GB2124045A - Power amplifier for driving a gas discharge lamp - Google Patents

Abstract

A source of regulated DC voltage (1) supplies a power amplifier section (3). The power amplifier section (3) operates in response to the output of an oscillator and amplifier drive (4) to drive a load (19) in the form of a gas discharge lamp. The power drawn by the power amplifier section (3) is monitored by a current sensor (7). A gain control circuit (6) is responsive to the sensor (7) to vary the oscillator and amplifier drive in a sense to maintain the power fed to the load (19) substantially constant. By this arrangement the power drawn by the lamp will be uneffected by ageing of the lamp or its replacement by another lamp. <IMAGE>

Description

SPECIFICATION Power amplifier for driving a gas discharge lamp BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power amplifier for driving a gas discharge light source or any other form of gas discharge device and more particularly to a novel power amplifier employing a minimum number of components to accomplish superior performance including that of driving the lamp with essentially sinusoidal current, of controlling the amount of energy supplied and of maintaining that amount of energy constant in response to external command without being affected by wide variations in the lamp characteristics either from lamp-to-lamp or changes caused by the lamp's aging.

2. Brief Description of the Prior Art A conventional gas discharge lamp such as a flourescent tube, high pressure sodium or mercury vapor outdoor and indoor lamps require a ballasting device to operate them since they have a negative impedance characteristic. In addition, special voltages are required to cause the lamp to originally ignite. Compensation must be provided for the wide variations in characteristics from lamp-to-lamp.

Conventionally, the ballasting of gas discharge type lamp sources have been accomplished with a reactive device that limits the current flow within the source. These devices have been improved gradually over the years to maintain an acceptable power factor by utilizing the transformer configuration with a high leakage reactance between the primary and secondary in the conjunction with capacitors connected to correct power factor. A high leakage reactance has the effect of producing a somewhat constant current in the discharge while limiting the primary input current. Improvements to the art as described in copending applications by this inventor utilize a solid state DC to AC inverter for producing a high frequency drive to the lamp. There are other devices that utilize square wave drive from an inverter directly to drive the lamp at a high frequency.The waveform may be modified by the insertion of reactive components between the output of the inverter and the lamp load, thus performs the same function as the conventional core and coil ballast but at a much smaller size due to the high frequency.

In the copending application a resonant network is placed between the output of the switching inverter and the lamp load to produce the sinusoidal wave desired. Although this device is effective and will accomplish the desired results, the number of components can be decreased by use of a novel amplifier concept disclosed herein.

SUMMARY OF THE INVENTION Accordingly, the above problems and difficulties are obviated by the present invention which provides a novel power amplifier means for lighting a gas discharge lamp wherein the special voltages and characteristics may be in a manner unlike conventional core and coil ballasts or even high frequency converter ballasts with or without resonant components between the ballast and the lamp.

In one form of the invention, a solid state amplifier is provided having a source of regulated DC voltage and a power amplifier section connected thereto which incorporates a gas discharge device as a load. An oscillator and amplifier drive circuit couples to the power amplifier for supplying appropriate drive signals thereto while a gain control circuit selectively sets the gain of the power amplifier in response to internal or external signals indicating either the amount of power delivered to the amplifier and/or indicating the desired power amplifier output power respectively.

It is among the primary objects of this invention to provide an amplifier type drive for a gas discharge device primarily employed as a light source.

It is the further object of this invention to reduce the number of components employed in a power amplifier for achieving this result below that previously accomplished.

Another object of this invention is to provide an amplifier that may respond rapidly to protect it components from damage that may result in response to rapid and sudden shifts in the lamp load characteristics.

Still another object of this invention is to provide a means of delivering constant power to a gas discharge device.

Yet another object of the inventive amplifier concept is to make sure that transistor switching in the power amplifier section is achieved at a time when there is no current flowing through it inspite of wide variations of load characteristics and sudden shifts of characteristics caused when the gas discharge lamp is first igniting.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the apended claims. The present invention both as to its organization and manner of operation together with further objects and advantages thereof may be best understood by reference to the following description taken in connection with the accompanying drawings, in which: Figure 1 is a block diagram showing the general components of the power amplifier for a gas discharge lamp incorporating the present invention; Figure 2 is a circuit schematic drawing of the- power amplifier section shown in block form in Fig. 1; Figure 3 is a circuit schematic drawing of an alternate network for the power amplifier illustrated in Fig. 2; and Figure 4 is a graph representation of current waveforms in the current sensing resistor of Fig. 2.

DESCRIPTION OF THE PREFERRED EMBODI MENTS The following detailed descriptions are af the best presently contemplated modes of carrying out the invention. This description is not to be taken in a limiting sense but is made merely for the purpose af illustrating the general principals of the invention since the scope of the invention is best described in the appended claims.

Refereing to Fig. 1, the block diagram incorporating the inventive concept includes a regulated source of DC power 1 coupled to a power amplifier section 3 via line 2. A gain control circuit 6 is provided between the power amplifier section 3 and an oscillator and amplifier drive circuit 4 which is snesitive to the current flowint through the power amplifier section 3. A feedback loop is established whereby the output from the oscillator and amplifier drive circuit is responsive to the signal from the gain control circuit 6. This latter circuit is also responsive to a signal derived from an external input along line 8 via the gain control circuit 6.

A load 19, which may be a gas discharge lamp, is operably connected to the output of the power amplifier section 3 and a current sensing network 7 is coupled between the load 19 and the gain control circuit 6 and the source of regulated DC power 1 via line 5.

The source of regulated DC power 1 may be any suitable circuit including the novel switching regulator disclosed in my copending application Patent Number 4,277,728 having the purpose of supplying a regulated filtered direct current for use by the power amplifier section 3. With a regulated voltage, the amount of energy transferred ta the load 19 may be controlled in response to an external control setting by the amount of current flowing through the amplifier. With the voltage from the regulative source 1 being held constant, the power into the amplifier section t will be directly proportional to the current amplitude.

In referring in detail to Fig. 2, the operation of the power amplifier is described as follows: a transistor 13 is driven "on" for a specified period of time and current commences to flow from the regulated source 1 through line 2, through an inductor 14, a transistor 13 and through current sensing resistor 7 to the common line 5. During this period of time, energy is stored in inductor 14. After sufficient energy has been stored in inductor 14, the transistor is biased to its "off" condition. The voltage at the collector of transistor 13, point 12, will commence to rise the rate primarily determined by the value of capacitor 16 and the inductance value of the inductor 14. Partial control of this period is provided by the combined values of capacitor 17, inductor 18, and the load 19.The voltage on capacitor 16 will rise substantially higher than the voltage supplied on input line 2. The energy now stored in capacitor 16 will flow into the network comprising capacitor 17, inductor 1 8 and load 1 9. Inductor 1 8 is selected such as the Q value of the circuit as related to load 19 and will be above 2.5 and preferably in the range of 4 to 5 for a reasonably undistorted sinusoidal wave form at the load.

A portion of the energy in capacitor 16 will flow backwards through inductor 14 to the regulated source 1 until the voltage of the collector transistor at point 12 is equal to that of the regulated source. At this point, resonant action of inductor 14 will continue to drive the voltage back towards common line 5 drawing the charge from capacitor 16. This, coupled with the action of the capacitor 1 7 and inductor 18, will attempt to pull the collector of the transistor below ground. This is prevented by the clamping action of a diode 1 5. The reactive action of capacitor 1 7 and inductor 18 continue to draw currect through the diode 15 until the current through inductor 14 can act as a supply.Prior to this happening, the transistor 13 is turned back to its "on" condition, thus the transistor 1 3 is turned "on" with no current flowing through it and no voltage across it and turned "off" with no voltage suddenly appearing across it.

This presents ideal operating conditions for the transistor.

In the event that the impedance of the load 19 is substantially different than that required for the normal operating voltages and a reasonable selection of values, the network of Fig. 3 incorporating a pair of inductors 22 and 23, a capacitor 24 and load 19 may be substituted at points 20 and 21 in Fig. 2.

Inductors 18 and 22 may be combined into a single inductor. The use of the network in Fig.

3 is also required in situations where the amplifier may be required to operate for an extended number of cycles into an open load condition such as with high intensity discharge lamps before striking. In this case, the network in Fig. 3 provides a reactive path through capacitors 16 and 24 and inductors 18 and 22 to the common line 5 and altows the amplifier to operate properly to give an open circuit voltage eapable of initiating conduction within the gas discharge lamp.

In the case of some gas discharges, it is necessary to have a high voltage trigger pulse in addition to the open circuit voltage to initiate conduction. One advantage of the circuit depected in both Figs. 2 and 3 is that the inductors 18 or 23 in series with the load 1 9 can also serve as ignition transformers utilizing a tap shown at 29 and 30 respectively where a momentary starting pulse may be introduced and transofrmed through the transformer action in the inductor to create a high voltage across the nonconducting load.

One of the unique qualities of this circuit is its ability to maintain a constant power output to the load regardless of changing load characteristics either through lamp aging or from different types of loads being applied. All the current flowing through the amplifier including the load passes through the current sensing resistor 7. The current waveform appears as shown in Fig. 4A under normal operation.

This saw-tooth waveform is integrated in the gain control circuit 6 for an average DC level which is directly proportional to the power since the voltage entering at on line 2 is constant. Thus, by controlling the gain of the amplifier as a function of the average value of the current flowing through it and maintaining that current constant, the power also maintained constant.

An external input entering at line 8 into the gain control circuit can adjust what level of current is desired and therefore what level of power is intended to be delivered to the load.

The current wave form shown in Fig. 4B is found under "open" load conditions when the circuit still draws power but the current flows both directions through the current sensing resistor returning that power to the regulated source each cycle. The amplitude of this negative current is directly proportional to the maximum voltage found at point 1 2 at the collector of transistor 1 3. It is therefore possible to detect this negative amplitude and adjust the drive to transistor 1 3 to maintain this voltage at a safe level. This will also control the maximum open circuit voltage presented to the load across points 20 and 21 for start-up purposes.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

Claims (11)

1. A power amplifier for a gas discharge device comprising the combination of: a source of regulated DC power; a gas discharge load; and power amplifier means operably coupled between said power source and said load for producing a sinusoidal output drive for operating said load in response to current through said power amplifier means.

2. The invention as defined in Claim 1 including: an oscillator and drive circuit for driving said power amplifier producing said output drive to said load.

3. The invention as defined in Claim 2 including: a plurality of electrical reactive elements operably connected to said power amplifier with a solid state component constituting a switching means for producing said sinusoidal output.

4. The invention as defined in Claim 3 including: a gain control circuit operably coupled between a current sensing means through said power amplifier means and said oscillator and drive circuit adapted to appropriately set the gain of said power amplifier means through control of the amount of drive from said oscillator and drive circuit.

5. The invention as defined in Claim 4 including: feedback means within said gain control circuit for indicating the amount of power delivered to said power amplifier means so as to set the aforementioned power gain.

6. The invention as defined in Claim 5 including: an external control coupled to said gain control circuit for indicating a desired power amplifier means output power.

7. The invention as defined in Claim 6 including: said power amplifier means being augmented by inclusion of additional reactive components for improving waveform load and power amplifier means performance.

8. The invention as defined in Claim 7 wherein: said reactive elements comprise at least a pair of inductors coupled in series with each other and said load and a capacitor connected between the junction of the two said inductors and the circuit common.

9. The invention as defined in Claim 8 wherein: the power amplifier consists of a transistor operably connected to an inductor whose other termination is a source of supply voltage and the drive to the base of said transistor being from the said oscillator and drive circuit; a capacitor connected between the collector and emitter of said transmitter; a diode, the anode of which is connected to the collector of said transistor and the cathode connected to the emitter of said transistor; and a second capacitor, second inductor both in series with each other and said load connected between the collector of said transistor and the emitter of said transistor.

10. The invention as defined in Claim 9 wherein: a resistor is placed in series with the emitter of said transistor and all elements connected thereto and the circuit common; and the wave form across said resistor being supplied to said gain control to effect the appropriate circuit operation.

11. A power amplifier for a gas discharge device substantially as hereinbefore described with reference to the accompanying drawings.