BACKGROUND

The present invention relates to load control devices for controlling the amount of power delivered to an electrical load, and more particularly, to a dimmer switch for controlling the power delivered to a lighting load, such as a high-efficiency load.

Prior art two-wire load control devices, such as dimmer switches, are coupled in series electrical connection between an alternating-current (AC) power source and a lighting load for controlling the amount of power delivered from the AC power source to the lighting load. A two-wire wall-mounted dimmer switch is adapted to be mounted to a standard electrical wallbox and comprises two load terminals: a hot terminal adapted to be coupled to the hot side of the AC power source and a dimmed hot terminal adapted to be coupled to the lighting load. In other words, the two-wire dimmer switch does not require a connection to the neutral side of the AC power source (i.e., the load control device is a “two-wire” device). Prior art “three-way” dimmer switches may be used in three-way lighting systems and comprise at least three load terminals, but do not require a connection to the neutral side of the AC power source.

The dimmer switch may comprise a toggle actuator for turning the lighting load on and off and an intensity adjustment actuator for adjusting the intensity of the lighting load. Examples of prior art dimmer switches are described in greater detail is commonly-assigned U.S. Pat. No. 5,248,919, issued Sep. 28, 1993, entitled LIGHTING CONTROL DEVICE; U.S. Pat. No. 6,969,959, issued Nov. 29, 2005, entitled ELECTRONIC CONTROL SYSTEMS AND METHODS; and U.S. Pat. No. 7,687,940, issued Mar. 30, 2010, entitled DIMMER SWITCH FOR USE WITH LIGHTING CIRCUITS HAVING THREE-WAY SWITCHES, the entire disclosures of which are hereby incorporated by reference.

The dimmer switch typically comprises a bidirectional semiconductor switch, e.g., a thryristor (such as a triac) or two field-effect transistors (FETs) in anti-series connection. The bidirectional semiconductor switch is coupled in series between the AC power source and the load, and is controlled to be conductive and non-conductive for portions of a half cycle of the AC power source to thus control the amount of power delivered to the electrical load. Generally, dimmer switches use either a forward phase-control dimming technique or a reverse phase-control dimming technique in order to control when the bidirectional semiconductor switch is rendered conductive and non-conductive to thus control the power delivered to the load.

With forward phase-control dimming, the bidirectional semiconductor switch is rendered conductive at some point within each AC line voltage half cycle and remains conductive until approximately the next voltage zero-crossing, such that the bidirectional semiconductor switch is conductive for a conduction time each half cycle. A zero-crossing is defined as the time at which the AC line voltage transitions from positive to negative polarity, or from negative to positive polarity, at the beginning of each half cycle. Forward phase-control dimming is often used to control energy delivered to a resistive or inductive load, which may include, for example, an incandescent lamp or a magnetic low-voltage transformer. The bidirectional semiconductor switch of a forward phase-control dimmer switch may be implemented as a thyristor, such as a triac or two silicon-controlled rectifiers (SCRs) coupled in anti-parallel connection (e.g., since a thyristor becomes non-conductive when the magnitude of the current conducted through the thyristor decreases to approximately zero amps).

Many forward phase-control dimmers include analog control circuits (such as timing circuits) for controlling when the thyristor is rendered conductive each half cycle of the AC power source. The analog control circuit typically comprises a potentiometer, which may be adjusted in response to a user input provided from, for example, a linear slider control or a rotary knob in order to control the amount of power delivered to the lighting load. The analog control circuit is typically coupled in parallel with the thyristor and conducts a small timing current through the lighting load when the thyristor is non-conductive. The magnitude of the timing current is small enough such that the controlled lighting load is not illuminated to a level that is perceptible to the human eye when the lighting load is off.

Thyristors are typically characterized by a rated latching current and a rated holding current, and comprise two main load terminals and a control terminal (e.g., a gate). When the current conducted through the main terminals of the thyristor exceeds the latching current, the thyristor may become fully conductive. If the current conducted through the main terminals of the thyristor remain above the holding current, the thyristor may remain in full conduction. Since an incandescent lamp is a resistive lighting load, a typical forward phase-control dimmer switch is operable to conduct enough current through the incandescent lamp to exceed the rated latching and holding currents of the thyristor if the impedance of the incandescent lamp is low enough. Therefore, prior art forward phase-control dimmer switches are typically rated to operate appropriately with lighting loads having a power rating above a minimum power rating (e.g., approximately 40 W), such that the thyristor will be able to latch and remain latched when dimming the lighting load.

Some prior art dimmer switches have included two triacs coupled together to overcome some of the problems related to the rated latching and holding currents of triacs, e.g., as described in greater detail in commonly-assigned U.S. Pat. No. 4,954,768, issued Sep. 4, 1990, entitled TWO WIRE LOW VOLTAGE DIMMER. Such a prior art dimmer switch may comprise a first triac characterized by a low power rating and low latching and holding currents, and a second triac characterized by a high power rating and high latching and holding currents. The main load terminals of the first triac are coupled between one of the main load terminals and the gate of the second triac. In addition, a resistor is coupled between the other main load terminal and the gate of the second triac. If the magnitude of the load current is small, the first triac is rendered conductive when a pulse of current is conducted through the gate and remains latched until the magnitude of the load current drops below the holding current of the first triac (e.g., at the end of a half cycle). If the magnitude of the load current is large, the first triac conducts a pulse of the gate current through the gate of the second triac to render the second triac conductive and the second triac conducts the load current. Since the voltage across the first triac drops to approximately zero volts when the second triac is conductive, the first triac becomes non-conductive after the second triac is rendered conductive. The second triac remains conductive until the magnitude of the load current drops below the holding current of the second triac (e.g., at the end of a half cycle).

When using reverse phase-control dimming, the bidirectional semiconductor switch may be rendered conductive at the zero-crossing of the AC line voltage and rendered non-conductive at some point within each half cycle of the AC line voltage, such that the bidirectional semiconductor switch is conductive for a conduction time each half cycle. Reverse phase-control dimming may be used to control energy to a capacitive load, which may include, for example, an electronic low-voltage transformer. Since the bidirectional semiconductor switch may be rendered conductive at the beginning of the half cycle, and may be able to be rendered non-conductive within the half cycle, the dimmer switch may comprise two FETs in anti-serial connection (or the like) in order to perform reverse phase-control dimming. A FET is operable to be rendered conductive and to remain conductive independent of the magnitude of the current conducted through the FET. In other words, a FET is not limited by a rated latching or holding current (e.g., as a thyristor may be). However, prior art reverse phase-control dimmer switches have either required neutral connections and/or advanced control circuits (such as microprocessors) for controlling the operation of the FETs. In order to power a microprocessor, the dimmer switch must also comprise a power supply, which is typically coupled in parallel with the FETs. These advanced control circuits and power supplies add to the cost of prior art FET-based reverse phase-control dimmer switches (e.g., as compared to analog forward phase-control dimmer switches).

Further, in order to properly charge, the power supply of such a two-wire dimmer switch may develop an amount of voltage across the power supply and may conduct a charging current from the AC power source through the electrical load, in many instances even when the lighting load is off. If the power rating of the lighting load is too low, the charging current conducted by the power supply through the lighting load may be great enough to cause the lighting load to illuminate to a level that is perceptible to the human eye when the lighting load is off. Therefore, prior art FET-based reverse phase-control dimmer switches are typically rated to operate appropriately with lighting loads having a power rating above a minimum power rating, such that the lighting load does not illuminate to a level that is perceptible to the human eye (e.g., due to the power supply current when the lighting load is off). Some prior art load control devices have included power supplies that only develop small voltages and draw small currents when charging, such that the minimum power rating of a controlling lighting load may be as low as ten Watts. An example of such a power supply is described in greater detail in commonly-assigned U.S. patent application Ser. No. 12/751,324, filed Mar. 31, 2010, entitled SMART ELECTRONIC SWITCH FOR LOW-POWER LOADS, the entire disclosure of which is hereby incorporated by reference.

Nevertheless, it is desirable to be able to control the amount of power delivered to electrical loads having different characteristics (e.g., having power rating lower than those able to be controlled by the prior art forward and reverse phase-control dimmer switches). For instance, in order to save energy, high-efficiency lighting loads, such as compact fluorescent lamps (CFLs) and light-emitting diode (LED) light sources, are being used in place of or as replacements for conventional incandescent or halogen lamps. High-efficiency light sources typically consume less power and provide longer operational lives as compared to incandescent and halogen lamps. A load regulation device (e.g., such as an electronic dimming ballast or an LED driver) may be coupled between the AC power source and the respective high-efficiency light source (e.g., the compact fluorescent lamp or the LED light source) for regulating the power supplied to the high-efficiency light source.

A dimmer switch controlling a high-efficiency light source may be coupled in series between the AC power source and the load control device for the high-efficiency light source. Some high-efficiency lighting loads may be integrally housed with the load regulation devices, e.g., in a single enclosure. Such an enclosure may have a screw-in base that allows for mechanical attachment to standard Edison sockets. The enclosure may provide electrical connections to the neutral side of the AC power source, and either the hot side of the AC power source or the dimmed-hot terminal of the dimmer switch (e.g., for receipt of the phase-control voltage). The load regulation circuit is operable to control the intensity of the high-efficiency light source to the desired intensity (e.g., in response to the conduction time of the bidirectional semiconductor switch of the dimmer switch).

The load regulation devices for the high-efficiency light sources may have high input impedances or input impedances that vary in magnitude throughout a half cycle. Therefore, when a prior-art forward phase-control dimmer switch is coupled between the AC power source and the load regulation device for the high-efficiency light source, the load control device may not be able to conduct enough current to exceed the rated latching and/or holding currents of the thyristor. When a prior-art reverse phase-control dimmer switch is coupled between the AC power source and the load regulation device, the magnitude of the charging current of the power supply may be great enough to cause the load regulation device to illuminate the controlled high-efficiency light source to a level that is perceptible by the human eye when the light source should be off.

The impedance characteristics of the load regulation device may negatively affect the magnitude of the phase-control voltage received by the load regulation device, such that the conduction time of the received phase-control voltage is different from the actual conduction time of the bidirectional semiconductor switch of the dimmer switch (e.g., if the load regulation device has a capacitive impedance). Therefore, the load regulation device may control the intensity of the high-efficiency light source to an intensity that is different than the desired intensity as directed by the dimmer switch. In addition, the charging current of the power supply of the dimmer switch may build up charge at the input of a load regulation device having a capacitive input impedance, thus negatively affecting the low-end intensity that may be achieved.

SUMMARY

As described herein, a load control device for controlling power delivered from an AC power source to an electrical load may comprise a thyristor adapted to be electrically coupled between the AC power source and the electrical load. The thyristor may include a first main terminal, a second main terminal, and a gate terminal. The first and second main terminals may be configured to conduct a load current to energize the electrical load. The gate terminal may be configured to conduct a gate current to render the thyristor conductive. The load control device may further comprise a gate current path connected to the gate terminal of the thyristor. The gate current path may include a gate coupling circuit configured to conduct gate current through the gate terminal of the thyristor. The load control device may further comprise a control circuit electrically coupled to control the gate coupling circuit. The control circuit may be configured to control the gate current path to conduct, at a firing time of a half-cycle of the AC power source, a pulse of the gate current through the gate terminal of the thyristor to render the thyristor conductive.

The control circuit may be configured to operate in a first gate drive mode and a second gate drive mode. During the first gate drive mode, the control circuit may render the gate current path non-conductive after the pulse time period during the half-cycle of the AC power source. During the second gate drive mode, the control circuit may maintain the gate current path conductive after the pulse time period to allow at least one other pulse of current to be conducted through the gate terminal of the thyristor after the pulse time period during the half-cycle.

The control circuit may be configured to operate in the first gate drive mode while turning on the electrical load and to operate in the second gate drive mode once the electrical load is in a steady state condition. For example, the control circuit may be configured to operate in the first gate drive mode over a first plurality of half-cycles of the AC power source during which the electrical load is being turned on. The control circuit may then switch to the second gate drive mode when the electrical load enters a steady state and may stay in the second gate drive mode over a second plurality of half-cycles of the AC power source. During the first plurality of half-cycles, the control circuit may adjust the pulse time period from a minimum pulse time period to a maximum pulse time period. During the second plurality of half-cycles, the control circuit may maintain the gate current path conductive for the maximum pulse time period. The value of the maximum pulse time period may be dependent upon the firing time of the second gate drive mode.

The control circuit may be configured to operate in the second gate drive mode while turning on the electrical load, detect a fault condition while turning on the electrical load using the second gate drive mode, switch to the first gate drive mode to turn on the electrical load, and operate in the second gate drive mode after the electrical load enters a steady state.

As described herein, the load control device may further comprise a zero-crossing detection circuit configured to generate a zero-cross signal at the zero-crossing of the AC power source. The control circuit may be configured to control the conductivity of the gate current path based on the zero-cross signal. For example, the control circuit may be configured to sample the zero-cross signal during a zero-cross window, and determine if the zero-cross signal indicated the zero-crossing of the AC power source. Based on a determination that there was a zero-crossing of the AC power source, the control circuit may control the gate current path to conduct the pulse of gate current through the gate terminal of the thyristor to render the thyristor conductive at the firing time of the present half-cycle.

As described herein, a method may be provided for turning on an electrical load through a load control device. The load control device may comprise a thyristor having first and second main terminals through which a load current from an AC power source can be conducted to energize the electrical load and a gate terminal through which a gate current can be conducted to render the thyristor conductive. The method may comprise applying a first one of a plurality of start-up routines via the load control device to turn on the electrical load. At least one of the plurality of start-up routines may comprise causing the load control device to conduct the gate current through the gate terminal of the thyristor for only a pulse time period during a half-cycle of the AC power source. At least another one of the plurality of start-up routines may comprise causing the load control device to conduct the gate current through the gate terminal of the thyristor for more than the pulse time period during the half-cycle of the AC power source.

The method may further comprise determining whether a fault condition has occurred in the electrical load while applying the first one of the plurality of start-up routines. Based on a determination that a fault condition has occurred in the electrical load in response to the first one of the plurality of start-up routines, the method may further comprise applying a second one of the plurality of start-up routines via the load control device to turn on the electrical load. Based on a determination that a fault condition has not occurred in the electrical load in response to the first one of the plurality of start-up routines, the method may further comprise storing the first one of the plurality of start-up routines in a memory location of the load control device.

