Patent Publication Number: US-6670776-B2

Title: Enhanced trim resolution voltage-controlled dimming LED driver

Description:
This application is a continuation of prior U.S. application Ser. No. 09/675,752 filed on Sep. 29, 2000, now U.S. Pat. No. 6,323,598. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to driver circuits for light emitting diodes (LEDs) and in particular to driver circuits which provide voltage controlled dimming. Still more particularly, the present invention relates to a driver circuit providing enhanced trim resolution in voltage controlled dimming of light emitting diodes. 
     2. Description of the Prior Art 
     Like many other display systems, aircraft instrumentation displays frequently employ illuminated indicators. Originally incandescent bulbs were employed for this purpose. However, a variety of factors have motivated replacement of incandescent bulbs with light emitting diodes (LEDs) in such applications, including improvements in power consumption, heat generation, and operating lifetime. 
     Standards exist for the luminance, or brightness level, of illumination sources in aircraft instrumentation displays. Generally, for example, the luminance required for a sunlight-readable indicator must be in the range of at least 300-500 foot-lamberts. Similarly, the accepted indicator luminance for commercial night-flying is approximately 15-20 foot-lamberts; for military night-flying, approximately 1 foot-lambert; and for night vision imaging system (NVIS) compatible flying, approximately 0.1 foot-lambert. Additionally, for voltage-controlled dimming of illuminated indicators, the various voltage levels at which these luminance ranges are achieved (e.g., sunlight readable illumination at approximately 28 volts and commercial night flying illumination at approximately 14-15 volts) have also become effectively standardized by industry expectations, since aircraft instrumentation designers would prefer to utilize existing analog circuitry implementing voltage-controlled dimming of illuminated indicators. 
     These luminance standards and corresponding voltage levels are based on incandescent bulbs as illumination sources within aircraft displays. However, LEDs have different luminance-power characteristics than incandescent bulbs. When replacing incandescent bulbs with LEDs in aircraft instrumentation, or when employing controls designed for incandescent bulbs with LEDs, alteration is required of the luminance-power characteristics of LEDs to satisfy the luminance standards and corresponding voltage level expectations. 
     One approach to satisfying the luminance standards and voltage level expectations when utilizing LEDs involves providing a mechanism for compensating for changing the portion of the applied input power which is actually transmitted to the LEDs. The portion of the applied input power which is transmitted to the LEDs changes across the operating range of input power to the LED illuminated indicator, matching the power transmitted to the LEDs to the power which is required by the LEDs to achieve approximately the same luminance as an incandescent bulb receiving the same input power. However, this approach negates at least some of the reduction in power consumption achieved by employing LEDs in lieu of incandescent bulbs. 
     An additional problem in satisfying the luminance standards and voltage level expectations with voltage controlled dimming of LEDs arises from the inherent luminance-power characteristics of series-connected LEDs. Parallel-connected LEDs may be employed to increase luminance, but require proportionally more power than a single LED. Series-connected LEDs, although requiring less additional power over a single LED than is required by parallel-connected LEDs, have inherent voltage-luminance characteristics which are poorly suited to voltage-controlled dimming, as illustrated in FIG.  6 . Curves  602   a  and  602   b  within FIG. 6 are voltage-luminance plots for two common types of incandescent bulbs. These curves provide significant changes in luminance across the operating range of input voltages as the applied input voltage is brought down from 28 volts to the minimum voltage required for illumination (about 2.5 volts). 
     Curves  604 ,  606 ,  608 ,  610  and  612  are voltage-luminance plots for, respectively, one LED, two series-connected LEDs, three series-connected LEDs, four series-connected LEDs, and eight series-connected LEDs, all with a series resistor to produce 300-500 foot lamberts at 28 volts. As can be seen from curve  604 , for example, the luminance of a single LED falls off very gradually as the applied input voltage is brought down from 28 volts to about 6 volts, after which the luminance fall off rather sharply up to the turn-on voltage for the LED. This provides poor trim characteristics for voltage-controlled dimming of the LED, offering only a narrow range of applied input voltages within which significant changes in luminance are achieved. This limitation is exacerbated by the addition of more LEDs within a series-connection, which increase the turn-on voltage for the LEDs. As a result, curve  612 , representing eight series-connected LEDs, provides extremely poor trim resolution for voltage-controlled dimming, and only within the upper half of a 28 volt operating range. 
     It would be desirable, therefore, to improve the trim characteristics of an LED illumination source to provide improved responsiveness of luminance to voltage changes across the entire operating range of applied input voltages. It would further be advantageous to employ series-connected LEDs for power savings and for improved trim characteristics. 
     SUMMARY OF THE INVENTION 
     Illumination sources, each including at least one light emitting diode are connected either in series or in parallel by a switching circuit, depending upon an applied input voltage. The switching circuit switches the illumination sources from series- to parallel-connection, or vice versa, when the applied input voltage crosses a threshold value in traversing the operating range of applied input voltages. Because the light emitting diodes within the illumination sources are switched from series to parallel connection at a defined kickover point, the voltage-luminance characteristic changes on opposite sides of the kickover point. The resulting overall voltage-luminance characteristic has greater variability in luminance across the entire operating range of applied input voltages, and luminance-variance is not limited to only a portion of the operating range. Greater trim resolution for voltage-controlled dimming of the light emitting diode is therefore provided, with industry standard luminances being achieved at appropriate applied input voltages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts an enhanced trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a plot of voltage-luminance characteristics of the enhance trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention; 
     FIG. 3 depicts an alternating current embodiment of the enhanced trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention; 
     FIGS. 4 and 5 are alternative enhanced trim resolution circuits for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention; and 
     FIG. 6 is a comparative plot of voltage-luminance characteristics for common incandescent bulbs and for different numbers of series-connected LEDs. 
    
    
     DETAILED DESCRIPTION 
     The following description details the structure, application and features of the present invention, but it will be understood by those of skill in the art that the scope of the invention is defined only by the issued claims, and not by any description herein. The process steps and structures described below do not form a complete process for manufacturing illuminated instrumentation indicators. The present invention my be practiced in conjunction with common illuminated instrumentation indicator fabrication techniques, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention. 
     With reference now to the figures, and in particular with reference to FIG. 1, an enhanced trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention is depicted. Voltage-controlled dimming circuit  102  includes input and output ports  104   a  and  104   b , across which the input voltage is applied. The input voltage applied across input and output ports  104   a  and  104   b  is variable and may be altered within a predefined operating range to change the luminance of light emitting diodes within circuit  102 . 
     Circuit  102  also includes first and second groups  106   a  and  106   b  of series connected light emitting diodes (LEDs). In the exemplary embodiment, LED groups  106   a  and  106   b  each include two series-connected LEDs; however, the present invention may be suitably employed with any positive, nonzero, integer number of LEDs within each LED group  106   a - 106   b , and the number of LEDs within one group  106   a  need not equal the number of LEDs within the other group  106   b . In the exemplary embodiment of FIG. 1, LED group  106   a  includes LEDs L 1  and L 2  while LED group  106   b  includes LEDs L 3  and L 4 . 
     Circuit  102  also includes a switching circuit  108  connected between and around LED groups  106   a - 106   b . Switching circuit  108  switches LEDs groups  106   a - 106   b  from series-connection between input and output ports  104   a - 104   b  to parallel-connection between ports  104   a - 104   b , or vice versa, as the applied input voltage across ports  104   a - 104   b  is varied across a threshold or “kickover” value. In the exemplary embodiment, switching circuit  108  includes a switching diode D 1  connected in series between LED groups  106   a  and  106   b , a first resistor R 3  connected in parallel with both LED group  106   a  and switching diode D 1 , and a second resistor R 4  connected in parallel with both LED group  106   b  and switching diode D 1 . The cathode of switching diode D 1  is connected to the anode of the last LED L 2  within LED group  106   a  and to one end of resistor R 4 ; the anode of switching diode D 1  is connected to the cathode of the first LED L 3  within LED group  106   b  and to one end of resistor R 3 . An opposite end of resistor R 3  is connected to the cathode of the first LED L 1  within LED group  106   a , and an opposite end of resistor R 4  is connected to the anode of the last LED L 4  within LED group  106   b.    
     LED groups  106   a  and  106   b  (comprising series-connected LED pairs L 1 /L 2  and L 3 /L 4 ) are connected by switching circuit  108  either in series or in parallel between input and output ports  104   a  and  104   b  depending on the voltage applied across the input and output ports  104   a  and  104   b . Switching circuit  108  provides kickover from parallel-connection to series-connection, and vice versa, of the LED groups  106   a  and  106   b . Switching diode D 1 , resistor R 3  (connected in parallel with LED group  106   a  and switching diode D 1 ), and resistor R 4  (connected in parallel with LED group  106   b  and switching diode. D 1 ) enable the switching mechanism. 
     In operation, the circuit  102  operates in two modes: high luminance mode above the kickover point, where the applied input voltage across ports  104   a  and  104   b  is greater than the combined forward voltage drops (turn-on voltages) of LEDs L 1 -L 4  an switching diode D 1 ; and low luminance mode below the kickover point, where the applied input voltage across ports  104   a  and  104   b  is less than the combined forward voltage drops of LEDs L 1 -L 4  and switching diode D 1  (but greater than the combined forward voltage drops of either of LED pairs L 1 /L 2  or L 3 /L 4 ). 
     In high luminance mode, switching diode D 1  conducts, and most of the current passes through the series-connected path of LED pair L 1 /L 2 , switching diode D 1 , and LED pair L 3 /L 4 . The primary current path for LED high luminance control is established by the high luminance resistor R 2 . 
     In low luminance mode, switching diode D 1  stops conducting and the current passes through two parallel paths: LED pair L 1 /L 2  and resistor R 4 , and resistor R 3  and LED pair L 3 /L 4 . Low luminance mode is therefore invoked by applying an input voltage which is insufficient to allow forward current to flow through switching diode D 1 . The primary current path for LED low luminance control is established by the low luminance resistors R 3 -R 4 . 
     Resistor R 1  provides a quiescent current path to prevent false or unintentional LED illumination at low current levels, and is located to allow the rise in current across the resistor with applied voltage to halt at the combined forward voltage drops of LEDs L 1 -L 4  and switching diode D 1 , reducing unnecessary power loss at higher voltage inputs. 
     Zener diodes Z 1  and Z 2 , in conjunction with high luminance resistor R 2 , provide circuit protection against transients, conducted electromagnetic susceptibility, or an electrostatic discharge event. Zener diodes Z 1  and Z 2  also prevent circuit failure should a single LED fail in an electrically open state, providing an alternate current path to maintain quadrant circuit integrity so that two LEDs will always remain illuminated under such a catastrophic failure condition. 
     Resistor R 2  serves to limit the current of a transient or overvoltage event and also serves to limit the operating current to safe levels to prevent a potential catastrophic failure of the display circuitry. 
     Exemplary values for the components depicted in FIG. 1 are: resistor R 1 =4.32 kΩ; resistor R 2 =1.5 kΩ; resistors R 3  and R 4 =20 kΩ; LEDs L 1 -L 4  forward voltage drop=2.5-3.3 V; zener didoes Z 1  and Z 2  rated for about 6.6 V; and applied input voltage across ports  104   a - 104   b  anywhere in an operating range of 6-28 V. These component values provide a kickover point between high luminance mode and low luminance mode at about 16 V. 
     Circuit  102 , which drives, dims, and protects the LEDs, is intended to improve low luminance level trimming resolution by configuring the circuit electrically in parallel, with two LEDs connected in series within each parallel circuit branch, for lower input voltages while allowing maximum power efficiency at high input voltages by electrically configuring the circuit so that all four LEDs are connected in series. The function of this dual mode, constant resistance LED driver circuit is to provide smooth, stable, voltage-controlled dimming from unlight conditions to total darkness by taking advantage of the inherent voltage, current and luminance characteristics of the LEDs. 
     Referring to FIG. 2, a plot of voltage-luminance characteristics of the enhanced trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention is illustrated. The plot presented in FIG. 2 is the voltage-luminance characteristic of the circuit depicted in FIG. 1 with the exemplary component values listed above. 
     The voltage-luminance characteristic  202  for circuit  102  across the operating range of input voltages has two portions: a first portion  202   a  follows the voltage-luminance characteristic for four series-connected LEDs in an upper portion of the voltage-luminance curve  202 , when circuit  102  is in high luminance mode above the kickover point  202   b ; a second portion  202   c  follows the voltage-luminance characteristic for two series-connected LEDs in a lower portion of the voltage-luminance curve  202 , when circuit  102  is in low luminance mode below the kickover point  202   b . The remaining portions  204  and  206  of the voltage-luminance characteristics for four series-connected LEDs and for two series-connected LEDs, respectively, are also shown in FIG.  2 . It should be noted that the voltage-luminance curve  202  for circuit  102  does not precisely follow either the voltage-luminance characteristic for four series-connected LEDs or the voltage-luminance characteristic for two series-connected LEDs in a region proximate to the kickover point  202   b  because of the voltage divider effect formed by high luminance resistor R 2  and quiescent current resistor R 1 . 
     Also illustrated in FIG. 2 for comparative reference are plots of the voltage-luminance characteristics  208   a  and  208   b  of two common incandescent bulbs, as well as reference lines for industry standard luminance ranges: the sunlight-readable rang  210   a  (at least 300-500 foot-lamberts) ; the commercial night-flying range  210   b  (about 15-20 foot-lamberts); the military night-flying range  210   c  (approximately 1 foot lambert); and the NVIS compatible range  210   d  (approximately 0.1 foot lambert). 
     As illustrated in FIG. 2, circuit  102  provides enhanced trim resolution for voltage-controlled dimming of LED illumination sources, with greater variability in luminance as a function of applied input voltage across the operating range of applied input voltages and variability in luminance across the entire operating range of applied input voltages. Industry standard luminance ranges are achieved by circuit  102 , and a corresponding voltage expectations, allowing circuit  102  to be employed with existing analog dimming controls. 
     Also illustrated in FIG. 2, circuit  102  does not attempt to match incandescent luminance for a particular applied input voltage or across the operating range of applied input voltages. Instead, circuit  102  follows classic LED voltage-luminance curves, switching from the voltage-luminance curve for four series-connected LEDs to the voltage-luminance curve for two series-connected LEDs at about the kickover point. 
     Within some tolerances, voltage-luminance characteristic  202  may be shaped by selection of appropriate resistance values for resistors R 1  through R 4  within circuit  102 . Resistor R 2  may be adjusted to control the highest luminance produced by circuit  102 . Resistors R 3  and R 4  control the highest luminance which might be produced by the two parallel connected LED groups  306   a  and  306   b , and therefore control the shape of the lower portion  202   c  of voltage-luminance characteristic  202 . Shunt resistor R 1  may be adjusted (relative to resistor R 2 ) to control the location of kickover point  202   b  within the operating range of applied input voltages and the quiescent current at low voltage levels. In this manner, the shape of voltage-luminance characteristic  202  and the quiescent current may be adjusted to suit particular implementations for circuit  102 . 
     With reference now to FIG. 3, an alternating current embodiment of the enhanced trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention is depicted. The example depicted is intended for operation with alternating current of 5-7 volts, and would employ components having values differing from those described above in connection with FIG.  1 . 
     Circuit  302  includes first and second LED groups  306   a  and  306   b  oriented in a one direction (with respect to anodes and cathodes of the LEDs) between input and output ports  304   a  and  304   b , as well as third and fourth LED groups  306   c  and  306   d  oriented in an opposite direction from LED groups  306   a  and  306   b  between input and output ports  304   a  and  304   b . Each LED group  306   a - 306   d  in this example includes only one LED. 
     Switching circuit  308  in circuit  302  includes two switching diodes D 1  and D 2  and resistors R 3  through R 6 . Switching diode D 1  is located between LED groups  306   a  and  306   b , oriented in the same direction as the LEDs (L 1  and L 2 , respectively) within those groups, while switching diode D 2  is connected between LED groups  306   c  and  306   d , oriented in the same direction as the LEDs (L 3  and L 4 , respectively) within those groups. Resistor R 3  is connected in parallel with LED group  306   a  and switching diode D 1 ; resistor R 4  is connected in parallel with LED group  306   b  and switching diode D 1 . Resistor R 5  is connected in parallel with LED group  306   d  and switching diode D 2 ; resistor R 6  is connected in parallel with LED group  306   c  and switching diode D 2 . 
     For positive cycles of the applied input voltage (when the voltage at input port  304   a  is positive with respect to the voltage at input port  304   b ), current flows through LED groups  306   a  and  306   b , switching diode D 1  (if conducting), and resistors R 3  and R 4  (if switching diode D 1  is not conducting). During negative cycles of the applied input voltage, current flows through LED groups  306   c  and  306   d , switching diode D 2  (if conducting) and resistors R 5  and R 6  (if switching diode D 2  is not conducting). In this manner, illumination is achieved during both positive and negative cycles of the applied input power. 
     Referring to FIG. 4, an alternative enhanced trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention is illustrated. Circuit  402  includes input and output ports  404   a - 404   b , three light emitting diode groups  406   a - 406   c , and a switching circuit  408 . Each light emitting diode group  406   a ,  406   b , and  406   c  contains only one LED L 1 , L 2  and L 2  in the example shown, although each group could contain more LEDs. 
     Switching circuit  408  includes: diode D 1  connected in series between LED groups  406   a  and  406   b ; diode D 2  connected in series between LED groups  406   b  and  406   c ; resistor R 3  connected in parallel with LED group  406   a  and diode D 1 ; resistor R 4  connected in parallel with LED group  406   c  and diode D 2 ; resistor R 5  connected in parallel with LED group  406   a , diode D 1 , LED group  406   b , and diode D 2 ; and resistor R 6  connected in parallel with diode D 1 , LED group  406   b , diode D 2 , and LED group  406   c    
     In operations, diodes D 1  and D 2  conduct when the voltage applied across input and output ports  404   a - 404   b  exceeds a threshold voltage, connecting LED groups  406   a  through  406   c  in series. Below the threshold voltage, diodes D 1  and D 2  stop conducting (preferably at the same time), leaving three parallel current paths between the input and output ports  404   a - 404   b : LED group  406   a  and resistor R 6 ; resistor R 3 , LED group  406   b , and resistor R 4 ; and resistor R 5  and LED group  406   c.    
     To achieve concurrent series- to parallel-connection switching, the resistances of resistors R 5  and R 6  should be the same, the resistances of resistors R 3  and R 4  should be the same, and he combined resistance of resistors R 3  and R 4  should equal the resistance of resistor R 5  (or R 6 ). Staggered series- to parallel-connection switching (e.g., LED group  406   a  switches from series-connection to parallel-connection with LED groups  406   b  and  406   c  at a first threshold, followed by LED group  406   b  switching from series-connection to parallel-connection with LED group  406   c  as a second threshold), although possible, will be accompanied by variance in LED luminance between different LED groups  406   a - 406   c.    
     With reference now to FIG. 5, another alternative enhanced trim resolution circuit for voltage-controlled dimming of light emitting diodes in accordance with a preferred embodiment of the present invention is depicted. Circuit  502  switches four LED groups  506   a - 506   d  from series connection between input/output ports  504   a - 504   b  to parallel connection utilizing switching circuit  508 . For concurrent parallel-to-series (or vice versa) connection switching, resistors R 3  and R 4  should have the same resistance, resistors R 5  and R 6  should have the same resistance, and resistors R 7  and R 8  should have the same resistance. Additionally, the combined resistance of resistors R 3  and R 6  (and therefore the combined resistance of resistors R 5  and R 4 ) should equal the resistance of resistor R 7  (or R 8 ). 
     Above the kickover point, diodes D 1  through D 3  within switching circuit  508  will all conduct, creating a series connection through LED groups  506   a - 506   d . Below the kickover point, none of diodes D 1  through D 3  will conduct, leaving four parallel current paths each including one of LED groups  506   a - 506   d  together with one or more resistors. 
     Although not depicted in either FIG. 4 or FIG. 5, quiescent current (shunt) resistors and overvoltage protection (e.g., zener diodes in parallel with LED groups) may be optionally added to any implementation of the present invention. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.