Technical Field

The present disclosure relates to lighting technology, and in particular to a driver for at least one LED and a luminaire comprising the same.

Background Art

LED drivers typically provide LED lighting means with an electrical quantity, such as a current or a voltage, in accordance with a nominal value of the electrical quantity, based on closed-loop control.

Especially if case of a relatively slow controller of the closed-loop control, LED drivers supplied off an AC mains supply may experience a disturbance of an internal DC link voltage (VBUS) in the form of an superimposed ripple (AVBUS), which originates from a rectification of the AC mains voltage and thus has twice an AC mains frequency.

This internal ripple may depend on a capacity of the internal DC link and may carry through to the provided electrical quantity depending on a loading of the LED driver, but as it can be measured it may be compensated for by appropriate adjustment of the control variable of the closed-loop control, known as feed-forward (FEFO) control.

The controller of the closed-loop control will try to compensate the fed-back output ripple, wherein it tries to compensate the control error with a 180° phase shift. As repetitive internal ripple keeps the controller busy, a correctly adjusted FEFO control can help to increase a control margin.

The adjustment of the control variable is based on a FEFO gain, which is usually measured during design and set depending on several parameters which can be for e.g. the measured current or voltages of the controlled plant. This compensation process is cumbersome and adds production cost, may be inaccurate due to component tolerances and lifetime issues, and may lead to even higher output ripple in case of overcompensation.

Summary

In view of the above-mentioned drawbacks and limitations, the present disclosure aims to improve LED drivers of the background art.

This is achieved by the embodiments as defined by the appended independent claims. Preferred embodiments are set forth in the dependent claims and in the following description and drawings.

A first aspect of the present disclosure relates to a driver for at least one LED. The driver comprises a switched-mode power supply, SMPS, configured to supply the at least one LED; a closed-loop control for regulating a controlled variable supplied to the at least one LED in accordance with a reference variable; and a feed-forward control for adjusting a control variable of the closed-loop control in dependence of a frequency component of a control error of the closed-loop control and a frequency component of a DC link voltage of the driver.

The feed-forward control may comprise a first narrow-band filter configured to filter the control error; and a frequency component derivation unit configured to determine the frequency component of the control error of the closed-loop control.

The frequency component derivation unit may comprise a peak detection circuit configured to determine a magnitude of a peak value of the filtered control error at a center frequency of the first narrow-band filter.

The frequency component derivation unit may comprise a frequency transform circuit configured to determine a magnitude of a frequency transform of the filtered control error at a center frequency of the first narrow-band filter. The feed-forward control may further comprise a second narrow-band filter configured to determine the frequency component of the DC link voltage of the driver.

The frequency component of the DC link voltage of the driver may comprise a filtered DC link voltage of the driver at a center frequency of the second narrow-band filter.

The respective center frequency of the first and second narrow-band filters may comprise a frequency in a range of twice an AC mains frequency ± 10%, and may preferably comprise a frequency of twice the AC mains frequency.

The AC mains frequency may comprise 50 Hz or 60 Hz.

The feed-forward control may further comprise a gain adjustment unit configured to switch a sign of a gain step width of the gain adjustment unit if the determined magnitude of the frequency component of the filtered control error exceeds a determined magnitude of the frequency component of the filtered control error of an immediately preceding control cycle of the closed-loop control; and adjust a value of a feed-forward gain by adding a value of the gain step width to the value of the feedforward gain of the immediately preceding control cycle of the closed-loop control.

The gain adjustment unit may further be configured to retrieve an initial value of the feed-forward gain in dependence of the determined magnitude of the frequency component of the filtered control error from a lookup table of the driver.

The gain adjustment unit may further be configured to retrieve an initial value of the gain step width from the lookup table of the driver.

The feed-forward control may further be configured to adjust the control variable of the closed-loop control by adding a product of the adjusted value of the feed-forward gain and the frequency component of the DC link voltage of the driver. The closed-loop control may further comprise a controller configured to provide the control variable in dependence of the control error; and a pulse width modulation, PWM, signal generator configured to provide a manipulated variable in dependence of the control variable. The controlled variable may comprise an average value of the current supplied to the at least one LED. The manipulated variable may comprise a PWM gate drive signal for power switches of the SMPS. The control variable may comprise a duty cycle of the PWM gate drive signal. The control error may comprise a differential of the controlled variable and the reference variable. The reference variable may comprise a nominal value of the current supplied to the at least one LED.

The controller may comprise a proportional-integral, PI, controller.

A second aspect of the present disclosure relates to a luminaire comprising a driver of the first aspect or any of its implementations; and at least one LED.

Advantageous Effects

The present disclosure provides an LED driver having a self-adjusting feed-forward control.

That is to say, a correct FEFO gain adjustment can be guaranteed by incrementing (or decrementing) a FEFO gain in discrete steps, depending on an observed decline (or increase) of the output ripple. This procedure takes place until zero actual ripple is achieved, and may be carried out again if ripple re-emerges due to a varying loading of the driver or components aging, for example.

This avoids an adjustment of the FEFO gain at the design stage, which is cumbersome and prone to inaccuracies.

More specifically, the improved LED driver features no production FEFO adjustment costs, no R&D effort for preadjustment of the FEFO control, a best-possible FEFO adjustment achieved, no additional costs due to SW strategy (no additional HW necessary), a robust stepper strategy, additional control phase margin for a Pl-based control due to optimal FEFO adjustment, and a robust driver.

The technical effects and advantages described above in relation with the LED driver equally apply to the luminaire comprising such an LED driver.

Brief Description of Drawings

The above-described aspects and implementations will now be explained with reference to the accompanying drawings, in which the same or similar reference numerals designate the same or similar elements.

The features of these aspects and implementations may be combined with each other unless specifically stated otherwise.

The drawings are to be regarded as being schematic representations, and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to those skilled in the art.

FIG. 1 illustrates a luminaire comprising a driver, both in accordance with the present disclosure; and

FIG. 2 illustrates a driver in accordance with the present disclosure; and

FIG. 3 illustrates a control approach in accordance with the present disclosure.

Detailed Descriptions of Drawings

FIG. 1 illustrates a luminaire 4 comprising a driver 1, both in accordance with the present disclosure. The luminaire 4 comprises at least one LED 5, and a driver 1 of the first aspect or any of its implementations configured to supply the at least one LED 5 off an AC mains supply 6 suggested in FIG. 1 by dashed lines. This driver 1 is described in more detail below.

FIG. 2 illustrates a driver 1 for an LED load in accordance with the present disclosure.

The driver 1 comprises a DC/DC converter 103 configured to provide a DC output current 114, ILED for the LED load in dependence of a DC link/bus voltage 113, VBUS and a duration of a switching period 115, THB of the DC/DC converter 103.

The DC/DC converter 103 may comprise a half-bridge (HB) resonant converter, which may in turn comprise a resonant tank circuit of LLC type, for example.

On an input side, the driver 1 of FIG. 1 may further comprise a power factor correction (PFC) converter 102, such as a boost converter, configured to provide the DC link/bus voltage 113, VBUS for the DC/DC converter 103, and a filter and rectifier circuitry 101 configured to filter electromagnetic noise and to rectify an AC mains input voltage at an input of the PFC converter 102.

On an output side, the driver 1 of FIG. 1 may further comprise an isolating transformer 104 and a subsequent rectifier and sensing circuitry 105 configured to provide, via a further isolating transformer 108, an indication of the DC output current 114, ILED for the LED load.

The driver 1 further comprises a control unit 106, 109 which will be explained in more detail in connection with FIG. 3 below. In the example of FIG. 1, the control unit 106, 109 includes an application-specific integrated circuit (ASIC) 106 and a microcontroller (pC) 109 and is supplied by a DC/DC converter 107. The pC 109 is configured to measure and detect an AC or DC mains input voltage by means of a mains detection circuitry 110. FIG. 1 further shows a digital addressable lighting interface (DALI) unit 111 interconnected with the control unit 106, 109 by means of optocouplers 112. The DALI unit 111 may receive dimming commands for varying an operating point of the DC/DC converter 103.

FIG. 3 illustrates a control approach in accordance with the present disclosure.

In particular, the control approach may be implemented by the control unit 106 shown in FIG. 2.

The driver 1 comprises a switched-mode power supply, SMPS, 27, 102, 103 configured to supply the at least one LED 5.

As used herein, an SMPS may refer to an electric circuit that is configured to transfer power from an AC source, such as an AC mains supply, to DC loads based on at least one switch that is continually toggled between low-dissipation, full-on and full-off states in accordance with a duty cycle.

The driver 1 further comprises a closed-loop control 2 for regulating a controlled variable 28 supplied to the at least one LED 5 in accordance with a reference variable 21.

As used herein, a closed-loop control may refer to a control system which adjusts its input (i.e., control error) to take account of how it affects the load.

The driver 1 further comprises a feed-forward control 3 for adjusting a control variable 24 of the closed-loop control 2 in dependence of a frequency component of a control error 22 of the closed- loop control 2 and a frequency component 29 of a DC link/bus voltage 113 of the SMPS 27, 102, 103.

As used herein, a feed-forward control may refer to an element within a control system which adjusts the controller’s output (i.e., the controlled variable) to take account of a known or detectable disturbance. The feed-forward control 3 may comprise a first narrow-band filter 31 configured to filter the control error 22; and a frequency component derivation unit 32 configured to determine the frequency component of the control error 22 of the closed-loop control 2.

As used herein, a narrow-band filter may refer to a band-pass filter having a fractional bandwidth (i.e., absolute bandwidth divided by the center frequency) of less than a predetermined ratio, wherein the absolute bandwidth is given by a differential between upper and lower cutoff frequencies of the filter.

In particular, the frequency component derivation unit 32 may comprise a peak detection circuit configured to determine a magnitude 33 of a peak value of the filtered control error at a center frequency of the first narrow-band filter 31, or alternatively a frequency transform circuit configured to determine a magnitude 33 of a frequency transform of the filtered control error at a center frequency of the first narrow-band filter 31. The resulting value represents the absolute output ripple.

The feed-forward control 3 may further comprise a second narrow-band filter 37 configured to determine the frequency component 29 of the DC link/bus voltage 113 of the SMPS 27, 102, 103.

In particular, the frequency component 29 of the DC link/bus voltage 113 of the SMPS 27, 102, 103 may comprise a filtered DC link voltage of the SMPS 27, 102, 103 at a center frequency of the second narrow-band filter 37.

The respective center frequency of the first and second narrow-band filters 31, 37 may comprise a frequency in a range of twice an AC mains frequency ± 10%, and may preferably comprise a frequency of twice the AC mains frequency, wherein the AC mains frequency may comprise 50 Hz or 60 Hz, depending on a nominal frequency of the applicable AC mains supply.

The feed-forward control 3 may further comprise a gain adjustment unit 34 configured to switch a sign of a gain step width 35 of the gain adjustment unit 34 if the determined magnitude 33 of the frequency component of the filtered control error exceeds a determined magnitude 33 of the frequency component of the filtered control error of an immediately preceding control cycle of the closed-loop control 2; and adjust a value of a feed-forward gain 36 by adding a value of the gain step width 35 to the value of the feed-forward gain 36 of the immediately preceding control cycle of the closed-loop control 2.

For example, switching the sign of the gain step width 35 may be achieved by a switch decider circuit which is configured to compare the determined magnitude 33 of the current control cycle of the closed-loop control 2 with its value of the immediately preceding control cycle. If the absolute output ripple improved due to the last action taken (+/-), the switch position is maintained, so that another addition (or subtraction) is applied to the feed-forward gain 36. Otherwise the switch position is toggled and a subtraction (or addition) is applied to the feed-forward gain 36.

The feed-forward control 3 may further be configured to adjust the control variable 24 of the closed- loop control 2 by adding a product of the adjusted value of the feed-forward gain 36 and the frequency component 29 of the DC link/bus voltage 113 of the SMPS 27, 102, 103.

This procedure takes place in every control cycle of the closed-loop control 2 until zero output ripple is achieved, and may be carried out over and over again if ripple re-emerges due to a varying loading of the driver, for example. When this happens, the switch position is toggled and therefore the sign of the gain step width 35 is changed.

In FIG. 3 it is further indicated by the arrows directed at itself that the gain adjustment unit 34 is configured to store the values of the gain step width 35 and the feed-forward gain 36 for retrieval as the determined magnitude 33 of the frequency component of the filtered control error “of the immediately preceding control cycle of the closed-loop control 2” and the feed-forward gain 36 “of the immediately preceding control cycle of the closed-loop control 2”, respectively.

The gain adjustment unit 34 may further be configured to retrieve an initial value of the feed-forward gain 36 in dependence of the determined magnitude 33 of the frequency component of the filtered control error from a lookup table of the driver 1 so as to achieve the optimal feed-forward gain 36 earlier. Furthermore, the gain adjustment unit 34 may further be configured to retrieve an initial value of the gain step width 35 from the lookup table of the driver 1.

The closed-loop control 2 may further comprise a controller 23, such as a proportional-integral, PI, controller, configured to provide the control variable 24 in dependence of the control error 22; and a pulse width modulation, PWM, signal generator 25 configured to provide a manipulated variable 26 in dependence of the control variable 24. In particular, the controlled variable 28 may comprise an average value of the DC output current 114, ILED supplied to the at least one LED 5; the manipulated variable 26 may comprise a PWM gate drive signal 26 for power switches of the SMPS 27, 102, 103; the control variable 24 may comprise a duty cycle of the PWM gate drive signal 26; the control error 22 may comprise a differential of the controlled variable 28 and the reference variable 21; and the reference variable 21 may comprise a nominal value of the current supplied to the at least one LED 5.