WO2016095194A1

WO2016095194A1 - Power conversion and power factor correction circuit for power supply device

Info

Publication number: WO2016095194A1
Application number: PCT/CN2014/094333
Authority: WO
Inventors: Carre Denise Scheidegger; Hongbin WEI; Gang Yao
Original assignee: GE Lighting Solutions, LLC
Priority date: 2014-12-19
Filing date: 2014-12-19
Publication date: 2016-06-23
Also published as: CN107210681B; CN107210681A

Abstract

A method and a power conversion and power factor correction circuit (100) includes at least one switching mode (90, 92) for power supply device, an AC/DC converter (50) and a DC/DC converter (60) that converts input power received from an input, into output power. The DC/DC converter (60) includes a converter switch (124) electrically coupled with the input. The power conversion and power factor correction circuit (100) also includes a circuit that receives a sensing signal (Vsense) selected by a user, converts the sensing signal (Vsense) into a control signal (Vcontrol) proportional to the output power, compares the control signal (Vcontrol) with a threshold signal, and controls the switching mode (90, 92).

Description

POWER CONVERSTION AND POWER FACTOR CORRECTION CIRCUIT FOR POWER SUPPLY DEVICE

I. Technical Field

The technical field relates generally to system having a power supply device (e.g., an indoor lighting system having a light emitting diode (LED) lighting driver or power supply) . In particularly, the present invention relates to controlling high power factor (PF) and total harmonic distortion (THD) LED lighting driver or power supply.

II. Background

Heat management plays an important role in an indoor lighting system. Therefore, dimming has become popular with the use of LEDs in the indoor lighting system.

PF is a ratio of the actual output power and the power drawn from the power source. A PF of “1” is typically desired. A current method for achieving PFC in low power applications is transition mode (TM) control.

Some integrated circuits (ICs) may include good PF ratio and low THD for a particular type of load such as a fixed load having a wide input voltage. In other ICs, PF and the THD worsens when the load changes to a wide range. For example, from full power to a low percentage (e.g., 3％) of the full power load.

Further, the PF and THD also worsen during deep dimming modes, especially for high line inputs. A boost circuit is typically used in the dimming circuit, and is controlled to operate at a TM with peak current control in order to save costs. When the power supply works at high line input and in deep diming mode, the current flowing through the boost circuit may be minimal, also the capacitance in the high line input may store a large amount of energy. Thus, during dimming operation, the energy stored may not be fully discharged causing poor THD and PF.

III. Summary of the Embodiments

The various embodiments of the present disclosure are configured to provide a power conversion circuit and a method of controlling ZCD voltage in order to switch mode operations at deep dimming range, for example, in order to achieve gain in the PF and a low THD.

In one exemplary embodiment, power conversion and power factor correction circuit for a power supply device is provided. The power conversion and power factor correction circuit includes a AC/DC converter and a DC/DC converter connected together and configured to convert received input power into output power, the DC/DC converter comprising a converting switch electrically coupled with the input. Also included is a circuit configured for receiving a sensing signal as selected by a user and converting the sensing signal into a control signal proportional to the output power and comparing the control signal with a threshold signal, and controlling a switching mode.

In one exemplary embodiment, a method is provided. The method includes selecting a sensing signal and converting the sensing signal into a control signal proportional to output power of the lighting driver. The method further includes comparing the control signal with a threshold signal of a threshold device； and controlling a switching mode.

The foregoing has broadly outlined some of the aspects and features of various embodiments, which should be construed to be merely illustrative of various potential applications of the disclosure. Other beneficial results can be obtained by applying the disclosed information in a different manner or by combining various aspects of the disclosed embodiments. Accordingly, other aspects and a more comprehensive understanding may be obtained by referring to the detailed description of the exemplary embodiments taken in conjunction with the accompanying drawings, in addition to the scope defined by the claims.

IV. DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a power conversion and power factor correction circuit for a power supply device (e.g., a lighting driver) of an indoor lighting system that can be implemented within one or more embodiments of the present invention.

FIG. 2 is a circuit schematic diagram illustration of an exemplary the power conversion and power factor correction circuit shown in FIG. 1 that can be implemented within one or more embodiments of the present invention.

FIG. 3 is a circuit schematic of a mode switching circuit of the power conversion and power factor correction circuit shown in FIG. 2 that can be implemented within one or more embodiments of the present invention.

FIG. 4 is a flow diagram illustrating an exemplary method of implementing an embodiment of the present invention.

FIG. 5A and 5B are illustrations of the switching point for switching control modes in accordance with embodiments of the present invention.

The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the disclosure. Given the following enabling description of the drawings, the novel aspects of the present disclosure should become evident to a person of ordinary skill in the art. This detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of embodiments of the invention.

V. DETAILED DESCRIPTION OF THE EMBODIMENTS

As required, detailed embodiments are disclosed herein. It must be understood that the disclosed embodiments are merely exemplary of various and alternative forms. As used herein, the word “exemplary” is used expansively to refer to embodiments that serve as illustrations, specimens, models, or patterns. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. In other instances, well-known components, systems, materials, or methods that are known to those having ordinary skill in the art have not been described in detail in order to avoid obscuring the present disclosure. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art.

Exemplary embodiments of the present invention provides method for controlling voltage of a lighting driver within an indoor lighting system by selecting in real-time, a sensing voltage to be used to sense output power to be supplied to the lighting elements of the lighting system, determining whether a voltage at a voltage point is greater than a threshold voltage, controlling the voltage at the voltage point based on a first control mode (e.g., a fixed on time mode (Ton) ) . The controlling occurs when the voltage is greater than the threshold voltage. The voltage is controlled at the voltage point based on a second control mode (e.g., a fixed off time mode (Toff) ) by performing a switching operation when the voltage is less than the threshold voltage.

FIG. 1 is a block diagram illustrating a power conversion and power factor correction circuit for a power supply device (e.g., a lighting driver) for an indoor lighting system that can be implemented within one or more embodiments of the present invention.

As shown in FIG. 1, the power conversion and power factor correction circuit 100 performs power factor correction (PFC) and controls output power in an LED lighting system. The power conversion and power factor correction circuit not limited to performing PFC and controlling power in any particular operations of the LED lighting system, and can be applicable to several operations thereof.

The power conversion and power factor correction circuit 100 includes a filter 20 and an alternate current-to-direct current (AC/DC) converter 50, a boost converter having a direct current-to-direct current (DC/DC) converter 60 and an output power sampling circuit 70. The power conversion and power factor correction circuit 100 provides a sensing voltage during performance of various operations, for example, a dimming operation, for example, of an LED lighting system. The power conversion and power factor correction circuit 100 also switches the working mode of the boost converter.

Input power to the LED lighting system is alternating current (AC) power and is input into the filter 20 for filtering. The filter 20 may be an electromagnetic interference (EMI) type filter for filtering the input power input to an LED lighting driver. The filter 20 includes a plurality of chokes, capacitors and inductors for filtering undesirable EMI noise.

Upon filtering, the AC power is then input into the AC/DC converter 50. During dimming, the voltage is varied to reduce the current flowing through the AC/DC converter 50. The DC voltage from the AC/DC converter 50 is transmitted to the DC/DC converter 60 and then output to LEDs of the lighting system.

According to the embodiments, a sensing voltage V_sense is selected and converted into a control voltage V_control is selected and input to the output power sampling circuit 70, from a selection point, for example, the DC/DC converter 60 (indicated by arrow 82) .

The sensing voltage V_sense may be a signal from a microcontroller, or other device being controlled by a smart signal.

The power conversion and power factor correction circuit 100 linearly senses the driver power of the LED driver in order to switch the operation of the boost converter (i.e., AC/DC converter 50) from a first control mode 90 (e.g., fixed on-time (Ton) ) to a second control mode 92 (e.g., fixed off-time (Toff) ) at low power (dimmed) applications. Thus, the boost PFC working mode is changed to achieve longer switching on time of the boost converter when the control voltage is lower than a threshold voltage, this process results in lower current distortion and high PF, as desired. By way of example, the AC/DC converter 50 may be changed from the fixed Ton control to the fixed Toff control when the control voltage linearly reflecting the output power is lower than the threshold voltage.

The present invention is not limited to the power conversion and power factor correction circuit 100 being applicable to a dimming operation of the LED lighting system. Further, embodiments of the present invention can implement any combination of modes, for example, fixed Toff to fixed Ton and fixed Ton to fixed Toff. The embodiments can also implement a boundary conduction mode (BCM) to continuous conduction mode (CCM) and vice versa.

Further, one or more embodiments of the present invention will now be discussed in Fig. 2 with reference to the power conversion and power correction circuit 100. By way of example, the power conversion and power correction circuit 100 is a dual mode circuit however the present invention is not limited hereto.

FIG. 2 is a circuit schematic diagram illustration of the exemplary power conversion and power factor correction circuit 100 shown in FIG. 1. The power conversion and power factor correction circuit 100 includes a boost converter (i. e., the DC/DC converter circuit 60) . The DC/DC converter circuit 60can be a form of a buck converter, a cuk converter and other type of converter suitable for the purpose set forth herein.

The circuit 100 also includes a bridge rectifier circuit 110 that includes a plurality of diodes for delivering a rectified voltage V_rect into the circuit 100. The DC/DC converter 60converts the rectified voltage V_rect into an output power at the output terminal OUT for output to a load. The DC/DC converter 60 also includes a transformer 115 comprising a primary winding 116 and secondary windings 118.

Additionally, the DC/DC converter 60having a first inductor 120 being the primary windings 116, a diode 122, a converting switch 124 and a plurality of

output capacitors

126 and 128. The converting switch 124 is coupled to the output terminal OUT. The converting switch 124 can include a metal oxide semiconductor field effect transistor (MOSFET) , an insulated gate bipolar transistor (IGBT) or other suitable switch circuits.

The circuit 100 also includes a power factor correction (PFC) control circuit 150 for shaping an input sinusoidal current, at the input of circuit 100, to be in phase with an input sinusoidal voltage and regulate the output power at the output terminal OUT.

According to the embodiments, the PFC control circuit 150 includes a mode switching circuit 130 and a switch controller 140. Details regarding the mode switching circuit 130 will be discussed below with reference to FIG. 3.

As depicted in FIG. 2, the switch controller 140 includes an integrated circuit (IC) and receives a plurality of electrical signals at a plurality of input pins thereof, from the DC/DC converter 60. The switch controller 140 also provides a switch signal to the converting switch 124. The input pins include, for example, a INV pin 1, COMP pin 2, MULT pin 3, CS pin 4, ZCD pin 5, Ground GND pin 6, Gate Drive (GD) pin 7 and VCC pin 8. The switch controller 140 is not limited to a particular type of switch controller and therefore include any switch controller suitable for the purpose set forth herein.

During operation of the circuit 100, the input sinusoidal voltage is applied across the bridge rectifier circuit 110 (i.e., the AC/DC converter 50) , filtered by a capacitor 160, and applied across a voltage divider 162. The controller supply voltage, Vcc, is applied to the switch controller 140 at the Vcc pin 8 and is used to power the switch controller 140.

The converting switch 124 is coupled between the first inductor 120, and when the converting switch 124 is closed the first inductor 120 is connected to ground via resistors 125a, 125b, 125c and 125d, forming a controlled power switch path for charging and discharging of the first inductor 120. A gate of the converting switch 124 is controlled by an output GD pin 7 of the switch controller 140. The gate control is a function of input signals at the plurality of input pins of the switch controller 140 including for example, the ZCD pin 5, the COMP pin 2, the INV pin 1 and the MULT pin 3.

The ZCD pin 5 is coupled to the second inductor 121 via the mode switching circuit 130 for performing zero current detection and mode switching operations. The switch controller 140 generates a startup signal at the GD pin 7 which switches on the converting switch 104. The INV pin 1 and the COMP pin 2 are coupled to an intermediate node between two

resistors

166 and 168. A feedback capacitor 170 is provided for frequency compensation. The switch controller 140 compares a sensed signal at the output terminal OUT with an internal reference voltage. The comparison maintains the output voltage at output terminal OUT constant by turning on and off of the converting switch 124. A voltage across resistors 125a, 125b, 125c and 125d is applied to the CS pin 4 to determine a time when the converting switch 124 is switched off. Further, the MULT pin 3 is coupled between resistors of the voltage divider 162 to receive a power signal from the rectified power line (i.e., a portion of the V_rect) . The power signal is used to set a peak current of the converting switch 104. Further, a capacitor 172 is coupled with the MULT pin 3 for filtering purposes.

As shown in FIG. 2, the mode switching circuit 130 is coupled with the switch controller 140 and works in concert with the switch controller 140 to maintain a high PF and low THD for wide input range and wide output range. The mode switching circuit 130 is coupled with the second inductor 121, and the ZCD pin 5 of the switch controller 140. The mode switching circuit 130 receives a sensed voltage (V_sense) from the second inductor 121, linearly reflecting the output power. The mode switching circuit 130 further receives a selected sensing voltage V_sense and converts the sensing voltage V_sense to a control voltage V_control and performs a comparison of the control voltage V_control and a threshold voltage to determine a switching point for switching between two switching modes. Additional details regarding the mode switching circuit 130 will be discussed below with reference to FIG. 3.

FIG. 3 is a circuit schematic of the mode switching circuit 130 of the power conversion and power factor correction circuit 100 shown in FIG. 2 that can be implemented within one or more embodiments of the present invention. The mode switching circuit 130 comprises a mode decision unit 180 and an external unit 200.

According to one or more embodiments, the mode decision unit 180 includes a rectifier diode 182 coupled to a voltage divider 184 having a plurality of

resistors

186 and 188, a threshold device (e.g., a Zener diode 190) which determines a threshold signal, a plurality of

capacitors

191 and 192 coupled between the voltage divider 184 and the threshold device 190 for filtering. The mode decision unit 180 also includes a switch 193, and a capacitor 194 and a resistor 195 coupled between an anode of the threshold device 190 and a gate of the switch 193.

The mode decision unit 180 is configured to receive sensing voltage V_sense from the second inductor 121 of the ZCD circuitry. The sensing voltage V_sense is sensing the voltage change across the second inductor 121. This voltage decreases as the output power decreases because less current is flowing through the primary winding 116 of the first inductor 120. Thus, the voltage linearly reflects the power by decreasing as the output power decreases, providing an indication of when the power supply device (e.g., the lighting driver) is dimming. Based on the sensing voltage, a control voltage V_control is determined from the output sampling circuit 70 depicted in FIG. 1, as determined by the user. The sensing voltage V_sense is rectified via the rectifier diode 182 and converted to a control voltage V_control. The control voltage V_control changes with the sensing voltage V_sense. That is, the control voltage V_control linearly reflects the sensing voltage V_sense. Thus, both the sensing voltage V_sensing and the control voltage Vcontrol is proportional to the output power. Since the output voltage is maintained constant, the output power is proportional to an output current or current flowing through the first inductor 120 or the second inductor 121 of the transformer 115. Thus, in one or more embodiments, a power sensor or current sensor may be coupled with the first or

second inductors

120 and 121 to sense the output power directly and output the sensing voltage V_sense.

The mode decision unit 180 then supplies the control voltage V_control to the threshold device 190 between the

resistors

186 and 188 of the voltage divider 184. A threshold signal is generated at the threshold device 190 and is selectable. The control voltage V_control is therefore tunable to any selected threshold signal. According to one embodiment, the threshold signal may be determined based on a reverse breakdown voltage of the threshold device 190. The

resistors

186 and 188 are configured to limit a current flowing through the threshold device 190 to protect the threshold device 190 and provide a voltage proportional to the control voltage V_control to the threshold device 190.

When the control voltage V_control is greater than the threshold signal, a voltage across the threshold device 190 maintains a constant value (e.g., 6V) . The current flowing through the threshold device 190 is then transmitted to the switch 193 and the voltage is supplied across the switch 193 and the switch is switched on. Power is then supplied to the external circuit 200 coupled with the mode decision unit 180.

According to one or more embodiments, the external circuit 200 includes a diode 202 having a cathode coupled with the ZCD pin 5 and an anode coupled with the drain of the switch 193. The anode of the diode 202 is also coupled with a resistor 204 at an intermediate node. When the switch 193 is switched on, the voltage level at the anode of the diode 202 is zero. As a result, the external circuit 200 is blocked, no electrical signal from the gate of the converting switch 104 (depicted in FIG. 2) flows into the ZCD pin 5 (also depicted in FIG. 2) , and the boost converter is operating in control mode 1 (Fixed on time (Ton) control mode) .

The external circuit 200 further includes a resistor 204 coupled with the cathode of the diode 202 at an intermediate node. The external circuit 200 also includes a plurality of

capacitors

206 and 207, a resistor 210, and a diode 212 coupled with the GD pin 7 of the switch controller 140.

Alternatively, when it is determined at the threshold device, that the control voltage V_control is less than the threshold signal, the threshold device 190 is blocked and the switch 193 is switched off. The resistor 195, and the switch 193 or the threshold device 190, the resistor 188 and the switch 193 form a circuit path for dissipating power in the switch 193. A drive voltage at the gate of the converting switch 124 is supplied to the ZCD pin 5 as the ZCD signal. The switch controller 140 is configured to generate a second switch signal to the converting switch 124 and switches to a control mode 2 (fixed off time (Toff) control mode) . That is, when the switch 193 is open (i.e., turned off) the external circuit 200 is therefore able to electrically supply the ZCD pin 5 directly. The external circuit 200 is based on the switch controller’s fixed off time mode (Toff) (control mode 2) . Since the circuit 200 is electrically supplying ZCD Pin 5 through diode 202, the PFC control circuit is altered to be Toff.

The illustrious embodiments therefore enables a specific sensing voltage V_sense to be selected as desired since the sensing voltage V_sense reflects the dimming of the output power. Thus, the output power can be sensed by a control voltage Vcontrol that is decreasing, to thereby control the switching point of the modes from control mode 1 to control mode 2 and vice versa.

FIG. 4 is a flow diagram illustrating an exemplary method 400 of implementing an embodiment of the present invention. The method 400 begins at operation 410 where a sensing voltage is selected and converted to a control voltage proportional to output power. From operation 410 the process continues to operation 420. In operation 420, a determination is made as to whether the control voltage in the mode decision unit 180, is greater than or less than a threshold voltage at a threshold device. In operation 430, an on and off state of the switch 193 is performed along with switching of the control modes based on the outcome of the determination.

FIG. 5A and 5B are illustrations of the switching point for switching control modes in accordance with the embodiments of the present invention.

In Fig. 5A, PF data for a standard lighting driver, as shown by line A, depicts the PF level at 0.6 when the output power is approximately 20.34 watts (W) . The PF data for a PF improved lighting driver is depicted by line B and shows PF level at 0.85 when output power is at approximately 20.34W.

Additionally, the PF level of the PF improved lighting driver is depicted by line C and shows the improved PF and a continued improvement in the PF after the switching mode point. Line C shows the added Fixed on Time (Ton) control mode being used at the lowest dimming ranges to improve PF and THD.

As shown in FIG. 5B, the THD plot shows at approximately 20.34 W, and the standard lighting driver at line A1 having a THD level of approximately 40％. The THD plot also shows PF improved lighting driver at line B1, having a THD level at approximately 30％and the PFC successfully initiated to Fixed on time (Ton) control mode with ZCD voltage sensing. A THD improvement of approximately 22％is depicted by line C1.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Parts List

Figure 1

100 –Power Conversion and Power Factor Correction circuit

20 –Filter

50 –AC/DC converter

60 –DC/DC converter

70 –Sampling circuit

82 –Arrow

90 –Control mode 1

92 –Control mode 2

Figure 2

100 –Power Conversion and Power Factor Correction circuit

50 –AC/DC converter

110 –Bridge rectifier circuit

115 - Transformer

116 –Primary winding

118 –Secondary windings

120 –First inductor

121 –Second inductor

122 –Diode

124 –Converting switch

125a, b, c, d -Resistors

126 –Output capacitor

128 –Output capacitor

130 –Mode Switching circuit

140 –Switch controller

150 –PFC control circuit

160 –Capacitor

162 –Voltage divider

166 –Resistor

168 –Resistor

170 –Feedback capacitor

172 –Capacitor

Figure 3

130 –Mode Switching circuit

182 –Rectifier diode

184 –Voltage divider

186 –Resistor

188 –Resistor

190 –Threshold device

191 –Capacitor

192 –Capacitor

193 –Switch

194 –Capacitor

195 –Resistor

200 –External circuit

202 –Diode

204 –Resistor

206 -Capacitor

207 –Capacitor

210 -Resistor

212 –Diode

Figure 4

400 –Exemplary method

410, 420, 430 –Steps

Claims

A power conversion and power factor correction circuit including at least one switching mode for power supply device comprising:

an AC/DC converter and a DC/DC converter connected together and configured to convert input power, into output power, and the DC/DC converter comprising a converter switch electrically coupled with the input；

a circuit configured to:

receive a sensing signal selected by a user,

convert the sensing signal into a control signal proportional to the output power,

compare the control signal with a threshold signal, and

control the switching mode.
The power conversion and power factor correction circuit of claim 1, wherein the power supply device comprises a lighting driver.
The power conversion and power factor correction circuit of claim 1, wherein when the control signal is greater than the threshold signal, the mode switch operation switches to a first control mode and when the control signal is less than the threshold signal, the mode switch operation switches to a second control mode.
The power conversion and power factor correction circuit of claim 3, wherein the first control mode is a fixed on time control mode, and the second control mode is a fixed off time control mode.
The power conversion and power factor correction circuit of claim 1, wherein the sensing signal is selected from an external control device.
The power conversion and power factor correction circuit of claim 1, wherein the circuit comprises:

a mode switching circuit configured to:

receive the sensing signal and convert the sensing signal into the control signal proportionally reflecting the output power； and

compare the control signal with the threshold signal to output a ZCD signal.
The power conversion and power factor correction circuit of claim 6, further comprising:

a switch controller coupled with the mode switching circuit and configured to drive the converting switch based on the ZCD signal received.
The power conversion and power factor correction circuit of claim 6, wherein the mode switching circuit comprises:

a mode decision unit comprising a switch and a threshold device coupled with a gate of the switch, and configured to compare the control signal to the threshold signal and determine the ZCD signal to be generated, and an on or off state of the switch.
The power conversion and power factor correction circuit of claim 8, wherein when the control signal is greater than the threshold signal, the switch is turned to an on state, and the DC/DC converter is operating in the first control mode, and when the control signal is less than the threshold signal, the switch is turned to an off state, and the DC/DC converter is operating in the second control mode.
The power conversion and power factor correction circuit of claim 8, wherein the threshold device comprises a Zener diode, and a reverse breakdown voltage of the Zener diode is the threshold signal.
The power conversion and power factor correction circuit of claim 7, wherein the mode switching circuit further comprises an external circuit in communication with the mode decision unit and configured to generate the ZCD signal when the control signal is less than the threshold signal.
A power factor correction circuit including at least one switching mode for a power supply device, comprising:

a mode switching circuit configured to:

receive a sensing signal selected by a user,

convert the sensing signal to a control signal proportional to output power,

compare the control signal with a threshold signal, and

control the at least one switching mode.
The power factor correction circuit of claim 12, wherein the power supply device comprising a lighting driver.
The power factor correction circuit of claim 12, wherein the mode switching circuit comprises:

a threshold device configured to receive the control signal, and to control a ZCD signal, by comparing the control signal with a threshold signal of the threshold device.
The power factor correction circuit of claim 14, wherein the mode switching circuit further comprises:

a mode decision unit comprising a switch and the threshold device coupled with a gate of the switch, and configured to compare the control signal to the threshold signal and determine the ZCD signal to be generated, and an on or off state of the switch.
The power factor correction circuit of claim 15, wherein when the control signal is greater than the threshold signal, the switch is turned to an on state, and the DC/DC converter is operating in the first control mode, and when the control signal is less than the threshold signal, the switch is turned to an off state, and the DC/DC converter is operating in the second control mode.
The power factor correction circuit of claim 15, wherein the threshold device comprises a Zener diode, and a reverse breakdown voltage of the Zener diode is the threshold signal.
The power factor correction circuit of claim 15, wherein the mode switching circuit further comprises an external circuit in communication with the mode decision unit and configured to generate the ZCD signal when the control signal is less than the threshold signal.
A method comprising:

selecting a sensing signal；

converting the sensing signal into a control signal proportional to output power；

comparing the control signal with a threshold

signal of a threshold device； and

controlling a switching mode.
The method of claim 19, further comprising:

switching, aswitch, between an on and off state based on the comparison of the control signal and the threshold signal,

wherein when the control signal is greater than the threshold signal, the switch is turned to an on state, and the switching mode includes switching to a first control mode, and

when the control signal is less than the threshold signal, the switch is turned to an off state, and the switching mode includes switching to a second control mode.
The method of claim 20, further comprising:

generating a ZCD signal by an external circuit when the control signal is less than the threshold signal.