MAGNETIC BALLAST ADAPTER CIRCUIT

Abstract: A method and apparatus for efficiently driving a load coupled to a ballast is described. A switching element coupled between a pair of ballast terminals converts a low frequency drive signal to a high frequency drive signal which is provided to the load.

Description: MAGNETIC BALLAST ADAPTER CIRCUIT FIELD OF THE INVENTION This invention relates generally to lighting systems and more particularly to methods and apparatus for starting and operating light sources. BACKGROUND OF THE INVENTION As is known in the art, a light source or lamp generally refers to an electrically powered man made element which produces light having a predetermined color such as a white or a near white. Light sources may be provided, for example, as incandescent light sources, fluorescent light sources and high-intensity discharge (HID) light sources such as mercury vapor, metal halide, high-pressure sodium and low-pressure sodium light sources. As is also known, fluorescent and HID light sources are driven by a ballast. A ballast is a device which by means of inductance, capacitance or resistance, singly or in combination, limits a current provided to a light source such as a fluorescent or a high intensity discharge light source, for example. The ballast provides an amount of current required for proper lamp operation. Also, in some applications, the ballast may provide a required starting voltage and current. In the case of so- called rapid start lamps, the ballast heats a cathode of the lamp prior to providing a strike voltage to the lamp. As is also known, a relatively common ballast is a so-called magnetic or inductive ballast. A magnetic ballast refers to any ballast which includes a magnetic element such as a laminated, iron core or an inductor. Magnetic ballasts are typically reliable and relatively inexpensive and drive lamps coupled thereto with a signal having a relatively low frequency. One problem with magnetic ballasts, however, is that the relatively low frequency drive signal which they provide results in a relatively inefficient lighting system. Furthermore, magnetic ballasts tend to incur substantial heat losses which further lowers the efficiency of lighting systems utilizing a low frequency magnetic ballast. In addition to efficiency, it is desirable, in some applications, to provide an instant-start lamp capability. Instant-start capability refers to the capability of starting a lamp within 50 milli-seconds (msec) of the time a strike voltage is provided to the lamp. To accomplish this, the ballast must provide a strike voltage typically in the range of about 500 V RMS. Magnetic ballasts are unable to produce a strike voltage which is large enough to cause the lamp to operate in an instant-start mode. In an attempt to overcome the low efficiency problem caused by the low frequency operating characteristic of magnetic ballasts as well as the inability to operate in an instant-start mode, attempts have been made to replace magnetic ballasts with electronic ballasts. Electronic ballasts drive the lamps with relatively high frequency drive signals and can provide strike voltages which allow instant-start lamp operation. One problem with electronic ballasts, however, is that they, however, utilize a relatively large number of circuit components which reduces reliability and maintainability of the electronic ballast while increasing cost of the ballast. Furthermore, in a lighting unit which includes a light source, a reflector, a housing and a magnetic ballast, it is relatively expensive to replace the magnetic ballast with an electronic ballast. This is due, at least in part, to the costs associated with the electronic ballast itself as well as the cost of labor required to physically disconnect the magnetic ballast from the lamp and electrically connect the electronic ballast to the lamp. Furthermore, some ballasts include a potting material which may include hazardous materials such as PCBs and asbestos. It is necessary to dispose of such hazardous waste materials in a particular manner and such disposal costs are significant. It would therefore, be desirable to provide a relatively inexpensive circuit which allows a magnetic ballast to provide high frequency operation of a light source and which is relatively easy and inexpensive to couple to existing light units which utilize magnetic ballasts. SUMMARY OF THE INVENTION In accordance with the present invention, a ballast adapter circuit for providing an excitation signal from a ballast source to a load includes a rectifier circuit having a first pair of terminals coupled to the ballast source and a second pair of terminals, a control circuit having a first and second terminal coupled to corresponding ones of the second pair of rectifier circuit terminals and an output terminal and a switching element having a pair of terminals coupled to the second pair of terminals of the rectifier circuit and a control terminal coupled to the output terminal of the control circuit. With this particular arrangement, an adapter circuit which converts a magnetic ballast signal having a relatively low frequency characteristic to a lamp drive signal having a relatively high frequency characteristic is provided. In one particular embodiment, the load is provided as a fluorescent lamp or a high energy discharge lamp and the control circuit is provided as a pulse width modulator circuit which controls the duty cycle of the switching element. The switching element is operated at a duty cycle which chops the supply current through the fluorescent lamp resulting in high frequency operation of the fluorescent lamp. High frequency operation of the lamp results in improved lighting efficiency and extends the operating life of the lamp. The rectifier circuit may be provided as a full wave bridge rectifier and the switch may be provided as a field effect transistor and in particular as a metal oxide semiconductor field effect transistor (MOS FET). The adapter circuit may also include a bias circuit coupled between the rectifier circuit and the pulse width modulator circuit. The bias circuit couples a portion of the rectified signal from the rectifier circuit and provides a supply signal to the pulse width modulator circuit. By generating a local DC supply voltage from the existing power supply lines, the adapter circuit is self powered and thus does not require any external DC supply source such as a battery. Moreover, by coupling the adapter circuit across the ballast source, there is no need to replace the ballast thus eliminating the need for disposing of the ballast. In accordance with a further aspect of the present invention, a method for coupling a source signal from a power source to a light source through a magnetic ballast includes the steps of providing the source signal to first and second input terminals of a magnetic ballast, providing, at first and second output terminals, a ballast signal having a first ballast signal frequency and a first ballast signal amplitude and chopping the first ballast signal to provide a light source drive signal having a drive signal frequency which is greater than the first ballast signal frequency. With this particular arrangement, a method of efficiently operating a light source is provided. The chopping step may be implemented by alternately providing low and high impedance signal paths between first and second output terminals of a magnetic ballast. Such operation may be accomplished by relatively biasing a switching element between its conduction and non-conduction states. In one particular embodiment, the chopping step includes the steps of rectifying the ballast signal, generating, from the rectified ballast signal, a control signal having a predetermined frequency and biasing a switching element between a conduction state and a non-conduction state at a frequency corresponding to the control signal frequency. The rectifying step may be accomplished by providing the ballast signal to a first pair of terminals of a rectifier circuit. The switching element may be provided as a switching transistor having a pair of bias terminals coupled to a second pair of terminals of the rectifier and a control terminal to which is provided the control signal. The switching transistor is switched between its conduction and non-conduction states to thus chop the current through the lamp. In accordance with a still further aspect of the present invention, an adapter circuit includes a rectifier circuit having a first pair of terminals coupled to first and second terminals of the adapter circuit and a second pair of terminals, a switching element having first and second switch terminals coupled to the second pair of terminals of the rectifier circuit and a control terminal and a controller having a first terminal coupled to the switching element control terminal, the controller for switching the switching element between a first state and a second state at a switch duty cycle. The adapter circuit further includes a signal detector coupled to the rectifier circuit for providing a detection signal in response to a drive signal having a predetermined signal level, a feedback signal generator coupled to the signal detector to receive the detection signal and to generate a feedback signal in response thereto, a comparator ' coupled to the feedback signal generator for comparing a reference signal to the feedback signal and for providing an output signal having a predetermined signal level based on the result of the comparison and means coupled to the controller for reducing the switch duty cycle in response to the capacitor output signal having a first one of first and second signal amplitudes. With this particular arrangement, a ballast adapter circuit which provides a relatively high frequency drive signal to a lamp and detects the removal of a lamp from a magnetic ballast circuit is provided. By detecting the removal of the lamp from the ballast circuit and reducing a switch duty cycle in response thereto, the controller effectively stops switching the switching element and thus prevents relatively high current signals from propagating through the switch thereby preventing a breakdown of the switch. This results in a relatively reliable adapter circuit. When the lamp is removed, the detector circuit acts as a peak voltage detector and the feedback signal generator provides a reference voltage to an input port of the comparator which compares the input voltage to a predetermined threshold voltage. In one embodiment, if the threshold voltage is greater than the feedback voltage then the pulse width modulator adjusts the duty cycle of the switching element accordingly to prevent the breakdown of the switching element. In accordance with a still further aspect of the present invention, a method for detecting removal of a lamp from a lighting unit includes the steps of detecting a drive signal, providing a protection signal in response to the drive signal having a predetermined signal level which at least equals a first threshold signal level, generating a feedback signal having a feedback signal characteristic corresponding to a signal characteristic of the detection signal, comparing the feedback signal to a reference signal, and changing the switch duty cycle in response to the comparison of the feedback signal and the reference signal. With this particular arrangement, a method of detecting lamp removal is provided. In accordance with a still further aspect of the present invention, an apparatus for providing a magnetic ballast as an instant-start magnetic ballast includes a switching element having a pair of switch terminals coupled to a pair of magnetic ballast output terminals and a control terminal coupled to an output terminal of a controller which provides a switching signal having a predetermined frequency to the switch control terminal. In accordance with this particular arrangement, an instant-start magnetic ballast is provided. When a light source is coupled to the ballast, the switching element is effectively coupled in parallel with the lamp. When the lamp is initially off, the controller biases the switching element into a first state in which the switching element provides a signal path having a relatively low impedance characteristic (e.g. a short circuit impedance characteristic) between the ballast terminals thereby effectively bypassing the lamp. This causes a charging current to flow through the ballast from a power source. The magnetic ballast, having a relatively large storage capacity, stores energy therein. Thus, with the switching element in its first state, the switching element provides a lamp bypass signal path and the ballast stores energy therein. When the switching element is biased into a second state, the switching element provides a signal path having a relatively high impedance characteristic (e.g. an open circuit impedance characteristic) between the ballast terminals. When the switching element is biased into the second state, the lamp presents an impedance to the ballast and the energy stored in the ballast causes an instantaneous voltage to be generated across the magnetic ballast. Also, when the switching element is biased from its first low impedance state to its second high impedance state, the voltage across the ballast reverses polarity and the voltage generated from the energy stored in the ballast combines with the source voltage. When the amplitude of the voltage generated from the energy stored in the ballast is approximately equal to the source voltage amplitude, a strike voltage available to the load is effectively doubled thus providing a lamp strike up voltage which is sufficiently large to allow instant-start up of a fluorescent light source without first warming light source filaments. It should be noted .that the adapter circuit of the present invention is coupled to ballast terminals and provides a signal path which is in parallel with a lamp or other load with which it will operate. In conventional lighting systems, magnetic ballasts provide a relatively low frequency drive signal to the lamp. The magnetic ballasts, however, present a finite impedance across the lamp. This ballast impedance loading causes distortion of a waveform of the drive signal provided by the ballast to the lamp. This results in the ballast having a relatively low power factor. The adapter circuit of the present invention, however, cooperates with the ballast to provide a relatively high frequency drive signal to the lamp. The higher frequency operation increases the impedance presented by the ballast to the lamp such that the ballast does not impedance load the lamp. This reduces the effect of the ballast impedance on the waveform shape of the drive signal thereby reducing the amount of drive signal distortion and improving the power factor of the ballast system. Furthermore, like lamps operate on like drive signals throughout the world. Ballasts, however, must accept electrical power signals from particular ones of different AC power sources which exists in different countries. For example, one ballast may accept 120 volt 60 Hertz power signals and a different ballast may accept 277 volt 60 Hertz power signals. Both ballasts, however may be required to provide an output signal which can drive the same lamp worldwide. The adapter circuit of the present invention, being coupled across ballast output terminals and lamp input terminals, may be coupled to any ballast and operate with any lamp without any modification regardless of the type of electrical power source used to power the lamp. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing features of this invention as well as the invention itself may be more fully understood from the following detailed description of the drawings in which: FIG. 1 is a block diagram of a lighting system using the adapter circuit of the present invention; FIG. 1A is a diagrammatic block diagram of a lighting system using the adapter circuit of the present invention; FIG. 2 is a block diagram of an adapter circuit coupled across a pair of terminals of a lamp; FIG. 2 A is block diagram of an adapter circuit coupled across a pair terminals of a load; FIG. 3 is a schematic diagram of an adapter circuit; FIG. 4 is a block diagram of a lighting system which includes an adapter circuit of the present invention; FIG. 5 is a block diagram of an adapter circuit; FIG. 6 is a schematic diagram of an adapter circuit coupled across a pair of input terminals of a load; FIG. 7 is a schematic diagram of an adapter circuit implemented as an integrated circuit; FIG. 8 is a diagram of a fluorescent lamp interconnected to a ballast and an adapter circuit; and FIG. 9 is a diagram of a pair of fluorescent coupled to a magnetic ballast and an adapter circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, an illumination system 10 includes a power source 12 coupled through power supply lines 13a, 13b generally denoted 13 to a pair of input terminals 14a, 14b of a ballast 14. Power source 12 provides a power signal through power lines 13 to ballast 14. The power signal may be provided, for example, as a 120 volt (V) 60 Hertz (Hz) alternating current signal. Alternatively, signal source 12 may provide one of a 277 V-60 Hz, 240 V-60 Hz, 240 V-50 Hz or 120 V-50 Hz signals to ballast 14. Ballast 14 is provided as one of the types which includes a magnetic component. For example, ballast 14 may correspond to a so-called magnetic or inductive or core-call ballast. Or, alternatively, ballast 14 may be provided as the type which includes a core-coil choke having an electronic starter. Alternatively still, ballast 14 may be provided as a reactor or lag ballast, a lead ballast or a magnetic regulator ballast. A pair of magnetic ballast output terminals 14c, 14d are coupled to a light source or lamp 18. Lamp 18 may be provided, for example, as a fluorescent lamp provided from a tube containing mercury vapor, an inert gas (e.g. argon) and having disposed on an inner wall thereof, a phosphor coating which fluoresces or glows when subjected to radiation from a mercury discharge. The color of the light so-produced (usually white or near white) depends upon the phosphors in the phosphor coating. Alternatively, lamp 18 may be provided as a high intensity discharge (HID) lamp which like a fluorescent lamp, requires a ballast for starting and limiting current to the lamp. Lamp 18 may be provided, for example, as a mercury vapor, metal halide, high-pressure sodium or a low-pressure sodium lamp. Ballast 14 provides a relatively high voltage signal required to start the arc for mercury discharge. Ballast 14 subsequently limits the current provided to the lamp 18 to be of sufficient amplitude to maintain the mercury discharge. Ballast 14 is coupled to lamp 18 at thermionic cathodes 18a, 18b with each of the cathodes having a pair of cathode terminals. Thus one of the terminals of thermionic cathode 18a is connected with ballast, output terminal 14c and one of the terminals of thermionic cathode 18b is connected with ballast output terminal 14d. Adapter circuit 16 is coupled in parallel between ballast 14 and lamp 18 as shown and includes a means for chopping the current to lamp 18. At a first predetermined period of time adapter circuit 16 provides relatively low impedance signal path between ballast terminals 14c, 14d and at a second predetermined period of time, adapter circuit 16 provides a relatively high impedance signal path between ballast terminals 14c, 14d. In one embodiment, the relatively low impedance corresponds to a short circuit impedance and the relatively high impedance corresponds to an open circuit impedance. When the current driven adapter circuit 16 provides the low impedance signal path between the ballast terminals, the adapter circuit 16 effectively bypasses the lamp load 18. Adapter circuit 16 thus drives current through lamp 18 at a so-called chopping frequency. This results in adapter circuit 16 driving lamp load 18 with a high frequency current signal which is superimposed on a low frequency current signal provided by ballast 14. Furthermore, the means for chopping the load current provides a continuous flow of sinusoidal low frequency currents through the magnetic ballast 14 and thus improves the power factor and total harmonic distortion performance of magnetic ballast 14. It should be noted that since adapter circuit 16 is coupled between ballast terminals 14c, 14d in parallel with lamp 18, in the event adapter circuit 16 ceases to operate, magnetic ballast 14 continues to drive lamp 18 and thus no loss of lighting capability is experienced by a user. Thus, when adapter circuit 16 is operational, a more efficient lighting system is provided. On the other hand, when adapter circuit 16 is not operational, light system 10 operates as any conventional lighting system which includes a magnetic ballast. The adapter circuit 16 can also improve the start up characteristics of ballast 14. Referring briefly to FIG. 1A, in which like reference elements of FIG. 1 are provided having like reference designations, ballast 14 is diagrammatically illustrated to include a pair of storage elements Cl, LI . Prior to the lamp starting, when the adapter circuit 16 provides the low impedance signal path between ballast terminals 14c, 14d the current flowing through the adapter circuit 16 causes an amount of energy to be stored in the inductive storage elements of magnetic ballast 14. On the other hand, when the adapter circuit 16 provides a relatively high impedance signal path between ballast terminals 14c and 14d, the energy stored in the magnetic ballast 14 generates a voltage superimposed on and about equal in amplitude to the output voltage thus doubling the striking voltage capability of the ballasting system. The increase in strike voltage provided by adapter circuit 16 thus allows the ballast 14 to operate as an instant-start magnetic ballast. For an instant-start application, one red wire e.g. wire 17c and one blue wire e.g. wire 17d are disconnected from the magnetic ballast 14. For a rapid start application, on the other hand, wires 17a-17d are coupled between the lamp 18 and the ballast 14 (i.e. two blue wires and two red wires). In this case, rapid start operation can be achieved by heating the lamp filaments prior to ballast 14 providing a strike voltage. Referring now to FIG. 2, a lighting unit 34 includes a ballast 35 having a pair of input terminals adapted to be coupled to a power source (not shown) and a pair of output terminals 35a, 35b coupled through signal paths 37a, 37b respectively to first and second terminals of a light source 42. A DC bias circuit 36 couples a portion of the signal propagating on signal paths 17a, 17b between the ballast and the load and generates a DC bias signal which is provided to a power terminal 38a of a controller 38. Controller 38 has an output terminal coupled to a control terminal 40a of a switching device 40. Switch arms 40b, 40c are coupled to the ballast signal paths 37a, 37b and thus switching device 40 is coupled in parallel with light source 42. DC bias circuit 36 receives an alternating current signal propagating between signal paths 37a, 37b and generates a DC bias signal from the AC signal. This may be accomplished, for example, by including in DC bias circuit 36, a rectifier circuit and filtering circuitry to generate a steady DC bias signal. Controller 38 provides a control signal to control terminal 40a of switching device 40. In response to the control signal having a first value, switching device 40 provides a relatively low impedance signal path between signal paths 37a, 37b. In response to the control signal having a second different value, switching device 40 is placed in a second switch state and provides a relatively high impedance signal path between signal paths 37a, 37b. Thus, by switching device 40 between its first and second switch states, switching device 40 alternately provides high and low impedance paths between signal paths 37a, 37b. Switching device 40 includes a switching element and circuitry which allows the switching element to switch on both positive and negative half cycles of the AC signal propagating between signal paths 37a, 37b. Such circuitry may include, for example, a rectifier circuit. Referring now to FIG. 2A, a portion of a lighting unit 20 includes an adapter circuit 16 having a pair of terminals 16a, 16b coupled to respective ones of signal paths 17a, 17b, generally denoted 17. A first end of signal path 17 is coupled to a ballast (not shown) to a lamp 32. Adapter circuit 16 includes a rectifier circuit 22 having a first pair of terminals 22a, 22b coupled to adapter circuit terminals 16a, 16b and a second pair of terminals 22c, 22d coupled to a DC bias circuit 24 at terminals 24a, 24b. DC bias circuit 24 receives a rectified signal from rectifier circuit 22 and at terminal 24c provides a DC bias signal to an input terminal 26a of a pulse width modulation controller 26. Pulse width modulation controller 26 is also coupled to terminals 22c, 22d of rectifier circuit 22. A control terminal 26d of controller 26 is coupled to a control terminal 30a of a switch 30. Switch inports 30b, 30c are coupled to terminals 22c, 22d of rectifier circuit 22. In operation, an alternating current (AC) signal is provided to rectifier circuit 22 at input ports 22a, 22b and a rectified signal is provided at output terminals 22c, 22d of rectifier circuit 22. DC bias circuit 24 couples a portion of the rectified signal and provides a DC bias signal which is used to provide power to controller 26 via input port 26a. Controller 26 also couples a portion of the rectified signal and provides a control signal having a predetermined frequency to control terminal 30a of switch 30. Switch 30 thus alternates between a conduction state and a non- conduction state at a pulse duty cycle determined by a pulse width modulation controller 26. In response to the control signal fed thereto, switch 30 alternately provides high and low impedance signal paths between rectifier circuit terminals 22c, 22d which results in a short circuit signal path being provided between terminals 16a, 16b of adapter circuit 16. This results in a chopping of the lamp drive signal being fed to lamp 32 at terminals 32a, 32b. A protection circuit 28 is coupled to an input port 26c of controller 26 and in response to lamp 32 being electrically decoupled from signal paths 17a, 17b, and thus from the ballast protection circuit 28 causes the pulse width modulation controller 26 to reduce the duty cycle of switch 30 to a relatively low value thereby effectively stopping the switching of switch 30. Referring now to FIG. 3, an adapter circuit 44 having terminals 44a, 44b optionally includes a protection device 46 having a first terminal coupled to terminal 44a and a second terminal coupled to a first terminal 48a of a rectifier circuit 48. Protection device 46 may be provided on a fuse or a circuit breaker, for example, and is serially coupled between terminal 44a and terminal 48a of rectifier circuit 48. Thus, in the event of a circuit company failure in adapter circuit 44, protection device 46 prevents excess current from flowing through adapter circuit 44. Rectifier circuit 48 is here provided as a full wave bridge rectifier including diodes D1-D4 coupled as shown. A second terminal 48b of rectifier 48 is coupled to adapter circuit terminal 44b. Rectifier circuit terminals 48c, 48d provide respective ones of positive and negative rails and are coupled to a bias circuit 50 provided from a capacitor Cl, a pair of diodes D5, D6, an inductor LI, and a capacitor C2 coupled as shown. An output port of the bias circuit 50 is coupled to a power terminal 52a of a controller circuit 52 which in this particular embodiment is provided as an integrated circuit. Controller 52 may be provided as a high performance current mode controller such as the types manufactured by Motoralla Company, Schaumborg, IL and identified as part numbers UC2844, UC2845, UC3844 and UC3845. Those of ordinary skill in the s * τt will appreciate that other controllers having similar functional and electrical characteristics may also be used. Those of ordinary skill in the art will also appreciate, of course, that the functions provided by controller 52 may also be provided from discrete circuit components rather than from an integrated circuit. An output terminal 52b of controller 52 is coupled to a switching element 54 which is here shown as a switching transistor 54. In one particular embodiment, transistor 54 is provided as a metal oxide semiconductor (MOS) field effect transistor (FET) having gate source and drain electrodes 56a, 56b and 56c, respectively. Transistor 54 may be provided, for example, as the type manufactured by National Rectifier and identified as part number IRF 730. Those of ordinary skill in the art will recognize, however, that other switching elements having similar switching speeds and power handling capabilities may also be used. Source electrode 54b is coupled to terminal 48c of rectifier circuit 48 and drain electrode 54c is coupled to ground through a bias resistor R7 as shown. A capacitor C5 is coupled between a cathode of diode D8 and ground as shown. In operation, full wave rectifier 48 generates a pulse voltage between terminals 48c, 48d. The pulse voltage is provided to DC bias circuit 50 which generates a relatively stable local DC power supply from the AC signal fed to rectifier input ports 48a, 48b. The local DC supply generated by DC bias circuit 50 is provided to power supply terminal 52a of controller 52 to thus power the controller 52. Controller 52 switches the switch 54 at a frequency which is preferably greater than a frequency which can be heard by humans. Thus, switch 54 is preferably switched at a frequency greater than 20 kilo-Hertz (kHz) and even more preferably at a frequency of about 60 kHz to thus avoid generating signals which interfere with infrared (IR) signals. The chopping frequency is thus selected to avoid producing an audible signal which can be heard by humans. By switching a switch 54 at a signal greater than 20 kHz, it is not necessary to drive a lamp (not shown) with which the adapter circuit 44 is coupled in parallel with a signal having a relatively pure sinusoidal waveform, since any harmonics generated by an asymmetric waveform shape of the control signal would be above frequencies which are audible to humans. During positive half cycles of the AC power signal provided at the output of the ballast, diodes D2 and D3 are biased into their conduction states. Thus, when switch 54 is biased into its conduction state, a low impedance signal path through diodes D2, D3 and switch 54 exists between terminals 44a, 44b. Thus during positive half cycles of the AC signal, diodes D2, D3 and switch 54 may provide a form a signal path which is in parallel with a lamp. Similarly, during negative half cycles of the AC signal, diodes DI, D4 are biased into their conduction states and when switch 54 is biased into its conduction state, a low impedance signal path exists between adapter circuit terminals 44a, 44b. Thus, during negative half cycles of the AC signal provided at the output of the ballast, diodes DI, D4 and switch 54 provide a signal path which is in parallel with a lamp. When switch 54 is biased into its conduction state, the parallel signal path is provided having a relatively low impedance characteristic in parallel with a lamp. Conversely, when switch 54 is biased into its non-conduction state, the parallel path is provided having a relatively high impedance characteristic in parallel with the lamp. Thus, by alternately biasing the switch 54 between its conduction and non- conduction states, a lamp or other load may be driven at a relatively high frequency which results in more efficient operation of the lamp or load. It should be noted that in this particular embodiment, since switch 54 is coupled to the ballast output signal paths and thus the lamp through the diodes of bridge rectifier 48, switch 54 performs both a switching function and a control function. A protection circuit 56 includes a detector diode D8 having an anode coupled to rectifier terminal 48c and a cathode coupled to a first terminal 57a of a voltage divider circuit 57 provided from resistors R4, R5. A second terminal 57b of the voltage divider circuit 57 is coupled to a reference potential here corresponding to ground and a third terminal 57c of voltage divider circuit 57 is coupled to a feedback voltage terminal 54c of controller 54. A stiffening capacitor C5 is coupled in parallel with voltage divider 57 with a first terminal of capacitor 65 coupled to the cathode of detector diode D8 and a second capacitor terminal coupled to the negative rail, as shown. The voltage divider 57 produces a feedback voltage at the third terminal thereof. The feedback voltage is coupled to feedback voltage terminal 54c of controller 54. When the feedback voltage at terminal 54c exceeds a predetermined threshold voltage, the controller 52 reduces the duty cycle at which switch 54 is operating until the duty cycle is such that switch 54 is effectively prevented from switching. Thus, when a lamp is de-coupled from the ballast lines, the feedback voltage rises above the threshold voltage thereby effectively maintaining the switch in a non-conduction state. In one particular embodiment, the threshold voltage is selected to be typically of about 2.5 volts (V). An optional diode D7 may be coupled between a FET electrode and the negative rail as shown to limit the duty cycle provided by controller 52 to a fixed duty cycle. The frequency and the maximum output duty cycle of the control signal provided at controller output terminal 52b are selected by appropriately selecting the resistance value of resistor Rl which is coupled to terminals 52d, 52e of controller 52. In one particular embodiment, resistor Rl may be provided having a resistance typically of about 10 K ohms which results in a chopping frequency typically of about 20 kHz. Decreasing the resistance value of resistor Rl increases the chopping frequency while increasing the resistance value of resistor Rl decreases the chopping frequency. A current sense terminal 52f is coupled through a resistor R6 to transistor drain electrode 54c. Controller 52 thus receives a signal having an amplitude proportional to the amplitude of the current signal through switch 54 and in response to a predetermined current amplitude, controller 52 terminates the transistor switch conduction by biasing transistor 54 into its non-conduction state. Referring now to FIG. 4, an illumination system 60 includes a power supply 62 coupled to a lamp 68 through a ballast 64 and an adapter circuit 66 . Adapter circuit 66 includes a switching device 67 which is coupled to lamp 68. Thus, in this particular embodiment, adapter circuit 66 is coupled between ballast 64 and lamp 68 and switching device 67 is disposed in a signal path which is parallel coupled to lamp 68. Ballast 64 receives a power signal from power supply 62 and provides a ballast output signal having a relatively low frequency to an input terminal of adapter circuit 66. Adapter circuit 66 receives the signal fed thereto from ballast 64 and operates switching device 67 such that switching device 67 drives lamp 68 with a relatively high frequency drive signal to thus improve the efficiency of illumination system 60. Referring now to FIG. 5, an adapter circuit 70 includes a rectifier circuit 72 having a pair of terminals corresponding to input terminals 70a, 70b of adapter circuit 70. Bridge rectifier 72 may be provided as a full wave rectifier or a half wave rectifier and creates a pulsed DC voltage across terminal 72a, 72b. Rectifier terminals 72a, 72b are coupled to a pair of terminals 74a, 74b of a high frequency inverter circuit 74. Inverter circuit 74 receives the pulsed DC voltage fed thereto and provides an inverter signal having a relatively high frequency to a control circuit 76. Control circuit 76 receives the high frequency inverter signal fed thereto and provides a control signal to a control terminal 78a of a switch 78. Switch output arms correspond to output terminals 70c, 70d of adapter circuit 70. In operation, terminals of a magnetic ballast (not shown) are coupled to adapter circuit terminals 70a, 70b and terminals of a lamp are coupled to adapter circuit terminals 70c, 70d. Thus, switch circuit 78 provides a signal path which is in parallel with a lamp coupled to adapter circuit terminals 70c, 70d. When switch circuit 78 is in its off or unbiased state, the signal path provided by switch circuit 78 has a relatively high impedance characteristic. When switch circuit is in its on or biased state, the signal path provided by switch circuit 78 has a relatively low impedance characteristic. Thus, by repetitively turning the switch on and off at a relatively high frequency via the control circuit 76, the switch drives a lamp coupled to terminals 70c, 70d with a relatively high frequency signal. Referring now to FIG. 6, a portion of a lighting unit 80 includes an adapter circuit 82 coupled to a load 84. Terminals 80a, 80b of adapter circuit 80 are coupled to output terminals of a ballast (not shown). A protection device 86, which may be provided as a fuse or a circuit breaker, for example, is coupled in series between adapter circuit terminal 80a and a first terminal of a rectifier circuit 88. Thus, if any circuit components of the adapter circuit 82 fail, protection device 86 prevents excess current from flowing through the adapter circuit which may present a fire hazard. Rectifier circuit 88 is here provided as a full wave bridge rectifier including diodes 90a-90d coupled as shown. A charge storage device 92, here provided as a capacitor 92, is coupled between third and fourth terminals 88c, 88d of rectifier circuit 88. A high frequency inverter 94 has a first terminal 94a coupled to terminal 88c of rectifier circuit 88 and a second terminal 94b coupled to terminal 88d of rectifier circuit 88. In this particular embodiment, high frequency inverter 94 is provided as a half-bridge self resonating type inverter provided from transistor 96, 98, resonating inductors LI , L2 and L3, series capacitors C3 and parallel resonating capacitor C4. Transistors 96, 98 are here provided as bipolar junction transistors (BJT). Each of the transistors 96, 98 has a bias circuit 97a, 97b, respectively, coupled to a control terminal thereof. As mentioned above, inductors LI , L2, L3 and capacitors C3, C4 form a resonant circuit generally denoted 95 coupled to transistors 96, 98 through transistor bias circuits 97a, 97b. The power transistors 96, 98 are driven by the voltage developed across the resonating inductors L1-L3. The switching sequence of the power transistors 96, 98 is achieved by the natural occurrence of current flowing in resonating circuits which include active components. Since the circuit does not include any saturable magnetic elements, it is relatively easy to control. The output of resonant circuit 95 is coupled through a resistor R3 to a base terminal of a bipolar junction transistor 100 and to a collector of a second bi-polar junction transistor 104. Transistors 100, 104 and 106 form a control circuit which controls the switching of a field effect transistor (FET) 102 by application of a control voltage to a gate electrode 102a of FET 102. In some embodiments FET 102 may be provided as a power MOS FET. With diode DI biased into its conduction state, in response to the control voltage having a first voltage level, FET 102 provides a high impedance signal path across load 84. Alternatively with diode DI biased into its non-conduction state, in response to the control voltage having a second voltage level FET 102 provides a relatively low impedance signal path across load 84. Thus, by switching FET 102 between its conduction and non-conduction states, FET 102 chops the drive signal to the lamp. The power control feature of adapter circuit 82 is thus achieved by means of diverting current from the load on a cycle by cycle basis. This is accomplished by turning on the power MOS FET transistor 102 for a portion of the high frequency cycle in response to a control signal provided by the control circuit formed from transistors 100, 104, 106. Once the load current is diverted through transistor 102, the power transfened to the load drops proportionally. Transistor 102 operates in the switching mode, therefore the power dissipated by this transistor is negligible. The load cunent waveform in sinusoidal, and thus once the load current crosses zero and becomes positive, transistor 106 is biased into its non-conduction state which initiates a charging sequence during which time a control capacitor C5 is charged from a DC source Vcc which is coupled to capacitor C5 through a resistor R8. When the base-emitter threshold voltage (V be ) for transistor 104 is exceeded, transistor 104 turns on which causes transistor 100 to turn off. The resistor R7 controls the timing when transistor 104 will turn on starting from the zero crossing of the lamp current. The turn off of transistor 100 causes transistor 102 to turn on which consequently causes the cunent from the load 84 to be diverted through transistor 102. The diverted cunent flows through transistor 102 and resister R8 which will maintain transistor 102 in its on state until the end of the cycle. Thus, this particular embodiment provides a power inverter which operates at a relatively high frequency (e.g. above 20 kHz) using a single active stage. The circuit also performs power transfer control without controlling the operating frequency of the power switching elements (i.e. the transistors). Furthermore, this particular circuit performs the control in a continuous fashion, preserving at the same time the crest factor for the load cunent. Referring now to FIG. 7, an adapter circuit 108 is here shown to be provided as an integrated circuit having a plurality of discrete circuit elements coupled thereto. Integrated circuit 108 is provided having similar or identical functional characteristics as adapter circuits 16, 35, 44, 66, 70 and 82 discussed above in conjunction with FIGs. 1-6 respectively. By providing an adapter circuit as an integrated circuit, however, the adapter circuit provides the adapter circuit as a relatively reliable, relatively inexpensive circuit. Referring now to FIG. 8, a lighting system 110 includes a source 112 coupled to a conventional magnetic ballast 114. Magnetic ballast 114 is coupled to a conventional lamp 116 at terminals 116a-116d, as shown. Lamp 116 may be provided, for example, as a fluorescent or an HID lamp. An adapter circuit 118 is coupled across first and second signal paths of magnetic ballast 114 to thus drive cunent through lamp 116 at a predetermined chopping frequency. Terminal 118a of adapter circuit 118 may be coupled, for example, to a so-called red wire of ballast 114 while adapter circuit terminal 118b is coupled to a so-called blue wire of ballast 114. Referring now to FIG. 9, a lighting system 120 includes a source 122 which provides an AC signal to a magnetic ballast circuit 124. Magnetic ballast circuit 124 is coupled to a pair of fluorescent lamps 126-128 in a manner which is generally known. An adapter circuit 130 has a pair of terminals 130a, 130b coupled to first and second terminals 124a, 124b of ballast circuit 124. In this particular example, ballast wire pair 125a may conespond to a pair of so-called red wires while wire pair 125b conesponds to so-called blue wires. Thus, adapter circuit terminals 130a, 130b are coupled to one red wire and one blue wire respectively. Wires leading from lamp terminals 126c, 126d, 128c, 128d to ballast 124 conespond to so-called yellow wires. Adapter circuit 130 is thus coupled to drive cunent through the fluorescent lamps 126, 128 at a predetermined chopping frequency. It should be noted that ballast 124 and lamps 126 and 128 may conespond to conventional ballast circuits and lamps. Thus, there is no need to disconnect the ballast circuit from the lamp in order to connect adapter circuit 130. Furthermore, adapter circuit 130 generates a local DC supply from the existing power lines and thus is self powered. The adapter circuit 130 controls the lamp operation frequency on a cycle basis. If the adapter circuit 130 stops operating, lamps 126, 128 are still driven by the low frequency signal provided from conventional ballast 124. However, with adapter circuit 130 in operation, the lamp operates in a more efficient manner. Having described prefened embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts may be used. It is felt, therefore, that these embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims.

Claims: . CLAIMS 1. An apparatus for coupling a source signal from a power source to a lamp, the source signal having a first source signal frequency and a first source signal amplitude and the apparatus comprising: a ballast having first and second ballast input terminals coupled to first and second power terminals of the power source and first and second ballast output terminals coupled to first and second terminals of the lamp, said ballast for receiving the source signal and for providing at the first and second ballast output terminals, a ballast signal having a first ballast signal frequency and a first ballast signal amplitude; and an adapter circuit having a first terminal coupled to the first output terminal of said ballast and a second terminal coupled to the second output terminal of said ballast, the adapter circuit for receiving the ballast signal having the first ballast signal frequency and the first ballast signal amplitude and for providing a drive signal to the lamp, the drive signal having a first drive signal frequency and a first drive signal amplitude. 2. The apparatus of Claim 1 wherein said adapter circuit comprises: a switching device having a first terminal coupled to the first terminal of said adapter circuit and a second terminal coupled to the second terminal of said adapter circuit thereby providing a signal path between the first and second terminals of said adapter circuit, said switching device having a first impedance characteristic in response to a first control signal having a first signal level and a second different impedance characteristic in response to the control signal having a second different signal level. 3. The apparatus of Claim 2 wherein said switching device comprises: a rectifier circuit having a first terminal coupled to the first terminal of said adapter circuit, a second terminal coupled to the second terminal of said adapter circuit, a third terminal and a fourth terminal; and a switch having a first terminal coupled to the third terminal of said rectifier circuit, a second terminal coupled to the fourth terminal of said rectifier circuit and a control terminal. 4. The apparatus of Claim 3 further comprising: a DC bias circuit having a first terminal coupled to the third terminal of said rectifier circuit, a second terminal coupled to the fourth terminal of said rectifier circuit and an output terminal; and a controller having a first terminal coupled to the third terminal of said DC bias circuit, second and third terminals coupled to respective ones of the third and fourth terminals of said rectifier circuit and a fourth terminal coupled to the control terminal of said switch. 5. The apparatus of Claim 4 wherein: said rectifier is provided as a full wave bridge rectifier; said controller is provided as pulse width modulation controller; and said switch is provided as a transistor. 6. The apparatus of Claim 5 wherein said transistor is provided as a field effect transistor. 7. The apparatus of Claim 6 further comprising a protection element having a first terminal coupled to a first terminal of said adapter circuit and a second terminal coupled to a first terminal of said full wave bridge rectifier. 8. The apparatus of Claim 7 wherein said DC bias circuit comprises: a first storage device having a first terminal coupled to the third terminal of said bridge rectifier and a second terminal; a first diode having a first terminal coupled to the second terminal of said storage device and a second terminal corresponding to the output terminal of said DC bias circuit; a second diode having a first terminal coupled to the first terminal of said first diode and a second terminal; a second storage device having a first terminal coupled to the second terminal of said second diode and a second terminal coupled to the fourth terminal of said rectifier circuit; and a third storage device having a first terminal coupled to the second terminal of said first diode and a second terminal coupled to the fourth terminal of said rectifier circuit. 9. The apparatus of Claim 8 wherein: said first storage device is provided as a first capacitor; said second storage device is provided as an inductor; and said third storage device is provided as a second capacitor. 10. The apparatus of Claim 9 wherein: the first terminal of said first diode conesponds to an anode and the second terminal of said first diode conesponds to a cathode; and the first terminal of said second diode conesponds to a cathode and the second terminal of said second diode conesponds to an anode. 11. A magnetic ballast adapter circuit having a first terminal and a second terminal, the magnetic ballast adapter circuit for providing an excitation signal from a magnetic ballast source to a load, the magnetic ballast adapter circuit comprising: a rectifier circuit having a first terminal coupled to the first terminal of the magnetic ballast adapter circuit, a second terminal coupled to the second terminal of the magnetic ballast adapter circuit, a third terminal and a fourth terminal; a control circuit having a first terminal coupled to the third terminal of said rectifier circuit, a second terminal coupled to the fourth terminal of said rectifier circuit and an output terminal; and a switching device having a first terminal coupled to the third terminal of said rectifier circuit, a second terminal coupled to the fourth terminal of said rectifier circuit and a control terminal coupled to the output terminal of said control circuit. 12. The adapter circuit of Claim 11 wherein: said rectifier circuit is provided as a full wave bridge rectifier; said controller is provided as pulse width modulation controller; and said switching device includes a transistor having a first, second and control electrodes conesponding to respective ones of the first, second and control terminals of said switching device. 13. The adapter circuit of Claim 12 wherein said transistor is provided as a field effect transistor. 14. The adapter circuit of Claim 13 further comprising a protection element having a first terminal coupled to a first terminal of the magnetic ballast adapter circuit and a second terminal coupled to a first terminal of said rectifier circuit. 15. The adapter circuit, of Claim 14 further comprising a DC bias circuit having first and second terminals coupled to respective ones of the third and fourth terminals of said rectifier circuit, and a third terminal coupled to a power supply terminal of said pulse width modulator circuit. 16. The adapter circuit of Claim 15 wherein said DC bias circuit comprises: a first storage device having a first terminal coupled to the third terminal of said bridge rectifier and a second terminal; a first diode having a first terminal coupled to the second terminal of said storage device and a second terminal conesponding to the output terminal of said DC bias circuit; a second diode having a first terminal coupled to the first terminal of said first diode and a second terminal; a second storage device having a first terminal coupled to the second terminal of said second diode and a second terminal coupled to the fourth terminal of said rectifier circuit; and a third storage device having a first terminal coupled to the second terminal of said first diode and a second terminal coupled to the fourth terminal of said rectifier circuit. 17. The adapter circuit of Claim 16 wherein: said first storage device is provided as a first capacitor; said second storage device is provided as an inductor; and said third storage device is provided as a second capacitor. 18. The adapter circuit of Claim 17 wherein: the first terminal of said first diode conesponds to an anode and the second terminal of said first diode conesponds to a cathode; and the first terminal of said second diode conesponds to a cathode and the second terminal of said second diode conesponds to an anode. 19. A method for coupling a source signal from a power source to a lamp through a ballast, the source signal having a first source signal frequency and a first source signal amplitude and the method comprising the steps of: providing the source signal to first and second input terminals of the ballast; providing, at first and second ballast output terminals, a ballast signal having a first ballast signal frequency and a first ballast signal amplitude; and chopping the first ballast signal to provide a lamp drive signal having a drive signal frequency wherein the drive signal frequency is greater than the first ballast signal frequency. 20. The method of claim 19 wherein said chopping step includes the step of: (a) providing a low impedance signal path between the first and second output terminals of the ballast for a first predetermined period of time; and (b) providing a high impedance signal path between the first and second output terminals of the ballast for a second predetermined period of time. 21. The method of Clajm 20 wherein said chopping step includes the steps of: providing the ballast signal to a rectifier circuit; rectifying the ballast signal; coupling the rectified ballast signal from the rectifier circuit to a control signal generator; generating a control signal from the rectified ballast signal, the control signal having a first control signal frequency; and alternately biasing a switching device between a first state and a second state at the control signal frequency, wherein the switching device is disposed in a signal path between the first and second output terminals of the magnetic ballast and wherein the switching device provides a low impedance signal path in the first state and a high impedance signal path in the second state. 22. The method of Claim 20 wherein the switching device has a first terminal, a second terminal and a control terminal and the rectifier circuit has a first pair of terminals coupled to respective ones of the first and second ballast output terminals and a second pair of terminals coupled to respective ones of the first and second terminals of the switching device. 23. An adapter circuit having first and second terminals, the adapter circuit comprising: a rectifier circuit having first and second terminals coupled to the first and second terminals of the adapter circuit and having third and fourth terminals; a switch having a control terminal, a first switch terminal coupled to the third terminal of said rectifier circuit and a second switch terminal coupled to the fourth terminal of said rectifier circuit; a controller, having a first terminal coupled to the control terminal of said switch, said controller for switching said switch between a first state and a second state at a switch duty cycle; a signal detector coupled to said rectifier circuit, said signal detector to provide a detection signal in response to a drive signal having a predetermined signal level wherein the predetermined signal level at least equals a first threshold signal level; a feedback signal generator, coupled to said signal detector to receive the detection signal from said signal detector and to generate a feedback signal in response thereto; a comparator, coupled to said feedback signal generator, for comparing a reference signal to the feedback signal wherein said comparator provides an output signal having a first signal level in response to an amplitude of the reference signal being greater than an amplitude of the feedback signal and having a second different signal level in response to an amplitude of the reference signal being less than an amplitude of the feedback signal; and means, coupled to said controller, for reducing the switch duty cycle in response to the comparator output signal having a first one of the first and second signal amplitudes. 24. The adapter circuit of Claim 23 wherein: said rectifier circuit includes a full wave bridge rectifier; said controller is provided as a pulse width modulation controller; and said switch is provided as a field effect transistor. 25. The circuit of Claim 24 further comprising a DC bias circuit coupled between said rectifier circuit and said pulse width modulator circuit. 26. The circuit of Claim 25 wherein said DC bias circuit comprises: a first storage device having a first terminal coupled to the third terminal of said bridge rectifier and a second terminal; a first diode having a first terminal coupled to the second terminal of said storage device and a second terminal conesponding to the output terminal of said DC bias circuit; a second diode having a first terminal coupled to the first terminal of said first diode and a second terminal; a second storage device having a first terminal coupled to the second terminal of said second diode and a second terminal coupled to the fourth terminal of said rectifier circuit; and a third storage device having a first terminal coupled to the second terminal of said first diode and a second terminal coupled to the fourth terminal of said rectifier circuit. 27. The circuit of Claim 26 wherein: said first storage device is provided as a first capacitor; said second storage device is provided as an inductor; and said third storage device is provided as a second capacitor. 28. The circuit of Claim 27 wherein: the first terminal of said first diode conesponds to an anode and the second terminal of said first diode conesponds to a cathode; and the first terminal of said second diode conesponds to a cathode and the second terminal of said second diode conesponds to an anode. 29. A method for detecting removal of a lamp, the method comprising: detecting a drive signal; providing a detection signal in response to the drive signal having a predetermined signal level which at least equals a first threshold signal level; generating a feedback signal having a feedback signal characteristic conesponding to a signal characteristic of the detection signal; and comparing the feedback signal to a reference signal. 30. The method of Claim 29 wherein the step of comparing the feedback signal to a reference signal includes the steps of: comparing an amplitude of the feedback signal to an amplitude of the reference signal; and generating a control signal having a first control signal level in response to the amplitude of the feedback signal being greater than the amplitude of the reference signal. 31. The method of Claim 30 further comprising the step of changing a switch duty cycle in response to the comparison of the feedback signal and the reference signal. 32. The method of Claim 31 wherein the step of changing the switch duty cycle includes the steps of: detecting the control signal; and in response to the first control signal having the first control signal level, reducing the switch duty cycle.

Inventors: MOISIN MIHAIL S, MOISIN, MIHAIL, S.
Applicants: ELECTRO MAG INT INC, ELECTRO-MAG INTERNATIONAL, INC.
Application year: 1997
Main classification: H05B41
IPC codes: H05B41