Patent Application
20130147387
The present disclosure is generally related to electronics and, more particularly, is related to dimmer switches.
A dimmer switch is a useful electrical component that allows a user to adjust light levels from nearly dark to fully lit by simply turning a knob or sliding a lever. Early dimmer switches had a pretty straightforward solution to adjusting light levels—a variable resistor. With the variable resistor, the total resistance of the resistor is adjusted by adjusting the distance that the charge has to travel through a resistive material. As the charge works to move through the resistor, energy is lost in the form of heat. A resistor in a series circuit causes a voltage drop in the circuit, decreasing the energy available to other loads (the light bulb, for example). Decreased voltage across the light bulb reduces its light output.
A problem with this solution is that a large amount of energy is lost to heat the resistor, which doesn't help light up the room but still costs in energy consumption. In addition to be being inefficient, these switches tend to be cumbersome and potentially dangerous, since the variable resistor releases a substantial amount of heat.
Instead of diverting energy from the light bulb into a resistor, modern dimmer switches rapidly shut the light circuit off and on to reduce the total amount of energy flowing through the circuit. The light bulb circuit is switched off many times every second. The switching cycle is built around the fluctuation of alternating current (AC). AC current has varying voltage polarity—in an undulating sine wave, it fluctuates from a positive voltage to a negative voltage. A modern dimmer switch automatically shuts the light bulb circuit off every time the current reverses direction—that is, whenever there is zero current running through the circuit. This happens twice per cycle, or 120 times a second. It turns the light circuit back on when the voltage climbs back up to a certain level.
This “turn-on value” is based on the position of the dimmer switch's knob or slider. If the dimmer is turned to a brighter setting, it will switch on very quickly after cutting off. The circuit is turned on for most of the cycle, so it supplies more energy per second to the light bulb. If the dimmer is set for lower light, it will wait until later in the cycle to turn back on. The central element in this switching circuit is a triode alternating current switch, or triac.
A triac is a small semiconductor device, similar to a diode or transistor. The triac has two terminals wired into two ends of the circuit and a third gate terminal used to trigger conduction. There is always a voltage difference between the two terminals, but it changes with the fluctuation of the alternating current. That is, when current moves one way, a first terminal is positively charged while the second terminal is negatively charged, and when the current moves the other way the first terminal is negatively charged while the second terminal is positively charged.
The gate is also wired into the circuit, by way of a variable resistor. This variable resistor works the same basic way as the variable resistor in the older dimmer switch design, but it doesn't waste nearly as much energy generating heat. The triac acts as a voltage-driven switch with the voltage on the gate controlling the switching action and the variable resistor controls the voltage on the gate.
When there is “normal” voltage across the terminals and little voltage on the gate, the triac will act as an open switch—it will not conduct electricity. Once a sufficient voltage is applied to the gate, it begins to conduct electricity. Triacs have a minimum “holding current” that is required for it to remain in conduction. If this holding current is not met the triac will turn off regardless of what voltage is presented across the dimmer or the load. If this holding current is not met during a typical AC period, the triac can “trigger” repeatedly creating undesired operation such as multiple on/off cycles.
The exact sequence varies depending on the direction of the current—that is, which part of the AC cycle is present across the triac. Using the triac with different light sources such as light emitting diodes (LEDs) introduces heretofore unaddressed needs with previous solutions.
Example embodiments of the present disclosure provide systems of systems and methods of LED dimmer compatibility. Briefly described, in architecture, one example embodiment of the system, among others, can be implemented as follows: at least one LED; and a power factor correction (PFC) controller connected between the LED illumination device and an AC input to the LED illumination device, the PFC controller configured to: determine whether the LED illumination device receives the AC input from a direct AC line, a trailing edge dimmer, or a leading edge dimmer; and control a gate transistor connected to the LED illumination device based on the determination.
Embodiments of the present disclosure can also be viewed as providing methods for systems and methods of LED dimmer compatibility. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: determining whether the LED illumination device receives an AC input from a direct AC line, a trailing edge dimmer, or a leading edge dimmer; and controlling a gate transistor of a power factor correction (PFC) circuit connected to the LED illumination device based on the determination.
Embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which example embodiments are shown. Embodiments of the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. The examples set forth herein are non-limiting examples and are merely examples among other possible examples.
Turning a dimmer switch knob pivots a contact arm (or contact plate) on a variable resistor, increasing or decreasing its total resistance. When the knob is set to “dim,” the variable resistor offers greater resistance so it “holds up” the current. As a result, the necessary boost voltage doesn't build up as quickly on the firing capacitor. By the time the capacitor is charged enough to make the triac conductive, the AC current cycle is well underway. If the knob is turned the other way, the variable resistor offers less resistance and the capacitor produces the necessary boost voltage earlier in the fluctuating cycle. When the current fluctuates back to the zero voltage point, there is nothing driving current through the triac, so the electrons stop moving. The depletion zones form again, and the triac loses its conductivity until the boost voltage builds up on the gate.
Modern LED lights do not work inherently with existing lighting infrastructure, such as dimmers. LEDs typically do not draw enough power to keep existing dimmers conducting, causing erratic behavior. If an LED illumination device that was not designed to work with a triac dimmer is plugged it into a triac-based dimmer, little, if any, light would be emitted from it, depending on the dimmer setting and the design of that dimmer. More often than not, the LEDs would flash repeatedly, mainly because the LED is not drawing enough power to keep the triac element of the dimmer conducting continuously. As provided in circuit 100 of
Once triac 125 turns on, it needs a certain amount of current to continually flow through it to remain in a conduction mode, for example 50 milliamps. Otherwise, triac 125 shuts off. The load from light bulb 130 maintains the holding current of triac 125. Triac 125 turns off close to the zero crossing of the AC input and the cycle repeats.
A challenge for LED luminaries is to first draw some reasonable level of current which looks like a low impedance to AC line input 110 to get the voltage on C1 to charge up to 20 volts at first. Once triac 125 is turned on, the LED luminare typically does not draw enough current to keep triac 125 conducting, thus shutting off triac 125. This challenge may be solved with extra circuitry or energy storage.
Other solutions have presented a low impedance to the line to trigger the triac and stored energy in a capacitor before the triac shuts off. These previous solutions may solve the challenges that the triac presents to an LED luminare, but when connected directly to an AC source, these circuits result in very low power factor. Additionally, these solutions may satisfy triac based dimmers, but they do not always work well with trailing edge dimmers. Another approach is to manipulate the system by wasting energy. Energy may be wasted by drawing excess current in an attempt to keep the triac in a conducting state. This achieves higher power factor but at the cost of efficiency. The disclosed systems and methods of LED dimmer compatibility offer LED systems that fit in applications regardless of whether a dimmer is present or the type of dimmer switch that is used. These disclosed systems and methods provide high power factor without wasting energy.
Not all dimmers are the same. (leading edge versus trailing edge dimmers). Different methods may be used in some applications to satisfy the current requirements of different dimmers. New regulations require LED lights to meet certain PF standard when directly connected to the AC grid (no dimmer). Most circuits to date need to waste power to achieve good PF and work with most dimmers. There are at least three types of wave forms that the LED illumination device may see. A compatibility circuit supply may be integrated into the LED illumination device such that the device may be screwed into any existing light socket. The device may be usable in a socket whether or not a dimmer is on the line. If a dimmer is present, it may be a trailing edge dimmer or a leading edge dimmer. If no dimmer is present on the line, the LED illumination device receives a direct AC line input. With no dimmer, the AC input rises slow and falls slow. However, with the trailing edge dimmer, the AC signal rises slowly and falls quickly when properly triggered. With a leading edge dimmer, the AC signal rises quickly and falls slowly when properly triggered.
Trailing edge dimmers, an example of which is provided in
Differences between leading edge dimming and trailing edge dimming are shown in
Leading and trailing edge dimmers are examples of phase angle based dimming. The brightness of the illumination is proportional to the dimmer conduction phase angle (tφ in
LED applications are required to have some level of PFC, when directly connected to an AC line. The PFC may be quantified by:
PF=Real Power(W)/Apparent Power(VA)
Recent Energy Star regulations in the United States specify power factor of no less than 0.7 for residential applications and 0.9 for commercial applications. Worldwide requirements increasingly specify power factor limits and in some cases, THD limits (EN61000-3-2 Class C, >25 W). PFC may be provided when a direct AC connection is determined to be present and not provided when there is a dimmer because operation of the dimmer counteracts the gains provided by the PFC. With a boost PFC, for example, near unity power factor may be achieved on the front end. Although boost PFC circuits are primarily referred to in this disclosure, other topologies such as flyback, buck, and forward among others may be used.
An example embodiment of boost PFC circuit 600 is provided in
In circuit 600, diode bridge or rectifier 620 rectifies AC input voltage 610 ahead of inductor 660. The output of the boost regulator is a constant voltage but the input current is programmed by the input voltage to be a half sine wave. The power flow into output capacitor 640 is not constant but is a sine wave at twice the line frequency since power is the instantaneous product of voltage and current. Output capacitor 640 stores energy when the input voltage is high and releases the energy when the input voltage is low to maintain the output power flow. The disclosed systems and methods of LED illumination compatibility are applicable in both isolated and non-isolated applications.
Additional intelligence in the LED illumination device is necessary to control the PFC circuit when a leading edge dimmer is detected. Trailing edge dimmer connections may be treated the same as a direct AC connection. The conduction of a trailing edge dimmer is controlled within the dimmer (timer 220 of
In an example embodiment, a boost PFC staged is applied to a leading edge dimmer. The gate transistor of the boost PFC circuit is held at substantially 100% duty cycle until the triac fires and then transitions to normal PFC operation (switching). After it is determined that sufficient energy has been transferred to the PFC output capacitor, the gate transistor of the boost PFC may be shut off. It is preferable for the PFC stage to run for a short period of time after the triac fires before being shut off to ensure reasonable energy transfer to the boost capacitor. If the PFC gate transistor is left running after the leading edge dimmer shuts off (due to insufficient current), a false triggering of the triac may result. In the example embodiment, the boost PFC circuit triggers the triac, producing a triac conduction angle that is consistent from AC line cycle to AC line cycle so that a leading edge application emulates a trailing edge application. It can be used in both isolated or non-isolated designs.
Using a PFC stage for energy storage allows for the transfer of energy used to trigger the triac to the output capacitor of the PFC and thus into the LED load. This is accomplished because the current used to charge the traic's RC network is stored in the PFC inductor via the 100% duty cycle of the PFC switch. When the TRIAC fires and the PFC stage becomes active (switching), the energy stored in the inductor is subsequently re-directed to the output capacitor. Although this is not a significant amount of power, when used in sub-10 W and 5 W systems, milliwatts of power can have an impact on overall efficiency.
PFC controller 825 may measure the conduction phase angle produced by dimmer 890 and provide intelligent control of PFC gate 830 for dimmer loading. If a direct AC connection is determined, PFC controller 825 may provide traditional PFC operation. If dimmer 890 is determined to be a trailing edge dimmer, PFC controller 825 may turn on switch 830 after trailing edge dimmer 890 is disabled, the disabling performed by asserting switch 210 of
In the case of a leading edge dimmer, hysteretic regulation of Vout may also be provided by PFC controller 825. In an example embodiment, an additional hysteretic voltage control loop is integrated with PFC controller 825 to enable the PFC circuit when the output voltage of the PFC stage is below a given threshold. Once the output voltage falls below this threshold, the PFC stage becomes active upon the trigger of the triac and remains active until the PFC output voltage exceeds a higher voltage threshold. The hysteresis loop ensures that a sufficient load is presented to the dimmer to maintain conduction. Selecting the amount of voltage hysteresis on the PFC output ensures proper operation. The greater the voltage differential between the thresholds, the more power required to charge it back to full value. The hysteresis algorithm may cause skipping (PFC remains disabled) of subsequent AC line cycles while the output voltage decays.
Although the present disclosure has been described in detail, it should be understood that various changes, substitutions and alterations can be made thereto without departing from the spirit and scope of the disclosure as defined by the appended claims.
