TW201419725A - Startup circuit of supply voltage for PFC circuit and switching power supply using the same - Google Patents

Abstract

The invention discloses a startup circuit of a supply voltage for a power factor correction (PFC) circuit and a switching power supply using the same. The startup circuit of the supply voltage includes an alternating-current (AC) input voltage sampling terminal, a PFC output voltage sampling terminal, a supply output terminal and an external supply voltage input terminal, a comparator circuit and a switch circuit. The startup circuit of the supply voltage samples an AC input peak voltage through the AC input voltage sampling terminal, samples a voltage at a positive terminal of a filter capacitor coupled to an output terminal of the PFC circuit through the PFC output voltage sampling terminal, and compares them to control whether or not the supply voltage received from the external supply voltage input terminal is transferred to the supply output terminal coupled to a control chip of the PFC circuit. The invention can prevent an inductor of the PFC circuit from saturation conducting under the condition of fast on/off so as to protect internal electric components of PFC circuit from damage.

Description

Supply voltage opening circuit of PFC circuit and switching power supply using same

The present invention relates to a switching power supply, and more particularly to a power factor correction circuit for a power factor correction (PFC) circuit and a switching power supply using the same.

Power Factor is the ratio of effective power to apparent power (total power consumption). Basically, the power factor can measure the extent to which power is effectively utilized. When the power factor is larger, it represents higher power utilization. For this reason, existing power products with large input power often need to add a power factor correction (PFC) circuit inside the power supply circuit to increase the power factor of the power input terminal and reduce the input current harmonic amount, thereby reducing the burden and interference to the power grid.

Please refer to FIG. 1. FIG. 1 is a circuit diagram of a conventional switching power supply with a PFC circuit, which can be applied to a liquid crystal display product. After the AC power supply (such as 90Vrms ~ 264Vrms) is input into the switching power supply, the EMI filtering is performed by the electromagnetic interference (ElectroMagnetic Interference (EMI) filter circuit 1 and the full-wave rectification is performed by the bridge rectifier circuit 2, and the voltage after full-wave rectification is performed. After boosting by the PFC circuit 3, a DC voltage of about 400V is generated at the positive terminal of the smoothing capacitor C1. The bridge rectifier circuit 2 includes diodes D1 to D4, and the PFC circuit 3 includes the inductor L1 and the diode D5. , power transistor Q1, control wafer U1 and resistor R1. The DC voltage of about 400V is subjected to voltage conversion by the main power conversion circuit 4, and then outputs a low-voltage DC voltage (such as 24V), which can be used as a power supply input terminal voltage of the liquid crystal panel lamp driving circuit of the liquid crystal display product. The DC voltage of about 400V is subjected to voltage conversion by the standby power conversion circuit 5, and then outputs another low-voltage DC voltage (for example, 5V), which can be used as a power supply input terminal voltage of the motherboard image processing circuit of the liquid crystal display product.

The standby power conversion circuit 5 has a power supply circuit (not shown) for generating a power supply voltage Vcc. The power supply voltage Vcc is input to the control chip (not shown) inside the standby power conversion circuit 5, and is also input to the power supply. Control circuit 6. The motherboard image processing circuit outputs a switch signal PS_on to control whether the power supply control circuit 6 outputs the supply voltage Vcc to the control chip U1 and the control chip (not shown) inside the main power conversion circuit 4. When the liquid crystal display product needs to enter the standby mode, the motherboard image processing circuit outputs a low level switch signal PS_on to control the power supply control circuit 6 to stop outputting the supply voltage Vcc to the control wafer U1 and the control chip inside the main power conversion circuit 4. Power is supplied to make the LCD product more power efficient in standby mode. When the liquid crystal display product works normally, the motherboard image processing circuit outputs a high level switch signal PS_on to control the power supply control circuit 6 to output the power supply voltage Vcc to the control wafer U1 and the control chip inside the main power conversion circuit 4.

Please continue to refer to Figure 1. The switching power supply shown in Figure 1 has a higher voltage (such as 220Vrms) on the input AC AC and the on/off condition of the switching power supply. When the AC input voltage of the switching power supply is turned off. (turn off), the positive terminal voltage of the filter capacitor C1 begins to decrease, and when the voltage drops to, for example, 80V, the standby power conversion circuit 5 can still operate normally, and the power supply control circuit 6 still supplies the supply voltage Vcc to the control wafer U1, so that the control chip U1 is controlled. Still working, at this time, if the voltage of the AC input terminal of the switching power supply is turned on again, the end of the inductor L1 connected to the negative terminals of the diodes D3 and D4 can instantaneously generate a very high voltage ( Such as 310V or more), a large voltage difference (such as 230V or more) is generated across the inductor L1, so that the inductor L1 instantaneously generates a large inrush current (such as 40A), and its current path is: inductor L1 → two The positive terminal D5 (also known as the anode terminal) → the negative terminal of the diode D5 (also known as the cathode terminal) → the positive terminal of the filter capacitor C1, so that the inductor L1 is in a fully saturated conduction state instantaneously, resulting in the inductor L1 The rapid decay of feeling in the moment and be close to non-inductive state. This is because when the current flowing through the inductor reaches a certain value, the magnetic flux density in the inductor core no longer increases in proportion to the strength of the magnetic field. At this time, the inductor begins to saturate and the inductance will be rapid. Attenuation causes the current flowing through the inductor to increase rapidly. When the inductor L1 is in a saturated conduction state, at this time, if the power transistor Q1 is controlled by the control wafer U1 to be turned on, a larger current will be generated across the inductor L1, and the current path is: inductor L1 → power The crystal Q1 汲 extreme → power transistor Q1 source terminal → resistor R1 eventually causes the power transistor Q1, the resistor R1 and the control wafer U1 to be destroyed.

Therefore, the existing PFC circuit design, as shown in FIG. 2, will add one between the input terminals (the negative ends of the diodes D3 and D4) and the output terminal (the positive terminal of the filter capacitor C1) of the PFC circuit 3 shown in FIG. The diode D7 enables the inrush current generated by the switching power supply to be fast-switched to the positive terminal of the smoothing capacitor C1 through the diode D7, thereby preventing the inductor L1 from generating a saturation conduction problem. The diode D7 usually adopts a diode such as the 1N5408 model, which adopts the DO-201AD package. Therefore, it is usually necessary to bend the two ends of the diode to 90 degrees and then insert it into the power board, but The DO-201AD package has a thicker lead. When the bent body is bent, the body of the diode is easily stressed and the inside of the body is slightly damaged. Such damage is difficult to find. Since the PFC circuit 3 belongs to the boosting circuit, that is, the voltage at the output of the PFC circuit 3 is always greater than the voltage at the input terminal during operation, the diode D7 is in a reverse bias state for a long time while the PFC circuit 3 is operating. If the diode D7 on the power supply board is defective due to processing or poor raw materials, the leakage current during reverse bias will be too large, causing the temperature of the diode D7 body to rise, and the temperature rise will cause The reverse leakage current continues to increase, causing the reverse leakage current to increase the vicious cycle, causing the diode D7 body to generate high temperature, so that the peripheral components and the power board of the diode D7 are carbonized at a high temperature, eventually causing the problem of the power board burning plate. .

The object of the present invention is to provide a power supply voltage turn-on circuit of a PFC circuit and a switching power supply using the same, which can prevent the inductor in the PFC circuit from being saturated and turned on under the condition that the switching power supply performs fast switching.

To achieve the above object, the present invention provides a power supply voltage turn-on circuit for a PFC circuit, which includes an AC input voltage sampling terminal, a PFC output voltage sampling terminal, a power supply output terminal, an external power supply voltage input terminal, a comparison circuit, and a switching circuit. The comparison circuit includes a diode, a capacitor, an operational amplifier, and first to fourth resistors. The sampling end of the AC input voltage is coupled to the positive end of the diode, the negative end of the diode is coupled to one end of the capacitor and one end of the first resistor, and the other end of the capacitor and one end of the second resistor are grounded, first and second. The other end of the resistor is coupled to the negative input terminal of the operational amplifier (also referred to as the inverting input terminal), the sampling end of the PFC output voltage is coupled to one end of the third resistor, and the other end of the third resistor is coupled to the positive input terminal of the operational amplifier. (also called non-inverting input terminal) and one end of the fourth resistor, the other end of the fourth resistor is grounded, the power supply input end of the operational amplifier is coupled to the external power supply voltage input terminal, and the ground of the operational amplifier is grounded. The first end of the switch circuit is coupled to the external power supply voltage input end, the second end of the switch circuit is coupled to the power supply output end, and the control end of the switch circuit is coupled to the output end of the operational amplifier, wherein the first end and the second end of the switch circuit are The signal level received by its control terminal is turned on or off.

In an embodiment of the invention, the switching circuit includes a first type of transistor, a second type of transistor, and fifth to seventh resistors. The control end of the switch circuit is coupled to one end of the fifth resistor, and the other end of the fifth resistor is coupled to the first type of transistor control end, the first end of the first type transistor is grounded, and the second end of the first type transistor The first end of the sixth resistor is coupled to one end of the seventh resistor and the second type of transistor control end, and the first end of the second type transistor is coupled to the other end of the seventh resistor and The first end of the switch circuit, the second end of the second type transistor is coupled to the second end of the switch circuit. The first end and the second end of the first type transistor are turned on or off according to the signal level received by the control end thereof, and the first end and the second end of the second type transistor are turned on according to the signal level received by the control end thereof. Or disconnected, the second type of transistor is turned on when the first type of transistor is turned on, and the second type of transistor is turned off when the first type of transistor is turned off.

In an embodiment of the invention, the first type of transistor is an NPN type transistor, the second type of transistor is a PNP type transistor, and the first end, the second end, and the control end of the first type and the second type of transistor respectively For the extremes of the shot, the extremes of the set and the base.

The invention also provides a switching power supply using the above-mentioned supply voltage opening circuit, which comprises an EMI filter circuit, a bridge rectifier circuit, a PFC circuit, a filter capacitor, a main power conversion circuit, a standby power conversion circuit, a power supply control circuit, and the above-mentioned power supply voltage Circuit. Among them, the municipal AC power is EMI filtered by the EMI filter circuit, and then the full-wave rectification by the bridge rectifier circuit is used as the input terminal voltage of the PFC circuit, the output end of the PFC circuit is coupled to the positive end of the filter capacitor, and the negative end of the filter capacitor is grounded, the PFC circuit The output terminal voltage is outputted by the main power conversion circuit to output the first DC voltage, and the output voltage of the PFC circuit is outputted by the standby power conversion circuit to output a second DC voltage, and the standby power conversion circuit outputs the supply voltage by the internal power supply circuit. And provided to the power supply control circuit, the power supply control circuit determines whether to output the power supply voltage to the external power supply voltage input terminal according to the switch signal, the AC input voltage sampling end is coupled to either end of the two output ends of the EMI filter circuit, and the PFC output voltage sampling end coupling Connected to the positive terminal of the filter capacitor, the power supply output terminal is coupled to the power supply input terminal of the PFC circuit and the power supply input terminal of the main power conversion circuit.

In an embodiment of the invention, the switching power supply can be applied to a liquid crystal display product, the first DC voltage is used as the power supply input terminal voltage of the liquid crystal panel lamp driving circuit of the liquid crystal display product, and the second DC voltage is used as the motherboard of the liquid crystal display product. The voltage at the power supply input of the image processing circuit.

In an embodiment of the invention, the PFC circuit includes an inductor, a rectifying diode, a power transistor, a control chip, and a current detecting resistor. One end of the inductor is coupled to the output end of the bridge rectifier circuit, and the other end of the inductor is coupled to the positive terminal of the rectifier diode and the first end of the power transistor, and the second end of the power transistor is coupled to the control chip current detection. One end of the current detecting resistor and the other end of the current detecting resistor are grounded, the power transistor control end is coupled to the control chip output end, the power supply output end is coupled to the control chip power supply input end, and the rectifying diode negative end coupling is coupled. Connect to the positive side of the filter capacitor.

In an embodiment of the invention, the power transistor is a field effect transistor, and the first end, the second end, and the control end of the power transistor are a 汲 terminal, a source terminal, and a gate terminal, respectively.

The power supply voltage opening circuit of the invention makes the inductor in the PFC circuit not in a saturated working state when the PFC circuit starts to work, and avoids the inductor in the PFC circuit 3 under the fast switching condition of the conventional switching power supply as shown in FIG. L1 will produce saturation and cause the destruction of electronic components such as power transistor Q1, and avoid the reverse leakage current caused by the conventional switching power supply shown in Fig. 2 due to the processing of the diode D7 component or the poor raw material itself. When working for a long time, the temperature of the body of the diode D7 continues to rise, which eventually causes the power board to be carbonized and burned by the diode D7. The switching power supply using the power supply voltage opening circuit of the invention not only ensures the service life of the power supply, but also improves the stability of the system, and has good practical value.

1. . . EMI filter circuit

2. . . Bridge rectifier circuit

3. . . PFC circuit

4. . . Main power conversion circuit

5. . . Standby power conversion circuit

6. . . Power supply control circuit

7. . . Supply voltage open circuit

71. . . AC input voltage sampling end

72. . . PFC output voltage sampling end

73. . . Power supply output

74. . . External supply voltage input

75. . . Comparison circuit

76. . . Switch circuit

761. . . First end

762. . . Second end

763. . . Control terminal

C1~C3. . . Capacitor

D1~D7. . . Dipole

L1. . . Inductor

OP1. . . Operational Amplifier

Q1. . . Power transistor

Q2. . . NPN type transistor

Q3. . . PNP type transistor

R1~R8. . . Resistor

U1. . . Control chip

CS. . . Current detection terminal

OUT. . . Output

VCC. . . Power input

AC. . . AC power

Vcc. . . Supply voltage

PS_on. . . Switch signal

1 is a circuit diagram of a conventional switching power supply having a PFC circuit.
2 is a circuit diagram of a switching power supply with a diode added between the input terminal and the output terminal of the PFC circuit shown in FIG. 1.
3 is a circuit diagram of an embodiment of a supply voltage turn-on circuit of a PFC circuit of the present invention.
4 is a circuit diagram showing an embodiment of a switching power supply of the power supply voltage turn-on circuit shown in FIG. 3 of the present invention.

Please refer to FIG. 3. FIG. 3 is a circuit diagram of an embodiment of a power supply voltage turn-on circuit of the PFC circuit of the present invention. The supply voltage opening circuit 7 of the PFC circuit of the present invention includes an AC input voltage sampling terminal 71, a PFC output voltage sampling terminal 72, a power supply output terminal 73, an external supply voltage input terminal 74, a comparison circuit 75, and a switching circuit 76. The comparison circuit 75 includes a diode D6, a capacitor C3, an operational amplifier OP1, a first resistor R2, a second resistor R3, a third resistor R4, and a fourth resistor R5. The AC input voltage sampling terminal 71 is coupled to the positive terminal of the diode D6, the negative terminal of the diode D6 is coupled to one end of the capacitor C3 and one end of the first resistor R2, and the other end of the capacitor C3 and the second resistor R3 are both ends. Grounding, the other end of the first resistor R2 and the other end of the second resistor R3 are coupled to the negative input terminal of the operational amplifier OP1, the PFC output voltage sampling end 72 is coupled to one end of the third resistor R4, and the third resistor R4 is further One end is coupled to the positive input terminal of the operational amplifier OP1 and the fourth resistor R5, and the other end of the fourth resistor R5 is grounded. The power supply input terminal of the operational amplifier OP1 is coupled to the external power supply voltage input terminal 74, and the ground terminal of the operational amplifier OP1 is grounded. In an embodiment, the operational amplifier OP1 can be replaced with a comparator.

The first end 761 of the switch circuit 76 is coupled to the external power supply voltage input terminal 74, the second end 762 of the switch circuit 76 is coupled to the power supply output end 73, and the control circuit 763 is coupled to the output end of the operational amplifier OP1. The first end 761 and the second end 762 of the switch circuit 76 are turned on or off according to the signal level received by the control terminal 763. In the present embodiment, the switch circuit 76 includes a first type transistor Q2, a second type transistor Q3, a fifth resistor R6, a sixth resistor R8, and a seventh resistor R7. The control terminal 763 is coupled to one end of the fifth resistor R6, and the other end of the fifth resistor R6 is coupled to the control end of the first type transistor Q2. The first end of the first type transistor Q2 is grounded, first The second end of the type transistor Q2 is coupled to one end of the sixth resistor R8, and the other end of the sixth resistor R8 is coupled to one end of the seventh resistor R7 and the control end of the second type transistor Q3, the second type transistor Q3 One end is coupled to the other end of the seventh resistor R7 and the first end 761 of the switch circuit 76. The second end of the second type transistor Q3 is coupled to the second end 762 of the switch circuit 76. The first end and the second end of the first type transistor Q2 are turned on or off according to the signal level received by the control end thereof, and the first end and the second end of the second type transistor Q3 are based on the signal received by the control end thereof. The bit is turned on or off. When the first type transistor Q2 is turned on, the second type transistor Q3 is turned on, and when the first type transistor Q2 is turned off, the second type transistor Q3 is turned off. In this embodiment, the first type transistor Q2 is an NPN type transistor, the second type transistor Q3 is a PNP type transistor, and the first type and the second type of the first type transistor Q2 and the second type transistor Q3 are used. And the control terminals are the emitter extreme, the collector extreme and the base extreme.

Referring to FIG. 4, FIG. 4 is a circuit diagram of an embodiment of a switching power supply of the power supply voltage opening circuit shown in FIG. 3, which can be applied to a liquid crystal display product. The switching power supply of the invention comprises an EMI filter circuit 1, a bridge rectifier circuit 2, a PFC circuit 3, a filter capacitor C1, a main power conversion circuit 4, a standby power conversion circuit 5, a power supply control circuit 6, and a supply voltage opening circuit shown in FIG. 7. After the city AC power (such as 90Vrms ~ 264Vrms) is input to the switching power supply, after EMI filtering by the EMI filter circuit 1, the bridge rectifier circuit 2 performs full-wave rectification and then acts as the input terminal voltage of the PFC circuit 3, and the voltage of the input terminal of the PFC circuit 3 passes. After the PFC circuit 3 is boosted, a DC voltage of about 400V is outputted at the output end of the PFC circuit 3. The output end of the PFC circuit 3 is coupled to the positive terminal of the smoothing capacitor C1, and the negative terminal of the smoothing capacitor C1 is grounded. The output terminal voltage of the PFC circuit 3 is subjected to voltage conversion by the main power conversion circuit 4, and then outputs a first DC voltage (for example, 24 V), which can be used as a power supply input terminal voltage of the liquid crystal panel lamp driving circuit of the liquid crystal display product. The output voltage of the PFC circuit 3 is subjected to voltage conversion by the standby power conversion circuit 5, and then outputs a second DC voltage (for example, 5 V), which can be used as a power supply input terminal voltage of the motherboard image processing circuit of the liquid crystal display product. The standby power conversion circuit 5 outputs the power supply voltage Vcc from its internal power supply circuit (not shown) and supplies it to the power supply control circuit 6. The power supply control circuit 6 determines whether to output the power supply voltage Vcc to the supply voltage open circuit 7 according to the switch signal PS_on. External supply voltage input 74. In addition, the AC input voltage sampling end 71 of the power supply voltage opening circuit 7 is coupled to either end of the two output ends of the EMI filter circuit 1, and the PFC output voltage sampling end 72 is coupled to the output end of the PFC circuit 3 (ie, the positive end of the filter capacitor C1). The power supply output end 73 is coupled to the control chip U1 of the PFC circuit 3 and the control chip (not shown) of the main power conversion circuit 4. In particular, the filter capacitor C1 needs to withstand high voltages above 400V and requires large capacity to provide sufficient voltage regulation filtering effect. Therefore, the filter capacitor C1 is often referred to as a high voltage large capacitor, and the filter capacitor C1 is usually as shown in the figure. An electrolytic capacitor having positive and negative polarities has its positive and negative ends referred to as a positive terminal and a negative terminal, respectively.

In the present embodiment, the PFC circuit 3 includes an inductor L1, a rectifying diode D5, a power transistor Q1, a control chip U1, and a current detecting resistor R1. Wherein, one end of the inductor L1 is coupled to the output end of the bridge rectifier circuit 2 (ie, the negative ends of the diodes D3 and D4), and the other end of the inductor L1 is coupled to the positive terminal of the rectifier diode D5 and the first of the power transistor Q1. The second end of the power transistor Q1 is coupled to one end of the current detecting terminal CS and the current detecting resistor R1 of the control chip U1, the other end of the current detecting resistor R1 is grounded, and the control end of the power transistor Q1 is coupled to the control chip. The U1 output terminal OUT, the power supply output terminal 73 of the power supply voltage opening circuit 7 is coupled to the power supply input terminal VCC of the control chip U1, and the negative terminal of the rectifier diode D5 is coupled to the positive terminal of the filter capacitor C1. In this embodiment, the power transistor Q1 is a field effect transistor, and the first end, the second end, and the control end of the power transistor Q1 are a 汲 terminal, a source terminal, and a gate terminal, respectively. In particular, the ground described in the present invention refers to the primary side ground in the switching power supply, that is, the primary side of the transformer inside the switching power supply.

Please continue to refer to FIG. 4, it can be seen from the figure that the switching power supply cancels the diode D7 in FIG. 2, and the power supply voltage opening circuit 7 is added to ensure that the inductor L1 is used when the switching power supply is performing fast switching. The saturation conduction problem does not occur when the PFC circuit 3 starts operating. The supply voltage turn-on circuit 7 samples the peak voltage of the input AC power through the diode D6 and the capacitor C3, wherein the diode D6 can be selected as a low-cost diode such as the FR107 or 1N4007 model. If the AC frequency of the input AC is 50 Hz and the voltage is 220 Vrms, the voltage across the capacitor C3 is V _C3 = 220 Vrms × √ 2 = 310 V, and it is necessary to satisfy: t = (R2 + R3) × C3 >> 1/50, That is: a charge and discharge time constant is much larger than one cycle time of the input AC AC, so the resistors R2 and R3 need to select a resistor of the order of MΩ or more, so that the voltage of the negative input terminal of the operational amplifier OP1 obtains a fixed voltage of V _- =V _C3 × R3/(R2+R3). Open circuit voltage supply 7 is sampled through the resistors R4 and R5 to the positive terminal of the smoothing capacitor C1 voltage V _C1, such that the positive input of the operational amplifier OP1 to detect a voltage across the smoothing capacitor C1 is _{V + = V C1 × R5 /} (R4 +R5). In addition, setting the ratio of R3/(R2+R3) needs to be slightly smaller than the ratio of R5/(R4+R5), so that when the switching power supply is performing fast switching, the voltage of the smoothing capacitor C1 is charged to the highest point of the AC input voltage ( If 310V is equivalent or slightly lower voltage (such as 300V), the output of the operational amplifier OP1 sends a high-level signal to turn on the NPN-type transistor Q2, so that the PNP-type transistor Q3 is turned on. At this time, the control chip U1 is turned on. The power supply input terminal VCC receives the power supply voltage Vcc output from the power supply control circuit 6, so that the PFC circuit 3 starts normal operation. Before the PFC circuit 3 starts to work, since the voltage on the filter capacitor C1 is substantially equal to the highest point of the AC input voltage, that is, the AC input current is substantially terminated by the inductor L1 and the diode D5 to the filter capacitor C1, and flows through the inductor. The current on L1 is already small, so that the inductor L1 is not in a saturated operating state when the PFC circuit 3 starts to operate, avoiding the inductor in the PFC circuit 3 under the fast switching condition of the conventional switching power supply as shown in FIG. L1 will produce saturation and cause the destruction of electronic components such as power transistor Q1, and avoid the reverse leakage current caused by the conventional switching power supply shown in Fig. 2 due to the processing of the diode D7 component or the poor raw material itself. When working for a long time, the temperature of the body of the diode D7 continues to rise, which eventually causes the power board to be carbonized and burned by the diode D7.

While the present invention has been described above in terms of the preferred embodiments thereof, it is not intended to limit the invention, and the present invention may be modified and modified without departing from the spirit and scope of the invention. The scope of protection is subject to the definition of the scope of the patent application.

7. . . Supply voltage open circuit

71. . . AC input voltage sampling end

72. . . PFC output voltage sampling end

73. . . Power supply output

74. . . External supply voltage input

75. . . Comparison circuit

76. . . Switch circuit

761. . . First end

762. . . Second end

763. . . Control terminal

C3. . . Capacitor

D6. . . Dipole

OP1. . . Operational Amplifier

Q2. . . NPN type transistor

Q3. . . PNP type transistor

R2~R8. . . Resistor

Claims (7)

A power factor correction circuit of a Power Factor Correction (PFC) circuit includes an AC input voltage sampling terminal, a PFC output voltage sampling terminal, a power supply output terminal, an external power supply voltage input terminal, a comparison circuit, and a Switching circuit, where:
The comparison circuit includes a diode, a capacitor, an operational amplifier, and a first to fourth resistor, wherein the AC input voltage sampling end is coupled to the diode positive end, the diode negative end The first end of the capacitor and the other end of the second resistor are coupled to the negative input terminal of the operational amplifier, and the other end of the capacitor is coupled to the negative input terminal of the operational amplifier. The PFC output voltage sampling end is coupled to one end of the third resistor, the other end of the third resistor is coupled to the operational amplifier positive input end and the fourth resistor end, and the other end of the fourth resistor is grounded. An operational amplifier input end is coupled to the external supply voltage input terminal, and the operational amplifier ground is grounded;
The first end of the switch circuit is coupled to the external power supply voltage input end, the second end of the switch circuit is coupled to the power supply output end, and the switch circuit control end is coupled to the output end of the operational amplifier, wherein the switch circuit One end and the second end are turned on or off according to the signal level received by the control terminal. The power supply voltage-on circuit of the PFC circuit of the first aspect of the invention, wherein the switch circuit comprises a first type of transistor, a second type of transistor, and a fifth to seventh resistor, wherein The switch circuit control end is coupled to one end of the fifth resistor, and the other end of the fifth resistor is coupled to the first type transistor control end, the first type of transistor first end is grounded, the first type of electricity The second end of the second transistor is coupled to one end of the sixth resistor, and the other end of the sixth resistor is coupled to one end of the seventh resistor and the second type transistor control end. Connected to the other end of the seventh resistor and the first end of the switch circuit, the second end of the second type transistor is coupled to the second end of the switch circuit, wherein the first type and the second end of the first type transistor The terminal is turned on or off according to the signal level received by the control terminal, and the first end and the second end of the second type transistor are turned on or off according to the signal level received by the control terminal, when the first type of electricity The second type of transistor is turned on when the crystal is turned on, and when the first type of transistor is turned off The second type of transistor is broken. The power supply voltage opening circuit of the PFC circuit according to claim 2, wherein the first type transistor is an NPN type transistor, and the second type transistor is a PNP type transistor, the first type And the first end, the second end and the control end of the second type transistor are an emitter end, a collector end and a base end, respectively. A switching power supply includes an electromagnetic interference (EMI) filter circuit, a bridge rectifier circuit, a PFC circuit, a filter capacitor, a main power conversion circuit, a standby power conversion circuit, a power supply control circuit, and an application The power supply voltage opening circuit according to the first item of the patent scope, wherein a commercial alternating current is subjected to EMI filtering by the EMI filter circuit, and then full-wave rectified by the bridge rectifier circuit is used as the input terminal voltage of the PFC circuit, and the PFC circuit The output end is coupled to the positive end of the filter capacitor, the negative end of the filter capacitor is grounded, and the voltage of the output end of the PFC circuit is subjected to voltage conversion by the main power conversion circuit to output a first DC voltage, and the output voltage of the PFC circuit passes through the standby The power conversion circuit outputs a second DC voltage after voltage conversion, and the standby power conversion circuit outputs a power supply voltage to the power supply control circuit by the internal power supply circuit, and the power supply control circuit determines whether to output the power according to a switch signal. Supply voltage to the external supply voltage input, the exchange The input voltage sampling end is coupled to either end of the output end of the EMI filter circuit, the PFC output voltage sampling end is coupled to the positive end of the filter capacitor, the power supply output end is coupled to the power supply input end of the PFC circuit and the main power conversion Circuit power input. The switching power supply of claim 4, wherein the switching power supply is applied to a liquid crystal display product, and the first DC voltage is used as a power supply input terminal voltage of the liquid crystal panel lamp driving circuit of the liquid crystal display product, The second DC voltage is used as the power supply input terminal voltage of the motherboard image processing circuit of the liquid crystal display product. The switching power supply of claim 4, wherein the PFC circuit comprises an inductor, a rectifying diode, a power transistor, a control chip and a current detecting resistor, wherein the inductor One end is coupled to the output of the bridge rectifier circuit, and the other end of the inductor is coupled to the positive terminal of the rectifier diode and the first end of the power transistor, and the second end of the power transistor is coupled to the control chip current The detection terminal and the current detecting resistor are connected to the control chip output end, and the power output terminal is coupled to the control chip power input terminal. The negative terminal of the rectifier diode is coupled to the positive terminal of the filter capacitor. The switching power supply of claim 6, wherein the power transistor is a field effect transistor, and the first end, the second end, and the control end of the power transistor are a 汲 terminal, a source terminal, and a gate terminal, respectively.