Abstract:
A constant current LED driver with adaptive startup voltage control is generally configured to provide output current to an LED load. A controller is configured to sense the output current, and to provide driving control signals as a function thereof to maintain the sensed current at a target current. During startup operation, the controller provides driving control signals further as a function of a first defined maximum output voltage value. During steady state operation, the driving control signals are provided as a function of a second defined maximum output voltage value. The maximum values may be set according to forward voltage drop values for the LED load in association with first and second temperatures, respectively. During transition operations, the maximum output voltage value is continuously adjusted between the first and second maximum output voltage values, such that the LED load exceeds its warm up time prior to steady state operation.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 61/944,667, filed Feb. 26, 2014, and which is hereby incorporated by reference. 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to circuitry and methods for providing constant current to a light source such as an LED load. More particularly, the present invention relates to an adaptive startup voltage control for a constant current LED driver. 
     Light emitting diode (“LED”) lighting is growing in popularity due to decreasing costs and long life compared to incandescent lighting and fluorescent lighting. LED lighting can also be dimmed without impairing the useful life of the LED light source, and is generally effective in extremely cold environments, such as for example −40 degrees Celsius (° C.). 
     Generally speaking, LED lighting applications are designed to control the output current in the load and not the voltage, as the forward voltage of an LED load can vary dramatically, for example as a function of temperature, and a fixed voltage output would likewise produce varying output current and lighting output levels. Conventional LED drivers therefore sense an output current provided to the load and a feedback circuit implemented by the controller maintains the output current to a substantially constant value. 
     However, as the current flow through the LED load may vary as a function of temperature, the brightness of the LED lighting output may change along with the temperature of the LED load even while the current supplied from the LED driver remains substantially constant. Therefore, at least for certain temperature variations, the output current from the driver circuit to the LED load must vary as a function of temperature in order to maintain appropriately constant lighting output. 
     A typical representation of forward voltage drop characteristics for an LED load versus temperature is shown in  FIG. 1 . As represented in  FIG. 1 , the incremental forward voltage drop for an LED load is relatively flat, or in other words substantially unchanging, after the temperature of the load exceeds about 20 degrees Celsius. However, the load voltage may be dramatically higher at −40 degrees Celsius than a voltage for the same LED load at a temperature such as 20 degrees Celsius, and typically it could approach levels 30% higher or more. 
     This forward drop voltage differential for loads at such different temperatures is a challenge for LED driver designers. When an LED load reaches steady state operation at full bright lighting output, it is generally hot enough to maintain a reasonably high temperature (for example &gt;20 degrees Celsius) because LEDs generate heat after a relatively short warm up time. LED driver designs typically include a maximum output voltage to limit the output power of the driver. This maximum output voltage limit typically is designed around the forward voltage drop at the steady state temperature (e.g., around 20° C.). 
     However, again with reference to the characteristic curve in  FIG. 1 , if the temperature at startup is around −40° C. the forward voltage of the LED load will be much greater than the voltage at 20° C. The driver circuit might not be able to start the LED load at −40° C. or otherwise maintain full current if the driver output voltage limit is set around Vf+20 (i.e., forward drop at +20° C.). If the LED driver design is accordingly modified to force the maximum output voltage limit to be the forward drop at −40° C., the LED driver output voltage will by necessity be over-designed for steady state operation, with a power limit substantially 30% higher or even more with respect to a desired configuration. This over-voltage design is highly undesirable, as it would typically increase the product cost and size dramatically. 
     BRIEF SUMMARY OF THE INVENTION 
     Circuits and methods as disclosed herein are implemented to reliably start an LED load at cold temperatures and to adaptively adjust the power output as the LED load approaches steady state operation, and thereafter, without electrically and thermally over-designing the LED driver circuit and associated power train components. LED drivers with a controller as disclosed herein therefore improve the driver starting capability while maintaining a relatively low product cost by allowing for the selection of low output voltage limits in power train design. 
     In an exemplary embodiment, a constant current LED driver with adaptive startup voltage control is generally configured to provide output current to an LED load. A controller is configured to sense the output current, and to provide driving control signals to the LED driver as a function thereof to maintain the sensed current at a target current. During startup modes of operation, the controller provides driving control signals further as a function of a first defined maximum output voltage value for the LED driver circuit. During steady state modes of operation, the driving control signals are provided as a function of a second defined maximum output voltage value. 
     In one aspect of the LED driver with adaptive startup voltage control, the controller may be programmed to operate in the startup mode for a first time duration after startup of the LED driver circuit. During a subsequent transition mode of operation, the maximum output voltage value may be continuously adjusted between the first and second maximum output voltage values over a second time duration, such that the LED load exceeds its warm up time prior to steady state operation. 
     In one aspect, the first time duration may be set such that the LED load exceeds its warm up time prior to steady state operation. In another and potentially alternative aspect, the first and second time durations may collectively be set to exceed a warm up time associated with the LED load. 
     In another aspect, the controller may be programmed wherein the startup mode of operation is initiated by the controller programming upon each startup of the LED driver circuit. In another aspect, the steady state mode of operation may be initiated by the controller programming upon completion of the second time duration and maintained for the duration of operation of the LED driver circuit. 
     In another aspect of the LED driver with adaptive startup voltage control, the first maximum output voltage value for the LED driver circuit may be associated with a forward voltage drop for the LED load at a first temperature. The second maximum output voltage value for the LED driver circuit may be associated with a forward voltage drop for the LED load at a second temperature. As one example of such an aspect, the first temperature may be about −40 degrees Celsius, and the second temperature may be about 20 degrees Celsius. 
     In another aspect of the LED driver with adaptive startup voltage control, the controller may be configured with an input to receive a dimming signal from a dimming circuit, and is further operable to control the current provided to the LED load further as a function of the received dimming signal by adjusting the target current as a function of the received dimming signal. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a graphical plot representing forward voltage for an LED load versus temperature. 
         FIG. 2  is a block diagram and partial schematic diagram of an exemplary light fixture including a light source and a ballast in accordance with an embodiment of the present disclosure. 
         FIG. 3  is a graphical plot representing an embodiment of an adaptive startup voltage control process for a constant current LED driver according to the present disclosure. 
         FIG. 4  is a graphical plot of average voltage of the resonant capacitor versus switching frequency for a driver circuit topology that does not include clamping diodes. 
         FIG. 5  is a plot of voltage of the resonant capacitor versus time for a switching frequency that is less than the resonant frequency of the resonant tank circuit. 
         FIG. 6  is a plot of voltage of the resonant capacitor versus time for a switching frequency that is greater than the resonant frequency of the resonant tank circuit. 
     
    
    
     Where the various figures may describe embodiments sharing various common elements and features with other embodiments, similar elements and features are given the same reference numerals and redundant description thereof may be omitted below. 
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     Referring to  FIG. 2 , a particular but non-limiting embodiment of a light fixture  100  of the present disclosure includes a housing  102 , an electronic ballast  106  and a light source L 2 . The light fixture  100  receives power from an alternating current (AC) power source  114  and provides light from the light source L 2 . The light source L 2  provides light in response to receiving current. The housing  102  is connected to the ballast  106  and the light source L 2 . In one embodiment, the housing  102  supports the ballast  106  and the light source L 2  in a predetermined spatial relationship. In preferred embodiments, the light source L 2  is a plurality of series and/or parallel connected light emitting diodes (LEDs). In one embodiment, the light fixture  100  also includes a dimming circuit that provides a dimming signal to a controller  120  of the ballast  106 . The dimming signal is indicative of a target current or light intensity level for the light source L 2 . 
     The ballast  106  provides current to the light source L 2  from the AC power source  114 . The ballast  106  includes an input rectifier  108  and a driver circuit  112 . The input rectifier  108  connects to the AC power source  114  and provides a DC power source having a power rail V_RAIL and a ground GND_PWR at an output of the input rectifier  108 . In one embodiment, the ballast  106  also includes a DC-to-DC converter  110  connected between the input rectifier  108  and the driver circuit  112 . The DC-to-DC converter  110  alters a voltage of a power rail V_RAIL of a DC power source provided by the input rectifier  108 . The driver circuit  112  provides current to the light source L 2  from the DC power source provided by the input rectifier  108 . 
     The driver circuit  112  includes a half-bridge inverter, a resonant tank circuit, an isolating transformer T 1 , an output rectifier  116 , and a controller  120 . The driver circuit  112  as shown is configured to operate as a constant current source with a self-limiting output voltage, but the clamping elements and associated operation as further described herein is merely exemplary and is particularly non-limiting with respect to the adaptive startup voltage control method as further disclosed below. 
     The driver circuit  112  is further optionally dimmable such that the constant current can be changed. The half-bridge inverter includes a first switch Q 1  (i.e., a high side switch) and a second switch Q 2  (i.e., a low side switch) and has an input and an output. The input of the half-bridge inverter connects to the power rail V_RAIL and the ground PWR_GND of the DC power source and provide an AC signal at the output of the half-bridge inverter. In one embodiment, the input of the half-bridge inverter is a high side of the high side switch. A low side of the low side switch (e.g., second switch Q 2 ) connects to the ground of the DC power source. 
     The resonant tank circuit conventionally includes a resonant inductor L 1  and a resonant capacitor C 1 , and further in accordance with the example shown in  FIG. 2 , includes a first clamping diode D 1  and a second clamping diode D 2 . The resonant tank circuit has an input and an output. The input of the resonant tank circuit (e.g., a first terminal of a resonant inductor L 1 ) is connected to the output of the half-bridge inverter. The resonant capacitor C 1  is connected in series with the resonant inductor L 1  between the output of the half-bridge inverter and the ground GND_PWR of the DC power source. The first clamping diode D 1  has an anode connected to a junction formed at the connection between the resonant inductor L 1  and a resonant capacitor C 1 . The cathode of the first clamping diode D 1  is connected to the power rail V_RAIL of the DC power source. The second clamping diode D 2  has an anode connected to the ground PWR_GND of the DC power source and a cathode connected to the junction between the resonant capacitor C 1  and the resonant inductor L 1 . In one embodiment, the resonant tank circuit includes a DC blocking capacitor C_dc connected between the junction of the resonant inductor L 1  and resonant capacitor C 1  and the output of the resonant tank circuit. The first clamping diode D 1  and the second clamping diode D 2  cooperate to limit the voltage at the junction between the resonant inductor L 1  and a resonant capacitor C 1  to a maximum voltage equal to the voltage of the power rail V_RAIL of the DC power source and a minimum voltage equal to the ground PWR_GND of the DC power source. 
     In an embodiment, an isolating transformer T 1  is connected to the output of the resonant tank circuit. The isolating transformer T 1  includes a primary winding T 1 P and a secondary winding T 1 S 1 , T 1 S 2 . The primary winding T 1 P is connected between the output of the resonant tank circuit and the ground PWR_GND of the DC power source. The output rectifier  116  has an input connected to the secondary winding T 1 S 1 , T 1 S 2  of the isolating transformer and an output operable to connect to the light source L 2 . In one embodiment, the turns ratio of the isolating transformer is selected as a function of a voltage of the power rail V_RAIL of the DC power source and a predetermined output voltage limit. In one embodiment, the output voltage limit is 60 VDC. 
     The controller  120  senses current provided to the light source L 2  from the output rectifier  116  and adjusts a switching frequency of the half-bridge inverter as a function of the sensed current to maintain the sensed current at a target current. In one embodiment, the target current is determined as a function of the dimming signal provided by the dimming controller  118 . The controller  120  further controls the current provided to the light source L 2  as a function of the received dimming signal by adjusting the target current as a function of the received dimming signal. In one embodiment, the controller  120  adjusts the current provided to the light source L 2  by adjusting a switching frequency of the half-bridge inverter. 
     Referring to an embodiment as shown in  FIG. 3 , the controller  120  employs an adaptive startup voltage limit control for a constant current power source that provides constant current output to a light source throughout at least a startup operating mode. In the following description for this process, the light source will be referenced in exemplary fashion as an LED load. In the example shown, upon LED startup, the controller  120  for the LED driver sets the output voltage limit V_output_limit around Vf−40 so that the driver can continuously provide the full amount of constant current I_LED_constant even when the relevant temperature is at −40° C. This high output voltage limit will remain as the setting for a certain amount of time, or otherwise stated for the duration of the startup mode of operation, until a transition mode of operation beginning with time T_adjust, a duration for which could be set long enough to warm up the LED load to 20° C. In some embodiments, the startup mode of operation itself may have a set duration to exceed the expected warm up time of the LED load. 
     By the time of T_adjust, LED driver will receive signals from the controller  120  to gradually reduce the output voltage limit from Vf−40 to Vf+20. At the time T_adjust_end, the LED driver would enter a steady state mode of operation and the maximum output voltage of LED driver will be permanently set at Vf+20. 
     In various embodiments, every time the LED driver is started, or otherwise initially is connected to and receives power input from the AC source  114 , the LED driver control operation will go through the output voltage adaptive adjustment curve shown in  FIG. 3 . Because the LED driver only needs to function for a short period of time at the high output voltage limit or higher output power limit, the power train doesn&#39;t have to be over-designed for higher thermal or electrical stress. This may typically have the effect of substantially reducing production cost and product reliability. 
     Referring again to  FIG. 2 , in one embodiment the secondary winding T 1 S 1 , T 1 S 2  of the isolating transformer is connected to a circuit ground CKT_GND which is isolated from the ground PWR_GND of the DC power source by the isolating transformer. Specifically, the secondary winding includes first secondary winding T 1 S 1  and second secondary winding T 1 S 2 , each connected to the circuit ground CKT_GND. The first secondary winding T 1 S 1  and the second secondary winding T 1 S 2  are connected out of phase with one another. 
     The output rectifier includes a first output diode D 3  and a second output diode D 5 . The first output diode D 3  has its anode connected to the first secondary winding T 1 S 1  and a cathode coupled to the light source L 2  (i.e., an output of the driver circuit  112  and ballast  106 ). The second output diode D 5  has an anode connected to the second secondary winding T 1 S 2  and a cathode coupled to the light source L 2  (i.e., the output of the driver circuit  112  and ballast  106 ). 
     In one embodiment, an output capacitor C 2  is connected between the output of the output rectifier  116  and the circuit ground CKT_GND to smooth or stabilize the output voltage of the driver circuit  112  and ballast  106 . In one embodiment, a current sensing resistor R 3  is connected between the circuit ground CKT_GND and the light source L 2 . A first terminal of the current sensing resistor R 3  is connected to the circuit ground CKT_GND, and a second terminal of the current sensing resistor is operable to connect to the light source L 2 . Thus, a voltage across the current sensing resistor is proportional to a current through the light source L 2 . The controller  120  is connected to the circuit ground CKT_GND and the second terminal of the current sensing resistor R 3  to monitor the voltage across the current sensing resistor and sense the current provided to the light source L 2  by the ballast  106 . 
     In one embodiment, the driver circuit  112  further includes a gate drive transformer. The gate drive transformer is operable to receive the gate drive signal from the controller  120  which controls the switching frequency of the half-bridge inverter. The gate drive transformer includes a primary winding T 2 P a first secondary winding T 2 S 1 , and a second secondary winding T 2 S 2 . In this embodiment, the first switch Q 1  and the second switch Q 2  of the half-bridge inverter each have a high terminal, a low terminal, and a control terminal. The high terminal of the first switch Q 1  is connected to the power rail V_RAIL of the DC power source. The low terminal of the second switch Q 2  is connected to the ground PWR_GND of the DC power source. The high terminal of the second switch Q 2  is connected to the low terminal of the first switch Q 1 . A gate drive capacitor C 6  is connected in series with the primary winding T 2 P of the gate drive transformer across a gate drive output (i.e., gate_H and gate_L) of the controller  120 . A first gate drive resistor R 1  is connected in series with the first secondary winding T 2 S 1  of the gate drive transformer between the control terminal of the first switch Q 1  and the output of the half-bridge inverter. A second gate drive resistor R 2  is connected in series with the second secondary winding T 2 S 2  of the gate drive transformer between the control terminal of the second switch Q 2  and the ground PWR_GND of the DC power circuit. The polarity of the first secondary winding T 2 S 1  and the second secondary winding T 2 S 2  of the gate drive transformer are opposites such that the first switch Q 1  and the second switch Q 2  are driven out of phase by the gate drive transformer. 
     Referring to  FIG. 4 , a plot of average resonant capacitor voltage V C   _   res  versus frequency is shown for a driver circuit topology that does not include the first clamping diode D 1  and the second clamping diode D 2 . Because the voltage V C   _   res  of the resonant capacitor C 1  is not clamped or limited, the voltage V C   _   res  of the resonant capacitor C 1  can be much greater than one-half the voltage of the power rail V_RAIL of the DC power source. 
     In contrast, in embodiments of the driver circuit  112  which include the first clamping diode D 1  and the second clamping diode D 2 , the average voltage V C   _   res  of the resonant capacitor C 1  is clamped at one-half the voltage of the power rail V_RAIL (i.e., V_RAIL/2 in  FIG. 3 ), regardless of the operating (i.e., switching) frequency of the half-bridge inverter because the instantaneous voltage of the resonant capacitor is clamped at the voltage of the power rail V_RAIL. The maximum frequency at which the first clamping diode D 1  and the second clamping diode D 2  limit the voltage of the resonant capacitor C 1  is the clamping frequency f op   _   clamp . Above the clamping frequency f op   _   clamp , the average voltage of the resonant capacitor C 1  is less than V_RAIL/2. 
     Thus, when the operating frequency (i.e., switching frequency) of the half-bridge inverter is less than the clamping frequency f op   _   clamp , the voltage V C   _   res  of the resonant capacitor C 1  is still limited to one-half the voltage of the power rail V_RAIL. So even when the switching frequency is at the resonant frequency f res  of the resonant tank, the average voltage V C   _   res  across the resonant capacitor C 1  will be limited to one-half the voltage of the power rail V_RAIL, but reducing the frequency below the clamping frequency f op   _   clamp  (e.g., to the resonant frequency f res  of the resonant tank circuit) can provide more current to the primary winding T 1 S 1  of the isolating transformer. The driver circuit  112  can thus provide additional current to the light source L 2  without increasing the output voltage of the driver circuit  112  by decreasing the operating frequency of the half-bridge inverter. 
     Referring now to  FIG. 5 , the voltage waveform for the resonant capacitor C 1  is shown for an operating frequency that is less than the resonant frequency f res  of the resonant tank circuit. Referring to  FIG. 6 , the voltage waveform for the resonant capacitor C 1  is shown for an operating frequency that is greater than the resonant frequency f res  of the resonant tank circuit. 
     Because the maximum voltage across the resonant capacitor C 1  is clamped, the maximum peak voltage across the isolating transformer primary winding T 1 S 1  is also clamped. Thus, if the turns ratio N of the isolating transformer is selected such that 
               N   =       V_RAIL   2     *     1   60         ,         
then the secondary winding voltage of the isolating transformer will never exceed 60 Volts (i.e., the UL Class-2 Limit). The half-bridge inverter and the resonant tank circuit is thus a voltage source with a self-clamped output voltage.
 
     Further, because the voltage V C   _   res  across the resonant capacitor C 1  is limited to V_RAIL/2, the half-bridge inverter that drives the resonant tank circuit will always operate in a soft-switching condition. This reduces the switching losses and increases the efficiency of the driver circuit  112 . 
     Referring again to  FIG. 4 , the voltage V C   _   res  across the resonant capacitor C 1  can vary between V_RAIL/2 and 0 volts. The driver circuit  112  can thus drive any light source L 2  (e.g., any LED configuration) from the predetermined maximum output voltage (e.g., 60 volts) to the minimum voltage (i.e., 0 volts). 
     In one embodiment, the driver circuit  112  includes a series resonant inverter that has a self-limited output voltage. The series resonant inverter exhibits half-bridge soft-switching under all operating conditions. The driver circuit  112  operates as a constant current source that has a self-limited output voltage and that that has a wide output voltage operating range. The driver circuit  112  can be controlled by an external reference signal (i.e., a dimming signal from a dimmer  130  interfacing with the dimming controller  118 ) to provide a dimming function. 
     Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. 
     The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. As used herein, “ballast” refers to any circuit for providing power from a power source to a light source. Additionally, “light source” refers to one or more light emitting devices such as fluorescent lamps, high intensity discharge lamps, incandescent bulbs, and solid state light-emitting elements such as LEDs, organic light emitting diodes, and plasmaloids. Terms such as “wire,” “wiring,” “line,” “signal,” “conductor,” and “bus” may be used to refer to any known structure, construction, arrangement, technique, method and/or process for physically transferring a signal from one point in a circuit to another. Also, unless indicated otherwise from the context of its use herein, the terms “known,” “fixed,” “given,” “certain” and “predetermined” generally refer to a value, quantity, parameter, constraint, condition, state, process, procedure, method, practice, or combination thereof that is, in theory, variable, but is typically set in advance and not varied thereafter when in use. 
     The terms “switching element” and “switch” may be used interchangeably and may refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, JFET, etc.), a switching diode, a silicon controlled rectifier (SCR), a diode for alternating current (DIAC), a triode for alternating current (TRIAC), a mechanical single pole/double pole switch (SPDT), or electrical, solid state or reed relays. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa. 
     The terms “power converter” and “converter” unless otherwise defined with respect to a particular element may be used interchangeably herein and with reference to at least DC-DC, DC-AC, AC-DC, buck, buck-boost, boost, half-bridge, full-bridge, H-bridge or various other forms of power conversion or inversion as known to one of skill in the art. 
     Terms such as “providing,” “processing,” “supplying,” “determining,” “calculating” or the like may refer at least to an action of a computer system, computer program, signal processor, logic or alternative analog or digital electronic device that may be transformative of signals represented as physical quantities, whether automatically or manually initiated. 
     The terms “controller,” “control circuit” and “control circuitry” as used herein may refer to, be embodied by or otherwise included within a machine, such as a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed and programmed to perform or cause the performance of the functions described herein. A general purpose processor can be a microprocessor, but in the alternative, the processor can be a controller, microcontroller, or state machine, combinations of the same, or the like. A processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of a new and useful invention, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.