High frequency ballast for gaseous discharge lamps

An electronic circuit for receiving input electrical power at a lower frequency and for energizing a load at a higher frequency. The electronic circuit has a rectifier for rectifying the input electrical power received on an input thereof and a pre-regulator for changing the rectified AC voltage provided by the rectifier to a source voltage. The electronic circuit further has a non-resonant inverter for providing electrical power at the high frequency to the load and includes first and second switches and is connected to the pre-regulator and to the load circuit. A logic circuit is responsive to a sensed signal representing only current flowing in the first and second switches for operating the first and second switches. The frequency of current in the load circuit varies for any substantial change in the magnitude of the source voltage, for any change in load impedance, and for any change in a set point value. A device for setting the set point value is connected to the logic circuit. Finally, a reactance circuit is provided and connected in circuit with the load. The peak amplitude of current in the load has a peak value determined by the set point value. The device for setting the set point value can be a replaceable element having a predetermined relationship to the type of load connected to the electronic circuit or to the desired light output level. Also, the device for setting the set point value can be a variable element for selecting the set point value from a range of set point values and thereby changing the peak amplitude of current in the load. The variable element can also be controlled by a signal originating remotely from the logic circuit in the ballast.

BACKGROUND OF THE INVENTION 
The present invention relates in general to circuits for energizing gaseous 
discharge lamps, such as fluorescent lamps or high intensity discharge 
lamps. More particularly, it relates to a ballast using solid state 
switches and adapted to energize the lamps with high frequency current. 
Ballast circuits of this type are normally designed to receive energy from 
a conventional 50 or 60 cycle power source as is commonly available, and 
by means of frequency inversion, generate a higher frequency signal in the 
range of 25 to 100 khz to energize the lamps. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved high 
frequency ballast which is adjustably matched to various types of gaseous 
discharge lamps. 
It is a further object of the present invention to provide a high frequency 
ballast which incorporates a pre-regulator for providing a substantially 
constant source of voltage for use by the inverter in the high frequency 
ballast. 
The present invention is an electronic circuit for receiving input 
electrical power at a lower frequency and for energizing a load at a 
higher frequency. In general, the electronic circuit has a rectifier means 
for rectifying the input electrical power received on an input thereof and 
a pre-regulator means for changing the rectified AC voltage provided by 
the rectifier means to a source voltage. The pre-regulator means has an 
input connected to the output of the rectifier means and the pre-regulator 
means provides the source voltage on a output thereof. The electronic 
circuit further has a non-resonant inverter means for providing electrical 
power at the high frequency to the load and includes first and second 
switching means. The inverter means has an input connected to the output 
of the pre-regulator means and an output connected to the load circuit. A 
logic circuit means for operating the first and second switching means is 
provided and is responsive to a sensed signal that represents current 
flowing only in the first and second switching means. This operation is 
such that the switching means are caused to conduct alternately by 
switching a conducting one of the switching means to a non-conducting 
state when the current flowing therein reaches a set point value and 
thereafter switching the other of the switching means to conduct until the 
current flowing therein reaches a set point value. The frequency of 
current in the load circuit varies for any change in load impedance, for 
any substantial change in the magnitude of the source voltage, and for any 
change in the set point value. Also provided is a means for setting the 
set point value and the means for setting is connected to the logic 
circuit means. Finally, a reactance circuit means is provided and is 
connected in circuit with the load. The operating frequency range of the 
inverter circuit means and the impedance of the reactance circuit means is 
such that for any substantial change in the magnitude of source voltage or 
load impedance, the operating frequency of the inverter circuit means 
changes and the resulting impedance of the reactance circuit means is such 
that the peak amplitude of current in the load current remains 
substantially constant. The peak amplitude of current has a peak value 
determined by the set point value. 
In a preferred embodiment of the present invention the pre-regulator means 
is a buck-boost power factor regulator. Also, the pre-regulator means has 
a means for converting the rectified AC voltage to a start up voltage for 
initiating the pre-regulator means when the input electrical power is 
initially applied to the electronic circuit. Also, the means for setting 
the set point value can be a replaceable element having a predetermined 
relationship to the type of load connected to the electronic circuit. 
Alternatively, the means for setting the set point value can be a variable 
element for selecting the set point value from a range of set point 
values, thereby changing the peak amplitude of current in the load. The 
variable element can be remotely located relative to the logic circuit 
means in the ballast.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention has general applicability but is most advantageously 
utilized in an electronic ballast of the type used for operating 
fluorescent lamps. 
FIG. 1 shows a general block diagram of the electronic ballast of the 
present invention. An electromagnetic interference filter 10 is connected 
to the AC line voltage source 12 by an input connector 14. The EMI filter 
10 is connected via rectifier 11 and pre-regulator 16 to an inverter 18. 
The inverter 18 is also connected to a module 20 via the input connector 
14. The module 20 determines the set point value for the inverter 18 and 
is related to the type of fluorescent lamps 22 connected to the inverter 
18. The lamps 22 are connected to the inverter 18, via the output 
connector 24. 
FIGS. 2A and 2B are a schematic diagram of the block diagram depicted in 
FIG. 1. As shown in this embodiment the module 20 is a resistor R9 which 
has a value related to the type of fluorescent lamps 22 connected to the 
inverter 18. The pre-regulator 16 utilizes a buck-boost power factor 
controller ML4813 manufactured by Micro-Linear as integrated circuit 30. 
The integrated circuit 30 is configured as a buck-boost power factor 
regulator with a start up circuit consisting of capacitor C7, resistor R3, 
diode D11 and L2 transformer. Operation of the buck-boost power factor 
regulator is described in the Mar. 1990 Advance Information publication by 
Micro-Linear (hereby incorporated by reference). 
The inverter 18 consists of switching devices Q1 and Q2 which are 
controlled by integrated circuit 32. The inverter 18 is disclosed in U.S. 
Pat. No. 4,873,471 hereby incorporated by reference. This U.S. Patent also 
sets forth in detail the operation of the inverter circuit. 
More specifically, the electronic circuit for receiving input electrical 
power at a lower frequency and for energizing a load at a higher frequency 
has the following functional elements. The rectifier 11 rectifies the 
input electrical power received on an input 34 thereof and provides a 
rectified AC voltage on an output 36 thereof. The pre-regulator 16 changes 
the rectified AC voltage to a source voltage and has an input 38 connected 
to the output 36 of the rectifier 11. The pre-regulator 16 provides the 
source voltage on an output 40 thereof. 
The non-resonant inverter 18 provides electrical power at the higher 
frequency to the load 22. The non-resonant inverter 18 includes first and 
second switching means Q1, Q2 and has an input 42 connected to the output 
40 of the pre-regulator 16 and has an output 44 connected to the load 
circuit. The logic circuit (integrated circuit 32 and associated elements) 
is responsive to a sensed signal representing only current flowing in the 
first and second switching means Q1, Q2. The logic circuit operates the 
first and second switching means Q1, Q2 to conduct alternately by 
switching a conducting one of the switching means to a non-conducting 
state when the current flowing therein reaches a set point value and 
thereafter switching the other of the switching means to conduct until the 
current flowing therein reaches the set point value. The frequency of 
current in the load circuit varies for at least any substantial change in 
the magnitude of the source voltage, for any change in the set point 
value, and for any change in the load impedance. A means (module 20) is 
provided for setting the set point value and is connected to the logic 
circuit. A reactance circuit means is connected in circuit with the load 
22, the operating frequency range of the inverter circuit 18 and the 
impedance of the reactance circuit means being such that for at least any 
substantial change in the magnitude of the source voltage or in the 
magnitude of the load impedance, the operating frequency of the inverter 
circuit 18 changes and the resulting impedance of the reactance circuit 
means is such that the peak amplitude of current in the load circuit 
remains substantially constant. The peak amplitude of current has a peak 
value determined by the set point value. 
In one embodiment (FIGS. 2A and 2B) the means 20 for setting the set point 
value is a replaceable element having a predetermined relationship to the 
type of fluorescent lamp 22 connected to the ballast. 
In another embodiment (FIG. 3) the means 20 for setting the set point value 
is a variable element for selecting the set point value from a range of 
set point values, thereby changing the peak amplitude of current in the 
fluorescent lamp 22. The means 20 is a plug-in module connected to the 
ballast 60 and receives a control signal S which determines the set point. 
As shown in FIG. 4 a control device 21 is used that outputs a variable 
electrical signal, such as a variable voltage level, that is received by 
electronically variable control element 23. The electronically variable 
control element 23 generates the proper set point value in response to the 
received electrical signal. The control device 21 can be remotely located 
relative to the electronically variable control element 23 and ballast 60. 
Also, in order to preserve the long life characteristic of the fluorescent 
lamps, a filament voltage control element 25 can be connected between the 
ballast 60 and lamp 22. This filament voltage control element 25 increases 
the lamps 22 filament voltage as the set point value is decreased. 
Decreasing the set point value facilitates the dimming of the lamps 22. 
The present invention can also be used in a high frequency electronic 
ballast system for receiving input electrical power at a lower frequency 
and for energizing at least one gaseous discharge lamp at a higher 
frequency. The system has at least one ballast circuit having rectifier 
means for rectifying the input electrical power and providing a rectified 
AC voltage, pre-regulator means for changing the rectified AC voltage to a 
source voltage, non-resonant inverter means for converting the source 
voltage to a high frequency voltage, logic circuit means for controlling 
the inverter means, reactance circuit coupled in circuit with the lamp, 
and means for setting at least one set point value connected to the 
inverter means, the set point value determining the peak amplitude of 
current in the fluorescent lamp. As shown in FIG. the ballast circuit can 
be connected to a plurality of fluorescent lamps 22 and the means 20 for 
setting provides a set point value as a function of the number and type of 
fluorescent lamps 22. When the fluorescent lamp is selected from a 
plurality of different types of fluorescent lamps, the means 20 for 
setting is selected from a plurality of means for setting corresponding to 
the different types of fluorescent lamps. The means 20 for setting is a 
plug-in module connected to the ballast. As depicted in FIG. 5, the system 
can have a plurality of ballast circuits 50 with associated electronic 
variable control elements 53, filament voltage control circuits 51 and 
fluorescent lamps 52 and one means 54 for remote control. For example, the 
means 54 for remote control can be a computer, photo-cell, an occupancy 
detector, or other electronic apparatus. 
The use of a buck-boost regulator 16 as a pre-regulator provides very 
efficient corrections for line power factor near 100% while maintaining a 
low total harmonic distortion of the line current of less than 10%. These 
characteristics are desireable since they reduce energy cost, reduce power 
loss in power distribution systems, increase the maximum number of 
ballasts that can be installed on each branch circuit, and reduce or 
eliminate the problem of a ballast interfering with other electronic 
equipment. The electronic ballast of the present invention can be easily 
adapted for different lamp types, even after installation in the field. 
For example, if an initial installation uses a 34 watt, T12 energy saving 
lamp in a 3 lamp fixture with the new ballast of the present invention, 
the ballast would be set at the factory to operate these lamps at an 
extremely well regulated current of 300 milliamps. At a later date in 
time, if the installation were up graded to a more efficient T8 lamp 
system this would present a problem to prior art electronic ballasts since 
the T8 lamp is optimally operated at approximately 190 milliamps. A prior 
art ballast would run the new lamps but at a current level similar to the 
initial T12 installation. This would result in more light output (by 
overdriving the lamps), but at the expense of increased energy consumption 
and shorter lamp life. This conversion would present minimal problems for 
the ballast of the present invention since the conversion means (module 
20) could be packaged with the ballast when shipped. The original module 
20 is simply replaced with a new module 20 which would have a different 
resistor value that relates to the T8 lamps. The T8 lamps may then be 
installed and operated at a well regulated approximately 190 milliamps, 
with the accompanying energy savings. 
In another embodiment of the present invention, the conversion from a 2 
lamp to a 1 lamp system could be accomplished in the same manner. 
Furthermore, since the module controls the set point for operation of the 
inverter 18, the ballast can be initially prepared for remote dimming or 
can be converted for dimming after installation. The conversion requires 
only the removal of the module 20, and the simple external mounting and 
connection of a low cost ballast dimming module and its associated wiring 
to the ballast. In one embodiment a twisted pair connection can be 
connected to a personal computer, facilitating centralized control of 
lighting. In another embodiment, a photo-cell can be mounted in each light 
fixture which would constantly monitor room light levels, modifying the 
lamp light output in response to changing light levels from other sources. 
In yet a further embodiment the twisted pair could be eliminated and the 
control of the ballast dimming module could be operated by power line 
carrier techniques. 
The invention is not limited to the particular details of the apparatus 
depicted and other modifications and applications are contemplated. 
Certain other changes may be made in the above described apparatus without 
departing from the true spirit and scope of the invention herein involved. 
It is intended, therefore, that the subject matter in the above depiction 
shall be interpreted as illustrative and not in a limiting sense.