CN212518788U

CN212518788U - Soft start circuit for switching power supply

Info

Publication number: CN212518788U
Application number: CN202021227960.9U
Authority: CN
Inventors: 杨志民; 杨佳斌
Original assignee: Mingzhi Electrical Appliances Taicang Co ltd
Current assignee: Mingzhi Electrical Appliances Taicang Co ltd
Priority date: 2020-06-29
Filing date: 2020-06-29
Publication date: 2021-02-09
Anticipated expiration: 2030-06-29

Abstract

The utility model relates to a soft start circuit for switching power supply, including steady voltage device, optoelectronic coupler, power end, output voltage sampling resistance R1, R2, R3, resistance R1, R2, R3 establish ties in proper order, optoelectronic coupler one end be connected with the power end, another termination is between resistance R2 and R3, is connected with the one end of steady voltage device simultaneously, the stabiliser other termination between resistance R2 and R3, the circuit still including parallelly connected soft start electric capacity C1 at resistance R1 both ends. Compared with the prior art, the utility model has the advantages of can enough avoid the start surge to strike too big, output voltage when can eliminate the start again overshoots.

Description

Soft start circuit for switching power supply

Technical Field

The utility model relates to a switching power supply especially relates to a soft start circuit for switching power supply.

Background

In a common switching power supply, a voltage control mode and a double-loop control mode added with a current control mode exist, however, no matter what mode is controlled, when the power supply is started, because a feedback circuit does not form a feedback path when the output voltage is low, a PWM controller can charge an output capacitor with a large duty ratio, when the output voltage reaches a set value, the feedback circuit is involved in feedback, and because the response of the feedback circuit is delayed, after the output voltage reaches the set value, the duty ratio of a power tube also exceeds the duty ratio required after the power supply reaches a steady state, the start-up output voltage is overshot and exceeds the set value.

The operating principle of the switching power supply is that the resistors R1, R2 and R3 are output voltage sampling resistors, and the divided voltage value Vout of the output voltage is multiplied by R3/(R1+ R2+ R3) which is used as an input signal of the three-terminal regulator IC1 and is compared with the internal reference voltage 2.5V of the three-terminal regulator IC 1: when the voltage division value is lower than 2.5V, the IC1 is cut off, the optical coupler does not draw current from the FB end, and the duty ratio of GATE output is maximum; IC1 begins to conduct when the divided voltage value is greater than 2.5V: due to the integral characteristic of the feedback circuit, the divided voltage value is higher and increases along with time, the more current the optocoupler draws from the FB end, the voltage Vfb at the FB end is reduced, the duty ratio of GATE output is reduced, and the output voltage Vout is gradually reduced; when the voltage division value is low and increases along with time, the current extracted by the optocoupler from the FB end is smaller, so that the voltage Vfb at the FB end is increased, the duty ratio of GATE output is increased, and the output voltage Vout is gradually increased. In this way, by continuously adjusting a feedback loop formed by the optocoupler and the PWM controller, the divided voltage value of the output voltage is finally equal to the internal reference value of the IC1, and Vout is equal to the internal reference value/R3 × (R1+ R2+ R3) of the IC1, thereby stabilizing the output voltage.

Although in the power-on startup process, the PWM controller controls the threshold value of the CS port to gradually rise from zero volts, and controls the duty ratio to gradually increase from zero, so that the surge current of startup is reduced. However, in the rising stage of the output voltage Vout, before the three-terminal regulator IC1 is turned off, the optical coupler has no current, the feedback loop is turned off, and the voltage at the FB terminal of the PWM controller reaches the maximum value due to the fact that the optical coupler does not pass the current, and therefore, the PWM controller has a very large overshoot with respect to the final steady-state value. When the output voltage Vout rises to reach a value very close to the final steady state, the IC1 starts to turn on the optical coupler to pass current, so that the voltage at the FB terminal starts to drop. Since the FB terminal voltage needs to be decreased from the maximum value to the steady-state value of the FB terminal, and the voltage swing is large, there is a delay in the voltage leakage of the FB terminal, where the delay time Td is (Cci × Δ Vfb)/Ici, where Δ Vfb is the swing of the FB terminal voltage, and Ici is the current drawn from the compensation capacitor Ca. It is because of this delay time that the duty cycle cannot be reduced rapidly in time, which leads to overshoot of the output voltage at start-up, especially during idle conditions.

SUMMERY OF THE UTILITY MODEL

The present invention is directed to a soft start circuit for a switching power supply, which overcomes the above-mentioned drawbacks of the prior art.

The purpose of the utility model can be realized through the following technical scheme:

a soft start circuit for a switching power supply comprises a voltage stabilizing device, a photoelectric coupler, a power supply end, an output voltage sampling resistor R1, R2 and R3, wherein the resistors R1, R2 and R3 are sequentially connected in series, one end of the photoelectric coupler is connected with the power supply end, the other end of the photoelectric coupler is connected between a resistor R2 and a resistor R3 and is simultaneously connected with one end of the voltage stabilizing device, the other end of the voltage stabilizer is connected between a resistor R2 and a resistor R3, and the circuit further comprises a soft start capacitor C1 connected in parallel with two ends of the resistor R1.

Preferably, the circuit further comprises a zener diode ZD1, wherein the anode of ZD1 is connected between resistor R1 and output voltage Vout, and the cathode is connected with soft-start capacitor C1.

Preferably, the voltage stabilizer is a three-terminal voltage stabilizer IC1, the control electrode of which is connected between the resistors R2 and R3, and the negative electrode of which is connected with the photocoupler.

Preferably, the voltage regulator is an operational amplifier, the output terminal of the operational amplifier is connected to the electric coupler, the positive input terminal of the operational amplifier is connected to the power supply terminal and the ground terminal, and the negative input terminal of the operational amplifier is connected between the resistors R2 and R3.

Preferably, the positive input terminal of the operational amplifier is connected to the power supply terminal through a resistor R7.

Preferably, the negative input terminal of the operational amplifier is connected to the ground terminal through a resistor R8.

Preferably, a resistor R6 is connected in parallel with two ends of the photoelectric coupler.

Preferably, one end of the photoelectric coupler is connected with a power supply end through a resistor R5.

Preferably, the other end of the photoelectric coupler is connected between the resistors R2 and R3 after passing through the capacitor C2 and the resistor R4 in sequence.

Preferably, the power supply end is a +5V power supply end.

Compared with the prior art, the utility model has the advantages of it is following:

1) the utility model discloses output voltage Vout when last start-up rises the stage, has a very little electric current to charge for soft start electric capacity, and three-terminal voltage regulator device IC1 can switch on in advance can the duty cycle size of pre-control PWM controller before output voltage reaches the setting value to can enough avoid the start surge to strike too big, can eliminate the output voltage when starting again and overshoot.

2) The utility model discloses after output voltage reached the stationary phase, there was not the electric current soft start circuit of flowing through, soft start electric capacity no longer influences feedback circuit's response characteristic, and soft start circuit and feedback circuit realize breaking away from, have improved the stability of power.

3) The utility model discloses when the load takes place the sudden change, soft start circuit can participate in the feedback rapidly to provide good load dynamic characteristic.

Drawings

FIG. 1 is a single stage PFC integrated circuit employing a soft start circuit;

fig. 2 is a circuit diagram of a zener diode ZD1 connected in series with the soft-start capacitor C1 of fig. 1;

fig. 3 is a circuit diagram of the three-terminal regulator IC1 of fig. 1 replaced by an operational amplifier.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, not all, of the embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by a person skilled in the art without creative efforts shall fall within the protection scope of the present invention.

As shown in fig. 1, the utility model relates to a soft start circuit for switching power supply, including voltage regulator device, optoelectronic coupler, +5V power end, output voltage sampling resistance R1, R2, R3, resistance R1, R2, R3 establish ties in proper order, optoelectronic coupler one end and +5V power end be connected, the other end connects between resistance R2 and R3, simultaneously with the one end of voltage regulator device, the voltage regulator other end connect between resistance R2 and R3, the circuit still include the soft start electric capacity C1 of connecting in parallel at resistance R1 both ends.

Fig. 1 shows a single-stage PFC integrated circuit using a soft start circuit, where the turning frequency of the single-stage PFC feedback circuit is about 18Hz and the dynamic response speed is very slow based on the PF value and the stability requirement. The output voltage is easy to overshoot, and a soft start capacitor C1 is generally added to the feedback circuit.

The function is as follows: when the power is on, the voltage of Vout rises, the voltage across the capacitor C1 cannot be abruptly kept to zero, at this time, the voltage at the input end of the IC1 is Vout × R3/(R2+ R3), the power output voltage Vout is equal to IC1 reference value/R3 × (R2+ R3), and since R2+ R3 is smaller than R1+ R2+ R3, Vout at this time is smaller than the final set value, i.e., IC1 reference value/R3 × (R1+ R2+ R3), that is, when the output voltage does not reach the set value, the IC1 has intervened feedback in advance, so that overshoot of the output voltage due to feedback lag is avoided. When the voltage across the capacitor C1 gradually increases from 0V to (IC1 reference value/R3 × R1) after charging, C1 ends charging, and the output voltage reaches the set value.

Fig. 2 is an improved circuit of fig. 1, in which a zener diode ZD1 is connected in series with the soft-start capacitor C1.

The function is as follows: VF voltage drop of ZD1 is ignored, voltage of Vout rises during power-up, voltage across capacitor C1 cannot be suddenly kept to zero, voltage at the input end of IC1 is Vout × R3/(R2+ R3), power output voltage Vout is IC1 reference value/R3 × (R2+ R3), and since R2+ R3 is smaller than R1+ R2+ R3, Vout at this time is smaller than final set value IC1 reference value/R3 × (R1+ R2+ R3), that is, when output voltage does not reach set value yet, IC1 has intervened in feedback in advance, and overshoot of output voltage due to feedback lag is avoided. When the voltage across the capacitor C1 gradually increases from 0V to (IC1 reference value/R3 × R1) after charging, C1 ends charging, and the output voltage reaches the set value.

Because the capacitor C1 is connected with the Zener diode ZD1 in series, when the power output reaches the set value, the proper Zener diode is selected, the Zener breakdown voltage Vz of the Zener diode is larger than the ripple value of the output voltage, ZD1 has no current flowing, the capacitor C1 has no charge-discharge equivalent to the disconnection of the resistor R1, and the characteristic of the feedback circuit can not be influenced.

When the load suddenly becomes heavy or light, so that the output voltage changes greatly, and exceeds the Zener breakdown voltage Vz of ZD1, the current flows through ZD1 and C1, the voltage at the two ends of C1 cannot suddenly change, the change value of the output voltage is deducted from Vz and then is directly added to R2 without passing through R1, according to the division of the feedback resistor, the feedback gain directly added to R2 is higher, and the dynamic response speed of the power supply can be improved.

When the output voltage drops after the input is powered off, the voltages at two ends of R2 and R3 drop, the currents flowing through R2 and R3 are reduced, the voltage at two ends of R1 is clamped on VC1-Vz, the current flowing through R1 is basically unchanged, the reduced current flowing to R2 and R3 from R1 is supplied to R1 by capacitor C1, and C1 discharges to R1. After power is supplied again, because the capacitor C1 finishes discharging, the voltage at the two ends is zero, and the soft start function can be realized again.

FIG. 3 is a soft start circuit of embodiment 2 modified from the circuit of FIG. 1; another embodiment using an op-amp for feedback.

The working principle of the three-terminal voltage regulator is equal to that of the three-terminal voltage regulator IC1 shown in figure 2, and the three-terminal voltage regulator IC1 is replaced by an operational amplifier. The input end of the inverse input end of the operational amplifier used as a signal is derived from the voltage division of the output voltage of a power supply, and the reference voltage is obtained by dividing 5V by R7 and R8 and is connected to the non-inverting input end of the operational amplifier.

The working process of the soft start device of the switching power supply comprises the following steps that in the output voltage rising stage of the power-on startup, the power supply provides a small current to charge the soft start capacitor, the three-terminal voltage stabilizing device IC1 is conducted in advance to control the duty ratio of the PWM controller in advance, so that the duty ratio of the PWM controller is gradually increased, and the voltage of the voltage feedback end is gradually increased.

As the utility model discloses soft starting drive's improvement has established ties zener diode in above-mentioned soft electric capacity return circuit that opens. When the output voltage reaches a steady state, the voltage at the two ends of the soft start capacitor reaches the maximum value, and the current does not charge the soft start capacitor any more; the soft start capacitor is also limited by the zener breakdown voltage of the zener diode and can not discharge, and the soft start circuit is equivalent to a circuit which is separated from the feedback circuit.

As the utility model discloses soft starting drive's improvement, output voltage descends after the power input outage, and the voltage on the soft start electric capacity is higher than divider resistance R1 voltage + stabilivolt zener voltage after and to discharging to divider resistance, soft start electric capacity both ends voltage decline, the soft start circuit also can normally work after going up the electricity once more.

As the improvement of the soft start device of the utility model, when the load is lightened suddenly, the voltage at the two ends of the soft start circuit can not be suddenly changed, and the increased value of the output voltage can be directly skipped over R1 and added to R2; when the load suddenly becomes heavy, the difference value of the output voltage drop value minus the zener voltage value of the regulator tube is also added to R2. According to the voltage division of the feedback resistor, the feedback gain directly added to the R2 is higher, and the dynamic response speed of the power supply can be improved.

The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily think of various equivalent modifications or replacements within the technical scope of the present invention, and these modifications or replacements should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A soft start circuit for a switching power supply comprises a voltage stabilizing device, a photoelectric coupler, a power supply end, an output voltage sampling resistor R1, R2 and R3, wherein the resistors R1, R2 and R3 are sequentially connected in series, one end of the photoelectric coupler is connected with the power supply end, the other end of the photoelectric coupler is connected between a resistor R2 and a resistor R3 and is simultaneously connected with one end of the voltage stabilizing device, the other end of the voltage stabilizer is connected between the resistor R2 and the resistor R3, and the soft start circuit is characterized by further comprising a soft start capacitor C1 connected in parallel with two ends of the resistor R1.

2. The soft-start circuit for the switching power supply as claimed in claim 1, further comprising a zener diode ZD1, wherein the anode of ZD1 is connected between the resistor R1 and the output voltage Vout, and the cathode is connected to the soft-start capacitor C1.

3. The soft-start circuit of claim 1, wherein the regulator is a three-terminal regulator IC1, the control terminal of which is connected between the resistors R2 and R3, and the negative terminal of which is connected to the photocoupler.

4. A soft-start circuit for a switching power supply as claimed in claim 1, wherein the voltage-stabilizing device is an operational amplifier having an output terminal connected to the electric coupler, positive input terminals connected to the power supply terminal and the ground terminal, respectively, and negative input terminals connected between the resistors R2 and R3.

5. A soft-start circuit for a switching power supply as claimed in claim 4, wherein the positive input terminal of the operational amplifier is connected to the power supply terminal through a resistor R7.

6. The soft-start circuit of claim 4, wherein the negative input terminal of the operational amplifier is connected to ground via a resistor R8.

7. The soft-start circuit for a switching power supply as claimed in claim 1, wherein a resistor R6 is connected in parallel across the photocoupler.

8. The soft-start circuit for a switching power supply as claimed in claim 1, wherein one end of said photo-coupler is connected to a power supply terminal through a resistor R5.

9. The soft-start circuit for switching power supply of claim 8, wherein the other end of said photo-coupler is connected between the resistors R2 and R3 after passing through the capacitor C2 and the resistor R4 in sequence.

10. A soft-start circuit for a switching power supply as claimed in claim 1, wherein said power supply terminal is a +5V power supply terminal.