Abstract:
An LED driver provides constant output power with wide range output current and voltage. A parallel resonant tank configuration is supplemented by resonant gain clamping circuit configured to partially cancel voltage across the resonant capacitor during half-bridge switching operation. Voltage between the resonant components is clamped to one-half a driver input voltage, ensuring inductive switching of the half-bridge. An output transformer has a primary winding coupled across the resonant capacitor, with a center tap defining first and second portions. An output voltage clamping circuit is coupled across the DC input power source and to the center tap, wherein maximum voltage across the primary is clamped based on a relationship between respective numbers of turns in the first and second portions, and maximum voltage across a secondary winding is clamped based on a relationship between the respective numbers of turns in the secondary winding and the first and second portions.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 62/305,362, dated Mar. 8, 2016, 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 a power supply for lighting devices. More particularly, the present invention relates to an LED (light emitting diode) driver providing constant output power across a wide range of output voltage and output current. 
     LED power supplies capable of providing constant output power are known in the art and desirable for, among other reasons, their output flexibility. For example, a 210 W constant power LED driver may be designed to drive LED loads ranging from 1.4 A/150V to 0.7 A/300V. The development time may accordingly be drastically reduced, and product consolidation can be maximized. 
     Referring to  FIG. 1 , a conventional example of an input stage for an isolated constant power LED driver  10  may be described. An input rail voltage V 1  may be provided from a DC source such as for example the output from a power factor correction (PFC) circuit. The rail voltage V 1  is provided as the DC voltage input across a half-bridge switching circuit including switching elements Q 1  and Q 2 . A primary resonant tank is connected to a node between the switching elements Q 1  and Q 2  to receive an output based on a switching frequency thereof, and includes a primary resonant inductor L 1  and resonant capacitor C 1 . A DC blocking capacitor C 2  and a primary winding T 1   p  for an output transformer are coupled in series with each other and across the resonant capacitor C 1 . In the example shown, output clamping diodes D 5  and D 6  are provided and limit the output voltage across the resonant capacitor C 1  to one half of the rail voltage V 1 . 
     There are at least two substantial drawbacks to this solution for constant power LED driver design. One such issue is that it is very difficult to optimize the turns ratio for the output transformer for a wide range of output voltage. A related problem is that it is very difficult to optimize the resonant current going through the resonant inductor for a desired wide range of output current and voltage. 
     Such problems may be illustrated herein by reference again to the example of a 210 W constant power LED driver. To drive a 300V/0.7 A LED load, the turns ratio of the output transformer will have to be at least 1:1.6 to provide the necessary output voltage. In this case, the resonant inductor current (I L1 ) reflected on the primary winding T 1   p  of the output transformer will be the output current (0.7 A)×the turns ratio (1.6)=1.12 A. Assuming the same turns ratio for the LED driver design, but now applying a 150V/1.4 A LED load, the resonant inductor current I L1  will be (1.4 A)×(1.6)=2.24 A. 
     Therefore, the resonant current in the 1.4 A output example is two times higher than that of a 0.7 A load at the same output power level 210 W. This makes the resonant inductor extremely difficult to design, and still further requires the use of switching elements (e.g., MOSFETs Q 1  and Q 2 ) with a very high current rating in order to handle the higher end of the current range. 
     If a dedicated driver is provided for each of 0.7 A and 1.4 A output loads, the output transformer can be optimized so that the resonant current will remain 1.12 A. But optimization of a circuit as shown in  FIG. 1  is impossible for constant power and wide-range-output applications because the voltage across the resonant capacitor is clamped. 
     Referring next to  FIG. 2 , designers may simply eliminate the clamping diodes (e.g., D 5  and D 6  from  FIG. 1 ) and optimize the turns ratio of the output transformer for 1.4 A (or the highest output current) to reduce the resonant current. However, the operating frequency for the 0.7 A example will be much higher than is the case when the load is 1.4 A. 
     As may further be demonstrated in  FIG. 3 , the frequency difference is primarily a result of the different quality factors (Q) for the resonant tank as corresponding to different LED loads. The natural frequency of the tank is typically high. When the output current is 0.7 A and the output voltage is 300V, the load is: 300V/0.7 A=429 ohms; whereas the load at 1.4 A, 150V is: 150V/1.4 A=107 ohms. As you can see there is four times the difference between load conditions. As a result, the resonant frequency and Q curve will differ greatly with changes in the load. The operating frequency may be very high when the load is 0.7 A, wherein the power converter will conceivably run out of frequency bandwidth in the case where dimming of the LED lighting output is required. 
     It would therefore be desirable to provide an LED driver circuit for which no large difference was produced on the operating frequency in response to changes in the load at full output, as represented in exemplary fashion in  FIG. 4 . 
     It would further be desirable to provide an LED driver circuit for producing a wide range of voltage and current output with constant power driven capability. For example, such an LED driver may desirably enable adjustable output current and output voltage capability across a wide range, for example 1.4 A/150V to 0.7 A, 300V. 
     It would further be desirable to provide such an LED driver to drive a 210 W lighting array and still further to be dimmable across the lighting range. 
     It would further be desirable to provide such an LED driver circuit with guaranteed half-bridge soft-switching behavior at all times. 
     It would further be desirable to provide an LED driver circuit for which no large difference was produced on the resonant current in response to changes in the load. 
     It would still further be desirable to provide such an LED driver circuit with a self-clamped output. 
     BRIEF SUMMARY OF THE INVENTION 
     A proposed LED driver topology in accordance with the present invention will satisfy desired features as listed above. 
     In one embodiment, an LED driver as disclosed herein is configured for providing output power to a load comprising one or more LEDs. First and second switching elements are coupled in series across a DC power input source. A resonant circuit including a resonant inductor and a resonant capacitor is coupled in series to a node between the first and second switching elements, and a controller is configured to regulate an operating frequency for the first and second switching elements in accordance with a sensed output current through the load and a nominal current value. A resonant gain clamping circuit is coupled between the resonant inductor and the resonant capacitor and is configured to cancel part of a voltage across the resonant capacitor during operation of the first and second switching elements, wherein a constant output power is maintained across an output current range from a first output current value to a second output current value. A primary winding of an output transformer is coupled in parallel with the resonant capacitor. 
     In one further aspect of an exemplary LED driver as disclosed herein, the resonant gain clamping circuit may be configured to limit a voltage between the resonant inductor and the resonant capacitor to one-half of an input voltage from the DC power input source, and ensure inductive switching operation for the first and second switching elements. 
     In another aspect of an exemplary LED driver as disclosed herein, a rectifier circuit may be coupled across a secondary winding of the output transformer. An output energy storage device is coupled across the rectifier circuit. First and second output terminals are configured to provide an output voltage from the rectifier circuit and the output energy storage device to the load, wherein the output voltage across the first and second output terminals exceeds a rail input voltage from the DC input power source. 
     In yet another aspect of an exemplary LED driver as disclosed herein, the primary winding may be formed of a first winding portion and a second winding portion coupled at a third node. An output voltage clamping circuit may be coupled across the DC input power source and to the third node, wherein the output voltage clamping circuit is configured to clamp a maximum voltage across the primary winding to a value based on a relationship between respective numbers of turns in the first and second portions of the primary winding. The output voltage clamping circuit may further be configured to clamp a maximum voltage across the secondary winding to a value based on a relationship between the respective numbers of turns in the secondary winding and the first and second portions of the primary winding. 
     In yet another aspect of an exemplary LED driver as disclosed herein, the nominal current value may be adjustable by the controller in accordance with an externally provided dimming input signal. The constant output power may be maintained for the nominal current value as between the first output current value and the second output current value. 
     In still another aspect of an exemplary LED driver as disclosed herein, the second output current value may be 2× the first output current value. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram representing a conventional example of an LED driver circuit. 
         FIG. 2  is a circuit block diagram representing another conventional example of an LED driver circuit. 
         FIG. 3  is a graphical representation of quality factor relative to operating frequency of an LED driver circuit for varying loads. 
         FIG. 4  is a graphical representation of a desired quality factor relative to operating frequency of an LED driver circuit for varying loads. 
         FIG. 5  is a circuit block diagram representing an embodiment of an LED driver circuit as disclosed herein. 
         FIG. 6  is a graphical representation of quality factor relative to operating frequency of an embodiment of an LED driver circuit as disclosed herein. 
         FIG. 7  is an equivalent circuit diagram for a resonant tank according to an embodiment of an LED driver as disclosed herein. 
         FIG. 8  is a graphical representation of a phase lag between a clamped voltage and resonant inductor current for an embodiment of an LED driver as disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring generally to  FIGS. 1-8 , various exemplary embodiments of an invention may now be described in detail. 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. 
     Referring first to  FIG. 5 , an embodiment of a wide output range LED driver  50  with constant power output capability may be described. An input stage as shown includes a number of components as previously described with respect to  FIG. 1 , including switching elements Q 1 , Q 2 , resonant inductor L 1 , resonant capacitor C 1 , and clamping diodes D 5 , D 6 . Further provided with the LED driver  50  of  FIG. 5  are gate drive resistors R 1  and R 2  each coupled in series with respective auxiliary windings T 2   s   2  and T 2   s   1  of a gate driver transformer T 2   p.    
     An output stage for the LED driver  50  includes a secondary winding T 1   s  of the output transformer which is magnetically coupled to the primary winding T 1   p  and provides the input of a full-wave rectifier diode bridge including diodes D 1 -D 4 . An AC voltage of the secondary winding T 1   s  will accordingly be rectified to a DC voltage across a high frequency filter capacitor C 4 . A load (e.g., LED array)  53  is coupled across the capacitor C 4 . A current sensing resistor R 3  is coupled in series with the load and feeds back the output current information to a controller  54 . The controller  54  is configured to control the output current with respect to the set point given by reference current value I_ref, regulating the driven frequency of the half-bridge converter through gate drive transformer T 2 . 
     In the embodiment shown in  FIG. 5 , a tank gain clamping circuit  51  and an output clamping circuit  52  are further added to the input stage. In the tank gain clamping circuit block  51 , a second resonant inductor L 2  is provided in series with primary winding T 1   p  and DC blocking capacitor C 2 . Clamping diodes D 5  and D 6  are coupled together at a node which is further between the first resonant inductor L 1  and the second resonant inductor L 2 , clamping the voltage across L 2  and C 1 . 
     The second resonant inductor L 2  may be characterized as cancelling part of the voltage across the resonant capacitor C 1  (inductive voltage cancels the capacitive voltage) so that the voltage across the resonant capacitor C 1  can actually exceed one-half of the input voltage (Vrail) from the voltage source V 1 . As a result, the turns ratio (Ns/Np) of the output transformer T 1  can be minimized to reduce the reflected current (from secondary to primary), and the output voltage may further exceed maximum output voltage values as are available in conventional circuits. 
     To further illustrate this feature, in an example as follows we assume a turns ratio for the output transformer of 1:1, wherein a maximum output voltage of a circuit such as represented in  FIG. 1  can only be V_rail. But for an LED driver  50  according to the present disclosure, the maximum output voltage may be much higher because the added resonant inductor L 2  cancels part of the voltage on the resonant capacitor C 1  in accordance with equations (1, 2) below:
 
 V rail/2 =V _ C 1 −V _ L 1  (1)
 
 V _ C 1 =V rail/2+ V _ L 1  (2)
 
     Therefore, as a direct result of integrating the tank gain clamping circuit  51  a wide range of driver output may be demonstrated with constant power driven capability. The turns ratio of the output transformer may further be optimized for maximum current, wherein changes in the resonant current may be minimized for variances in the load. 
     The clamping diodes D 5  and D 6  will also limit the voltage across the resonant inductor L 2  and the resonant capacitor C 1  when it reaches Vrail/2. Accordingly, and by illustrative reference to the graphical diagram of  FIG. 6 , the gain of the tank will be substantially flattened out, as a result of which the operating frequencies at different load conditions will be much closer to each other than in embodiments lacking the clamping circuitry (see, e.g., the graphical diagram representing gain (quality factor) with respect to operating frequency in  FIG. 3 ). One potential advantage of such a feature is that the power converter is far less likely to run out of frequency bandwidth in the case where dimming of the LED lighting output is required. 
     Another aforementioned advantage of the LED driver configuration as provided herein is inherent soft-switching of the half-bridge components. The clamping diodes D 5  and D 6  prevent the voltage across the resonant inductor L 2  and the resonant capacitor C 1  from ramping out of control and too high or otherwise beyond circuit specifications. In an embodiment as shown, the voltage after the main resonant inductor L 1  for example is always less than Vrail/2. With reference to  FIG. 7 , an equivalent circuit of the resonant tank may be demonstrated in the case when diodes D 5  and D 6  are provided for clamping the voltage. If the current through the resonant inductor L 1  is defined as:
 
 I _ L 1= I*ejA=I *(cos( A )+ j *sin( A )) with −90&lt; A&lt; 90;
 
and wherein V_in is the reference input with a phase angle=0:
 
 V _clamped= V _in* I*ej ( A−B )= V _in*(cos( A−B )+ j *sin( A−B ))
 
     B is a very small positive number, which is the phase lag between V_clamped and I_L 1 , as shown in  FIG. 8 . According to Kirchoff&#39;s Voltage law,
 
 V _in= j*wL _res* I _ L res+ V _clamped;
 
= wL _res*(cos( A+ 90)+ j *sin( A+ 90)+ V _in*(cos( A−B )+ j *sin( A−B ))
 
     The left side of the equation is a real number and does not have any imaginary component, so it follows that:
 
 wL _res*sin( A+ 90)=− V _in*sin( A−B )
 
since −90&lt;A&lt;90, 0&lt;A&lt;180 and sin(A+90)&gt;0. As a result:
 
− V _in*sin( A−B )&gt;0
 
     Further upon acknowledging that:
 
sin( A−B )&lt;0
 
and taking into account that B is a very small number, it follows that:
 
−90 &lt;A&lt; 0.
 
     Accordingly, the phase angle is a negative number, which means that the inductor current I_L 1  is always lagging the input voltage of the resonant tank. Thus, the LED driver will always guarantee soft-switching behavior of the half-bridge switch pair Q 1 , Q 2 , regardless of any load characteristics and changes in the load. 
     Referring again to  FIG. 5 , the output clamping circuit  52  as disclosed herein may further enable embodiments of the LED driver  50  to achieve automatic clamping of the output voltage to the load. The output clamping circuit  52  includes an energy storage device such as for example capacitor C 3  connected on a first end to a center tap of the primary winding (yielding a first primary winding portion T 1   p   1  and a second primary winding portion T 1   p   2  on opposing ends of the center tap) of the output transformer. The capacitor C 3  is coupled on a second end to circuit ground GND  1  through a charging diode D 7 . The total voltage across the capacitor C 3  and primary winding portion T 1   p   2  is then clamped to the input voltage rail V_rail by another diode D 8 . 
     If we provide or otherwise assume the turns ratio between primary winding portion T 1   p   2  and the entire primary winding (T 1   p =T 1   p   1 +T 1   p   2 ) is 1:N, the voltage relationship in the output clamping circuit can be shown as:
 
 V _c_charge+ V _To_ p 2 =V _rail
 
 V _c_charge= V _To_ p 2
 
Accordingly:
 
 V _To_ p 2= V _rail/2
 
 V _To_ p 2/ V _To_ p= 1 /N  
 
And finally:
 
 V _To_ p=N*V _To_ p 2 =N*V _rail/2
 
     Therefore it may be appreciated that the maximum voltage across the primary winding T 1   p  of the output transformer is clamped at N*V_rail/2. 
     If the turns ratio between the primary windings and the secondary winding of the output transformer is defined as Np:Ns=1:Nps, the maximum voltage at the secondary may be characterized as:
 
 V _To_ s=Nps*N*V _rail/2
 
     Therefore it may be further appreciated that the output clamping circuit  52  provides automatic limiting of the output voltage. 
     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 “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. 
     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. 
     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, IGFET, 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. 
     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 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. 
     Conditional language used herein, such as, among others, “can,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     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.