Patent Publication Number: US-2022216783-A1

Title: Control circuit with high power factor and ac/dc converter

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This application is a continuation of International Patent Application No. PCT/CN2020/117483, filed on Sep. 24, 2020, entitled “CONTROL CIRCUIT WITH HIGH POWER FACTOR AND AC/DC CONVERSION CIRCUIT”, and the entire disclosure of which is incorporated herein. 
     TECHNICAL FIELD 
     The present invention relates to control circuits and AC/DC converters, and more particularly to a high power factor control circuit and AC/DC converter. 
     BACKGROUND 
     A single stage light-emitting diode (LED) controller with power factor correction that has the advantages of a good power factor, satisfactory harmonic current control, a streamlined circuit architecture and high cost effectiveness. However, despite the high power factor of such LED controllers, they tend to suffer from significant line frequency ripple in the output current due to a relative small DC bus capacitance and a bus voltage having the line frequency. 
       FIG. 1  shows a conventional LED control circuit  90  including a bridge rectifier  91 , a DC bus capacitor  92 , a buck converter  93 , a high power factor controller  94 , a load  95  and a switching component  96 . When receiving an AC mains supply, the bridge rectifier  91  produces a DC current, which flows into the DC bus capacitor  92 , resulting in a bus voltage (V bus ). As previously mentioned, this bus voltage (V bus ) is a voltage having the line frequency. Waveforms of a current flowing through an inductive element (I L ) and an output current flowing through the load (I O ) in the buck converter  93  are shown in  FIG. 2 . The output current (I O ) is averaged from the inductor current (I L ), and the output current flowing through the load have ripple relative with the AC mains. A value of the ripple is positively correlated with a maximum peak value of the inductor current (I L ). 
     Ripple may not only shorten a service life of an affected component but may also become a reason for flickering of an LED. Therefore, for single stage LED controllers with power factor correction, it is important to control their ripple within a reasonable range. 
     SUMMARY OF THE INVENTION 
     It is a principal object of the present invention to solve the problem of excessive line frequency ripple with the conventional single stage LED controllers with power factor correction. 
     The above object is attained by a high power factor control circuit according to the present invention, which is used in an AC/DC converter. The AC/DC converter includes a rectification module, a conversion module coupled to the rectification module and a load driven by the conversion module. The rectification module is configured to receive AC power and to rectify it into a DC current. The conversion module is configured to convert the DC current to drive power as desired by the load and to provide it to the load and includes a conversion element and a switching element coupled to the conversion element. The conversion element includes an inductive element, and the switching element is configured to regulate a current flowing through the load, wherein the current flowing have ripple relative with the AC power. The control circuit includes: a peak limiting signal generator configured to receive a reference signal and a sample signal, the sample signal indicating the output current flowing through the load, and to output at least one peak limiting signal indicating a peak current according to the peak limiting signal and the sample signal; and a switching element control module coupled to the switching element, the switching element control module configured to control switching of the switching element based on the peak limiting signal so that, within at least half a line-frequency period, a value of the ripple in the output current flowing through load is not greater than a limit value. 
     According to one embodiment, the high power factor control circuit may further include a feedback unit, wherein the feedback unit may include: a sample circuit coupled to the switching element, the sample circuit configured to sample the output current though the load and to output the sample signal. 
     According to one embodiment, the feedback unit may include: a sampler circuit coupled to the switching element, the sampler circuit configured to sample the current though the inductive element and output a first sample signal; and a sample processor configured to generate the sample signal according to the first sample signal. 
     According to one embodiment, the peak limiting signal generator may further include a compensation module configured to produce a compensation signal from the reference signal and the sample signal. 
     According to one embodiment, the compensation module may include an error amplifier and a filter coupled to the error amplifier, the error amplifier configured to receive the reference signal and the sample signal, and to generate an error from the reference signal and the sample signal, the error being provided to the filter, the filter outputting the compensation signal. 
     According to one embodiment, the peak limiting signal generator may include a current limiting module configured to receive the compensation signal and to generate the peak limiting signal according to the compensation signal. 
     According to one embodiment, the switching element control module may further include a minimum OFF time generator configured to produce a minimum OFF time signal according to the compensation signal. 
     According to one embodiment, the current limiting module may include an amplifier and a resistor string, the amplifier including an output terminal, a first input terminal for receiving the compensation signal and a second input terminal coupled to the output terminal, the resistor string coupled to the output terminal and configured to output the peak limiting signal. 
     According to one embodiment, the current limiting module may include an amplifier and a resistor string, the amplifier including an output terminal, a first input terminal for receiving the compensation signal and a second input terminal coupled to the resistor string, wherein the peak limiting signal is output from a node between the output terminal of the amplifier and the resistor string. 
     According to one embodiment, the switching element control module may include a logic module and a driver coupled to the logic module, the logic module configured to produce a switching signal for controlling the switching element and to provide it to the driver. 
     According to one embodiment, the switching element control module may further include a delay configured to receive an ON signal and generate a first control signal for a constant-on-time mode of the AC/DC converter, the first control signal reflecting a maximum ON time for the switching element. 
     According to one embodiment, the switching element control module may further include a comparator configured to generate the second control signal based on a comparison of the peak limiting signal and the sensed signal. 
     According to one embodiment, the first and second control signals may determine an OFF control signal reflecting OFF timing for the switching element. 
     According to one embodiment, the switching element control module may further include a demagnetization detector including an input terminal coupled to the switching element and an output terminal configured to output a demagnetization signal to the logic module. 
     According to one embodiment, the high power factor control circuit may further include a dimming module configured to output the reference signal to the switching element control module, wherein the load is an LED lamp dimmed by the reference signal. 
     According to one embodiment, the peak limiting signal may vary with at least one factor selected from a group including the sample signal, the reference signal and the load. 
     The above object is also attained by a high power factor AC/DC converter, including: a rectification module configured to receive AC power and rectify it into a DC current; a conversion module coupled to the rectification module and configured to convert the DC current to drive power as desired by the load and provide it to the load, wherein the conversion module comprises a conversion element and a switching element coupled to the conversion element, the conversion element comprising an inductive element, the switching element configured to regulate a current flowing through the load, wherein the current flowing have ripples relative with the AC power; and a control module including: a peak limiting signal generator configured to receive a reference signal and a sample signal, the sample signal indicating the output current flowing through the load, and to output at least one peak limiting signal indicating a peak current according to the peak limiting signal and the sample signal; and a switching element control module coupled to the switching element, the switching element control module configured to control the switching element on and off, wherein the switching element control module controls the state of switching element according to the peak limiting signal and a sensed signal indicating a current flowing through the inductive element so that, within at least half a line-frequency period, a value of the ripple in the output current flowing through the load is not greater than a limit value. 
     According to one embodiment, the conversion module may be selected from a group consisting of a floating buck converter, a boost converter, a flyback converter and a buck-boost converter. 
     According to one embodiment, the load may be an LED lamp. 
     According to the present invention, the peak limiting signal produced from the sample signal varies with the sample signal, meaning that it depends on the luminous brightness, the bus voltage, the output voltage or the output current. In this way, both a peak current though the inductor and the peak threshold value of the current reflected by the sample signal vary over each line-frequency period, resulting in reduced ripple in the output current, which in turn extends the service life of any affected component and mitigates the LED&#39;s flickering issue. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional LED control circuit. 
         FIG. 2  schematically illustrates waveforms of an inductor current (I L ) and an output current (I O ) in a conventional LED control circuit. 
         FIG. 3  is a schematic illustration of a high power factor control circuit according to another embodiment of the present invention. 
         FIG. 4  is a schematic illustration of a high power factor control circuit according to another embodiment of the present invention. 
         FIG. 5  is a schematic illustration of a high power factor control circuit according to another embodiment of the present invention. 
         FIG. 6  is a schematic illustration of a high power factor control circuit according to another embodiment of the present invention. 
         FIG. 7  is a schematic illustration of a high power factor control circuit according to another embodiment of the present invention. 
         FIG. 8A  schematically illustrates an arrangement of a current limiting module according to an embodiment of the present invention. 
         FIG. 8B  schematically illustrates an arrangement of the current limiting module according to another embodiment of the present invention. 
         FIG. 9  schematically illustrates waveforms of an inductor current (I L ) and an output current (I O ) in a control circuit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described in detail below in connection with the accompanying drawings. 
     The present invention discloses a high power factor control circuit and an AC/DC converter, the high power factor control circuit can be applied in the AC/DC converter  10 .  FIG. 3  is a schematic illustration of a high power factor control circuit according to an embodiment of the present invention. The AC/DC converter  10  includes a rectification module  11 , a conversion module  12  and a load  13 . The conversion module  12  is coupled to the rectification module  11  and is configured to drive the load  13 . The rectification module  11  is configured to receive an alternating current AC and rectify it into a direct current DC. The rectification module  11  includes a rectifier  111  and a capacitor  112 . The conversion module  12  is configured to convert the direct current DC to drive power as required by the load  13  and provide the drive power to the load  13 . The conversion module  12  includes a conversion element  121  and a switching element  122 . The conversion element  121  includes an inductor  123 , a capacitor  124  and a diode  125 . The switching element  122  may be a power switching device such as, without limitation, a metal-oxide-semiconductor field-effect transistor (MOSFET). Alternatively, the power switching device may be one or more triodes. The switching element  122  is configured to switch between an ON state and an OFF state. The switching element  122  includes a drain terminal D, a source terminal S and a gate terminal G. In this embodiment, the load  13  is an LED lamp. 
     The control circuit includes a peak limiting signal generator  20  and a switching element control module  30 . The peak limiting signal generator  20  is configured to receive a reference signal and generate, from a sample signal from the switching element  122 , at least one peak limiting signal. In this embodiment, the switching element control module  30  is coupled to the gate terminal G of the switching element  122 . The switching element control module  30  is configured to control the switching element  122  on and off, wherein the switching element control module controls the state of switching element according to the peak limiting signal and a sensed signal indicating a current flowing through the inductive element so that, within at least half a line-frequency period, when a voltage on the inductor  123  is higher than a threshold, ripple in a current flowing through the inductor  123  is not greater than a limit value, which may vary with at least one factor selected from at least any of the sample signal, the reference signal and the load  13 . In this embodiment, the control circuit further includes a feedback unit  40  and a dimming module  50 . The feedback unit  40  is configured to sample a current though the inductor  123  and responsively output the sample signal. The feedback unit  40  may include a sampling resistor  41  and a sample processor  42 . The sampling resistor  41  may be coupled to the switching element  122  and configured to sample the current though the inductor  123  and responsively output a first sample signal. The sample processor  42  may produce a second sample signal from the first sample signal and output it to the peak limiting signal generator  20 . According to the present invention, the sample signal may be either the first sample signal or the second sample signal, depending on circumstances as appropriate. Further, the dimming module  50  may be controlled internally or externally to output the reference signal which may be considered as a dimming signal. 
     In this embodiment, the peak limiting signal generator  20  includes a compensation module  21  and a current limiting module  22 . The compensation module  21  includes an error amplifier  211  and a filter  212 , the filter  212  is coupled to the error amplifier  211 . The error amplifier  211  is configured to receive the reference signal from the dimming module  50  and the sample signal, generate an error from the reference signal and the sample signal and provide the error to the filter  212 . In this embodiment, the error amplifier  211  is configured to receive the reference signal and the second sample signal. The filter  212  is configured to output a compensation signal, which is then received by the current limiting module  22  that is coupled to the compensation module  21 . The current limiting module  22  is configured to generate, according to the compensation signal, the peak limiting signal that reflects a voltage limit. According to the present invention, the filter  212  may be replaced with another circuit (e.g., a digital low-pass filter) or an analog low-pass filter, which is capable of filtering the error and outputting a DC current signal. For example, it may be provided by a combination of a transconductance amplifier and a capacitor. 
     The switching element control module  30  includes a driver  31 , a logic module  32 , a demagnetization detector  33 , a first control module  34  and a second control module  35 . The first control module  34  is coupled to the current limiting module  22  in the peak limiting signal generator  20 . The first control module  34  includes a comparator  341 , a delay  342  and an OR gate  343 . The driver  31  is coupled to the gate terminal G of the switching element  122  and configured to switch on or off the switching element  122  based on a switching signal from the logic module  32 . The first control module  34  is configured to produce an OFF control signal for controlling OFF timing (Turn-off timing) for the switching element  122 , and the second control module  35  is configured to produce a minimum OFF time signal for setting a minimum OFF time (Turn-off duration) for the switching element  122 . 
     The delay  342  is configured to receive the switching signal and generate a first control signal for a constant-on-time mode of the AC/DC converter, the first control signal reflecting a maximum ON time for the switching element  122 . The first control signal is used to determine the OFF timing for the switching element  122 . The comparator  341  has a first input terminal  341   a,  a second input terminal  341   b  and an output terminal  341   c.  The first input terminal  341   a  is configured to receive a sensed signal indicating a current flowing through the inductor. For example. The first input terminal  341   a  is configured to receive the first sample signal, and the second input terminal  341   b  is configured to receive the peak limiting signal from the current limiting module  22 . The comparator  341  is configured to produce a second control signal from both the peak limiting signal and the first sample signal and output it at the output terminal  341   c  to the logic module  32 . In other words, the comparator  341  determines, based on a comparison drawn between the peak limiting signal and the first sample signal, and outputs the second control signal that reflects a comparison result between the current sample signal (voltage or current) and a threshold limit (voltage or current limit), which is used to determine the OFF timing for the switching element  122 . In this embodiment, current sample signal and the threshold limit are both voltages, and the first input terminal  341   a  is configured to receive the first sample signal. Accordingly, the comparator  341  is configured to produce, based on the peak limiting signal and the first sample signal, and output the second control signal. The first and second control signals are in turn fed to the OR gate  343 , the OR gate  343  then produces the OFF control signal from the first and second control signals and provides it to the logic module  32 . The OFF control signal determines the OFF timing for the switching element  122 . 
     The second control module  35  is coupled to the compensation module  21  in the peak limiting signal generator  20 . The second control module  35  may be implemented as a minimum OFF time generator, which is configured to produce, according to the compensation signal received from the compensation module  21 , and provide, to the logic module  32 , a third control signal reflecting the minimum OFF time for the switching element  122 . That is, the third control signal provides the minimum OFF time signal. In other words, the third control signal is used to determine an ON time for the switching element  122 . The demagnetization detector  33  is coupled between an output terminal of the logic module  32  and the driver  31 . The demagnetization detector  33  has an input terminal and an output terminal. The demagnetization detector  33  is coupled to an output terminal of the driver  31  and configured to produce a demagnetization signal from an output from the driver  31  and provide the demagnetization signal to the logic module  32 . 
     The logic module  32  is configured to receive the OFF control signal, the minimum OFF time signal and the demagnetization signal and to produce the switching signal therefrom, which is fed to the driver  31 . As described above, the OFF control signal functions to switch off the switching element  122  when either of the following occurs (whichever is earlier): expiry of the maximum ON time for the switching element  122 ; and reaching of the threshold limit (voltage or current limit) by the current sample signal (voltage or current). On the other hand, the minimum OFF time signal functions to cause the switching element  122  to operate in a discontinuous conduction mode (DCM) in response to both a low luminous brightness and a short ON time. 
     According to the present invention, the converter  12  may be implemented as a floating buck converter, a boost converter, a flyback converter or a buck-boost converter, and specific circuit examples of some of these configurations will be explained below.  FIG. 4  schematically shows a high power factor control circuit according to another embodiment of the present invention. In this embodiment, the converter  12  is implemented as a floating buck converter, with the switching element control module  30  being coupled to the gate terminal G of the switching element  122 . The conversion element  121  includes an inductor  123 , a capacitor  124 , a diode  125  and a resistor  126 . The capacitor  124 , the diode  125  and the load  13  are connected in parallel, and the inductor  123  and the resistor  126  are connected in series between one end of the capacitor  124  and one end of the diode  125 . The other end of the capacitor  124  is grounded.  FIG. 5  schematically illustrates a high power factor control circuit according to another embodiment of the present invention. In this embodiment, the conversion module  12  is implemented as a boost converter, with the switching element control module  30  being coupled to the gate terminal G of the switching element  122 . 
     The conversion element  121  includes an inductor  123 , a capacitor  124  and a diode  125 . The capacitor  124  and the load  13  are connected in parallel, and the inductor  123  and the diode  125  are connected in series between the capacitor  124  and the capacitor  112 . The drain terminal D of the switching element  122  is coupled between the inductor  123  and the diode  125 . 
       FIG. 6  schematically illustrates a high power factor control circuit according to another embodiment of the present invention. In this embodiment, the conversion module  12  is implemented as a flyback converter, with the switching element control module  30  being coupled to the gate terminal G of the switching element  122 . The conversion element  121  includes an inductor  123 , a capacitor  124  and a diode  125 . The capacitor  124  and the load  13  are connected in parallel, and the inductor  123  is a transformer having a first side and a second side. The first side is coupled to both the capacitor  112  and the drain terminal D of the switching element  122 , and the second side is connected in parallel with the capacitor  124 . The diode  125  is coupled to between the capacitor  124  and the second side.  FIG. 7  schematically illustrates a high power factor control circuit according to another embodiment of the present invention. In this embodiment, the conversion module  12  is implemented as a buck-boost converter, with the switching element control module  30  being coupled to gate terminal G of the switching element  122 . The conversion element  121  includes an inductor  123 , a capacitor  124  and a diode  125 . The inductor  123 , the capacitor  124  and the load  13  are connected in parallel, and the diode  125  is coupled between the inductor  123  and the capacitor  124 . The above configurations are merely exemplary, and in other embodiments of the present invention, other converter configurations are also possible. 
     According to the present invention, the current limiting module  22  may be configured as shown in  FIG. 8A  to include an amplifier  221 , a first resistor  222  and a second resistor  223 . The amplifier  221  includes a first input terminal  221   a,  a second input terminal  221   b  and an output terminal  221   c.  The first input terminal  221   a  is configured to receive the compensation signal, and the second input terminal  221   b  is coupled to the output terminal  221   c.  One end of the first resistor  222  is coupled to the output terminal  221   c  of amplifier  221 , and the other end is coupled to the second resistor  223  that is grounded. The peak limiting signal (CS_limit) is output from a node between the first resistor  222  and the second resistor  223 . Alternatively, the current limiting module  22  may be configured as shown in  FIG. 8B . In this configuration, an output from a node between the first resistor  222  and the second resistor  223  is fed to the second input terminal  221   b,  and the peak limiting signal (CS_limit) is output from a node between the output terminal  221   c  and the first resistor  222 . The above configurations are merely exemplary, and in other embodiments of the present invention, other configurations of the current limiting module are also possible. 
       FIG. 9  schematically illustrates waveforms of a current flowing through an inductive element (I L ) and an output current flowing through the load (I O ) in a control circuit according to the present invention. Under the effect of the peak limiting signal produced by the peak limiting signal generator from the sample signal, as can be seen in  FIG. 9 , the current flowing through the inductor is limited within a peak threshold value (IL_limit), thereby reducing ripple in the current. Moreover, the minimum OFF time signal produced by the second control module can prevent an undesirable short ON time at low brightness, despite limiting the current within the peak threshold value (IL_limit). 
     Further, the peak limiting signal produced from the sample signal varies with the sample signal, meaning that it depends on the luminous brightness, the bus voltage, the output voltage or the output current. In this way, both a peak current though the inductor and the peak threshold value of the current reflected by the sample signal vary over each line-frequency period, resulting in reduced ripple in the output current, which in turn extends the service life of any affected component and mitigates the LED&#39;s flickering issue. Furthermore, the same amount of reduction in output current ripple can be achieved by a smaller output capacitor, resulting in a reduction in cost.