WO2015048834A1

WO2015048834A1 - Method for dimming at least one led

Info

Publication number: WO2015048834A1
Application number: PCT/AT2014/050230
Authority: WO
Inventors: Phillip Jermyn
Original assignee: Tridonic Gmbh & Co Kg
Priority date: 2013-10-01
Filing date: 2014-10-01
Publication date: 2015-04-09
Also published as: GB2518852A; GB201317422D0

Abstract

A converter module for dimming LEDs, comprising a switched/clocked converter, e.g a flyback, adapted to supply power to at least one LED, the converter being operated in a borderline mode and comprising a transformer with an additional bifilar winding coupled to its primary winding, a detection means (an auxiliary winding inductively coupled to the transformer) adapted to generate an activation signal based on a detected current/voltage input of the converter, a switching stage adapted to connect the bifilar winding to the negative supply rail (through a diode) according to the activation signal.

Description

Method for dimming at least one LED

The invention relates to converter modules for allowing dimmable operation of LEDs, 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 according to the invention includes 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 input voltage.

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 WO200/8137460 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 voltage. The switching power comprises a flyback converter configuration, a buck converter configuration, or a boost converter configuration, and the load may comprise an LED-based light source.

There are dimmable also known control units, which show a variety of approaches to the problem of controlling Triac behavior/phase control dimming and converting the phase angle into an LED current. A disadvantage of these approaches is that they require a high bill of materials (BOM) and, e.g. require double-sided printed circuit boards (PCBs)

It is hence a goal of the invention, to provide a LED operating device that allows dimming of LEDs connected 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 converter module for dimming of at least one LED, comprising a switched/clocked converter supplying power to the at least one LED, the converter comprising a transformer and advantageously being operated in a borderline mode, the converter comprising a bifilar primary winding, wherein the bifilar primary winding can be activated depending on the current value of the input voltage and wherein the converter module being configured to be controlled by a phase controlled/triac dimmer.

In another aspect, the invention provides a converter module for dimming of at least one LED, comprising a switched/clocked converter supplying power to the at least one LED, the converter comprising a transformer with a bifilar primary winding and advantageously being operated in a borderline mode, a detection means for detecting a current/voltage input to the converter, in particular on its primary side, and a switching stage adapted to, activate the bifilar primary winding depending on the current/voltage detected at the detection means.

The converter may be a flyback converter. The converter can be controlled by a control unit. In borderline mode, the converter switch may be switched on, when the input current reaches a predefined threshold level, e.g. 0 Ampere.

A detection means can be provided to switch the converter switch dependent on the input current/voltage of the converter.

The detection means can be an auxiliary winding, in particular galvanically separated from a converter winding, especially arranged on the converter's primary side. The detection means can feed back a parameter indicating indirectly or directly the input current/voltage of the converter.

The control unit may be a PFC fixed controller and/or a PFC fixed output controller.

The control unit can comprise a DC-bus capacitor and/or a shaping circuit adapted to control an input current/voltage drawn from a supply voltage source during a half- cycle.

A PFC, in particular a passive PFC circuit, can be comprised in the control unit and/or in the converter module. The control unit may comprise a switch of the flyback converter.

A feedback signal can be supplied to the control unit from the detection means.

The detection means may be provided on the primary side of a flyback converter and preferably feeds back, directly or indirectly, a parameter to the control unit indicative of the current/voltage through the primary side of the flyback converter. The detection means can feed back a current/voltage dependent on the input current/voltage to the module as an activation signal, and the fed back current/voltage may connect via a switching stage the bifilar primary winding to the negative supply rail.

The bifilar primary winding may be connected via a Zener diode to the negative supply rail. The Zener diode may be arranged in parallel to the switching stage. The Zener diode may pass through a current from the bifilar primary winding but take over a certain voltage from the bifilar winding in case the switching stage is blocked.

An energy storage unit is provided, can be particular on the primary side of the flyback converter.

In particular on the secondary side of the flyback converter, a storage capacitor can be provided dimensioned to compensate ripple on the input current/voltage supplied to the converter module.

The switch-off threshold of the control unit and/or the flyback switch can be adaptively set as a function of the input current/voltage of the converter to facilitate PFC functionality.

In the feedback path from the detecting means to the control unit, a second Zener diode may be connected to clamp the negative current/voltage fed back to the control unit to a maximal value. The clamping may be additionally or alternatively performed internally in the control unit and/or by software of the control unit.

In another aspect, a method for dimming at least one LED is provided, comprising the steps of supplying a converter module with an input current/voltage controlled by a phase controlled/triac dimmer, supplying power to the at least one LED by a switched/clocked converter, the converter comprising a transformer and advantageously being operated in a borderline mode, the converter comprising a bifilar primary winding, wherein the bifilar primary winding can be activated depending on the current value of the input voltage and wherein the converter module being configured to be controlled by a phase controlled/triac dimmer.

In yet another aspect, a method for dimming at least one LED is provided, comprising the steps of supplying a converter module with an input current/voltage controlled by a phase controlled/triac dimmer, supplying power to the at least one LED by a switched/clocked converter, the converter comprising a transformer with a bifilar primary winding, and detecting, by a detection means, a current/voltage input to the converter, in particular on its primary side, activate the bifilar primary winding depending on the current/voltage detected by the detection means through control unit.

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 an exemplary dimming driver based on a PFC fixed output controller; Fig. 2 shows an exemplary input current waveform;

Fig. 3 shows an exemplary waveform of the switching stage;

Single-stage fixed output flyback driver control units (IC) with build-in power factor correction are known, however, it turns out that this control unit type can be used to make a simple and elegant dimming solution, with very good performance, as described herein.

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 amounts of parts and bill of material, which at the same time can be adapted to work with different ICs. Also, no high-voltage electrolytic capacitor needs to be present.

To achieve this, the circuit provided in the converter module for controlling the at least one LED, may provide a power factor correction (PFC) functionality, e.g. a "true" PFC circuit, a flyback converter with PFC functionality, etc. In particular, the invention exploits the high power factor of a PFC circuit. If a flyback converter is used, the current/voltage on the secondary side of the flyback converter, driving the at least one LED, mirrors the dimming performed via phase cutting or phase firing at the input of the primary side of the converter.

In this, the problem exists that the PFC circuit sometimes does not draw enough current, which is needed by the (Triac) dimmer used for phase cutting or phase firing dimming as holding current. Bleeding circuits are known to address this symptom and to guarantee the required holding current through their power loss. Typically these bleeding circuits are formed as conductive pathways that can be selectively activated or deactivated.

If a bifilar winding is coupled to the transformer or the converter and is directly connected to the rails of the input stage of such circuit, the supply voltage (input voltage) will be less than the reflected voltage from the bifilar winding for much of the time. Energy flows back during the periods when the input voltage is low, preventing supply current flowing in from the input during these periods. This impairs power factor and harmonics, and the supply waveform becomes distorted, which reduces dimming performance. Efficiency is also reduced due to the energy which is circulated between input and output.

Fig. 1 shows a setup, which is basically similar to the circuit covered by the patent application DE 102012220779.1. The description of DE 102012220779.1 is referenced to describe the circuit of this invention. Fig. 1 hence shows a circuit for a converter module including a PFC fixed output controller. The circuit with PFC functionality is realized by using a control unit Ul, which may be an integrated circuit, e.g. the flyback converter driver, optionally with an integrated switch. The circuit shown in Fig. 1 may drive one or several LED connected to the output connectors of the circuit.

However, the circuit of Fig. 1 shows a very low complexity of the converter itself using the latest fixed output control units. The circuit comprises a transformer LI, with a bifilar winding Lib arranged on the primary side. The bifilar winding Lib is an additional winding on the primary side of the transformer LI inductively coupled to the primary winding of the transformer LI. Further, an auxiliary winding Lla is provided, which is inductively coupled to the transformer LI of the converter and in which a current/voltage is generated depending on the supply voltage of the LED operating device. This generated voltage is supplied as an activation signal, and the fed back current/voltage may connect via a switching stage the bifilar primary winding to the winding to the negative supply rail. The switching stage preferably comprises a darlington pair formed by two bipolar transistors Q3 and Q4.

In order to facilitate dimming operation as function of a leading phase angle of the supply voltage, a feedback signal is fed back as an activation signal to the switching stage from a detecting means, e.g. an auxiliary winding Lla of the converter. By this, the switching stage may connect the bifilar primary winding Lib to the negative supply rail.

In addition the bifilar primary winding Lib may be connected via a Zener diode Z2 to the negative supply rail. The Zener diode Z2 may be arranged in parallel to the switching stage. The Zener diode Z2 may pass through a current from the bifilar primary winding Lib in case the switching stage is blocked. As the Zener diode Z2 shall have a sufficiently high zener voltage (e.g. 150V) it will clamp the voltage over the bifilar winding and will block this voltage from the input.

The bifilar winding may be wound in the standard way, but instead of being connected directly to the input rails (via D5), it is connected to the switching stage and the Zener diode Z2. When the switching stage with the darlington pair of transistors Q3 and Q4 is switched on, it connects the bifilar winding directly to the negative supply rail via diode D5. When the switching stage is switched off, the bifilar winding current must pass through Zener diode Z2. This zener diode Z2 has a high enough voltage (about 150 V) to block the reflected output voltage at the minimum supply bus voltage. The switching stage with the darlington pair Q3 and Q4 is controlled by the voltage provided by the transformer's auxiliary winding. When the main switch Ml is switched on, the auxiliary winding output is a negative voltage proportional to the bus voltage (input voltage of the converter). This is rectified by rectifying diode D6, creating a negative replica of the bus voltage across capacitor C9. This voltage drives the base of bipolar transistor Q4 via zener diode Z3, which sets the bus voltage at which turns the darlington pair of Q3 and Q4 on - e.g. at 230V. This value may be chosen to match the reflected output voltage.

Figure 4 shows the bus voltage (the input voltage of the converter) as upper trace and the voltage across transistor Q3 of the switching stage as lower trace. Note that it is negative. The voltage is zero during the mains peaks (peaks of the input voltage where the switching stage is on) and clamped to 150V by zener diode Z2 during the valleys of the input voltage.

If the converter is e.g. realized as a flyback converter, the circuitry of the converter module may provide an energy storage (capacitor) on the secondary side of the converter, the flyback providing a inductive separation between components arranged on its primary side and components arranged on its secondary side. The auxiliary winding as e.g. shown in Fig. 5 may also be located on the secondary side.

A flyback converter with constant on-time (T_on) could also realize the PFC functionality without a feedback supplied to the driver IC from the auxiliary winding. However, the power factor would not be as good. An example for a control unit used is HVLED805 by ST Microelectronics.

A single stage power factor corrected control unit may also have a small DC-bus capacitor in the region of about lOOnF. A flyback converter therefore receives as its input unsmoothed rectified mains half cycles which may descend all the way to 0V.

Application of such a driver IC allows for an excellent dimming performance with the majority of leading and trailing edge dimmers. The circuitry will typically fit onto a single-sided printed circuit board, and, since the technique will work with almost any PFC fixed controller, designs can be easily adapted to suit applications using the latest, low-cost ICs. As a control factor, the control unit generates a current/voltage, which is used to modify the current of the flyback converter on its primary input side. In particular, a dimming operation depends on the phase cutting as the peak current, i.e. the current at the flyback converter switch-off is modified, and the peak current is controlled to be basically proportional of the amplitude of the input voltage present at the moment.

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

It should be noted that the converter module circuit also may be implemented with other converters, such as buck converter topologies. Also, the control unit may comprise power factor correction (PFC) but this is not mandatory. However, improving the power factor of the converter module circuit is desirable and hence in the following a method to add power factor correction to any control unit is described, especially for control units with an external peak current sense resistor.

A control unit with only source input may be used (e.g. NXP SSL21083, which in contrast, e.g. HVLED815 has only one source input)

The control unit may contain a control system which includes a current sense resistor to measure a peak inductor current during the time that the main switch is on.

Figs. 2 shows the waveform of the input voltage of the converter (over capacitor CI) as the upper trace over some cycles of the mains voltage and the input current of the converter (lower trace).

Claims

1. A converter module for dimming of at least one LED, comprising

a switched/clocked converter supplying power to the at least one LED, the converter advantageously being operated in a borderline mode,

the converter comprising a transformer with a bifilar primary winding, and wherein

wherein the bifilar primary winding can be activated depending on the current value of the input voltage.

2. A converter module for dimming of at least one LED, comprising

the converter comprising a transformer with a bifilar primary winding, a detection means for detecting a current/voltage input to the converter, in particular on its primary side, and

and a switching stage adapted to activate the bifilar primary winding depending on the current/voltage detected at the detection means.

3. Converter module according to claim 1 or 2, wherein the converter is a flyback converter.

4. Converter module according to any one of the preceding claims, wherein the converter is controlled by a control unit.

5. Converter module according to any one of the preceding claims, wherein in borderline mode, the converter switch is switched on, when the input current reaches a predefined threshold level, e.g. 0 Ampere.

6. Converter module according to any one of the preceding claims, wherein a detection means is provided to switch the converter switch dependent on the input current/voltage of the converter.

Converter module according to any one of the preceding claims, wherein the detection means is an auxiliary winding, in particular galvanically separated from a converter winding, especially arranged on the converter's primary side, and wherein the detection means feeds back a parameter indicating indirectly or directly the input current/voltage of the converter.

Converter module according to any one of the preceding claims, wherein the control unit is a PFC fixed controller and/or a PFC fixed output controller.

Converter module according to any one of the preceding claims, wherein the control unit comprises a DC-bus capacitor and/or a shaping circuit adapted to control an input current/voltage drawn from a supply voltage source during a half-cycle.

Converter module according to any one of the preceding claims, wherein a PFC, in particular a passive PFC circuit, is comprised in the control unit, and wherein the control unit comprises a switch of the flyback converter.

11. Converter module according to any one of the preceding claims, wherein a feedback signal is supplied to the switching stage from the detection means.

12. Converter module according to any one of the preceding claims, wherein the detection means is provided on the primary side of a flyback converter and preferably feeds back, directly or indirectly, a parameter to the switching stagi indicative of the current/voltage through the primary side of the flyback converter.

13. Converter module according to any one of the preceding claims, wherein the detection means feeds back a current/voltage dependent on the supply current/voltage input to the module as a negative current/voltage to the switching stage, the fed back current/voltage being indicative of the current/voltage through the primary side of the flyback converter.

14. Converter module according to any one of the preceding claims, wherein the switching stage is adapted to activate the bifilar primary winding depending on the current/voltage detected at the detection means.

15. Converter module according to any one of the preceding claims, wherein the control unit influences the input current/voltage supplied to the converter as a function of a phase cutting by modifying the peak current, i.e. the

current/voltage that changes on switching-off the flyback converter, and in particular by modifying the peak current/voltage to be substantially proportional to the currently applied amplitude of the input current/voltage.

16. Converter module according to any one of the preceding claims, wherein an energy storage unit is provided, in particular on the primary side of the flyback converter.

17. Converter module according to any one of the preceding claims, wherein, in particular on the secondary side of the flyback converter, a storage capacitor is provided dimensioned to compensate ripple on the input current/voltage supplied to the converter module.

18. Converter module according to any one of the preceding claims, wherein the switch-off threshold of the control unit and/or the flyback switch is adaptively set as a function of the input current/voltage of the converter to facilitate PFC functionality.

19. Converter module according to any one of the preceding claims, wherein in the feedback path from the detecting means to the control unit, a Zener diode is connected to clamp the negative current/voltage fed back to the switching stage to a maximal value and/or wherein the clamping is performed internally in the control unit and/or by software of the control unit.

20. Converter module according to any one of the preceding claims, wherein the converter is a buck converter and wherein the auxiliary winding only has a few windings, e.g. 1-7, especially at least two windings, adapted to reduce the current/voltage supplied to the control unit and/or to reduce the power loss at the control unit.

21. A method for dimming at least one LED, comprising the steps of

supplying a converter module, according to anyone of the preceding claims, with an input current/voltage controlled by a phase controlled/triac dimmer, supplying power to the at least one LED by a switched/clocked converter, the converter comprising a transformer and advantageously being operated in a borderline mode, and

detecting, by a detection means, a current or voltage input to the converter, in particular on its primary side,

the converter comprising a bifilar primary winding, wherein the bifilar primary winding can be activated depending on the current value of the input voltage or input curent.

22. A system comprising a phase controlled/triac dimmer and a converter module according to any one of the claims 1- 21.