GB2042830A - Ballast circuit for discharge lamp - Google Patents

Abstract

A tuned output high frequency inverter (9) drives a lamp load (11). A voltage corresponding to the load current is derived in feedback rectifier 15, and fed to storage capacitor 91. This voltage then smoothes the pulsed d.c. input voltage derived from rectifier 7, by means of isolating diode 93 and also stabilises the output voltage of the inverter. The starting circuit (19) including diac 101, provides a negative pulse to transistor 43, turning transistor 41 on via the transformer 63 upon which, storage capacitor 91 is charged from the feedback rectifier 15 and the diac can no longer breakdown. Hence the starting circuit is effectively isolated whilst the oscillator is running. <IMAGE>

Description

SPECIFICATION Direct drive ballast with starting circuit This invention relates to ballast circuitry for fluorescent lamp loads and more particularly to directly driven ballast circuitry wherein a high frequency inverter dependent upon current flow in a load circuit is energized by a relaxation-type oscillator starting circuit.

Ballast circuitry' for a great many fluorescent lamp systems is of the auto-transformer type which is undesirably heavy, cumbersome, and expensive as compared with most electronic-type circuitry. Moreover, auto-transformer type ballast circuitry tends to be relatively inefficient of energy causing undesired heating which is obviously detrimental. Also, such apparatus operates in the audible frequency range which results in undue and undesired noise and is annoying to a user.

As to electronic type ballast circuitry, one form of such circuitry is set forth in Patent Specification No. 9906/78. Therein, a charge storage and charge storage isolating capability is provided in apparatus which includes a high frequency inverter circuit. However, the high frequency inverter circuit is independent of unexpected load changes which is a less than satisfactory operational condition.

In another known form of electronic ballast circuitry, the high frequency inverter circuit is load dependent which enhances the operational capability. However, the drive system for the high frequency inverter circuit is relatively complex which, in turn, undesirably increases the component and assembly costs. Moreover, circuit complexity is usually in diametric opposition to enhanced reliability.

In still another form of electronic ballast circuitry, a load dependent high frequency inverter is utilized in conjunction with a charge storage and charge storage isolating circuit. Moreover, the high frequency inverter drive circuitry is relatively uncomplicated and a starting circuit initiates operation of the high frequency inverter. However, the starting circuit requires an amplifier system which adds complexity and expense to the apparatus.

In one aspect of the present invention, an improved direct drive electronic ballast circuit includes a high frequency inverter circuit coupled to a pulsating DC potential source connected to an AC potential source. The high frequency inverter circuit is coupled to a load and the load is coupled by a drive circuit to the high frequency inverter circuit. A charge storage and isolating circuit shunts the high voltage rectifier and is coupled to a feedback rectifier circuit and to the high frequency inverter circuit. Moreover, an improved oscillator starting circuit for the high frequency inverter is directly coupled to the rectifier circuit, the charge storage and isolating circuit and the feedback rectifier circuit, and is AC coupled to the high frequency inverter circuit. The oscillator starter circuit may include a voltage breakdown device, e.g. a diac.

The invention is illustrated by way of example in the accompanying drawing, the single figure of which is a schematic illustration of a direct drive ballast circuit having a starting circuit in accordance with the invention.

Referring to the drawing, a preferred form of direct drive ballast circuitry suitable for use with a lamp load includes an AC potential source 3 coupled by a line conditioner circuit 5 to a rectifier circuit 7 for providing a pulsed DC potential. The rectifier circuit 7 is coupled to a high frequency inverter circuit 9 which is, in turn, coupled to a lamp load circuit 11. The load circuit 11 is directly connected to a high frequency inverter drive circuit 1 3 coupled to the high frequency inverter circuit 9.

A feedback rectifier circuit 1 5 in series connection with the output of the high frequency inverter circuit 9 provides energy to a charge storage and charge isolating circuit 1 7 shunting the rectifier circuit 7. A starting oscillator circuit 19 is directly coupled to the rectifier circuit 7, the charge storage and charge isolating circuit 17, and the feedback rectifier circuit 1 5. Also, the starting oscillator circuit 1 9 is AC coupled to the high frequency inverter circuit 9.

More specifically, the line conditioner circuit 5 includes a transient suppressor 21, which may be in the form of a metal oxide varistor or back-toback transistors for example, shunting the AC source 3. One side of the AC source 3 line is coupled via an overload switch 23 to a first inductor 25 while the other side of the AC source line is coupled to a second inductor 27. Both the first and second inductors 25 and 27 are preferably affixed to the same core to maximize the mutual inductance therebetween. Also, a capacitor 29 is coupled across the first and second inductors 25 and 27.

The rectifier circuit 7 is preferably in the form of a full-wave bridge-type rectifier. Specifically, the rectifier circuit 7 has a first pair of diodes 31 and 33 connected to one line and a second pair of diodes 35 and 37 connected to the opposite line of the line conditioner circuit 5. A filter capacitor 39 is shunted across the diodes 35 and 37.

Connected to the rectifier circuit 7 is the high frequency inverter circuit 9 which includes a pair of series connected substantially identical transistors 41 and 43 shunting the rectifier circuit 7. The junction 45 of the series connected transistors 41 and 43 is coupled to a series resonant circuit including a capacitor 47 and the primary winding 49 of a second transformer 51 as well as to a center-tapped inductive winding 53.

Also, each of the transistors 41 and 43 has emitter and base electrodes coupled to a drive winding 55 and 57 shunted by a damping resistor 59 and 61 respectively. Moreover, these drive windings 55 and 57 are the secondary windings of a first transformer 63.

The high frequency inverter circuit 9 has a high frequency inverter drive circuit 1 3 wherein the secondary windings 55 and 57 of the first transformer 63 are energized by the primary windings 65, 67 and 69 respectively which are, in turn, directly connected to a load 11. Therein, the secondary windings 71 and 73 and filament windings 75, 77 and 79 respectively of the first transformer 51 are series connected to a pair of lamps 81 and 83.

Also, a feedback rectifier circuit 1 5 in the form of a voltage-doubler circuit includes the centertapped winding 53 in series connection with the primary winding 49 of the second transformer 51.

This center-tapped winding 53 is coupled by a capacitor 85 to the junction of a pair of diodes 87 and 89 forming a voltage doubler circuit.

Moreover, the center-tapped winding 53 is adjustable in order to control the energy feedback of the system.

Shunting the rectifier circuit 7 and coupled to the voltage-doubler circuit 1 5 is a charge storage and charge isolating circuit 17. Therein a charge storage capacitor 91 and charge isolating diode 93 are in series connection across the rectifier circuit 7 with the junction 95 therebetween coupled to the diode 89 of the feedback rectifier circuit 1 5 and to a resistor 97 shunting the capacitor 91.

Additionally, a starting oscillator circuit 1 9 includes a series connected first impedance 99 and diac 101 connected to the rectifier circuit 7 and to the feedback rectifier circuit 1 5 as well as to the junction 95 of the charge storage and charge isolating circuit 17. From the junction of the first impedance 99 and diac 101, a second impedance 103 and capacitor 105 are series connected to the transistor 43 of the high frequency inverter circuit 9.

As to operation, a potential from the AC source 3 is filtered by the line conditioner circuit 5. This line conditioner circuit 5 serves as a transient signal filter as well as a radio frequency interference (RFI) filter. Therein, the transient suppressor 21 provides a "clipping" capability for undesired transient signal spikes appearing at the AC source 3. These "clipped" signals are then filtered by the first and second instructors 25 and 27. Moreover, these first and second inductors 25 and 27 acting in conjunction with the capacitor 29 provide an RFI filter capability which inhibits the appearance of such undesired signal features at the rectifier circuit 7. Thus, the potential applied to the rectifier circuit 7 is essentially devoid of undesired transient spikes and RFI signals.Also, capacitor 29, inductors 25 and 27 filter RFI, generated by the high frequency inverter, which prevents RFI from getting out on the AC source.

The rectifier circuit 7 which is in the form of a bridge-Type full-wave rectifier responds to the applied AC potential to provide a pulsating DC potential at a frequency of about 1 20Hz. In turn, this pulsating DC potential is altered, in a manner to be explained hereinafter, to provide a relatively steady-state DC potential which is applied to the high frequency inverter circuit 9.

The high frequency inverter circuit 9 is in the form of a chopper or square wave oscillator having a pair of substantially similar transistors 41 and 43 which switch in a push-pull mode. The chopper or oscillator has a series resonant output circuit which includes the capacitor 47 and primary winding 49 of the second transformer 51. This series resonant circuit has a resonant frequency of about 20 KHz, which is well above the audio range and therefore removed from the area of deleterious effect upon the consumer. Also, the series resonant output circuit provides a low impedance path to current flow therethrough and any such increase in current flow is accompanied by the usual increase in current flow in the secondary windings 71 and 73 of the second transformer 51.

Importantly, increased current flow in the secondary windings 71 and 73 of the load circuit 11 is accompanied by an increased current flow in the primary windings 65, 67 and 69 of the first transformer 63. In turn, the secondary drive windings 55 and 57 provide increased base drive for the series connected transistors 41 and 43 of the high frequency inverter circuit 9. Thus, the high frequency inverter circuit 9 not only derives drive potentials from the series resonant loop of capacitor 47 and inductor 49 but is also dependent upon and driven by current flowing in the load circuit 11.

Also, increased current flow in the resonant circuit including the winding 49 is accompanied by an increased current flow in the inductive winding 53. This increased current flow in the inductive winding 53 is rectified by the voltage doubler circuit, including diodes 87 and 89, and applied to the charge storage capacitor 91 of the charge storage and charge isolating circuit 1 7.

Therein, the charge storage capacitor 91 serves to store energy while the charge isolating diode 93 isolates the capacitor 91 from the pulsating DC potential source 7 so long as the pulsating DC potential remains greater than a given reference level. However, when the pulsating DC potential does decrease below the given reference level, energy is supplied from the storage capacitor 91 via the diode 93 to the rectifier circuit 7 whereby a relatively steady state DC potential is provided for the high frequency inverter circuit 9.

Further, it has been found that the switching capability of the transistors of a high frequency inverter circuit is enhanced when driven directly from a transformer rather than through a complex base biasing arrangement. However, it has also been found that the high frequency inverter circuit 9 would not self-start when a direct drive system was employed. Moreover, it was also found that minimizing the component count of the starting circuit would reduce costs, facilitate mechanized assembly and increase the reliability factor of the circuit.

As to operation of the starting circuit 19, there is no energy feedback to the charge storage capacitor 91 prior to operation of the high frequency inverter circuit 9. However, the AC source 3 provides energy which causes development of a relatively high voltage across the capacitor 39.

This relatively high voltage, developed at the capacitor 39, causes development of an increasing charge on the capacitor 105 of the oscillator starting circuit 1 9 via the first and second impedances 99 and 103 and the winding 57 of the first transformer 63. Moreover, the high frequency inverter circuit 9 has not yet started to oscillate and no charge is present on the charge storage capacitor 91 of the charge storage and charge isolating circuit 1 7.

When the voltage at the capacitor 105 exceeds the breakover voltage of the diac 101, the capacitor 105 discharges through the impedance 103, the diac 101, the capacitor 91 and the winding 57 of the first transformer 63. The transformer 63 transmits this discharge current appearing at the winding 57 to the emitter-base junction of the transistor 41 of the high frequency inverter circuit 9, biasing the transistor 41 on and starting the oscillator of the high frequency inverter circuit 9. Thereupon, the high frequency inverter circuit 9 charges the charge storage capacitor 91. Thus, the charge on the capacitor 91 is sufficient to prevent the voltage across the isolating diode 93 from reaching a value sufficient to effect breakover of the diac 101.As a result, the starting circuit 1 9 is, for all practical purposes, removed from the operational circuitry upon accomplishment of the task of starting the high frequency inverter circuit 9.

Thus, there has been provided a direct drive electronic ballast circuit having an enhanced starting circuit capability. The ballast circuit is also load dependent whereby alteration in the load causes an immediate effect upon the operation of the apparatus and prevents development of undesired high currents and destruction of the componenty of the apparatus. Moreover, the enhanced starting circuit is inexpensive, reliable and improves the assembly of the apparatus.

While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.

Claims (10)

1. A direct drive ballast circuit coupled to an AC potential source and comprising a rectifier circuit means providing a pulsating DC potential to a high frequency inverter circuit; a load circuit; a high frequency inverter drive circuit coupling the load circuit to the high frequency inverter circuit; a charge storage and isolating circuit shunting the rectifier circuit means and coupled to a feedback rectifier means connected to the high frequency inverter circuit means; and an oscillator starter circuit means directly coupled to said rectifier circuit means, said feedback rectifier circuit means and said charge storage and isolating circuit, and AC coupled to said high frequency inverter circuit means.

2. A direct drive ballast circuit as claimed in Claim 1, wherein said oscillator starter circuit includes a voltage breakdown device directly coupled to said rectifier circuit means, said feedback rectifier circuit means and said charge storage and isolating circuit, and AC coupled to said high frequency inverter circuit.

3. A direct drive ballast circuit as claimed in Claim 1 or 2, wherein said oscillator starter circuit includes a series connected diac and impedance coupled to said rectifier circuit means to the junction of said feedback rectifier circuit means and said charge storage and isolating circuit means.

4. A direct ballast circuit as claimed in Claim 1 or 2, wherein said oscillator starter circuit includes a series connected diac and impedance with a capacitor coupling the junction of said series connected diac and impedance to said high frequency inverter circuit.

5. A direct drive ballast circuit as claimed in Claim 1, wherein the arrangement is such that said starter circuit means responds to said AC source to activate said high frequency inverter circuit and energize said load circuit.

6. A circuit as claimed in Claim 5, wherein said oscillator starter circuit means includes a diac directly coupling said rectifier circuit means to said feedback rectifier means and to said charge storage and isolating circuit and AC coupled to said high frequency inverter circuit.

7. A circuit as claimed in Claim 5, wherein said oscillator starter circuit includes a diac directly coupling said rectifier circuit means to said feedback rectifier circuit means and to said charge storage and isolating circuit and a capacitor coupling the junction of the rectifier circuit means and the diac to said high frequency inverter circuit.

8. A circuit as claimed in Claim 5, wherein said oscillator starter circuit includes a series connected first impedance and diac connected to said rectifier circuit means and to the junction of said charge storage and isolating circuit and said feedback rectifier circuit means, and a series connected second impedance and capacitor coupling the junction of said first impedance and diac to said high frequency inverter circuit.

9. A direct drive ballast circuit substantially as described herein with reference to the accompanying drawing.

10. The features as herein described, or their equivalents, in any novel selection.