Electronic fluorescent lamp ballast with overload protection

Subject invention relates to an inverter-type electronic fluorescent lamp ballast wherein a series-resonant LC circuit connected across the inverter's output is used for matching the inverter's operating characteristics to those of the fluorescent lamp--the fluorescent lamp being connected in parallel with the tank-capacitor of this LC circuit. More particularly, the invention relates to the use of a Varistor coupled in parallel with this tank-capacitor, thereby limiting the voltage developed thereacross to a magnitude suitable for proper lamp starting. Moreover, by providing for means whereby the inverter shuts itself off in case current flows through this Varistor for a longer time than it should take for a fluorescent lamp to start, inverter overload protection is obtained. Without such overload protection the inverter would self-destruct in case the fluorescent lamp failed to start or if it were removed from the circuit.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to inverter-type electronic ballasts for 
fluorescent lamps, particularly of the type using series-resonant LC 
circuit means for matching the inverter's operating characteristics to 
those of the fluorescent lamp. 
2. Description of Prior Art 
Inverter-type fluorescent lamp ballasts in which a series-resonant LC 
circuit is used for matching the inverter's operating characteristics to 
those of the fluorescent lamp (i.e., series-resonance-loaded inverter 
ballasts) have been described in published literature, such as in U.S. 
Pat. No. 3,710,177 to Richard Ward or U.S. Pat. No. 4,370,600 to Zoltan 
Zansky. 
In these ballasts, the fluorescent lamp is typically connected in parallel 
with the tank-capacitor of the series-resonant LC circuit, and the high 
voltage developed across this tank-capacitor (due to so-called 
Q-multiplication) is used for starting and operating the fluorescent lamp. 
However, if the load is removed from such a series-resonant LC circuit, 
which occurs whenever the lamp is removed therefrom or whenever the lamp 
fails to operate (as is bound to happen toward end of normal lamp life), 
the voltage developed across the tank-capacitor (and thereby the power 
drawn by the series-resonant circuit) will become so high as to cause 
circuit damage--except if some form of over-voltage or over-current 
protection has been provided. 
While Ward does not describe any specific means of over-voltage protection, 
Zansky does provide for such means in the form of a voltage-clamping 
arrangement; which arrangement operates to limit the voltage across the 
tank-capacitor by way of using the inverter's DC supply-voltage as a load 
for the series-resonant LC circuit whenever the magnitude of the voltage 
across the tank-capacitor exceeds a certain pre-determined multiple of the 
magnitude of the DC supply-voltage. Of course, to permit proper lamp 
starting, it is necessary that this clamping-voltage limit be set higher 
than the worst-case lamp starting-voltage for the minimum anticipated DC 
supply-voltage. 
In summary, although the best arrangement that can be found among all known 
prior art voltage-clamping circuits does indeed provide a degree of 
mitigation against over-voltage and excess power-draw as resulting from 
having an unloaded series-resonant LC circuit connected directly across 
the output of an inverter, it provides far from a fully satisfactory 
solution. Some of its more serious limitations are as follows: 
(a) Although the typical prior art voltage-clamping arrangement 
significantly limits the net power drawn by the overall ballasting system 
in case of an inoperative lamp, the amount of power that has to be handled 
by the inverter itself has been limited to a much lesser degree: it still 
has to handle on a continuous basis all the power associated with having 
the voltage-clamping arrangement connected across the 
tank-capacitor--which voltage-clamping arrangement, as far as the inverter 
and the LC circuit are concerned, is just another load. This implies that 
the transistors of the inverter have to be able to handle on a continuous 
basis several times the amount of power that they have to handle in order 
to provide just for the continuous operation of the regular fluorescent 
lamp load. (Note: For a fixed input voltage to a series-resonant LC 
circuit, the power absorbed by a load connected across the tank-capacitor 
of the LC circuit is substantially proportional to the magnitude of the 
voltage developed across the tank-capacitor.) 
(b) With the DC supply-voltage being used as the voltage-clamping means, 
the clamping-voltage limit is fully dependent upon the magnitude of this 
DC supply-voltage. Thus, in cases where the magnitude of the DC 
supply-voltage may change (as it invariably will in situations where the 
DC supply-voltage is derived from an ordinary electric utility power 
line), it is necessary to arrange for the clamping-voltage to be 
adequately high for proper lamp starting even at the lowest anticipated 
level of DC supply-voltage; which implies that the clamping-voltage will 
be that much higher for the maximum anticipated level of DC 
supply-voltage. Thus, the demands on the inverter in terms of power 
handling capabilities increase in proportion to the ratio of 
maximum-to-minimum levels of DC supply-voltage. (Or, conversely, there is 
a distinct limitation on the minimum level of DC supply-voltage for which 
the ballast will be able to start the fluorescent lamp). 
(c) Even though the inverter in the ballast is able to handle on a 
continuous basis the excess current that results with an inoperative 
fluorescent lamp, there is a sufficient amount of wasted power associated 
with doing so; which implies that--in case such an inoperative fluorescent 
lamp is left connected with the ballast for an extended period of time, as 
in indeed apt to occur near end of lamp life--a significant amount of 
energy is wasted. 
Of course, it would be possible to use an ordinary voltage clamping means 
(such as a Varistor or a Zener diode) for providing the requisite 
voltage-clamping effect; except that this would result in such a gross 
amount of excess power dissipation as to be non-feasible both from the 
viewpoint of size and cost of the voltage-clamping means itself as well as 
from the viewpoint of excessive energy waste. 
Yet, this approach does have the advantage of providing the fluorescent 
lamp with a starting voltage of substantially constant magnitude 
regardless of relatively wide variations in the magnitude of the DC 
supply-voltage. 
Thus, while there are several inherent and potentially important advantages 
associated with resonance-loaded inverter ballasts, the several 
limitations associated with such ballasts according to prior art are 
severe enough to prevent their widespread application. 
SUMMARY OF THE INVENTION 
1. Objects of the Invention 
A first object of the present invention is that of providing a basis for 
designing cost-effective inverter-type ballasts for gas discharge lamps. 
A second object is that of providing a basis for designing an inverter 
circuit that is capable of being loaded safely and effectively by way of a 
series-resonant circuit. 
A third object is that of providing a basis for designing safe, 
cost-effective and high-efficiency series-resonance-loaded inverter 
ballasts for fluorescent lamps. 
These as well as other objects, features and advantages of the present 
invention will become apparent from the following description and claims. 
2. Brief Description 
In its preferred embodiment, subject invention constitutes a 
series-resonance-loaded fluorescent lamp ballast comprising the following 
key component parts: 
a source of DC voltage, which DC voltage is derived by rectification of the 
AC voltage from a regular 60 Hz power line; 
an inverter connected with said source of DC voltage and operative to 
provide across an output a relatively high-frequency squarewave voltage, 
said inverter comprising a disable-means operative on receipt of a 
disable-signal to disable the inverter and thereby to remove said 
squarewave voltage from said output while also substantially reducing the 
power drawn by the inverter from said source of DC voltage; 
a series LC circuit connected across said output, said LC circuit being 
substantially series-resonant at the fundamental frequency of said 
squarewave voltage; 
means for disconnectably connecting a fluorescent lamp across the 
tank-capacitor of said LC circuit, said fluorescent lamp requiring for 
proper starting a voltage of magnitude above a certain threshold level; 
a Varistor voltage-clamping means and a current-sensor means connected in 
series across said tank-capacitor, said voltage-clamping means being 
operative to limit the magnitude of the voltage across said tank-capacitor 
to a level not substantially higher than said threshold level, said 
current-sensor being operative to sense the clamping-current flowing 
through said voltage-clamping means and to provide a clamping-current 
output whenever clamping-current is flowing through said voltage-clamping 
means; and 
integration-means connected in circuit between said clamping-current output 
and said inverter disable-means, said integration-means being operative to 
provide said disable-signal to said disable-means whenever 
clamping-current has been flowing for a period of time longer than that 
normally required for starting said fluorescent lamp; 
whereby, if for some reason the fluorescent lamp fails to present a load to 
said series LC circuit for more than a brief period, said inverter becomes 
disabled. 
Comparing the operation of the ballast of subject invention with that of 
resonance-loaded ballasts of prior art, such as that described by Zansky, 
it is noted that all the indicated limitations have been mitigated: 
(a) The amount of power that has to be handled by the inverter on a 
continuous basis has been limited to just the amount required for 
operating the fluorescent lamp; 
(b) With a Varistor used as the voltage-clamping means, the magnitude of 
the starting voltage presented to the fluorescent lamp is essentially 
independent of the magnitude of the DC supply-voltage, which implies that 
the ballast of subject invention will be able to start and operate the 
fluorescent lamp over a wide range of DC supply-voltage magnitudes; 
(c) Since the inverter is disabled in case the fluorescent lamp fails to 
operate (or if the lamp is removed), no significant amount of energy is 
wasted even if an inoperative fluorescent lamp is left connected for an 
extended period of time. 
For the same reason, the power rating of the requisite Varistor may be 
quite modest.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
1. Description of the Drawing 
In FIG. 1, a source S of 120 Volt/60 Hz voltage is applied to a full-wave 
bridge rectifier BR, the unidirectional voltage output of which is applied 
directly between a B+ bus and a B- bus, with the positive voltage being 
connected to the B+ bus. 
Between the B+ bus and the B- bus are connected a series-combination of two 
transistors Q1 and Q2 as well as a series-combination of two 
energy-storing capacitors C1 and C2. 
The secondary winding CT1s of positive feedback current transformer CT1 is 
connected directly between the base and the emitter of transistor Q1; and 
the secondary winding CT2s of positive feedback current transformer CT2 is 
connected directly between the base and the emitter of transistor Q2. 
The collector of transistor Q1 is connected directly with the B+ bus; the 
emitter of transistor Q2 is connected directly with the B- bus; and the 
emitter of transistor Q1 is connected directly with the collector of 
transistor Q2, thereby forming junction QJ. 
One terminal of capacitor C1 is connected directly with the B+ bus, while 
the other terminal of capacitor C1 is connected with a junction CJ. One 
terminal of capacitor C2 is connected directly with the B- bus, while the 
other terminal of capacitor C2 is connected directly with junction CJ. 
An inductor L and a capacitor C are connected in series with one another 
and with the primary windings CT1p and CT2p of current transformers CT1 
and CT2. 
The series-connected primary windings CT1p and CT2p are connected directly 
between junction QJ and a point X. Inductor L is connected with one of its 
terminals to point X and with the other of its terminals to one of the 
terminals of capacitor C. The other terminal of capacitor C is connected 
directly with junction CJ. 
A fluorescent lamp FL is connected, by way of lamp sockets S1 and S2, in 
parallel-circuit across capacitor C. 
A Varistor V and primary winding CT3p of current transformer CT3 are 
connected in series across capacitor C. 
One terminal of the secondary winding CT3s of transformer CT3 is connected 
with the B- bus; the other terminal of this secondary winding is connected 
with the anode of a high speed rectifier HSR1. The cathode of rectifier 
HSR1 is connected to the positive terminal of an energy-storing capacitor 
EC. The negative terminal of capacitor EC is connected directly to the B- 
bus. A bleeding resistor R1 is connected directly across capacitor EC. 
A Diac D1 is connected between the cathode of rectifier HSR1 and the 
cathode of another high speed rectifier HSR2. The anode of rectifier HSR2 
is connected to the B- bus. 
Between the cathode of rectifier HSR2 and the base of an auxiliary 
transistor Qa is connected a resistor R2. 
The collector of transistor Qa is connected directly to the base of 
transistor Q2, and the emitter of transistor Qa is connected directly to 
B- bus. 
The combination of varistor V, current transformer CT3, rectifier HSR1, 
capacitor EC, Resistor R1, Diac D1, rectifier HSR2, resistor R2 and 
transistor Qa is referred to as sub-assembly A. 
A series-combination of a capacitor C3 and a Diac D2 is connected between 
the B+ bus and the base of transistor Q2. 
Values and designations of the various parts of the circuit of FIG. 1 are 
listed as follows: 
Output of Source S: 120 Volt/60 Hz; 
Bridge rectifier BR: a bridge of four 1N4004's; 
Capacitors C1 & C2: 100 uF/100 Volt Electrolytics; 
Transistors Q1 & Q2: Motorola MJE13002's; 
Capacitor C: 15 nF/1000 Volt (High-Q); 
Fluorescent Lamp FL: Sylvania Octron F032/31K; 
Varistor V: 511 Volt/80 Joule; 
High-speed Rectifiers HSR1 & HSR2: 1N4154's; 
Resistor R1: 68 KOhm/0.25 Watt; 
Capacitor EC: 33 uF/35 Volt Electrolytic; 
Diacs D1 & D2: 1N5760's; 
Resistor R2: 1 kOhm/0.25 Watt; 
Transistor Qa: 2N4401; 
Capacitor C3: 22 nF/200 Volt; 
Transformers CT1 & CT2: Wound on Ferroxcube Toroids 213T050 of 3E2A Ferrite 
Material with three turns of #26 wire for the primary windings and ten 
turns of #30 wire for the secondary windings; 
Inductor L: 140 turns of three twisted strands of #30 wire on a 
3019P-L00-3C8 Ferroxcube Ferrite Pot Core with a 120 mil air gap; 
Transformer CT3: Wound on Magnetics Toroid 40503-TC of W Ferrite Material 
with five turns of #28 wire for the primary winding and 20 turns of #32 
wire for the secondary winding; 
The frequency of inverter oscillation associated with the component values 
identified above is approximately 33 kHz. 
2. Description of Operation 
In FIG. 1, the source S represents an ordinary electric utility power line, 
the voltage from which is applied directly to the bridge rectifier 
identified as BR. This bridge rectifier is of conventional construction 
and provides for the rectified line voltage to be applied to the inverter 
circuit by way of the B+ bus and the B- bus. 
The two energy-storing capacitors C1 and C2 are connected directly across 
the output of the bridge rectifier BR and serve to filter the rectified 
line voltage, thereby providing for the voltage between the B+ bus and the 
B- bus to be substantially constant. Junction CJ between the two 
capacitors serves to provide a power supply center tap. 
The inverter circuit of FIG. 1, which represents a so-called half-bridge 
inverter, operates in a manner that is analogous with circuits previously 
described in published literature, as for instance in U.S. Pat. No. 
4,184,128 entitled High Efficiency Push-Pull Inverters. 
Upon initial application of power to the circuit, inverter oscillation is 
initiated by way of one or a few trigger pulses applied to the base of 
transistor Q2 by way of the combination of capacitor C3 and Diac D2. Of 
course, once the magnitude of the B+ voltage has stabilized, no further 
trigger pulses will be provided; and, if for some reason the inverter 
ceases to oscillate, the only way to get it restarted is to remove and 
then re-apply the power line voltage. (To permit speedy inverter 
re-starting, a bleeding resistor may be connected between the B+ bus and 
the B- bus.) 
The output of the half-bridge inverter is a substantially squarewave 33 kHz 
AC voltage provided between point X and junction CJ. Directly across this 
output is connected a resonant or near-resonant L-C series circuit--with 
the fluorescent lamp connected in parallel with the tank-capacitor 
thereof. 
The resonant or near-resonant action of the L-C series circuit provides for 
appropriate lamp starting and operating voltages, as well as for proper 
lamp current limiting; which is to say that it provides for appropriate 
lamp ballasting. 
The essential feature of the present invention, which involves that of 
disabling the inverter in case the inverter output power remains at an 
excessive magnitude for more than a very brief period, is accomplished by 
way of the sub-assembly referred to as A in FIG. 1. 
More particularly, when the inverter is operating, the voltage developed 
across the tank-capacitor is essentially only limited by the 
voltage-clamping characteristics of either the fluorescent lamp FL or the 
Varistor V--i.e., by the one which clamps at the lower voltage. If the 
lamp is inoperable, or if the lamp is removed from the circuit, or during 
the brief period before the lamp ignites, the Varistor acts as the 
principal voltage-clamping means; and the circuit load current then flows 
through this Varistor. As soon as the lamp gets into operation, however, 
the voltage across the tank-capacitor (and thereby across the Varistor) 
falls to a magnitude that is so low that current will no longer flow 
through the Varistor. 
In the arrangement of FIG. 1, the various relevant voltage and current 
magnitudes are approximately as follows: (i) maximum required lamp 
starting voltage: 500 Volt RMS for not more than 100 milli-Second; (ii) 
Varistor RMS and peak clamping voltage, as well as energy-handling 
capability: 511 Volt RMS, 750 Volt and 80 Joules, respectively; lamp 
operating voltage and current: 140 Volt RMS and 0.2 Amp RMS, respectively. 
In an LC series-resonant circuit, the power provided to a resistive load 
connected in parallel with the circuit tank-capacitor is approximately 
proportional to the magnitude of the load resistance. Hence, in FIG. 1, as 
long as the parameters of the LC circuit have been arranged to provide the 
fluorescent lamp with its required 0.2 Amp operating current at 140 Volt 
RMS (which corresponds to 28 Watt), the load power resulting at higher 
voltages will be roughly proportionately larger. Thus, at the point where 
the Varistor is clamping (at about 511 Volt RMS), the power provided to 
the Varistor is on the order of 100 Watt. However, since the fluorescent 
lamp is supposed to start within 100 milli-Second, the total cumulated 
energy dissipation in the Varistor is limited by the lamp to about 10 
Joule. 
That is, under normal conditions, current will flow through the Varistor 
for but a very brief period of time. Thereafter, the lamp starts and the 
Varistor in effect gets disconnected. 
However, if the lamp is inoperative or not connected, the amount of energy 
that would be dissipated in the Varistor would rapidly exceed its 
energy-handling capability. In particular, for the parameters indicated 
above, the maximum energy capable of being absorbed by the Varistor would 
be reached in only 0.8 Second. 
As long as current is flowing through the Varistor, it also flows through 
the primary winding CT3p of current-transformer CT3; which roughly implies 
that a corresponding output current can be obtained from the secondary 
winding CT3s. By way of rectifier HSR1, the positive component of this 
output current is used for charging energy-storing capacitor EC; which, 
after a brief period, accumulates a charge and develops a corresponding 
voltage. After this capacitor voltage has reached a magnitude high enough 
to cause the Diac D1 to break down, the accumulated charge on the 
capacitor is discharged into the base of transistor Qa--the magnitude of 
the discharge current being limited by the resistance of R2. 
With a Diac breakdown voltage of about 30 Volt and a capacitance value of 
33 uF for the energy-storing capacitor EC, the amount of charge 
accumulated at the point of breakdown is about 1 milli-Coulomb. Thus, if 
the breakdown is to occur in a time period of about 250 milli-Second 
(which is chosen as being a suitable value), the magnitude of the current 
supplied to the capacitor would have to be about 10 milli-Amp; which is 
indeed what is approximately provided in the circuit of FIG. 1. 
Now, as the Diac breakes down, the 1 milli-Coulomb charge on capacitor EC 
discharges into the base of Qa--limited mainly by the 1 kilo-Ohm 
resistance of R2. With the Qa transistor being thusly switched into a 
conductive state, albeit for just a brief moment, a very low impedance 
path is provided between the base and the emitter of transistor Q2. As a 
result, the inverter feedback path is broken and the inverter stops 
oscillating. 
And, of course, once it has stopped oscillating, the inverter will not 
restart until trigger pulses are provided by way of Diac D2; and these 
trigger pulses will not occur until the B+ voltage is made to change 
significantly. Thus, without having made other provisions, the inverter 
will not restart until the power line voltage has been removed and then 
re-applied at a later time--after much of the charge on the filter 
capacitors has had a chance to leak off. 
Of course, to permit a speedier re-initiation of inverter operation, a 
bleeding resistor may be connected across the two filter capacitors. 
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.