Phase control with adaptive parameters

#FIG. 1 illustrates a prior art circuit for controlling the flow of AC power to a load. An AC power switch, shown here as a Triac TR, is series connected between the power source 12 and the load 14. A controller 16 operates the Triac to regulate the RMS power transferred to the load using a phase control technique as shown in FIG. 2.

#The voltage available from the AC power source is shown as a broken line in FIG. 2. Each line cycle of the AC power source has a positive half cycle beginning at a first zero crossing at time t0 and ending at a midpoint zero crossing at t2. The AC line cycle then has a negative half cycle beginning at t2 and ending at another zero crossing at t4. For common 60 Hz power, the entire line cycle from t0 to t4 lasts 1/60th of a second.

#At the beginning of the line cycle illustrated in FIG. 2, the Triac remains off during a delay period TD. At time t1, the Triac is turned on to connect the power source to the load. The portion of the AC voltage waveform actually applied to the load is shown as a solid line. The Triac continues conducting power to the load during conduction period TC until it switches off at the zero crossing at t2. Triacs are self-commutating devices, which means that they turn themselves off when the current through the device falls below a holding level as described below. The same process is repeated for the negative half cycle where the Triac turns on at t3 and off at t4. Generally, if the load is purely resistive, the current IL flowing through the load has essentially the same waveform as the portion of the AC voltage applied to the load as shown in FIG. 3.

#FIG. 1 is a block diagram of a prior art circuit for controlling the flow of AC power to a load.

#FIGS. 2-5 illustrate a prior art phase control technique for controlling the flow of AC power to a load.

#FIGS. 6-8 illustrate the construction and operation of a prior art phase control device.

#In the circuit of FIG. 1, the controller 16 includes electronic circuitry that requires a power supply for operation. The controller and Triac or other phase-control switch are typically packaged in a common assembly 18 as shown in FIG. 6. The assembly must include a line terminal 20 and a load terminal 22 to be able to operate as a switch. If the assembly can include a connection to a neutral terminal 24, then the full AC power source is always available to generate a power supply for the controller. In many situations, however, a neutral connection is not available, so the power supply for the controller must be derived from the current path that flows through the load. This places constraints on the manner in which the power supply for the controller can be derived.

#For example, the circuit of FIG. 6 may be used in a two wire phase control dimmer with no neutral connection for lighting applications. During time periods when the Triac is off, the full AC line voltage appears across the line and load terminals. This voltage may be used to generate the power supply for the controller, but in the case of a two wire dimmer, any current consumed by the power supply also flows through the load 14 and may interfere with the normal off state of the load. For example, a compact fluorescent lamp (CFL) typically includes an input rectifier and capacitor, as well as its own control electronics. Even a small power supply current flowing through the lamp in the off state may keep charging the input capacitor until it reaches a voltage level that causes the lamp to ignite briefly. This may possibly cause visible flicker and reduce the operating life of the lamp.

#During time periods when the Triac is on, the line and load terminals of the dimmer are essentially shorted, so there may be insufficient voltage available to generate a power supply for the controller. Therefore, very little power is typically available for power supplies in two wire devices.