WO2013074913A2

WO2013074913A2 - Led anti-flicker circuitry

Info

Publication number: WO2013074913A2
Application number: PCT/US2012/065498
Authority: WO
Inventors: Ronald J. Lenk; Carol Lenk
Original assignee: Reliabulb, Llc
Priority date: 2011-11-16
Filing date: 2012-11-16
Publication date: 2013-05-23
Also published as: WO2013074913A3

Abstract

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

Description

LED ANTI-FLICKER CIRCUITRY

TECHNICAL FIELD

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

BACKGROUND ART

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 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.

Triacs generically require some holding current to remain on. That is, they will continue conducting, and thus keep the AC line applied to the incandescent filament, only so long as they have adequate current flowing through them. Indeed, this is how they are supposed to work, since at AC line crossing the current necessarily goes to zero, and the triac turns off until retriggered.

Triacs, however, are not ideal devices. Even in their off state, there is some leakage current passing through them. This leakage current can interact with the LED driver to cause flicker. In particular, a typical LED driver integrated circuit (IC) will only work when the voltage applied to its power pin exceeds some threshold. When the line voltage is below this threshold, the IC turns off, and no power is provided to the LEDs. However, once the IC is off, the triac leakage current will charge up the IC's power pin capacitor. At some point, the capacitor will have enough voltage that the IC will turn on again, momentarily running the LEDs. Then, as the power in the capacitor is exhausted, the IC again turns off. This on-again off-again behavior is observed visually as flicker. A straightforward solution to this would be to include a large capacitor for the voltage, which would provide continuous power to the circuit during the time when the line voltage is below the IC threshold. However, LED lights are typically required to be power factor corrected, and this requirement is incompatible with having large capacitors on the AC line.

For LED bulbs operating on dimmers, it would be desirable to be able to prevent the triac's leakage current from causing flicker.

SUMMARY OF INVENTION

The present disclosure is directed to LED lighting that uses simple, small and inexpensive circuitry to prevent flicker during dimming. The circuitry consists of two pieces. For one piece, the circuitry includes a small current sink in parallel with the LEDs. In normal operation, the current shunted by this sink is a few milliamps and is not large enough to have a significant effect on the LED operation, nor does it dissipate a significant amount of power. When the line voltage is low and the IC is off, however, it provides a path for the triac leakage current to bypass the LEDs. In one embodiment, the current sink may consist of a current regulating diode. In a preferred embodiment, the current sink may consist of discrete resistors and transistors, relying on the base-emitter junction voltage of one of the transistors divided by the resistance of one of the resistors to set the current level.

The second piece of the circuitry ensures that the switching MOSFET remains on when the IC is off. In one embodiment, this second piece of circuitry is a high-value resistor on the gate of the switching MOSFET to pull up the gate to the line voltage. This causes the MOSFET to be on even when the IC is off, providing a path to ground for the triac leakage current. Triac leakage current passes through the current sink and then through the MOSFET to ground. In this state, the LEDs receive no current. When the line voltage returns to being high enough to supply both the current sink and the IC with power, the IC re-starts, and the LEDs are re-lit.

In another embodiment of the second piece of the circuitry, the IC is powered by a diode rectifier and holdup capacitor from the AC line. When the AC line is low, the holdup capacitor retains enough energy to power the IC. The IC in turn holds the MOSFET on since it detects that there is insufficient current in the LEDs. This again provides a path to ground for the triac leakage current.

BRIEF DESCRIPTION OF DRAWINGS

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 schematic of an LED driver that shows why the LEDs may flicker when operated on a dimmer according to one or more embodiments shown or described herein.

FIG. 2 is a schematic of a circuit that prevents LEDs from flickering when operated on a dimmer according to one or more embodiments shown or described herein.

FIG. 3 is a schematic of a circuit that prevents LEDs from flickering when operated on a dimmer according to one or more embodiments shown or described herein.

DESCRIPTION OF EMBODIMENTS

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 schematic of an LED driver 100 that shows why the LEDs 110 may flicker when operated on a dimmer 120. As shown in FIG. 1, the dimmer 120 produces a chopped version 130 of the AC line 131. This chopped version 130 is rectified by a diode bridge 140, producing a rectified chopped AC line 132. The rectified chopped AC line 132 powers an IC 150, whose power pin 151 is bypassed by a capacitor 160. During the portion of time of the rectified chopped AC line 132 when voltage is not present, the IC 150 turns off. There is a small leakage current from the dimmer 120 during this time, which gradually recharges the capacitor 160. When the voltage on the capacitor 160 reaches the operating threshold voltage of the IC 150, the IC 150 turns on. It quickly uses up the energy stored in capacitor 160, and IC 150 turns off again. This on-off cycle may repeat itself multiple times during the portion of time of the rectified chopped AC line 132 when voltage is not present. This on-off cycle may give rise to optical flickering of the LEDs 110.

FIG. 2 is a schematic of a circuit 210 that prevents LEDs 110 from flickering when operated on a dimmer 120. As shown in FIG. 2, the circuit 210 consists of two sub- circuits 220 and 230. Sub-circuit 220 is a low-dropout voltage current sink, in parallel with the LEDs 110. Sub-circuit 220 consists of two bipolar junction transistors (BJTs) 221 and 222 and two resistors 223 and 224. BJT 221 is the device that passes the current. It is powered by resistor 223, which provides base current. The amount of current passed by BJT 221 is set by resistor 224. When the current through resistor 224 is sufficient to turn on the base of BJT 222, BJT 222 begins taking current away from the base of BJT 221, thus providing negative feedback. The sub-circuit 220 thus pulls a current which is approximately set by the base-emitter voltage of BJT 222 divided by the resistance of resistor 224. In practice, the total voltage drop across sub-circuit 220 may be one volt or less.

Sub-circuit 230 consists of a pull-up resistor 231 on the gate of the switching power MOSFET 240. During the portion of time of the rectified chopped AC line when voltage is not present, the leakage current from the dimmer 120 is sufficient to hold the gate of the MOSFET 240 on at approximately its threshold voltage. The leakage current from the dimmer 120 thus has a path to ground, through the sub-circuit 220, the inductor, and the MOSFET 240. The circuit 210 thus will take all the leakage current from the dimmer 120, bypassing the LEDs 110 and shunting it through current sink 220, the inductor, and MOSFET 240 to ground. The LEDs 110 thus receive no current during this time and remain off.

FIG. 3 is a schematic of a circuit 310 that prevents LEDs 110 from flickering when operated on a dimmer 120. As shown in FIG. 3, the circuit 310 consists of two sub- circuits 220 and 330. Sub-circuit 220 is a low-dropout voltage current sink, shown here as a discrete current diode, in parallel with the LEDs 110. Sub-circuit 330 consists of a diode 331 and a capacitor 332. During the portion of time of the rectified chopped AC line when voltage is present, diode 331 charges up capacitor 332. During the portion of time of the rectified chopped AC line when voltage is not present, diode 331 prevents capacitor 332 from discharging back to the rectified chopped AC line. Capacitor 332 is selected to be of a sufficiently large value that it can supply current to the IC 150 during the entire portion of time of the rectified chopped AC line when voltage is not present. Since IC 150 is on during this entire portion of time, MOSFET 240 is also held on, since IC 150 senses that there is insufficient current in the LEDs 110. Thus, the circuit 310 will take all the leakage current from the dimmer 120, bypassing the LEDs 110 and shunting it through current sink 220, the inductor, and MOSFET 240 to ground. The LEDs 110 thus receive no current during this time and remain off.

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 driver circuit, comprising:

at least one LED;

a current shunt in parallel with said at least one LED;

a MOSFET;

a control circuit to turn said MOSFET on; and

wherein said control circuit turns on said MOSFET during times when line voltage is not present, and said current shunt passes leakage current to said MOSFET.

2. An LED driver circuit as set forth in Claim 1, wherein said current shunt has a voltage drop less than that of said at least one LED.

3. An LED driver circuit as set forth in Claim 1, wherein said MOSFET is the main power switching device for said LED driver circuit.

4. An LED driver circuit as set forth in Claim 1, wherein said control circuit is a pull-up resistor to input power on the gate of said MOSFET.

5. An LED driver circuit as set forth in Claim 1, wherein said control circuit includes a rectifier and a hold-up capacitor to provide power to hold said MOSFET on when said line voltage is not present.