Patent Application
20110127925
The present application claims priority to and the benefit of Chinese Patent Application No. 200910310660.9, filed Nov. 30, 2009, the disclosure of which is incorporated herein by reference in its entirety.
The present technology generally relates to circuits and methods for driving light emitting diodes (“LEDs”), and in particular, relates to circuits and methods for driving white LEDs (“WLEDs”) with Triac dimmer used for realizing the dimming function.
Currently, one major trend of WLED application is to replace existing traditional lamps. One problem to solve is to achieve smooth dimming of WLED with standard Triac dimmers which are conventionally designed for pure resistive lamp loads, such as incandescent or halogen light bulbs.
However, WLED does not appear as a resistive load to the Triac dimmer. Thus, when dimming WLED with conventional Triac dimmer, the dimming performance is often unsatisfactory.
Generally, a control signal is provided to turn on the Triac and a current will flow through it. When said current flowing through the Triac decreases to a determined value, the Triac turns off automatically. Electronic transformer 103 receives said high AC voltage and converts it into a low AC voltage. Rectifier 105 rectifies said low AC voltage and generates a low DC voltage to power said WLED driver 107 which drives the WLED in operation. As discussed in more detail below, several characteristics of the foregoing operation can cause the WLED to flicker. Accordingly, several improvements in circuits and methods for driving WLEDs may be desirable.
The following detailed description of the embodiments of the present disclosure can best be understood when read in conjunction with the following drawings, in which the features are not necessarily drawn to scale but rather are drawn as to best illustrate the pertinent features, wherein:
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Various embodiments of the technology will now be described. In the following description, some specific details, such as example circuits and example values for these circuit components, are included to provide a thorough understanding of embodiments of the technology. One skilled in the relevant art will recognize, however, that the technology can be practiced without one or more specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the technology.
Several embodiments of the present technology are directed to Triac dimmer compatible WLED driving circuits that can address asymmetrical AC voltage generation and/or lost conduction angle in conventional Triac dimmer circuits. Referring to
In addition, if some of the conduction angles are lost during the dimming process, the DC voltage generated from the rectifier 105 may further contain low frequency voltage ripples with frequencies lower than 50 Hz. Without being bound by theory, it is believed that a conduction angle may be lost during a cycle if the control signal comes late. For example, if the control signal comes nearly at the end of the cycle, the Triac may not have sufficient time to be fully turned on and consequently the conduction angle which should have represented the short conduction time in this case maybe lost. The low DC voltage that contains the low frequency AC voltage ripples, when supplied to the WLED driver 107, may cause the WLED 109 to be flickering during the dimming process.
As illustrated in
During a dimming process, Triac dimmer 202 receives the high AC supply voltage U1 (
Electronic transformer 204 detects said conduction angles via said conduction angle detection module 301 and generates said first PWM signal Ua (
Comparing said DC voltage signal Udc with a triangle waveform, a second PWM signal Um (
Said WLED driver 206 drives said WLED 208 with constant current when said PWM low DC voltage U3 is in a high level, and does not supply current to said WLED 208 when said PWM low DC voltage U3 is in low level. Current (ILED) flowing through WLED 208 is illustrated in
According to the present disclosure, said PWM low DC voltage U3 from the electronic transformer 204 has a frequency and a duty cycle that are the same as or at least generally similar to those of said second PWM signal Um, thus, the frequency of said PWM low DC voltage U3 is higher than that of the conduction angles. Therefore, said PWM low DC voltage U3 generally does not contain low frequency AC voltage ripples that are of frequency of 50 Hz or lower, which at least reduces the risk of flicking by the WLED 208 during the dimming process. As such, embodiments of the Triac dimmer compatible driving circuit 200 and associated methods thereof can achieve smooth dimming for WLEDs with satisfactory dimming performance.
According to one embodiment of the present disclosure, said conduction angle detection module 301 can be implemented by a circuitry 500 comprising a rectifier circuit 501 and an analogous linear regulator circuit 502 as illustrated in
Analogous linear regulator circuit 502 comprises a first resistor R1, a first Zener diode D5, a second Zener diode D6, a transistor Q1, a second resistor R2 and a capacitor C1. Said first resistor R1 is coupled to node L1 at one terminal and to the cathode of said first Zener diode D5 at the other terminal; the anode of said first Zener diode D5 is coupled to the cathode of said second Zener diode D6 and the gate terminal of said transistor Q1; the anode of said second Zener diode D6 is coupled to ground; the drain terminal of said transistor Q1 is coupled to node L1 and the source terminal of said transistor Q1 is coupled to ground via said second resistor R2 and said capacitor C1 which are coupled in parallel.
The source terminal of transistor Q1 is configured as the output terminal of said circuitry 500. In operation, rectifier circuit 501 converts the original negative part of said high AC voltage U2 (
When the voltage across Zener diode D6 also reaches its reverse break down voltage, it stays at its reverse break down voltage. This allows the voltage Ua to be clamped to a voltage that is nearly of the reverse break down voltage of Zener diode D6, generally the reverse break down voltage of Zener diode D6 minus the gate to source voltage of transistor Q1. The reverse break down voltages of Zener diodes D5 and D6 are typically not very high, and thus are quick to reach, so the rising edge of the voltage Ua is basically in accordance with the moment when the internal Triac of the Triac dimmer 202 is turned on. Similarly, the falling edge of the voltage Ua is basically in accordance with the moment when the internal Triac of the Triac dimmer 202 is tuned off. Thus, the voltage Ua is a pulse signal whose pulse width is in accordance with the ON time of the internal Triac of the Triac dimmer 202, and accordingly implements the detection of conduction angles of said high AC voltage U2.
It should be understood by those skilled in the art that various modifications and variations can be made to circuitry 500, for example, it is possible to remove said first Zener diode D5 without influencing the conduction angle detection function, and it is also possible to replace said transistor Q1 with any other controllable transistor devices, such as a bi-polar junction transistor (“BJT”).
According to certain embodiments of the present disclosure, it is to be understood by those skilled in the art that a zero cross detection comparator can be used to replace said analogous linear regulator circuit 502 in said circuitry 500. In this case, said zero cross detection comparator receives and processes said line voltage UL1 to generate said first PWM signal Ua so that once said line voltage UL1 is higher than zero, said first PWM signal Ua changes to high level, once said line voltage UL1 falls to or below zero, said first PWM signal Ua changes to low level. In this way, said first PWM signal Ua represents conduction angles of said high AC voltage U2 in its pulse width.
In other embodiments of the present disclosure, said conduction angle detection module 301 comprises a low pass filter and a PWM comparator. Said low pass filter is configured to receive said first PWM signal Ua and convert it into said DC voltage signal Udc; said PWM comparator is configured to receive said DC voltage signal Udc and compare it with a triangle signal to generate said second PWM signal Um. In further embodiments of the present disclosure, said converter module can be any AC to DC converter that converts a high AC voltage into a low DC voltage.
The above detailed description of the embodiments of the technology is not intended to be exhaustive or to limit the technology to the precise form disclosed above. While specific embodiments of, and examples for, the technology are described above for illustrative purposes, various equivalent modifications are possible within the scope of the technology, as those skilled in the relevant art will recognize. For instance, while specific component values and voltage values are provided herein, it is to be appreciated that these values are for the sake of illustration and explanation. Various embodiments of the technology may utilize values that are different from what is specified herein.
These modifications can be made to the technology in light of the above detailed description. The terms used in the following claims should not be construed to limit the technology to the specific embodiments disclosed in the specification and claims. Rather, the scope of the technology is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200910310660.9 | Nov 2009 | CN | national |
