WO2013052605A1

WO2013052605A1 - Led dimming circuitry

Info

Publication number: WO2013052605A1
Application number: PCT/US2012/058658
Authority: WO
Inventors: Ronald J. Lenk; Carol Lenk
Original assignee: Reliabulb, Llc
Priority date: 2011-10-04
Filing date: 2012-10-04
Publication date: 2013-04-11

Abstract

Circuitry for dimming the light emitted from light emitting diodes (LEDs).

Description

LED DIMMING CIRCUITRY

The present invention relates to circuitry for dimming the light emitted from light emitting diodes (LEDs).

LEDs are highly energy-efficient light sources, which may make LEDs a suitable replacement for other light sources, for example, incandescent lighting. It would be desirable for LED lighting to reproduce incandescent lighting in every way, including when operated on a dimmer. However, incandescent lighting has very specific and unusual characteristics when operated on a dimmer, and LED circuits to date have not been able to reproduce these characteristics without large, complex and expensive circuitry.

A typical dimmer uses a triac to turn off the AC line during specific portions of each line cycle. The period of off-time is controlled by the dimmer control, typically a potentiometer. Since the AC line is applied to the incandescent filament, as the size of the off-time increases, the power applied to the filament decreases.

As the power applied to the filament decreases, several changes occur in the light output of the incandescent bulb. For one thing, the light output decreases. In a simplistic model in which the incandescent filament is a fixed resistor, it is clear that light output from the filament must decrease as the square of the RMS voltage, since power = voltage^A2 / resistance. However, the filament is not a constant-valued resistor. In fact, as less power is applied to it, its resistance increases. Thus, as the RMS voltage to the filament is decreased, the power dissipated, and thus the light output, decreases faster than quadratically. In actual incandescent filaments used for lighting, this decrease is somewhere between cubic and quartic in the voltage.

A second change in the light output of an incandescent bulb as power to it is decreased is that it reddens, that is, its coordinated color temperature (CCT) decreases. This corresponds to the primitive concepts of 'white hot' vs. 'red hot'. In actual incandescent filaments used for lighting, the decrease in CCT with power is

approximately linear.

A typical requirement for LED lighting is that it be power factor corrected (PFC).

In practice, this requires that the current drawn by the lighting be proportional to the line voltage. When the line voltage is sinusoidal, so must be the input current; when the line voltage is cut off for a certain portion of the line cycle, the input current must also be cut off. Thus, LED lighting with PFC naturally dims as line voltage is cut by a triac dimmer.

However, this dimming is approximately inversely proportional to line voltage, not quadratic and certainly not cubic or quartic. Thus, the dimming of an LED light does not well- approximate the brightness characteristics of an incandescent light.

Typical LED lighting is built with white LEDs. Although some systems use mixtures of different colors, this adds to the cost and complexity of the lighting, and is used only for special purposes. As white LEDs dim their CCT does not appreciably change. Some LED lighting attempts to lower CCT during dimming by adding in addition to the white LEDs additional long-wavelength LEDs, such as amber or red. These may either be operated at the same current as the other LEDs, or at a constant current. However, since the LED brightness as a function of dimming is incorrect for these bulbs, the CCT vs. dimming is also incorrect.

Thus, for LED bulbs using white LEDs, it would be desirable to be able to control the brightness and CCT of the bulbs as a function of dimming so as to match that of an incandescent bulb.

The present disclosure is directed to LED lighting that uses simple, small, inexpensive circuitry to control both the brightness and the CCT of the lighting during dimming. The circuitry includes a sub-circuit which finds either the time-averaged input voltage, or the average percentage of time the input voltage is present, where the time average is taken over multiple line cycles. In one embodiment, the time-averaged input voltage is determined by integrating the rectified line voltage. In another embodiment, the average percentage of time the input voltage is present is determined by integrating a voltage representative of the time during which the input voltage is present.

In one embodiment, this time-average is used to control a non-linear resistance, which is in parallel with the resistance that sets the LED current. Since the resistance of the non-linear resistance is approximately constant over a single line cycle, the PFC continues to work. Over multiple cycles, however, the resistance changes non-linearly. The non-linearity of the resistance is selected to approximate the appropriate non- linearity of the dimming of an incandescent light. In another embodiment, one or more long-wavelength LEDs are added in parallel with the white LEDs. As the time-average voltage changes, the same sub-circuit may control a linear or non-linear resistance which sets the current, and thus the brightness, of the long-wavelength LEDs. The resistance is selected such that the combination of the white LEDs and the long-wavelength LEDs together approximates the CCT of the dimming of an incandescent light.

In a preferred embodiment, the current through one or more long- wavelength LEDs is controlled by a fixed resistor in conjunction with a switch. When the average voltage is normal, the switch is off so that the long-wavelength LEDs are off. This saves power in undimmed operation. When the average voltage is decreased, the switch and the long-wavelength LEDs turn on, the current through the long-wavelength LEDs together with the dimming white LEDs approximating the CCT of the dimming of an incandescent light.

The accompanying drawings are included to provide a further understanding of the present disclosure, and is incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments of the present disclosure and, together with the detailed description, serve to explain the principles of the present disclosure.

FIG. 1 is a drawing of the brightness and CCT of an incandescent bulb as a function of dimming level.

FIG. 2 is a schematic of a circuit that measures the average rectified line voltage, and uses it to control the brightness of an LED light according to one or more

embodiments shown or described herein.

FIG. 3 is a schematic of a circuit that measures the percentage of time that the line voltage is present, and uses it to control the brightness of an LED light according to one or more embodiments shown or described herein.

FIG. 4 is a schematic of a circuit that measures the percentage of time the line voltage is present, and uses it to control the CCT of an LED light according to one or more embodiments shown or described herein.

FIG. 5 is a schematic of a circuit that uses the percentage of time the line voltage is present to control the brightness of an LED light, while using a fixed resistance and a switch to control the CCT according to one or more embodiments shown or described herein.

Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

According to the design characteristics, a detailed description of the preferred embodiment is given below.

FIG. 1 is a drawing of the brightness 100 and CCT 110 of an incandescent bulb as a function of dimming level 120. As shown in FIG. 1, the brightness 100, measured as a percentage of the nominal brightness, decreases strongly non-linearly as a function of dimming level 120, measured as a percentage of the nominal voltage. The CCT 110, measured in degrees Kelvin, decreases approximately linearly as a function of dimming level 120.

FIG. 2 is a schematic of a circuit 210 that measures the average rectified line voltage 220, and uses it to control the brightness of an LED light 251. As shown in FIG. 2, the line voltage 230 is rectified by a diode bridge 231. The rectified line voltage 232 is scaled by a resistive voltage divider 240, and is used to control the current 250 drawn by the LED light 251. As the rectified line voltage 232 rises and falls, so does the current 250 drawn by the LED light 251 , and thus the LED light is PFC.

The rectified line voltage 232 is also scaled and averaged by the resistive voltage divider 260 and the capacitor 261. The time constant of the resistive voltage divider 260 and the capacitor 261 is set to be a number of line cycles. The average rectified line voltage 220 is used to control a non-linear resistor 270, shown as a JFET. The values of the minimum resistance of the JFET 270, and of the resistance of the resistive voltage divider 240, are selected such that the current 250 drawn by the LED light 251 generates a brightness that approximates that of an incandescent light as a function of average rectified line voltage 220.

FIG. 3 is a schematic of a circuit 310 that measures the percentage of time that the line voltage 230 is present, and uses it to control the brightness of an LED light 251. As shown in FIG. 3, the line voltage 230 is rectified by a diode bridge 231. The rectified line voltage 232 powers a control IC 320. The control IC 320 produces a regulated voltage 330. During the time the rectified line voltage 232 is present, the control IC 320 is powered and the regulated voltage 330 is present. During the time the rectified line voltage 232 is not present, the control IC 320 is not powered, and the regulated voltage 330 is absent.

The regulated voltage 330 is integrated by a resistor-capacitor network 340. The time constant of the resistive-capacitor network 340 is set to be a number of line cycles. The average voltage 350 on the capacitor 341 is used to control a non-linear resistor 270, shown as a JFET. The values of the minimum resistance of the JFET 270, and of the resistance of the resistive voltage divider 240, are selected such that the current 250 drawn by the LED light 251 generates a brightness that approximates that of an incandescent light as a function of average rectified line voltage 220.

FIG. 4 is a schematic of a circuit 410 that measures the percentage of time the line voltage is present, and uses it to control the CCT of an LED light 411. The LED light 411 consists of both white LEDs 450 and long- wavelength LEDs 430. As shown in FIG. 4, the rectified line voltage 232 powers a control IC 320. The control IC 320 produces a regulated voltage 330. The regulated voltage 330 is integrated by a resistor- capacitor network 340. The average voltage 350 on the capacitor 341 is used to control a non-linear resistor, shown as a JFET 270. One or more long-wavelength LEDs 430 are in series with the JFET 270. As the average voltage 350 decreases, the JFET 270 decreases in resistance, increasing the current 340 and thus the brightness of the long-wavelength LEDs 430. The value of the resistance of the JFET 270 is selected such that the current 340 drawn by the long- wavelength LEDs 430 generates long-wavelength light which, in combination with the other LEDs 450 of the LED light 411, generates a CCT that approximates that of an incandescent light as a function of percentage of time the line voltage is present. The long-wavelength LEDs 430 may be in one or more strings, and each string may have one or more wavelength LEDs.

FIG. 5 is a schematic of a circuit 510 that uses the percentage of time the line voltage is present to control the brightness of an LED light 411, while using a fixed resistance 520 and a switch 530 to control the CCT. As shown in FIG. 5, the rectified line voltage 232 powers a control IC 320. The control IC 320 produces a regulated voltage 330. The regulated voltage 330 is integrated by a resistor-capacitor network 340. The average voltage 350 on the capacitor 341 is used to control a non-linear resistor 270, shown as a JFET.

The LED light 411 consists of both white LEDs 450 and long-wavelength LEDs 430. The long-wavelength LEDs 430 are in series with a fixed resistance 520 which sets the current through the long-wavelength LEDs 430. The fixed resistance 520 is in series with a switch 530. The switch 530 is controlled by the average voltage 350 to be off when the line voltage is not being dimmed and to be on when the line voltage is dimmed. When the switch 530 is off, the long-wavelength LEDs 430 are also off. When the switch 530 is on, the series combination of the LEDs 430 and the fixed resistance 520 is in parallel with some or all of the white LEDs 450. Since the voltage across the white LEDs 450 is approximately independent of current, the current through the long-wavelength LEDs 430 will be approximately constant independent of the current through the white LEDs 450. Thus, as the white LEDs 450 dim as a function of the percentage of time the line voltage is present, the long- wavelength LEDs 430 maintain an approximately constant current, with the result that the CCT of the LED light 411 decreases

approximately linearly as a function of the percentage of time the line voltage is present. The fixed resistance 520 may be selected such that the CCT of the LED light 411 approximates that of an incandescent light as a function of the percentage of time the line voltage is present. The long-wavelength LEDs 430 may be in one or more strings, and each string may have one or more wavelength LEDs.

It will be apparent to those skilled in the art that various modifications and variation can be made to the structure of the present disclosure without departing from the scope or spirit of the embodiments disclosed herein. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of the

embodiments provided they fall within the scope of the following claims and their equivalents.

Claims

1. An LED light, comprising:

at least one LED;

a power circuit for controlling current to said as least one LED;

an averaging circuit that determines the average input line voltage;

a controllable resistance; and

wherein said averaging circuit controls said resistance to control the current generated by said power circuit to said at least one LED as a function of average input line voltage.

2. An LED light as set forth in Claim 1, wherein said power circuit uses a measurement of line voltage to control current drawn from the line.

3. An LED light as set forth in Claim 1, wherein said averaging circuit measures said input line voltage to determine its average.

4. An LED light as set forth in Claim 1, wherein said averaging circuit measures a signal which is present when said input line voltage is present and is not present when said input line voltage is not present.

5. An LED light as set forth in Claim 1, wherein said averaging circuit consists of a resistor divider and a capacitor.

6. An LED light as set forth in Claim 1, wherein said controllable resistance is a JFET.

7. An LED light as set forth in Claim 1, wherein said averaging circuit controls said controllable resistance to control the current generated by said power circuit to said at least one LED to approximate the dimming performance of an incandescent light as a function of dimming level.

8. An LED light, comprising:

at least one broad- spectrum LED;

at least one long- wavelength LED;

an averaging circuit that measures the average input line voltage;

at least one resistance; and

wherein said averaging circuit controls said at least one resistance to control the current to said at least one long- wavelength LED as a function of average input line voltage.

9. An LED light as set forth in Claim 8, wherein said averaging circuit measures said input line voltage to determine its average.

10. An LED light as set forth in Claim 8, wherein said averaging circuit measures a signal which is present when said input line voltage is present and is not present when said input line voltage is not present.

11. An LED light as set forth in Claim 8, wherein said averaging circuit consists of a resistor divider and a capacitor.

12. An LED light as set forth in Claim 8, wherein said at least one resistance is a JFET.

13. An LED light as set forth in Claim 8, wherein said at least one resistance is a constant resistance.

14. An LED light as set forth in Claim 13, wherein said at least one resistance is in series with a switch, and wherein said switch is controlled by said averaged signal.

15. An LED light as set forth in Claim 8, wherein said averaging circuit controls said at least one resistance to control the current to said at least one long- wavelength LED to approximate the CCT of an incandescent light as a function of input line voltage.

16. An LED light, comprising:

at least one broad- spectrum LED;

at least one long- wavelength LED;

a power circuit for controlling current to said as least one broad-spectrum LED; at least one averaging circuit that measures the average input line voltage;

a first controllable resistance;

at least one second resistance;

wherein said at least one averaging circuit controls said first controllable resistance to control the current generated by said power circuit to said at least one broad- spectrum LED as a function of average input line voltage; and

wherein said at least one averaging circuit controls said at least one second resistance to control the current to said at least one long- wavelength LED as a function of average input line voltage.

17. An LED light as set forth in Claim 16, wherein said at least one second resistance is at least one constant resistance.

18. An LED light as set forth in Claim 17, wherein said at least one second resistance is in series with a switch, and wherein said switch is controlled by said at least one averaged signal.

19. An LED light as set forth in Claim 16, wherein said at least one averaging circuit controls said controllable first resistance to control the current generated by said power circuit to said at least one broad-spectrum LED to approximate the dimming performance of an incandescent light as a function of dimming level.

20. An LED light as set forth in Claim 18, wherein said at least one constant resistance is selected to control the current to said at least one long- wavelength LED in coordination with the current to said at least one broad- spectrum LED to approximate the CCT of an incandescent light as a function of dimming level.