CN100525040C - Sample-taking feedback circuit of switch electric power output electric voltage - Google Patents

Abstract

The invention proposes a sampling feedback circuit for the switching power supply output voltage applied to a switching power supply circuit, comprising separate components. The structure is simple, the system is stable and the cost is low. The invention includes a voltage sampling circuit, an error amplifying circuit and an insulating and converting circuit. The output voltage (Vo) of the switching power supply is imported through the voltage-signal input end of the voltage sampling circuit, and the sampling signal is achieved by voltage division; and the output voltage (Vo) also acts as the power supply for the error amplifying circuit; the sampling signal is imported through the sampling-signal input end of the error amplifying circuit, converted into a current-mode error-signal that is exported to the error current signal input end of the insulating and converting circuit, and a feedback signal (Vfb) is exported by electrical insulation.

Description

A Sampling Feedback Circuit of Output Voltage of Switching Power Supply

technical field

The invention relates to a voltage signal feedback circuit, in particular to a sampling feedback circuit for the output voltage of a switching power supply.

Background technique

Power supply is an indispensable part of various electronic equipment. Switching power supply occupies an important position in power supply technology. Most modern power supplies use this technology. Its advantages are reduced raw material consumption, miniaturization, fast dynamic response of the system, high efficiency, and effective Suppress noise pollution.

Most of today's switching power supplies use PWM pulse width modulation technology, which first performs basic rectification and filtering on the mains to convert AC signals into DC. Then through the high-frequency switching action of the switching device, the DC voltage is converted into a high-frequency AC voltage, and an isolated high-frequency AC voltage is induced on the output side of the power supply by using the isolation and coupling effect of the transformer, and rectified and filtered , generating a DC voltage supplied to the load.

In order to stabilize the output DC voltage, the error signal of the output voltage of the power supply must be sampled and amplified, and fed back to the input terminal. Through the PWM comparator, the on-off duty cycle of the switching device is changed, and a negative feedback is completed to adjust the output. The stability of the voltage controls the size of the output voltage ripple.

The invention is a sampling and feedback circuit of the output voltage of a switching power supply, which can sample and amplify the error signal of the output voltage deviating from the stable voltage value, and feed back to the input terminal of the power supply through the isolation of the optical coupler. The circuit can be widely used in various switching power supply equipment, and the feedback signal generated by it can well reflect the error signal, and then accurately control the duty cycle of the switching device in the switching power supply, thereby stabilizing the output voltage.

For example, a power supply with a single-ended forward structure in Figure 1. Vin is the DC voltage after rectification and filtering of the mains. The MOS tube acts as a switch. Its high-frequency switching action will cause a high-frequency AC voltage to be generated on the primary winding of the transformer. Voltage. Using the coupling effect of the transformer, a high-frequency AC voltage will be induced on the secondary side, and through the rectification and filtering of the diode and capacitor, a DC voltage Vo is generated to supply the load. If the duty cycle of the MOS tube switch is constant, Vo cannot be stabilized when Vin or load changes, which requires the voltage sampling feedback circuit to amplify the error disturbance signal of Vo and feed it back to the input side of the switching power supply. Through the PWM modulator, the feedback signal is compared with a triangle wave with a constant frequency and volt value, and a square wave signal with an adjustable duty ratio is output, and then the power drive circuit controls the duty ratio of the MOS tube switch to control the power supply. The output voltage Vo is regulated. Since this is a negative feedback process, Vo can be stabilized within a certain range of requirements.

In a switching power supply, the output voltage sampling feedback circuit is a key part, and its design directly affects the entire circuit

The stability and performance of the power supply system, the requirements of today's switching power supply system for its feedback circuit are basically:

1. It has fast input and output dynamic response and high stability;

2. Very high output voltage accuracy;

3. It can realize the electrical isolation of input and output.

In the development process of switching power supply, the output voltage feedback circuit has produced various structures and deformations, which can be mainly summarized as follows:

(1) From the point of view of today's application, the simplest structure is to use resistor voltage divider, but this structure cannot achieve electrical isolation, and the precision is low, the magnification cannot be changed and frequency compensation can not be performed, and the stability is poor. So this method is rarely used.

(2) At present, a small number of power supplies use operational amplifiers for error amplification feedback, but this method still cannot achieve electrical isolation, and the requirements for operational amplifiers are relatively high.

(3) Although the circuit using transformer auxiliary winding for voltage sampling and feedback can realize electrical isolation, when the load of the power supply changes greatly, the voltage stabilization effect is very poor. Moreover, the response speed of the circuit is relatively slow and cannot adapt to high-frequency switching power supplies.

(4) Linear voltage regulator TL431 combined with optocoupler isolation is the most used voltage feedback circuit in small and medium power switching power supplies. However, because the withstand voltage of t1431 is only about 40V, if the output voltage of the switching power supply is high, even if the resistor divider is used, it will often be damaged by the peak voltage when starting up. Therefore, it is necessary to add an auxiliary winding to provide the working voltage of the optocoupler and t1431, which increases the complexity of the transformer winding, and in some applications will interfere with the winding of the main output circuit.

Contents of the invention

Aiming at the above-mentioned problems in the prior art, the present invention proposes a sampling feedback circuit for the output voltage of a switching power supply with a simple structure and strong stability.

The present invention adopts following technical scheme:

A sampling feedback circuit for the output voltage of a switching power supply, including a voltage sampling circuit, an error amplifier circuit, and an isolation conversion circuit. The output voltage Vo of the switching power supply is input through the voltage signal input terminal of the voltage sampling circuit, and the sampling signal is obtained by dividing the voltage, and the voltage The input signal Vo is also used as the power supply of the error amplifier circuit, and the sampling signal is input through the sampling signal input terminal of the error amplifier circuit, converted into a current-type error signal and output to the error current signal input terminal of the isolation conversion circuit, and electrically isolated to obtain the feedback output signal Vfb .

Described voltage sampling circuit comprises sliding rheostat VR1, resistance R5 and resistance R6, and one end of resistance R6 is used as voltage signal input terminal, connects voltage input signal Vo, and the other end is connected in series with sliding rheostat VR1, one end of resistance R5, and the other end of resistance R5 One end is grounded, and the tap of the sliding rheostat VR1 serves as the sampling signal output terminal of the voltage sampling circuit and is connected to the sampling signal input terminal of the error amplifier circuit.

Described error amplification circuit comprises resistance R1, R2, R3, R4, capacitor C1, Zener diode ZD1, triode Q1; Wherein, the base of triode Q1 is as the sampling signal input end of error amplification circuit, resistance R3 and Zener tube ZD1 is connected in series, its connected end is connected to the emitter of transistor Q1, the anode of Zener diode ZD1 is grounded, the other end of resistor R3 is used as the power supply input terminal of the error amplifier circuit to connect to the output voltage Vo of the switching power supply, and one end of resistor R1 is connected to the switching power supply output voltage Vo of the error amplifier circuit, the other end is used as the upper end of the error current signal output of the error amplifier circuit and is connected to the upper end of the error current signal input of the isolation conversion circuit, and the collector of the transistor Q1 is used as the lower end of the error current signal input of the error amplifier circuit and the error of the isolation conversion circuit The lower end of the current signal output is connected, one end of the resistor R2 is connected to the collector of the transistor Q1, the other end is connected to the other end of the resistor R1, one end of the resistor R4 is connected in series with one end of the capacitor C1, the other end of the resistor R4 is connected to the base of the transistor Q1, and the capacitor The other end of C1 is connected to the collector of transistor Q1.

Described isolation conversion circuit comprises optocoupler OP1, resistance R7, R8, R9, capacitor C2, diode D1; Wherein, the light-emitting diode anode of optocoupler OP1 is used as the error current signal input upper end of isolation conversion circuit, the light-emitting diode of optocoupler OP1 The cathode is used as the lower end of the error current signal output of the isolation conversion circuit. The phototransistor of the optocoupler OP1 is connected in series with the diode D1 and the resistor R7. The anode of the diode D1 is connected to the power supply Vc, and the resistor R9 and the capacitor C2 are connected in series. The ends are connected in parallel, one end of the resistor R8 is used as the feedback signal output end of the isolation conversion circuit to output the signal Vfb, and the other end is grounded.

Compared with prior art, the present invention has following advantage:

1. The present invention is completely composed of discrete components, with simple structure and low cost. In the error amplification circuit, the electrical signal amplification principle of the triode is used, and the voltage regulator tube is used to ensure that the emitter voltage of the triode is constant, and the voltage error signal reflected between the base and emitter is amplified into the collector current signal of the triode for feedback. In this way, there is no need to use a linear regulator chip such as TL431 or an operational amplifier chip to amplify the error signal, which simplifies the circuit structure and saves costs.

2. The present invention has high stability. In this circuit, resistor R1 is used to limit the current of the light-emitting diode in the optocoupler. When the triode is turned off, resistor R2 is used to bypass the light-emitting diode, which prevents false triggering of the light-emitting diode and A large reverse bias voltage is generated on it. Therefore, it is only necessary to select a triode with a suitable withstand voltage, and the output voltage Vo of the switching power supply can be directly used to provide the working voltage for the optocoupler and the triode, instead of using the auxiliary winding of the transformer to provide voltage separately, especially in the output voltage Vo higher case. This also simplifies the circuit structure further, improves system stability, and has wide adaptability, and is applicable to switching power supplies with an output of ten volts to several hundred volts.

3. In the present invention, the frequency compensation circuit of the error amplifier composed of the capacitor C1 and the resistor R4 is used in the feedback circuit of the present invention, and the amplitude-frequency and phase-frequency characteristics of the error amplifier circuit can be changed by using different values. Reasonable value selection can ensure that the transfer function of the circuit has sufficient phase margin, so that the switching power supply system using the feedback circuit can greatly reduce the generation of oscillation phenomenon, and further improve the stability of the system operation.

4. The input and output terminals of the present invention use optocouplers for signal transmission, which is electrical-optical-electrical conversion, so there is no direct electrical connection between the two ends, greatly reducing interference, and making the system more stable.

5. The present invention uses resistor R9 and capacitor C2 at the output end of the optocoupler to form a discharge path for high-frequency signals, which can effectively filter out high-frequency interference and enhance the stability of feedback signals.

6. The present invention uses the resistor R8 to limit the maximum value of the feedback output Vfb. When the current is large, its resistance is much greater than the equivalent resistance of the phototransistor, and it is bypassed and does not work; when the current is small, its resistance is smaller than the equivalent resistance of the phototransistor, and the current mainly flows through the resistor R8 , the maximum output value of the feedback output Vfb is the value obtained by dividing the voltage between the resistor R8 and the resistor R7. By limiting this value, the circuit of the present invention can more flexibly match the entire switching power supply system, and limit the maximum duty ratio of the pulse width modulation signal. In addition, only one resistor is needed to realize this function, and the circuit structure is simple.

Description of drawings

Figure 1 is a circuit diagram of a switching power supply.

Fig. 2 is a structural block diagram of the present invention.

Figure 3 is an electrical schematic diagram of the present invention.

Fig. 4 is a circuit diagram of an embodiment of the present invention.

FIG. 5 is a waveform test diagram of key nodes when Vo has a normal error fluctuation of plus or minus 6V in an embodiment of the present invention.

FIG. 6 is a waveform test chart of key nodes when Vo has a large error fluctuation of plus or minus 50V in another embodiment of the present invention.

Detailed ways

The purpose, circuit structure and advantages of the present invention will be further described below through specific embodiments of the present invention in conjunction with the accompanying drawings.

As shown in Figure 1, it is a basic circuit structure of a widely used forward switching power supply. The DC output voltage generated by the switching power supply is the voltage input signal Vo. The voltage input signal Vo is input to the sampling feedback circuit of the present invention, and the voltage feedback signal Vfb is obtained after processing, which is used as the output signal of the circuit of the present invention and fed back to the PWM (pulse width modulation) module of the switching power supply.

As shown in Figure 2, a sampling feedback circuit for the output voltage of a switching power supply includes a voltage sampling circuit 1, an error amplifier circuit 2, and an isolation conversion circuit 3;

The function of the voltage sampling circuit 1 is to divide and sample the voltage input signal Vo. The voltage input signal Vo is input through the voltage signal input terminal of the voltage sampling circuit 1 , and the sampling signal of Vo obtained through voltage division by resistors is output to the error amplifier circuit 2 . It includes a sliding rheostat VR1, a resistor R5 and a resistor R6; one end of the resistor R6 is used as the voltage signal input end of the voltage sampling circuit 1, and is connected to the voltage input signal Vo, and the other end is connected in series with the sliding rheostat VR1 and one end of the resistor R5. The other end is grounded, and the tap of the sliding rheostat VR1 is used as the sampling signal output terminal of the voltage sampling circuit 1 to be connected to the sampling signal input terminal of the error amplifier circuit 2, that is, the base of the triode Q1;

The function of the error amplification circuit 2 is to amplify the error of the sampling signal. The voltage input signal Vo is used as the power supply of the error amplifier circuit 2, and is connected to its power supply input terminal. The sampling signal is input through the sampling signal input terminal of the error amplifier circuit 2, converted into a current-type error signal, and output from the upper end of the error current signal output of the error amplifier circuit 2. The upper end of the error current signal input to the isolation conversion circuit 3. The error amplifier circuit 2 includes resistors R1, R2, R3, R4, capacitor C1, Zener diode ZD1, and triode Q1; the resistor R3 is connected in series with the Zener transistor ZD1, and its connected end is connected to the emitter of the transistor Q1, and the Zener diode The anode of ZD1 is grounded, the other end of the resistor R3 is used as the power supply input terminal of the error amplifier circuit 2 to connect to the output voltage Vo of the switching power supply, one end of the resistor R1 is connected to the output voltage Vo of the switching power supply, and the other end is used as the error current signal output of the error amplifier circuit 2 The upper end is connected to the light-emitting diode anode of the optocoupler OP1 as the upper end of the error current signal input of the isolation conversion circuit 3, and the collector of the triode Q1 is used as the error current signal input of the error amplifier circuit 2. The lower end is output as an error current signal of the isolation conversion circuit 3 The cathode of the light-emitting diode of the optocoupler OP1 at the lower end is connected to form a current loop; one end of the resistor R2 is connected to the collector of the transistor Q1, and the other end is connected to the other end of the resistor R1; one end of the resistor R4 is connected in series with one end of the capacitor C1, and the resistor R4 The other end of the capacitor C1 is connected to the base of the transistor Q1, and the other end of the capacitor C1 is connected to the collector of the transistor Q1.

The function of the isolation conversion circuit 3 is to electrically isolate the current-type error signal output by the error amplifier circuit 2, convert it into a voltage feedback signal Vfb, and output it from the feedback signal output terminal. It includes optocoupler OP1, resistors R7, R8, R9, capacitor C2, and diode D1; the phototransistor of optocoupler OP1 is connected in series with diode D1 and resistor R7, the anode of diode D1 is connected to power supply Vc, resistor R9 and capacitor C2 are connected in series, and Two ends of the phototransistor of the optocoupler OP1 are connected in parallel, one end of the resistor R8 is used as the feedback signal output end of the isolation conversion circuit 3 , the output signal is Vfb, and the other end is grounded.

The working process and principle of the circuit of the present invention:

When the switching power supply is working, its DC output voltage is Vo. In order to stabilize the output voltage Vo, it is necessary to sample and amplify the error signal that deviates from the voltage regulation value through the sampling feedback circuit, and then feed it back to the front-end control terminal of the switching power supply for PWM (pulse width modulation), and then use negative feedback to realize Voltage regulation control.

The voltage sampling circuit composed of resistor R5, resistor R6, and sliding rheostat VR1 divides and samples the voltage input signal Vo, and the sampling voltage follows the fluctuation of the voltage input signal Vo, thereby controlling the base voltage of the transistor Q1. Since the emitter of the triode Q1 is connected to a Zener diode ZD1, the emitter of the triode Q1 is stabilized at a relatively fixed potential. Therefore, the base-emitter junction voltage of the transistor Q1 varies with its base voltage, that is, it is proportional to the variation of the voltage input signal Vo.

According to the characteristics of the triode, the collector current is proportional to the base-emitter junction voltage, and the collector current of the triode Q1 is also proportional to the variation of the voltage input signal Vo. The light-emitting diode of the optocoupler OP1 is connected in series with the collector of the triode Q1, and the current flowing through it is basically the same. It can be seen that the current of the light-emitting diode of the optocoupler OP1 is proportional to the variation of Vo. In the circuit of the present invention, the resistor R1 is used to limit the current of the optocoupler OP1, and the resistor R3 is used to limit the current of the regulator tube ZD1. Resistor R2 plays a protective role. When the transistor Q1 is turned on, the impedance of the light-emitting diode is much smaller than the resistor R2, and the resistor R2 is bypassed and has no effect; when the transistor Q1 is turned off, the impedance of the light-emitting diode is much larger than the resistor R2. More, the resistor R2 bypasses the light-emitting diode, and the small leakage current mainly flows through the resistor R2, ensuring that the voltage at both ends of the light-emitting diode is close to 0V, making it non-conductive, preventing false triggering and a large current on the diode reverse bias. The characteristic of this circuit is that the triode Q1 is used as an error amplifier, its emitter is equivalent to the positive terminal of the operational amplifier, the base is equivalent to the negative terminal, and the emitter current is equivalent to the output of the error amplifier. Adding resistor R4 and capacitor C1 between the collector and base of transistor Q1 is equivalent to adding negative feedback between the output and input of the error amplifier to play the role of frequency compensation, and can be set according to the transfer function of different main power supply structures. The value of the switching power supply makes the entire switching power supply regulation system more stable. If it is not set properly or there is no such frequency compensation network, the system may be unstable and oscillate.

Since the input and output terminals of the switching power supply will have a lot of electromagnetic interference, electrical isolation is very necessary. The circuit of the present invention uses a photocoupler to convert electrical signals into optical signals and then convert them back to electrical signals to complete the electrical isolation at both ends. Under the power supply of Vc, the phototransistor of the optocoupler OP1 can flow a current proportional to the light emitting diode. Resistor R7 acts as a current limiter, and diode D1 prevents voltage backfeeding. When the current flowing is different, the voltage drop of resistor R7 is different, and the voltage drop of Vc and diode D1 is basically constant, then the small signal voltage change of the output error amplification feedback signal Vfb is proportional to the current change of the phototransistor. Proportional. It can be seen from the above that the feedback signal Vfb is proportional to the disturbance of the output voltage Vo of the switching power supply, which can well reflect the error of the output voltage of the switching power supply and amplify it, and play the role of feedback of the output voltage Vo error signal of the switching power supply. Resistor R9 and capacitor C2 play a role in eliminating high-frequency interference. Resistor R8 is connected between Vfb and ground. When the current is large, its resistance value is much greater than the equivalent resistance of the phototransistor, and it is bypassed and does not work; when the current is small, its resistance value is smaller than that of the phototransistor, etc. The effective resistor, the current mainly flows through the resistor R8, which can make Vfb not exceed a certain set value, and limit the output duty cycle of the downstream PWM controller.

The advantages of the present invention are illustrated below with a specific embodiment:

1. Input settings:

The stable value of the output DC voltage Vo of the switching power supply is 195V;

Ripple voltage: within 3%, that is, the normal fluctuation range of Vo is 195V plus or minus 6V.

2. Parameter selection:

(1) The voltage stabilization value of the voltage regulator tube ZD1 can take any value between 2V-10V, and other component parameters are calculated according to this value. In this example, a voltage regulator tube with a voltage regulation value of 6.2V is selected.

Assuming the regulated voltage value of Zener tube ZD1 is Vz, then when Vo is normal, the sampling divided voltage of resistors R5, R6 and sliding rheostat VR1 should be Vz+0.7V, that is, the voltage drop of a triode emitter junction higher than Vz. The resistance value range of the resistor R5 is generally 1K-10K ohms (optional setting), and the resistor R6 and the sliding rheostat VR1 should be calculated according to the resistance value of the resistor R5. In this example, resistor R5 is 5K ohms. In order to make the circuit composed of resistors R5, R6 and sliding rheostat VR1 get a voltage of 6.9V, which is 0.7V higher than the voltage regulator value of the 6.2V regulator tube, in this example, take resistor R6 as 250K ohms, and sliding rheostat VR1 as 20K ohm sliding rheostat, by adjusting the sliding rheostat VR1, the divided voltage can be 6.9V.

(2) Select the β value of the triode Q1 according to the requirements of the error amplification factor, generally about 100, and the withstand voltage value of the triode Q1 is 20% higher than the maximum value of the output voltage Vo. In this example, the NPN tube of model q2n3439 is selected.

(3) The current-limiting resistors R1 and R3 are selected according to the size of the output voltage, generally 500 ohms of the Vo value. Take 100K ohms in this example.

(4) The protective resistor R2 is generally about 10K ohms.

(5) Frequency compensation resistor R4 and capacitor C1 are generally determined by the phase margin required by the system. The value range of resistor R4 is 200K-500K ohms, and the capacitor C1 is 2nF-10nF. In this example, resistor R4 is 270K ohms, and capacitor C1 is 6.8nF.

(6) Optocoupler OP1 selects the current transfer ratio as about 1. In this example, the selected model is PC817B.

(7) The diode D1 can be an ordinary diode. In this case it is 1N4007.

(8) Different current-limiting resistors R7 can be selected according to the difference of Vc, generally 1000 ohms of the value of Vc. Take 4.3K ohms in this example.

(9) Resistor R9 and capacitor C2 are selected according to the bandwidth requirements for eliminating high-frequency interference. Generally, resistor R9 is several hundred to several thousand ohms, and capacitor C2 is between several nF and 1uF. In this example, the resistor R9 is 2.2K ohms, and the capacitor C2 is 47nF.

(10) The selection of resistor R8 should be based on the maximum value of Vfb. If the maximum value of Vfb is Vfbmax, then Vc×R7/(R7+R8)=Vfbmax, and the value of resistor R8 is selected according to this formula. In this example, 12K ohms is used to limit the maximum value of Vfb to no more than 2.5V.

The specific parameters are summarized as follows:

Resistor R1: 100K ohms; Resistor R2: 10K ohms;

Resistor R3: 100K ohms; Resistor R4: 270K ohms;

Resistor R5: 5K ohms; Resistor R6: 250K ohms;

Resistor R7: 4.3K ohms; Resistor R8: 12K ohms;

Resistor R9: 2.2K ohms; sliding rheostat VR1: 20K sliding rheostat;

Capacitor C1: 6.8nF/400V; Capacitor C2: 0.047uF/100V;

Diode D1: 1n4007; Zener diode ZD1: 6.2V regulator;

Transistor Q1: NPN tube q2n3439;

Optocoupler OP1: PC817B.

3. Analysis of results

Figure 5 is a waveform test diagram of key nodes when the input voltage Vo (195V) has a normal error fluctuation of plus or minus 6V; where, Vo is the output voltage of the switching power supply; Vbe is the base-emitter voltage on the transistor Q1; The current on the diode; Ic is the current induced on the phototransistor; Vfb is the error amplification signal output by the voltage feedback circuit.

It can be seen from Figure 5 that when Vo fluctuates at plus or minus 6V around its regulated voltage value of 195V, Vfb can follow the disturbance very well, the phase is opposite, it is negative feedback, the response is fast, there is almost no delay, and its fluctuation range is 1.5 V ~ 2V, suitable for the input requirements of the PWM comparator behind it.

This is the case when the power supply is functioning normally. It can be seen from Figure 5 that when Vo fluctuates at plus or minus 6V around its regulated voltage value of 195V, Vfb can follow the disturbance signal of Vo very well, and the phase is opposite, which is negative feedback, and the response is fast, almost without delay, and the amplitude The value fluctuates within the range of 1.5V to 2V, and the change is relatively smooth without spikes or burrs. These test waveforms all show that the circuit has the advantage of high stability.

Figure 6 is a waveform test diagram of key points when the input voltage Vo (195V) has a large error fluctuation of plus or minus 50V; the definitions of each signal are shown in Figure 5.

The situation shown in Figure 6 is a fluctuation of plus or minus 50V around the voltage regulation value of Vo 195V, which belongs to a large signal response, and is generally the case when the power supply is turned on, turned off or not operating normally.

It can be seen from Figure 6 that Vfb can still follow the fluctuation of Vo very well, and has a high response speed.

When Vo is too low, the transistor Q1 is cut off, the current in the light-emitting diode drops to 0, the phototransistor does not induce current, and the Vfb output saturates to the maximum voltage. Due to the effect of the resistor R8, the maximum Vfb is 2.5V instead of Vc 4V, which plays a role in limiting the maximum pulse width output by the PWM comparator in the latter stage.

When Vo is too high, the current of the light-emitting diode is large, the phototransistor is in a saturated state, the collector-emitter saturation voltage drop is about 0.3V, and the minimum output voltage of Vfb is 0.3V, which is lower than the volt value of the sawtooth wave at the forward end of the PWM comparator. The duty cycle of its output waveform is 0. You can turn off the switching MOS tube for a period of time, and wait for Vo to drop to the normal 195V voltage regulation range before modulating.

It can be seen that Vfb can still follow the fluctuation of Vo well under abnormal conditions, has a high response speed, and will not affect the work stability due to the sharp change of Vo.

Claims (3)

1, a kind of sampling feedback circuit of switch power source output voltage, comprise voltage sampling circuit (1), error amplifying circuit (2), isolate change-over circuit (3), the output voltage V o of Switching Power Supply is by the voltage signal input input of voltage sampling circuit (1), carry out dividing potential drop and obtain sampled signal, and the voltage input signal Vo while is as the power supply of error amplifying circuit (2), sampled signal is by the sampled signal input input of error amplifying circuit (2), be converted to the current mode error signal and output to the error current signal input part of isolating change-over circuit (3), carry out electrical isolation and obtain feedback loop output signal Vfb, it is characterized in that: described isolation change-over circuit (3) comprises optocoupler (OP1), resistance R 7, resistance R 8, resistance R 9, electric capacity (C2), diode (D1); Wherein, the light-emitting diodes tube anode of optocoupler (OP1) is as the error current signal input upper end that isolates change-over circuit (3), the light-emitting diodes tube cathode of optocoupler (OP1) is as the error current signal output lower end that isolates change-over circuit (3), the collector electrode of the phototriode of optocoupler (OP1) respectively with resistance R 7, resistance R 8, one end of resistance R 9 connects the back and constitutes the feedback output end of isolating change-over circuit (3), isolate change-over circuit (3) output feedback loop output signal Vfb, the other end of resistance R 7 is connected with the negative electrode of diode (D1), the anode of diode (D1) meets power supply Vc, the other end of resistance R 9 with after electric capacity (C2) is connected respectively with the other end of resistance R 8, the emitter of the phototriode of optocoupler (OP1) connects ground connection.

2, the sampling feedback circuit of a kind of switch power source output voltage according to claim 1, it is characterized in that: described voltage sampling circuit (1) comprises slide rheostat (VR1), resistance R 5 and resistance R 6, one end of resistance R 6 is as the voltage signal input, meet voltage input signal Vo, the other end is connected with an end of slide rheostat (VR1), resistance R 5, the other end ground connection of resistance R 5, the tap of slide rheostat (VR1) is connected with the sampled signal input of error amplifying circuit (2) as the sampled signal output of voltage sampling circuit (1).

3, the sampling feedback circuit of a kind of switch power source output voltage according to claim 1, it is characterized in that: described error amplifying circuit (2) comprises resistance R 1, resistance R 2, resistance R 3, resistance R 4, electric capacity (C1), voltage stabilizing didoe (ZD1), triode (Q1); Wherein, the base stage of triode (Q1) is as the sampled signal input of error amplifying circuit (2), resistance R 3 is connected in series with voltage-stabiliser tube (ZD1), its end that links to each other links to each other with the emitter of triode (Q1), the plus earth of voltage stabilizing didoe (ZD1), the other end of resistance R 3 is as the output voltage V o of the power supply input termination switch power supply of error amplifying circuit (2), the output voltage V o of resistance R 1 one termination Switching Power Supplies, the other end is connected with the error current signal input upper end that isolates change-over circuit (3) as the error current signal output upper end of error amplifying circuit (2), the collector electrode of triode (Q1) is connected with the error current signal output lower end that isolates change-over circuit (3) as the error current signal input lower end of error amplifying circuit (2), the collector electrode of one termination triode (Q1) of resistance R 2, the other end of other end connecting resistance R1, one end of resistance R 4 is connected with an end of electric capacity (C1), the base stage of another termination triode (Q1) of resistance R 4, the collector electrode of another termination triode (Q1) of electric capacity (C1).

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