EP3105995B1 - Driver circuit for leds - Google Patents

Description

In particular, the present invention relates to a control circuit and a converter for the operation of at least one light source, e.g. a driver circuit for the operation of at least one LED, and a corresponding method for operating a light source.

The US 2013/0285565 A1 discloses a multichannel constant current source having two controllable stages for operating an array of light emitting diodes and a lighting source.

The WO 2013/072784 A1 discloses a system and method for controlling the maximum output operating voltage of a solid state lighting device.

The publication " AN-9729th LED Application Design Guide Using Half-Bridge LLC Resonant Converter for 100W Street Lighting "(November 16, 2012, pages 1-14, XP002707759 Fairchild discloses an LED operating system using a half-bridge LLC resonant converter for high power LED lighting applications such as outdoor or street lighting.

A driver circuit for operating LEDs is basically known from the prior art. Such a driver circuit is powered by an electrical supply source, and includes a resonant circuit, such as a resonant circuit. an LLC converter, which is responsible for transmitting power via a galvanic barrier or galvanic barrier from a primary side to a secondary side of the galvanic barrier. The purpose of this transfer of electrical energy is the supply of a switched on the secondary side LED track with electricity.

The invention is based on such an LLC topology, e.g. a half-bridge inverter with the following resonant circuit comprises. The resonant circuit feeds a transformer, starting from the secondary side in turn an LED track can be supplied.

From the prior art, however, it is known to detect the current through the primary side of the transformer as the actual value of the LED current for constant control of the LED current. This measurement is, however, fraught with the Uncertainty of the magnetization current whose exact size is not determined in this topology.

In this case, a known correction of this measurement is that the voltage across the LED path is measured on the secondary side and, if appropriate with further measured values, such as, for example, the temperature is sent from a secondary side microcontroller via the SELV barrier to the microcontroller on the primary side. This microcontroller thus corrects the primary current measurement by this correction contribution, which is calculated using empirical, e.g. in a look-up table stored measured values is determined from the secondary-side LED voltage measurement. Finally, the microcontroller controls the frequency of the half-bridge depending on the current and voltage measurement via an ASIC as manipulated variable. The disadvantage here is that the look-up table for determining the correction value can only approximately determine the correct correction contribution. In addition, the solution is relatively expensive in terms of components, in particular to overcome the SELV barrier.

With such a driver circuit, dimming of the LED path can be achieved by briefly switching off the high-frequency clocking of the switches of the half-bridge. This is achieved by a low-frequency PWM modulation. However, it is problematic here that the levels of PWM modulation are limited. The resolution of the stages of the PWM modulation, eg 7-bit, causes so-called dithering, ie the frequency of the LLC converter can not be controlled precisely enough. Consequently, the desired LED current and thus the desired brightness can only be approximately adjusted.

The invention is based on the technical problem of specifying a control circuit or driver circuit for operating light sources, in particular LEDs, in which a desired value for the brightness is better implemented.

This problem underlying the invention will now be solved by the combination of the features of the independent claims. The dependent claims further advantageously form the central idea of the invention.

According to a first aspect of the invention, a driver circuit for lighting means is provided, in particular for one or more LEDs. The driver circuit comprises a voltage-supplyable circuit which is clocked by means of at least one switch and supplies a resonant circuit for supplying the lighting means with power. The driver circuit further comprises a control circuit for controlling the luminous flux, comprising a regulator. The controller generates a manipulated variable for controlling the luminous flux in response to a feedback signal representing the current through the luminous means and a signal representing a setpoint value for the luminous flux. The driver circuit comprises a PWM modulator for modulating the manipulated variable with a PWM signal. The modulation of the manipulated variable by the PWM signal causes the control loop to be activated continuously and also in the switch-off periods of the PWM signal.

Further, the drive circuit includes a PWM dimming unit for setting a duty ratio for the PWM modulation depending on a dimming command. Furthermore, the driver circuit has an amplitude dimming unit for determining the setpoint value for the luminous flux in dependence on the dimming command.

Preferably, the control loop is activated continuously and in particular also in the turn-off periods of the PWM signal. As already stated above, the modulation of the manipulated variable by the PWM signal leads to the control circuit being activated continuously, and in particular also during the switch-off periods of the PWM signal.

Preferably, the control circuit has a time constant that is slower than the time duration of a period of the PWM modulation.

Preferably, the time constant of the control loop is much slower than the duration of a period of the PWM modulation, e.g. at least 5 times slower, especially between 5 and 10 times slower.

The driver circuit preferably has a low-pass filter for filtering the feedback signal. Thus, the controller sets the manipulated variable depending on the low-pass filtered feedback signal.

Preferably, the time constant of the low-pass filter is slower than the duration of a period of the PWM modulation.

At this time, the time constant of the control loop may be slower than the period of one period of the PWM modulation that the time constant of the low-pass filtering by the low-pass filter is slower than the period of one period of the PWM modulation.

Preferably, the time constant of the control algorithm implemented in the controller is slower than the duration of a period of the PWM modulation.

The time constant of the control loop can thereby be slower than the time duration of a period of the PWM modulation that the time constant of the control algorithm implemented in the control algorithm is slower than the duration of a period of the PWM modulation.

Preferably, the PWM modulation due to the slow time constant, in particular due to the slowdown by the low-pass filter, by the control loop is not adjustable.

As already stated above, the driver circuit has a PWM dimming unit for setting a duty ratio for the PWM modulation depending on a dimming command. DB). The duty cycle is preferably set independently of feedback variables, in particular from the area of the driver circuit.

In this case, the PWM modulation can be done in particular open-loop. That the PWM signal is modulated only by control and not by control on the manipulated variable. In other words, the PWM modulation is done without feedback.

As already explained above, the driver circuit has an amplitude dimming unit for determining the setpoint value for the luminous flux in dependence on the dimming command.

The setpoint can be set independently of the set duty cycle for the PWM modulation.

Preferably, in a first dimming range of the dimming command, the duty cycle for the PWM modulation is 100%. Preferably, in a second dimming range, the duty cycle is reduced stepwise or continuously (continuously or quasi-continuously esp. In a digital embodiment).

Preferably, a cascade control is provided. The first control circuit comprising the regulator for regulating the luminous flux is interleaved with a second control circuit having a further regulator. The second control loop may have a faster time constant than the first control loop. The second control circuit can in particular serve to regulate a residual ripple of the voltage.

The driver circuit preferably has a transformer following the resonance circuit for transmitting electrical energy from a primary winding coupled to the resonance circuit to a secondary winding, from which the lighting means can be supplied with power.

Preferably, the feedback signal indirectly reflects the luminous flux, in which it is inductively coupled out on the secondary side of the transformer.

The driver circuit preferably has a driver for output, based on the PWM-modulated manipulated variable, at least one on / off drive signal for the control of the at least one switch of the clocked circuit.

The manipulated variable preferably reproduces the frequency and / or the duty ratio of the control of the at least one switch of the clocked circuit.

Furthermore, a control unit for operating a driver circuit for lighting means, in particular for one or more LEDs, is disclosed. The control unit comprises an input for a return signal representing the current through the lighting means. The control unit comprises a control circuit for regulating the luminous flux, comprising a regulator. The controller generates a manipulated variable for the regulation of the luminous flux as a function of the feedback signal and of a signal representing a setpoint value for the luminous flux. The control unit comprises a PWM modulator for modulating the manipulated variable with a PWM signal.

Preferably, the control unit is in the form of an integrated circuit, in particular ASIC or microcontroller or a hybrid version thereof.

In accordance with a further aspect of the invention, a method is provided for regulating the current through lighting means, in particular by one or more LEDs. A return signal representing the current through the lamps is tapped off. Depending on the feedback signal and of a signal representing a setpoint for the luminous flux, a manipulated variable for the regulation of the luminous flux is generated.

A PWM signal is modulated onto the manipulated variable. In this case, the modulation of the manipulated variable by the PWM signal causes the control loop to be activated continuously and also in the turn-off periods of the PWM signal; a duty cycle for the PWM modulation is set depending on a dimming command; and the setpoint for the illuminant current is set in response to the dimming command.

The aspects mentioned above with respect to the driver circuit are also applicable to the control unit and to the method.

The invention thus preferably proposes that the PWM signal does not temporarily switch off the control loop or modulate the entire half-bridge driver, but rather modulate the output signal of the half-bridge control loop.

Preferably, a control of the LED current by changing the frequency of the half-bridge drive is present. This control loop runs constantly, ie also in the turn-off periods of the PWM modulation. This control has a time constant that is significantly slower than the frequency / duration of the PWM modulation.

In a preferred embodiment, the targeted 'slowing down takes place by the low-pass filtering of the feedback signal before the comparator with the desired signal. However, this time constant can also be implemented after the comparator or in the control algorithm itself.

Overall, there is preferably a type of combined PWM / AM dimming. However, since the feedback signal is low-pass filtered and the control loop is continuously active, the LED current remains substantially constant over a period of PWM modulation, resulting in improved color continuity of the LED path.

• Fig. 1 zeigt schematisch den Aufbau einer erfindungsgemäßen Treiberschaltung zur Versorgung von Leuchtmitteln, insbesondere zur Versorgung von LEDs bzw. einer LED-Strecke,
• Fig. 2 zeigt eine alternative Ausführungsform der erfindungsgemäßen Treiberschaltung,
• Fig. 3 zeigt eine alternative Ausführungsform der erfindungsgemäßen Treiberschaltung,
• Fig. 4 zeigt eine alternative Ausführungsform der Erfassung des Stroms durch die LEDs bei der erfindungsgemäßen Treiberschaltung
• Fig. 5 zeigt den Einfluss eines Dimmwerts auf Parameter der erfindungsgemäßen Regelung, und
• Fig. 6 ein weiteres Ausführungsbeispiel einer Treiberschaltung gemäß der vorliegenden Erfindung.

The invention will also be described below with reference to the figures.

• Fig. 1 shows schematically the structure of a driver circuit according to the invention for the supply of lighting means, in particular for the supply of LEDs or an LED track,
• Fig. 2 shows an alternative embodiment of the driver circuit according to the invention,
• Fig. 3 shows an alternative embodiment of the driver circuit according to the invention,
• Fig. 4 shows an alternative embodiment of the detection of the current through the LEDs in the driver circuit according to the invention
• Fig. 5 shows the influence of a dimming value on parameters of the control according to the invention, and
• Fig. 6 another embodiment of a driver circuit according to the present invention.

In Fig. 1 an embodiment of a driver circuit 1 for the supply of lighting means is shown, in particular in the form of an LED converter for the supply of LEDs or an LED track.

The driver circuit 1 is fed on the input side by an input voltage Vdc. The input voltage Vdc is preferably a rectified, and optionally filtered, AC voltage or mains voltage. Preferably, this rectified mains voltage is still supplied to a converter in the form of, for example, a power factor correction circuit (not shown) before it supplies the driver circuit 1. The input voltage Vdc is in this case an approximately constant bus voltage possibly having a residual ripple. In the embodiment of Fig. 1 The input voltage Vdc has an amplitude of 400V. The input voltage can also be called bus voltage or DC link voltage. Alternatively, the input voltage Vdc may also be a DC voltage or a constant voltage such as a battery voltage.

On the input side, a switching regulator is provided in the driver circuit 1, which is dependent on the input voltage Vdc is fed. The input voltage Vdc supplies, in particular, a clocked circuit or an inverter, which can be designed, for example, in the form of a half-bridge circuit 2. The half-bridge circuit 2 shown has a potential-lower switch LS and a higher-potential switch HS. According to the invention, the inverter 2 has at least one switch. As an inverter with a switch, for example, a flyback converter (not shown) may be provided.

The series-connected switches LS, HS of the half-bridge circuit 2 may be implemented as transistors, e.g. FET or MOSFET be configured. The switches LS, HS are controlled by respective control signals S / LS, S / HS, starting from a half-bridge driver 12 of a control unit ST. The switches LS, HS are preferably switched on and off alternately by the control signals S / LS, S / HS and by the half-bridge driver 12. The mean value of the current through the LEDs can be adjusted by changing the drive frequency ASF of the switches LS, HS, and / or by changing the duty cycle of the drive. The potential-lower switch LS is connected to a primary-side ground. At the half-bridge circuit 2, the input voltage Vdc is applied.

Between the two switches LS, HS, ie at the midpoint of the half-bridge circuit 2, a resonant circuit 3 in the form of, for example, a series resonant circuit is connected. Alternatively, according to the invention, a parallel resonant circuit may also be connected at the midpoint of the half-bridge circuit 2. The in Fig. 1 shown resonant circuit 3 is designed as a series resonant circuit and includes Inductance and capacitance elements. In particular, between the primary-side ground and the center of the half-bridge circuit 2 is a series circuit comprising a first coil Lr, a second coil La and a capacitor Cr. The resonant circuit 3 is referred to in this case as the LLC resonant circuit. The coil Lr and the capacitor Cr preferably form an LC resonant circuit and are referred to as a resonant coil and a resonant capacitor. 7

The second coil La connected in series with the coil Lr and the capacitor Cr is preferably the primary winding of a transformer T serving as a transformer for galvanic isolation. The transformer T is an example of a galvanic barrier used in Fig. 1 is shown as a safety extra-low voltage barrier or SELV barrier 7 (Safety Extra Low Voltage). The transformer T forms a total of a galvanic barrier between a primary side having the primary winding La and a secondary side comprising the secondary winding Lb of the transformer T. In Fig. 1 the transformer T is shown as an ideal transformer, wherein the primary winding of the real transformer T can have a leakage inductance and a main inductance for guiding the magnetizing current.

The secondary winding Lb of the transformer T has a tapping, in particular a center tap or center tap, this center tap can serve as a secondary side ground. Alternatively, the secondary winding Lb may consist of two separate windings, in which case the center of these separate windings corresponds to the center tap.

One terminal of the secondary winding Lb is connected to a first detection winding L1, and the other terminal of the secondary winding Lb is connected to a second detection winding L1 '. The first detection winding L1 and the second detection winding L1 'are preferably identical. Preferably, the respective number of turns nL1_sec, nL1'_sec of the detection windings L1, L1 'are the same. In series with the first detection winding L1, a first diode D1 is connected. In series with the second detection winding L1 ', a second diode D1' is connected. The detection coils L1, L1 'are connected to the anode of the diodes D1, D1'.

The respective cathodes of the diodes D1, D1 'are brought together, so that these diodes D1, D1' form a rectifier circuit 4. In operation, an AC current, i.e., AC current flows through the secondary winding Lb of the transistor T, preferably. an alternating current. Depending on the direction of this alternating current, a current flows through the first diode D1 or through the second diode D1 '. At the output of the rectifier circuit 4, i. at the connection point of the diodes D1, D1 'thus flows a rectified current. The rectifier is also referred to as the center rectifier.

The rectifier circuit 4 supplies a storage capacitor C2 on the output side. This storage capacitor C2 is preferably connected between the connection point of the diodes D1, D1 'and the center tap of the secondary winding Lb. As storage capacitor C2, an electrolytic capacitor can preferably be used because of its comparatively high capacity. The secondary side current through the detection windings L1, L1 'is thus rectified to operate the LEDs initially by the rectifier circuit 4 and then preferably filtered or low-pass filtered.

Parallel to the storage capacitor C2, the lighting means, preferably LEDs or an LED track, are connected. The driver circuit 1 has correspondingly two output terminals K1, K2 for connecting the LEDs. In Fig. 1 the illustrated LED should be representative of one or more LEDs. Preferably, the LED circuit operated by the driver circuit 1 may comprise a series connection of a plurality of LEDs. Alternatively, parallel LEDs or a combination of LEDs connected in parallel and in series can also be supplied.

At the output of the rectifier circuit 4 and the storage capacitor C2, further components may be provided for filtering. Exemplary is in this Fig. 1 a coil L2 shown. This coil L2 may preferably be arranged in series with the LEDs, this series circuit being connected in parallel with the capacitor C2. The coil L2 is preferably connected between the output terminal K1 on the one hand and the connection point of the diodes D1, D1 'on the other hand. Between two output terminals K1, K2, the driver circuit may have a further storage or filter capacitor C3. In addition, a resistor R3 may be provided between the output terminal K2 and the center tap of the secondary winding Lb.

The detection windings L1, L1 'are coupled to a primary-side winding L1 ", and the alternating current flowing through the secondary winding Lb of the transistor T is thus supplied from the secondary side provided detection windings L1, L1 'transformed into a current flowing through the winding L1 "primary-side current .These three windings L1, L1', L1" form a detection transformer or a preferably isolated sense transformer T1. The current through the primary-side winding L1 "represents the current through the secondary-side windings L1, L1 ', ie also the current through the LEDs .. At least averaged over time, the current through the primary-side winding L1" is a representation of the mean value of the current through the LEDs. Of course, the ratio of the number of turns of the corresponding primary and secondary windings to each other is taken into account. Preferably, the number of turns nL1 "_prim, nL1_sec, nL1'_sec of the primary and secondary side detection windings L1", L1, L1 'are equal.

Preferably, the secondary winding Lb on the one hand and the detection windings L1, L1 'on the other hand are formed as separate windings. That the secondary winding Lb and the sense windings L1, L1 'form two separate transformers. This results, in particular, from the requirement that the detection transformer L1, L1 ', L1 "is designed as a current transformer The windings of the detection transformer are in particular designed to enable a detection of the secondary-side alternating current which is as loss-free as possible Current transformer formed detection transformer L1, L1 ', L1 "have the lowest possible impedance.

The alternating current through the detection windings L1, L1 'generates an alternating current in the coupled primary-side detection winding L1 " Evaluation circuit 6 is connected to the primary-side detection winding L1 "to generate a measured value Im for the current through the LEDs This measurement value Im is fed back to the control circuit ST Based on the obtained feedback value Im, the control circuit ST generates the control signals S / LS, S / HS for the switches LS, HS Starting from the actual value Im, the control circuit ST performs a current regulation to a desired setpoint value ILS, in which the half-bridge circuit 2 is clocked accordingly.

The evaluation circuit 6 serves in principle to evaluate or process the information supplied by the detection winding L1 "about the current through the LEDs and then to return it to the control circuit ST.On the secondary side, a signal representing the current through the secondary side is inductively decoupled and applied to the signal Where it is rectified, averaged and then fed to the control circuit ST, the detection winding L1 "is connected to a rectifier 5 for this purpose. The rectifier 5 may be e.g. in the form of a full-bridge rectifier having four diodes (not shown) configured.

At the output of the rectifier 5, in turn, a resistor or measuring resistor Rshunt is connected, which reproduces the current through the secondary side and through the LEDs. After a low-pass filtering by a low-pass filter LPF, the actual value signal Im of the LED current of the control circuit ST is fed back. The low-pass filter LPF can be configured, for example, as an RC element with a resistor and a capacitor, wherein the capacitor is preferably connected in parallel with the filtered signal Im. This filtered actual value Im gives the mean value of the LED power again. The analog average actual value Im of the LED current is preferably converted by an analog-to-digital converter ADC into a digital actual value. The analog-to-digital converter ADC is preferably designed as a 12-bit converter.

The measured actual value Im of the LED current is subtracted from the control unit ST a setpoint value for the LED current ILS. The control unit ST comprises means such as e.g. a comparator 9 for comparing the set value ILS and the actual value Im or for forming the difference of these values. This results in a control difference RDF for controlling the current through the LEDs.

Meanwhile, the set value ILS for the LED current can be set internally by the control unit ST. Alternatively, a dimming command B, as in FIG Fig. 1 shown, externally specified. For example, the control unit ST may be connected to a line in order to receive the dimming command B via this line and to derive therefrom the current setpoint ILS. In particular, this line may be a data line or a data bus for data transmission between the control unit ST and an external communication unit (not shown). The data transmission can take place analogously or preferably digitally by means of a protocol for the control of photometric operating devices. For example, DALI (Digital Addressable Lighting Interface) or DSI (Digital Serial Interface) can be used as the protocol. The received dimming command B is converted or converted by an amplitude dimming unit 8 into the setpoint value ILS. The amplitude dimming unit 8 preferably generates a desired value ILS in digital form, for example as a 12-bit value.

The control difference RDF is fed to a controller 10, in which a control algorithm for the regulation of the LED current is implemented. The regulator 10 is preferably designed as a digital regulator and can be used e.g. be configured in the form of a PI controller. Depending on the supplied control difference RDF, the controller generates a control variable, by means of which the half-bridge driver 12 is driven. As manipulated variable, e.g. the drive frequency ASF of the switches LS, HS, and / or the duty cycle of the control of the switches LS, HS be provided. In this case, the switches LS, HS of the half-bridge 2 are switched to high-frequency, typically in a frequency range of about 10 kHz. The drive frequency ASF is thus typically higher than 10 kHz and may be e.g. up to a few MHz.

The control unit ST comprises parallel to the amplitude dimming unit 8 for determining the set value ILS for the LED current in response to the dimming command B nor a PWM dimming unit 8 '. This PWM dimming unit 8 'serves to convert the received dimming command B into a duty cycle TVH for PWM modulation (pulse width modulation). The frequency of the PWM modulation is low-frequency compared with the drive frequency ASF, typically in the range of 100-1000 Hz. The duty cycle TVH is fed to a PWM modulator 11.

The PWM modulator 11 receives on the input side, on the one hand, the value of the duty cycle TVH for the PWM modulation and, on the other hand, the manipulated variable ASF or a signal which reproduces this manipulated variable.

The duty cycle TVH is preferably dependent only on the dimming command B and in particular not on the set value ILS. In the PWM dimming unit 8 ', a look-up table may be provided, in which a suitable duty cycle TVH is stored for different dimming commands B.

The duty cycle TVH affects the PWM modulation by the PWM modulator 11, but not the current regulation by the controller 10. In other words, the controller 10 receives as input only the set value ILS for the LED current and not the duty cycle TVH. Thus, the controller 10 is also not turned off during a turn-off period of the PWM modulation.

Thus, there is a kind of combined PWM / AM dimming, the PWM dimming being done by modulating the PWM signal through the PWM modulator 11 and the AM dimming or amplitude dimming by controlling the amplitude of the LED Current through the controller 10 takes place. However, since the actual value signal Im is low-pass filtered and the control loop is continuously active, the LED current is higher in the on-time periods than in true AM / PWM dimming, where the control is turned off in a PWM-off period. Thus, the LED current remains substantially constant - except for a sawtooth ripple - resulting in improved color consistency of the LED path.

According to the invention, the control loop of the LED current is selected, for example, between 5 and 10 times slower than the low-frequency PWM modulation of the operation of the inverter 2. This PWM modulation is superimposed on the high-frequency operation of the half-bridge inverter for dimming the LED path.

The signal representing the LED current is, according to the invention, so low-pass filtered or averaged that the time constant is substantially slower than the frequency of the PWM signal.

The controller topology according to the invention now lies in the fact that the control loop with the actual value signal 'low-pass filtered LED current' and the manipulated variable 'frequency of the half-bridge inverter' is continuously activated, ie in particular in the turn-off of the PWM signal.

The PWM signal, which thus stops the operation of the half-bridge during the switch-off periods, is applied to the output of the control algorithm. The control loop is unable to correct the PWM modulation due to the slowdown due to the low-pass filtering of the feedback signal.

In the embodiment of Fig. 1 the low-pass filter LPF is provided outside the control unit ST. Preferably, the low-pass filter is designed analogously. Alternatively, the low-pass filter can also be connected within the control unit ST. For example, the low-pass filter LPF may be after the analog-to-digital converter ADC, in which case the low-pass filter is implemented digitally. In this case, the low-pass filter may be interconnected either between the analog-to-digital converter ADC and the comparator 9, or between the comparator 9 and the controller 10.

Fig. 2 shows an alternative embodiment for the secondary side of the driver circuit. In particular, in this Fig. 2 an alternative construction of the rectification of the secondary-side current is shown.

The detection winding L1 is, as in the embodiment of Fig. 1 , in series with the secondary winding Lb, which in contrast to the embodiment of the Fig. 1 is designed as a single winding without center tap. The current through the secondary winding Lb is supplied to a rectifier circuit 20, which is thus coupled on the input side to the secondary winding Lb or to the series circuit of the secondary winding Lb and the detection winding L1. The rectifier circuit 20 is designed as a bridge rectifier or full-bridge rectifier with four diodes (not shown).

The circuit at the output of the rectifier circuit 20 in turn corresponds to the circuit at the output of in Fig. 1 shown rectifier circuit 4. The two output terminals of the rectifier circuit 20 are in particular connected to the storage capacitor C2. The LEDs can be connected to terminals K1, K2.

Fig. 3 shows a further alternative embodiment for the secondary side of the driver circuit. In particular, in this Fig. 3 shown another alternative construction of the rectification of the secondary-side current.

Similar to in Fig. 2 the detection winding L1 is connected in series with the secondary winding Lb. The terminal of the secondary winding Lb which is not connected to the detection winding L1 is connected to two diodes 30, 31, respectively. The secondary winding Lb is connected to the anode of the first diode 30 and to the cathode of the second diode 31 connected. A first storage capacitor C30 is connected between the cathode of the first diode 30 and the detection winding L1. A second storage capacitor C31 is connected between the anode of the second diode 31 and the detection winding L1. The diodes D30, D31 form a rectifier circuit 30, which the rectifier circuit 4 of Fig. 1 equivalent. Depending on the direction of flow of the current flowing through the secondary winding Lb current blocks the first or the second diode D30, D31. The storage capacitors C30, C31 are preferably the same and corresponding to the in Fig. 1 shown storage capacitor C2.

Fig. 4 shows an alternative embodiment of the detection of the current through the LEDs in the driver circuit according to the invention.

While in the embodiments of the Figs. 1 to 3 the secondary-side current is fed back to the primary side via a detection transformer T1, the embodiment of FIG Fig. 4 on the detection transformer T1 and thus on the secondary-side detection winding and secondary-side detection windings Lb. A shunt or measuring resistor Rshunt connected in series with the LEDs is used here for current measurement.

At the measuring resistor Rshunt a signal is tapped. that reflects the current through the LEDs. This signal is preferably low-pass filtered, for example by an RC element consisting of a capacitor C40 and a resistor R40. Namely, the tapped signal can be used to charge the capacitor C40 serving as an example of implementation of integration of the current.

The charging voltage of the capacitor C40 is supplied to a secondary-side control unit ST2 as a signal provided to the LEDs. The secondary-side control unit ST2, which is thus arranged on the secondary side of the transformer T, comprises an analog-to-digital converter 40 for converting the measurement signal into digital data. The analog-to-digital converter 40 is preferably designed as a 12-bit converter. This digital data is e.g. returned to the primary side of the transformer 7 via an optocoupler 41 via the SELV barrier. Alternatively to the optocoupler 41, e.g. a digital isolator, e.g. ADUM digital isolator. In particular, the digital data representing current through the LEDs are fed back to the primary-side control unit ST, which in turn activates the half-bridge circuit 2 on the one hand by these feedback actual values of the LED current and on the other hand by the current setpoint ILS or by the dimming command B.

Fig. 5 shows an embodiment for the dimming by the amplitude dimming unit 8 and the PWM dimming unit 8.

Preferably, a dimming of the LED track is now as follows:
From dimming value 100% to a certain dimming value range DWB of, for example, 35%, the LED current is reduced continuously. Here, the duty cycle TVH preferably remains at 100%, so that the PWM modulator 11 has no influence on the manipulated variable.

From a certain dimming value DWB of, for example, 35%, the duty cycle of the PWM modulation is then set below 100%. In other words, between this threshold value DWB of 35% and a lower threshold value USW of, for example, 1%, the duty cycle TVH is gradually reduced. In the PWM dimming unit 8, for example, the correspondence between dimming value B and duty cycle TVH is stored in a look-up table. The duty cycle TVH remains constant for a specific dimming range. For example, in dimming range DWB - USW (35% -x%), the duty cycle is the value y% etc., s. Fig. 5 ,

Thus, if a dimming value below the dimming value DWB, for example, below 35% is specified, a linearly decreasing setpoint for the LED current is fed to the controller. At the same time starting from this dimming value, the duty cycle of the PWM modulator is reduced starting at 100%.

The dimming value specification signal B is thus converted, on the one hand, into a variable PWM duty cycle and, on the other hand, supplied to the control algorithm as a continuously decreasing setpoint value for the time-averaged LED current. The PWM duty cycle and the setpoint preferably do not affect each other. Rather, both values preferably depend only on the dimming value B. This is in Fig. 5 recognizable in which the setpoint ILS decreases linearly with the dimming value B. The duty cycle TVH, however, decreases gradually with the dimming value B, preferably from the dimming value DWB, for example, 35%.

The PWM modulation is thus modulated open loop. The steadily running controller 10 tries to correct this type of disturbance, but this Ausregeling narrow limits set due to the low-pass filtering, for example, the actual value signal. Fig. 5 Figure 4 shows how the controller 10 attempts to control the PWM modulation in which the controller 10 attempts to average the LED current to the value ILED_R.

Thus, the setpoint value ILS for the current controller 10 is continuously reduced. However, the current through the LEDs remains substantially constant over the turn-on periods of the PWM signal, except for a small sawtooth ripple. However, this ripple is not present in the time course, but considered in continuous dimming, since the controller tries in the steadily shortening dimmer with longer dimming durations to control the average value of the current constant.

Fig. 6 shows a further embodiment of a driver circuit according to the present invention.

The driver circuit 60 of the Fig. 6 is basically based on the structure of Fig. 1 , Actual value ILED_ _is the secondary-side LED current measured as in Fig. 4 shown. Similar to in Fig. 1 the actual value is subtracted from the desired value ILS by a comparator 61 and a controller 62, for example in the form of a P, PI or PID controller supplied. The output of the regulator 62 is supplied to a second comparator or subtractor 63. Alternatively, to measure the LED current, the feedback topology of the Fig. 1 be used with the detection transformer T1.

An actual value for the current through the LLC resonant circuit, or for the current through the coil Lr, the primary winding La or through the capacitor Cr, is measured.

In particular, the AC on the primary side is passed through the LLC resonant circuit of e.g. a diode D60 rectified. The rectified current value is then lowpass filtered by an RC element 64, i. averaged, and converted by an analog-to-digital converter 65 into a digital actual value, preferably into a 12-bit value.

The measured actual value of the primary-side current is subtracted from the output of the regulator 62 and fed to the controller 10. The output of the regulator 10 is as in Fig. 1 the PWM modulator 11 and the half-bridge driver 12 supplied.

As explained in the beginning, the input voltage Vdc may be of e.g. 400 V have a residual ripple. When rectifying a mains voltage results in particular a ripple of 100 Hz.

The first control loop with the regulator 62 preferably refers to a slow regulation of the LED current. As a result, e.g. the influence of the temperature, in particular the influence of the ambient temperature on the LEDs, be compensated.

The second interleaved loop with the controller 64, on the other hand, implements a faster algorithm, e.g. to correct the residual ripple of the input voltage Vdc.

The interleaving of the two control loops or by the cascade control thus results in even faster compensation options in addition to the relatively slow current control loop. For example, the Effect of the 100 Hz ripple in the bus voltage is detected and compensated or compensated.

Advantageously, the PWM modulation according to the invention can be used to limit the maximum switching frequency of the LLC resonant circuit.

With the PWM modulation according to the invention, a better efficiency of the converter or the LLC resonant circuit can be achieved.

The PWM modulation according to the invention can be used in combination or as an alternative to amplitude dimming without the need for additional circuitry.

When employing the PWM modulation according to the invention, color shifts are avoided, in particular in the range of low dimming values. This is because the controller 10 regulates to a higher LED current. This is eg in Fig. 5 can be seen where is regulated to a current ILED_R, which is significantly higher than the setpoint ILS, so that the occurring at low current values color shifts can be avoided according to the invention.

The low resolution of the PWM modulation is compensated by controlling the mean value of the LED current.

Claims (12)

1. A driver circuit (1) for lamps, in particular for one or several LEDs, comprising:

   - a circuit (2) that can be supplied with voltage (Vdc) and is clocked by means of at least one switch (LS, HS), which circuit feeds a resonance circuit (3) serving to supply the lamps with current,

   - a control circuit for controlling the lamp current having a controller (10), wherein the controller (10), depending on a feedback signal (lm), that reproduces the current through the lamp, and depending on a signal (ILS) that reproduces a target value for the lamp current, generates a control variable (ASF) for controlling the lamp current, and

   - a PWM modulator (11) for modulating the control variable (ASF) with a PWM signal;

   characterized in that

   - the modulation of the control variable (ASF) by the PWM signal leads to the control circuit being activated continuously and even in the switched-off time durations of the PWM signal,

   - the driver circuit has a PWM dimming unit (8') for adjusting a duty cycle (TVH) for the PWM modulation depending on a dimming command (B), and

   - the driver circuit has an amplitude-dimming unit (8) for establishing the target value (ILS) for the lamp current depending on the dimming command (B).
2. The driver circuit (1) according to Claim 1,
   wherein the control circuit has a time constant, which is slower than the time duration of a period of the PWM modulation.
3. The driver circuit (1) according to Claim 2,
   wherein the time constant of the control circuit is substantially slower than the time duration of a period of the PWM modulation, for example, at least 5 times slower, in particular, between 5 and 10 times slower.
4. The driver circuit (1) according to any one of the preceding claims,
   having a low-pass filter (LPF) for filtering the feedback signal (lm), so that the controller (10) adjusts the control variable (ASF) depending on the low-pass filtered feedback signal (lm).
5. The driver circuit (1) according to Claim 4,
   wherein the time constant of the low-pass filter (LPF) is slower than the time duration of a period of the PWM modulation.
6. The driver circuit (1) according to any one of the preceding claims,
   wherein the time constant of the control algorithm implemented in the controller (10) is slower than the time duration of a period of the PWM modulation.
7. The driver circuit (1) according to any one of the preceding claims,
   wherein the duty cycle (TVH) is adjusted independent of the feedback variables in particular from the region of the driver circuit (1).
8. The driver circuit (1) according to any one of the preceding claims,
   wherein in a first dimming range (100%-DWB) of the dimming command (B) the duty cycle (TVH) for the PWM modulation is 100%, and in a second dimming range (DWB-USW) the duty cycle (TVH) is reduced preferably step-by-step.
9. The driver circuit (1) according to any one of the preceding claims,
   wherein the first control circuit having the controller (10) for controlling the lamp current is interleaved with a second control circuit having a further controller (64), wherein the second control circuit has a faster time constant than the first control circuit, and serves for correcting a residual ripple of the voltage (Vdc).
10. The driver circuit (1) according to any one of the preceding claims,
    having the resonance circuit (3) and a transmitter (T) following the resonance circuit (3) for transmitting electrical energy from a primary winding (La) coupled with the resonance circuit (3) to a secondary winding (Lb), based upon which the lamps can be supplied with current.
11. The driver circuit (1) according to any one of the preceding claims,
    wherein the control variable (ASF) reproduces the frequency and/or the duty cycle of the control of the at least one switch (LS, HS) of the clocked circuit (2).
12. A method for controlling the current through lamps, in particular through one or several LEDs, in which

    - a feedback signal (lm) reproducing the current through the lamps is tapped,

    - depending on the feedback signal (lm) and a signal (ILS) reproducing a target value for the lamp current a control variable (ASF) for the control of the lamp current is generated, and

    - a PWM signal is modulated onto the control variable (ASF),

    characterized in that

    - the modulation of the control variable (ASF) by the PWM signal leads to the control circuit being activated continuously and even in the switched-off time durations of the PWM signal,

    - a duty cycle (TVH) for the PWM modulation is adjusted depending on a dimming command (B), and

    - the target value (ILS) for the lamp current is established depending on the dimming command (B).

Images