GB2533303A

GB2533303A - Method for dimming at least one LED

Info

Publication number: GB2533303A
Application number: GB1422294.7A
Authority: GB
Inventors: Jermyn Philip
Original assignee: Tridonic GmbH and Co KG
Current assignee: Tridonic GmbH and Co KG
Priority date: 2014-12-15
Filing date: 2014-12-15
Publication date: 2016-06-22

Abstract

A trailing-edge dimmer and method for dimming at least one lamp, preferably a LED, by controlling an amount of power delivered from an alternating current (AC) power source to the lamp LOAD via a semiconductor switch Q4, comprises means R1, R5, C2 for integrating the voltage delivered by the AC power source after the start of each mains half-cycle and a comparator U3 for comparing the integrated voltage across capacitor C2 with a dimming reference value set by potentiometer VR1, whereby the dimming reference value is defined by the dimming level setting of a control module and the semiconductor switch is opened when the integrated voltage becomes equal to the dimming reference value.

Description

Method for dimming at least one LED * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ** The invention relates to converter modules for allowing dimmable operation of LED; which are dimmable by a phase control dimmer (Triac). Triacs are still widely used as infrastructure for dimming of lighting bulbs.

It is a known fact that lighting bulbs directly convert the leading phase angle to a corresponding light output. The operation of a dimmable converter module is different as the leading phase angle cannot be converted directly to a corresponding variable light output in a typical LED operating device.

The converter module can be configured for operating of at least one LED or one LED bulb, e.g. a LED retrofit lamp, which may show a limited dimming functionality. Alternatively or additionally, the converter module may also be part of an LED lamp or LED bulb or LED retrofit lamp, e.g. as a module integrated into the bulb/lamp.

The converter module may include a control unit/converter driver, which either controls a flyback converter with a galvanic separated primary and secondary side, which e.g. is operated in borderline mode. Other converter topologies are also possible to be used with the inventive concepts presented.

Typically, the supply current/voltage is a mains current/voltage, but may also be a different input voltage. In operation, a variation or fluctuation of the supply current voltage, e.g. resulting from a phase cutting dimming action, may result in a variation or fluctuation of the LED current.

Power control methods and apparatuses in which a switching power supply provides power factor correction and an output voltage to a load via control of a single switch, without requiring any feedback information associated with the load are known from W0200/8137460 A2. There, a single switch may be controlled without monitoring either the output voltage across the load or a current drawn by the load, without regulating either the output voltage across the load or the current drawn by the load.

The RMS value of an AC input voltage to the switching power supply may be varied via a conventional AC dimmer to in turn control the output current. The switching power supply comprises a flyback converter configuration, a buck converter configuration, or a boost converter configuration, and the load may comprise an LED-based light source.

The existing methods have several distinct drawbacks, which have not been fully solved through LED driver design. These problems include instabilities resulting out of the phase-cut waveform whereas those instabilities are causing flickering at various frequencies. In addition fluctuations of the brightness may occur at low dimming levels due to variations of the mains voltage and noise transmitted on the mains voltage. This may result as a shimmering effect in the light, ranging in frequency from * ..

* * * * a few hertz down to step changes in brightness lasting minutes or more. Another * * * * * problem may be the unsatisfactory brightness control by the dimmer control as the dimming range might be limited. Often the light will not go to full brightness and/or it cannot turn down to very low dimming levels. *

It is hence a goal of the invention, to provide a control module for dimming control of a lamp, preferably a LED operating device that allows dimming of LEDs connected 20 thereto to be dimmable by a phase control/phase-cutting leading-edge/Triac dimmer, while keeping the additional costs as low as possible.

This problem is solved with a device, method and system according to the independent claims. Further aspects of the invention are addressed in the dependent claims.

In a first aspect, the invention provides a control module for dimming control of a lamp, preferably a LED by controlling an amount of power delivered from an alternating current (AC) power source to the lamp, comprising a semiconductor switch configured to be coupled between the AC power source and the electrical load; and a control circuit operatively coupled to the semiconductor switch, the control circuit configured to: integrate the voltage delivered by the AC power source after start of each mains cycle (zero cross of the voltage); compare the integrated voltage with a dimming reference value; whereby the dimming reference value is defined by the * * * * * * * . . * * ** * * * dimming level setting of the control module, and open the semiconductor switch if the integrated voltage is equal to the dimming reference value.

* * * ** * * The control module may be configured to close the semiconductor switch after start of 5 each mains cycle.

The dimming level setting is set by a potentiometer.

The control module may comprise a rectifier bridge formed by four diodes, which is 10 directly connected with the LIVE connection of the AC power source with the first input and with the other input to the lamp, whereby the lamp is connected with the NEUTRAL connection of the AC power source.

* The outputs of the rectifier bridge may be connected to the semiconductor switch. * *

* * **** **** * 15 In another aspect, a method for dimming at least one lamp, preferably a LED by * * controlling an amount of power delivered from an alternating current (AC) power source to the lamp is provided, comprising the steps of integrate the voltage delivered * * * * * by the AC power source after start of each mains cycle; compare the integrated voltage * * * **** with a dimming reference value; whereby the dimming reference value is defined by the dimming level setting of the control module, and open a semiconductor switch if the integrated voltage is equal to the dimming reference value.

In yet a further aspect, a system comprising a phase controlled/triac dimmer and a converter module as described above is provided.

Further aspects of the invention are now explained with reference to the drawings. In particular, Fig. 1 shows waveform of the dimmer signal according to the prior art; Fig. 2 shows an exemplary waveform of the dimmer signal according to the invention; Fig. 3 shows an exemplary dimmer circuit according to the invention; In general, the invention aims at providing a smooth and flicker-free dimming, which can be achieved in connection with a great majority of dimmers, decreasing the 5 amounts of parts and bill of material, which at the same time can be adapted to work with different ICs.

Fig. 1 shows diagrammatically the waveform generated by a conventional phase cut dimmer.

Fig. 1 shows the behavior of the conventional dimmer at normal mains input and with a fluctuation of the mains voltage provided by the AC power source. Phase cut * * ** * dimmers used with LEDs are usually of the trailing-edge type, in order to eliminate *.* * sharp turn-on current transients which are typically seen when triac-based leading-edge * *** 15 dimmers drive capacitive loads. These dimmers begin each mains half-cycle in the * * *-** * "on" state, and switch off after a period of time determined by the dimmer brightness * * setting. The "on" time is measured by a simple timer, which is reset at the start of each half-cycle. If the mains voltage changes, the mean output voltage from the dimmer also * * * changes, because the "on" time remains constant. * *

* ** 20 Because the dimmer output voltage changes, the LED driver tends to respond by changing the light output. This can be prevented, but an expensive multi-stage driver is required.

Fig. 2 shows exemplary the waveform generated by a phase cut dimmer according to this invention.

The dimming approach according to this invention also uses the trailing-edge method. However the "on" period is not measured by a simple timer. Instead, the time is determined according to the integrated voltage over the.time following the start of each mains half cycle of the voltage provided by the AC power source. The start of each mains half cycle is the zero cross of the voltage provided by the AC power source.

When the integral reaches a value determined by the dimmer setting, the dimmer switches off If the mains voltage changes, the mean output voltage from the dimmer does not change, because the "on" time adjusts to maintain a constant integral value. As a result, fluctuations in the light output are greatly reduced.

Figure 3 shows an exemplary dimmer circuit according to the invention.

In order to perform the integration function, the dimmer must be able to "see" the full mains voltage at all times. A neutral connection is therefore required, something that is generally absent on standard dimmers. However the current drawn by the control module and thus delivered to the neutral line is extremely small for the circuit according to the invention, so a ground (earth) connection could be used instead. * CO * * *

* * * . In addition to eliminating the light shimmering effect seen with standard dimmers at * * low brightness, an additional advantage of the new method is that the timing circuitry **** * * * * * * is self-contained and operates independently of the load. Standard dimmers without * * * neutral connections rely on the load to provide the dimmer's operating current while the dimmer is off. This works well with resistive loads, but causes problems with the *r o.

* * reactive inputs presented by LED drivers. The result is the flickering and poor dim The circuit part to the left of the line shows the control circuit which forms voltage integrating timer function of the control module. The circuitry to the right of the line performs the mains switching of the voltage provided by the AC power source 25 required for phase-cut control.

The timing circuitry is powered by a 3V regulated supply U1, supplied by a resistive voltage dropper (R49, R47, Z1) from the rectified mains provided by the AC power source. The integration of the mains voltage (the voltage provided by the AC power source) is done by the simple RC timer (capacitor C2, RI and R5). Capacitor C2 charges up to around 2.8V during a complete mains half-cycle, after which it is discharged by Ml ready for the next cycle. ** * * *

** * 20 profile often encountered.

The operational amplifier U3 acts as a threshold detector, changing state from Low to High when the voltage across capacitor C2 reaches the dimming reference value set by potentiometer VR1.

Control of the discharge transistor M1 is done by operational amplifier U2, which acts as another threshold detector. It compares a divided down fraction of the mains waveform (R8, R9, R2) with the low voltage provided by divider R3 and R4. The operational amplifier U2 gives a short positive-going pulse each time the mains waveform descends to zero, turning on MI to discharge capacitor C2. * **

* * * ** * * * * **** ** *** * * * * * **,*** * * *** * * 0 * * The output of operational amplifier U3 is therefore Low from the start of each mains half cycle, until the voltage across U2 reaches the required threshold set by VRI. The operational amplifier U3 then switches to High, and remains there until the end of the half cycle.

The mains switching circuitry (to the right of the red line) is based around the semiconductor switch Q4, preferably a bipolar power transistor, which is connected to the output of bridge rectifier RB. The bridge directs the mains current to pass through the semiconductor switch Q4 in the correct direction at all times as it changes polarity.As explained the control module comprises a rectifier bridge RB formed by four diodes, which is directly connected with the LIVE connection of the AC power source with the first input and with the other input to the lamp, whereby the lamp is connected with the NEUTRAL connection of the AC power source. The outputs of the rectifier bridge RB are connected to the semiconductor switch Q4.

The semiconductor switch Q4 is connected in a darlington configuration with transistors Ql, Q2, Q3. The darlington combination Ql, Q2, Q3 has an enormous current gain. As the darlington combination Q1, Q2, Q3 is connected via the resistors R7 and R10 to the positive output Doutl of the bridge rectifier RB, so it is held in Rill conduction by 1 megohm resistors R7 and R10. The dimmer conduction state of the control module is therefore "normally on" (which equals the switching state of semiconductor switch Q4), but it can be turned off by switching on transistor Q5 which is part of the control circuit.

a * * 10 * * * ** * * * * * * * * * * . * * * * ** When the output of operational amplifier U3 goes high, transistor Q5 is switched on and thus the control output of the control circuit and the semiconductor switch Q4 is opened and thus dimmer ceases conduction until the beginning of the next half cycle.

In result, depending on the phase cut, a voltage characteristic as exemplarily shown in Fig. 2 might be generated.

Therefore a method for dimming at least one lamp, preferably a LED by controlling an amount of power delivered from an alternating current (AC) power source to the lamp is possible, comprising the steps of integrate the voltage delivered by the AC power source after start of each mains cycle; compare the integrated voltage with a dimming reference value; whereby the dimming reference value is defined by the dimming level setting of the control module, and open a semiconductor switch if the integrated voltage is equal to the dimming reference value. 1. * ** * * *** * * * Of* * * * ** ** * * 2. * * ** *

Claims

* * * 20 * ** 3. 4. 5.Claims A control module for dimming control of a lamp, preferably a LED by controlling an amount of power delivered from an alternating current (AC) power source to the lamp, comprising a semiconductor switch configured to be coupled between the AC power source and the electrical load; and a control circuit operatively coupled to the semiconductor switch, the control circuit configured to: integrate the voltage delivered by the AC power source after start of each mains cycle; compare the integrated voltage with a dimming reference value; whereby the dimming reference value is defined by the dimming level setting of the control module, and open the semiconductor switch if the integrated voltage is equal to the dimming reference value.

A control module according to claim 1, wherein the control circuit is configured to close the semiconductor switch after start of each mains cycle.

Control module according to claim 1 or 2, wherein the dimming level setting is set by a potentiometer.

Control module according to any one of the preceding claims, wherein the control module comprises a rectifier bridge formed by four diodes, which is directly connected with the LIVE connection of the AC power source with the first input and with the other input to the lamp, whereby the lamp is connected with the NEUTRAL connection of the AC power source.

Control module according to claim 4, wherein the outputs of the rectifier bridge are connected to the semiconductor switch.

6. A method for dimming at least one lamp, preferably a LED by controlling an amount of power delivered from an alternating current (AC) power source to the lamp, comprising the steps of integrate the voltage delivered by the AC power source after start of each mains cycle; compare the integrated voltage with a dimming reference value; whereby the dimming reference value is defined by the dimming level setting of the control module, and open a semiconductor switch if the integrated voltage is equal to the dimming reference value. * * * * * ** * * * * Ogee * * 0000 * * * * * * * * * * **