LED driving circuit

An LED driving circuit configured to drive an LED lamp having first and second loads, can include: a power converter; a dimmer configured to control the power converter to output a driving current to the LED lamp; and a current distribution circuit configured to adjust a proportion of current from the driving current that flows through each of the first and second loads of the LED lamp, in order to adjust the color temperature or the brightness of the LED lamp.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 201710302777.7, filed on May 3, 2017, which is incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of power electronics, and more particularly, to light-emitting diode (LED) driving circuitry.

BACKGROUND

With increasingly use of LED lights, multi-stage switching converters can typically be used in order to adjust the brightness of the light sources. In addition, each stage may deal with the power in total, which can increase product costs of the LED drivers. In order to reduce such costs, some techniques divide the secondary winding at the output side of a flyback converter into two groups, and a DC-DC converter of the second stage may be connected to only one of the two groups. In one LED luminance system, in order to achieve current balance of each LED branch circuit, a linear regulator (LDO) may be coupled with the LED branch circuit in order to regulate the LED driving current.

DETAILED DESCRIPTION

As a relatively new lighting tool, light-emitting diode (LED) lighting has become increasingly used in various lighting fields due to advantages of high lighting efficiency, long lifetime, environmental protection, and so on. LED dimming methods can generally be divided into two categories: DC dimming and pulse-width modulation (PWM) dimming. PWM dimming has widely been utilized due to its advantages of constant light color and good stability at low brightness. For some special light source environments, not only may the brightness of the LED lamp need to be adjusted, but the color temperature of the LED lamp may need to be adjusted.

Referring now toFIG. 1, shown is a schematic block diagram of an example LED driving circuit. In this example, AC voltage Vac may be converted to voltage Vin through a rectifier and filter circuit01. Power converter02can receive voltage Vin, and may output constant voltage Vout under the control of a constant voltage control circuit. Constant voltage Vout can be used as an input voltage of constant current driving circuits03and04. The first current control circuit can control constant current driving circuit03to output a constant driving current to a cool-color temperature LED light string LED1, and the second constant current control circuit can control constant current driving circuit04to output a constant driving current to a warm-color temperature LED light string LED2. Constant current driving circuits03and04can respectively receive pulse-width modulation signals PWM1and PWM2generated from a pulse-width modulation signal generator, in order to control the average current of light strings LED1and LED2, thereby achieving dimming and color adjustment of the LED lamp.

In this approach, the LED driving circuit may need an independent constant current driving circuit in order to drive the LED light strings with corresponding color light, and each constant current driving circuit may need to be equipped with a constant current control circuit. In addition, since a constant current driving circuit is typically constructed by a plurality of devices, such as including an energy storage device, more electronic components and increased circuit complexity can result. Thus, realizing such an LED driving circuit with dimming and color adjusting functions may have relatively high production costs, and system size, which makes integration more difficult.

In one embodiment, an LED driving circuit configured to drive an LED lamp having first and second loads, can include: (i) a power converter; (ii) a dimmer configured to control the power converter to output a driving current to the LED lamp; and (iii) a current distribution circuit configured to adjust a proportion of current from the driving current that flows through each of the first and second loads of the LED lamp, in order to adjust the color temperature or the brightness of the LED lamp.

Referring now toFIG. 2, shown is a schematic block diagram of an example LED driving circuit, in accordance with embodiments of the present invention. This particular example LED driving circuit can be used to drive an LED lamp having two loads, and can include power converter21, dimmer22, and current distribution circuit23. Power converter21can output predetermined driving current IINto the LED lamp according to a dimming instruction. This example LED driving circuit may also include rectifier circuit24. Power converter21can include power switch M1with its switch states being controlled by dimmable control circuit211, such that power converter21may output a desired driving current IIN. Rectifier circuit24can receive an AC voltage AC, and output a DC voltage Vin to power converter21. Power converter21can convert DC voltage Vin into the driving current, and may output the driving current.

Power converter21may be implemented with an isolated topology or a non-isolated topology. In this particular example, power converter21can be implemented with a flyback power converter. Power converter21can also include transformer T with primary winding Np and secondary winding Ns. Primary winding Np can connect to power switch M1. Dimmable control circuit211can control the on state and off state of power switch M1, such that power converter21outputs predetermined driving current IIN. Any suitable approach of making power converter21output the predetermined driving current by controlling the switching states of power switch M1through dimmable control circuit211can be employed in certain embodiments. For example, by detecting the phase angle of silicon-controlled dimmer22, an output current (e.g., driving current IINof power converter21) can be regulated by adjusting a current reference in dimmable control circuit211according to the detected phase angle. The output current of power converter21can also be regulated by dimmable control circuit211directly according to the dimming instruction of dimmer22′.

The LED lamp can include two loads (e.g., load1and load2) coupled in parallel. The two loads can include two LED light strings with different color temperatures, or one LED light string and one other types of load (e.g., a resistor). The sum of current IOUT1flowing through load1and current IOUT2flowing through load2can be referred to as driving current IIN. In this particular example, load1can be a cool-color temperature LED light string, and load2can be a warm-color temperature LED light string. Current distribution circuit23can adjust the current flowing through each of the loads in the LED lamp according to driving current IIN. In this case, the proportions of current IOUT1flowing through load1, and current IOUT2flowing through load2in driving current IINmay be respectively adjusted, thereby adjusting the color temperature or brightness of the LED lamp.

Current distribution circuit23can connect in series with one of the two loads. Current distribution circuit23can include transistor Q1, resistor R1, resistor R2, and feedback circuit231. For example, load2, transistor Q1, and resistors R1and R2may be sequentially connected in series. That is, one terminal of transistor Q1can connect to one terminal of load2, and a first terminal of resistor R1can connect to the other terminal of transistor Q1. A second terminal of resistor R1can connect to a first terminal of resistor R2, and a second terminal of resistor R2may be grounded. For example, the other of the two loads can connect to the common node of resistors R1and R2.

Feedback circuit231can receive feedback voltage Vsen at the common node of resistor R1and transistor Q1, and reference voltage Vref. Feedback circuit231may generate control signal Vg of transistor Q1by comparing feedback voltage Vsen against reference voltage Vref. Feedback circuit231can include comparator CMP, switch K1, and current source I1. For example, the inverting input terminal of comparator CMP can receive reference voltage Vref, the non-inverting input terminal can receive feedback voltage Vsen, and the output terminal may provide comparison signal Vcmp. Switch K1can be controlled by comparison signal Vcmp, and a first terminal of switch K1can connect to gate G of transistor Q1. A second terminal of switch K1can connect to a first terminal of current source I1, and a second terminal of current source I1may be grounded. The first terminal of switch K1may output control signal Vg of transistor Q1.

In normal operation, the sum of current IOUT1flowing through load1, and current IOUT2flowing through load2may be driving current IIN. That is, IOUT1+IOUT2=IIN(Formula 1). Current IOUT1flowing through load1, and current IOUT2flowing through load2may form a voltage drop across resistors R1and R2. Due to the presence of feedback circuit231, the voltage at the first terminal of resistor R1(e.g., feedback voltage Vsen) can be maintained at reference voltage Vref. Thus, there is (R1+R2)*IOUT2+R2*IOUT1=Vref (Formula 2). Therefore, according to Formulas 1 and 2, the proportions of current IOUT1flowing through load1, and current IOUT2flowing through load2in driving current IINcan be respectively obtained for different driving currents IIN.

Referring now toFIG. 3, shown is an example current distribution curve of an example LED driving circuit, in accordance with embodiments of the present invention. In this example, it can be seen that the proportions of current IOUT1flowing through load1, and current IOUT2flowing through load2in driving current IINmay change for different driving currents IIN. The current distribution curve may have two turning points: turning point “a” and turning point “b” in the example ofFIG. 3. In this example, driving circuit IINat turning point “a” is threshold Ia, and driving circuit IINat turning point “b” is threshold Ib. As shown, when driving current IINis less than threshold Ia, only load2operates. When driving current IINis greater than threshold Ia and less than threshold Ib, both load1and load2operate. When drive current IINis greater than threshold Ib, only load1operates.

In this particular example, load1is a cold color temperature LED light string, and load2is a warm color temperature LED light string. Due to the above current distribution characteristics, when driving current IINis relatively small, only load2(e.g., the warm-color temperature LED light string) operates, and the light of the LED light string can be warm white. When driving current IINincreases to be greater than threshold Ia and less than threshold Ib, current IOUT2of load2(e.g., the warm-color temperature LED light string) may gradually decrease, current IOUT1of load1(e.g., the cool-color temperature LED light string) may gradually increase, and the light of the LED light string can gradually change from warm white to cool white. When driving current IINis greater than threshold Ib, only load1(e.g., the cold-color temperature LED light string) operates, and the light of the LED light string can be cool white. Thus, according to different driving currents IIN, it is possible to assign different currents to the two loads in order to adjust the color temperature of the LED lamp, which may be particularly suitable for sunset lamps.

In particular embodiments, if load1or load2is set as an LED light string, and the other load is set as a resistor, the LED driving circuit can still operate and the brightness of the LED lamp may be adjusted. As discussed above, when driving current IINis different, the proportions of current IOUT1flowing through load1, and current IOUT2flowing through load2in driving current IINcan change accordingly, such as per the current distribution curve shown inFIG. 3. If load1is the LED light string, and load2is the resistor, according to the relationship of current IOUT1flowing through load1varying with driving current IIN, it is known that after turning point “a,” the greater driving current IINis, the greater current IOUT1flowing through load1is, and the brightness of the LED lamp is also brighter. If load2is the LED light string and load1is the resistor, according to the relationship of current IOUT2flowing through load2varying with the driving current IIN, it is known that before turning point “a,” the greater driving current IINis, the greater current IOUT2flowing through load2is, and the brightness of the LED lamp is also brighter. After the turning point “a” and before turning point “b,” the greater driving current IINis, the smaller the current IOUT2flowing through load2is, and the brightness of the LED lamp may also be reduced.

It should be noted that turning point “a” and turning point “b” are not fixed. In the circuit, threshold Ia of driving current IINat turning point “a” is Ia=Vref/(R1+R2), and threshold Ib of circuit current IINat turning point “b” is Ib=Vref/R2. Therefore, by adjusting the resistance of resistor R1and/or resistor R2, the positions of turning point “a” and turning point “b” can be changed. That is, for the same driving current IIN, the proportion of the current flowing through each load of the LED lamp in the driving current can be changed. Based on this, the turning points of the LED lamp's color temperature or brightness can be flexibly changed according to different requirements in any given application.

Referring back toFIG. 2, current distribution circuit23can also include ripple removing circuit232, which can include capacitor C1and resistor R3. For example, resistor R3can connect in parallel between drain D and gate G of transistor Q1, and capacitor C1can connect in parallel between gate G of transistor Q1and ground. When driving current IINis less than threshold Ia, only load2may operate, and the voltage across load2can be less than the voltage across load1. When the values of capacitor C1and resistor R3are relatively large, the LED driving circuit can also have the function of eliminating output jitter (e.g., caused by the city electric jitter, or the strobe caused by the dimmer).

Referring now toFIG. 4, shown is a schematic block diagram of another example LED driving circuit, in accordance with embodiments of the present invention. The difference between the LED driving circuits shown inFIGS. 2 and 4lies in the different implementations of feedback circuit231. In this particular example, feedback circuit231can include transconductance amplifier GM and transistor Q2. For example, the inverting input terminal of transconductance amplifier GM can receive reference voltage Vref, the non-inverting input terminal can receive feedback voltage Vsen, and the output terminal may output error amplified signal Verr. Transistor Q2can be controlled by error amplified signal Verr, and the first power terminal of transistor Q2can connect to gate G of transistor Q1. The second power terminal of transistor Q2can connect to ground. The first power terminal of transistor Q2can output signal Vg of transistor Q1.

In particular embodiments, an LED driving circuit may provide a predetermined driving current to the LED lamp through a power converter, and then adjusts the proportion of each load current of the LED lamp in the driving current through the current distribution circuit according to the driving current, in order to adjust the color temperature or brightness of the LED lamp. In addition, the LED driving circuit may eliminate output jitter, such as may be caused by the city electric jitter or the strobe caused by the dimmer when the driving current is small. Therefore, the LED driving circuit of particular embodiments can adjust the color temperature and the brightness of the LED lamp without needing to equip an independent constant current driving circuit and a constant current control circuit to each load. The circuit structure is thus relatively simple and easily integrated, uses fewer peripheral devices, and results in a relatively small circuit volume with relatively low production costs.