Flickering suppressor system for a dimmable LED light bulb

A flicker suppression system for a dimmable LED bulb. In one embodiment, the system includes a rectifier circuit having input terminals and output terminals. The rectifier circuit is configured to rectify a line voltage to generate a rectified voltage at its output terminals. A resistor and switch are also included and coupled in series. A switch control circuit is directly coupled between the output terminals and configured to control the switch only as a function of the rectified voltage.

BACKGROUND OF THE INVENTION

Dimmers are devices that allow users to adjust the amount of power delivered to light bulbs in various lighting applications (e.g., home, commercial, etc.). Many types of conventional dimmers are often mounted on a wall and have a user interface such as a knob or a slider, which can be manipulated by a user. Typically, the user interface is mechanically coupled to a variable resistor, and as a user manipulates the user interface the resistance of the variable resistor increases or decreases, which in turn increases or decreases the power delivered to the light bulb.

SUMMARY OF THE INVENTION

A flicker suppression system for a dimmable LED bulb. In one embodiment, the system includes a rectifier circuit having input terminals and output terminals. The rectifier circuit is configured to rectify a line voltage to generate a rectified voltage at its output terminals. A resistor and switch are also included and coupled in series. A switch control circuit is directly coupled between the output terminals and configured to control the switch only as a function of the rectified voltage.

DETAILED DESCRIPTION

Many lighting systems are powered by an alternating current (AC) source, commonly referred to as “line voltage” (e.g., 120 volts RMS at 60 hertz). A conventional AC dimmer typically receives line voltage as an input, and provides an adjusted output voltage for a light bulb.

Conventional dimmers can control power delivered to a light bulb in different ways. Most commonly, adjustments by the user causes the dimmer to adjust the duty cycle of the output (e.g., by chopping out portions of the AC voltage cycles). This technique is sometimes referred to as phase angle control. The most commonly used dimmers of this type employ a triode-for-alternating current (TRIAC), which is an electronic component that can conduct current in either direction when it is triggered (turned on). When employed in a dimmer, the TRIAC chops off rising portions of the AC voltage half cycles (e.g., portions after 0 volt crossings and before peaks) depending on where the user interface (e.g., slider or knob) is set as will be more fully described below.

FIG. 1illustrates a lighting system that employs a TRIAC-based dimmer. The lighting system includes voltage source100that delivers a line voltage V1to TRIAC-based dimmer104, which in turn controls the power delivered to light bulb102. Dimmer104is one example of one of many types of dimmers that can be employed for controlling power to a light bulb, such as light bulb102shown inFIG. 1. It should be noted that the term dimmer should not be limited to dimmer104shown withinFIG. 1.

Dimmer104includes a variable resistor R, the resistance of which can be varied by a user via an interface (not shown) such as a wall-mounted slider. Variable resistor R is coupled in series to resistor112, the combination of which is coupled to capacitor114. A diode-for-alternating current (DIAC)116is coupled between a capacitor114and a gate of TRIAC120. Light bulb102is connected in series with TRIAC120.

A DIAC is a diode that activates or turns-on only after its breakover voltage has been reached. When this occurs, there is a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains active or “on” until the current through it drops below a value characteristic for the device, called the DIAC holding current. Below this value, the diode switches back to its high-resistance (non-conducting) off state. DIACs are mainly used for triggering (turning on) TRIACs. TRIACs can be triggered by either a positive or a negative current applied to its gate electrode. However, a minimum amount of current (latching current) is required to maintain the TRIAC in the on-state immediately after a TRIAC is triggered. Moreover, a minimum current (holding current) is required to maintain the TRIAC in the on-state, not allowing it to turn off.

Typical TRIAC-based dimmers can dim light through phase angle control as mentioned above. Initially, assume TRIAC120is turned off so that no power flows to light bulb102. As line voltage V1increases from zero (at the start of every half wave), capacitor114charges. When the voltage on capacitor114exceeds the breakover voltage of DIAC116, DIAC116activates and conducts current from capacitor114to the gate of TRIAC120and turns it on. The DIAC116is active for a short period of time while discharging capacitor114. Eventually the voltage across DIAC116drops and it deactivates, which terminates the gate current to TRIAC120. If current flowing through TRIAC120exceeds its latching current when the gate current terminates, the TRIAC will remain on and continue to conduct current to light bulb102so long as the TRIAC's holding current is exceeded. The current to light bulb102will eventually fall below the TRIAC's holding current as the line voltage V1drops to zero near the end of the half cycle, at which point TRIAC120will turn off. A similar process repeats for the next half cycle.

FIG. 2illustrates wave forms relevant to the operation of dimmer104. In particular,FIG. 2shows the line voltage V1, and the chopped output voltage202provided to light bulb102as TRIAC120turns on and off as described above.FIG. 2also shows pulses during which DIAC is activated and conducting current to the gate of TRIAC120. TRIAC120turns on with each pulse of DIAC116as described above. A time constant is formed by capacitor114and the series combination of variable resistor R and resistor112. The time at which the DIAC pulses occur depends on the time constant, which can be adjusted by changing the resistance of variable resistor R. One of ordinary skill in the art understands that the input wave form can be clipped at different phase angles than that shown withinFIG. 2by adjusting the resistance of variable resistor R.

Light emitting diode (LED) based light bulbs are becoming more popular due to their long service life and high energy efficiency. LED light bulbs can be made interchangeable with other types of light bulbs such as incandescent light bulbs. Some LED light bulbs are made with identical bases so that they are directly interchangeable with incandescent light bulbs.

LEDs operate based on substantially DC power sources. In other words, LEDs need to be powered in constant current/constant voltage mode. InFIG. 1, light bulb102takes form in an LED light bulb that includes a LED converter circuit106and LEDs110. The LED converter106is configured to receive the chopped AC output of dimmer104and provide constant current and voltage to the LED110. In one embodiment, the LED controller106may include a rectifier, a low pass filter, and a DC converter (not shown). In this embodiment, the output of the DC converter tries to provide a stable DC voltage to LED110sas duty cycle changes for the output voltage provided by dimmer104. Reducing the duty cycle of the voltage may not achieve light dimming since the LED controller106tries to adjust for the missing portion of the input voltage. If dimmer104outputs a voltage having a duty cycle of 50% “on” or if dimmer104outputs a voltage having a duty cycle of 25% “on” the difference in light output by light bulb102may not be perceivable by the human eye. In other words, a user may not notice a perceptible difference in the light output as the user adjusts down the variable resistor R due to the action of the DC converter until under-voltage circuitry (not shown) of controller106kicks in, at which point the LED converter106turns off and no light is generated by LEDs110at all.

To resolve this problem, the LED controller106may include a circuit (e.g., a microcontroller) that reads the phase angle of the chopped input voltage and adapts the DC converter to reduce the power delivered to LEDs110, which results in perceivable dimming. Unfortunately, this means the dimming range for LED light bulb102is dependent upon the dynamic range of LED controller106and not dimmer104. While dimming LED light bulbs, it is not uncommon to reduce the power applied to the LED light bulbs by 99% or more in order to achieve a comparable dimming effect that one would experience when dimming incandescent light bulbs. That means for a 9-watt LED light bulb, when fully dimmed, the power provided by a dimmer (e.g., dimmer104) could be well below 1 watt.

As a user continues to reduce power provided to LED light bulb102via dimmer104, the current flow through TRIAC120will fall accordingly. At some point, the current may fall below the TRIAC's holding current, or there will not be enough current to latch TRIAC120on when current is injected into the gate as DIAC116pulses. When this occurs, TRIAC120may suddenly turn off or not trigger at all, and the power delivered to LED light bulb102is interrupted. For the next few cycles, the input current to TRIAC120might be higher, which allows converter106to restart and power up LEDs110. This condition creates “flickering” or an instability that results in a rapid turn on and turn off of LEDs110.

Flickering can be reduced or eliminated by maintaining the current flow above a minimum when TRIAC120is triggered by a pulse from DIAC116. Current in the TRIAC can be maintained above the minimum if a load such as a resistor is permanently coupled between the outputs of dimmer104and thus in parallel with light bulb102. However, this solution leads to low efficiency due to the power loses in the permanently coupled resistor, particularly when full power is being delivered to light bulb102.

Alternatively, a load such as a resistor can be selectively coupled in parallel with light bulb102when needed.FIGS. 3-5illustrate alternative solutions in which a flicker suppressor circuit selectively couples a load such as a resistor in parallel with light bulb102. The selectively coupled resistor can operate to maintain a minimum TRIAC current when needed, which should reduce or eliminate flickering.

FIGS. 3-5illustrate lighting systems each of which contain a flicker suppressor circuit (FSC). In one embodiment, the FSC circuits in each of the Figures are identical to each other. In an alternative embodiment the FSC circuits may be different from each other. InFIG. 3, the FSC is contained within dimmer302, and inFIG. 4, the FSC is between dimmer104and light bulb102. It is noted that dimmer302shown inFIG. 3is nearly identical to dimmer104shown inFIG. 1, except for the addition of FSC304. InFIG. 5, LED converter circuit502may be similar to the LED converter106circuit shown inFIG. 3or4, except for the addition of FSC304. In one embodiment, the FSC304in LED converter circuit502may be identical to FSCs304shown withinFIGS. 3 and 4. In an alternative embodiment, the FSC304inFIG. 5may be different. For example, the FSC304inFIG. 5may share a bridge rectifier (more fully described below) with other components of LED convertor circuit502.

The FSC304can maintain a minimum current in TRIAC120by inserting a bleeder resistor (not shown inFIGS. 3-5) in parallel with LED light bulb106when needed. When not needed, the FSC disconnects the bleeder resistor so that current does not flow there through. In one embodiment, the FSC can maintain current in TRIAC120by inserting the bleeder resistor just before TRIAC120is activated by a pulse from DIAC116. The FSC can maintain the bleeder resistor in parallel with light bulb102for a short time after the DIAC pulse ends to insure that TRIAC latching current is meet. The bleeder resistor should be sized so that a minimum current (e.g., current that is greater than the latching or holding current for TRIAC120) flows through it. This will insure proper latching of TRIAC120for the remaining portion of the voltage cycle and maintain the power provided to the LED light bulb106, which in turn should preclude flickering. In one embodiment, the FSC is a stand-alone system, which means that it operates independently of LED converter106shown inFIG. 3or4or the other electronics contained in LED converter502. In other words, FSC does not rely on a control signal from of LED converter106shown inFIG. 3or4or the other electronics contained in LED converter502when selectively inserting a bleeder resistor in between the outputs of dimmer104or302.

FIG. 6illustrates an example FSC that can be employed within any of the lighting systems shown inFIGS. 3-5. More particularly, the FSC includes a bridge rectifier circuit600. A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Inputs to bridge rectifier600are coupled to conductors108R and108L as shown. The FSC further includes a bleeder resistor602and switch604. Lastly, the FSC includes a switch control circuit606that controls switch604.

In one embodiment, the switch control circuit606closes switch604just before a gate pulse is provided to TRIAC120via DIAC116. The switch control circuit606continues to maintain switch604in the closed position for a short time period after the gate pulse completes. While switch604is closed the minimal current needed to turn on or maintain TRIAC120in the on state can flow through bleeder resistor602. Once TRIAC120turns on, switch control circuit606opens switch604.

FIG. 7illustrates in an example embodiment of the FSC shown inFIG. 6. Rectifier600contains diodes arranged in a bridge to convert the AC input to DC output, which is provided to the series combination of bleeder resistor602and switch604. In one embodiment, switch604takes form in an integrated gate bipolar transistor (IGBT). The IGBT combines a FET712for the control input, and a bipolar power transistor714for the switching mechanism. The switch control circuit606takes form in MOSFET702having a drain coupled to a gate of FET712. The switch control circuit606includes a pair of resistors704and706, and a capacitor710. Lastly, capacitor710is coupled in parallel with a zener diode716and MOSFET702as shown. Zener diode716is coupled between the gate of MOSFET702and common node c. A zener diode is a special kind of diode which allows current or flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the zener breakdown voltage.

Bleeder resistor602is inserted in parallel with the LED light bulb102when switch604is activated or turned on. Switch604is controlled by MOSFET702, which in turn is controlled by the voltage at common node c. To illustrate, assume MOSFET702is off or inactive (e.g., the gate to source voltage Vgs of MOSFET702is below its threshold voltage Vt). With MOSFET702off, switch704is closed, which couples bleeder resistor602in parallel with light bulb106via bridge600. Also, assume TRIAC120is initially inactive or turned off, and as a result no current flows therethrough or through bleeder resistor602. On the other hand, because switch604is closed prior to activation of TRIAC120, TRIAC120can draw current through bleeder resistor602when TRIAC120is first activated.

As line voltage V1increases, capacitor114integrates charge, which in turn increases the voltage across DIAC116. Eventually, this voltage exceeds the DIAC breakover voltage, at which point DIAC116conducts current to the gate of TRIAC120during the pulse as described above. With the pulse, current begins flowing through bleeder resistor602and TRIAC120. Bleeder resistor602should be sized so that the current flowing TRIAC120exceeds a latching current when the DIAC pulse ends.

The voltage at common node c within switch circuit606continues to increase. More particularly, as current flows through resistor704and into capacitor710, the voltage on capacitor710increases. Eventually, as the voltage at common node c exceeds the zener breakdown voltage of zener diode716, the voltage at the gate of MOSFET702increases. Once the gate voltage of MOSFET702exceeds its threshold voltage Vt, MOSFET702activates, which in turn deactivates switch604by virtue of deactivating FET712. With switch604off, current no longer flows through bleeder resistor602. However, at that point, current flow through TRIAC120should exceed its latching current, and TRIAC120turns on.

It is noted that current can flow through resistors704and706within switch control circuit606when MOSFET702is turned on. However, if resistor704is sufficiently sized (e.g., 560 k ohms) relative to resistor706, the power consumed by the combination of resistors704and706should be minimal. It is also noted that capacitor710is provided to increase the speed at which MOSFET702will activate by virtue of current flow from capacitor710to the gate of MOSFET702via zener716, since resistor704, which has a high resistance, restricts the flow of current to common node c, and thus to the gate of MOSFET702via zener diode716.

As the line voltage V1begins to drop toward the zero crossing and begin the next half-phase cycle, the voltage at the common node c also begins to drop. When the voltage at node c falls below the zener breakdown voltage of zener diode716, MOSFET702will deactivate once its gate voltage drops below Vt. When MOSFET702deactivates, the voltage at the gate of FET712raises and eventually activates FET712, which in turn closes switch604and places bleeder resistor602in parallel with the light bulb106once again. It is noted that current may flow through bleeder resistor602before line voltage V1reaches the zero crossing. However, since line voltage V1is reducing toward zero, this current flow through bleed resistor602is small, which in turn minimizes the power consumption of bleeder resistor602prior to the zero crossing. Further, as the line voltage V1reduces toward zero, eventually the current flow through TRIAC120falls below its holding current and TRIAC120will turn off as a result. Once turned off, current will no longer flow through bleeder resistor602. However, switch604remains on by virtue of the voltage remaining across capacitor710. The time during which switch604is closed thus inserting bleeder resistor602in parallel with light bulb106can be controlled by the size of resistor706. As the resistance of resistor706increases, the time during which switch604remains closed increases, and vice versa.