US 20080074053 A1
A phase-control power controller converts a line voltage to an RMS load voltage and includes a phase-control clipping circuit that clips a load voltage to provide the RMS load voltage. The power controller may be in a base of a lamp and connected to a lamp terminal. The phase-clipping circuit establishes a phase conduction angle that determines the RMS load voltage for the lamp and includes a transistor switch and a microcontroller that operates the transistor switch, where ON/OFF periods of the transistor switch define the phase conduction angle. The microcontroller senses a load voltage at the lamp terminal, compares the sensed load voltage to a reference RMS voltage, and adjusts the ON/OFF periods of the transistor switch in response to the comparison to cause the load voltage to approach the reference RMS voltage. The circuit may be used for reverse, forward, or forward/reverse hybrid phase clipping.
1. A phase-control power controller that converts a line voltage to a RMS load voltage, the controller comprising:
line terminals for a line voltage and load terminals for a load voltage; and
a phase-clipping circuit connected to said line and load terminals and establishing a phase conduction angle that determines an RMS load voltage,
said phase-clipping circuit comprising a transistor switch and a microcontroller that operates said transistor switch, wherein ON/OFF periods of said transistor switch define the phase conduction angle,
said microcontroller being arranged and adapted to sense the load voltage and to compare the sensed load voltage to a reference RMS voltage and to adjust the ON/OFF periods of said transistor switch in response to the comparison to cause the load voltage to approach the reference RMS voltage.
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The present invention is directed to a power controller that supplies a specified power to a load, and more particularly to a voltage converter for a lamp that converts line voltage to a voltage suitable for lamp operation.
Some loads, such as lamps, operate at a voltage lower than a line (or mains) voltage of, for example, 120V or 220V, and for such loads a voltage converter (or power controller) that converts line voltage to a lower operating voltage must be provided. The power supplied to the load may be controlled with a phase-control clipping circuit that typically includes an RC circuit. Moreover, some loads operate most efficiently when the power is constant, or substantially so. However, line voltage variations are magnified by these phase-control clipping circuits due to their inherent properties (as will be explained below) and the phase-control clipping circuit is desirably modified to provide a more nearly constant RMS load voltage.
A simple four-component RC phase-control clipping circuit demonstrates a problem of conventional phase-control clipping circuits. The phase-controlled clipping circuit shown in
In operation, a clipping circuit such as shown in
The voltage across the diac 24 is analogous to the voltage drop across the capacitor 22 and thus the diac will fire once breakover voltage VBO is achieved across the capacitor. The triac 26 fires when the diac 24 fires. Once the diac has triggered the triac, the triac will continue to operate in saturation until the diac voltage approaches zero. That is, the triac will continue to conduct until the line voltage nears zero crossing. The virtual short circuit provided by the triac becomes the second state of the clipping circuit as illustrated in
Triggering of the triac 26 in the clipping circuit is forward phase-controlled by the RC series network and the leading portion of the line voltage waveform is clipped until triggering occurs as illustrated in
Accordingly, the RMS load voltage and current are determined by the resistance and capacitance values in the clipping circuit since the phase at which the clipping occurs is determined by the RC series network and since the RMS voltage and current depend on how much energy is removed by the clipping.
With reference to
Define Virrms as RMS line voltage, Vorms as RMS load voltage, T as period, and ω as angular frequency (rad) with ω=27πf.
Line voltage may vary from location to location up to about 10% and this variation can cause a harmful variation in RMS load voltage in the load (e.g., a lamp). For example, if line voltage were above the standard for which the voltage conversion circuit was designed, the triac 26 may trigger early thereby increasing RMS load voltage. In a halogen incandescent lamp, it is particularly desirable to have an RMS load voltage that is nearly constant.
Changes in the line voltage are exaggerated at the load due to a variable conduction angle, and conduction angle is dependent on the rate at which the capacitor voltage reaches the breakover voltage of the diac. For fixed values of frequency, resistance and capacitance, the capacitor voltage phase angle (θC) is a constant defined by θC=arctan (−ωRC). Therefore, the phase of VC is independent of the line voltage magnitude. However, the rate at which VC reaches VBO is a function of Virrms and is not independent of the line voltage magnitude.
Changes in Virrms leading to exaggerated or disproportional changes in Vorrms are a direct result of the relationship between conduction angle and line voltage magnitude. As Virrms increases, Vorrms increases due to both the increase in peak voltage and the increase in conduction angle, and as Virrms decreases, Vorrms decreases due to both the decrease in peak voltage and the decrease in conduction angle. Thus, load voltage is influenced twice, once by a change in peak voltage and once by a change in conduction angle, resulting in unstable RMS load voltage conversion for the simple phase-control clipping circuit.
When the phase-control power controller is used in a voltage converter of a lamp, the voltage converter may be provided in a fixture to which the lamp is connected or within the lamp itself. U.S. Pat. No. 3,869,631 is an example of the latter, in which a diode is provided in the lamp base for clipping the line voltage to reduce RMS load voltage at the light emitting element. U.S. Pat. No. 6,445,133 is another example of the latter, in which transformer circuits are provided in the lamp base for reducing the load voltage at the light emitting element.
An object of the present invention is to provide a novel phase-control power controller that converts a line voltage to an RMS load voltage and uses a microcontroller to adjust the voltage conversion in response to variations in line voltage magnitude.
A further object is to provide a novel phase-control power controller with a phase-control clipping circuit that establishes a phase conduction angle that determines an RMS load voltage, where the phase-clipping circuit includes a transistor switch and a microcontroller that operates the transistor switch, where ON/OFF periods of the transistor switch define the phase conduction angle, and in which the microcontroller senses the load voltage and compares the sensed load voltage to a reference RMS voltage and adjusts the ON/OFF periods of the transistor switch in response to the comparison to cause the load voltage to approach the reference RMS voltage. The circuit may be used for reverse, forward, or forward/reverse hybrid phase clipping.
With reference to
With reference to
Conventional RC phase-control clipping circuits are very sensitive to fluctuations in the line voltage magnitude. The present invention provides a power controller that makes adjustments in response to changes in the line voltage magnitude by changing the ON periods of the transistor switch that triggers conduction in response to sensed changes, thereby reducing variation of the RMS load voltage compared to conventional RC phase-control circuits. Additionally, this control technique makes it possible to use a forward/reverse hybrid of phase-control clipping by which the effects of electromagnetic interference (EMI) and total harmonic distortion (THD) are reduced in comparison to forward-only phase-control clipping.
Microcontroller 42 preferably includes an analog-to-digital converter (ADC) that converts the load voltage to a digital signal, a comparator that compares the output from the ADC to the reference RMS voltage (or a corresponding reference value), and a program (e.g., in a hardwired and/or programmable circuit) that adjusts the ON time of the transistor switch to adjust the RMS load voltage based on an output from the comparator so as to approach the reference RMS voltage. The ADC is connected to the load voltage through a current limiting resistor. The microcontroller samples the load voltage waveform applied to the lamp and automatically increases or decreases the conduction times such that the RMS load voltage is nearly always at a desired level. The reference RMS voltage is preset to a value that provides the desired RMS load voltage for the lamp. The structure and operation of microcontroller 42 need not be described in detail as such microcontrollers are known in the art and are commercially available from various sources, including Microchip Technology, Inc. under the PIC trademark (e.g., a PIC™ 8-pin 8-bit CMOS microcontroller, such as PIC 12F683).
With reference now to
The phase-clipping circuit may be used for reverse, forward, or forward/reverse hybrid phase clipping. With reference to
With reference to
Alternatively and with reference to
Similarly, the microcontroller may be used to control the transistor switch to provide forward phase clipping that removes power from the region of the load cycle from a polarity change and through a peak load voltage. The conduction angle convention for reverse clipping is shown in
While embodiments of the present invention have been described in the foregoing specification and drawings, it is to be understood that the present invention is defined by the following claims when read in light of the specification and drawings.