WO1995012300A1

WO1995012300A1 - Double resonant driver ballast for gas lamps

Info

Publication number: WO1995012300A1
Application number: PCT/US1994/012387
Authority: WO
Inventors: David A. Christian
Original assignee: Marshall Electric Corp
Current assignee: Marshall Electric Corp
Priority date: 1993-10-28
Filing date: 1994-10-28
Publication date: 1995-05-04
Anticipated expiration: 1996-04-28
Also published as: CN1060307C; CN1134216A; CA2173873C; AU8128194A; CA2173873A1

Abstract

The present invention involves a parallel double resonant power supply (20') for a gas filled discharge lamp (22, 22'). The control section (32') provides input undervoltage lockout, excess primary voltage lockout, and major output current imbalance lockout functions which de-energize the power supply in the event of faulty operating conditions being sensed.

Description

DOUBLE RESONANT DRIVER BALLAST FOR GAS LAMPS BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electronic ballast circuitry, and, more particularly, to ballast circuits for 5 driving gas-filled arc-type lamps.

2. Description of the related art.

Gas-filled arc-type lamps, e.g., neon lamps, require high voltage input signals for proper operation. Such lamps are commonly driven by output transformer ballast

10 inverters. Electronic ballast circuits generally include a transformer and an oscillation device which accept an input power source and operate to alter the input power source to provide a high frequency, high voltage output having proper characteristics to drive the gas-filled arc-

15 type lamp load. Electronic ballast circuits utilize electronic switches in the form of transistors to satisfy the high frequency requirement, typically controlled by pulse-width modulation. The transistors are connected to operate in a push/pull mode, thereby providing a high

__0 frequency oscillating signal. High frequency operation provides the advantage of reduced power consumption by way of iirproved lighting efficiency and reduced power dissipation in the ballast.

The voltage outputs of prior art ballast designs may

25 rise to potentially hazardous levels in the event of a fault condition, for example, an overvoltage input or circuit energization across an open load. Under such circumstances, prior art circuits may suffer internal damage. Prior art ballast control circuitry for gas-

30 filled arc-type lamps typically derives power directly from the input transformer power source, resulting in unnecessary power consumption and heat build up. Another potential problem traditionally associated with ballast circuits involves the squealing and humming caused by

35 internal circuit electronic interferences. Standing wave patterns or "bubbles" are a problem associated with gas-filled arc-type lamps. The power supplied to the lamp resonates and causes zones of the gas inside the lamp tubing to become ionized, with the length of the ionized zones being related to the speed of sound inside the lamp and the internal geometry of the lamp tubing. This resonant condition results in dark and light zones in the gas filled lamp which appear as "bubbles" visible to the human eye. What is needed is circuitry which provides a voltage- limited output to prevent damage or injury caused from operation into open or short circuits.

Another need is to monitor the output for operation into impermissible loads and shut down the system operation upon the occurrence of such conditions.

Another need is for an inverter which substantially eliminates electronic interference, and thereby eliminates the squeal and hum associated with such gas filled lamps. Another need is for an output signal that automatically disperses "bubbles" or visible standing waves when the lamp is illuminated without the need of operator adjustment.

Another need is for a control circuit which reduces power consumption and temperature rise. SUMMARY OF THE INVENTION

The present invention provides a fullwave push/pull double parallel resonant inverter circuit for use as a gas-filled arc-type lamp driver ballast. The inverter circuit provides a high voltage, high frequency, low current output signal to drive the gas filled lamp load and disperse standing wave patterns, or "bubbles". The inverter is comprised of a control section which derives its power from a secondary winding opposite the in-line transformer DC power supply inductor choke which supplies a reduced level power supply to the control circuit to reduce power consumption and eliminate unnecessary temperature rises. The inverter control section is comprised of an op-amp circuit which provides an input undervoltage lockout section, an excess primary overvoltage lockout, and a major output current imbalance lockout to self-protect the device against faulty operation. These and other features of circuit allow its use with a great range of arc-type lamps without requiring special installation measurements and adjustments. The design includes the following functional sections: DC power source, start-up bias, control circuit power source, oscillator control circuit, output transformer section, and load side signal balancing section.

The DC power section accepts a standard 120 VAC, 60 Hz power source. This signal is rectified through a fullwave bridge rectifier in parallel with a filter capacitor such that a rectified filtered DC power source is supplied to the start-up bias section. The start-up bias section comprises a large value resistor used to develop the start-up bias for the control circuitry. As opposed to traditional electronic ballast circuits which drive the oscillator control circuitry source directly from the DC supply, in this instance approximately 150 V, the present invention includes a secondary winding opposite the series current choke inductor which provides a biased DC source of 10 V to 15 V. This biased DC control circuit power source greatly reduces temperature rise and power consumption. The control circuit power source drives the op-amp devices and the switching transistors.

The present invention provides for the dispersion of standing waves, thereby eliminating the degradation of the lamp appearance caused by unsightly gas bubbles. Standing wave dispersion is achieved by introducing an even order harmonic distortion to the primary waveform which causes an equivalent distortion on the secondary side. Due to the sensitive nature of the gas, a constant flow of ionized gas is achieved. Even order harmonic distortion may be accomplished by several means, including, but not limited to, using a capacitor or inductor in parallel with the primary output transformer winding resulting in an unbalanced conductance with respect to the tap of the primary. This unbalancing of the admittance on the primary side has the effect of "moving" the bubbles along through the lamp rendering such bubbles invisible to the human eye. The type or shape of the tube is determinative of the degree of harmonic distortion required to sufficiently disperse the problematic standing waves. The present invention is configured to provide proper dispersion over a wide variation in lamp tube design.

The present invention features a quad op-amp device which provides an input undervoltage lockout, excess primary voltage lockout, and a major output current imbalance lockout. The input undervoltage lockout device disables the circuit by preventing the switching transistors from switching to a conducting state and thereby prevents current flow through the inverter. The excess primary voltage lockout senses the current allowed through the Zener diode voltage clamp and, upon sensing an excessive level, resets the control circuit, thereby placing the switching transistors in a non-conducting state. The major output current imbalance lockout senses the input and output sides of the transformer, and shuts down the switching transistors when insufficient power is delivered to the output side.

One embodiment of the invention provides a resistor between the center tap of the output winding and the internal circuit ground which is monitored by an op-amp. When the secondary load becomes imbalanced, current flows through the resistor. As the imbalance increases, the voltage across the resistor rises and, upon attaining a prescribed level, the circuit sets the lockout circuit, thereby placing the switching transistors in a non-conducting state.

The present invention provides for enhanced voltage limited operation. If the voltage level across the primary exceeds a certain level, the series zener diode voltage clamp conducts and connects the path to circuit ground. The clamp current is monitored and an excessive level sets the control circuitry. Due to the tight coupling of the primary and secondary windings in the current fed ballast and the use of the balancing capacitors, the output voltage accurately reflects that of the primary winding. By limiting the primary side voltage, the circuit correspondingly limits the output voltage.

The present invention also provides reduced electrical interference. Due to the high frequency resonant nature of the device, the output lamps tend to stay at least partially ionized through both halves of each cycle. This results in less arcing noise, less arc establishment noise, less broad band static, and less radiated noise.

The high voltage circuitry of the present invention is encased in a dense, resilient, hydrophobic compound which exhibits superior high voltage and high frequency insulation characteristics.

One object of the present invention is to provide a voltage limited inverter with a performance characteristic resulting in a voltage limited output. Another objective of the invention is to provide monitoring and safety disabling capabilities in the event of impermissible loads, input undervoltage start-up lockout, and current limiting overvoltage shunting in order to reduce damage potential. Another . ject of the invention is to provide reduced temperature rise and power consumption and therefore a more efficient lighting circuit.

Another objective of the invention is to provide the end user with simplicity and increased safety in operation. Another object of the invention is to significantly reduce electrical interference in the form of noise and standing waveforms.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

Figure 1 is a schematic circuit diagram of a first embodiment of the neon driver ballast circuitry; and

Figure 2 is a schematic circuit diagram of a second embodiment of the neon driver ballast circuitry. Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates preferred embodiments of the invention, in two forms, and such exemplification is not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION The preferred embodiments disclosed below are not intended to be exhaustive or limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may utilize their teachings.

Figure 1 schematically illustrates a first embodiment of gas-filled arc-type lamp, such as neon, driver ballast 20 in which tuned transformer Tl delivers a sufficient high frequency AC output across contact points W2 and W3 to excite the neon or other gas in discharge lamp 22. Driver ballast 20 consists of DC power source 24 having start-up section 26, high frequency series push-pull inverter circuit 28, and transformer Tl.

Transformer Tl is comprised of windings PI through P4, SI and S2, and a ferrite core possessing characteristics such that the primary and secondary windings are tightly coupled resulting in a double parallel resonant circuit. This tight coupling allows for reliable sensing of the secondary load conditions at the primary winding.

DC power source 24 derives its power from standard 120 VAC 60 Hz power source Wl. On/off switch SI energizes and de-energizes electronic ballast system 20. DC power source 24 additionally consists of surge inrush current lim:ter SRI, low frequency filter capacitor Cl, over teιr,_ arature fuse FI, serpentine fuse F2, and inductors LI and L2. Full wave bridge rectifier circuit 30 includes a bridge circuit formed of diodes D1-D4 and MOV1, which is connected across the AC main as a protection device to guard against voltage spikes. Rectifier circuit 30 in conjunction with capacitor C2 provides a rectified and filtered DC power source to drive inverter traⁿsformer Tl. Start-up section 26 includes capacitor C2, re., _-tor Rl, and diode D5. This input rectifier and filter arrangement provides DC voltage and blocks reverse conduction of high frequency noise. The resulting DC output of the full wave bridge rectifier is approximately 150 V.

Inductor L3 is placed in series with ^C power source 24 and serves as a DC current choke to primary windings PI and P2 of transformer Tl. Rather than feed control circuitry 32 directly with DC power source 24 of approximately 150 V, secondary choke winding L4 is utilized to generate a DC bias voltage of 10 V to 15 V. Stepping down from 150 V to the range of 10 V to 15 V serves to lower the 15 to 25 watts of control bias normally required down to 3 to 4 watts thereby conserving power and reducing temperature rise. Start-up bias resistor Rl develops the start-up bias for control circuitry 32. Inductor L3 also provides a current source along path 34.

Inductor L3 feeds DC supply current to primary windings PI and P2, providing a high impedance, a low DC resistance, and a filtering function in conjunction with capacitor C2. Accordingly, resonant switching wave forms are not allowed into the main supply. Capacitor C5 provides an unbalanced capacitance with respect to tap 36 of the primary winding, as will be discussed in detail below.

Control circuitry 32 is comprised primarily of op- amps U1A, U1B, U1C, and U1D which serve as an input undervoltage lockout, a set/reset flip/flop device, an unbalance current lockout, and an impermissible load lockout. Undervoltage lockout op-amp U1A prevents system operation until main DC supply 24 reaches a sufficient level to drive ballast system 20. Until such time, driver U1C is in an "off" state providing no bias current to the Darlington pair Q3 and Q4. By depriving the Darlington circuit of sufficient bias current, current is not allowed to pass from collector through emitter of transistors Q3 and Q4 and consequently current is not allowed to flow through the inverter circuit. Op-amp U1D acts as an impermissible load sensor and, in conjunction with current sensing resistor R20, de-energizes the control circuit in the event of insufficient delivered load power by setting the flip/flop circuit of op-amp U1B. By setting op-amp U1B, switching transistors Q3 and Q4 are deprived of sufficient bias current along current path 34 to drive switching transistors Q3 and Q4 to their respective conducting states. By placing switching transistors Q3 and Q4 in their respective nonconducting states, current is not allowed to flow through collector to emitter of the switching transistors thereby precluding primary winding current flow through the current path 38.

Push/pull full wave oscillator 28 comprises oscillation transistors Ql and Q2. Oscillation transistors Ql and Q2 act in a push/pull manner with feedback windings P3 and P4 to provide an oscillation feedback signal inversely corresponding to the voltage apparent at primary windings Pi and P2. This feedback signal alternately switches power transistors Ql and Q2 to their respective conducting states, thereby alternately allowing primary winding current to pass through collector to emitter. Diodes D6 and D7 rectify the voltage signal produced by primary windings PI and P2, and zener diodes D23 and D24 act as an overvoltage clamp. If the voltage across the primary windings exceed the threshold limit of the zener diode series combination D23 and D24, then these devices will conduct and allow current to flow through to circuit ground. In series with zener diodes D23 and D24 is resistor R12. Should the current through resistor R12 exceed a predetermined voltage level, then op-amp U1B is triggered and the flip/flop circuit is set.

The frequency of push/pull fullwave oscillator 28 is determined by primary side capacitor C3 in parallel with the primary windings. These elements acting in parallel resonate the collectors of oscillator transistors Ql and Q2. Primary windings P3 and P4 provide feedback to the bases of oscillator transistors Ql and Q2. Secondary windings SI and S2 provide high voltage sine waveform to output resonating capacitors C7 and C8 and the load, lamp 22, at outputs W2 and W3. Resonating capacitors C7 and C8 also act as ballasting reactances for the load, and provide blocking of direct and low frequency alternating current to the load. Output windings SI and S2 are connected in series to resonating capacitors C7 and C8 and in series with the load resulting in gas discharge lamp 22 being an element in the resonant ballast circuit with the resonating capacitors C7 and C8 tuning the output frequency.

The preferred embodiment of the transformer calls for having primary winding PI, P2 coaxially positioned about a core, such as an E-type core. The secondary winding SI, S2 should be coaxially disposed about the primary winding with both primary and secondary windings being mounted on a common core. This provides tight coupling between the primary and secondary windings and overall reduction in size. In addition, the tight coupling provides self- sheilding, resulting in virtual elimination of magnetic field radiation and associated power losses. In this manner, the output of the secondary winding will emulate the waveform associated with the primary winding and the secondary waveform may be accurately monitored from the primary side.

In order to avoid the formation of bubbles within the gas filled tubes, that is to avoid degrading the appearance of the lamps due to standing waveforms, an even order harmonic distortion is introduced to the output waveform. This is achieved by causing an imbalance on the primary windings of transformer Tl. This imbalance can be created by any of several different means including but not limited to: clipping, clamping, and alternating voltages. In the preferred embodiment, capacitor C3 connected between the primary windings and common achieves the desired unbalancing. This imbalance results in distorted sine waves. One half-cycle will be slightly longer than the other with slightly lower peak voltage.

This results in similar distortion in the current flowing through gas discharge lamp 22 at W2 and W3. The gas inside the tube is very sensitive to this distortion such that it results in a constant flow of ionized gas toward one end or the other. This flow disperses standing waves and serves to inhibit the formation of bubbles within the lamp tube.

Secondary side load current imbalance monitoring circuit 50 includes capacitor Cll and C12, diodes D8 and D9, and resistor R21. Capacitors Cll and C12 and resistor R21 are coupled between center tap 52 of secondary windings SI and S2 and circuit ground. If the load is reasonably balanced, e.g., plus or minus 20% with respect to circuit ground, then insufficient current flows through resistor R21 to develop sufficient voltage to set op-amp U1B. In the event of a major current imbalance, resistor R21 generates sufficient voltage to set op-amp U1B. Once op-amp UIB is set, the power transistor arrangement of high frequency oscillation circuit 28 is placed in a non-conducting state thereby disallowing current flow through the primary windings.

The high voltage components are encased in a dense, resilient, hydrophobic compound exhibiting superior high voltage and high frequency insulation characteristics. In the exemplary embodiment, the encasing compound is polyurethane based potting compound.

The following is a list of component parts used in the neon driver ballast circuit as embodied in Figure 1.

Table Of Component Values Used In Figure 1

SYMBOL VALUE

D1-D4 800V, 3A

D5, D25- -D26 100V, 1A (Fast)

D6-D7 800V, 1A (Fast)

D17 24V, 1W, 5% Zener

D8, D9, D18-D20, D22, D27 100V, 0.1A

D21 5.1V, 1/2W, 5% Zener

D23, D24 300 V Zener

Cl 0.47UF

C2 220UF

C3 18nF

C4 1.0UF

C5 4.7nF

C6 220uF

C7-8 lOOpF

C9-10 luF

Cll 0.027UF

C12 3300pF

Rl 47KΩ

R2 2.7Ω

R3, R17 10KΩ

R4-R7, R8-R12 100KΩ

R13 12KΩ

R14 780KΩ

R15 1.2MΩ

R16 1.8KΩ

R18 1MΩ

R19 240Ω

R20 0.47Ω

R21 1KΩ

Ql, Q2 700V, 5A (MJE13005)

Q3 TIP-41, -41A, -4IB or 41C Q4 2N4401 Ul LM324N(Quad Op-Amp 14 pin DIP)

MOV1 150VAC, 7.0 j MOV2 360VDC, 7.0 j MOV3 240VDC, 7.0 j SRI 3A, SG-220 (Surge

Inrush Limiter)

FI 3A, 250V(250deg F INT) F2 (Serpentine Trace Fuse)

LI 95-100 turns, 23.5 gage L2 95-100 turns, 23.5 gage L3 270-300 turns, 24 gage L4 45 turns, 30 gage

PI 40 turns, 24 gage P2 48 turns, 24 gage P3 1 turn, 24 gage P4 1 turn, 24 gage

SI 1100 turns, 39 gage (heavy build)

S2 1100 turns, 39 gage (heavy build)

The embodiment shown in Figure 2 is a schematic block diagram representation of the ballast circuit shown in Figure 1 with an alternative monitoring circuit for detecting significant current imbalances with respect to the secondary load. The ballast circuit of Figure 2 includes a DC power source, control circuitry, transformer section, oscillation transistor section, and output section.

Ballast circuit 20' accepts AC power source 24' and rectifies the signal by full wave bridge rectifier 30'. The resulting rectified and filtered DC power source is coupled to current choke inductor L3' and is supplied along current path 34' to the primary windings of ballast transformer Tl• . Secondary choke inductor L4* is coupled to current choke inductor L3• and steps down the DC power source through start-up section 26' to provide a reduced power source to power control circuitry 32' at input 40.

Control circuit 32' monitors the DC power source via undervoltage lockout input 42 whereby ballast circuit operation is precluded upon sensing an insufficient source of power being supplied to the primary windings of ballast transformer Tl^• . Where the voltage across the primary windings exceeds the threshold limit of overvoltage clamp 44', then current flows to circuit ground. Coupled to overvoltage clamp 44' is resistor R12 ' which, upon developing an excessive voltage level, may reset control circuit 32• through input 46 and interrupt ballast circuit operation. As current flows through the primary side of transformer Tl', oscillation circuit 28*, and control circuitry 32', resistor R20¹ develops a voltage signal which is introduced to control circuit 32• at input 48. Upon the occurrence of an undercurrent condition, the voltage developed across resistor R20' is sufficient to set control circuit 32' and thereby interrupt ballast circuit operation.

Transformer Tl' is comprised of windings PI' through P4• , SI• and S2' , and a ferrite core possessing characteristics such that the primary and secondary windings are tightly coupled resulting in a double parallel resonant circuit. This tight coupling allows for reliable sensing of the secondary load conditions on the primary side.

Secondary side load current imbalance monitoring circuit 50' includes DC isolation capacitor C9, diode D28, and resistor R21. Capacitor C9 and resistor R21¹ are coupled between center tap 52' of secondary windings SI and S2 and circuit ground. If the load is reasonably balanced, e.g., plus or minus 20% with respect to circuit ground, then insufficient current flows through resistor R21 to develop sufficient voltage to set control circuit 32 at input 54. In the event of a major current imbalance, resistor R21 generates sufficient voltage to set control circuit 32' at input 54. Once control circuit 32* is set, the power transistor arrangement of high frequency oscillation c_rcuit 28' is placed in a non-conducting state thereby disallowing current flow through the primary windings.

Feedback windings P3' and P4' provide high frequency oscillation circuit 28' with a feedback signal inversely (i.e., anti-phase) corresponding to the voltage apparent at the primary windings. This feedback winding alternately switches the transistor arrangement found within oscillation circuit 28' to a conducting state, thereby allowing primary winding current to pass through oscillation circuit 28' to circuit ground.

The frequency of oscillation circuit 28' is determined by primary side capacitor C3• in parallel with the primary windings. These elements acting in parallel resonate the collector(s) of the oscillator transistor(s) associated with oscillation circuit 28'. Windings P3• and P4• provide feedback to the base(s) of the oscillator transistor(s) associated with oscillation circuit 28'. Windings SI• and S2* provide high voltage sine waveform to output resonating capacitors C7' and C8' and the load at outputs W2' and W3• . Secondary windings SI¹ and S2' are connected in series to resonating capacitors C7' and C8• and in series with the load resulting in gas discharge lamp 22' being an element in the resonant ballast circuit. In order to avoid the formation of bubbles within the gas filled tubes, that is to avoid degrading the appearance of the lamps due to standing waveforms, an even order harmonic distortion is introduced to the output waveform. This is achieved by causing an imbalance at the primary coil of transformer Tl' . This imbalance can be created by any of several different means including but not limited to; clipping, clamping, and alternating voltages. In the preferred embodiment, capacitor C3' is connected between the primary coil and common to achieve the desired unbalancing. This imbalance results in distorted sine waves. One half-cycle will be slightly longer than the other, with slightly lower peak voltage. This results in a similar distortion in the current flowing through gas discharge lamp 22* . The gas inside the tube is very sensitive to this distortion such that it results in a constant flow of ionized gas toward one end or the other. This dispersion of standing waves serves to inhibit the formation of bubbles within the lamp tube.

While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application i therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

Claims

1. A power supply device for a gas-filled arc-type lamp (22; 22') comprising: a transformer (Tl; Tl•) having a core with a primary coil (PI, P2; PI', P2•) and a secondary coil (SI, S2; SI' S2•) ; a power source (24, 28, 30; 24', 28', 30') coupled to said primary coil of said transformer core; said power source providing a generally sinusoidal signal; means for connecting (W2, W3) said secondary coil of said transformer to the gas-filled arc-type lamp; and characterized by introducing means

(C3;C3*) for introducing even order harmonic distortion on said generally sinusoidal signal on said primary coil of said transformer and generating a generally sinusoidal distorted signal whereby a flow of ionized gas is created within the gas-filled arc-type lamp to disperse any bubbles and standing waves in the lamp.

2. The power supply of Claim 1, characterised in that said transformer secondary coil is coaxially disposed about said primary coil and said primary and secondary coils are mounted on a common core.

3. The power supply of Claim 1, characterised in that said power source comprises a rectifying means (30; 30') for receiving an AC power source (24; 24') and providing a DC power supply, and an oscillating means (28; 28•) for generating the generally sinusoidal signal, said oscillating means being coupled to said transformer primary coil.

4. The power supply of claim 1, characterised in that said introducing means operates by initiating a waveform unbalancing as measured at said primary coil of said transformer thereby resulting in a DC bias equivalency in the gas-filled arc-type lamp, said introducing means including one of an unbalanced inductance and unbalanced capacitance (C3; C3 ') with respect to a non-centered tap of said primary coil resulting in the dispersion of standing waves in the gas- filled arc-type lamp.

5. The power supply of claim 1, characterised by control means (32; 32') for controlling said power source, current choking means (L3; L3') for coupling said primary coil of said transformer to said power source, and secondary choking means (L4; L4') for providing power to said control means, said secondary choking means being operatively associated with said current choking means.

6. The power supply of claim 5, characterised in that said control means comprises switching transistors (Q3, Q4) and an op-amp network (U1A, UIB, U1C, U1D) , said op-amp network includes undervoltage lockout means (U1A; 32', 42) for monitoring said power source and deactivating said oscillating means upon sensing insufficient power being supplied to said transformer.

7. The power supply of claim 5, characterised in that said control means comprises switching transistors (Q3, Q4) and an op-amp network (U1A, UIB, U1C, U1D) , said op-amp network includes underload limiting means for sensing current passing through said primary coil of said transformer, said undercurrent limiting means (UIB, UlD; 32') including a resistive element (R20; R20') connected to the input of said op-amp network such that the output of said op-amp network deactivates said oscillating means upon the occurrence of insufficient primary coil current flow.

8. The power supply of claim 5, characterised in that said control means comprises switching transistors (Q3, Q4) and an op-amp network (U1A, UIB, U1C, UlD), said op-amp network includes current imbalance monitoring means (50; 50') for monitoring output load current imbalances, said current imbalance monitoring means including a resistive element (R21; R21¹) coupled between a center tap (52; 52') of said secondary coil and circuit ground, said resistive element generating a voltage signal sufficient to de-energize said power supply upon the occurrence of a major current imbalance.

9. The power supply of claim 3, characterised by feedback means (P3, P4; P3' , P4*) for generating feedback signals to alternately activate said oscillating means, said feedback means being coupled to said transformer and comprising a center tapped feedback coil.

10. The power supply of claim 9, characterised in that said oscillating means comprises power transistors (Ql, Q2) , and said power transistors are alternately driven to their respective conducting states by said feedback signals in a push/pull manner; said power transistor collectors are coupled in parallel to said primary coil of said transformer and an unbalanced capacitor (C3; C3') , whereby said unbalanced capacitor determines the frequency of said oscillating means; and further characterised by two resonating capacitors (C7,

C8; C7' , C8') coupled between said secondary coil and the gas-filled arc-type lamp.