FI129861B - Method and arrangement for deep dimming of leds - Google Patents

Abstract

Liitäntälaite ledivalonlähteille (101) käsittää tehotulon ja valaistuslähdön ja niiden välissä ensimmäisen jännitemuuntimen (201) ja toisen jännitemuuntimen (202, 302). Ensimmäinen jännitemuunnin (201) on sovitettu tuottamaan välijännite (VBUS) jännitelähdössä. Toinen jännitemuunnin (202, 302) muuntaa mainitun välijännitteen (VBUS) lähtöjännitteeksi (VOUT) ensimmäisen lähtövirta-alueen (IOUT) tuottamiseksi. Ohjausyksikkö (203, 303) ohjaa ainakin mainittua toista jännitemuunninta (202, 302). Liitäntälaite käsittää ohjattavan, resistiivisen ohitusvirtareitin (204, 305) mainitun välijännitelähdön ja mainitun valaistuslähdön välillä. Se voi johtaa ohjattavasti toisen lähtövirtaalueen (IOUT) mainittuun valaistuslähtöön riippumatta mainitun toisen jännitemuuntimen (202, 302) toiminnasta. Ohjausyksikkö (203, 303) vastaa himmennyskomentoon vähentämällä mainitun toisen jännitemuuntimen (202,302) toimintaa ja aktivoimalla mainitun ohitusvirtareitin (204, 305).

Description

METHOD AND ARRANGEMENT FOR DEEP DIMMING OF LEDS

FIELD OF THE INVENTION The invention relates to the field of dimmable led lighting. In particular, the invention relates to a driver device capable of dimming a led light source to very low levels of emitted light without flickering or other undesired effects.

BACKGROUND OF THE INVENTION Fig. 1 is a simplified block diagram of a typ- ical dimmable luminaire, in which the light source con- sists of leds 101. The driver device 102 is of the two- stage type. There is a first stage 103, commonly re- ferred to as the PFC stage because it performs power factor correcting. The first stage 103 receives an input voltage VIN, which may be an AC grid voltage for example. It produces an intermediate voltage VBUS, which is often called the bus voltage. The bus voltage goes into a second stage 104, which typically has the topology of a switched-mode DC/DC converter, for example a buck con- verter. The output of the second stage 104 provides the output voltage VOUT and output current IOUT to the leds

101. The driver device 102 comprises a control unit N 105, which may receive control inputs from various N sources, like a lighting control bus, one or more sen- S sors, a wireless lighting controller, or the like. The S 30 control unit 105 may receive measurement signals 106 and I 107 from the first and second stages 103 and 104. Among * such measurement signals may be signals indicative of & the magnitude of the output voltage VOUT and current = IOUT, separately shown in fig. 1 as 108 and 109 respec- S 35 tively. The control unit 105 controls the operation of the first and second stages 103 and 104 by delivering control signals to them.

Dimming the leds 101 is basically possible by changing the output voltage VOUT, the output current IOUT, or both.

In many cases the output voltage VOUT is not controlled at all but allowed to assume a value equal to the sum of the forward threshold voltages of the leds 101. Instead, the output current IOUT is con- trolled.

Earliest dimmable led luminaires used a con- trollable resistor in series with the leds, with poor energy efficiency as an obvious consequence.

Two main types of more modern dimming are known, called PWM (pulse width modulation) and CCR (constant current re- duction) dimming.

In PWM dimming the nominal value of the output current IOUT stays the same but it is re- peatedly switched on and off rapidly enough for the human visual system to only perceive a steady light, the brightness of which depends on the duty cycle of the switching.

CCR dimming involves providing a continuous output current but changing its magnitude.

The control unit 105 responds to dimming commands that it received as control inputs, by changing the operation of the second stage 104 correspondingly.

Dimming is involved not only in changing the overall amount of emitted light but also in changing the colour or colour temperature of emitted light.

For ex- ample, so-called tunable white light is produced by N driving a first set of leds emitting very warm white N light and a second set of leds emitting very cold white S 30 light and changing the relative brightness of the first S and second sets.

A led luminaire that emits light of Ek variable colour or colour temperature may have separate, * controllable driver devices for each set of leds.

Al- & ternatively, a driver device may have some common parts, = 35 like a common first stage for example, and parallel set- S specific parts like parallel, controllable second stages.

The known dimming techniques are not optimal for dimming the leds 101 to very low brightness, like 13 or below. It is difficult to perform PWM or CCR dimming without causing undesired phenomena like per- ceivable flickering or random fluctuations in the amount of emitted light.

SUMMARY Taken the known drawbacks of prior art, it is an objective to provide a method and arrangement for deep dimming of leds at good accuracy and pleasant vis- ual effect. Another objective is to enable dimming over a very wide brightness range with good energy effi- ciency. A further objective is to achieve said aims with a driver device that is simple to manufacture and reli- able in operation.

These and further advantageous objectives are achieved by equipping the driver device with a resistive bypass path, through which small currents can be con- trollably directed to the leds from an internal voltage of the driver device, effectively bypassing its second stage.

According to a first aspect there is provided a driver device for providing output current to one or more led light sources. The driver device comprises a power input and a lighting output. Coupled between said — power input and said lighting output are a first voltage N converter and a second voltage converter. The first 5 voltage converter comprises an intermediate voltage out- 2 30 put and is configured to produce an intermediate voltage N at said intermediate voltage output from an input volt- E age received through said power input. Said second volt- © age converter is configured to convert said intermediate 3 voltage into an output voltage and to produce a first N 35 range of output currents at said lighting output. The N driver device comprises a control unit coupled to at least said second voltage converter and configured to control its operation. The driver device comprises a controllable, resistive bypass current path between said intermediate voltage output and said lighting output, for controllably conducting a second range of output currents to said lighting output independent of the op- eration of said second voltage converter. The control unit is configured to respond to a dimming command by reducing the operation of said second voltage converter and activating said bypass current path.

According to an embodiment, said reducing of the operation of the second voltage converter involves stopping the operation of the second voltage converter. This enables very simple and straightforward control of the production of output current in deep-dimmed opera- tion.

According to an embodiment, said reducing of the operation of the second voltage converter involves making the second voltage converter operate in a burst mode. This involves the advantage that not all electric energy needs to go through the bypass current path, which may help to achieve low losses and simpler dimen- sioning of the bypass current path.

According to an embodiment, said control unit is configured to only activate said bypass current path in response to said dimming command being one the exe- cution of which involves dimming said one or more led light sources below a predetermined threshold brightness N level. This involves the advantage of simplicity in cir- N cuit design, because the bypass current path can be S 30 optimised for very small output currents only. e According to an embodiment, said bypass current I path bypasses the second voltage converter and comprises * at least one of: a controllable resistance responsive & to control signals from said control unit by changing = 35 its resistance, a switch responsive to control signals S from said control unit by preventing or allowing current to flow through said bypass current path. This involves the advantage that designing the bypass current path is simple.

According to an embodiment said bypass current path goes at least partly through said second voltage 5 converter but is configured to allow said second range of output currents to flow independent of the operation of the second voltage converter.

This involves the ad- vantage that some components may serve multiple uses.

According to an embodiment, said second voltage converter is a switched-mode converter and comprises a converter switch and an inductance.

The inductance may then serve for temporarily storing energy into a mag- netic field and releasing such temporarily stored energy during each switching cycle of the switched-mode con- verter.

A switching cycle is a period during which said converter switch is first on and then off.

The bypass current path may then go through said inductance but not through said converter switch.

This involves the ad- vantage that the inductance may be utilized as a part of an output current filtering arrangement.

According to an embodiment, said second voltage converter comprises a switch driver circuit configured to provide said converter switch with switching pulses.

The bypass current path may then go through a part of said switch driver circuit.

This involves the advantage that parts of the bypass current path may be utilized for purposes that are advantageous from the viewpoint N of the switch driver circuit.

N According to an embodiment, said switch driver S 30 circuit comprises a start voltage input for making said e switch driver circuit achieve a normal operating status I at start-up.

The bypass current path may then go through * said start voltage input.

A part of said bypass current & path may also constitute a start voltage path for con- = 35 trollably providing said switch driver circuit with a S start voltage at said start voltage input.

This involves the advantage that reliable operation of the switch driver circuit can be ensured. According to a second aspect there is provided a method for deep dimming of leds. The method comprises operating a second voltage converter stage of a two- stage driver device to provide said leds with output currents in a first range and responding to a dimming command by reducing the operation of the second voltage converter stage and activating a controllable, resistive bypass current path between an intermediate voltage and the leds. The intermediate voltage appears between the stages of the two-stage driver device. Thus, said leds are provided with output currents in a second range lower than said first range.

According to an embodiment, said reducing of the operation of the second voltage converter stage in- volves stopping its operation. This enables very simple and straightforward control of the production of output current in deep-dimmed operation.

According to an embodiment, said reducing of the operation of the second voltage converter stage in- volves making it operate in a burst mode. This involves the advantage that not all electric energy needs to go through the bypass current path, which may help to achieve low losses and simpler dimensioning of the by- pass current path.

According to an embodiment, said activating of N said bypass current path involves closing a switch on N said bypass current path and/or reducing the value of a S 30 controllable resistance on said bypass current path. e This involves the advantage that relatively straight- Ek forward and well understood methods are available for * controlling the operation of the driver device in deep- & dimmed states. = 35 According to an embodiment, said activating of S said bypass current path further involves repeatedly deactivating and re-activating said bypass current path according to a duty cycle that corresponds to an in- tended deep-dimmed brightness of the leds. This involves the advantage that the brightness of the leds can be controlled in a wide range during deep-dimmed operation. According to an embodiment, the method com- prises using a part of said bypass current path to pro- vide a start voltage to a switch driver circuit of said second voltage converter stage at start-up. This in- volves the advantage that reliable operation of the switch driver circuit can be ensured.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate em- bodiments of the invention and together with the de- scription help to explain the principles of the inven- tion. In the drawings: Figure 1 illustrates a known driver device, figure 2 illustrates a driver device according to an embodiment, figure 3 illustrates a driver device according to an embodiment, figure 4 illustrates parts of a driver device according to an embodiment, figure 5 illustrates parts of a driver device — according to an embodiment, O figure 6 illustrates parts of a driver device < according to an embodiment, 2 30 figure 7 illustrates parts of a driver device N according to an embodiment, E figure 8 illustrates parts of a driver device © according to an embodiment, and 3 figure 9 illustrates a switch driver circuit. N 35

N

DETAILED DESCRIPTION Fig. 2 illustrates a driver device for provid- ing output current to one or more led light sources 101. The led light sources do not need to be part of the driver device, although also integrated implementations are possible in which the driver device and light sources form an integrated entity. The driver device comprises a power input, to which an input voltage VIN may be coupled. The driver device may be configured to accept AC or DC or any of these as the input voltage VIN. Typically, the input voltage VIN is the AC grid voltage that is available in a building, vessel, outdoor construction, or other con- structed environment. The driver device comprises also a lighting output, to which the led light sources 101 may be coupled. Only one lighting output and only one set of led light sources 101 are shown in fig. 2 for simplicity, but there may be two or more of both.

Coupled between the power input and the light- ing output are a first voltage converter 201 and a second voltage converter 202. These may also be called the first stage and the second stage of the driver device. The first voltage converter 201 is also designated as the PFC stage in fig. 2, because typically —- but not necessarily — it may be configured to perform power — factor correction. The second voltage converter 20? is N also designated as the buck converter in fig. 2, because 5 the buck converter topology is one that is frequently 2 30 used for the output stage of a driver device for led N sources. This designation is, however, given as an ex- E ample only, as the second voltage converter 202 may have © some other topology like boost, buck-boost, flyback, 3 cuk, or the like. N 35 The first voltage converter 201 comprises an N intermediate voltage output, which in many driver device implementations may be referred to as the bus voltage output. The first voltage converter 201 is configured to produce an intermediate voltage - also called the bus voltage VBUS - at said intermediate voltage output. Be- ing a voltage converter, the first voltage converter 201 converts the input voltage (for example 230 V AC) into the intermediate voltage. A significant detail is that in many cases - although not mandatorily in all cases - a voltage converter used as the first stage of a driver device may be optimized to maintain the intermediate voltage VBUS relatively exactly at a specific value, such as 400 V DC for example. Such regulation of the intermediate voltage may be inherent to the first volt- age converter 201. If there are sufficient couplings for measurement and control signals between the first volt- age converter 201 and a control unit 203 of the driver device, the regulation of the intermediate voltage VBUS may involve control action taken by the control unit

203.

The second voltage converter 202 is configured to convert the intermediate voltage VBUS into the output voltage VOUT. The second voltage converter 202 is also configured to produce a first range of output currents IOUT at the lighting output. The first range of output currents extends from a maximum output current (100% lighting brightness) to a lower limiting value, such as a current that will produce a 1% lighting brightness. Especially if the buck topology is used for the second N voltage converter 202, possible values of the output N voltage VOUT are smaller than the intermediate voltage S 30 VBUS. The second voltage converter 202 may be designed e for a fixed value of the output voltage VOUT. Alterna- Ek tively, the design of the second voltage converter 202 * may be such that it allows the output voltage VOUT to & assume (within certain minimum and maximum voltage lim- = 35 its) a value equal to the sum of the forward threshold S voltages of the serially connected leds in the led light source.

As already pointed out above, the driver device comprises a control unit 203 that is coupled to at least the second voltage converter 202 and configured to con- trol its operation. In particular, the control unit 203 may be configured to control the production of output current in the second voltage converter 202 so that, as a result, the brightness of the led light source(s) changes between certain limits, like the 100% and 1% limits mentioned above. Advantageously, but not manda- torily, such controlling of the production of the output current may be based on dimming commands that the con- trol unit 203 receives through its control input. The other, although less frequently encountered alternative is that the control unit 203 executes some kind of an internal dimming control program, so that the production of the output current may be based on dimming commands generated internally within the control unit 203. As a difference to the prior art implementation in fig. 1, the driver device of fig. 2 comprises a controllable, resistive bypass current path 204 between the intermediate voltage output and the lighting output. The purpose of the bypass current path 204 is to make the driver device capable of controllably conducting a second range of output currents to the lighting output independent of the operation of the second voltage con- verter 202. The control unit 203 is configured to re- spond to certain kind(s) of dimming command(s) by re- N ducing the operation of the second voltage converter 202 N and activating the bypass current path 204.

S 30 Characterising the bypass current path 204 as e resistive means that it is basically just an ohmically Ek conductive connection. In clear contrast to the second * voltage converter 202, the bypass current path 204 does & not perform voltage conversions of other kind than the = 35 combined voltage drop that occurs in its resistive el- S ements according to Ohms’s law, possibly augmented by voltage drops that may occur at semiconductor junctions through which the current may pass.

Characterising the bypass current path 204 as controllable means that the control unit 203 (or some other controlling entity within the driver device, ded- icated for this task) may control the amount of current that passes through it.

The basic idea of having a controllable, re- sistive bypass current path 204 and the possibility of reducing the operation of the second voltage converter 202 is as follows.

For larger values of desired output current, the second voltage converter 202 is operational and the bypass current path 204 is not activated.

The second voltage converter 202 is operated by applying some known kind of dimming, like PWM or CCR dimming for example, to vary the output current in the first range of output currents.

For smaller values of desired output current, the bypass current path 204 is activated.

Sim- ultaneously reducing the operation of the second voltage converter 202 results in the second range of output currents being produced by varying the amount of current that passes through the bypass current path 204. Basi- cally this involves inferior energy efficiency when pro- ducing the second range of output currents, because the combined resistance of the bypass current path 204 con- verts into heat an amount of electric energy that is relatively large compared to that simultaneously con- N verted into light in the led light source(s). However, N in absolute value the ohmic losses remain small, because S 30 the second range of output currents involves only very e small currents. =E Reducing the operation of the second voltage * converter 202 may involve stopping its operation alto- & gether.

If that is the case, the whole output current = 35 in the second range of output currents flows through the S bypass current path 204. Another possibility is that reducing the operation of the second voltage converter

202 involves making the second voltage converter operate in a burst mode, i.e. intermittently so that a burst of switching pulses turn a converter switch in the second voltage converter repeatedly on and off for a short duration of time, after which the converter switch is completely off for another short duration of time, and so on. In such a case the operating bursts of the second voltage converter 202 serve to repeatedly recharge a capacitor at or close to the output of the second voltage converter 202, so that between the bursts a portion of the (small) output current at the lighting output comes from said capacitor.

As the bypass current path 204 is only dimen- sioned for relatively small currents, it may be worth- while to maintain it completely deactivated when higher brightness levels are required. That is, the control unit 203 would be configured to only activate the bypass current path 204 in response to dimming commands the execution of which involves dimming the one or more led light sources below a predetermined threshold brightness level.

In the embodiment of fig. 2 the bypass current path 204 bypasses the second voltage converter 202 al- together. The symbol of controllable resistance on the bypass current path 204 should not be taken as restrict- ing the actual implementation of the bypass current path

204. The bypass current path 204 may comprise a con- N trollable resistance that is responsive to control sig- N nals from the control unit 203 by changing its re- S 30 sistance. Additionally or alternatively, the bypass cur- e rent path 204 may comprise a switch responsive to con- Ek trol signals from the control unit 203 by preventing or * allowing current to flow through the bypass current path S 204. | | N 35 Fig. 3 illustrates an alternative embodiment, S in which the bypass current path goes at least partly through the second voltage converter 302. Despite going at least partly therethrough, the bypass current path is configured to allow the second range of output cur- rents to flow independent of the operation of the second voltage converter 302. Figuratively this is illustrated in fig. 3 so that within the second voltage converter 302 there are a normal operating path 304 and a (highly) resistive bypass current path 305, and the control unit 303 may select one of these for operation by setting the controllable switches either both up or both down.

In this simplified graphical representation, it should be noted that activating the bypass current path 305 does not necessarily preclude the operation of the second voltage converter altogether.

In other words, even if the switches would be in the lower position in fig. 3, the second voltage converter 302 may still operate, typ- ically in a reduced operating mode.

Fig. 4 illustrates one example of how the prin- ciple shown in fig. 3 can be implemented in practice.

Fig. 4 shows the second voltage converter 302, the con- trol unit 303, and the led light sources 101 in the form of a simplified circuit diagram.

The second voltage con- verter 302 is a switched-mode converter and comprises a converter switch 401, such as a field-effect transistor for example.

Additionally, the second voltage converter 302 comprises an inductance 402 for temporarily storing energy into an electromagnetically induced magnetic field and releasing such temporarily stored energy dur- N ing each switching cycle of the switched-mode converter.

N A switching cycle is a period during which the converter S 30 switch 401 is first on and then off. e As highlighted with arrow 403, the bypass cur- Ek rent path goes through the inductance 402 but not * through the converter switch 401 in the embodiment of & fig. 4. Thus, the control unit 303 may e.g. stop the = 35 operation of the second voltage converter 302 altogether S by making it keep the converter switch 401 non-conduc- tive, and a current may still flow through the bypass current path. The control unit 303 may set the amount of current that flows through the bypass current path by setting the value of the controllable resistance 404 accordingly.

Fig. 5 illustrates an embodiment that is oth- erwise similar to that of fig. 4 but instead of a con- trollable resistance the bypass current path comprises a constant resistance 501 and a switch 502 that is re- sponsive to control signals from the control unit 303 by preventing or allowing current to flow through the bypass current path. In such an embodiment the control unit 303 may set the effective amount of current that flows through the bypass current path by repeatedly closing and opening the switch 50? according to a de- sired duty cycle. The pulse frequency of such PWM switching can be set high enough so that there is no risk of perceivable flickering. As an example, a pulse frequency in the order of 2 kHz is high enough.

The principles shown in figs. 4 and 5 can be combined, so that a bypass current path could comprise both a controllable switch and a controllable re- sistance.

In figs. 4 and 5 the second voltage converter 302 is configured for PWM dimming. It comprises a switch driver circuit 405 configured to provide the converter switch 401 with switching pulses. The switch driver cir- cuit 405 comprises an enabling input, to which comes a N control signal line 406 from the control unit 303. When N the operation of the switch driver circuit 405 is ena- S 30 bled, it operates by providing the converter switch 401 e with switching pulses at a pulse freguency that may be Ek in the order of 50 to 150 kHz. For setting the switching * moments of those switching pulses the switch driver cir- & cuit 405 employs current feedback control, taking output = 35 current readings across a current measurement resistor S 407. The control unit 303 enables and disables the switch driver circuit 405 at a smaller pulse frequency,

in the order of 1 to 2 kHz, and at a duty cycle that corresponds to the desired dimming level.

If the operation of the second voltage con- verter follows a principle resembling that explained above, the control unit 303 may reduce its operation by maintaining a disabling control signal on the control signal line 406 either altogether, stopping the opera- tion of the second voltage converter 302, or for pro- longed periods separated by short enabling intervals, which results in burst mode operation.

Fig. 6 shows an example of how the use of a bypass current path can be equally applied in combina- tion with a second voltage converter configured for CCR dimming. In fig. 6, that part of the driver device that includes the control unit 303 and the second voltage converter 302 differs from the embodiment of figs. 4 and 5 regarding the current feedback coupling and the con- trol method applied by the control unit 303. No control signal line to any enabling input of the switch driver circuit 405 is needed (although one is not excluded either). Instead, a control signal line 601 goes from the control unit 303 to an amplifier 601, which is also coupled to receive the current measurement signal. The amplified difference of these two goes to the current feedback input of the switch driver circuit 405. This allows the control unit 303 to tamper with the current feedback measurement that the switch driver circuit 405 N utilizes in setting the switching moments of the pulses N that in turn operate the converter switch 401.

S 30 In a circuit like that of fig. 6 the control e unit 303 may reduce the operation of the second voltage Ek converter 302 by increasing an analog voltage on the * control signal line 601. A small increase leads to ex- & aggerating the current feedback signal to the switch = 35 driver circuit 405, making it shorten the switching S pulses to the converter switch 401. A larger increase may lure the switch driver circuit 405 into believing that there is an overcurrent situation, stopping the generating of switching pulses altogether. Burst mode operation follows if the control unit 303 keeps the analog voltage on the control signal line 601 high for most of the time but allows it to drop sufficiently every now and then so that the switch driver circuit 405 provides corresponding bursts of switching pulses to the converter switch 401. The bypass current path shown in fig. 6 resem- bles that of fig. 5 and is available for similar kind of operation. Other kinds of bypass current paths, like those explained earlier above, could be used as well.

Fig. 7 shows the second voltage converter 302 and the control unit 303 of a driver device according to an embodiment, as well as the led light sources 101, in the form of a simplified circuit diagram. In this embodiment, the bypass current path goes through a part of the switch driver circuit 405. Another difference, which is shown here as an example and which does not need to be associated with the bypass current path going through a part of the switch driver circuit, is that the operating voltage of the switch driver circuit 405 does not come from the bus voltage line 701 but from a sep- arate operating voltage line 702. In fig. 7 the bypass current path comprises a switch 703 responsive to control signals from the con- trol unit 303, as well as a constant resistance 704. The N series connection of the switch 703 and resistance 704 N is coupled between the bus voltage line 701 and one pin S 30 of the switch driver circuit 405. A controllable re- S sistance could be used in place of the constant re- Ek sistance 704, or such a controllable resistance could * replace the series connection of the switch 703 and the & resistance 704. The bypass current path goes through a = 35 part of the switch driver circuit 405 and continues from S another pin thereof to a point between the converter switch 401 and the inductance 402. Thus, also in this embodiment the bypass current path goes through the main inductance but not through the converter switch of the second voltage converter 302. In general, making the bypass current path go through an inductance and past a capacitance on its way to the lighting output is advantageous, because the in- ductance and capacitance may act as parts of a filter that filters out undesired high-frequency interference from the bypass current.

Utilising an inductance and a capacitance of the second voltage converter is advanta- geous, because it avoids having to provide separate fil- tering components solely for the purpose of the bypass current path.

Making the bypass current path not go through the converter switch involves the natural ad- vantage that the bypass current path remains independent of whether the converter switch is open or closed.

Making the bypass current path go through at least a part of the switch driver circuit involves the advantage that the bypass current path can be utilized also for other purposes.

One possible other purpose of that kind is to make a part of the bypass current path constitute a start voltage path for controllably provid- ing the switch driver circuit with a start voltage, also referred to as a kick-start pulse.

Fig. 8 illustrates a second voltage converter 302 and a bypass current path of a driver device that applies the principle shown in fig. 7. The switch 703 N of the bypass current path is implemented with two bi- N polar junction transistors.

The switch driver circuit S 30 405 of the second voltage converter 302 has the pin S organization - and is assumed to have the internal to- Ek pology — of a circuit IRS25411 available from Infineon * Technologies, Inc.

Its HO pin is coupled to the gate of & a MOSFET that constitutes the converter switch 401. The = 35 EN pin of the switch driver circuit 405 is coupled to S local ground, meaning that the switch driver circuit 405 is constantly enabled.

Its current feedback control scheme follows the model explained above with reference to figs. 6 and 7. The VS pin of the switch driver circuit 405 of fig. 8 is coupled to a point between the converter switch 401 and the inductance 402.

The upper transistor in the switch 703 and the adjacent resistor 704 constitute that part of the bypass current path that is coupled between the bus voltage line 701 and (the VB pin of) the switch driver circuit

405. This part of the bypass current path is then also utilized as a start-up circuit, as described in more detail below. The operating voltage VCC of the switch driver circuit 405 comes through an operating voltage line 702.

Fig. 9 shows how a circuit with the internal topology of the IRS25411 comprises a Zener diode coupled in reverse direction between the VB and VS pins. This means that a sufficiently high voltage on the VB pin will propagate through the circuit to the VS pin. In other words, in the circuit diagram of fig. 8 the bypass current path begins at the bus voltage line 701, goes through the upper transistor in the switch 703 and through the resistor 704, into the switch driver circuit 405 through its VB pin, out of the switch driver circuit 405 through its VS pin, and through the inductance 402 (and the additional inductance) to the lighting output.

In practice it has been found that despite re- ceiving an appropriate operating voltage at their VCC N pin, circuits like the IRS24511 may not always begin to N operate appropriately. The VB pin acts as a start volt- S 30 age input for making the switch driver circuit achieve e a normal operating status at start-up. Thus, a kick- =E start pulse to their VB pin may aid circuits like the * IRS24511 in starting normal operation. In a topology & like that of fig. 8, a part of the bypass current path = 35 also constitutes a start voltage path for controllably S providing the switch driver circuit 405 with a start voltage at its start voltage input VB.

Method embodiments comprise operating a second voltage converter stage of a two-stage driver device to provide the leds of one or more led light sources with output currents in a first range. The method embodiments also comprise responding to a dimming command by reduc- ing the operation of the second voltage converter stage and activating a controllable, resistive bypass current path between an intermediate voltage and the leds. The intermediate voltage meant here appears between the stages of the two-stage driver device. Activating the bypass current path results in providing said leds with output currents in a second range that includes currents of lower value than said first range.

In said method embodiments, reducing the oper- ation of the second voltage converter stage may involve stopping its operation altogether or making it operate in a burst mode. Activating the bypass current path may involve closing a switch on the bypass current path and/or reducing the value of a controllable resistance on the bypass current path. If further control of the current through the bypass current path is desired, it can be accomplished for example by repeatedly deactivat- ing and re-activating said bypass current path according to a duty cycle that corresponds to an intended deep- dimmed brightness of the leds. If the circuit topology and selection of components is suitable, the method may also comprise using a part of the bypass current path N to provide a start voltage to a switch driver circuit N of the second voltage converter stage at start-up. S 30 A smooth transition should be made to occur e between a first state, in which the second voltage con- Ek verter operates normally to provide output currents in * the first range, and a second state, in which the bypass & current path is active to produce a output currents in = 35 the second range. This can be accomplished in a variety S of ways, one of which is careful designing of the circuit and dimensioning of the electronic components. One may also utilize the measurements of output voltage and/or output current that the control unit may be equipped to make anyway.

As an example, we may consider a driver device designed to allow its output voltage to assume — within a certain allowable range —- a value equal to the sum of the forward threshold voltages of the serially connected leds. The driver device is designed with means for mak- ing the control unit aware of the actual value of the output voltage, which in an exemplary case may be for example 100 volts. On the other hand, the control unit knows that the value of the intermediate voltage will be relatively exactly 400 volts at all times. Thus, based on the measurement mentioned above, the control unit knows that a voltage drop of 300 volts should occur in the resistive part of the bypass current path. If the bypass current path comprises a controllable resistance, and if the threshold current at which a transition from said first state to said second state should occur is

3.5 mA, the control unit knows that it should set the value of said controllable resistance to 85.7 kilo-ohms when the transition between the states occurs. Corre- spondingly, if the output voltage was measured to be only 65 volts, the other values remaining the same, the control unit should set the value of the controllable resistance to 95.7 kilo-ohms when the transition between the states occurs. N Adaptive properties of the control unit, like N the appropriate setting of the value of the controllable S 30 resistance on the bypass current path, can be imple- e mented by programming the control unit correspondingly. =E It is obvious to a person skilled in the art * that with the advancement of technology, the basic idea & of the invention may be implemented in various ways. The = 35 invention and its embodiments are thus not limited to S the examples described above, instead they may vary within the scope of the claims. For example, even if a buck converter has been used as an example of the to- pology of the second voltage converter in the drawings, this is not a restrictive feature. The same principles concerning e.g. possibly making the bypass current path go through an inductance in the second voltage con- verter, can be applied regardless of which topology is selected for the second voltage converter.

N O

N <+ <Q ©

N

I a a ©

O +

LO N O N

Claims (15)

1. A driver device for providing output cur- rent to one or more led light sources (101), compris- ing: - a power input and a lighting output, - coupled between said power input and said lighting output a first voltage converter (201) and a second voltage converter (202, 302), of which: -- said first voltage converter (201) comprises an in- termediate voltage output and is configured to produce an intermediate voltage (VBUS) at said intermediate voltage output from an input voltage (VIN) received through said power input, and -- said second voltage converter (202, 302) is config- ured to convert said intermediate voltage (VBUS) into an output voltage (VOUT) and to produce a first range of output currents (IOUT) at said lighting output, and - a control unit (203, 303) coupled to at least said second voltage converter (202, 302) and configured to control its operation, characterized in that: - the driver device comprises a controllable, resis- tive bypass current path (204, 305) between said in- termediate voltage output and said lighting output, for controllably conducting a second range of output currents (IOUT) to said lighting output independent of N the operation of said second voltage converter (202, O 302), and + - said control unit (203, 303) is configured to re- = 30 spond to a dimming command by reducing the operation T of said second voltage converter (202, 302) and acti- = vating said bypass current path (204, 305) to make a 00 voltage drop of a predetermined magnitude between said > intermediate voltage (VBUS) and said output voltage N 35 (VOUT) occur on said bypass current path (204, 305).

N

2. A driver device according to claim 1, wherein said reducing of the operation of the second voltage converter (202, 302) involves stopping the op- eration of the second voltage converter (202, 302).

3. A driver device according to claim 1, wherein said reducing of the operation of the second voltage converter (202, 302) involves making the sec- ond voltage converter (202, 302) operate in a burst mode.

4. A driver device according to any of the preceding claims, wherein said control unit (203, 303) is configured to only activate said bypass current path (204, 305) in response to said dimming command being one the execution of which involves dimming said one or more led light sources (101) below a predeter- mined threshold brightness level.

5. A driver device according to any of the preceding claims, wherein said bypass current path (204) bypasses the second voltage converter (202) and comprises at least one of: a controllable resistance responsive to control signals from said control unit (203) by changing its resistance, a switch responsive to control signals from said control unit (203) by preventing or allowing current to flow through said bypass current path (204).

N N

6. A driver device according to any of claims x 1 to 4, wherein said bypass current path (305) goes at N least partly through said second voltage converter r (302) but is configured to allow said second range of E 30 output currents (IOUT) to flow independent of the op- & eration of the second voltage converter (302).

LO N

7. A driver device according to claim 6, N wherein:

- said second voltage converter (302) is a switched- mode converter and comprises: -- a converter switch (401) and -- an inductance (402) for temporarily storing energy into a magnetic field of said inductance (402) and re- leasing such temporarily stored energy during each switching cycle of the switched-mode converter, said switching cycle being a period during which said con- verter switch (401) is first on and then off, - said bypass current path goes through said induct- ance (402) but not through said converter switch (401).

8. A driver device according to claim 7, wherein: - said second voltage converter (302) comprises a switch driver circuit (405) configured to provide said converter switch (401) with switching pulses, and - said bypass current path goes through a part of said switch driver circuit (405).

9. A driver device according to claim 8, wherein: - said switch driver circuit (405) comprises a start voltage input (VB) for making said switch driver cir- cuit (405) achieve a normal operating status at start- up, and N - said bypass current path goes through said start O voltage input (VB); + wherein a part (703, 704) of said bypass current path = also constitutes a start voltage path for controllably T 30 providing said switch driver circuit (405) with a E start voltage at said start voltage input (VB). 00 S 10. A method for deep dimming of leds (101), = comprising: N - operating a second voltage converter stage (202, 302) of a two-stage driver device to provide said leds

(101) with output currents (IOUT) in a first range, characterized in that the method comprises: - responding to a dimming command by reducing the op- eration of the second voltage converter stage (202, 302) and activating a controllable, resistive bypass current path (204, 305) between an intermediate volt- age (VBUS) and the leds (101) to make a voltage drop of a predetermined magnitude between said intermediate voltage (VBUS) and said output voltage (VOUT) occur on said bypass current path (204, 305), which intermedi- ate voltage (VBUS) appears between the stages (201, 202, 302) of the two-stage driver device, thus provid- ing said leds (101) with output currents (IOUT) in a second range lower than said first range.

11. A method according to claim 10, wherein said reducing of the operation of the second voltage converter stage (202, 302) involves stopping its oper- ation.

12. A method according to claim 10, wherein said reducing of the operation of the second voltage converter stage (202, 302) involves making it operate in a burst mode.

13. A method according to any of claims 10 to 12, wherein said activating of said bypass current path (204, 305) involves at least one of: closing a N switch on said bypass current path, reducing the value N of a controllable resistance on said bypass current S path.

N T 14. A method according to claim 13, wherein E 30 said activating of said bypass current path further & involves repeatedly deactivating and re-activating O said bypass current path according to a duty cycle O that corresponds to an intended deep-dimmed brightness of the leds.

15. A method according to any of claims 10 to 14, additionally comprising using a part of said by- pass current path to provide a start voltage to a switch driver circuit of said second voltage converter stage at start-up.

N

N

O

N <+ <Q

N

I jami a 00

O +

O

N

O

N