Patent Application
20130154491
A. Technical Field
The present invention relates to a switch-mode regulator, and more particularly, to systems, devices and methods of using a switch-mode regulator to regulate an LED current in order to improve overall system efficacy and suppress power consumption of a dimmable light emitting diode (hereinafter, “LED”) illumination system. Such a switch-mode regulator may also be used in applications other than the LED illumination system.
B. Background of the Invention
Semiconductor-based solid-state lighting (SSL), until recently associated mainly with simple indicator lamps in electronics and toys, has become more common as SSL solutions continue to improve when compared to other lighting technologies. In particular, the enormous technology improvements have been achieved on light emitting diodes (LEDs) over the past years. LEDs have been available for various wavelengths, and suitable for white illumination. Lifetime of LEDs is also extended to more than 100 thousand hours, and can work up to many watts input power.
When compared to conventional lamps, LEDs are relatively smaller, require significant lower amount of power, and have a longer operating life. LEDs are normally connected in series as an LED string for use in lighting applications. Each power LED in the LED string used for illumination requires a nominal LED current in the range of 35-1400 mA, a forward voltage drop of 3V, and large manufacturing tolerances. The amount of power for LEDs to operate is much less than that of a typical halogen lamp. A halogen lamp may operate within a range of 20-50 Watts, while an LED at about 5-10 Watts is sufficient to provide a similar level of brightness. However, a need always exists to further enhance the energy efficiency of an LED illumination system.
In the LED illumination system 100, an input power PIN provided by the supply 106 is not fully converted to the illumination power of the LED light module 104. Both the LED driver 102 and the LED light module 104 dissipate some energy in the format of heat and invisible radiation during their respective process for voltage conversion and LED illumination. In particular, the LED driver 102 is typically based on a linear or switch-mode regulator. The linear regulator exhibits poor efficiency characteristics across its load range. The switch-mode regulator exhibits better efficiency than the linear regulator, but may still suffer from reduced efficiency when it is loaded below a maximum efficiency operating point due to inherent switching and quiescent bias losses within the switch-mode regulator.
The LED system efficacy ESYS refers to the overall illumination efficiency of the LED illumination system 100. Since any energy loss in the LED drive process has to be accounted, the LED system efficacy ESYS is a combination of the driver efficiency ηDRV and an efficacy ELED of the LEDs in the LED module 104. Hence, the LED system efficacy, the efficacy of the LEDs and the driver efficiency may be respectively represented as
wherein POUT is the output power from the LED driver 102.
Various solutions are adopted to drive a dimmable LED illumination system. In one solution, the LED current 108 is generated as a pulse waveform having a frequency, magnitude and duty cycle that are modulated according to a dimming control. The magnitude of the LED current 108 alternates between zero and the maximum LED current IMAX during LED dimming operation. Thus, the LED driver 102 either is disabled or works at a driver efficiency associated with the maximum LED current IMAX, and however, the efficacy ηLED of the LEDs is compromised to work at a relatively low end of the efficacy curve 152 in such a solution. In another solution, the LED current 108 is generated as a direct current (DC) according to the dimming control. Although the LEDs function efficiently under a DC driver current, the LED driver 102 and its driver efficiency ηDRV is compromised in order to provide such a DC current. Therefore, both solutions cannot reach a preferred overall LED system efficacy upon receiving the varying dimming control.
Although it originates from the dimmable LED illumination system based on a switch-mode regulator, the above efficiency regulation issue commonly exists in a switch-mode regulator or power supply that generates an adjustable average output load current. A need exists to maintain preferred driver efficiency when the switch-mode regulator generates the adjustable average output load current.
The present invention relates to a switch-mode regulator, and more particularly, to systems, devices and methods of using a switch-mode regulator to regulate an LED current in order to improve overall LED system efficacy and suppress power consumption of a dimmable LED illumination system. Such a switch-mode power supply or regulator may also be used in applications other than the LED illumination system.
In the LED illumination system, a high brightness mode and a moderate brightness mode are implemented. In the high brightness mode, the LED current is larger than a preferred LED current IDMAX. In the moderate brightness mode, an output current is substantially equal to the preferred LED current IDMAX; upon receiving further dimming control, an LED driver based on the switch-mode regulator operates at a preferred driver efficiency ηDOPT, while the LED current remains as a direct current.
One aspect of the present invention is an LED illumination system that comprises an LED driver, an LED module, a current sink and an output capacitor. The LED driver is coupled to receive a DC power supply and a dimming control, and generates a drive voltage and a current control according to the dimming control. In particular, the LED driver operates in a brightness mode selected from a high brightness mode and a moderate brightness mode. The LED module is coupled to the LED driver and driven by an LED current that is associated with the drive voltage. The current sink is arranged in series with the LED module and limits the LED current to a substantially direct current according to the current control. The output capacitor is coupled in parallel with the LED module and the current sink.
Another aspect of the present invention is an LED driver that comprises a clock generator, a current regulation module, a switching regulator, and a rectifier and filter. The current regulation module receives a dimming control, and generates a current control and a mode control. The switching regulator modulates an AC drive voltage that is generated by the clock generator in a brightness mode selected from a high brightness mode and a moderate brightness mode according to the mode control. The rectifier and filter rectifies and filters the modulated AC drive voltage to generate a drive voltage to drive the LED module. The LED current that is a substantially direct current according to the current control.
Another aspect of the present invention is a method of generating an LED current to drive an LED module in an LED driver. The method comprises the steps of (1) receiving a DC supply voltage and a dimming control, (2) generating a current control according to the dimming control, (3) generating an AC drive voltage, (4) modulating the AC drive signal according to a brightness mode selected from a high brightness mode and a moderate brightness mode, and (5) rectifying and filtering the modulated AC drive voltage to generate an LED current to drive the LED module. The LED current is a substantially direct current according to the current control.
Certain features and advantages of the present invention have been generally described in this summary section; however, additional features, advantages, and embodiments are presented herein or will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. Accordingly, it should be understood that the scope of the invention shall not be limited by the particular embodiments disclosed in this summary section.
Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
In the following description, for purposes of explanation, specific details are set forth in order to provide an understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, described below, may be performed in a variety of ways and using a variety of structures. Those skilled in the art will also recognize additional modifications, applications, and embodiments are within the scope thereof, as are additional fields in which the invention may provide utility. Accordingly, the embodiments described below are illustrative of specific embodiments of the invention and are meant to avoid obscuring the invention.
Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearance of the phrase “in one embodiment,” “in an embodiment,” or the like in various places in the specification are not necessarily all referring to the same embodiment.
Furthermore, connections between components or between method steps in the figures are not restricted to connections that are effected directly. Instead, connections illustrated in the figures between components or method steps may be modified or otherwise changed through the addition thereto of intermediary components or method steps, without departing from the teachings of the present invention.
Various embodiments of the present invention relates to a switch-mode regulator, and more particularly, to systems, devices and methods of using a switch-mode regulator to regulate an LED current in order to improve overall LED system efficacy and suppress power consumption of a dimmable LED illumination system. One of those skilled in the art knows that such a switch-mode power supply or regulator is not limited to drive LEDs, and may also be used in other applications. Moreover, an LED driver based on the switch-mode regulator may be implemented in a buck, boost or buck-boost topology.
In the moderate brightness mode, operation of the illumination system is based on alternating the LED driver between active and inactive durations, and brightness of the LEDs is adjusted by modulating the width of the active durations according to the dimming control. During each active duration, the LED driver is switched on to provide an output current IOUT equal to the preferred LED current IDMAX, and thus, to operate at a preferred driver efficiency ηDOPT. The LEDs are driven to generate illumination at a proper level using part of the output current IOUT, and an output capacitor is charged up by the rest of the output current IOUT. This proper level of illumination is determined according to the dimming control. During a subsequent inactive duration, the LED driver is switched off, and charge accumulated by the output capacitor is used to provide a supplement current. The supplement current is used as the LED current to drive the LEDs and generate illumination at the proper level.
Generally, excessive power is conserved in each active duration and used to provide illumination power in the subsequent inactive duration. Even though the width of the active durations is adjusted according to the dimming control, the LED current is sustained as a DC current, and the LED driver remains at the preferred driver efficiency ηDOPT. By this means, the overall LED system efficacy may be improved for the dimmable LED illumination system.
In one embodiment, the LED driver 202 is based on a switch mode converter that converts a DC voltage to a direct current, i.e., the output current IOUT. In a prior art LED illumination system, the switch mode converter includes a continuous AC voltage that has a suitable magnitude, e.g., 3V, and a relatively high frequency, e.g., several MHz. The switching duty cycle and resulting switching current of this continuous AC voltage is adjusted according to the dimming control, such that when this AC voltage is appropriately rectified and smoothed, the LED driver 202 may generate an adjustable DC current to drive the LED module 204. The driver efficiency ηDOPT of the LED driver 202 may be compromised even though the LED module 204 is driven efficiently using a direct current.
In various embodiments of the inventions, two brightness modes, a high brightness mode and a moderate brightness mode, are incorporated in the LED driver 202. In a high brightness mode, the dimming control requires an LED current that is larger than the preferred LED current IDMAX in
However, in the moderate brightness mode, the dimming control requires an LED current ILED that is smaller than the preferred LED current IDMAX in
In such a moderate brightness mode, the continuous AC signal is modulated with a duration control signal VIC that determines the periodic active and inactive durations. The modulated AC driver voltage is enabled during active durations, and disabled during inactive durations. During the active durations, the magnitude, frequency, switching duty cycle and switching current of the AC drive voltage are determined, such that the LED driver 202 operates at the preferred driver efficiency ηDOPT. The modulated AC drive voltage is rectified and smoothed to generate a drive voltage VDR and an output current IOUT. The output current IOUT is associated with the preferred LED current IDMAX. During the inactive durations, the LED driver 202 is disabled, and thus, consumes no power. As a result, the LED driver 202 periodically switches between the active and inactive durations, and may sustain its preferred driver efficiency ηDOPT once it is enabled.
The current sink 206 is coupled in series with the LED module 204 between the drive voltage VDR and the ground, while the output capacitor COUT is biased in parallel between VDR and the ground. The current sink 206 is coupled to receive a current control signal VICON from the LED driver 202. This current control signal VICON may be either a multiple bit digital signal or an analog voltage for a current sink 206 that is implemented in a digital or analog configuration. During the active durations, the output current IOUT splits to an LED current ILED and a charging current ICOUT which are used to drive the LED module 204 and charge the output capacitor COUT, respectively. During the inactive durations, the output current IOUT generated by the LED driver 202 is disabled, and a supplement current IDCOUT is provided by the output capacitor COUT to drive the LED module 204. Regardless of the active or inactive durations, the current control signal VICON controls the current sink 206 to limit the LED current ILED to a value determined by the dimming control.
This continuous AC drive voltage is a continuous periodic sinusoidal or square wave signal that has a high frequency around several MHz. The clock generator 304 is used to generate this AC drive voltage. In one embodiment, the magnitude is set at the level of the supply voltage VSUP, the frequency and the duty cycle are randomly selected at several MHz and 50%, respectively. The resulting LED current is less than the maximum LED current IMAX. In particular, the magnitude, frequency and switching duty cycle and switching current are adjusted to result in the preferred LED current IDMAX.
The current regulation module 302 is coupled to receive the dimming control; and determines the current control signal VICON, a mode control signal, and the critical voltages VCH and VCL. The current control signal VICON is associated with the dimming control, and applied to limit the LED current ILED in the LED module 204. The current regulation module 302 may further comprise a memory which is applied to store the value of a preferred dimming control associated with the preferred LED current IDMAX, such that an incoming diming control can be compared to the value to determine the brightness mode.
The mode control signal is generated according to the brightness mode. When the dimming control is associated with an LED current ILED that is equal to or less than the preferred LED current IDMAX, the mode control signal is set to enable the moderate brightness mode in the switching regulator 308. In accordance with the dimming control, the critical voltages VCH and VCL are provided to the output voltage sensing circuit 306 Likewise, when the dimming control is associated with an LED current ILED that is higher than the preferred LED current IDMAX, the mode control signal is set to enable the high brightness mode in the switching regulator 308, and however, the critical voltages VCH and VCL are not needed in this mode. Therefore, the current regulation module 302 functions as a local core for the driver to determine the brightness mode and provide reference voltages, i.e., VCH and VCL.
The output voltage sensing circuit 306 is coupled to receive the drive voltages VDR and the critical voltages VCH and VCL, and generates the duration control VIC that is used to determine the active and inactive durations of the AC drive voltage. In the high brightness mode, no duration control is needed. The output voltage sensing module 306 is either disabled or outputs a high logic 352. In the moderate brightness mode, the drive voltage VDR is compared to both of the critical voltages VCH and VCL. In accordance with curve 354, the duration control VIC is enabled when the drive voltage VIC increases from the low critical voltage VCL to the high critical voltage VCH, and disabled when the drive voltage VIC drops from VCH to VCL. The period and duty cycle of this duration control VIC may vary with the brightness level as determined by the dimming control, and its frequency is much smaller than the frequency of the AC drive voltage.
The switching regulator 308 is coupled to the current regulation module 302, the clock generator 304, and the output voltage sensing circuit 306. The switching regulator 308 functions at both brightness modes, i.e., the high or moderate brightness modes, and the AC drive voltage is modulated in two corresponding manners. The modulated AC drive voltage is further rectified and smoothed by the rectifier and filter 310 to provide the DC drive voltage VDR and the DC output current IOUT.
In the high brightness mode, the switching regulator 308 is used to modulate the switching duty cycle of each cycle within the AC drive voltage. In various embodiments of the invention, the higher brightness level is associated with a larger duty cycle. When the duty cycle is increased to a maximum value, the LED current ILED reaches its maximum current IMAX and the LED module 204 shows its highest brightness level. Apparently, in the high brightness mode, both the driver efficiency ηDRV and the efficacy ELED of the LED module 204 drop as the LED current ILED increases up.
Such duty cycle modulation in the high brightness mode is employed in most of the prior art dimmable LED illumination systems that are based on DC dimming. In the prior art systems, such modulation is applied within the entire illumination range. In particular, when the LED current ILED is less than the preferred LED current ILED, the driver efficiency ηDRV is compromised to ensure the efficacy ELED of the LED module 204.
In various embodiments of the invention, the duration control VIC used in the moderate brightness mode to control the active and inactive durations has a relatively lower frequency than that of the AC drive voltage. In each active duration, the modulated AC drive voltage is generated to maintain the magnitude, frequency and duty cycle that are associated with the preferred LED current IDMAX; in each inactive duration, the modulated AC drive voltage is disabled. In some embodiments, the AC drive voltage is predetermined according to the preferred LED current IDMAX, and therefore, modulation of active durations is sufficient to generate the modulated AC drive voltage. In some embodiments, the AC drive voltage is predetermined according to the maximum LED current IMAX. In addition to modulation of the active durations, an alternative modulation is needed on each cycle of the AC drive voltage to tune down the output current IOUT from the maximum LED current IMAX to the preferred LED current IDMAX during the active durations. Regardless of the modulations, a substantially constant output current IOUT may be generated at the level of the preferred LED current IDMAX during the active durations in the moderate brightness mode. In accordance, a substantially direct current is delivered to drive the LED module 204, while the LED driver 202 sustains its preferred driver efficiency 11 DOPT by preferably operating in the active durations.
Load condition A is associated with the high brightness mode in which relatively high brightness is provided by the LED module 204. The resulting LED current ILED is IMAX, which is larger than the preferred LED current IDMAX. The AC drive voltage is modulated at the level of each high frequency cycle. No active duration modulation is involved. In some embodiments, it may also be regarded that the active duration is maximized such that adjacent active durations merge with each other. The drive voltage VDR is stabilized at a substantially constant voltage. In general, both the duty cycle of the AC drive voltage and the level of the drive voltage VDR increase as the brightness level increases.
Load condition O is associated with a preferred driver efficiency ηDOPT that occurs when the LED current reaches the preferred LED current IDMAX. The switching duty cycle is not modulated for each high frequency cycle, but is set at a fixed duty cycle according to the preferred driver efficiency μ efficiency ηDOPT.
Load condition B is associated with the moderate brightness mode that is based on the preferred switch-mode efficiency obtained at IDMAX. The LED module 204 provides moderate brightness as determined by the dimming control. In one embodiment, the drive voltage VDR is controlled between the high critical voltage VCH and the low critical voltage VCL. Once the LED current is less than IDMAX between load conditions O and C, modulation of active duration is enabled to control the preferred AC drive voltage and switching currents. The modulated AC drive voltage adopts the magnitude, frequency, duty cycle and switching currents of a preferred AC drive voltage. The preferred AC drive voltage and switching currents are associated with the preferred LED current IDMAX. Although the instantaneous output current IOUT is equal to the preferred LED current IDMAX, the LED current ILED is limited by the current sink 206 according to the dimming control. An excessive current is directed to charge the output capacitor, and the drive voltage VDR increases.
As illustrated in
As illustrated in
For load conditions B and C in the moderate brightness mode, the drive voltage VDR is controlled to oscillate between a high critical voltage VCH and a low critical voltage VCL. In certain embodiment, the critical voltages VCH and VCL are determined according to brightness requirement set by the dimming control in the current regulation module 302. A higher brightness level is associated with larger critical voltages VCH and VCL. However, in a preferred embodiment, the critical voltages VCH and VCL are fixed regardless of the dimming control, and both of them are less than a preferred drive voltage in association with the preferred LED current IDMAX.
Both the width and the period of the active durations are determined by the difference of VCH and VCL, and in particular, a larger difference of VCH and VCL are associated with a larger width and a longer period for the active durations. Once VCH and VCL are fixed, various load conditions, e.g., B and C, may slightly impact the period, and however, causes a significant variation to the width of the active durations.
In certain embodiments, the current regulation module 306 directly determines the period and width of the active durations according to the dimming control. In the moderate brightness mode, the period of the active durations may be fixed, while the width TACT is extended longer for high brightness, e.g., load condition B, than low brightness, e.g., load condition C. A similar oscillation may also be observed for the drive voltage VDR. In this embodiment, the current regulation module 302 generates the duration control VIC directly from the dimming control, and provides it to the switching regulator 308; and thus, the drive voltage VDR may not be monitored.
Regardless of the load condition, the preferred driver efficiency ηDOPT is maintained in the moderate brightness mode in the dimmable LED illumination system 200. During the active duration, i.e., between times T0-T1, the modulated AC drive voltage and switching currents are enabled, and the LED module 204 and the output capacitor COUT are driven with a preferred LED current IDMAX. The preferred driver efficiency ηDOPT is maintained in the active durations. Thereafter, during the inactive duration, the modulated AC drive voltage is disabled. Rather, the output capacitor COUT discharges its accumulated charges to drive the LED module 204 at an LED current ILED as determined by the current control IICON. By this means, illumination power collected during the active duration is spread evenly to the entire period to provide a desired brightness level as specified by the dimming control. The preferred driver efficiency ηDOPT is maintained, while the LED module 204 is driven by a direct current to sustain a desired system efficacy.
Although the voltage VDR fluctuates between the critical voltages VCH and VCL, the LED current ILED is preferably a direct current to avoid flickering of the LEDs in the LED module 204 and to maintain the preferred LED efficacy in the LED module 204. The current sink 206 enforces a substantially constant LED current and thus eliminates most of the noises including output voltage ripples caused by the VCH to VCL deviation. The rectification and filter unit 208 is capable of attenuating high frequency ripple in the voltage VDR, and avoids its coupling to the LED current ILED. Even if a variation exists in the LED current ILED due to modulations of active/inactive durations, an associated brightness variation may be naturally averaged by our eyes. As far as the period of such variation is controlled within one sampling/integration period of human eyes, this brightness variation may not be noticed by our visual systems, while the absolute brightness level is substantially constant as determined by the dimming control. In various embodiments of the invention, the frequency for the active and inactive durations is maintained above an audible noise threshold, i.e., 20 kHz. As a result, oscillation in the drive voltage VDR may not lead to noticeable visual artifacts in illumination.
In the moderate brightness mode, the LED current needs to be smaller than the preferred LED current IDMAX. The moderate brightness mode is associated with alternating active and inactive durations. The LED driver maintains the preferred driver efficiency during the active duration, and the output capacitor provides a supplemental current as the LED current during the inactive duration. A high critical voltage VCH and a low critical voltage VCL are generated at step 614. If the AC drive voltage is not associated with the preferred LED current IDMAX, then the duty cycle of each cycle in the AC drive voltage is adjusted such that the preferred LED current IDMAX is generated when no dimming control is applied at step 616. However, if the AC drive voltage is already associated with the preferred LED current IDMAX at step 614, step 616 is optional; and the drive voltage VDR is directly monitored, and used to generate a control signal VIC according to the critical voltages VCH and VCL at step 618. The control signal VIC determines active and inactive durations. At step 620, the AC drive voltage is disabled or enabled according to the control signal VIC, and thereafter, converted to the modulated AC drive voltage.
In another embodiment, the dimming control signal is applied to determine an active duration width TACT and a period TLED for LED current control, rather than VCH and VCL, at step 614. At step 618, the control signal VIC is thus generated according to the active duty duration width TACT and period TLED.
One of those skilled in the art will recognize that the method 600 can be applied to address the efficiency regulation issue commonly existing in a switch-mode regulator or power supply that generates an adjustable average current. The switch-mode regulator drives a load that relies on the regulator to provide a load current, i.e., the adjustable average current. The efficiency of the switch-mode regulator reaches a preferred driver efficiency ηODRV at a preferred load current IDMAX, if modulation is applied on a duty cycle level within the regulator. In accordance with the high and moderate brightness modes, two current control modes are used in the regulator, and are differentiated by the preferred load current in this switch-mode regulator. Particularly, in a moderate current control mode, active and inactive durations are alternated in the switch-mode regulator to avoid modulation on the duty cycle level, such that the preferred efficiency is maintained.
It will be appreciated to those skilled in the art that the preceding examples and embodiments are exemplary and are for the purposes of clarity and understanding and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, combinations, and improvements thereto that are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present invention. It is, therefore, intended that the claims in the future non-provisional application will include all such modifications, permutation and equivalents as fall within the true spirit and scope of the present invention.
