Protected low-pressure discharge lamp operating circuit

A fluorescent lamp control circuit which provides power at high frequency, or example in the order of 35 kHz, includes a push-pull oscillator circuit having two transistors (T1, T2), an inductance element (L1, L1'), and a capacitor (C1, C1') to supply the lamps (3, 3'). To prevent dangerous high voltages upon removal of the lamp load, connected to the oscillatory circuit and to the series resonance circuit, upon removal of failure of a lamp, a protective circuit is provided formed by a thyristor (TH) connected to short-circuit at least one of the transistors (T1) of the oscillatory circuit. Control energy is derived directly from the high-voltage supply by a rectifier (D1, D1') connected to the inductance (L1, L1') of the series resonance circuit, so that sufficient and reliable switching energy for the protective thyristor (TH) is always available, the firing criterion therefor being determined by the breakdown voltage of a breakdown element such as a diac (22) connected to a voltage divider (20, 21) and receiving its control voltage also from the high-voltage connection of the series resonance circuit (4) including the inductance, which also feeds the power supply capacitor (C3) for the thyristor (TH).

Reference to related applications, assigned to the assignee of this 
invention, the disclosure of which is hereby incorporated by reference: 
U.S. Ser. No. 193,254, filed Oct. 1, 1980, by the inventor hereof; and 
U.S. Ser. No. 352,781, filed Feb. 26, 1982, by the inventor hereof 
"RAPID-START, LOW-PRESSURE DISCHARGE LAMP OPERATING CIRCUIT". 
The present invention relates to a discharge lamp operating circuit, for 
example to an operating circuit for a fluorescent lamp, capable of use 
with one or more fluorescent lamps, and which utilizes a self-oscillating 
push-pull switch having two transistors. 
BACKGROUND 
The referenced application Ser. No. 193,254, of Oct. 1, 1980, by the 
inventor hereof, discloses a circuit of this type having two similary 
poled switching transistors which are connected to bridge the terminals of 
a direct current supply source. If more than one lamp is operated in such 
a system, each one of the lamp operating circuits has its own series 
resonance circuit which is formed by respective ballast inductances and a 
capacitor. The respective lamp operating circuits and the associated 
series resonance circuits are connected in parallel. 
The referenced application discloses a system which is suitable both for 
single-lamp as well as for multiple-lamp operation which includes a 
protective circuit so that, in case of malfunction for example upon 
non-firing or starting of a lamp, the transistor push-pull switch is 
disconnected, so that damage to the lamps or to the system is prevented. 
Preferably, the protective circuit includes a thyristor which is connected 
to the base of one of the transistors-for example one which is connected 
to the positive terminal of the direct supply, and then to the negative 
terminal of the direct voltage supply. 
The protective circuit as described in the referenced application Ser. No. 
193,254 provides operating voltages for the disconnect transistor solely 
from the filter capacitor of the d-c supply. This, for example, is an 
electrolytic capacitor. Such a filter capacitor is connected to the 
network filter circuit of the bridge rectifier. It has been found that the 
voltage on the filter capacitor changes with loading, and thus the 
switching function of the thyristor will depend on loading on the lamp. In 
case of malfunction, the direct current supply is loaded to a greater 
extent so that, if load interruption should cause such higher loading, 
which is frequently the case when the disconnect thyristor must become 
operative, the switching conditions under which the disconnect thyristor 
operates is impaired due to the lower holding current available therefor. 
The drop in voltage on the filter capacitor of a direct supply network is 
due to the current limiting effect of the filter choke which is provided 
to remove ripple from the network supply. The filter capacitor is capable 
of supplying increased current into the circuit for a short period of time 
without substantial drop in voltage upon shedding of load during normal 
operating conditions, that is, still within the response period of the 
protective circuit. Consequently, the switching conditions are usually 
suitable for emergency switching. If, however, switching conditions occur, 
for example if one lamp is not connected or not properly connected during 
initial connection of the entire unit, no charge with increased voltage 
can build up on the filter capacitor so that, then, the protective circuit 
may not be able to respond due to insufficient energy supply thereto. 
THE INVENTION 
It is an object to improve the circuit described in the aforementioned 
referenced application Ser. No. 193,254, suitable both for single-lamp as 
well as for multiple-lamp operation, by improving the protective circuitry 
therefor so that it will reliably respond regardless of operating 
conditions of the lamp and/or the auxiliary circuit. 
Briefly, a separate capacitor is provided to supply operating power for the 
disconnect thyristor. The separate disconnect capacitor receives its power 
from a rectifier circuit deriving, in turn, energy from the ballast 
inductances. 
The voltage supply in accordance with the invention has the advantage that, 
upon interruption of a load, high voltage peaks will result due to the 
formation of a series resonance circuit formed by the ballast inductances 
and the operating capacitors of the network, so that particularly good 
circuit conditions will obtain leading to switching of the disconnect 
thyristor. This increased voltage in the series resonance circuit can be 
used as a disconnect criterion for the thyristor itself so that separate 
control windings necessary to trigger firing of the thyristor need hot be 
provided. Such additional control windings were usually applied to the 
ballast inductances.

Referring to FIG. 1 showing the general principle of the power system for 
two lamps 3, 3': Terminals 1, 2 provide direct current power. They are 
connected through the collector-emitter paths of two serially connected 
similarly poled high-voltage switching transistors T1, T2. The transistor 
circuit operates as a self-oscillating push-pull oscillator to supply 
power at a frequency substantially elevated above that of power line 
frequency of low-pressure discharge lamps 3, 3', for example standard 
fluorescent lamps. Any desired number of lamps may be supplied from the 
power supply unit upon suitable dimensioning of the components thereof. 
Each one of the lamps has its own ballast inductance L1, L1' and is 
coupled to its own individual series resonance circuit 4, 4', which 
includes the respective inductance L1, L1' and a capacitor C1, C1', 
respectively. The respective lamp operating circuits and the associated 
series resonance circuits are all connected in parallel. If only a single 
lamp is to be operated from the system, only a single accessory circuit 4 
with the resonance circuit L1, C1 is associated with the single lamp. 
Optimum operation is obtained if the d-c voltage supply has a ripple of not 
over about 20%. The transistor push-pull circuit formed by transistors T1, 
T2 is supplied with d-c power from a rectifier 5. The rectifier 5 is 
connected to any suitable power supply network, for example 220 V/50 Hz 
or, for example, 110 V/60 Hz. The rectifier 5 preferably is constructed as 
a bridge rectifier and includes a filter capacitor as well as a network 
ripple filter. When supplied, for example, with 220 V network voltage, it 
provides at output terminals 1, 2 a d-c voltage of about 
.sqroot.2.multidot.220 V. For operation with 110 V/60 Hz, the rectifier 
preferably includes a symmetrical voltage doubler circuit and, if needed, 
a filter network to remove network voltage ripple. The transistor 
push-pull switches T1, T2 apply only half the voltage to the respective 
lamp operating circuits, with alternating polarity. The series resonance 
circuits 4, 4' provide a substantially higher voltage to the respective 
lamps 3, 3', particularly during starting. They also supply a sufficiently 
high operating voltage for continuous operation of the lamps 3, 3'. 
The system can also be used for direct connection to a d-c network (not 
shown). The rectifier and operating supply unit 5 then is not needed. 
The first electrodes 6, 6' of the lamps 3, 3' are connected through the 
respective ballast inductances L1, L1' or, rather, the resonant-tuned 
circuit inductances, to a common supply bus S1 which is connected to the 
center tap or junction 8 between the two transistors T1, T2 of the 
push-pull switching system. The electrodes 6, 6' of the lamps 3, 3' are 
additionally connected to the oscillating capacitors C1, C1' which, in 
turn, are connected to a common bus S2 which is connected to one terminal, 
as shown to the negative terminal 2 of the d-c supply source formed by the 
rectifier and operating supply unit 5. The arrangement may be varied, and 
modified arrangements and positions of the oscillating capacitors C1, C1' 
are shown in FIGS. 2 and 3, to be described in detail below. The second 
electrodes 7, 7' of the lamps 3, 3' are connected to a common bus S3, 
which is in turn connected through a capacitor C2 of relatively high 
capacity to the aforementioned bus S2, in this case the negative bus of 
the d-c power supply unit 5. The capacitor C2 has at least 50 times the 
capacity value of the oscillating capacitors C1, C1'. Consequently, the 
common bus S3 is practically at center voltage with respect to d-c supply 
voltage. The lamps 3, 3' are supplied with alternating power of a 
frequency which is within the audio or high-frequency range. The impedance 
of the capacitor C2 is extremely low at the operating frequency. The 
capacitor C2 acts only as a reactive power load or impedance element. The 
oscillating frequency of the transistor oscillator is determined by the 
series resonant oscillating circuit 4, 4'. The respective lamps 3, 3' form 
the damping load for the respective oscillating circuits. 
The inductances of the oscillating circuits provide control energy for the 
switching transistors T1, T2 of the push-pull circuit over additional 
control windings. Control windings L2, L2', and L3, L3' are inductively 
coupled with the inductances L1, L1' and connected over a network formed 
by rapid switching diodes, resistances, and inductances with the bases of 
the respective switching transistors T1, T2. The control networks for the 
transistors T1, T2 are shown only schematically in FIG. 1, and 
collectively by block 9a, 9b. 
Starting of the transistor oscillator is effected by a start oscillating 
circuit 10 which triggers transistor T2, that is, the transistor which is 
connected to the negative terminal 2 of the power supply, by providing a 
trigger voltage to the base thereof. When the switching transistor T2 is 
rendered conductive by a pulse from the trigger circuit 10, the 
oscillating circuits 4, 4' will start to oscillate, and the feedback 
currents maintain oscillation of the push-pull oscillator T1, T2. 
The operation of the trigger circuit 10 is described in detail in 
referenced application Ser. No. 352,781, by the inventor hereof, filed 
concurrently herewith, entitled "RAPID-START, LOW-PRESSURE DISCHARGE LAMP 
OPERATING CIRCUIT". 
The series resonance circuits 4, 4' operate stably and with low inherent or 
internal losses as long as a lamp 3, 3' is connected as a load and absorbs 
energy. The oscillating circuits 4, 4' must be so damped that the 
switching transistors T1, T2 are suitably controlled. Upon interruption of 
a load, for example if a lamp is to be exchanged, the voltage rises 
rapidly, and substantial losses will result within the accessory 
apparatus. Unless precautions are taken, the accessory apparatus may be 
damaged or destroyed. To prevent destruction, a protective circuit 11 is 
provided which disconnects the transistors T1, T2 rapidly, for example in 
about 1/2 second or less, after an interruption in normal operation 
occurs. Simultaneously, upon removal of one of the lamps 3, 3', for 
example, from the lamp socket, disconnection of the transistors T1, T2 
prevents dangerously high voltages from occurring at the lamp sockets. The 
protective circuit 11 includes a controlled switching element TH, for 
example a thyristor, which is connected in the connection line between the 
base of the switching transistor T1, that is, the transistor connected to 
the positive terminal 1 of the d-c operating power supply unit, and the 
negative terminal 2 thereof. 
The present invention is directed to the specific network of the protective 
circuit 11, and control of the controlled switching element TH, and will 
be described below with reference to FIGS. 2 and 3. 
The resonance capacitors C1, C1' will have a high voltage when the lamps 3, 
3' are not yet ignited or have not yet fired. This high voltage is also 
the idling voltage for the lamp. Due to this high voltage, the lamps 3, 3' 
will be of the "rapid-start" type, thus will fire or illuminate without 
preheating of the electrodes. The lamps, thus, are cold-starting. In order 
to insure reliable switching, starter switches 12, 12' may be supplied 
which bridge the lamps 3, 3' when the network voltage is first connected 
in order to preheat the electrodes 6, 7; 6', 7', as well known. The 
starter switches 12, 12', after the lamps have been preheated, will open 
and thus insure firing. The switches 12, 12' may be rapid-switching 
four-layer diodes, starter switches with a glow element, which have a high 
glow current, and which prevent ignition of the lamp unless the electrodes 
6, 7; 6', 7' have been preheated. Thus, the system can be used both with 
rapid-start cold-firing, as well as with starter-type lamp circuits. 
Network arrangements which by suitable connection of the capacitors C1, 
C1' permit elimination of the starter switches 12, 12' are shown in FIGS. 
2 and 3. 
General circuit arrangement of FIGS. 2 and 3: The rectifier and operating 
power supply unit 5 provides operating power for one (FIG. 2) or two (FIG. 
3) fluorescent lamps 3, 3' of 50 W, 1.5 m length, having a firing or 
ignition voltage of over 800 V. The operating frequency for the lamps is 
35 kHz, and each one of the lamps, including accessory equipment, have a 
power consumption of 56 W, so that, for two-lamp operation together with 
the circuits (FIG. 3), 112 W load will be placed on the power network. The 
rectifier operating power supply unit 5 is shown in FIG. 2 as a bridge 
rectifier 13 to which a large network filter capacitor 14 is connected. A 
filter network 15 protects the network against high-frequency and 
excessively high current peaks or needle pulses. FIG. 3 illustrates a 
rectifier and operarting power supply unit 5', adapted for connection to 
110 V/60 Hz, which has a voltage doubler circuit 13', a network 
high-frequency filter protective circuit 15, and a filter capacitor 14. 
FIGS. 2 and 3, further, show different possibilities for connection of the 
capacitors C1, C1' of the series resonance circuits 4, 4'. 
The control windings L2, L2' and L3, L3' are so subdivided that each one of 
the control circuits has to supply, for n lamps, only the n-th portion of 
the control energy. Thus, for one-lamp operation (FIG. 2) the entire 
control energy must be supplied by the coils L2, L3; for multiple-lamp 
operation, for example two-lamp operation, the windings L2, L2', L3, L3' 
need supply only 1/2 or, generally, 1/n of the control energy. The 
summation of the voltage components optimally should be so high that the 
switching transistors T1, T2 are controlled into at least 
quasi-saturation. The control windings L2, L2' and L3, L3' for the 
respective switching transistors T1, T2 are galvanically separated from 
each other. If more than one lamp is to be operated--see FIG. 3--the 
respective inductances L2, L2'; L3, L3' are serially connected in a group 
of series circuits which bridge the base-emitter path of the respective 
switching transistor T1, T2. If only a single lamp is used, only the 
control windings L2 and L3 are coupled with the ballast inductance L1--see 
FIG. 2. 
The control networks 9a, 9b (FIG. 1) of the switching transistors T1, T2 
receive their energy from the windings L2, L2' and L3, L3' on the 
inductances L1, L1', respectively. 
The exact construction and function of the control networks 9a, 9b is 
described in detail in the referenced application Ser. No. 352,781, filed 
Feb. 26, 1982, by the inventor hereof, entitled "RAPID-START, LOW-PRESSURE 
DISCHARGE LAMP OPERATING CIRCUIT" which also describes in detail the 
oscillation trigger circuit 10. 
In accordance with a feature of the present invention, the protective 
network 11 receives its power supply from the high-voltage portion of the 
network, preferably over an individual charge and power capacitor C3 
(FIGS. 2, 3) which is connected to receive its charge over a rectifier 
circuit, for example formed by diodes D1, D1' connected to the 
high-voltage supply of the network. 
The basic element for the protective circuit 11 is a controlled 
semiconductor switch, as shown and preferably a thyristor TH (FIGS. 2, 3), 
connected to be polarized for conduction to the negative terminal 2. The 
anode of thyristor TH is connected through current limiting resistors 17, 
18 to a diode 16 which is connected to the base of the transistor T1. 
In accordance with a feature of the invention, the ignition and turn-off 
energy for the thyristor TH is derived from a separate capacitor C3 which 
is connected in parallel to the thyristor TH or, rather to the tap or 
junction point between the resistors 17, 18. The capacitor C3, which may 
have a value of between 0.5 and 1 .mu.F, is provided to supply d-c voltage 
and energy for the thyristor TH when the thyristor is to be rendered 
conductive. The direct parallel connection of the capacitor C3 with the 
thyristor TH--as shown via the current limiting resistor 18--reliably 
prevents high-frequency voltage loading of the thyristor TH and consequent 
possible damaging and undesired stray high-frequency triggered firing. 
Charging energy for the capacitor C3, in accordance with a feature of the 
invention, is derived from the high-voltage operating network of the 
circuit. High-voltage rectifier diodes D1 and D1' (FIGS. 2, 3) are used. 
The high-voltage rectifier diodes D1, D1' have to be designed to accept 
the highest possibly arising voltage peaks, that is, for a peak value of 
greater than 2 kV, and reliably block even under such high-voltage peaks. 
The diodes D1, D1' are poled for conduction in the same direction as the 
thyristor TH. Their anodes are connected to the junction between the 
ballast inductance L1, L1', respectively, and the series tank circuit 
capacitor C1, C1'. The cathodes of the diodes are connected in parallel. 
The junction of the cathodes is connected over a charge resistor 19--for 
current limiting--directly to the terminal of the capacitor C3 which is 
also connected in parallel with the thyristor TH, that is, to the anode of 
the thyristor, through the resistor 18. 
The thyristor control circuit, which is supplied by the capacitor C3 from 
high-volage d-c, includes a voltage divider 20, 21 connected in parallel 
to the capacitor C3. The voltage division ratio is so selected that, in 
ordinary normal operation, the voltage across resistor 21 is below the 
breakdown voltage of a diac 22. The gate or control electrode of the 
thyristor TH is connected through the diac 22 to the junction between the 
resistors 20, 21, that is, to the tap point of the voltage divider. The 
diac 22 provides for the switching characteristics, and change-over 
between normal operation and excess voltage, that is, operation with the 
load (the lamp) removed. When the breakdown voltage of the diac 22 is 
reached, diac 22 becomes conductive and fires thyristor TH. Firing or 
ignition energy for the diac is supplied by a capacitor 23 connected in 
parallel to the resistor 21 which, together with the resistor 20, forms a 
timing circuit providing for some time delay prior to response of the 
protective circuit. The dimensioning of the timing circuit 20, 23 is so 
selected that response of the thyristor TH is reliably inhibited during 
the igniting phase of the lamp. For normal lamp operation, and also during 
normal lamp ignition time, the trigger threshold for the thyristor is not 
reached. 
Let it be assumed that one of the lamps is removed, or fails, or there is a 
circuit interruption. Upon removal of the lamp, that is, removal of the 
load on the circuit, the capacitor C3 will charge, although the voltage at 
the filter capacitor 14 of the power rectifier drops, since the capacitor 
C3 will be charged over the rectifier network D1, D1' directly from the 
high-voltage terminal of the inductance. If only one-lamp operation is 
used, only one of the rectifier diodes D1 will be supplying high voltage. 
The capacitor C3, thus, will be charged to a higher voltage through the 
charge resistor 19, serially connected with the diode D1, and D1', 
respectively, for multiple-lamp operation. This circuit, thus, utilizes 
the high voltage increase on the series resonance circuit 4, or 4', 
respectively, which occurs if the resonance circuit is undamped, a 
situation pertaining upon removal of the load formed by the lamp. The 
increase in voltage at the capacitor C3 then reliably will trigger the 
thyristor TH as soon as the breakdown voltage of diac 22 is reached, thus 
firing the thyristor. When the thyristor has fired, the base of the 
transistor T1 will no longer receive control energy since the base will, 
effectively, be granted. Transistor T1 will block and, consequently, the 
oscillatory circuit 4, 4' will be deenergized. 
A resistor 24 is provided, connecting the thyristor TH with the positive 
terminal 1 of the d-c voltage supply. The thyristor TH, thus, will 
continue to receive power, and therefore will remain conductive since it 
will have holding current flowing therethrough. This prevents renewed 
oscillations from starting. Diode 16 blocks the alternating voltage from 
the anode circuit of the thyristor TH. The capacitor 25 in the circuit 
prevents undesired triggering of the thyristor TH in case of occurrence of 
stray or interference peaks, and prevents their application to the control 
electrode of the thyristor. Capacitor 25 is connected between the gate 
electrode and the cathode of the thyristor. Resistor 26, in parallel to 
capacitor 25, prevents response of the protective circuit 11 at 
excessively high inherent or leakage current of the diac 22 and/or of the 
thyristor control electrode; effectively, it bleeds off any gate offset 
current. 
After the circuit has responded, it can be reset only by disconnecting the 
power supply from the rectifier and operating power supply unit 5 or, 
otherwise, by removing voltage from the supply terminals 1, 2. The 
thyristor then will revert to blocking condition, and the system is again 
ready for operation. 
Various changes and modifications may be made, and features shown and 
described in connection with any one of the embodiments may be used with 
any of the others, within the scope of the inventive concept.