soft-start circuit. The soft-start circuit eliminates inrush current and avoids output voltage overshoot by limiting the duty cycle of the control signal or limiting the current value of the switching transistor in the switching power supply.

[0005] In some technical solutions, digital circuits such as triggers are used to achieve soft start. However, digital circuits such as triggers have high delays, making it difficult to effectively control the linearity of the output voltage during the startup phase.

[0006] As switching power supplies become increasingly complex, higher demands are placed on their power-on timing and power-on smoothness. The power-on state of the DC+ input terminal must match the soft-start process to prevent the soft-start from starting before the DC+ input terminal is powered on. If the power-on time of the DC+ input terminal is later than the start time of the soft-start, the soft-start may be unable to control the output voltage after the DC+ input terminal is powered on, making it difficult to suppress potential inrush currents. In other words, if the soft-start process and the power-on state of the DC+ input terminal are mismatched, inrush current generation cannot be effectively suppressed.

[0007] However, there is no existing circuit capable of soft-starting a switching power supply in response to the power-on state of its input DC+ terminal.

[0008] Therefore, there is an urgent need in the field for a circuit that can soft-start a switching power supply in response to the power-on state of its input terminal DC+, so that the soft-start process of the switching power supply matches the power-on state of its input terminal DC+. Summary of the Invention

[0009] This disclosure provides a soft-start circuit and method for a switching power supply, which solves the technical problem in the prior art that there is no circuit capable of soft-starting the switching power supply in response to the power-on state of the input terminal DC+ of the switching power supply, so that the soft-start process of the switching power supply matches the power-on state of the input terminal DC+ of the switching power supply.

[0010] In a first aspect, this disclosure provides a soft-start circuit for a switching power supply, characterized in that it includes:

[0011] The first comparison module is connected to the power supply VCC and is used to sample the input voltage of the switching power supply to determine whether the switching power supply is in the power-on state. When the switching power supply is in the power-on state and the voltage of the power supply is greater than a preset voltage threshold, a soft-start signal is output.

[0012] The main control module is connected to the output of the first comparison module and is used to perform soft start on the switching power supply based on the soft start signal.

[0013] In some embodiments, the first comparison module includes:

[0014] The first comparator has its non-inverting input connected to the power supply.

[0015] The second comparator has its non-inverting input connected to the output of the first comparator, its inverting input connected to the reference voltage pin of the main control module, and its output connected to the reset pin of the main control module.

[0016] The third comparator has its inverting input connected to the inverting input of the first comparator, its non-inverting input connected to the power supply, and its output connected to the soft-start pin and reference voltage pin of the main control module.

[0017] The soft-start capacitor has one end connected to the connection line between the output of the third comparator and the soft-start pin of the main control module, and the other end grounded.

[0018] In some embodiments, the first comparator, the second comparator, and the third comparator are used for:

[0019] When the switching power supply is not powered on or when the switching power supply is powered on and the voltage of the power supply is not greater than the preset voltage threshold, a low-level signal is output. This low-level signal enables the main control module to start the reset function to discharge the soft-start capacitor.

[0020] In some embodiments, the first comparator, the second comparator, and the third comparator are used for:

[0021] When the switching power supply is powered on and the voltage of the power supply is greater than the preset voltage threshold, a high-level signal is output, which causes the main control module to charge the soft-start capacitor based on the reference voltage, thereby realizing the soft start of the switching power supply.

[0022] In some embodiments, the first comparison module further includes: a first resistor, a second resistor, and a third resistor;

[0023] The first resistor and the second resistor are connected in series between the input terminal of the switching power supply and ground to sample the input voltage of the switching power supply.

[0024] One end of the third resistor is connected to the connection line between the first resistor and the second resistor, and the other end of the third resistor is connected to the connection line between the output terminal of the first comparator and the non-inverting input terminal of the second comparator.

[0025] In some embodiments, it also includes:

[0026] The second comparison module is connected to the shutdown pin of the main control module and is used to output a shutdown pin control signal when the switching power supply is in the power-on state, so as to disable the shutdown pin.

[0027] The main control module is also used to control the main control module to not be turned off when powered on, based on the control signal of the shutdown pin.

[0028] In some embodiments, the second comparison module includes:

[0029] The fourth comparator has its inverting input connected to the reference voltage pin of the main control module, and its non-inverting input grounded.

[0030] The fifth comparator has its inverting input connected to the output of the fourth comparator.

[0031] The sixth comparator has its non-inverting input connected to the output of the fifth comparator and its inverting input connected to the inverting input of the fourth comparator.

[0032] The seventh comparator has its inverting input connected to the output of the sixth comparator, its non-inverting input connected to the reference voltage pin of the main control module, and its output connected to the shutdown pin of the main control module.

[0033] The reference voltage pin of the main control module is also connected to the non-inverting input of the fourth comparator, the connection line between the output of the fifth comparator and the non-inverting input of the sixth comparator, the inverting input of the sixth comparator, and the connection line between the output of the seventh comparator and the shutdown pin of the main control module. The power supply is connected to the connection line between the output of the sixth comparator and the inverting input of the seventh comparator.

[0034] Secondly, this disclosure provides a soft-start method for a switching power supply, implemented based on the soft-start circuit of the switching power supply in the first aspect, the method comprising:

[0035] The input voltage of the switching power supply is sampled to determine whether the switching power supply is powered on.

[0036] A soft-start signal is output in response to the switching power supply being powered on and the voltage of the power supply being greater than a preset voltage threshold.

[0037] The main control module performs a soft start on the switching power supply based on the soft start signal.

[0038] In some embodiments, it also includes:

[0039] In response to the switching power supply not being powered on or the switching power supply being powered on and the voltage of the power supply not exceeding a preset voltage threshold, the main control module's reset function is activated to discharge the soft-start capacitor.

[0040] In some embodiments, it also includes:

[0041] The shutdown pin of the main control module is disabled when the switching power supply is powered on.

[0042] This disclosure provides a soft-start circuit and method for a switching power supply. By setting up a first comparison module and a main control module, and connecting the first comparison module to the power supply VCC, the input voltage of the switching power supply is sampled to determine whether the switching power supply is in a powered-on state. When the switching power supply is in a powered-on state and the voltage of the power supply is greater than a preset voltage threshold, a soft-start signal is output. The main control module is connected to the output of the first comparison module to perform a soft-start of the switching power supply based on the soft-start signal, enabling the soft-start process of the switching power supply to match the powered-on state of the DC+ input terminal of the switching power supply. Attached Figure Description

[0043] The present disclosure will be described in more detail below based on embodiments and with reference to the accompanying drawings:

[0044] Figure 1 A block diagram of the soft-start circuit of the switching power supply in this disclosure is shown;

[0045] Figure 2A schematic diagram of a pulse width modulator according to the present disclosure is shown;

[0046] Figure 3 A schematic diagram of a four-channel differential circuit comparator of this disclosure is shown;

[0047] Figure 4 The logic circuitry within the main control module of this disclosure is shown;

[0048] Figure 5 A schematic diagram of a first comparison module of this disclosure is shown;

[0049] Figure 6 A schematic diagram of a second comparison module of this disclosure is shown;

[0050] Figure 7 A schematic diagram of the soft-start circuit of the switching power supply in this disclosure is shown.

[0051] In the accompanying drawings, the same parts are referred to by the same reference numerals, and the drawings are not drawn to scale. Detailed Implementation

[0052] To enable those skilled in the art to better understand the present disclosure and to fully understand and implement the process of how the present disclosure applies technical means to solve technical problems and achieve corresponding technical effects, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present disclosure, not all embodiments. The embodiments of the present disclosure and the various features therein can be combined with each other without conflict, and the resulting technical solutions are all within the protection scope of the present disclosure. Based on the embodiments of the present disclosure, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present disclosure.

[0053] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this disclosure are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this disclosure described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0054] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0055] Using a soft-start circuit to control the startup of a switching power supply to eliminate inrush current involves the following design principle: limiting the useful current. If no load is being driven, the startup current of a switching power supply is typically very small. In many cases, the actual startup current may be smaller than the steady-state operating current maintained using this method.

[0056] Example 1

[0057] Figure 1 This is a block diagram of the soft-start circuit of the switching power supply in this embodiment. Figure 1 As shown, a soft-start circuit for a switching power supply includes:

[0058] The first comparison module is connected to the power supply VCC and is used to sample the input voltage DC+ of the switching power supply to determine whether the switching power supply is in the power-on state. When the switching power supply is in the power-on state and the voltage of the power supply is greater than the preset voltage threshold, a soft start signal is output.

[0059] The main control module is connected to the output of the first comparison module and is used to perform soft start on the switching power supply based on the soft start signal.

[0060] Figure 2 A schematic diagram of a pulse width modulator according to this embodiment is shown. Figure 3 A schematic diagram of a four-channel differential circuit comparator according to this embodiment is shown. In this embodiment, the main control module may be, but is not limited to, [the following]. Figure 2 The pulse width modulator shown may have a first comparison module that is, but is not limited to, the following: Figure 3 The four-channel differential circuit comparator shown.

[0061] like Figure 7 As shown, this embodiment provides a soft-start circuit for a switching power supply. By setting a first comparison module 100 and a main control module, and connecting the first comparison module 100 to the power supply VCC, the circuit samples the input voltage of the switching power supply to determine whether the switching power supply is in a powered-on state. When the switching power supply is in a powered-on state and the voltage of the power supply is greater than a preset voltage threshold, a soft-start signal is output. The main control module is connected to the output of the first comparison module 100 to perform a soft start on the switching power supply according to the soft-start signal, so that the soft-start process of the switching power supply matches the powered-on state of the input DC+ of the switching power supply.

[0062] To facilitate understanding of the technical solution in this embodiment, the following time points are defined:

[0063] At time 0, the power supply VCC is not connected;

[0064] At time 0+, the power supply VCC is connected, but the voltage of the power supply VCC does not reach the normal operating range, that is, it is not greater than the preset voltage threshold.

[0065] At time T1, the power supply VCC is connected, and the voltage of the power supply VCC has reached the normal operating range, which is greater than the preset voltage threshold, for example, the preset voltage threshold is 15V.

[0066] Figure 4 The logic circuitry within the main control module is shown. (Combined with...) Figures 2 to 4 As can be seen, the main control module U4 in this embodiment has the following logic:

[0067] 1. A reference voltage V is generated based on a reference voltage circuit that includes a temperature-compensated Zener diode. REF At time T1, the reference voltage V REF It is 5V ± 1%.

[0068] 2. When the pin is turned off When the reset pin is set low, it suppresses all PWM output pulses of the main control module U4; when the reset pin is set low... When set low, the soft-start capacitor C SOFTSTART Discharge is performed.

[0069] 3. For example Figure 4 As shown, when the power supply VCC of the switching power supply is turned on, at time 0+, the main control module U4 powers on and the voltage of the power supply VCC does not reach the normal operating range. At this time, the undervoltage lockout circuit built into the main control module U4, through the third transistor Q3, locks out the voltage. Reset pin 5 remains at a low potential, thus activating the reset function. Transistor Q1 is turned on, and the soft-start capacitor C is activated through its first collector. SOFTSTART The output of the error amplifier (ERROR AMP) is clamped to ground potential through the second collector of the first transistor Q1, ensuring that the main control module U4 outputs a signal with zero duty cycle (i.e., zero output for OUTPUT-A and OUTPUT-B). That is, at time 0+, the undervoltage lockout circuit inside the main control module U4 resets the module and activates the soft-start capacitor C. SOFTSTART Both the PWM output and the output are zero.

[0070] 4. When the voltage of the power supply VCC reaches the normal operating range (greater than the preset voltage threshold), at time T1, the undervoltage lockout function of the main control module U4 is no longer enabled. At this time, the soft-start circuit of this embodiment will activate the main control module U4. The reset pin is set to high level, disabling the reset function and turning off the first transistor Q1. This allows the soft-start capacitor C to be switched on via the 100μA current source within the main control module U4 and the soft-start circuit of this embodiment. SOFTSTART During charging, the output of the error amplifier is clamped to a value higher than C by the second transistor Q2. SOFTSTART Voltage 1 times V BE (i.e., clamped to VC) SOFTSTART +V BE When the soft-start voltage (VC) SOFTSTART +V BE When the voltage rises to the set value (high level), the duty cycle of the PWM increases linearly to the design value required for voltage modulation.

[0071] Example 2

[0072] Based on the above embodiments, Figure 5 A schematic diagram of a first comparison module is shown. (For example...) Figure 5 As shown, the first comparison module 100 includes:

[0073] The first comparator U1B has its non-inverting input connected to the power supply.

[0074] The second comparator U1C has its non-inverting input connected to the output of the first comparator U1B, its inverting input connected to the reference voltage pin of the main control module U4, and its output connected to the reset pin of the main control module U4.

[0075] The third comparator U1D has its inverting input connected to the inverting input of the first comparator U1B, its non-inverting input connected to the power supply, and its output connected to the soft-start pin and reference voltage pin of the main control module U4.

[0076] The soft-start capacitor C3 is connected at one end to the connection line between the output of the third comparator U1D and the soft-start pin Css of the main control module, and at the other end to ground.

[0077] In some embodiments, the first comparator U1B, the second comparator U1C, and the third comparator U1D are used for:

[0078] When the switching power supply is not powered on or when the switching power supply is powered on and the voltage of the power supply is not greater than the preset voltage threshold, a low-level signal is output. This low-level signal enables the reset function of the main control module U4 to discharge the soft-start capacitor.

[0079] When the switching power supply is powered on and the voltage of the power supply is greater than the preset voltage threshold, a high-level signal is output, which causes the main control module U4 to charge the soft-start capacitor based on the reference voltage, thereby realizing the soft start of the switching power supply.

[0080] In some embodiments, the first comparison module 100 further includes: a first resistor R1, a second resistor R31, and a third resistor R3;

[0081] Among them, the first resistor R1 and the second resistor R31 are connected in series between the input terminal DC+ of the switching power supply and ground to sample the input voltage of the switching power supply.

[0082] One end of the third resistor R3 is connected to the connection line between the first resistor R1 and the second resistor R31, and the other end of the third resistor R3 is connected to the connection line between the output terminal of the first comparator U1B and the non-inverting input terminal of the second comparator U1C.

[0083] like Figure 5 As shown, in the first comparison module of this disclosure, the first resistor R1 and the second resistor R31 are connected in series between the input terminal DC+ of the switching power supply and ground; one end of the third resistor R3 is connected to the connection line of the first resistor R1 and the second resistor R31, and the other end of the third resistor R3 is connected to the non-inverting input terminal of the second comparator U1C; the inverting input terminal of the second comparator U1C is connected to the reference voltage pin V. REF The output of the second comparator U1C is connected to the reset pin of the main control module U4. A diode CR1 is connected in series on the connection line between the non-inverting input of the first comparator U1B and the power supply VCC. One end of capacitor C20 is connected to the connection line between diode CR1 and the power supply VCC, and the other end is grounded. One end of capacitor C16 is connected to the ground line of capacitor C20, and the other end is connected to one end of the third resistor R3. A resistor R4 is connected in series on the connection line between the non-inverting input of the third comparator U1D and the power supply VCC. Resistors R5 and R50 are connected in series on the line connecting the output of the third comparator U1D and the soft-start pin Css. The reference voltage pin V... REF Resistor R21 is connected to the connection line between resistors R5 and R50. One end of the soft-start capacitor C3 is connected to the line connecting resistor R50 and the soft-start pin Css, and the other end is grounded; the reference voltage pin V... REF Connect the inverting input terminal and the output terminal of the second comparator U1C respectively, and connect them to the reference voltage pin V. REF A resistor R22 is also connected to the connection line with the second comparator U1C.

[0084] In practical applications, the soft-start circuit of this embodiment samples the input terminal DC+ of the switching power supply. This sampling can be achieved by connecting the first resistor R1 = 47kΩ and the second resistor R31 = 17.8kΩ in series to obtain the sampled voltage.

[0085] For example, when the DC+ input of the switching power supply reaches its rated value of 74V, the sampling voltage is approximately 20.3V. This sampling voltage is current-limited through the third resistor R3 = 560kΩ and input to the non-inverting input (pin 9) of the second comparator U1C, which is higher than the V at the inverting input (pin 8) of the second comparator U1C. REF A 5V level causes pin 14 of the second comparator U1C to output a high level, which in turn resets the main control module U4's reset pin. Setting it to high level disables the reset function and enables the soft-start function.

[0086] For example, when the input voltage DC+ of the switching power supply is lower than the threshold voltage of 18.2V, the non-inverting input of the second comparator U1C (pin 9 of the second comparator U1C) is lower than the reference voltage V at the inverting input of the second comparator U1C (pin 8 of the second comparator U1C). REF When the voltage level is 5V, the output of the second comparator U1C (pin 14 of the second comparator U1C) will be low, thus resetting the reset pin of the main control module U4. Set to low level to activate the reset function and disable the soft-start function of the main control module U4.

[0087] When the DC+ input terminal of the switching power supply is not powered on or does not exceed the preset voltage threshold, Figure 5 The non-inverting input (pin 5) of the first comparator U1B is lower than the potential of the inverting input (pin 4) of U1B, causing the output (pin 2) of the first comparator U1B to output a low level. This also causes the non-inverting inputs of the second comparator U1C and the third comparator U1D to be lower than their inverting inputs, resulting in their outputs also being low. This activates the reset function of the main control module U4 and discharges the soft-start capacitor. In other words, when the DC+ input of the switching power supply is not powered on or is below the threshold voltage, Figure 5 The circuit initiates the reset function of the main control module U4, thereby ensuring that the soft start time is not earlier than the power-on time of the input terminal DC+ of the switching power supply.

[0088] When the DC+ input terminal of the switching power supply is powered on and reaches the preset voltage threshold, Figure 5The potential at the non-inverting input (pin 5) of the first comparator U1B shown is VCC + 0.7V (15V + 0.7V ≈ 15.7V), which is higher than the potential at the inverting input (pin 4) of the first comparator U1B. REF (Approximately 5V), the output terminal U1B2 of the first comparator U1B outputs a high level. Since the output terminal (pin 2) of the first comparator U1B is connected to the non-inverting input terminal (pin 9) of the second comparator U1C, the non-inverting input terminal (pin 9) of the second comparator U1C is at a high level, higher than the potential V of the inverting input terminal (pin 8) of the second comparator U1C. REF (Approximately 5V), the output terminal (pin 14) of the second comparator U1C outputs a high level, which resets the main control module U4's reset pin. The non-inverting input (pin 11) of the third comparator U1D is set to a high level, higher than the inverting input (pin 10) of the third comparator U1D. The output (pin 13) of the third comparator U1D then outputs a high level. At this time, the reference voltage V... REF Give soft-start capacitor C3(C SOFTSTART The charging time constant τ1 = (R21 + R50) × C3 can be adjusted by setting the values ​​of R21, R50, and C3. This allows control over the soft-start slope.

[0089] After power-on (at time T1), the main control module U4... The terminal is set to high level; the soft-start capacitor C3 is gradually charged to high level, and the switching power supply performs a soft start.

[0090] It should be noted that if directly in If a pull-up resistor is connected to the reset pin 5, the soft-start process of the switching power supply cannot be matched with the power-on state of the DC+ input terminal of the switching power supply.

[0091] Example 3

[0092] Figure 6 A schematic diagram of a second comparison module according to this embodiment is shown. Figure 7 A schematic diagram of the soft-start circuit of the switching power supply of this disclosure is shown, which includes... Figure 5 and Figure 6 The schematic diagram shown illustrates the connection relationship between the main control module U4 and the circuit. For example... Figure 6 As shown, based on the above embodiments, the soft-start circuit further includes:

[0093] The second comparison module 200 is connected to the shutdown pin of the main control module U4 and is used to output a shutdown pin control signal when the switching power supply is in the power-on state, so as to disable the shutdown pin.

[0094] The main control module U4 is also used to control the main control module U4 to not be turned off when powered on, based on the control signal of the shutdown pin.

[0095] In some embodiments, the second comparison module 200 includes:

[0096] The fourth comparator U7D has its inverting input connected to the reference voltage pin of the main control module U4, and its non-inverting input grounded.

[0097] The fifth comparator U7A has its inverting input connected to the output of the fourth comparator U7D;

[0098] The sixth comparator U7B has its non-inverting input connected to the output of the fifth comparator U7A, and its inverting input connected to the inverting input of the fourth comparator U7D.

[0099] The seventh comparator U7C has its inverting input connected to the output of the sixth comparator U7B, its non-inverting input connected to the reference voltage pin of the main control module U4, and its output connected to the turn-off pin of the main control module U4.

[0100] The reference voltage pin of the main control module U4 is also connected to the connection line between the output of the fifth comparator U7A and the non-inverting input of the sixth comparator U7B, and the connection line between the output of the seventh comparator U7C and the shutdown pin of the main control module U4. The power supply is connected to the connection line between the output of the sixth comparator U7B and the inverting input of the seventh comparator U7C.

[0101] Main control module U4 The shutdown pin (responsible for controlling PWM output) is high after power-on (at time T1), and the OUTPUT of the main control module U4 enables PWM pulse output;

[0102] like Figure 6 As shown, the non-inverting input (pin 5) of the sixth comparator U7B is connected to the output (pin 1) of the fifth comparator U7A, and the output (pin 2) of the sixth comparator U7B is connected to the inverting input (pin 8) of the seventh comparator U7C.

[0103] The non-inverting input (pin 9) of the seventh comparator U7C is connected to the reference voltage pin, the inverting input (pin 8) of the seventh comparator U7C is connected to the output (pin 2) of the sixth comparator U7B, and the output (pin 14) of the seventh comparator U7C is connected to the shutdown pin of the main control module U4.

[0104] The second comparison module also includes resistors R58 and R59 connected in series between the reference voltage pin and ground. The inverting input (pin 4) of the sixth comparator U7B and the inverting input of the fourth comparator U7D are respectively connected to the connection lines of resistors R58 and R59. The non-inverting input of the fourth comparator U7D is grounded through capacitor C27. One end of resistor R17 is connected to the power supply VCC, and the other end is connected to the connection line between the output of the fourth comparator U7D and the inverting input of the fifth comparator U7A. At the connection point of resistor R17 on this line, the positive terminal of diode CR15 is also connected. The negative terminal of diode CR15 is grounded through capacitor C11. The COMP pin of the main control module U4 is connected to the connection line between the negative terminal of diode CR15 and capacitor C11. One end of resistor R21 is grounded, and the other end is connected to one end of resistor R20. The non-inverting input of the fifth comparator U7A is connected to the connection line of resistors R21 and R20. Resistor R56 is also connected in series with the connection line between the output of the fifth comparator U7A and the non-inverting input of the sixth comparator U7B. The reference voltage pin V... REF The circuit is connected via resistor R54 to the connection between the output of the fifth comparator U7A and resistor R56. One end of capacitor C5 is also connected to this connection point, and the other end is grounded. The anode of diode CR16 is connected to the connection between the non-inverting input of the sixth comparator U7B and resistor R56. The cathode of diode CR16 is connected to the other end of resistor R20. The power supply VCC is connected via resistor R57 to the connection between the output of the sixth comparator U7B and the inverting input of the seventh comparator U7C. The anode of diode CR17 is connected to resistor R57, and the cathode of diode CR17 is connected to the anode of diode CR16. Reference voltage pin V REF Connected to the output and turn-off pin of the seventh comparator U7C via resistors R60 and R61 and diode CR18. The connection point on the lines connecting resistors R60 and R61 is connected to the connection point on the line connecting the non-inverting input of the fourth comparator U7D and capacitor C27. The cathode of diode CR16 is also connected to resistor R22 to achieve connection to the reference voltage pin V via resistor R22. REF .

[0105] At the instant the switching power supply is powered on (0+ moment), the reset pin of the main control module U4... The main control module U4 is in reset state as it is not yet set to high level. The non-inverting input (pin 5) of the sixth comparator U7B is clamped to low level (the non-inverting input (pin 5) of the sixth comparator U7B is connected to the reset pin through clamping diode CR16). Connected, clamped to at time 0+ The low level), is lower than the potential of the inverting input (pin 4) of the sixth comparator U7B (≈V).REF The output of the sixth comparator U7B (pin 2) is low, meaning the inverting input (pin 8) of the seventh comparator U7C is also low, lower than the potential of the non-inverting input (pin 9) of the seventh comparator U7C. The output of the seventh comparator U7C (pin 14) is high, meaning the main control module U4... The pin (which can interrupt PWM output when enabled) is ready to change from low level to high level at the moment of startup (0+ moment), and the output pin OUTPUT of the main control module U4 enables PWM pulse output.

[0106] like Figure 6 As shown, resistors R20 and R21 are connected in series at the reset pin. Between the fifth comparator U7A and ground, the non-inverting input terminal U7A7 of the fifth comparator U7A is connected to the connection line of the series resistors R20 and R21, the inverting input terminal (pin 6) of the fifth comparator U7A is connected to the output terminal (pin 13) of the fourth comparator U7D, and the output terminal (pin 1) of the fifth comparator U7A is connected to the non-inverting input terminal (pin 5) of the sixth comparator U7B.

[0107] The non-inverting input (pin 11) of the fourth comparator U7D is connected to the sampling voltage buffer circuit, and the output (pin 13) of the fourth comparator U7D is connected to the inverting input (pin 6) of the fifth comparator U7A.

[0108] The sampling voltage buffer circuit includes a sampling voltage buffer capacitor C27 and a sampling voltage buffer resistor R60. The sampling voltage buffer capacitor C27 is connected in series between the non-inverting input terminal (pin 11) of the fourth comparator U7D and ground, and the sampling voltage buffer resistor R60 is connected to the reference voltage pin.

[0109] The reset pin of the main control module U4 at time T1 When the voltage level changes to high, the non-inverting input (pin 7) of the fifth comparator U7A is set to high, higher than the potential of the inverting input (pin 6) of the fifth comparator U7A. Therefore, the output (pin 1) of the fifth comparator U7A outputs a high level. At this time, the reference voltage V... REF The fifth capacitor C5 will be briefly charged with a second charging time constant, τ2 = R54 × C5. The second charging time constant τ2 can be adjusted by setting the values ​​of resistor R54 and / or the fifth capacitor C5.

[0110] After the switching power supply is powered on (at time T1), the reference voltage V REFThe sampling voltage buffer capacitor C27 is charged with a third charging time constant, τ3 = R60 × C27. The third charging time constant τ3 is adjustable by setting the values ​​of resistor R60 and capacitor C27. When the potential of the non-inverting input terminal (pin 11) of the fourth comparator U7D rises to a level greater than that of the inverting input terminal (pin 10) of the fourth comparator U7D, the output terminal (pin 13) of the fourth comparator U7D flips from a low level to a high level, and the voltage on the COMP pin continues to rise.

[0111] Soft-start capacitor C3 (in some cases referred to as C) SOFTSTART During the charging process to the stable stage (charging time constant is τ1) T1+τ1, the voltage V of the soft-start capacitor C3 is... CSS The voltage will rise from ground potential to a set value, known as the soft-start ramp voltage; the output of the error amplifier of the main control module U4 is connected to pin 3 COMP of the main control module U4, therefore the output of the error amplifier of the main control module U4 is always clamped to V. CSS +V BE Therefore, the potential of pin 3 COMP (output terminal) of the main control module U4 is also clamped to V. CSS +V BE As the COMP terminal potential rises, the potential at pin 6 of the inverting input of the fifth comparator U7A slowly rises from 0. When the potential at pin 6 rises above the potential at pin 7, the fifth comparator U7A flips, changing from an initial high level to a low level output. At this time, the potential at the non-inverting input (pin 11) of the fourth comparator U7D is higher than the potential at the inverting input (pin 10), and the fourth comparator U7D maintains a high level output. Meanwhile, the potential at pin 6 of the inverting input of the fifth comparator U7A is higher than the potential at pin 7 of the non-inverting input, and the fifth comparator U7A output remains low. The normal startup logic is established, and the switching power supply enters normal operating mode.

[0112] Example 4

[0113] Based on the above embodiments, this embodiment provides a soft-start method for a switching power supply, the method comprising:

[0114] The input voltage of the switching power supply is sampled to determine whether the switching power supply is powered on.

[0115] A soft-start signal is output in response to the switching power supply being powered on and the voltage of the power supply being greater than a preset voltage threshold.

[0116] The main control module U4 performs a soft start on the switching power supply based on the soft start signal.

[0117] In some embodiments, in response to the switching power supply not being powered on or the switching power supply being powered on and the voltage of the power supply not exceeding a preset voltage threshold, the reset function of the main control module U4 is activated to discharge the soft-start capacitor.

[0118] In some embodiments, the shutdown pin of the main control module U4 is disabled when the switching power supply is powered on.

[0119] The startup logic of this disclosure is as follows:

[0120] 1) Upon power-up of the main control module U4, the soft-start circuit in the above embodiment begins to shut down the pin. Raising prices includes:

[0121] At time 0+, via the reset pin The soft-start circuit of the above embodiment will turn off the pin. Raise;

[0122] At time T1, the soft-start circuit described in the above embodiment will turn off the pin. Raise;

[0123] 2) Soft start only begins after both the main control module U4 and the DC+ input of the switching power supply are powered on, including:

[0124] At 0+, the U4's built-in undervoltage lockout circuit is activated and reset, and the soft-start capacitor is discharged.

[0125] At time T1, if the DC+ input terminal of the switching power supply is not powered on, the soft-start circuit in the above embodiment will reset the pin. Pull low; if the DC+ input of the switching power supply is already powered on, the soft-start circuit in the above embodiment will reset the pin. Pull it up and charge the soft-start capacitor.

[0126] like Figure 7 As shown, this disclosure provides a soft-start circuit for a switching power supply based on a pulse width modulator and a four-channel differential comparator. The pulse width modulator is the main control module U4 of the soft-start circuit. Based on the four-channel differential comparator, two sets of interconnected first and second comparison modules are designed, enabling the soft-start of the switching power supply to only begin when the DC+ input terminal is powered on and exceeds a designed preset voltage threshold; and enabling the shutdown pin... The voltage starts to rise during undervoltage lockout, even if the pin needs to be turned off. The pin goes high before the soft-start process begins, thus ensuring that the soft-start process is not interrupted by the shutdown pin. This interruption enabled the implementation of a soft-start circuit for a switching power supply.

[0127] Based on the above, the soft-start method for switching power supplies provided in this embodiment can at least bring the following technical effects:

[0128] 1. The soft-start circuit disclosed herein can effectively protect the power transistors and rectifier diodes of the switching power supply from the surge current impact when the power is turned on, ensuring the safe operation of the switching power supply and improving the service life of electronic components.

[0129] 2. The control principle of the soft-start circuit disclosed herein is based on the IC chip combination design of differential circuit comparator and pulse control modulator, which reduces the scale and complexity of circuit design. It has the advantages of high circuit design integration, high hardware resource utilization, and strong logic combination function matching, which is conducive to the miniaturization, lightweighting and low cost design of switching power supplies.

[0130] 3. By precisely setting the values ​​of the soft-start capacitor and charging resistor, and optimizing the design of the RC filter time constant, the normal startup sequence of the switching power supply is ensured, which helps to improve the anti-interference capability of the switching power supply during startup (preventing EMI from causing malfunctions of the switching power supply).

[0131] 4. The soft-start circuit disclosed herein is constructed from analog circuits and has the advantages of simplified principle structure (fewer external components), clear logic function (designed using a four-channel differential comparator integrated circuit), adjustable parameter configuration (sampling resistor, comparison reference voltage, time constant, etc.), and low cost.

[0132] 5. The electronic components disclosed herein are selected from discrete device packages (MOSFETs, resistors, capacitors, etc. selected from THT package type). The electronic components have high power dissipation, which is conducive to improving heat dissipation performance under natural cooling conditions, adapting to high temperature environment (above +75℃), and improving product reliability and service life.

[0133] It should be noted that, in this disclosure, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element limited by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0134] While the embodiments disclosed herein are as described above, the foregoing content is merely for the purpose of facilitating understanding of this disclosure and is not intended to limit this disclosure. Any person skilled in the art to which this disclosure pertains may make any modifications and changes in form and detail of the implementation without departing from the spirit and scope of this disclosure; however, the scope of patent protection of this disclosure shall still be determined by the scope defined in the appended claims.

Claims

1. A soft start circuit for a switching power supply, characterized by include: The first comparison module is connected to the power supply and is used to sample the input voltage of the switching power supply to determine whether the switching power supply is in a powered-on state. When the switching power supply is in a powered-on state and the voltage of the power supply is greater than a preset voltage threshold, a soft-start signal is output. The main control module is connected to the output terminal of the first comparison module and is used to perform soft start on the switching power supply according to the soft start signal; The first comparison module includes: The first comparator has its non-inverting input connected to the power supply. The second comparator has its non-inverting input connected to the output of the first comparator, its inverting input connected to the reference voltage pin of the main control module, and its output connected to the reset pin of the main control module. The second comparator is used to: output a low-level signal to activate the reset function of the main control module and discharge the soft-start capacitor when the switching power supply is not powered on or the voltage of the power supply is not greater than a preset voltage threshold; and output a high-level signal to release the reset of the main control module when the switching power supply is powered on and the voltage of the power supply is greater than the preset voltage threshold, allowing the main control module to charge the soft-start capacitor based on the reference voltage, thereby achieving a soft start for the switching power supply. The third comparator has its inverting input connected to the inverting input of the first comparator, its non-inverting input connected to the power supply, and its output connected to the soft-start pin and the reference voltage pin of the main control module. The soft-start capacitor has one end connected to the connection line between the output of the third comparator and the soft-start pin of the main control module, and the other end grounded. The first comparison module further includes: a first resistor, a second resistor, and a third resistor; The first resistor and the second resistor are connected in series between the input terminal of the switching power supply and ground to sample the input voltage of the switching power supply. One end of the third resistor is connected to the connection line between the first resistor and the second resistor, and the other end of the third resistor is connected to the connection line between the output terminal of the first comparator and the non-inverting input terminal of the second comparator.

2. The circuit of claim 1, wherein, Also includes: The second comparison module is connected to the shutdown pin of the main control module and is used to output a shutdown pin control signal when the switching power supply is in the power-on state, so as to disable the shutdown pin. The main control module is also used to control the main control module not to be turned off when powered on, according to the control signal of the shutdown pin.

3. The circuit of claim 2, wherein, The second comparison module includes: The fourth comparator has its inverting input connected to the reference voltage pin of the main control module, and its non-inverting input grounded. The fifth comparator has its inverting input connected to the output of the fourth comparator. The sixth comparator has its non-inverting input connected to the output of the fifth comparator and its inverting input connected to the inverting input of the fourth comparator. The seventh comparator has its inverting input connected to the output of the sixth comparator, its non-inverting input connected to the reference voltage pin of the main control module, and its output connected to the shutdown pin of the main control module. The reference voltage pin of the main control module is also connected to the non-inverting input of the fourth comparator, the connection line between the output of the fifth comparator and the non-inverting input of the sixth comparator, the inverting input of the sixth comparator, and the connection line between the output of the seventh comparator and the shutdown pin of the main control module. The power supply is connected to the connection line between the output of the sixth comparator and the inverting input of the seventh comparator.

4. A soft start method of a switching power supply, characterized by, The method is based on the soft-start circuit of the switching power supply according to any one of claims 1 to 3, and includes: The input voltage of the switching power supply is sampled to determine whether the switching power supply is in a powered-on state. A soft-start signal is output when the switching power supply is powered on and the voltage of the power supply is greater than a preset voltage threshold. The main control module performs a soft start on the switching power supply based on the soft start signal.

5. The soft-start method for a switching power supply according to claim 4, characterized in that, Also includes: In response to the switching power supply not being powered on or the switching power supply being powered on and the voltage of the power supply not exceeding a preset voltage threshold, the reset function of the main control module is activated to discharge the soft-start capacitor.

6. The soft-start method for a switching power supply according to claim 4, characterized in that, Also includes: The shutdown pin of the main control module is disabled when the switching power supply is powered on.

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