WO2014060872A1

WO2014060872A1 - Driver device and driving method for driving a load, in particular an led unit, using a resonant converter

Info

Publication number: WO2014060872A1
Application number: PCT/IB2013/058566
Authority: WO
Inventors: Andrew Ulrich RUTGERS
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-10-18
Filing date: 2013-09-16
Publication date: 2014-04-24

Abstract

The present invention relates to a driver device (10) for driving a load (18), comprising input terminals (14, 16) for connecting the driver device (10)to a voltage supply (12) and for receiving an input voltage (V10) from the voltage supply (12), an output terminal for connecting the driver device (10)to the load (18)for powering the load (18), two controllable switches (20, 22) connected to the input terminals (14, 16)for converting the input voltage (V10) to a drive voltage (V11), an electromagnetic converter unit (24) connected to the controllable switches (20, 22)for converting the drive voltage (V11)to a secondary voltage (V14, V16) and for providing a secondary current (I18) for powering the load (18), and a control unit (28) for controlling the controllable switches (20, 22)such that consecutive voltage cycles are provided for a first time period (ΔT1) as the drive voltage (V11)to the electromagnetic converter unit (24)followed by a constant voltage level or a floating voltage provided for a second time period (ΔT2) as the drive voltage (V11)to the electromagnetic converter unit (24), wherein a frequency of the consecutive voltage cycles is set to a level at which the secondary current (I18) is provided only during a single voltage cycle or a plurality of initial voltage cycles of the first time period (ΔT1).

Description

DRIVER DEVICE AND DRIVING METHOD FOR DRIVING A LOAD, IN PARTICULAR AN LED UNIT, USING A RESONANT CONVERTER

FIELD OF THE INVENTION

The present invention relates to a driver device and a corresponding driving method for driving a load, in particular an LED unit, comprising one or more LEDs. Further, the present invention relates to a light apparatus.

BACKGROUND OF THE INVENTION

In the field of LED drivers for offline applications solutions are demanded to drive the LEDs over a large power range with a high reliability and solutions are demanded to drive the LEDs at very low power below 1 % of the full power.

In the field of LED driver resonant converters such as LLC converters are commonly known for driving LEDs. The LED converter control the output power provided to the load by switching two controllable switches creating an alternating or pulsating input voltage to an electromagnetic converter. The energy transferred by the LLC converter is proportional to the energy change in a capacitor between the two switching states. The energy provided to the load is controlled by switching the controllable switches.

The output power of LLC converters can be controlled by changing the switching frequency of the controllable switches. Alternatively, US 2011/0164473 Al discloses an LCC converter, wherein the output power is controlled by varying the duty cycle of the controllable switches.

Very low power operation using LLC converter in a continuous mode operation is usually challenging since the operation can be highly sensitive to the input voltage and output load, resulting in an unstable operation which may cause a flicker of the LEDs connected to the LLC converter.

For providing low power operation WO 2007/148271 discloses to operate an

LLC converter in a discontinuous mode, wherein the controllable switches provide a single voltage pulse to the electromagnetic converter for providing a load output power. To deliver such single voltage pulses a control unit is required, which is able to provide single pulses which is technically complex and costly.

The disadvantage of the known LLC converter in the range of low power operation is that the technical effort for driving the LLC converter is very high and the LLC converter are highly sensitive to input and output voltages resulting in an unstable operation, which may cause light flicker in the case of LED applications.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a driver device and a corresponding driving method for driving a load, in particular an LED unit comprising one or more LEDs providing a reliable low power operation with low technical effort. Further, it is an object of the present invention to provide a corresponding light apparatus.

According to one aspect of the present invention a driver device for driving a load, in particular an LED unit having or more LEDs is provided, comprising:

input terminals for connecting the driver device to a voltage supply and for receiving an input voltage from the voltage supply,

an output terminal for connecting the driver device to the load for powering the load,

- two controllable switches connected to the input terminals for converting the input voltage to a drive voltage,

an electromagnetic converter unit connected to the controllable switches for converting the drive voltage to a secondary voltage and for providing a secondary current for powering the load, and

- a control unit for controlling the controllable switches such that consecutive voltage cycles are provided for a first time period as the drive voltage to the electromagnetic converter unit followed by a constant voltage level or a floating voltage provided for a second time period as the drive voltage to the electromagnetic converter unit, wherein a frequency of the consecutive voltage cycles is set to a level at which the secondary current is provided only during a single voltage cycle or a plurality of initial voltage cycles of the first time period.

According to another aspect of the present invention a driving method for driving a load, in particular an LED unit comprising one or more LEDs is provided, wherein the driving method comprises the steps of:

- converting an input voltage to a drive voltage by means of two controllable switches,

providing consecutive voltage cycles for a first time period as the drive voltage to an electromagnetic converter unit, providing a constant voltage or a floating voltage for a second time period as the drive voltage to the electromagnetic converter unit.

converting the drive voltage by means of the electromagnetic converter unit to a secondary voltage and providing a secondary current for powering the load, wherein a frequency of the consecutive voltage cycles is set to a level at which the secondary current is provided only during a single voltage cycle or a plurality of initial voltage cycles of the consecutive voltage cycles of the first time period.

According to still another aspect of the present invention, a light apparatus is provided comprising a light assembly having one or more light units, in particular an LED unit having one or more LEDs, and a driver device for driving said light assembly as provided according to the present invention.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method has similar and/or identical preferred

embodiments as the claimed device and as defined in the dependent claims.

The present invention is based on the idea to provide very low output power to the load by operating the electromagnetic converter unit using a plurality of consecutive voltage cycles having a constant duty cycle wherein only the first few voltage cycles transmit electrical power. After the first few cycles, the output voltage reaches the load voltage causing the impedance of the resonant circuit apparent to the drive voltage to change. With the apparent impedance changed the amount of electrical power is limited and the output current slowly decreases. Following the voltage cycles a constant voltage level or a floating voltage level is provided as the drive voltage to the electromagnetic converter unit so that the resonance elements of the electromagnetic converter unit are slowly discharged so that the small amount of electrical power can be transmitted during the following consecutive voltage cycles for powering the load. Hence, due to the combination of the voltage cycles and the constant voltage electrical power is principally delivered during a first few cycles of the consecutive voltage cycles followed by a time frame during which the electromagnetic converter unit oscillates without delivering electrical power. The resonant elements discharge during the constant voltage time period to prepare the electromagnetic converter unit for the next consecutive voltage cycle. Since the consecutive voltage cycles force the operation of the electromagnetic converter unit into a decaying oscillation mode, small bursts of energy can be delivered, and a more stable load power behavior of the driver device is obtained.

In a preferred embodiment, the control unit is adapted to switch the controllable switches alternating to provide a pulsating voltage as the consecutive voltage cycles to the electromagnetic converter unit. This is a practical possibility to provide the voltage cycles to the electromagnetic converter unit with low technical effort.

In a further preferred embodiment, the control unit is adapted to switch one of the controllable switches continuously to a conductive state for the second time period to provide the constant voltage level as the drive voltage to the electromagnetic converter unit. This is a simple solution to provide a defined constant voltage level as the drive voltage to the electromagnetic converter unit with low regulation effort.

In a further preferred embodiment, the control unit is adapted to set the duration of the second time period such that resonance elements of the electromagnetic converter unit can be substantially discharged during the second time period, hence, the second period provides sufficient time for the resonant elements to discharge and permits a higher inrush current when the consecutive voltage cycling is restarted for a subsequent first time period.

In a further embodiment, the control unit is adapted to control the electrical output power by variation of the duration of a total interval formed by a sum of the first time period and the second time period. This is a possibility to control the output power with low technical effort since the number of pulses of the consecutive voltage cycles influences the output power.

In a further embodiment, the control unit is adapted to control the electrical output power by variation of a duty cycle of the first time period and the second time period. This is a practical solution to control the output power with low technical effort.

In a further preferred embodiment, the control unit is adapted to set the frequency of the consecutive voltage cycles to a value close to or above a no-load frequency of the electromagnetic converter unit. This provides a small current peak during only the first few voltage cycles. This is a simple possibility to deliver a small amount of electrical power to the load.

It is further preferred if the control unit is adapted to set the on-time of the controllable switches to predefined values to provide the consecutive voltage cycles. This is a possibility to control the controllable switches with high reliability since the switching is less sensitive to external interferences and noise.

According to a further preferred embodiment, the control unit is adapted to control a first of the controllable switches on the basis of a primary voltage or a primary current of the electromagnetic converter unit and a threshold level to provide the consecutive voltage cycles. This is a possibility to control the voltage cycles without a control unit which is able to deliver single control pulses for setting the voltage cycles.

In a further preferred embodiment, the control unit is adapted to control the second of the controllable switches on the basis of the primary voltage or the primary current and a second threshold level. This is a simple solution to control the two controllable switches with low technical effort.

In a preferred embodiment, the control unit is adapted to set the on-time of the second of the controllable switches to a predefined value. Either the high or low side switch can be time controlled wherein the opposite switch is threshold controlled. This is a simple solution to control the second controllable switch with a high reliability and low sensitivity to interferences and noise, since the second controllable switch is time-controlled.

According to a preferred embodiment, the control unit is adapted to control the controllable switches such that the on-times of the two controllable switches have different durations. This is a practical possibility to precisely set the electrical power provided to the load to a predefined level.

It is further preferred if the electromagnetic converter unit comprises a capacitor and a primary winding connected in series to each other. This is a practical solution to provide a primary resonant circuit.

In a further preferred embodiment, the electromagnetic converter unit comprises at least one secondary winding and an output capacitor connected between the secondary winding and the output terminal for providing a constant output voltage to the load for powering the load. This is a practical solution to provide a secondary oscillator circuit and to provide a filter capacitor to provide a constant voltage to the load for powering the load.

As mentioned above, the present invention provides an improved driver device for driving a load, wherein a small amount of electrical power is provided during a first few cycles of the consecutive drive voltage cycles by the electromagnetic converter unit, so that very low electrical power can be provided to the load. This allows a very deep dimming of e.g. an LED unit. To prepare the electromagnetic converter unit to the next time period of consecutive voltage cycles, a constant voltage, or open circuit, is applied as the drive voltage to the electromagnetic converter unit to allow the resonant energy to dissipate, wherein the time period for discharging has to be sufficient that the resonant energy is reduced to a desired level that a current peak can be provided during the following consecutive voltage cycles. Hence, very low electrical power can be provided to the load with a high reliability and low technical effort. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings:

Fig. 1 shows a schematic block diagram of a driver device for driving a load;

Fig. 2 shows a diagram illustrating the power control with voltage pulses and a constant voltage level as drive voltage;

Fig. 3 shows a diagram illustrating a plurality of power pulses provided to the load;

Fig. 4 shows a diagram illustrating an asymmetric control of the driver device on the basis of two threshold levels; and

Fig. 5 shows a diagram illustrating the asymmetric control of the driver device on the basis of one threshold level and an on-time control.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a schematic block diagram of a driver device generally denoted by 10. The driver device 10 is connected to a voltage supply 12 which provides a supply voltage V10. The driver device 10 is connected to the voltage supply 12 by means of the input terminals 14, 16. The driver device 10 converts the input voltage V10 to an output voltage V20 for powering a load 18, which is in the particular embodiment shown in Fig. 1 formed as an LED unit 18. The driver device 10 is a resonant converter and preferably an LLC converter.

The driver device 10 comprises two controllable switches 20, 22 and an electromagnetic converter unit 24 for converting the input voltage VI 0 to the output voltage V20 for powering the load 18. The input voltage VI 0 is a direct current voltage or a rectified voltage rectified by means of a rectifier (not shown) connected to an AC voltage supply. The controllable switches 20, 22 are connected in series to each other and are connected in parallel to the input terminals 14, 16. The controllable switches 20, 22 are connected to each other to form a half bridge, wherein a node 26 between the controllable switches 20, 22 forms an output terminal of the half bridge. The driver device 10 comprises a control unit 28 for controlling the controllable switches 20, 22. The control unit 28 switches the controllable switches 20, 22 alternating to provide an alternating or a pulsating voltage to the node 26 and to the electromagnetic converter unit 24 as explained below.

The node 26 of the half bridge is connected to an input capacitor 30 of the electromagnetic converter unit 24. The electromagnetic converter unit 24 is formed of a transformer comprising a primary winding 32, two secondary windings 34, 36 and an electromagnetic coupling member 38 for coupling the primary winding 32 and the second windings 34, 36. The electromagnetic converter unit 24 may also include an additional inductor in series with the primary winding 32 to provide additional inductance. The electromagnetic converter unit 24 can also be configured with the capacitor 30 and the primary winding 32 in the opposite order with the primary winding 32 connected to the node 26 and the capacitor 30 connected to the second input terminal and/or a primary ground. The primary winding 32, the capacitor 30 and any additional inductor have a series resistance, which is not explicitly shown in Fig. 1. The electromagnetic converter unit 24 comprises a measurement device 40 for measuring a primary voltage V12 at the primary winding 32. The measurement device 40 is connected to the control unit 28 for providing a measurement signal to the control unit 28 for controlling the controllable switches 20, 22. Alternatively, a capacitor voltage V30 across the capacitor 30 or the primary current 112 through the capacitor 30 and the primary winding 32, can be measured by a measurement unit and provided to the control unit 28 for controlling the controllable switches 20, 22. The primary winding 32 is connected to a primary ground 41. The primary voltage V12 and a primary current 112 in the primary winding 32 are transformed to two secondary voltages V14, V16 and two secondary currents 114, 116 provided by the secondary windings 34, 36, respectively. The secondary windings 34, 36 are each connected via a diode 42, 44 and an output capacitor 46 to the load 18 for providing a direct charge current 118 or secondary current 118 to the output capacitor 46 and a direct voltage as the output voltage V20 and a direct current as the output current 120 to the load 18 for powering the load.

The controllable switches 20, 22 are switched alternating to provide an alternating or direct pulsating drive voltage VI 1 to the electromagnetic converter unit 24. The output voltage V20 and the secondary currents 114, 116 are dependent on the wave form of the primary voltage V12 and can be controlled by switching the controllable switches 20, 22 and a duty cycle of an on-time of the controllable switches 20, 22. The control unit 28 receives a measurement signal from the measurement device 40 and controls the controllable switches 20, 22 on the basis of the primary voltage V12 in some embodiments. Fig. 2 shows a diagram of a control signal V22 provided by the control unit 28 for switching the second controllable switch 22, the resulting capacitor voltage V30, the output voltage V20, the output current 120, and the secondary current 118.

The control unit 28 starts to trigger the controllable switches 20,22 at tl, wherein the controllable switches 20, 22 are switched alternating during a first time period ΔΤ1 so that the drive voltage VI 1 is provided to the input capacitor 30 comprising a plurality of consecutive voltage cycles or pulses corresponding to the control signal V22. The control unit 28 switches one of the switches 20 or 22 continuously on at t2 to provide a constant voltage level as the drive voltage VI 1 to the input capacitor 30 for a second time period ΔΤ2. The first controllable switch 20 is switched off during the second time period ΔΤ2. The cycle duration of the frequency of the consecutive voltage cycles is constant between tl and t2. During the first time period ΔΤ1 between tl and t2, when the voltage pulses are provided as the drive voltage VI 1 to the input capacitor 30 the primary voltage V12 and the capacitor voltage V30 are continuously oscillating at a driven frequency corresponding to the frequency of the consecutive voltage pulses of the drive voltage VI 1. Corresponding to the input voltage VI 2, the primary current 112 is oscillating with a corresponding frequency. During the second time period ΔΤ2 the primary voltage V12 and the capacitor voltage V30 oscillate at a natural frequency corresponding to the resonance frequency of the resonance elements of the electromagnetic converter unit 24 and slowly damps out due to the resistance of the resonance elements. The first time period ΔΤ1 is divided in two time periods a charge period Tc and a steady state period Ts. During the charge period Tc between tl and tp, when the output current 120 reaches a peak value, the output capacitor 46 is charged by means of the secondary current 118. Since the apparent inductance of the input capacitor 30 and the primary winding 32 is low at the beginning of the first time period ΔΤ1, the secondary current 118 comprises in this certain embodiment one or more peaks during the charge period Tc corresponding to current peaks of the secondary current 114, 116. The peaks of the secondary current 118 during the charge period Tc charges the output capacitor 46 and increases the output voltage V20 and the output current 120 correspondingly. When the output voltage V20 reaches the load voltage causing the diodes 42, 44 to conduct and the impedance of the resonant circuit formed by the input capacitor 30 and the primary winding 32 apparent to the drive voltage VI 1 to change. When the impedance is increased and the diodes 42, 44 are conducting, the secondary currents 114, 116 charge the output capacitor 46 increasing the output voltage V20. During subsequent cycles of VI 1, due to the increase of V20, no further electrical energy can be transmitted by the electromagnetic converter unit 24 and the secondary current 118 is reduced to substantially zero as shown in Fig. 2.

At tp, the output current 120 and the output voltage V20 reaches a peak value and due to the increased impedance, no further electrical energy can be transmitted by the electromagnetic converter unit 24. During the steady state period Ts between tp and t2, the output capacitor 46 remains charged while the output voltage V20 slowly decreases and the output current 120 decreases.

Hence, the electrical power provided to the load 18 is substantially limited to the electrical power provided during the first few voltage pulses until the threshold voltage is reached.

After the triggering the controllable switches 20, 22 is stopped at t2 and the constant voltage, or floating input, is provided as a drive voltage VI 1 to the input capacitor 30, the primary voltage V12, the capacitor voltage V30 and the primary current 112 oscillate with the resonance frequency corresponding to the capacity of the input capacitor 30 and the inductance of the primary winding 32 and any series inductor. The series resistance of the circuit causes the resonance to damp out. The primary voltage V12 or the capacitor voltage V30 and the primary current 112 have a phase shift and a continuously decreasing amplitude in ΔΤ2 after the switching as stopped as shown in Fig. 2. The constant voltage is provided for a second time period ΔΤ2 between t2 and a following tl' when the control unit 28 starts triggering the controllable switches 20, 22 again corresponding to tl . During the second time period ΔΤ2, the output current 118 continuously decreases as the resonance is damped by the series resistance. Since the output capacitor 46 is discharged or partially discharged during the second time period ΔΤ2, the peak of the output current 118 can be provided during the first voltage pulses of a following first time period ΔΤ1 starting at ti'.

The electrical power provided to the load 18 can be controlled by means of the frequency of the voltage cycles provided during the first time period ΔΤ1, and by means of the frequency of pulse trains, the reciprocal of the sum of the two time periods (ΔΤ1+ΔΤ2). The frequency of the voltage pulses within the first time period ΔΤ1 is set to a level close to or higher than the threshold frequency at which no power is transmitted by the

electromagnetic converter unit for a given output voltage in steady state such that the electrical power is transmitted only during the few voltage pulses, when the resonance is not yet fully charged. The threshold frequency is determined by the output resonance circuitry formed by the output windings 34, 36 and the output capacitor 46 and the output voltage. In other words, the threshold or no-load frequency it is the frequency where the maximum voltage oscillation across the inductor multiplied by the turns ratio of the windings of the converter 24 is smaller than the minimum output voltage V20.

The no-load frequency can be calculated, assuming no parasitic effects, for a 50% duty cycle, via a natural resonance ωθ of the electromagnetic converter unit 24, which is given by:

wherein Ls and Lm are the leakage and mutual inductances respectively of the primary winding 32 and C_out is the capacitance of the Capacitor 46. Ls can also include an additional external inductor in series with the electromagnetic converter unit 24 and the capacitor 46. The no-load frequency can be calculated by:

ωθ

f m

Π0

4 arccos max

2 - nx - V20

wherein V10_max is the maximum value of the input Voltage VI 0, V20_mi_n is the minimum output Voltage V20 and nx is the effective turns ratio calculated by:

Lp + Ls

nx = np

Lp

wherein np is an ideal turn ratio of the electromagnetic converter unit 24.

Fig. 3 shows a diagram illustrating the control signal V22 controlling the second controllable switch 22, the charge current 118, the output voltage V20 and the primary voltage VI 2.

In Fig. 3 two first time periods ΔΤ1 are shown, during which the consecutive voltage pulses are provided as a drive voltage VI 1 to the input capacitor 30 and two second time periods ΔΤ2, during which the constant voltage is provided as the drive voltage VI 1 to the input capacitor 30.

The charge current 118 oscillate during the first few voltage pulses of the drive voltage VI 1 of the first time period ΔΤ1 and decreases after one or a few sinusoidal peaks to zero. The output voltage V20 rapidly increases at tl and decreases after a few voltage pulses of the drive voltage VI 1 exponentially. The primary voltage V12 oscillates during the first time period ΔΤ1, when the voltage pulses are provided as the drive voltage VI 1 to the input capacitor 30, wherein the first few voltage cycles of the primary voltage V12 have a larger amplitude than the following voltage cycles. The amplitude of the oscillating primary voltage V12 continuously decreases after the constant voltage is provided as the drive voltage VI 1 to the input capacitor 30 during the second time period ΔΤ2, until the primary voltage V12 is approximately 0 at or before tl'. At tl', the triggering of the controllable switches 20, 22 starts again and the consecutive voltage pulses are provided as the drive voltage VI 1 to the input capacitor 30.

Hence, a small amount of electrical power can be provided to the load 18 during the first few voltage pulses and the resonant energy damped during the second time period ΔΤ2 to be prepared for the following consecutive power pulses.

The frequency of the consecutive voltage pulses during the first time period ΔΤ1 is constant or fixed and close to or above the threshold frequency or the no-load frequency at which no electrical power can be transmitted by the electromagnetic converter unit.

Fig. 4 shows a diagram illustrating the on-time of the controllable switches 20 and 22 during the first period ΔΤ1 and the resulting primary voltage V12 in threshold controlled mode. The controllable switches 20, 22 are voltage-controlled by means of the respective control signals S_H, S_L shown in Fig. 4 and switched on the basis of the primary voltage V12 and an upper threshold level 48 and a lower threshold level 50.

At tl, the upper controllable switch 20 is switched on. This leads to an increase of the primary voltage V12 until a peak value 51 is reached followed by a decrease of the primary voltage V12. At t2 the primary voltage V12 drops below the upper threshold level 48 and the upper controllable switch 20 is switched off. After a fixed time period or a dead time, the lower controllable switch 22 is switched on at t3. This leads to a further dropping of the primary voltage V12 until a peak value is reached followed by an increase of the primary voltage V12 until the primary voltage V12 reaches the lower threshold level 50 at t4. When the primary voltage V12 exceeds the lower threshold level 50, the lower controllable switch 22 is switched off again at t4. After a fixed time period or a dead time, the upper controllable switch 20 is switched on again at tl ' Hence, the on-time of the

controllable switches 20, 22 and the duty cycle of the primary voltage V12 can be voltage- controlled by setting the threshold levels 48, 50 to certain values.

Since the threshold levels 48, 50 are set to different absolute values (e.g. the upper threshold 48 to +100V and the lower threshold 50 to -400V), the on-times of the controllable switches 20, 22 have different durations. This asymmetric setting of the threshold levels 48, 50 leads to an asymmetric triggering of the controllable switches 20, 22, whereby a lower electrical power can be provided to the load 18, since the whole output current flows through one of the secondary windings 34, 36. Further, the output power is more linear connected to the settings of the threshold levels 48, 50 and the technical effort to control the controllable switches 20, 22 is reduced.

Fig. 5 shows a diagram illustrating the hybrid threshold/period control of the controllable switches 20, 22 during the first time period ΔΤ1, 22 by means of the respective control signals S_H, S_L shown in Fig. 5 on the basis of the primary voltage V12, one threshold level 52 and an on-time control of one of the controllable switches 20, 22.

The on-time of the upper controllable switch 20 is controlled by means of the control signal S_H on the basis of the primary voltage V12 and the threshold level 52 as described above. At tl, the upper controllable switch 20 is switched on. The primary voltage V12 increases until a peak voltage 54 is reached and drops again below the threshold level 52 at t2. When the primary voltage V12 drops below the threshold level 52, the upper controllable switch 20 is switched off at t2 and after a dead time or a fixed time period, the lower controllable switch 22 is switched on at t3. After a predefined or a set time toFF, the lower controllable switch 22 is switched off at t4 and after a dead time, the upper controllable switch 20 is switched on again at tl ' and the primary voltage V12 increases.

Hence, the operation of the controllable switches 20, 22 during the first time period ΔΤ1 is controlled on the basis of one threshold level 52 and the on-time duration of one of the controllable switches 20, 22. In this case shown in Fig. 3, the upper controllable switch 20 is voltage-controlled by the threshold level 52 and the on-time duration of the lower controllable switch 22 is time-controlled. In an alternative embodiment, the lower controllable switch 22 is voltage-controlled on the basis of a lower threshold level and the on-time duration of the upper controllable switch 20 is time-controlled.

Hence, an asymmetric triggering of the controllable switches 20, 22 can be achieved by either two threshold levels 48, 50 set to different absolute values in an asymmetric fashion or by means of one threshold level 52 and an on-time control of the respective other controllable switch 20, 22. This leads to a more stable output power, in particular for a very low power levels.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Driver device (10) for driving a load (18), comprising:

input terminals (14, 16) for connecting the driver device (10) to a voltage supply (12) and for receiving an input voltage (V10) from the voltage supply (12),

an output terminal for connecting the driver device (10) to the load (18) for powering the load (18),

two controllable switches (20, 22) connected to the input terminals (14, 16) for converting the input voltage (V10) to a drive voltage (VI 1),

an electromagnetic converter unit (24) connected to the controllable switches (20, 22) for converting the drive voltage (VI 1) to a secondary voltage (V14, V16) and for providing a secondary current (118) for powering the load (18), and

a control unit (28) for controlling the controllable switches (20, 22) such that consecutive voltage cycles are provided for a first time period (ΔΤ1) as the drive voltage (VI 1) to the electromagnetic converter unit (24) followed by a constant voltage level or a floating voltage provided for a second time period (ΔΤ2) as the drive voltage (VI 1) to the electromagnetic converter unit (24), wherein a frequency of the consecutive voltage cycles is set to a level at which the secondary current (118) is provided only during a single voltage cycle or a plurality of initial voltage cycles of the first time period (ΔΤ1).

2. Driver device as claimed in claim 1, wherein the control unit (28) is adapted to switch the controllable switches (20, 22) alternating to provide a pulsating voltage as the consecutive voltage cycles to the electromagnetic converter unit (24).

3. Driver device as claimed in claim 1 or 2, wherein the control unit (28) is adapted to switch one of the controllable switches (20, 22) to a conductive state for the second time period (ΔΤ2) to provide the constant voltage level as the drive voltage (VI 1) to the electromagnetic converter unit (24).

4. Driver device as claimed in any of claims 1 to 3, wherein the control unit (28) is adapted to set the duration of the second time period (ΔΤ2) such that resonance elements (34, 36, 46) of the electromagnetic converter unit (24) can be substantially discharged during the second time period (ΔΤ2).

5. Driver device as claimed in any of claims 1 to 3, wherein the control unit (28) is adapted to control the electrical output power by variation of the duration of a total interval formed by the first time period (ΔΤ1) and the second time period (ΔΤ2).

6. Driver device as claimed in any of claims 1 to 5, wherein the control unit (28) is adapted to control the electrical output power by variation of a duty cycle of the first time period (ΔΤ1) and the second time period (ΔΤ2).

7. Driver device as claimed in any of claims 1 to 6, wherein the control unit (28) is adapted to set the frequency of the voltage cycles to a value close to or above a no-load frequency of the electromagnetic converter unit (24).

8. Driver device as claimed in any of claims 1 to 7, wherein the control unit (28) is adapted to set the on-time of the controllable switches (20, 22) to predefined values to provide the consecutive voltage cycles.

9. Driver device as claimed in any of claims 1 to 7, wherein the control unit (28) is adapted to control a first of the controllable switches (20, 22) on the basis of a primary voltage (V12) or a primary current (112) of the electromagnetic converter unit (24) and a threshold level (48; 52) to provide the consecutive voltage cycles.

10. Driver device as claimed in claim 9, wherein the control unit (28) is adapted to control a second of the controllable switches (20, 22) on the basis of the primary voltage (VI 2) or a primary current (112) and a second threshold level (50).

11. Driver device as claimed in 9, wherein the control unit (28) is adapted to set the on-time of a second of the controllable switches (20, 22) to a predefined value (toFF) ·

12. Driver device as claimed in any of claims 8 to 11, wherein the control unit is adapted to control the controllable switches (20, 22) such that the on-times of the controllable switches (20, 22) have different durations.

13. Driver device as claimed in any of claims 1 to 12, wherein the electromagnetic converter unit (24) comprises a capacitor (30) and a primary winding (32) connected in series to each other.

14. Driver device as claimed in any of claims 1 to 13, wherein the electromagnetic converter unit (24) comprises at least one secondary winding (34, 36) and an output capacitor

(46) connected between the secondary winding (34, 36) and the output terminal for providing a constant output voltage to the load (18) for powering the load (18).

15. Driving method for driving a load (18), comprising the steps of:

- converting an input voltage (V10) to a drive voltage (VI 1) by means of two controllable switches (20, 22),

providing consecutive voltage cycles for a first time period (ΔΤ1) to an electromagnetic converter unit (24),

providing a constant voltage or a floating voltage for a second time period (ΔΤ2) as the drive voltage (VI 1) to the electromagnetic converter unit (24),

converting the drive voltage (VI 1) to a secondary voltage (V14, V16) and providing a secondary current (118) by means of the electromagnetic converter unit (24) for powering the load (18), wherein a frequency of the consecutive voltage cycles is set to a level at which the secondary current (118) is provided only during a single voltage cycle or a plurality of initial voltage cycles of the consecutive voltage cycles of the first time period (ΔΤ1).