A soft start circuit and method for switching power supply

Abstract

The present invention relates to a soft-start circuit and method for a switching power supply, wherein the circuit includes a voltage stabilizing device, a photocoupler, a power supply terminal, and output voltage sampling resistors R1, R2, and R3. The resistors R1, R2, and R3 are connected in series in sequence. One end of the photocoupler is connected to the power supply terminal, and the other end is connected between the resistors R2 and R3. The photocoupler is also connected to one end of the voltage stabilizing device, and the other end of the voltage stabilizing device is connected between the resistors R2 and R3. The circuit also includes a soft-start capacitor C1 connected in parallel across the resistor R1. During the rising phase of the output voltage Vout at power-on startup, a small current is passed to charge the soft-start capacitor C1, thereby enabling the voltage stabilizing device to conduct in advance and pre-controlling the duty cycle of the PWM controller before the output voltage reaches a set value. Compared with the prior art, the present invention has the advantages of avoiding excessive startup surges and eliminating output voltage overshoot during startup.

Description

Technical Field

[0001] The present invention relates to a switching power supply, and in particular to a soft starting circuit and method for the switching power supply. Background Art

[0002] Commonly used switching power supplies include voltage control mode and dual-loop control mode with current control mode added. However, regardless of the control mode, when the power is turned on, the feedback circuit has not yet formed a feedback path when the output voltage is low. The PWM controller will charge the output capacitor with a large duty cycle. When the output voltage reaches the set value, the feedback circuit will intervene to provide feedback. Due to the lag in the feedback circuit's response, after the output voltage reaches the set value, the duty cycle of the power tube exceeds the duty cycle required for the power supply to reach steady state, which will cause the startup output voltage to overshoot and exceed the set value.

[0003] The working principle of the switching power supply is: resistors R1, R2 and R3 are output voltage sampling resistors, and the output voltage divided value Vout×R3 / (R1+R2+R3) is used as the input signal of the three-terminal voltage regulator IC1 and compared with the internal reference voltage of 2.5V of the three-terminal voltage regulator IC1: When the divided voltage value is lower than 2.5V, IC1 is cut off, the optocoupler does not draw current from the FB terminal, and the duty cycle of the GATE output is the largest; when the divided voltage value is greater than 2.5V, IC1 starts to turn on: Due to the integral characteristic of the feedback circuit, the divided voltage value is high and increases with time. The more current the optocoupler draws from the FB terminal, the lower the FB terminal voltage Vfb, the smaller the duty cycle of the GATE output, and the gradually decreasing output voltage Vout; when the divided voltage value is low and increases with time, the smaller the current the optocoupler draws from the FB terminal, the higher the FB terminal voltage Vfb, the larger the duty cycle of the GATE output, and the gradually increasing output voltage Vout. In this way, through the continuous adjustment of the feedback loop formed by the optocoupler and the PWM controller, the output voltage division value is finally made equal to the internal reference value of IC1. Correspondingly, Vout = IC1 internal reference value / R3 × (R1 + R2 + R3) to achieve output voltage regulation.

[0004] Although the PWM controller reduces startup inrush current by gradually increasing the CS threshold from zero volts and the duty cycle from zero during power-up, during the rising phase of the output voltage Vout, before the three-terminal voltage regulator IC1 turns on, there is no current flowing through the optocoupler, and the feedback loop is disconnected. Because the optocoupler does not pass current, the voltage at the PWM controller's FB terminal reaches its maximum value, significantly overshooting the final steady-state value. When the output voltage Vout rises and reaches very close to its final steady-state value, IC1 turns on the optocoupler, allowing current to flow, causing the FB terminal voltage to begin to decrease. Because the FB terminal voltage needs to decrease from its maximum value to its steady-state value, the voltage swing is large, resulting in a delay in the voltage discharge at the FB terminal. This delay is calculated as Td = (Cci.ΔVfb) / Ici, where ΔVfb is the FB terminal voltage swing and Ici is the current drawn from the compensation capacitor Ca. It is precisely because of the existence of this delay time that the duty cycle cannot be reduced quickly and in time, resulting in output voltage overshoot during startup, which is particularly serious under no-load conditions. Summary of the Invention

[0005] The purpose of the present invention is to provide a soft start circuit and method for a switching power supply in order to overcome the above-mentioned defects in the prior art.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A soft start circuit for a switching power supply includes a voltage stabilizing device, a photoelectric coupler, a power supply terminal, and output voltage sampling resistors R1, R2, and R3. The resistors R1, R2, and R3 are connected in series in sequence. One end of the photoelectric coupler is connected to the power supply terminal, and the other end is connected between the resistors R2 and R3. The other end of the photoelectric coupler is connected to one end of the voltage stabilizing device, and the other end of the voltage stabilizer is connected between the resistors R2 and R3. The circuit also includes a soft start capacitor C1 connected in parallel across the resistor R1.

[0008] During the rising phase of the output voltage Vout at power-on startup, a small current is used to charge the soft-start capacitor C1, thereby enabling the voltage regulator to be turned on in advance and pre-controlling the duty cycle of the PWM controller before the output voltage reaches the set value.

[0009] Preferably, the circuit further comprises a voltage stabilizing diode ZD1 , wherein the positive electrode of the voltage stabilizing diode ZD1 is connected between the resistor R1 and the output voltage Vout, and the negative electrode of the voltage stabilizing diode ZD1 is connected to the soft-start capacitor C1 .

[0010] Preferably, the voltage stabilizing device is a three-terminal voltage stabilizing device IC1 , a control electrode of which is connected between resistors R2 and R3 , and a cathode of which is connected to a photocoupler.

[0011] Preferably, the voltage stabilizing device is an operational amplifier, whose output terminal is connected to the electric coupler, the positive input terminal is connected to the power supply terminal and the ground terminal respectively, and the negative input terminal is connected between the resistors R2 and R3.

[0012] Preferably, the positive input terminal of the operational amplifier is connected to the power supply terminal through a resistor R7, and the negative input terminal of the operational amplifier is connected to the ground terminal through a resistor R8.

[0013] Preferably, a resistor R6 is connected in parallel at both ends of the photoelectric coupler.

[0014] Preferably, one end of the photocoupler is connected to the power supply end through the resistor R5, and the other end is connected between the resistors R2 and R3 through the capacitor C2 and the resistor R4 in sequence, wherein the power supply end is a +5V power supply end.

[0015] A soft start method using the soft start circuit for a switching power supply, the method being used in a PWM controller having a voltage feedback input terminal, the method comprising:

[0016] During the output voltage rising stage of power-on startup, the output voltage provides a small current to charge the soft-start capacitor C1. At the same time, the voltage across the soft-start capacitor C1 is low, causing the voltage feedback circuit and the optocoupler to conduct, thereby limiting the rise in the voltage at the voltage feedback input terminal of the PWM chip. As the voltage across the soft-start capacitor C1 gradually rises, the voltage at the voltage feedback input terminal of the PWM chip also gradually increases, the duty cycle gradually increases, and the output voltage gradually rises.

[0017] Preferably, after the output voltage of the soft start circuit reaches a steady-state value, the voltage across the soft start capacitor C1 reaches a maximum value. Limited by the series-connected Zener diode ZD1, no charge or discharge current flows through the soft start circuit, and the soft start circuit is equivalent to being disconnected.

[0018] Preferably, the soft start method further includes the soft start circuit being able to promptly participate in the voltage feedback circuit when the load suddenly changes and the power supply output voltage changes drastically, thereby accelerating the response speed of the circuit.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1) In the present invention, during the rising phase of the output voltage Vout at power-on startup, a very small current charges the soft-start capacitor, allowing the three-terminal voltage regulator IC1 to be turned on in advance. This allows the duty cycle of the PWM controller to be controlled in advance before the output voltage reaches the set value, thereby avoiding excessive startup surges and eliminating output voltage overshoot during startup.

[0021] 2) In the present invention, after the output voltage reaches a stable stage, no current flows through the soft start circuit, the soft start capacitor no longer affects the response characteristics of the feedback circuit, and the soft start circuit and the feedback circuit are separated, thereby improving the stability of the power supply.

[0022] 3) When a sudden change occurs in the load, the soft start circuit of the present invention can quickly participate in feedback, thereby providing good load dynamic characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a single-stage PFC overall circuit using a soft start circuit;

[0024] Figure 2 for Figure 1 The circuit diagram of the soft-start capacitor C1 connected in series with the voltage regulator diode ZD1;

[0025] Figure 3 for Figure 1 The three-terminal voltage regulator IC1 is replaced by an operational amplifier. DETAILED DESCRIPTION

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0027] like Figure 1 As shown, the present invention provides a soft start circuit for a switching power supply, including a voltage stabilizing device, a photoelectric coupler, a +5V power supply terminal, and output voltage sampling resistors R1, R2, and R3. The resistors R1, R2, and R3 are connected in series in sequence. One end of the photoelectric coupler is connected to the +5V power supply terminal, and the other end is connected between the resistors R2 and R3, and is also connected to one end of the voltage stabilizing device. The other end of the voltage stabilizer is connected between the resistors R2 and R3. The circuit also includes a soft start capacitor C1 connected in parallel across the resistor R1.

[0028] Figure 1 For a single-stage PFC circuit using a soft-start circuit, the feedback circuit's corner frequency is around 18Hz due to PF and stability requirements, resulting in very slow dynamic response. This can easily lead to output voltage overshoot, so a soft-start capacitor C1 is typically added to the feedback circuit.

[0029] Its function is as follows: When the power is turned on, the Vout voltage rises, and the voltage across capacitor C1 cannot suddenly change and remain zero. At this time, the voltage at the input of IC1 is Vout×R3 / (R2+R3), and the output voltage of the power supply Vout=IC1 reference value / R3×(R2+R3). Since R2+R3 is less than R1+R2+R3, Vout at this time is less than the final set value=IC1 reference value / R3×(R1+R2+R3). That is, before the output voltage reaches the set value, IC1 has already intervened in the feedback in advance, avoiding the overshoot of the output voltage caused by feedback lag. Then capacitor C1 charges, and when the voltage across it gradually increases from 0V to (IC1 reference value / R3×R1), C1 charging is completed and the output voltage reaches the set value.

[0030] Figure 2 To improve Figure 1 In the circuit, a voltage regulator diode ZD1 is connected in series with the soft-start capacitor C1.

[0031] Its function is as follows: ZD1's VF voltage drop is negligible. When power is applied, the Vout voltage rises, and the voltage across capacitor C1 cannot suddenly change to zero. At this time, the voltage at IC1's input is Vout×R3 / (R2+R3), and the power supply output voltage Vout = IC1 reference value / R3×(R2+R3). Since R2+R3 is less than R1+R2+R3, Vout at this time is less than the final set value = IC1 reference value / R3×(R1+R2+R3). In other words, before the output voltage reaches the set value, IC1 has already intervened in the feedback in advance, avoiding the overshoot of the output voltage caused by feedback lag. Then, capacitor C1 charges, and when the voltage across it gradually increases from 0V to (IC1 reference value / R3×R1), C1 charging is completed and the output voltage reaches the set value.

[0032] Since the voltage regulator diode ZD1 is connected in series with the capacitor C1, when the power supply output reaches the set value, a suitable voltage regulator diode is selected so that the Zener breakdown voltage Vz of the voltage regulator diode is greater than the output voltage ripple value. No current flows through ZD1, and the capacitor C1 is not charged or discharged, which is equivalent to being disconnected from the resistor R1, and will not affect the characteristics of the feedback circuit.

[0033] When the load suddenly becomes heavier or lighter, causing the output voltage to change significantly, exceeding the Zener breakdown voltage Vz of ZD1, current will flow through ZD1 and C1. The voltage across C1 cannot change suddenly. The change in output voltage minus Vz will be directly added to R2 without passing through R1. According to the voltage divider of the feedback resistor, the feedback gain directly added to R2 is higher, which can improve the dynamic response speed of the power supply.

[0034] When the input power is turned off, the output voltage drops, and the voltage across R2 and R3 drops, reducing the current flowing through R2 and R3. However, the voltage across R1 is clamped at VC1-Vz, and the current flowing through R1 remains essentially unchanged. The reduced portion of the current flowing from R1 to R2 and R3 is supplied to R1 by capacitor C1, which discharges the current into R1. When power is turned on again, the voltage across capacitor C1 is zero due to the complete discharge of capacitor C1, enabling the soft-start function to be realized again.

[0035] Figure 3 To improve Figure 1 Circuit embodiment 2 soft start circuit; another embodiment using an operational amplifier for feedback.

[0036] Its working principle is equivalent to Figure 2 , replace the three-terminal voltage regulator IC1 with an op amp. The op amp's inverting input serves as the signal input terminal, derived from the divided voltage of the power supply output voltage. The reference voltage is obtained by dividing 5V by R7 and R8 and connected to the op amp's non-inverting input terminal.

[0037] The soft-start method of a switching power supply includes the following steps: during the output voltage rising phase of power-on startup, the power supply provides a very small current to charge the soft-start capacitor, and the three-terminal voltage regulator IC1 is turned on in advance to pre-control the duty cycle of the PWM controller, so that the duty cycle of the PWM controller gradually increases and the voltage at the voltage feedback terminal gradually rises.

[0038] As an improvement to the soft-start method of the present invention, a voltage-stabilizing diode is connected in series with the soft-start capacitor circuit. When the output voltage reaches a steady state, the voltage across the soft-start capacitor reaches its maximum value, and current no longer charges the soft-start capacitor. Furthermore, the soft-start capacitor is limited by the Zener breakdown voltage of the voltage-stabilizing diode and cannot discharge. This effectively decouples the soft-start circuit from the feedback circuit.

[0039] As an improvement to the soft-start method of the present invention, the output voltage drops after the power input is turned off. The voltage on the soft-start capacitor is higher than the sum of the voltage of the voltage-dividing resistor R1 and the Zener voltage of the voltage-stabilizing diode, and then discharges to the voltage-dividing resistor. The voltage across the soft-start capacitor drops, and the soft-start circuit can work normally after power is turned on again.

[0040] As an improvement to the soft-start method of the present invention, when the load suddenly becomes lighter, the voltage across the soft-start circuit cannot change suddenly. The increase in the output voltage is directly applied to R2, bypassing R1. When the load suddenly becomes heavier, the difference between the output voltage drop and the Zener voltage of the voltage regulator is also applied to R2. Based on the voltage divider of the feedback resistors, it can be seen that the feedback directly applied to R2 has a higher gain, which can improve the dynamic response speed of the power supply.

[0041] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A soft start method using a soft start circuit for a switching power supply, the soft start circuit comprising a voltage stabilizer, a photocoupler, a power supply terminal, and output voltage sampling resistors R1, R2, and R3, wherein the resistors R1, R2, and R3 are sequentially connected in series, one end of the photocoupler is connected to the power supply terminal, and the other end is connected between the resistors R2 and R3, and is connected to one end of the voltage stabilizer, and the other end of the voltage stabilizer is connected between the resistors R2 and R3, characterized in that: The soft start circuit further includes a soft start capacitor C1 connected in parallel across the resistor R1; During the rising phase of the output voltage Vout at power-on startup, a small current is used to charge the soft-start capacitor C1, thereby enabling the voltage regulator to be turned on in advance and pre-controlling the duty cycle of the PWM controller before the output voltage reaches the set value. The method is used for a PWM controller having a voltage feedback input terminal, and the method comprises: During the output voltage rising phase when the machine is powered on, the output voltage provides a small current to charge the soft-start capacitor C1. At the same time, the voltage across the soft-start capacitor C1 is low, causing the voltage feedback circuit and the optocoupler to conduct, thereby limiting the rise in the voltage at the voltage feedback input of the PWM chip. As the voltage across the soft-start capacitor C1 gradually rises, the voltage at the voltage feedback input of the PWM chip also gradually increases, the duty cycle gradually increases, and the output voltage gradually rises. After the output voltage of the soft start circuit reaches a steady-state value, the voltage across the soft start capacitor C1 reaches a maximum value. Limited by the series-connected Zener diode ZD1, no charge or discharge current flows through the soft start circuit, and the soft start circuit is equivalent to being disconnected. The soft start method further includes the following steps: when the load suddenly changes and the power supply output voltage changes dramatically, the soft start circuit can promptly participate in the voltage feedback circuit to speed up the circuit response; The soft start circuit further includes a voltage stabilizing diode ZD1 , wherein the positive electrode of the voltage stabilizing diode ZD1 is connected between the resistor R1 and the output voltage Vout, and the negative electrode of the voltage stabilizing diode ZD1 is connected to the soft start capacitor C1 .

2. The soft start method according to claim 1, characterized in that: The voltage stabilizing device is a three-terminal voltage stabilizing device IC1, the control electrode of which is connected between the resistors R2 and R3, and the negative electrode is connected to the photoelectric coupler.

3. The soft start method according to claim 1, wherein: The voltage stabilizing device is an operational amplifier, whose output end is connected to the electric coupler, the positive input end is connected to the power supply end and the ground end respectively, and the negative input end is connected between the resistors R2 and R3.

4. The soft start method according to claim 3, characterized in that: The positive input terminal of the operational amplifier is connected to the power supply terminal through the resistor R7, and the negative input terminal of the operational amplifier is connected to the ground terminal through the resistor R8.

5. The soft start method according to claim 1, wherein: A resistor R6 is connected in parallel to both ends of the photoelectric coupler.

6. The soft start method according to claim 1, characterized in that: One end of the photoelectric coupler is connected to the power supply terminal through the resistor R5, and the other end is connected between the resistors R2 and R3 through the capacitor C2 and the resistor R4 in sequence, wherein the power supply terminal is a +5V power supply terminal.

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