WO2007141741A1

WO2007141741A1 - Circuitry for dimming led illumination devices

Info

Publication number: WO2007141741A1
Application number: PCT/IB2007/052131
Authority: WO
Inventors: Matthieu M. J. Verstraete; Alexandre P. GÉBLEUX
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-06-08
Filing date: 2007-06-06
Publication date: 2007-12-13

Abstract

Proposed is a current supply circuitry (1) for at least one LED (10), comprising at least one inductive reactance connected as a storage choke (20) in series with the LED(10), a free running current path (30), connected parallel to the series connection of the LED (10) and the storage choke (20), a switch (40) for switching between a charging and a discharging process occurring in the storage choke (20). The current supply circuitry (1) is characterized in that the circuitry comprises control means (50) arranged to change the current supplied to the LED (10) from a continuous conduction mode to a discontinuous conduction mode at an adjustable dimming level of the LED (10).

Description

Circuitry for dimming LED illumination devices

FIELD OF THE INVENTION

This invention relates to a current supply circuitry for dimming illumination devices comprising LEDs as light sources. More particular, it relates to a current supply circuitry comprising at least one LED, at least one inductive reactance acting as an energy storage choke, a free running current path and a switch for switching between a charging and a discharging process occurring in the storage choke.

BACKGROUND OF THE INVENTION

A current supply circuitry of the kind set forth is known per se, for instance from US2006/ 0072324. This document discloses a LED driving semiconductor circuit including a first input terminal connected to a LED, a switching device block having a first FET and a first switching device. Furthermore, the circuit includes a reference voltage terminal which is connected to the first FET and outputs a reference voltage and a start/stop circuit which outputs a start signal when the reference voltage is equal to or larger than a predetermined value and outputs a stop signal when the reference voltage is less then the predetermined value. Furthermore, the circuit includes a current detection circuit which detects the current flow through the first switching device and a control circuit which controls ON/ OFF of the first switching device intermittently at the predetermined frequency based on the output signal of the start/stop circuit so that the constant current flows through the LED. A drawback of the circuitry disclosed in US2006/ 0072324 is that the current through the LEDs is continuous and modulated down to low levels when a high dimming of the LEDs is required. As a consequence the light generating efficiency of the LEDs is low, especially for high power LEDs. Hence there is a need for a current supply circuitry for dimming illumination devices comprising LEDs as light sources having a high efficiency in generating light, even when low light levels (equivalent to high dimming levels) are required.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a current supply circuitry for at least one LED of the kind set forth, which enables the LEDs to efficiently generate light even at low light output levels. This object is achieved with the a current supply circuitry according to the invention as defined in claim 1. The a current supply circuitry according to the invention characterized in that the circuitry comprises control means arranged to change the current supplied to the LED from a continuous conduction mode to a discontinuous conduction mode at an adjustable dimming level of the LED. Advantageously, the invention provides a circuitry capable of supplying a current to the LED(s) in adequacy with the LED specifications even at very high dimming levels. At these levels the circuitry switches over to the discontinuous conduction mode.

In an embodiment of the invention the control means is arranged to turn the switch off at a predetermined current I_peak through the LED and to turn the switch on after a controllable delay time td_elay. Advantageously, the current I_peak is the rated or nominal design current of the LED.

In an embodiment of the invention the LED dimming level is arranged to be determined by the delay time tdeiay- Advantageously, the delay time tdeiay between the point where the current through the LEDs reaches I_peak(and the switch is turned off) to the point where the switch is turned back on determines the average light output of the LED.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of the invention are disclosed in the following description of exemplary and preferred embodiments in connection with the drawings.

Fig. 1 shows a circuitry according to the invention Fig. 2 shows the current flow in an embodiment of the invention Fig. 3 shows the current through the LEDs as a function of time for different delay times

Fig. 4 shows a detail of an embodiment of the invention Fig. 5 shows details of two different embodiments of the invention Fig. 6 shows a detail of an embodiment of the invention Fig. 7 shows an embodiment for controlling the delay time tdeiay Fig. 8 shows another embodiment for controlling the delay time tdeiay

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 is the schematic of a circuit that has been implemented and tested in a real LED application. Some useful but not essential electronics components have been removed for a better understanding. The right part of the design is dedicated to the power flux, the left part of the design is dedicated to the control means 50. How does the power flux stage work ? The functioning mode of the power stage is similar to a BUCK topology in which the output capacitor would have been removed. The main components of this stage are :

DELl ... DELN : Light Emitting Diodes 10 (load) Tl : switch 40 - a transistor working in commutation, Ll : a 'smoothing' self working as a storage choke 20 Dl : a free-wheel diode working as a free running current path 30 (switch with automatic commutation)

RS 1 : resistor to measure current (used to detect peak current), RS2 : resistor to measure current (used to measure average current in the load)

The components external to the circuit :

V PO WER : voltage supply, V CTR l and V CTR 2 : control voltages (set points)

Finally, the arrows on I MAX and I ME AN are indicating the voltage, not the direction of the currents in the circuit

The power transfer to the LEDs 10 is done in 2 phases (see Fig. 2): (i) Phase « A » - "main conduction" - the switch 40 (transistor Tl) is conductive, the energy is supplied by the power supply V POWER and (ii) Phase « B » - « relaxation » - the choke 20 (self Ll) gives back its energy through the free-wheel diode, working as a free running current path 30.

When the switch 40 is conductive (phase « A »), the current through the LED 10 increases linearly. When the current reaches the predetermined "I_peak" value, the switch 40 is opened (controlled by the control means 50 comprising a microcontroller) for an adjustable time tdeiay (also controlled by the microcontroller). When the switch 40 goes to OFF state (phase « B »), the current in the load (i.e. through the LEDs 10) is decreasing linearly.

At this stage there are 2 possible behaviors: (i) if the delay time tdeiay with the next transition (where the switch goes to ON state) is long enough, the current reaches zero (discontinuous conduction) and (ii) if the delay time tdeiay is short enough, phase "A" starts again before that the current has decreased to zero (continuous conduction). Continuous and discontinuous modes are known in electronics, but the combination of the two modes in a system have never been used for LEDs 10.

The light flux emitted by the LEDs 10 is proportional to the average current that goes through the LEDs. Fig. 3 shows that it is possible to obtain a particular flux by spacing out the current pulses. The low limit of the period (the high limit of tdeiay) is determined by the retinal perception, so few hundreds hertz, while the high limit (the low limit of tdeiay) is linked to the technology, its choice is a compromise between economical and EMC constraints, for example 140 kHz, would give a suitable dynamic range for flux variation. Fig. 3 shows that for (very) low flux, the current through the LEDs - and thus their light output - is pulsed (Figs. 3.1, 3.2 and 3.3), whereas it becomes continuous for high flux (Figs. 3.4 and 3.5).

For a pre-determined current and a stable design in terms of component values (the supply being very often used for several LEDs 10 [possibly with different colors] and fixed to a constant value) there are a lot of possible combinations of the 2 influencing parameters: (i) The delay time tdeiay and (ii) the peak current I_Peak. Regarding I_peak, the choice is limited by the specification of the LED 10. The peak current must be low enough to not damage the connections, and high enough to generate a pulse of flux that is optically fitting with specification. As a general rule, the pulse of flux must be compatible with the electrical and optical specifications of the LED 10.

It is very important to notice that the flux is controlled by only one timing parameter tdeiay. For a predetermined flux, the spectrum analysis obtained would come from the triangular component of the waveform. The current solution is not similar at all to a "switching supply" (with its own switching frequency), which would be itself modulated by a second frequency (PWM mode...). For a known I_Peak, the flux variation is a continuous process of increasing one (only) commutation frequency, to get in the load (LED 10) the suitable average current by pulse shapes in adequacy with the LED specifications.

From a topology point of view, the power stage looks like BUCK supply topology, for which the output capacitor would have been removed. Switching from one conduction mode to the other, based on a current peak detection looks close to the functioning mode of FLYBACK supply, called "currents mode". Based on tdeiay and I_peak it is possible to use and adapt a wide range of regulators from the market to the topology described. The schematic in Fig. 4 shows a well-known topology. A significant advantage compared to Fig. 1 is to use only one resistor (« RS ») to measure both peak current and average current. In Fig. 1, 2 resistors where used for practical reason. An important parameter to take into account in the choice of the topology to implement is the supply voltage. With the topology presented in Fig. 1, the switch is easily controlled related to zero volt, whereas in Fig. 4 it requires a voltage translation. Actually, it is possible to create many different implementations by simply permuting components.

For example, on Fig. 5 the choke 20 (self Ll) is on the switch 40 side (Fig. 5A), or on the supply side (Fig. 5B). Those permutations can result in different behaviors of the circuit.

From a flux point of view these 2 topologies can be considered as being equivalent. From an electrical point of view, however, in version "A" none of (X, Y) points is at a higher potential than the power supply. In version "B", Yl point goes over the voltage supply from a value equal to the voltage between the terminals of the LEDs 10, this can be a problem for applications (EMC, electrical security...).

Fig. 3 shows that it is possible to have an important flux variation, providing that the power stage can work in both continuous conduction mode and discontinuous conduction mode. The control means 50 for the control of the flux generally comprises a microcontroller that receives orders (from a controller) for flux variation. Those orders are sometimes fast. It is therefore necessary to have a way of controlling that handles the switch 40 from one conduction mode to the other, that is also able to quickly reach the required flux level, while furthermore stabilizes quickly when that level has been reached. Going from one conduction mode to the other is very difficult. Controlling the flux of the average current requires a filter to smooth the measuring of the current, which means a time constant that is not compatible with the required response time. The solution is to modulate the flux by controlling the delay time tdeiay between the time point where the current reaches its peak value and the next transition of the switch to ON state.

An example of principle of implementation is represented on Fig. 6. The control voltages V CTR l and C CTR 2 are supplied by a microcontroller. The V CTR 2 voltage determines the I_peak current, the V_CTR_1 voltage determines the delay time tdeiay, UlOl works like a flip-flop, it is based on a cheap timer very well known in electronics. When the switch 40 is ON, the current through the LEDs 10 is increasing linearly. When the current reaches the controlled value I_peak, determined by V CTR 2/ RS, the comparator UlOOA switches the OUT output of UlOl to zero, and opens the switch 40. The output of the flip-flop controls simultaneously the transistor (TlOl), keeping the capacitor ClOO unloaded. At the same time as the switch 40 (TlOO) goes to the OFF state, the loading of the capacitor ClOO through RlOO starts. When this voltage reaches the value V CTR l, the output of the comparator UlOO-B switches again the output of the flip-flop to high state, and the switch 40 is back to ON state.

When measuring the variation of the current in the LEDs 10 depending on V CTR l voltage (the others parameters being fixed) it will be clear that this variation is not linear. The variation increases at high flux, which reminds the flux curves made to adapt the flux variation to the sensitivity of the eyes (not linear); although in practice it is seen as a difficulty.

There are many ways to generate a delay time td_elay. The advantage of the method presented in Fig. 7 is its cost effectiveness. One of the advantages of this principle is its possibility to evolve to digital implementation. The delay time tdeiay can of course be obtained accurately by a counter in a FPGA or similar device. The fast determination of the control values (I_peak and tdeiay) from a current

(flux) is not compatible with a control in close-loop. RS2, used to measure the average current in the LEDs 10, is mainly used during calibration of the product. The non-linearity of the transfer- function complicates the determination of the functioning to control the current. The measurement of the current through RS2 complicates the design when the supply voltage goes over few tens of voltage.

Advantageously, for a known application, the values are choosen and fixed according to the voltage values and the maximum current in the loads (LEDs 10). Knowing those values and the variation curve, it is possible to estimate the control values (I_peak and tdeiay) to reach the fitting current and voltage for the load (LEDs 10). The possibility to determine the functioning point without direct measurement of the current simplifies the circuit, and therefore reduces the costs. Advantageously, the solution consists on having the possibility in the circuit to detect the value of tdeiay (Ipeak is fixed by the LEDs 10) for which the system switches from discontinuous conduction mode to continuous conduction mode at a predetermined dimming level of the LEDs 10. As the other parameters of the load are known, the value of tdeiay only depends on the voltage at the terminals of the LEDs 10. By knowing the LEDs voltage it is possible to estimate the tdeiay value to reach the desired current (the voltage can be very different from one LED 10 to the other, but is very stable in time). Fig. 8 shows the simplicity to implement the solution. Just after that the switch 40 is in conduction mode, the voltage at the terminals of the resistor of I max measurement is compared to a value close to zero. The result of the comparison makes possible to estimate the conduction mode. The resistor (RS2 Fig. 1) to measure the average current and the associated translators are removed.

Claims

CLAIMS:

1. A current supply circuitry (1) for at least one LED (10), comprising:

At least one inductive reactance connected as a storage choke (20) in series with the LED(IO),

A free running current path (30), connected parallel to the series connection of the LED (10) and the storage choke (20),

A switch (40) for switching between a charging and a discharging process occurring in the storage choke (20), characterized in that:

The circuitry comprises control means (50) arranged to change the current supplied to the LED (10) from a continuous conduction mode to a discontinuous conduction mode at an adjustable dimming level of the LED (10).

2. A current supply circuitry (1) according to claim 1, wherein the control means (50) is arranged to turn the switch (40) off at a predetermined current I_peak through the LED (10) and to turn the switch (40) on after a controllable delay time tdeiay

3. A current supply circuitry (1) according to claim 2, wherein the LED (10) dimming level is arranged to be determined by the delay time td_elay.