Linear constant current LED power supply

Abstract

A linear constant current LED power supply is provided, including a first capacitor, a second capacitor, a first switch, a second switch, and a third switch. The first and second output terminals of the linear constant current LED power supply are used to couple to the two ends of an LED load. The first terminal of the first capacitor is coupled to the rectified input voltage via a first diode and to the first output terminal via a third diode. The second terminal of the first capacitor is coupled to ground via the first switch and a first current sensing resistor. The first terminal of the second capacitor is coupled to the rectified input voltage via a second diode. The second terminal of the second capacitor is coupled to ground via the second switch and the first current sensing resistor. The third switch is coupled between the second terminal of the first capacitor and the first terminal of the second capacitor. By time-division constant current charging of the first and second capacitors and then discharging them in series, the conduction phase angle of the system input current of the LED power supply is increased, and the power factor (PF) and system efficiency of the LED power supply are improved.

Description

Technical Field

[0001] This invention relates to the field of circuits, and more specifically to a linear constant current light-emitting diode (LED) power supply. Prior Technology

[0002] Linear constant current LED power supplies are widely used in LED lighting due to their simple and reliable structure and low system cost. Figure 1 shows a schematic diagram of the topology of a traditional linear constant current LED power supply. In the linear constant current LED power supply 100 shown in Figure 1, the current sensing resistor Rcs samples the current flowing through the transistor S1 to generate a load current sampling voltage Vcs characterizing the current flowing through the LED load. In steady state, the average current flowing through the output capacitor Co is 0, and the average current flowing through the transistor S1 is equal to the average current flowing through the LED load. Controlling the transistor S1 to operate in saturation conduction based on the load current sampling voltage Vcs ensures that the current flowing through the LED load is constant. Only when the rectified input voltage Vin is higher than the output voltage Vo on the LED load will there be a system input current Iin and a current flowing through the transistor S1. Therefore, the conduction phase angle of the system input current Iin is small. In order to improve the system power factor... The factor (PF) value requires the addition of transistor S2 and bleed resistor Rb. When the rectified input voltage Vin is lower than the output voltage Vo on the LED load (i.e., no current flows through transistor S1), transistor S2 is controlled to be in the conducting state, forming a bleed current on the bleed resistor Rb, which can make the system input current Iin follow the change of the rectified input voltage Vin.

[0003] Figure 2 shows a schematic diagram of the working waveforms of the voltage and current signals in the linear constant current LED power supply shown in Figure 1. As shown in Figures 1 and 2, when the rectified input voltage Vin is less than the output voltage Vo on the LED load, the system input current Iin flows through the branch where the bleeder resistor Rb is located. All of this energy is lost in the bleeder resistor Rb and cannot be transmitted to the LED load. Therefore, the system efficiency of the linear constant current LED power supply 100 is low. Summary of the Invention

[0004] According to an embodiment of the present invention, a linear constant current LED power supply includes a first capacitor, a second capacitor, a first switch, a second switch, and a third switch, wherein: a first output terminal and a second output terminal of the linear constant current LED power supply are used to couple the two ends of an LED load; a first terminal of the first capacitor is coupled to the rectified input voltage of the linear constant current LED power supply via a first diode and coupled to the first output terminal via a third diode; a second terminal of the first capacitor is coupled to ground via the first switch and a first current sensing resistor; a first terminal of the second capacitor is coupled to the rectified input voltage via a second diode; a second terminal of the second capacitor is coupled to ground via the second switch and the first current sensing resistor; and a third switch is coupled between the second terminal of the first capacitor and the first terminal of the second capacitor; the first switch, the second switch, and the third switch are configured to: charge one of the first capacitor and the second capacitor when the rectified input voltage is lower than the output voltage on the LED load but higher than the voltage on at least one of the first capacitor and the second capacitor; and discharge the series circuit of the first capacitor and the second capacitor to the LED load when the rectified input voltage is lower than the output voltage on the LED load and lower than the voltage on both the first capacitor and the second capacitor. Simple Explanation of the Diagram

[0005] The invention can be better understood from the following description of specific embodiments of the invention in conjunction with the accompanying drawings, wherein: Figure 1 shows a schematic diagram of the topology of a traditional linear constant current LED power supply. Figure 2 shows a schematic diagram of the working waveforms of the voltage and current signals in the linear constant current LED power supply shown in Figure 1. Figure 3 shows a schematic diagram of the topology of a linear constant current LED power supply according to an embodiment of the present invention. Figure 4 shows a schematic diagram of the working waveforms of the voltage and current signals in the linear constant current LED power supply shown in Figure 3. Figure 5 shows the equivalent circuit diagram of the linear constant current LED power supply shown in Figure 3 during the first and fifth time periods shown in Figure 4. Figure 6 shows the equivalent circuit diagram of the linear constant current LED power supply shown in Figure 3 during the second and fourth time periods shown in Figure 4. Figure 7 shows the equivalent circuit diagram of the linear constant current LED power supply shown in Figure 3 during the third time period shown in Figure 4. Figure 8 shows the equivalent circuit diagram of the linear constant current LED power supply shown in Figure 3 during the sixth time period shown in Figure 4. Implementation

[0006] The features and exemplary embodiments of various aspects of the present invention will now be described in detail. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without requiring some of these specific details. The following description of embodiments is merely intended to provide a better understanding of the invention by illustrating examples of the invention. The invention is by no means limited to any specific configurations and algorithms presented below, but covers any modifications, substitutions, and improvements to elements, components, and algorithms without departing from the spirit of the invention. Well-known structures and techniques are not shown in the drawings and the following description in order to avoid unnecessarily obscuring the invention.

[0007] In view of one or more problems of the linear constant current LED power supply shown in Figure 1, a linear constant current LED power supply according to an embodiment of the present invention is proposed, which improves the system power factor (PF) and system efficiency while increasing the conduction phase angle of the system input current.

[0008] Figure 3 shows a schematic diagram of the topology of a linear constant current LED power supply according to an embodiment of the present invention. As shown in Figure 3, the linear constant current LED power supply 300 includes capacitor C1, capacitor C2, switch Sa, switch Sb, and switch Sc, wherein: the first output terminal and the second output terminal of the linear constant current LED power supply 300 are used to couple the two ends of the LED load; the first end of capacitor C1 is coupled to the rectified input voltage Vin via diode D1 and to the first output terminal via diode D3; the second end of capacitor C1 is coupled to ground via switch Sa and current sensing resistor Rcs2; the first end of capacitor C2 is coupled to the rectified input voltage Vin via diode D2; the second end of capacitor C2 is coupled to ground via switch Sb and current sensing resistor Rcs2. Resistor Rcs2 is coupled to ground, and switch Sc is coupled between the second terminal of capacitor C1 and the first terminal of capacitor C2. Switches Sa, Sb, and Sc are configured such that: when the rectified input voltage Vin is lower than the output voltage Vo on the LED load but higher than the voltage on at least one of capacitors C1 and C2, the rectified input voltage Vin charges one of capacitors C1 and C2; and when the rectified input voltage Vin is lower than the output voltage Vo on the LED load and lower than the voltage on both capacitors C1 and C2, the series circuit of capacitors C1 and C2 discharges to the LED load. Here, the current sensing resistor Rcs2 is used to sample the sum of the charging / discharging currents of capacitors C1 and C2 and generate a charging / discharging current sampling voltage Vcs2.

[0009] As shown in FIG. 3, in some embodiments, the linear constant-current LED power supply 300 further includes an output capacitor Co, a load transistor S1, and a current sensing resistor Rcs. The two ends of the output capacitor Co are respectively coupled to the first output terminal and the second output terminal. The load transistor S1 and the current sensing resistor Rcs are serially coupled between the second output terminal and the ground. Here, the current sensing resistor Rcs is used to sample the current flowing through the load transistor S1 (i.e., the current flowing through the LED load) and generate a load current sampling voltage Vcs. It should be noted that the load transistor S1, the switches Sa, Sb, and Sc can all be implemented by semiconductor switching devices.

[0010] As shown in FIG. 3, in some embodiments, the linear constant-current LED power supply 300 further includes an operational amplifier EA1 and a compensation capacitor Ccomp1. The operational amplifier EA1 generates an output loop compensation voltage Vcomp_a on the compensation capacitor Ccomp1 based on the reference voltage Vref_a and the charge / discharge current sampling voltage Vcs2 on the current sensing resistor Rcs2. The output loop compensation voltage Vcomp_a is used to control the switch Sa to operate in a fully or saturated conduction state or in an off state.

[0011] As shown in FIG. 3, in some embodiments, the linear constant-current LED power supply 300 further includes an operational amplifier EA2 and a compensation capacitor Ccomp2. The operational amplifier EA2 generates an output loop compensation voltage Vcomp_b on the compensation capacitor Ccomp2 based on the reference voltage Vref_b (where Vref_a < Vref_b) and the charge / discharge current sampling voltage Vcs2 on the current sensing resistor Rcs2. The output loop compensation voltage Vcomp_b is used to control the switch Sb to operate in a fully or saturated conduction state or in an off state.

[0012] As shown in FIG. 3, in the linear constant-current LED power supply 300, the switches Sa and Sb respectively control the charging currents of the capacitors C1 and C2. When the switch Sa operates in a saturated conduction state under the control of the output loop compensation voltage Vcomp_a, the charging current of the capacitor C1 is constant. When the switch Sb operates in a saturated conduction state under the control of the output loop compensation voltage Vcomp_b, the charging current of the capacitor C2 is constant. Since Vref_a < Vref_b, the charging current of the capacitor C1 is less than the charging current of the capacitor C2, and the voltage on the capacitor C1 is less than the voltage on the capacitor C2.

[0013] As shown in FIG. 3, in the linear constant-current LED power supply 300, when the rectified input voltage Vin is lower than the output voltage Vo on the LED load, higher than the voltage on the capacitor C1, and lower than the voltage on the capacitor C2, the constant-current loop of the operational amplifier EA1 comes into play. The switch Sa operates in the saturation conduction state under the control of the output loop compensation voltage Vcomp_a. The charging current of the capacitor C1 is constant, and the charging current sampling voltage Vcs2 on the current sensing resistor Rcs2 is Vcs2 = Vref_a. Since Vref_a < Vref_b, then Vcs2 < Vref_b, and the operational amplifier EA2 operates in the open-loop state. The output loop compensation voltage Vcomp_b reaches the maximum, and the switch Sb operates in the fully-conducted state. Since the rectified input voltage Vin is lower than the voltage on the capacitor C2, even when the switch Sb is in the fully-conducted state, the charging current of the capacitor C2 is zero.

[0014] As shown in FIG. 3, in the linear constant-current LED power supply 300, when the rectified input voltage Vin is lower than the output voltage Vo on the LED load, higher than the voltage on the capacitor C1, and higher than the voltage on the capacitor C2, the constant-current loop of the operational amplifier EA2 comes into play. The switch Sb operates in the saturation conduction state under the control of the output loop compensation voltage Vcomp_b. The charging current of the capacitor C2 is constant, and the charging current sampling voltage Vcs2 on the current sensing resistor Rcs2 is Vcs2 = Vref_b. Since Vref_a < Vref_b, then Vcs2 > Vref_a, and the operational amplifier EA1 operates in the open-loop state. The output loop compensation voltage Vcomp_a reaches the minimum, the switch Sa is in the off state, and the charging current of the capacitor C1 is zero.

[0015] FIG. 4 shows a schematic diagram of the working waveforms of the voltage and current signals in the linear constant-current LED power supply shown in FIG. 3. Among them, Vo represents the output voltage on the LED load, Vin represents the rectified input voltage, Vcs represents the load current sampling voltage generated by the current sensing resistor Rcs sampling the current flowing through the LED load, ISa represents the current flowing through the switch Sa, ISb represents the current flowing through the switch Sb, Iin represents the system input current, Sa_gate represents the control voltage for controlling the switch Sa, Sb_gate represents the control voltage for controlling the switch Sb, and Sc_gate represents the control voltage for controlling the switch Sc.

[0016] As shown in FIGS. 3 and 4, the working process of the linear constant-current LED power supply 300 is as follows:

[0017] [At] [t0 - t1] [,] [t4 - t5] [Time period:] When the rectified input voltage Vin is lower than the output voltage Vo on the LED load, lower than the voltage on the capacitor C2, and higher than the voltage on the capacitor C1, the rectified input voltage Vin charges the capacitor C1 with a constant current through the diode D1 and the switch Sa, and the current flowing through the LED load is provided by the output capacitor Co. Figure 5 shows the equivalent circuit diagram of the linear constant-current LED power supply shown in Figure 3 during the time periods of t0 - t1 and t4 - t5 shown in Figure 4, where the current flow is as indicated by the arrows. As shown in Figure 5, during the time periods of t0 - t1 and t4 - t5, the switch Sc is in the off state; the switch Sa operates in the saturated conduction state under the control of the output loop compensation voltage Vcomp_a; since the rectified input voltage Vin is lower than the voltage on the capacitor C2 and higher than the voltage on the capacitor C1, the charging current of the capacitor C2 is zero, the system input current Iin is equal to the charging current of the capacitor C1, the current sensing resistor Rcs2 only samples the charging current of the capacitor C1 to generate the charging current sampling voltage Vcs2, and Vcs2 = Vref_a; since Vref_a < Vref_b, so Vcs2 < Vref_b, the operational amplifier EA2 operates in the open-loop state, and the switch Sb operates in the fully conduction state.

[0018] [[ID=A]][At] [[ID=B]] [t1 - t2] [,] [t3 - t4] [Time period:] When the rectified input voltage Vin is lower than the output voltage Vo on the LED load, higher than the voltage on capacitor C1, and higher than the voltage on capacitor C2, the rectified input voltage Vin charges capacitor C2 with a constant current via diode D2 and switch Sb, and the current flowing through the LED load is provided by the output capacitor Co. Figure 6 shows the equivalent circuit diagram of the linear constant-current LED power supply shown in Figure 3 during the time periods t1 - t2 and t3 - t4 shown in Figure 4, where the current flow is as indicated by the arrows. As shown in Figure 6, during the time periods t1 - t2 and t3 - t4, switch Sc is in the off state; switch Sa is in the off state under the control of the output loop compensation voltage Vcomp_a; switch Sb works in the saturated conduction state under the control of the output loop compensation voltage Vcomp_b; the system input current Iin is equal to the charging current of capacitor C2, the charging current of capacitor C1 is zero, the current sensing resistor Rcs2 only samples the charging current of capacitor C2 to generate the charging current sampling voltage Vcs2, and Vcs2 = Vref_b; since Vref_a < Vref_b, so Vref_a < Vcs2, the operational amplifier EA1 works in the open-loop state, and switch Sa is in the off state.

[0019] [At] [t2 - t3] [Time period:] When the rectified input voltage Vin is higher than the output voltage Vo on the LED load, the current flowing through the LED load is provided by the rectified input voltage Vin, the current flowing through the load transistor S1 is equal to the system input current Iin, and switches Sa, Sb, and Sc are all in the off state. Figure 7 shows the equivalent circuit diagram of the linear constant-current LED power supply shown in Figure 3 during the time period t2 - t3 shown in Figure 4, where the current flow is as indicated by the arrows. As shown in Figure 7, during the time period t2 - t3, the rectified input voltage Vin charges the output capacitor Co while providing current to the LED load via the load transistor S1 and the current sensing resistor Rcs; the load current sampling voltage Vcs generated by sampling the current flowing through the load transistor S1 based on the current sensing resistor Rcs controls the magnitude of the current flowing through the load transistor S1, making the load transistor S1 work in the saturated conduction state.

[0020] [At] [t5 - t6] [Time Period:] When the rectified input voltage Vin is lower than the output voltage Vo on the LED load, lower than the voltage on capacitor C1, and lower than the voltage on capacitor C2, the rectified input voltage Vin can neither provide current to the LED load nor charge capacitors C1 or C2. The system input current Iin is zero, and capacitors C1 and C2 discharge to the LED load in series. Figure 8 shows the equivalent circuit diagram of the linear constant current LED power supply shown in Figure 3 during the t5-t6 time period shown in Figure 4, where the current flow direction is as shown by the arrows. As shown in Figure 8, during the t5-t6 time period, switches Sb and Sc are in the fully on state, the load transistor S1 is in the saturated on state, and switch Sa is in the off state. The current flowing through the load transistor S1 is controlled based on the load current sampling voltage Vcs, thereby controlling the discharge current of capacitors C1 and C2 (i.e., the current flowing through the LED load). The current sensing resistor Rcs2 samples the discharge current of capacitors C1 and C2 to generate the discharge current sampling voltage Vcs2.

[0021] In the linear constant current LED power supply 300 according to an embodiment of the present invention, by time-sharing constant current charging of capacitors C1 and C2 and then discharging them in series, the conduction phase angle of the system input current Iin is increased, and the system power factor (PF) and system efficiency are improved. When the rectified input voltage Vin is so low that it can neither provide current to the LED load nor charge capacitors C1 or C2, capacitors C1 and C2 discharge in series to the LED load, discharging the charging energy to the LED load. This can improve system efficiency while achieving low total harmonic distortion (THD). Specifically, the larger the charging current of capacitors C1 and C2, the lower the THD and the lower the system efficiency; the smaller the charging current of capacitors C1 and C2, the higher the THD and the higher the system efficiency.

[0022] This invention can be implemented in other specific forms without departing from its spirit and essential characteristics. For example, the algorithm described in a particular embodiment may be modified without departing from the basic spirit of the invention. Therefore, the present embodiments are to be regarded as exemplary rather than limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and all changes falling within the meaning of the claims and their equivalents are thus included within the scope of the invention.

[0023] 100, 300: Linear constant current LED power supply ACIN: AC input voltage source C1, C2: Capacitors Ccomp1, Ccomp2: Compensation capacitors Co: Output capacitor D1, D2, D3: Diodes EA1, EA2: Operational amplifiers Iin: System input current Isa, ISb: Current LED: Light Emitting Diode Rb: Leakage resistor Rcs, Rcs2: Current sensing resistors S1, S2: Transistors Sa, Sb, Sc: Switches Sa_gate, Sb_gate, Sc_gate: Control voltage Vcomp_a, Vcomp_b: Output loop compensation voltages Vcs: Load current sampling voltage Vcs2: Charging / discharging current sampling voltage Vin: Rectified input voltage Vo: Output voltage Vref_a, Vref_b: Reference voltage

Claims

1. A linear constant current LED power supply, comprising a first capacitor, a second capacitor, a first switch, a second switch, and a third switch, wherein: The first and second output terminals of the linear constant current LED power supply are used to couple the two ends of an LED load. The first terminal of the first capacitor is coupled to the rectified input voltage of the linear constant current LED power supply via a first diode and to the first output terminal via a third diode. The second terminal of the first capacitor is coupled to ground via a first switch and a first current sensing resistor. The first terminal of the second capacitor is coupled to the rectified input voltage via a second diode. The second terminal of the second capacitor is coupled to ground via a second switch and the first current sensing resistor. The third switch is coupled between the second terminal of the first capacitor and the first terminal of the second capacitor. The first switch, the second switch, and the third switch are configured to: charge one of the first capacitor and the second capacitor when the rectified input voltage is lower than the output voltage on the LED load but higher than the voltage on at least one of the first capacitor and the second capacitor; and discharge the series circuit of the first capacitor and the second capacitor to the LED load when the rectified input voltage is lower than the output voltage on the LED load and lower than the voltage on both the first capacitor and the second capacitor.

2. The linear constant current LED power supply as described in claim 1, wherein, When the rectified input voltage is higher than the output voltage on the LED load, the first switch, the second switch, and the third switch are all configured to be in the off state, so that the rectified input voltage supplies power to the LED load via the first diode and the third diode.

3. The linear constant current LED power supply as described in claim 1, wherein, When the rectified input voltage is lower than the output voltage on the LED load but higher than the voltage on both the first capacitor and the second capacitor, the first switch and the third switch are configured to be in the off state, and the second switch is configured to operate in the saturated conduction state, so that the rectified input voltage charges the second capacitor.

4. The linear constant current LED power supply as described in claim 1, wherein, When the rectified input voltage is lower than the output voltage on the LED load and the voltage on the second capacitor but higher than the voltage on the first capacitor, the first switch is configured to operate in a saturated conduction state, the second switch is configured to operate in a fully conduction state, and the third switch is configured to be in a de-energized state, so that the rectified input voltage charges the first capacitor.

5. The linear constant current LED power supply as described in claim 1, wherein, When the rectified input voltage is lower than the output voltage on the LED load and lower than the voltage on both the first capacitor and the second capacitor, the first switch is configured to be in the off state, and the second switch and the third switch are configured to operate in the fully on state, so that the series circuit of the first capacitor and the second capacitor discharges to the LED load through the third diode.

6. The linear constant current LED power supply as described in claim 1, further comprising a first operational amplifier and a first compensation capacitor, wherein, The first operational amplifier generates a first output loop compensation voltage on the first compensation capacitor based on the first reference voltage and the charging or discharging current sampling voltage on the first current sensing resistor. The first output loop compensation voltage is used to control the first switch to operate in a fully or saturated conduction state or in a turn-off state.

7. The linear constant current LED power supply as described in claim 6 further includes a second operational amplifier and a second compensation capacitor, wherein, The second operational amplifier generates a second output loop compensation voltage on the second compensation capacitor based on the second reference voltage and the charging or discharging current sampling voltage on the first current sensing resistor. The second output loop compensation voltage is used to control the second switch to operate in a fully or saturated conduction state or in a turn-off state. The second reference voltage is greater than the first reference voltage.

8. The linear constant current LED power supply as claimed in claim 1 further includes an output capacitor, a load switch, and a second current sensing resistor, wherein the two ends of the output capacitor are respectively coupled to the first output terminal and the second output terminal, and the load switch and the second current sensing resistor are connected in series between the second output terminal and ground.

9. The linear constant current LED power supply as described in claim 8, wherein, When the rectified input voltage is lower than the output voltage on the LED load but higher than the voltage on at least one of the first capacitor and the second capacitor, the output capacitor discharges to the LED load.

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