LED LIGHT SOURCE
LED light source suitable for use with a phase cut dimmer (III), comprising •input terminals (K1, K2) for connection to an AC supply voltage, •a rectifier (I) coupled to the input terminals, 108 a series arrangement comprising N LEDs (LL1, LL2, LL3) coupled to the rectifier, •control circuitry (EX) for sequentially activating (deactivating) the LEDs as the supply voltage increases (decreases), wherein the control circuitry comprises: •N controllable current regulators, each regulator being coupled between the cathode of a LED and ground and comprising a transistor and one impedance in series, •a global current control circuit (X) coupled to the control electrodes of said transistors and comprising circuitry for adjusting the voltage at said control electrodes in dependency of the phase angle, •a series arrangement of a switch (S) and a capacitor (C1) coupled- to the rectifier and •means (VIII) for rendering said switch conductive in case the AC supply voltage drops below a reference value.
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The invention relates to a cheap and simple LED light source comprising N LED loads that is directly connectable to a supply source supplying a low frequency AC voltage such as the mains supply.
BACKGROUND OF THE INVENTIONSuch a LED light source is known from U.S. Pat. No. 7,081,722 B1. The LED loads are LED arrays comprising series arrangements and possibly parallel arrangements of individual LEDs. The known LED light source comprises a rectifier for rectifying the low frequency AC supply voltage. A series arrangement comprising the N LED loads is connected to output terminals of the rectifier. During operation a periodical DC voltage with an instantaneous value varying between zero Volt and a maximum amplitude is present between the output terminals of the rectifier. The known LED light source is equipped with control means for subsequently making the LED loads conduct a current, one by one and starting with a first LED load that is closest to a first end of the series arrangement, in dependency of the instantaneous value of the low frequency AC supply voltage when the instantaneous value increases and for subsequently making the LED loads stop conducting a current, one by one and starting with the Nth LED load, in dependency of the instantaneous value of the low frequency AC supply voltage when the instantaneous value decreases. These control means typically comprise N control strings, each comprising a transistor and being coupled between the cathode of one of the LED loads and an output terminal of the rectifier.
When the instantaneous value of the periodical DC voltage is zero Volt, all of the transistors comprised in the control strings are conductive but none of the LED loads carries a current. When the instantaneous value of the periodical DC voltage increases, a voltage is reached at which a first LED load and the first transistor comprised in the first control string start conducting a current. Similarly, when the instantaneous value of the periodical DC voltage has increased further to a high enough value, the second LED load and the transistor in the second control string start conducting. In order to minimize power dissipation it is desirable to make sure that the current through the first control string is reduced and preferably stopped.
In the case of further increase of the instantaneous value of the periodical DC voltage, the remaining LED loads and the transistors comprised in the control strings connected to the cathodes of these LED loads subsequently start conducting a current. When the nth control string carries a current, the control means ensure that the currents in the first n−1 control strings are reduced or stopped. When all of the LED loads conduct a current, the Nth transistor conducts a current and the instantaneous value of the periodical DC voltage increases further until the maximum amplitude is reached. After that the instantaneous value of the periodical DC voltage starts decreasing. While the instantaneous value decreases, the LED loads stop conducting a current one by one in reversed order (first the Nth LED load stops conducting and the first LED load is the last to stop conducting). When the nth LED load stops conducting, the (n−1)th control string starts conducting a current. The nth transistor remains conductive but no longer carries a current or only carries a strongly reduced current. After the first LED load has stopped conducting, all transistors are conductive but none conducts a current, the instantaneous value of the periodical DC voltage decreases further to zero and then the cycle described here-above is repeated. The known LED light source is very compact and comparatively simple. Furthermore, it can be directly supplied from a low frequency AC supply voltage source such as the European or American mains supply.
Dimming of a LED light source according to the invention can for instance be effected by adjusting the voltage present at the control electrodes of the transistors comprised in the control strings via the global current control circuit. However, it is generally desirable that the LED light source is not just dimmable but suitable for use with a phase cut dimmer such as a TRIAC dimmer.
SUMMARY OF THE INVENTIONIt is an object of the present invention to provide a LED light source that can be dimmed making use of a phase cut dimmer such as a TRIAC dimmer.
According to a first aspect of the invention A LED light source is provided, suitable for use with a phase cut dimmer such as a TRIAC dimmer, and comprising
-
- a first input terminal and a second input terminal for connection to a supply voltage source supplying a low frequency AC supply voltage with frequency f,
- a rectifier coupled to the input terminals for rectifying the low frequency AC supply voltage,
- a series arrangement comprising N LED loads, a first and second end of said series arrangement being coupled to a first output terminal and a second output terminal of the rectifier,
- control circuitry for subsequently making the LED loads conduct a current, one by one, in dependency of the instantaneous value of the low frequency AC supply voltage when the instantaneous value increases and for subsequently making the LED loads stop conducting a current, one by one, in dependency of the instantaneous value of the low frequency AC supply voltage when the instantaneous value decreases, wherein the control circuitry comprises
- a global current control circuit coupled to the controllable current regulators for controlling the instantaneous value of the current through the control strings, wherein the controllable current regulators each comprise a series arrangement of a transistor and at least one impedance, and wherein the global current control circuit is coupled to the control electrodes of the transistors comprised in the controllable current regulators and comprises circuitry for adjusting the voltage present at the control electrodes of the transistors comprised in the control strings, in dependency of the adjusted phase angle of the phase cut dimmer, and
- a series arrangement of a switch and a capacitor coupled between the output terminals of the rectifier and control circuitry for rendering the switch conductive in case the instantaneous value of the low frequency AC supply voltage drops below a reference value.
During operation, the capacitor is charged to nearly the peak value of the rectified mains voltage and it supplies energy to the LED loads when the rectified mains voltage has dropped below the reference value. Stroboscopic effects are thereby suppressed. By controlling the voltage at the control electrodes of the transistors comprised in the controllable current regulators in dependency of the adjusted phase angle of the phase cut dimmer, it is possible to dim the LED light source by adjusting the phase angle of the phase cut dimmer.
According to a further aspect, a method is provided for supplying a LED light source, comprising the steps of
-
- providing a supply voltage source supplying a low frequency AC supply voltage with frequency f,
- rectifying the low frequency AC supply voltage,
- providing a series arrangement of a switch and a capacitor coupled between the output terminals of the rectifier and control circuitry for rendering the switch conductive in case the instantaneous value of the low frequency AC supply voltage drops below a reference value,
- supplying the rectified AC supply voltage to a series arrangement of N LED loads,
- supplying the voltage across the capacitor to the series arrangement of N LED loads, when the instantaneous value of the low frequency AC supply voltage drops below the reference value,
- subsequently making the LED loads conduct a current, one by one, in dependency of the instantaneous value of the supplied voltage when the supplied voltage increases and subsequently making the LED loads stop conducting a current, one by one, in dependency of the instantaneous value of the supplied voltage when the instantaneous value of the supplied voltage decreases,
- providing N control strings comprising a series arrangement of a transistor and an impedance and coupled between the cathode of a LED load and the second output terminal of the rectifier, and
- controlling the instantaneous value of the current through the control strings by controlling the voltages at the control electrodes of the transistors comprised in the control strings.
The method has the same advantages as the LED light source according to the first aspect.
Preferably a LED light source according to the invention comprises a current bleeder for providing a conductive path for a TRIAC dimmer, when used in combination with such a dimmer. Wrong triggering of the TRIAC comprised in a TRIAC dimmer causing light flickering is thereby prevented. The current bleeder draws a current higher than the holding current of the TRIAC so that multi-triggering of the TRIAC (also causing light flickering) is also prevented.
Preferably a LED light source according to the invention comprises a clamping circuit to enable correct functioning of a timer comprised in a TRIAC dimmer, when the LED light source is used in combination with such a TRIAC dimmer.
More preferably the current bleeder and the clamping circuit are combined into one linear current source.
A LED light source according to the invention preferably comprises a damping circuit for damping an oscillation in an oscillation circuit formed by an inductive element and a capacitor comprised in an EMI filter in a TRIAC dimmer, when the LED light source is used in combination with such a TRIAC dimmer. These oscillations, when not sufficiently damped, cause malfunctioning of the TRIAC dimmer.
Preferably a LED light source according to the invention comprises an RC shunt circuit to draw a current from a TRIAC dimmer when the TRIAC is triggered, in the case that the LED light source is used in combination with such a dimmer. It is thereby prevented that the firing of the TRIAC is unsuccessful which might result in multi-firing of the TRIAC which in turn causes light flicker. In addition, the RC shunt circuit helps also to effectively damp the oscillation at the moment when the TRIAC is triggered.
As explained hereabove, a LED light source according to the invention comprises a series arrangement of a switch and a capacitor, coupled between the output terminals of the rectifier, and control circuitry for rendering the switch conductive in case the instantaneous value of the low frequency AC supply voltage drops below a reference value.
The capacitor is referred to as fill-in capacitor and is used as a supply voltage source, when the instantaneous value of the low frequency AC supply voltage is too low for causing a current to flow through any of the LED loads. When the LED light source is not dimmed, this happens directly before and after the zero crossing of the mains supply voltage. Both the occurrence of light flicker and stroboscopic effect are thereby prevented. In case the LED light source is used in combination with a TRIAC dimmer, the LED light source preferably further comprises a linear regulator to control the charging current of the capacitor. The linear regulator controls the charging current at a level that is low compared to the repetitive current flowing through the RC shunt, so a too high total current causing damage to the dimmer and possibly audible noise generated by the dimmer is prevented.
A LED light source according to the invention comprising a number of the features or all the features mentioned here-above to make the LED light source compatible with a TRIAC dimmer preferably comprises one or more half bridge rectifiers. By configuring different features such as for instance the fill-in capacitor and the RC shunt at the output of different rectifiers, the operation of each feature is not interfered with by the operation of the others, so that proper operation of all features is obtained.
Embodiments of a LED light source according to the invention will be further discussed with reference to a drawing.
In the drawing,
embodiment shown in
dimmer and the power consumed by the LED loads.
In
Fill in capacitor C1 and switch S are connected between output terminals of the rectifier I. Circuit part VIII is coupled to switch S and is the control circuitry for switch S. LL1, LL2 and LL3 are three LED loads in series arrangement and circuit part IX represents control circuitry for controlling the current through one or more LED loads in dependency of the instantaneous value of the rectified AC voltage. This control circuitry typically comprises N control strings, each comprising a transistor and coupled between the cathode of one of the LED loads and an output terminal of the rectifier and circuitry for controlling the conductive state of the transistors. A possible implementation of such circuitry is for instance disclosed in
In
The top curve V1 in
When the TRIAC is triggered, the instantaneous value of the phase cut input voltage is higher than zero again and both transistors Qb and Qc are maintained in a conductive state until the instantaneous value of the phase cut input voltage drops to a value equal to the setpoint for bleeding once more. The cycle is illustrated in
For TRIAC dimmer compatibility it is necessary that the conduction angle of the TRIAC dimmer is translated into a voltage that is present on the control electrodes of the transistors comprised in the control strings of the control circuitry of the LED light source. The instantaneous value of this signal determines the current through the LED loads. This signal must be varied as a function of the dimmer conduction angle and can for instance be derived from the dimmer conduction angle as follows. The dimmer conduction angle is detected by sensing the rectified input voltage through a comparator. This angle information then controls a transistor and resistor network coupled to the low voltage supply. The output voltage of this network is a pulse width modulated signal, whose duty cycle corresponds to the dimmer conduction angle, i.e. maximum duty cycle at dimmer highest setting (maximum output power), and smallest duty cycle at lowest setting. The resulting signal is then averaged through an RC network. Furthermore, a saturation circuit is placed; the averaged signal will be clamped at certain level. As a result, the dimming curve shows a flat part in the region close to the maximum output power. This is useful to achieve a matched maximum light output between different dimmers (the maximum achievable conduction angle is different from dimmer to dimmer). Finally, the averaged DC signal is used to control the control strings through a voltage follower which performs impedance matching and increases driving capability.
Claims
1. A LED light source, for use with a phase cut dimmer comprising: wherein the control circuitry comprises
- a first input terminal and a second input terminal for connection to a supply voltage source supplying a low frequency AC supply voltage with frequency f,
- a rectifier coupled to the input terminals for rectifying the low frequency AC supply voltage,
- a series arrangement comprising N LED loads, a first and second end of said series arrangement being coupled to a first output terminal and a second output terminal of the rectifier,
- control circuitry for subsequently making the LED loads conduct a current, one by one, in dependence on the instantaneous value of the low frequency AC supply voltage when the instantaneous value increases and for subsequently making the LED loads stop conducting a current, one by one, in dependence on the instantaneous value of the low frequency AC supply voltage when the instantaneous value decreases,
- each N control strings comprising a controllable current regulator and being coupled between the cathode of a respective LED load and the second output terminal of the rectifier, and
- a global current control circuit coupled to the controllable current regulators for controlling the instantaneous value of the current through the control strings, wherein the controllable current regulators each comprise a series arrangement of a transistor and at least one impedance, and wherein the global current control circuit is coupled to the control electrodes of the transistors comprised in the controllable current regulators and comprises circuitry for adjusting the voltage present at the control electrodes of the transistors comprised in the control strings, in dependency of the adjusted phase angle of the phase cut dimmer, wherein
- a series arrangement of a switch and a capacitor coupled between the output terminals of the rectifier and control circuitry for rendering the switch conductive in case the instantaneous value of the low frequency AC supply voltage drops below a reference value; and
- wherein the phase cut dimmer is a TRIAC dimmer and the LED light source comprises an RC shunt circuit for providing reliable latching of the TRIAC dimmer.
2. The LED light source as claimed in claim 1, wherein the LED light source comprises a current bleeder for providing a conductive path for the phase cut dimmer.
3. The LED light source as claimed in claim 1, wherein the LED light source comprises a clamping circuit to enable correct functioning of a timer comprised in the phase cut dimmer.
4. The LED light source as claimed in claims 2, wherein the current bleeder and the clamping circuit are combined into one linear current source.
5. The LED light source as claimed in claim 1, wherein the LED light source comprises a damping circuit for damping an oscillation in an oscillation circuit formed by an inductive element and a capacitor comprised in an EMI filter in the phase cut dimmer.
6. (canceled)
7. The LED light source as claimed in claim 1, wherein the LED light source further comprises a linear regulator to control the charging current of the capacitor.
8. The LED light source as claimed in claim 7, wherein the LED light source comprises two half bridge rectifiers.
9. A method for supplying a LED light source, comprising the steps of:
- providing a supply voltage source supplying a low frequency AC supply voltage with frequency f,
- rectifying the low frequency AC supply voltage,
- supplying the rectified AC supply voltage to a series arrangement of N LED loads,
- supplying the voltage across the capacitor to the series arrangement of N LED loads, when the instantaneous value of the low frequency AC supply voltage drops below the reference value,
- subsequently making the LED loads conduct a current, one by one, in dependency of the instantaneous value of the supplied voltage when the supplied voltage increases and subsequently making the LED loads stop conducting a current, one by one, in dependency of the instantaneous value of the supplied voltage when the instantaneous value of the supplied voltage decreases,
- providing N control strings, each control string comprising a series arrangement of a transistor and an impedance and coupled between the cathode of a respective LED load and the second output terminal of the rectifier, and
- controlling the instantaneous value of the current through the control strings by controlling the voltages at the control electrodes of the transistors comprised in the control strings, further comprising the steps of
- providing a series arrangement of a switch and a capacitor coupled between the output terminals of the rectifier and control circuitry for rendering the switch conductive in case the instantaneous value of the low frequency AC supply voltage drops below a reference value,
- providing a TRIAC dimmer, wherein the LED light source comprises an RC shunt circuit for providing reliable latching of the TRIAC dimmer.
Type: Application
Filed: May 30, 2012
Publication Date: Dec 25, 2014
Applicant: KONINKLIJKE PHILIPS N.V. (EINDHOVEN)
Inventors: Haimin Tao (Eindhoven), Henricus Marius Joseph Maria Kahlman (Dongen), Ralph Kurt (Eindhoven)
Application Number: 14/124,731
International Classification: H05B 33/08 (20060101);