HK1164623B - Method and apparatus for controlling brightness of light emitting diodes - Google Patents

Description

Method and apparatus for controlling brightness of light emitting diode

CROSS-REFERENCE TO RELATED APPLICATIONS

This PCT international application claims priority from U.S. provisional patent application No.61/262,582, filed on 11/19/2009, which is incorporated herein by reference.

Background

The present invention relates to an apparatus and method for controlling the brightness of a Light Emitting Diode (LED).

It is often desirable to have a dimming function in the power supply to the lighting device. One commonly used dimmer is the well known triac dimmer. A triac dimmer reduces its load power by chopping the load voltage, which drives current to the load in each half-cycle.

Fig. 1 shows a typical connection of a triac dimmer 14 to an LED light source 10. One end of the LED light source 10 is connected to a neutral line N of the AC power source 12. The other end of the LED light source 10 is connected to a triac dimmer 14, which in turn is connected to the line L of the AC power source 12 via an optional switch 16. The most common dimmer is the leading edge type, in which the conductance at each half-cycle is delayed from the zero-crossing, so that the first voltage presented to the load after the zero-crossing is the leading edge of the voltage.

The upper waveform of fig. 2 shows the output voltage V of the leading edge type triac dimmer 14 at the maximum brightness setting_TDAnd (4) waveform. The firing angle, which is the phase angle of the voltage at which the triac conducts, is at its minimum. The lower waveform in fig. 2 shows the input current waveform ILS for the LED light source 10 when the triac dimmer 14 is at its maximum.

As the dimmer adjusts, the firing angle changes. Fig. 3 shows the voltage V when the ignition angle is set to 90 degrees (about half brightness)_TDAnd current I_LSWaveform, and fig. 4 shows the voltage V when the ignition angle is set to the maximum ignition angle (minimum brightness)_TDAnd current I_LSAnd (4) waveform.

However, most dimmers cannot be powered up if the dimmer is preset to a maximum ignition angle. The user must turn the dimmer on to power up properly and then down to the desired brightness. Further, at maximum ignition angles, even if the dimmer is able to power up on its own, the dimmer output power may be too low to push down the LED light source circuit.

There is a need for a power supply method and apparatus that does not suffer from these drawbacks.

Disclosure of Invention

In view of the above, according to a first aspect of the present invention, a method of driving a light emitting diode, LED, light source, the method comprising: providing an alternating voltage source having a positive half cycle and a negative half cycle; adjusting a dimmer having an ignition angle to provide current to the LED when the ignition angle has been reached after starting the positive or negative half-cycle; waiting a time period after starting the positive half cycle or the negative half cycle; and reducing the current supplied to the LED at the end of the time period, wherein the time period does not exceed the end of either the positive or negative half-cycles.

In another aspect, the period of reduced current provided to the LED is a fixed period of time.

In another aspect, the period of reduced current provided to the LED is a variable time period.

In another aspect, the period of the reduced current provided to the LED is set to be proportional to the firing angle of the dimmer.

According to another aspect of the present invention, a circuit for driving a Light Emitting Diode (LED) includes: an alternating current voltage source comprising a triac dimmer having a firing angle and output terminals for providing electrical energy to the LEDs; a zero-crossing detector that detects a change in polarity of an alternating voltage supplied by the alternating voltage source and provides a zero-crossing output signal indicative of a zero-crossing; a timer triggered by the zero-crossing output signal, the timer generating a timer output signal during a time period of the timer; and an LED power supply circuit that reduces a current supplied to the light emitting diode based on a timing characteristic of the timer output signal.

In another aspect, the circuit further comprises: a second voltage detector detecting the dimmer voltage and outputting a voltage detector output; and a logic circuit logically combining the voltage detector output and the timer output signal and generating a charging signal to charge and discharge a timing capacitor coupled to the LED power circuit for switching between a normal current and a reduced current provided to the LED.

In another aspect, a logic circuit includes: a logic gate receiving and logically combining the voltage detector output and the timer output signal; one or more resistors controlling the charging and discharging currents of the timing capacitor; and one or more diodes separating the charging path and the discharging path.

In another aspect, the circuit further includes a pulse shaping amplifier that converts the timing capacitor voltage to a digital signal that is output to the LED power circuit.

Drawings

The drawings are for illustrative purposes only and are not necessarily drawn to scale. The invention itself, however, may best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram showing an existing connection of a triac dimmer and an LED light source;

FIG. 2 is a graph showing waveforms at minimum firing angles for the dimmer output voltage and LED light source input current for the prior art circuit of FIG. 1;

FIG. 3 is a graph showing waveforms at 90 degree firing angle for the dimmer output voltage and LED light source input current for the prior art circuit of FIG. 1;

FIG. 4 is a graph showing waveforms at maximum firing angle for the dimmer output voltage and LED light source input current for the prior art circuit of FIG. 1;

FIG. 5 is a graph illustrating waveforms at maximum brightness of a dimmer output voltage and an LED light source input current in accordance with an aspect of the present invention;

FIG. 6 is a graph illustrating waveforms at half brightness of a dimmer output voltage and an LED light source input current in accordance with an aspect of the present invention;

FIG. 7 is a graph illustrating waveforms at minimum brightness of a dimmer output voltage and an LED light source input current in accordance with an aspect of the present invention;

FIG. 8 is a schematic diagram of a dimmable LED power supply according to an aspect of the present invention;

FIG. 9 shows waveforms of an input voltage, a trigger voltage, and a mode voltage for use in the power supply shown in FIG. 8;

FIG. 10 is a graph illustrating waveforms at maximum brightness of a dimmer output voltage and an LED light source input current according to an aspect of the present invention with variable idle time;

FIG. 11 is a graph illustrating waveforms at half brightness of a dimmer output voltage and an LED light source input current according to an aspect of the present invention with variable idle time;

FIG. 12 is a graph illustrating waveforms at minimum brightness of a dimmer output voltage and an LED light source input current according to an aspect of the present invention with variable idle time;

FIG. 13 is a diagram of a dimmable LED power supply according to another aspect of the invention with variable cycle functionality; and

fig. 14 is a diagram illustrating signal waveforms of functional blocks of the power supply shown in fig. 13 according to an aspect of the present invention.

Detailed Description

According to an aspect of the present invention, an apparatus and method of driving an LED light source is presented such that the driven LED light source draws a reduced current from a dimmer when the phase angle of the half-cycle voltage exceeds some predetermined value. A fixed period of reduced current is inserted at the end of the half cycle. In accordance with actuation according to aspects of the present invention, the user merely adjusts the dimmer to a predetermined angle for minimum brightness without adjusting the dimmer down all the way to a maximum ignition angle. At a predetermined angle, the dimmer can be powered up and the dimmer output voltage is high enough to activate the LED light source circuit.

FIG. 5 illustrates a driving method according to an aspect of the present invention, a dimmer voltage V when the brightness is set to the maximum brightness_TDSum current waveform I_LS. As can be seen from the figure, the dimmer output voltage V_TDThe waveforms are the same as in fig. 2. However, the current waveform I_LSIs different. Current I_LSAt each tail ofAnd decreases significantly at half-cycles. FIG. 6 illustrates a driving method according to an aspect of the present invention, a dimmer voltage V when the luminance is set to half luminance_TDSum current waveform I_LS. FIG. 7 illustrates a driving method according to an aspect of the present invention, a dimmer voltage V when the brightness is set to the minimum brightness_TDSum current waveform I_LS. Due to the reduced current, the minimum brightness ignition angle is smaller compared to fig. 4. Dimmer output voltage V_TDAlso higher than shown in figure 4. At such minimum brightness ignition angles, the dimmer should be able to power up and the dimmer output voltage should be high enough to activate the LED light source circuit.

Fig. 8 shows a dimmable LED power supply circuit 100, which provides a way to implement the dimming method, where a zero-crossing detector is used to detect a polarity change of the AC input voltage. The power supply circuit 100 according to an embodiment of the present invention includes an AC input voltage source 102, a triac dimmer 106, a zero-crossing detector 104, a timer 108, a power supply circuit 112, and an LED 114. The polarity change detected by the zero-crossing detector 104 triggers the timer 108, which generates a signal to switch the power circuit 112 from the normal current mode to the reduced circuit mode.

Fig. 9 shows signal waveforms of each functional block of the circuit of fig. 8. The use of the power supply circuit 100 thus results in a reduced current as illustrated by the waveforms of fig. 5-7.

A variant of the proposed method according to another embodiment of the invention uses a variable off period (off period) instead of a fixed off period at the end of the half period. According to this aspect of the invention, the off-period is selected to be proportional to the firing angle of the triac dimmer. If the firing angle of the triac dimmer is θ, the angular offset within the off period will be k θ, where k is a constant.

Fig. 10-12 show the voltage V for the variable off-period embodiment of the present invention when the brightness is set to maximum, half and minimum, respectively_TDSum current waveform I_LS。

It can be appreciated that with the variable idle time mechanism of this aspect of the invention, the ignition angle of the dimmer at minimum brightness can also be offset to a smaller value. Furthermore, the power factor at full brightness will be superior to the fixed idle time mechanism discussed above.

Fig. 13 illustrates a dimmable LED power supply circuit 200 that provides a way to implement a variable off-cycle mechanism in accordance with this aspect of the invention. In the circuit 200, a zero crossing voltage detector 204 is used to detect a change in polarity of the AC input voltage provided by the AC voltage source 202. This polarity change triggers the timer 208. The second voltage detector 210 is used to detect the voltage from the dimmer 206. The output of the voltage detector 210 is logically combined with the output of the timer 208 using a logic circuit 209. In this circuit, a diode is used to separate the charging path and the discharging path, and a resistor R_CAnd R_DFor controlling the charging current and the discharging current. The resulting signal is used to charge and discharge a timing capacitor C_T。R_CControlling charging current while R_DThe discharge current is controlled. Different R's may be used_CAnd R_DThe values are selected to select different charging and discharging currents. The capacitor voltage is then pulse shaped by a pulse shaping amplifier 211 to be converted into a digital signal. The signal is then fed to the power supply circuit 212 for switching between the normal current mode and the reduced current mode.

Fig. 14 shows signal waveforms of each functional block of the circuit shown in fig. 13. The use of circuit 200 results in the variable off-cycle current waveforms of fig. 10-12.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Thus, the invention is limited only by the claims and the equivalents thereof.

Claims (8)

1. A method of driving a Light Emitting Diode (LED) light source, the method comprising:

providing an alternating voltage source having a positive half cycle and a negative half cycle;

adjusting a dimmer having an ignition angle to provide current to the LED when the ignition angle has been reached after starting the positive or negative half-cycle;

waiting a time period after starting the positive half-cycle or the negative half-cycle; and

the current supplied to the LED is reduced at the end of said time period,

wherein the time period does not exceed the end of either the positive or negative half-cycles.

2. The method of claim l, wherein the period of reduced current provided to the LED is a fixed period of time.

3. The method of claim 1, wherein the period of reduced current provided to the LED is a variable time period.

4. The method of claim 3, wherein the period of the reduced current provided to the LED is set to be proportional to the firing angle of the dimmer.

5. A circuit for driving a Light Emitting Diode (LED), the circuit comprising:

an alternating current voltage source comprising a triac dimmer having a firing angle and output terminals for providing electrical energy to the LEDs;

a zero-crossing detector that detects a change in polarity of an alternating voltage supplied by the alternating voltage source and provides a zero-crossing output signal indicative of a zero-crossing;

a timer triggered by the zero-crossing output signal, the timer generating a timer output signal during a time period of the timer; and

an LED power circuit that reduces the current supplied to the light emitting diode at the end of the time period of the timer.

6. The circuit of claim 5, further comprising:

a second voltage detector detecting the dimmer voltage and outputting a voltage detector output; and

a logic circuit logically combines the voltage detector output and the timer output signal and generates a charging signal to charge and discharge a timing capacitor coupled to the LED power circuit for switching between a normal current and a reduced current provided to the LED.

7. The circuit of claim 6, wherein the logic circuit comprises:

a logic gate receiving and logically combining the voltage detector output and the timer output signal;

one or more resistors controlling the charging and discharging currents of the timing capacitor; and

one or more diodes separating the charging path and the discharging path.

8. The circuit of claim 6, further comprising a pulse shaping amplifier that converts the timing capacitor voltage to a digital signal that is output to the LED power circuit.