WO1999030539A2

WO1999030539A2 - Method and apparatus for power factor correction

Info

Publication number: WO1999030539A2
Application number: PCT/US1998/022260
Authority: WO
Inventors: John R. Shannon
Original assignee: Electronic Lighting Incorporated
Priority date: 1997-12-08
Filing date: 1998-11-20
Publication date: 1999-06-17
Also published as: WO1999030539A3

Abstract

A power control circuit for controlling the power drawn by a load such as a lamp control ballast from a source of rectified A.C. voltage source provided on a voltage supply line. The power control circuit includes a keep-alive portion having a capacitor which receives and stores charge during intervals when the rectified A.C. voltage exceeds a threshold level, and discharges to support operation of the ballast during intervals when the rectified A.C. voltage is below the threshold level. The capacitor receives charge from the current flowing through the primary winding of the ballast transformer. The power control circuit also includes a feedback capacitor connected between the primary winding of the ballast transformer and the active line of the power supply to store a feedback charge when the current through the primary winding of the ballast transformer flows in a first direction and to discharge the feedback charge to the voltage supply line when the current through the primary winding of the ballast transformer flows in a second direction.

Description

METHOD AND APPARATUS FOR POWER FACTOR CORRECTION

This application is a continuation-in-part application of Serial No. 08/523,387, filed September 5, 1995.

Field of the Invention The present invention relates generally to a method and apparatus for power factor control. More particularly, the present invention is directed to an apparatus and method for controlling the load current supplied to a load such that the load current is substantially sinusoidal and in phase with an AC supply voltage.

Background of the Invention In order to control the current drawn by a load, numerous AC power control circuits have been developed. Typical AC power control circuit designs seek to ensure that the load draws an current which is substantially sinusoidal and in phase with an AC supply voltage. Such circuits have a wide variety of applications, including as a control circuit for a dimming ballast to control a gas- discharge lamp. Gas-discharge lamps generate light when an electric current passes through the gas contained within the lamp. Gas-discharge lamps have a negative resistance (that is, resistance which decreases as the current increases), and are typically provided with a ballast circuit for controlling the current supplied to the lamp such that an AC current is supplied to the lamp which is substantially sinusoidal and in phase with the AC supply voltage. As is known in the art, a high-frequency current generates light more efficiently than the 60 Hz frequency of a standard A.C. supply voltage. Dimming ballast control circuit designs seek to maintain a high power factor as the current supplied to a gas-discharge lamp is adjusted to dim the lamp. The term "power factor" refers to the ratio of active power to apparent power measured at the input to the ballast. The apparent power is the product of the root mean square values for the input current and input voltage, respectively, at the input to the ballast. The active power is the product of the root mean square of the in-phase component of the input current and the root mean square of the input voltage.

Early power control circuits included active power factor correction (PFC) circuits, in which a converter stage converts an AC supply voltage into a DC voltage. The regulated DC voltage is supplied to a ballast circuit for driving a lamp. These PFC circuits provide a relatively low total harmonic distortion (THD), typically less than 10% . However, active PFC circuits require many parts and are therefore complex and expensive. Further, active PFC circuits suffer from a high electro-magnetic interference (EMI) and a high inrush current, which can damage switch gears and circuit breakers.

Another known AC control circuit is a passive low pass filter, in which capacitors and inductors having relatively large values are used to filter the input current. The filtered input current is rectified by a diode bridge and supplied to the ballast circuit. While this design is simple, durable, and requires relatively few parts, it has the disadvantages of relatively high power loss, noise, and excessive weight due to the required use of large size inductors and capacitors. Further, the degree of power factor correction varies depending on the load. That is, the circuit may work well for certain loads under certain conditions, but may perform poorly for other loads and conditions. Yet another known power control circuit is the parallel-discharge series- charge circuit, in which a capacitor/diode arrangement causes the capacitors to be charged from a rectified A.C. voltage source in series, and discharged in parallel. These circuits are low in cost and durable, but suffer from performance limitations. In particular, while the power factor is approximately 0.90, the total harmonic distortion is on the order of 35 % .

Numerous other power control circuits exist which have attempted to exploit advantages of the foregoing approaches, but which have done so at the expense of circuit size, complexity, and/or performance. For example, a circuit disclosed in U.S. Patent 5,345,164 combines the parallel-discharge series-charge approach with the passive low pass filter approach. This combination permits the sizes of the inductors and capacitors in the passive low pass filter to be reduced, thereby reducing cost and weight. Although this circuit has a power factor of approximately 0.95 and a total harmonic distortion of approximately 14% , it suffers from the drawback of a power factor correction which is load-dependent. U.S. Patent 5,057,749 discloses a power control circuit wherein a sustaining, or "keep alive", capacitor receives and stores charge supplied from the lamp drive circuit during the high intervals of the supply voltage. The capacitor discharges to support operation of the lamp driver during the low intervals of the AC supply voltage. Although the circuit is simple, low in cost, and reduces the inrush current, it offers only limited performance, with a power factor of about 0.94 and about 40% total harmonic distortion.

U.S. Patent 4,808,887 discloses another power control circuit having a feedback capacitor which receives charge from the lamp driver oscillator and which provides the stored charge to increase the voltage stored on a main capacitor. While this circuit offers generally good performance at a low cost, it suffers from limited performance in dimming applications and has a relatively large inrush current.

U.S. Patent 5,063,331 discloses a power control circuit which includes an extra winding associated with the lamp driver circuit for charging a "keep alive" capacitor, and also includes a diode bridge configured in a feedback loop from the lamp driver circuit. However, this circuit is highly complex and requires additional windings and other parts.

It would therefore be desirable to provide a simple AC power control circuit which can be used in a gas-discharge lamp ballast to control the current supplied to the lamp, and which can offer a high power correction factor, low total harmonic distortion, limited inrush current, light weight, and low cost. Further, since typical low cost power factor correction circuits offer good performance only for specific input voltages and load values, it would be desirable to provide a voltage control circuit which can offer consistent performance for many combinations of input voltages and load values, so that high performance can be achieved for variable load conditions, such as those which exist when the circuit is used as a dimming ballast application.

Summary of the Invention

The present invention is directed to overcoming the above-noted problems, while at the same time providing other advantages, with an apparatus and method for controlling the current supplied to a load such as a lamp. According to exemplary embodiments, the apparatus includes a voltage source for providing a rectified A.C. voltage on a supply line and a coupling means for coupling the supply line to a modulation circuit. The modulation circuit has alternately conducting switches for controlling the direction of current flow through the primary winding of a transformer. The current induced in the secondary winding of the transformer provides power to the lamp. Alternate exemplary embodiments may also include a keep-alive circuit in which a charging capacitor receives and stores a charge from a current flowing through the load, which is preferably the current flowing through the primary winding of the transformer, during intervals when the supply voltage supplied to the coupling means exceeds a threshold level. The charging capacitor discharges to provide current to the modulation circuit and transformer, and thus support the operation of the ballast, during intervals when the supply voltage is below the threshold level. Alternate embodiments may further include a feedback portion in which a feedback capacitance is connected between the transformer and the input to the coupling means. The feedback capacitance maintains current flow through the coupling means by receiving and storing charge pulses from the supply line when the current through the primary winding flows in a first direction, and discharging the stored feedback charge when the current through the primary winding flows in the opposite direction.

Exemplary embodiments of the present invention require no additional windings, and are therefore simple and low in cost. Further improved performance can be attained using known AC voltage control circuits. More particularly, exemplary embodiments of the present invention using the combination of the keep-alive circuitry with the simple feedback circuit achieves a power correction factor of about 0.98 or greater and a total harmonic distortion of only about 12% or less. Because the power factor correction performed by the keep-alive and feedback circuits is derived from the current through the primary winding of the transformer, a current which is proportional to the lamp current induced in the secondary winding, the power factor remains high for variable loads, such as when the lamp current is decreased during a dimming operation.

Exemplary embodiments of the present invention may further include a control interface for controlling the oscillation frequency of an modulation circuit including a current sampling resistor for sampling the current through a load, a transformer for generating a transformer current proportional to the sampled current, a rectifier network for generating a rectified voltage from the transformer current, an optocoupler for comparing the rectified voltage to a threshold voltage to generate a control current, and an oscillation controller for changing the oscillation frequency of the modulation circuit in response to changes in the control current.

Brief Description of the Drawings

The present invention will be more fully understood with reference to the following description and accompanying drawings, wherein like elements are provided with like reference numerals, and in which:

FIG. 1 is a block diagram of a ballast incorporating a voltage control circuit according to the present invention; and

FIG. 2 is a more detailed schematic diagram of the circuit of FIG. 1.

Detailed Description of the Preferred Embodiments Referring now to FIG. 1, a ballast incorporating a voltage control circuit according to the present invention is shown. In the ballast circuit, an AC voltage supply 100 supplies an alternating current voltage on an active or "hot" line 102 and a neutral line 104. Hot line 102 and neutral line 104 supply A.C. voltage to electro-magnetic interference (EMI) filter 106, which outputs a filtered AC voltage onto second active and neutral lines 108 and 110. Lines 108 and 110 provide a filtered AC voltage input to diode bridge 112, which rectifies the AC voltage to provide a pulsed D.C. voltage on third active line 114 and third circuit common line 116. The pulsed D.C. voltage on active line 114 is provided as an input to diode 118, which functions as a coupling means to supply current on active line 114 to input line 119 of modulation circuit 120. It will be appreciated that a transistor or other suitable coupling means may be used instead of diode 118. Modulation circuit 120 controls the direction of the current flow through the primary winding 122 of transformer 124. Current flowing through the primary winding 122 of transformer 124 causes an induced current, proportional to the current flowing through primary winding 122, to flow through secondary winding 126 of transformer 124. The induced current through secondary winding 126 provides power to a load such as a gas-discharge lamp 128. It will be appreciated that while the circuit of FIG. 1 is described in terms of a lighting control circuit, the circuit may be used in a variety of applications, and is not limited to lighting control. It will be further appreciated that a plurality of lamps may be driven by the ballast circuit of the present invention.

The ballast circuit of FIG. 1 also includes a control interface 130 for comparing a voltage representative of the current supplied to lamp 128 with a threshold voltage. Based on this comparison, the interface 130 outputs a control signal on line 131 to oscillator 132. Oscillator 132 determines the frequency at which modulation circuit 120 changes the direction of the primary winding current based on the control signal on line 131. Modulation circuit 120 includes first and second switching elements which operate 180° out of phase with each other to control the direction of the current flowing through primary winding 122 of transformer 124. If the comparison made by control interface 130 indicates that the lamp needs to be dimmed, for example, the control signal causes oscillator 132 to supply a signal on line 228 which increases the oscillation frequency of the switching elements of modulation circuit 120, thus causing the current through primary winding 122 to change directions more frequently and reducing the current supplied to the lamp 128 to cause the lamp to dim. It will be appreciated that other power control conventions may be used. EMI filter 106, diode bridge 112, modulation circuit 120, control interface 130, and oscillator 132 will be described in detail later.

The ballast circuit of FIG. 1 further includes a keep-alive circuit 135 which may be implemented by diodes 136 and 138, and charging capacitance 140, or other suitable elements. In the embodiment of FIG. 1, diodes 136 and 138 are connected in series between the primary winding 122 of transformer 124 and the output of diode 118. Capacitance 140 is connected between a node 142, defined at the connection between diodes 136 and 138, and circuit common line 116. When the voltage on active line 114 exceeds a threshold level, the voltage on line 114 supplies power to modulation circuit 120 and transformer 124 to drive the lamp 128. During the interval when the voltage on active line 114 exceeds the threshold voltage and when the current through primary winding 122 flows in a direction from modulation circuit 120 to diode 136, charge capacitance 140 receives and stores a charge. When the voltage on line 114 falls below the threshold level and the current through primary winding 122 flows in the direction from modulation circuit 120 toward diode 136, the charge stored by capacitance 140 is discharged through diode 138 to line 119 and modulation circuit 120 to drive the lamp 128. The operation of keep alive circuit 135 will be described in more detail below.

The ballast circuit of FIG. 1 further includes a feedback circuit 137 connected between the primary winding 122 of transformer 124 and active line 114. The feedback circuit 137 includes a feedback capacitance 134 or other suitable elements. Modulation circuit 120 operates in cycles, and each cycle includes two half-cycles. In one half-cycle of each cycle, current flows through primary winding 122 of transformer 124 in the direction from modulation circuit 120 to keep-alive circuit 135. In the other half-cycle of each cycle, current flows through primary winding 112 of transformer 124 in a direction from feedback circuit 137 to modulation circuit 120. During the half cycle in which the current through primary winding 122 flows in the direction from feedback circuit 137 to modulation circuit 120, feedback capacitance 134 draws a current pulse from active line 114 and stores a charge. During the half-cycle in which the current through primary winding 122 flows in the direction from the modulation circuit 120 to the keep-alive circuit 135, feedback capacitance 134 discharges the stored feedback charge to active line 114. Feedback circuit 137 draws only a small amount of current from the circuit, and therefore has a significant effect on the operation of the circuit only at the zero crossings of the A.C. supply voltage. The operation of the feedback circuit will be described in more detail below.

Referring now to FIG. 2, a more detailed schematic diagram of the ballast circuit of FIG. 1 is shown. It will be appreciated that many of the elements of the ballast circuit shown in FIG. 2 are described in detail in a copending application entitled "Control Circuit With Improved Functionality For Non-Linear and Negative Resistance Loads", assigned to the assignee of the present invention, the entirety of which is hereby incorporated by reference. As shown in FIG. 2, EMI filter 106 includes inductances 202, 204 and capacitance 206. Inductances 202 and 204 have inductance values of approximately 700 microhenries each, and capacitance 206 has a value of approximately .47 microfarads in the preferred embodiment. It will be appreciated that other types of filters, other filtering elements, or other capacitance and inductance values may be suitable to provide a filtered supply voltage for use in the present invention.

Diode rectifier bridge 112 includes diodes 208, 210, 212, and 214. Diode 208 is connected between neutral line 110 and active line 108, and diode 210 is connected in a forward conducting direction on active line 108. Diode 212 is connected between neutral line 110 and active line 114, in parallel to diode 208. Diode 214 is connected between neutral line 110 and circuit common line 116. It will be appreciated that other diode networks or other rectifier elements may be suitable to provide a rectified A.C. voltage for use in the ballast circuit of the present invention. It will be further appreciated that A.C. voltage supply 100, filter 106, and diode bridge 112 may be replaced by any suitable source of rectified A.C. voltage, such as a source of pulsed D.C. voltage.

The rectified A.C. voltage is supplied through active line 114 and diode 118 to modulation circuit 120. Modulation circuit 120 is of a type generally known in the art and includes a filtering capacitor 222 connected between input line 119 and circuit common line 116. Diodes 224 and 226 are connected in series from circuit common line 116 to input line 119, and the modulation circuit 120 also includes series-connected capacitances 225 and 227. These elements 224-227 prevent damage from occurring to the circuit if a lamp is removed while the circuit is operation. While modulation circuit 120 also includes a number of additional supporting components, the operation of the modulation circuit 120 will be described with reference only to the most significant components.

Modulation circuit 120 receives an oscillating input signal from oscillator 132 on line 228. The oscillating input signal oscillates between high and low levels at a frequency determined by oscillator circuit 132, and this frequency determines the oscillation frequency of switching elements 232 and 236 of modulation circuit 120. Specifically, the oscillating input signal on line 228 is provided to the gate of transistors 230, and to the gate of transistor 232 through resistor 231. When the input on line 228 from oscillator 132 is high, transistors 230 and 232 are rendered conductive, causing transistor 234 to be rendered conductive and transistor 236 to be rendered nonconductive. When the input on line 228 from oscillator 132 is low, transistors 230, 232, and 234 are rendered nonconductive, and transistor 236 is rendered conductive via transistor 238. Thus, transistors 236 and 232 are rendered conductive 180° out of phase with each other to change the direction of the current flowing through primary winding 122. The switching frequency at which transistors 236 and 232 are alternately rendered conductive and nonconductive is independent of the frequency of the input supply voltage 100 or the rectified A.C. voltage on active line 114. As the switching frequency of transistors 232 and 236 is increased, the induced current through lamp 128 will decrease, and the lamp will dim. As the switching frequency of transistors 232 and 236 is increased, the induced current through lamp 128 will be increased, and the lamp will brighten. It will be appreciated that an alternate convention may be used to implement the present invention.

When transistor 236 is conductive and transistor 232 is nonconductive, current flows from input line 119 through transistor 236, inductance 240, and primary winding 122 of transformer 124. When the current through primary winding 122 is flowing in this direction, feedback circuit 137 discharges the charge stored by capacitance 134 to the active line 114. If, during the interval while transistor 236 is conductive and transistor 232 is nonconductive, the rectified A.C. voltage on active line 114 exceeds the threshold level, the current flowing through input line 119, transistor 236, and primary winding 122 is provided from line 114 through diode 118. The current flowing through primary winding 122 is provided to the keep-alive circuit 135, in which charge is accumulated and stored by capacitance 140. If, during the interval while transistor 236 is conductive and transistor 232 is nonconductive, the rectified A.C. voltage on line 114 is less than the threshold level, the current through input line 119, transistor 236, and primary winding 122 is provided from the charging capacitor 140 of keep-alive circuit 135.

When transistor 236 is nonconductive and transistor 232 is conductive, the current flow through the primary winding 122 is reversed such that current flows from line 114 through feedback circuit 137, primary winding 122 of transformer 124, inductance 240, and transistor 232 to circuit common line 116. When the current through primary winding 122 flows in this direction, feedback circuit 137 draws a current pulse from active line 114 and stores a charge on feedback capacitance 134. If, while transistor 236 is nonconductive and transistor 232 is conductive, the rectified A.C. voltage on active line 114 exceeds the threshold level, current does not flow through line 119 or diode 118. Since transistor 236 is nonconductive, keep-alive circuit 135 receives no current and the charge on its capacitor remains substantially the same. If, while transistor 236 is nonconductive and transistor 232 is conductive, the rectified A.C. voltage on active line 114 is below the threshold level, the current does not flow through line 119, and the charge stored by capacitance 140 remains substantially the same.

It will be appreciated that the modulation circuit 120 shown in FIG. 2 is but one example of a suitable modulation circuit, and that many other circuits may be suitable for achieving the purpose of controlling the current flow through the primary winding 122 of transformer 124. The current flow through primary winding 122 of transformer 124 causes an induced current to flow through secondary winding 126 of transformer 124 to provide power to the lamp 128.

The keep-alive circuit 135 includes diodes 136 and 138 connected in series between the primary winding 122 of transformer 124 and active line 119. Charging capacitance 140 is connected between node 142, defined by the output of diode 136 and the input of diode 138, and circuit common line 116. As shown in FIG. 2, keep-alive circuit 135 may further include inductance 270 connected between capacitance 140 and the node between diodes 136 and 138. Keep-alive circuit 135 may also include a second capacitance 272. It will be appreciated that inductance 270 and second capacitance 272 form a high pass filter which functions to reduce high frequency ripple currents which might flow from capacitance 140 during discharge. The high pass filter formed by inductance 270 and capacitance 272 is optional, and these elements may be omitted or replaced by other suitable filtering elements. In the preferred embodiment, capacitance 140 is approximately 100 microfarads, inductance 270 is approximately 220 microhenries, and second capacitance 272 is approximately .47 microfarads. Keep-alive circuit 135 may also include an overvoltage protection circuit 274, which may include resistors 276, 278, and diode 280. When a short circuit condition occurs in the lamp 128, current is not dissipated and charge continues to accumulate in charging capacitance 140. When the charge stored on charging capacitance 140 exceeds a threshold level, the current flowing through overvoltage protection circuit 274 to oscillator 132 will increase to indicate a short circuit condition. Oscillator 132 can then suspend operation of the modulation circuit 120. During the intervals when transistor 236 is conductive, charging capacitance 140 receives a current pulse from primary winding 122 through diode 136, and stores this charge. During the intervals when transistor 232 is conductive and the current through primary winding 122 is reversed, charging capacitor 140 receives no current pulse. Charging capacitance 140 is charged to a level which is about 50% of the peak level of the rectified A.C. voltage on active line 114. When the rectified A.C. voltage on active line 114 exceeds the charge stored by capacitance 140, the current on line 114 is supplied through diode 118, line 119, modulation circuit 120, and primary winding 122 to cause an induced current to flow through secondary winding 126 and provide power to lamp 128. When the rectified A.C. voltage on active line 114 falls below the charge stored on capacitance 140 (e.g., the voltage on active line 114 drops below 50% of its peak value), the charge stored on charging capacitance 140 is discharged (optionally filtered by inductance 270 and second capacitance 272) through diode 138, line 119, modulation circuit 120, and primary winding 122 of transformer 124. Thus, keep-alive circuit 135 functions to reduce deviations in the current supplied to modulation circuit 120 on active line 119. By reducing deviations of the current supplied on active line 119, a high power factor (about 0.94) may be maintained as the frequency of the direction change of the current flowing through primary winding 122 is adjusted to control the dimming or brightening of the lamp 128.

The feedback circuit 137 includes a first feedback capacitance 134 connected between the primary winding 122 of the transformer 124 and the input of diode 118 on active line 114. Feedback circuit 137 may also include a filtering capacitance 237 to enhance the performance of the feedback circuit 137, but this is not essential to the operation of the circuit. Capacitance 134 may have a value of approximately .15 microfarads, and filtering capacitance 237 may have a value of approximately .047 microfarads. Feedback capacitance 134 stores charge from active line 114 during intervals when transistor 232 is conductive, as current flows from active line 114 through feedback circuit 137, primary winding 122, inductance 240, and transistor 232. Feedback capacitance 134 discharges the stored feedback charge during intervals when the transistor 236 is conductive, as current flows from active line 114 through diode 118, transistor 236, inductance 240, primary winding 122 and keep-alive circuit 135. The implementation of the feedback circuit 137 in conjunction with the keep-alive circuit 135 enables a power factor of approximately 0.98 or more to be achieved, and the total harmonic distortion of the circuit may be reduced to about 12% or less. Because the feedback control and keep alive functions are accomplished using the current flowing through the primary winding 122, which is proportional to the current flowing through secondary winding 126 and lamp 128, the high power factor and low total harmonic distortion may be maintained during dimming or brightening of the lamp 128.

It will be appreciated that feedback circuit 137 draws a relatively small percentage (e.g., about 20%) of the peak current flowing through active line 114. Because the current drawn by feedback circuit 137 is relatively small, feedback circuit 137 has a significant effect on the power factor and total harmonic distortion only at the zero crossings of the A.C. input voltage. During these periods, feedback circuit 137 functions to force current to flow through active line 114 and smooth the effects of the current direction reversal through the primary winding 122. As described above, the feedback circuit 137 in combination with the keep-alive circuit 135 provides a very high power correction factor and low total harmonic distortion.

Control interface 130 and oscillator 132 control the oscillation frequency at which transistors 236 and 232 are rendered conductive and nonconductive. Control interface 130 compares a voltage representative of the current flowing through secondary winding 126 and lamp 128 to a threshold level, and outputs a control signal on line 131 to oscillator 132. The control signal on line 131 causes the oscillator 132 to change the switching frequency of transistors 232 and 236 of modulation circuit 120 by means of a control signal on line 228. The details of control interface 130 will now be described. Control interface 130 includes a current sampling resistor 242 connected between secondary winding 126 of transformer 124 and lamp 128. The current flowing through current sampling resistor 242 is applied to the primary winding of a transformer 244 to cause an induced current, proportional to the current flowing through current sampling resistor 242 to flow through the secondary winding of transformer 244. As shown in FIG. 2, the current flowing through current sampling resistor 242 from the secondary winding 126 is returned to the lamp 128. The current flowing through the secondary winding of transformer 244 is rectified by a diode network 246, which includes diodes 248, 250, 252, and 254, along with zener diode 256, and a rectified voltage proportional to the lamp current is generated across lines 258 and 260. An optocoupler 262 includes a diode 264 connected to line 258, and a transistor 266 which has a collector connected to receive current from active line 119 through a resistor 267 and an emitter which provides the control signal on line 131 to oscillator 132. Diode 264 is connected in series with a capacitor 268 between line 258 and line 260. Diode 264 and capacitor 268 are in parallel with a photosensor 269 through a diode 271. The photosensor 269 may be implemented by any device which outputs a voltage or current signal in proportion to the intensity of received input light, such as a 4N25 photosensor known in the art. A single photosensor may be used in connection with any number of ballast circuits. When the level of light received by photosensor 269 is increased, the photosensor output decreases. If the output of the photosensor 269 is less than the rectified voltage proportional to the lamp current across diode 264 and capacitor 268, then additional current from the photosensor 269 will flow through diode 264. The current through optocoupler diode 264 controls the conduction of optocoupler transistor 266, such that an increase in current flow through diode 264 will cause an increase in the conduction of transistor 266 to cause an increased current to flow from active line 119 tiirough transistor 266 and control signal line 131 to oscillator 132. It will be appreciated that the optocoupler 262 may be implemented by other suitable devices, and that the control interface 130 may be implemented by any of a number of suitable circuits to compare the lamp current to a photosensor output.

Oscillator 132 is well-known in the art, and may be implemented by any number of circuits which receive a variable input signal, such as the control signal on line 131, and generate an output oscillating signal having a frequency indicative to the magnitude of the control signal. In the preferred embodiment of the present invention, oscillator 132 may be implemented by a UC3884 integrated circuit, along with necessary supporting circuitry as will be appreciated by one of ordinary skill in the art. It will be appreciated that the specific capacitance and inductance values included in the foregoing description assume an input voltage of 120 volts A.C. and the load 128 includes two lamps. It should be readily apparent that the capacitance and inductance values will necessarily change if the load or input voltage is changed. It will be further appreciated that digital as well as analog circuitry may be used to implement the present invention.

While the foregoing description includes many specificities, it is to be understood that the disclosed embodiments are for purposes of explanation and illustration only. Many modifications, some of which have been suggested in the above description and many others not, will be readily apparent to those of ordinary skill in the art which do not depart from the spirit and scope of the invention, as defined by the appended claims and their legal equivalents.

Claims

WHAT TS CLAIMED IS:

1) A circuit for controlling the current applied to a load, comprising: a voltage supply for supplying a rectified A.C. voltage on a voltage supply line; coupling means for coupling the voltage supply line to the load; and a charging capacitor for storing charge from a current through the load during an interval when the rectified A.C. voltage is above a threshold level and discharging the stored charge to the voltage supply line when the rectified A.C. voltage is below the threshold level.

2) The circuit of claim 1, wherein the threshold level is approximately 50% of the peak value of the rectified A.C. voltage.

3) The circuit of claim 1 , wherein the charging capacitor is approximately 100 microfarads.

4) The circuit of claim 1, wherein the coupling means includes a diode.

5) The circuit of claim 1, wherein the load is a lamp control ballast including a transformer having primary and secondary windings.

6) The circuit of claim 5, further comprising a first diode connected between the primary winding and the charging capacitor and a second diode connected between the charging capacitor and the voltage supply line.

7) A method for controlling a current applied to a load, comprising the steps of: generating a rectified A.C. voltage; supplying the rectified A.C. voltage to the load through a coupling means; storing, in a charging capacitor, charge from a current through the load when the rectified A.C. voltage exceeds a threshold level; and discharging the stored charge to the coupling means when the rectified A.C. voltage is below the threshold level.

8) The method of claim 7, wherein the load includes a transformer having a primary winding and a secondary winding, and wherein the current through the load is the current through the primary winding.

9) The method of claim 7, wherein the threshold level is approximately

50% of the peak value of the rectified A.C. voltage.

10) The method of claim 7, wherein the capacitor is approximately 100 microfarads.

11) The method of claim 7, wherein the step of storing is performed through a first diode and the step of discharging is performed through a second diode.

12) A circuit for controlling the current applied to a load, comprising: a voltage supply for supplying a rectified A.C. voltage on a voltage supply line; coupling means for coupling the voltage supply line to the load; and a feedback capacitance for storing a feedback charge received from a current through the voltage supply line when a current through the load flows in a first direction, and discharging the stored feedback charge to the coupling means when the current through the load flows in a second direction. 13) The circuit of claim 12, wherein the load is a lamp control ballast which includes a transformer having primary and secondary windings, and the feedback capacitance is connected between the primary winding and the coupling means.

14) The circuit of claim 12, wherein the threshold level is approximately

50% of the peak level of the rectified A.C. voltage.

15) The circuit of claim 12, wherein the feedback capacitance includes a .15 microfarad capacitor.

16) the circuit of claim 12, wherein the coupling means includes a diode.

17) A method for controlling the current applied to a load, comprising the steps of: generating a rectified A.C. voltage on a voltage supply line; coupling the voltage supply line to the load through a coupling means; storing a feedback charge from a current through the voltage supply line when a current through the load flows in a first direction; and discharging the feedback charge to the coupling means when the current through the load flows in a second direction.

18) The method of claim 17, wherein the load is a lamp control ballast which has a transformer having a primary winding and a secondary winding, and wherein the step of storing a feedback charge is performed by a feedback capacitance connected between the primary winding and the coupling means.

19) A circuit for controlling the current applied to a load, comprising: a voltage supply for supplying a rectified A.C. voltage to the load on a supply line; a charging capacitor for storing charge received from a current flowing through the load in a first direction when the rectified A.C. voltage exceeds a threshold level and for discharging the stored charge to the load when the current through the load is flowing in the first direction and the rectified A.C. voltage is below the threshold level; and a feedback capacitor for storing a feedback charge received from the supply line when the current through the load is flowing in the first direction and for discharging the stored feedback charge to the voltage supply line when the current through the load is flowing in a second direction.

20) The circuit of claim 19, wherein the load is a lamp control ballast including a transformer having a primary winding and a secondary winding, the feedback capacitor is connected between the primary winding and the supply line, and the current through the load is the current through the primary winding of the transformer.

21) The circuit of claim 19, wherein the threshold level is approximately 50% of the peak level of the rectified A.C. voltage.

22) A circuit for controlling the oscillation frequency of an modulation circuit, comprising: a current sampling resistor for sampling the current through a load; a transformer for generating a transformer current proportional to the sampled current; a rectifier network for generating a rectified voltage from the transformer current; an optocoupler for comparing the rectified voltage to a threshold voltage, the optocoupler including a diode for conducting a diode current indicative of the rectified voltage and a transistor for conducting a control current, the diode current controlling the amount of control current flow through the transistor; and an oscillation controller for causing oscillations of the modulation circuit to occur at an oscillation frequency, the oscillation frequency changing in response to changes in the control current flow through the transistor.