A self-oscillating inverter-type fluorescent lamp ballast has two modes of operation: (a) a first mode in which the inversion frequency is about 70 kHz and is resonant with a first tuned circuit by which power is supplied to the cathodes of the fluorescent lamp; and (b) a second mode in which the inversion frequency is about 30 kHz and is resonant with a second tuned circuit by which main lamp power is supplied. When the ballast is initially powered-up, it starts operation in its first mode, thereby providing cathode heating power without yet providing main lamp power. About one second later, after the cathodes have reached full incandescence, the inverter automatically changes into its second mode, thereby providing main lamp power while at the same time removing cathode heating power. If for some reason the lamp were not to ignite within about 10 milli-seconds, the inverter reverts back into its first mode; thereafter cycling between its two modes until the lamp does ignite.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electronic ballasts for gas discharge 
lamps, particularly to ballasts wherein the load is powered by way of a 
series-excited parallel-loaded resonant L-C circuit. 
2. Description of Prior Art 
There are two predominant types of electronic ballasts for gas discharge 
lamps: (a) a first type may be referred-to as the parallel-resonant type 
and involves the use of a current-excited (i.e., parallel-excited) 
parallel-loaded resonant L-C circuit; and (b) a secnd type that may be 
referred-to as the series-resonant type and involves the use of a 
voltage-excited (i.e., series-excited) parallel-loaded resonant L-C 
circuit. 
An example of the parallel-resonant type of electronic ballasts is 
described in U.S. Pat. No. 4,277,726 to Burke. An example of the 
series-resonant type of electronic ballasts is described in U.S. Pat. No. 
4,538,095 to Nilssen. 
Of these two types of electronic ballasts, the parallel-resonant type is 
conducive to yielding a stable easy-to-control self-oscillating 
inverter-type ballast; whereas the series-resonant type, although 
potentially simpler and more efficient, is harder to control in that it 
has a natural tendency to self-destruct in case the lamp load be removed. 
To mitigate this tendency to self-destruct under no-load conditions, 
various protection circuits have been developed, such as for instance 
described in U.S. Pat. No. 4,638,562 to Nilssen. 
GENERAL PURPOSE OF PRESENT INVENTION 
The general purpose of the present invention is that of providing a method 
for cost-effectively controlling the operation of a series-resonant 
electronic inverter-type ballast for fluorescent lamps. 
SUMMARY OF THE INVENTION 
1. Objects of the Invention 
An object of the present invention is the provision of a cost-effective 
control arrangement for attaining proper operation of an electronic 
ballast wherein the lamp load is powered by way of a series-excited 
predominantly parallel-loaded resonant L-C circuit. 
This as well as other objects, features and advantages of the present 
invention will become apparent from the following description and claims. 
2. Brief Description 
A self-oscillating inverter-type fluorescent lamp ballast has two modes of 
operation: (a) a first mode in which the inversion frequency is about 70 
kHz and is resonant with a first tuned L-C circuit by which power is 
supplied to the cathodes of the fluorescent lamp; and (b) a second mode in 
which the inversion frequency is about 30 kHz and is resonant with a 
second tuned L-C circuit by which main lamp power is supplied. 
When the ballast is initially powered-up, it starts operation in its first 
mode, thereby providing cathode heating power without yet providing main 
lamp power. About one second later, after the cathodes have reached full 
incandescence, the inverter automatically changes into its second mode, 
thereby providing main lamp power while at the same time removing cathode 
heating power. If for some reason the lamp were not to ignite within about 
10 milli-seconds, the inverter reverts back into its first mode; 
thereafter cycling (with a period of about one second) between its two 
modes until the lamp does ignite. 
Thus, the first tuned L-C circuit is resonant at 70 kHz; and, due to 
inherent frequency-selectivity characteristics, this first tuned circuit 
provides cathode heating power only when being excited at or near 70 kHz. 
Likewise, the second tuned L-C circuit provides main lamp starting voltage 
and operating power only when being excited at or near 30 kHz.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
1. Details of Construction 
The drawing schematically illustrates the preferred embodiment of the 
invention in the form of a half-bridge inverter-type two-mode electronic 
ballast for a fluorescent lamp. 
In the drawing, 277 Volt/60 Hz power line voltage from an ordinary electric 
utility power line PL is provided to the AC power input terminals of a 
rectifier and filter means RFM, the DC output from which is applied 
between a B+ bus and a B- bus. 
A filter capacitor FCa is connected between the B+ bus and a junction J1; a 
filter capacitor FCb is connected between junction J1 and the B- bus. A 
tank capacitor TC is connected between junction J1 and a junction J2. An 
auxiliary inductor AI is connected between junction J2 and a junction J3; 
and a main tank inductor TI is connected between junction J3 and a 
junction J4. 
Junction J4 is connected with a junction J5 by way of series-connected 
primary windings SCTap and SCTbp of saturable current transformers SCTa 
and SCTb, respectively. 
A first main inverter transistor Qa is connected with its collector to the 
B+ bus and with its emitter to junction J5; a second main inverter 
transistor Qb is connected with its collector to junction J5 and with its 
emitter to the B- bus. 
Secondary winding SCTas of saturable current transformer SCTa is connected 
between the base of transistor Qa and a junction Ja. A capacitor Ca is 
connected between junctions Ja and J5. A Zener diode Za is connected with 
its anode to junction Ja and with its cathode to junction J5. An auxiliary 
transistor AQa is connected with its collector to junction Ja and with its 
emitter to junction J5. A resistor Ra is connected between the B+ bus and 
the base of transistor Qa. The base of auxiliary transistor AQa is 
designated a. 
Secondary winding SCTbs of saturable current transformer SCTb is connected 
between the base of transistor Qb and a junction Jb. A capacitor Cb is 
connected between junction Jb and the B- bus. A Zener diode Zb is 
connected with its anode to junction Jb and with its cathode to the B- 
bus. An auxiliary transistor AQb is connected with its collector to 
junction Jb and with its emitter to the B- bus. A resistor Rb is connected 
between junction J5 and the base of transistor Qb. The base of auxiliary 
transistor AQb is designated b. 
Tank inductor TI has a secondary winding SWt, which has a center tap CTt 
connected with the collector of a control transistor CQ. The emitter of 
control transistor CQ is connected with the B- bus. 
The terminals of secondary winding SWt are connected with the cathodes of 
two diodes D1 and D2; whose anodes are connected with the terminals of a 
secondary winding SWc of a control transformer Tc; which secondary winding 
has a center tap CTc connected with the B- bus. 
A fluorescent lamp FL has two thermionic cathodes TCx and TCy; which has 
power input terminals x--x and y--y, respectively. One of the power input 
terminals of cathode TCx is connected with junction J1 by way of primary 
winding PWc of control transformer Tc. One of the power input terminals of 
cathode TCy is connected with junction J2. 
Power input terminals x--x and y--y of cathodes TCx and TCy are connected 
with power output terminals x--x and y--y of secondary windings SWx and 
SWy of auxiliary inductor AI, all respectively; which secondary windings 
have series-connected capacitors Cx and Cy, also respectively. 
A resistor R1 is connected between the B+ bus and a junction J6; and a 
capacitor C1 is connected between junction J6 and the B- bus. A resistor 
R2 and a Diac D4 are connected in series between junction J6 and the base 
of control transistor CQ. A resistor R3 is connected between the base of 
transistor CQ and the B- bus. 
A resistor R4 is connected between junction J6 and the collector of a 
transistor Qc, whose emitter is connected with the B- bus. A resistor R5 
is connected between the base of transistor Qc and the B- bus. A resistor 
R6 is connected between the base of transistor Qc and a junction J7. A 
capacitor C2 is connected between junction J7 and the B- bus. A diode D3 
is connected with its anode to the anode of diode D2 and with its cathode 
to junction J7. 
Control transformer Tc also has two secondary windings SWa and SWb. The 
terminals of secondary winding SWa are connected between base a of 
transistor AQa and junction J5; the terminals of secondary winding SWb are 
connected between the B- bus and base b of transistor AQb. 
2. Details of Operation 
The operation of the circuit arrangement schematically illustrated by the 
drawing may be explained as follows. 
In the arrangement of the drawing, ordinary 277 Volt/60 Hz power line 
voltage is provided from the power line (PL) and is rectified and filtered 
by conventional rectifier and filter means RFM such as to provide a DC 
voltage between the B+ and the B- buses, with the B+ bus carrying the 
positive polarity. 
The half-bridge inverter, which principally consists of capacitors FCa and 
FCb, transistors Qa and Qb, and saturable current feedback transformers 
SCTa and SCTb, is self-oscillating and functions in a substantially 
ordinary manner, such as for instance described in conjunction with FIG. 8 
of U.S. Pat. No. Re. 31,758 to Nilssen. 
The output of the half-bridge inverter is provided to and between junctions 
J1 and J4; between which junctions are connected in series: tank capacitor 
TC, tank inductor TI, and auxiliary inductor AI. 
Auxiliary inductor AI is tuned to about 70 kHz by way of capacitors Cx and 
Cy; which two capacitors are connected with the secondary windings of the 
auxiliary inductor as well as with the loads connected to the output of 
these secondary windings. Thus, only when lamp cathodes TCx and TCy are 
indeed connected with the two secondary windings is the auxiliary inductor 
tuned to about 70 kHz. As a result, at about 70 kHz, the auxiliary 
inductor appears like a parallel-resonant circuit as viewed from between 
junctions J2 and J3. 
Tank inductor TI is tuned to series-resonate with tank capacitor TC at 
about 30 kHz; which is to say that the total impedance between junctions 
J1 and J4 appears substantially like a series-resonant circuit at about 30 
kHz. 
At 30 kHz, the impedance of auxiliary inductor AI is inductive and 
relatively small, and is at that frequency simply considered as a small 
part of tank inductor TI. 
At 70 kHz, the impedance of tank capacitor TC is capacitive and relatively 
small, whereas the impedance of tank inductor TI is inductive and 
relatively high. 
Thus, when the inverter oscillates at 70 kHz, its output voltage is applied 
by way of high-impedance tank inductor TI to the parallel-resonant circuit 
represented by auxiliary inductor AI; which parallel-resonant circuit then 
operates to power the two thermionic cathodes of fluorescent lamp FL. 
During this mode, the power provided to these two cathodes is about two 
watts, and the magnitude of the current then drawn from the inverter 
output is quite small. As a result, the magnitude of the 70 kHz voltage 
resulting across tank capacitor TC is very small. 
On the other hand, when the inverter oscillates at 30 kHz, essentially no 
power is provided to the thermionic cathodes. However, at that frequency, 
the resonant series-tuned L-C circuit then loading the inverter's output 
causes a 30 kHz voltage of very large magnitude to develop across the tank 
capacitor. The magnitude of this 30 kHz voltage is so large as to cause 
the fluorescent lamp to ignite; whereafter the magnitude of the 30 kHz 
voltage across the tank capacitor will be determined by the 
current-voltage characteristices of the fluorescent lamp. In reality, at 
the 30 kHz series-resonance, the output provided from the output terminals 
to the fluorescent lamp (i.e., from junctions J1 and J2) will essentially 
be a 30 kHz constant-magnitude current. 
The frequency of inverter oscillation is determined by the saturation 
characteristics of saturable current transformers SCTa and SCTb in 
conjunction with the magnitude of the voltage presented to their secondary 
windings SCTas and SCTbs. 
The magnitude of the voltage presented to secondary windings SCTas and 
SCTbs will be determined by the base-emitter voltage of transistors Qa and 
Qb in combination with the magnitude of the voltage present at junctions 
Ja and Jb as referenced to the emitters of transistors Qa and Qb, 
respectively. 
With no control signals provided to the bases a and b of auxiliary 
transistors AQa and AQb, the magnitude of the voltage at junctions Ja and 
Jb will be determined by the Zener voltages of Zener diodes Za and Zb; 
which Zener voltages are chosen to be about 4.0 Volt each. However, when 
sufficient control current is provided to each of bases a and b, 
transistors AQa and AQb become conductive and therefore operative to shunt 
Zener diodes Za and Zb, thereby to cause the magnitudes of the voltages at 
junctions Ja and Jb to become very low (about 1.0 Volt each). 
Thus, absent control currents at bases a and b, the inverter will oscillate 
at about 70 kHz; whereas, with control currents, the inverter will 
oscillate at about 30 kHz. 
When the inverter is initially powered-up, no lamp current is flowing 
through the primary winding of control transformer Tc and control 
transistor CQ is non-conductive; which means that no control currents are 
provided to bases a and b of transistors AQa and AQb. Thus, when initially 
powered-up, the inverter will initiate oscillations at a frequency of 
about 70 kHz. 
However, after about one second, capacitor C1 will have reached a voltage 
high enough to cause Diac D4 to break down; which, in turn, causes 
capacitor C1 to discharge into the base of control transistor CQ, thereby 
causing this transistor to become conductive. 
Control transistor CQ will remain conductive for a period of about 10 
milli-seconds; and, during this period, current from secondary winding SWt 
of tank inductor TI will flow through secondary winding SWc of control 
transformer Tc; thereby--via secondary windings SWa and SWb on control 
transformer Tc--providing control currents to bases a and b of transistors 
AQa and AQb; thereby causing capacitors Ca and Cb to discharge to a 
voltage level of about 1 Volt; thereby, in turn, to cause the inverter's 
oscillating frequency to become about 30 kHz. 
With the inverter frequency at 30 kHz, the magnitude of the voltage 
provided between the lamp's cathodes becomes large enough to cause lamp 
ignition within the 10 milli-second period; which, in turn, gives rise to 
the flow of lamp current; which lamp current flows through primary winding 
PWc of control transformer Tc, thereby continuing to provide control 
currents to bases a and b of transistors AQa and AQb; thereby continuing 
to maintain the inverter's oscillation frequency at 30 kHz. 
On the other hand, if the fluorescent lamp were to fail to ignite within 
the 10 milli-second time-window during which the control transistor CQ be 
conductive, control currents to bases a and b would not be sustained; 
thereby causing the inverter to revert to its 70 kHz oscillating 
frequency. 
In short, with a properly operational fluorescent lamp connected, the 
ballast arrangement of the drawing operates as follows. 
(1) Upon initial connection to the power line, the inverter starts 
oscillating at a 70 kHz frequency; which, via a 70 kHz resonating circuit 
associated with auxiliary inductor AI, therefore causes cathode heating 
power to be provided to the thermionic cathodes of the fluorescent lamp. 
(2) After about one second, at which time the cathodes are fully 
thermionic, control transistor CQ suddenly becomes conductive and 
thereafter remains conductive for a period of about 10 milli-seconds. With 
transistor CQ conductive, control current is provided to auxiliary 
transistors AQa/AQb; which then become conductive, thereby to cause a 
reduction in the magnitudes of the voltages across capacitors Ca/Cb; 
which, in turn, causes the frequency of inverter oscillation to reduce to 
30 kHz and to remain at 30 kHz for at least 10 milli-seconds. 
(3) With the inverter oscillating at 30 kHz, series-resonance occurs 
between tank capacitor TC and tank inductor TI (including the net 
inductance of auxiliary inductor AI and its associated circuitry); which 
series-resonance, due to so-called Q-multiplication effects, results in a 
high-magnitude 30 kHz voltage developing across the tank capacitor. 
(4) The high-magnitude 30 kHz voltage developing across the tank capacitor 
is applied across the fluorescent lamp and, because its cathodes are 
already thermionic, causes it to ignite immediately. The resulting lamp 
current will then, via control transformer Tc, continue to provide control 
current to auxiliary transistors AQa/AQb; thereby, even after the initial 
10 millisecond period, ensuring that the inverter's frequency of 
oscillation remains at 30 kHz. 
(5) When the inverter is operating at 30 kHz, essentially no power is being 
delivered to the cathodes of the fluorescent lamp, thereby providing for 
improved energy efficiency as compared with the situation where cathode 
power be supplied on a continuous basis. 
(6) If the lamp were to be removed or if lamp current otherwise were to 
fail to flow, control current would cease to be provided to auxiliary 
transistors AQa/AQb, thereby causing the inverter's oscillation frequency 
to revert to 70 kHz. Thus, as long as no lamp current is flowing, the 
inverter will alternate between two modes: a first mode of oscillating at 
70 kHz and a second mode of oscillating at 30 kHz, spending about one 
second (1000 milli-seconds) at 70 kHz for each 10 milliseconds at 30 kHz. 
3. Additional Comments 
(a) To protect against possible self-destruction of the inverter circuit 
(which might occur if the circuit were to operate for a period of time 
without being connected with a properly functioning lamp load), it may be 
advantageous to connect a voltage-limiting means, such as a Varistor, in 
parallel with the tank capacitor. 
(b) For further details relative to the biasing arrangement used in 
connection with main inverter transistors Qa/Qb, reference is made to FIG. 
3 of U.S. Pat. No. 4,307,353 to Nilssen. 
(c) By providing for additional levels of adjustment for the magnitude of 
the bias voltage (i.e., the voltage across capacitors Ca/Cb), 
corresponding adjustment of the magnitude of lamp current may be attained, 
thereby to provide for lamp dimming. 
(d) It is believed that the present invention and its several attendant 
advantages and features will be understood from the preceeding 
description. However, without departing from the spirit of the invention, 
changes may be made in its form and in the construction and 
interrelationships of its component parts, the form herein presented 
merely representing the preferred embodiment.