GB2093613A

GB2093613A - D.C. power supply

Info

Publication number: GB2093613A
Application number: GB8200200A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-05-08
Filing date: 1982-01-05
Publication date: 1982-09-02
Also published as: GB2093613B

Abstract

The D.C. power supply, which is suitable as a D.C. power source for a power supply for a gas discharge lamp, receives A.C. line power and includes rectification means (13) for converting the A.C. line power to pulsating D.C., which is supplied, and filtered, as the input to a switching regulator (16a) which is designed so as to be deliberately nonresponsive to the frequency of the pulsating D.C. so as not to compensate for the variation of input voltage at the pulsating frequency. The output (17, 18) of the switching regulator (16) is filtered by a large capacitor (19) which is sufficiently large so as to filter at the frequency of the rectified A.C. input. The regulator switching duty cycle is controlled in response to the average D.C. level across this filter capacitor and in this way a unity power factor is achieved without third harmonic distortion. <IMAGE>

Description

SPECIFICATION D.C. power supply The present invention relates to a d.c. power source suitable for a power supply for a gas discharge lamp, and particularly to such a device employing a switching regulator reflecting a unity power factor and no third harmonic distortion to the a.c. line.

Various types of gas discharge lamps are widely used for lighting purposes. These include fluorescent lamps, high intensity discharge lamps of different types including the metal halide varieties and sodium lamps of both high and low pressure. A common feature of all these lamps is that they require some type of ballast for operation. Ordinarily ballast transformers are used. This approach has several shortcomings. For operation at line frequency, the ballast must be of substantial physical size and weight, resulting from the large magnetic transformers and capacitors that are required. Efficiency is low. The ballast must be operated at the rated line voltage, and any serious deviation can cause either the ballast to overheatorthe lampto flicker.

One approach of the prior art to overcome these difficulties has been the use of switching regulators to provide to the lamp a direct current that is switched on and off at a high frequency. While d.c.

will light the lamp adequately, a specially designed lamp is required if the lamp lifetime is not to be sacrificed considerably. Moreover, such circuits require that the a.c. line voltage first be rectified and filtered for input to the regulator.

If inductive filtering is used, a very poor power factor will result, and the inductor may have to be the same large size as the original ballast. Also, line distortion is created by the combination of the inductor and the bridge rectifier. If capacitive filtering is used, all of the current will be conducted during the peak of the a.c. line cycle. This produces "third harmonic distortion" which heats up the pole transformers and requires extra heavy wiring between the device and the power source.

Another approach of the prior art is shown in the U.S. Patent No. 3,999,100. This supply uses a switching regulator in conjunction with a commutator to provide power to a metal halide lamp. The commutator is operated at or near the a.c. line frequency.

The object of the present invention is to provide an improved d.c.power supply, such as suitable for use in a power supply for a gas discharge lamp, particularly wherein no third harmonic distortion is produced and the supply has a near unity power factor.

According to the present invention there is provided a direct current power supply comprising: a source of D.C. voltage which includes rectification means for converting A.C. line power to pulsating D.C.; a switching regulator; connection means allowing the essentially unfiltered puslating D.C. to be the source for said switching regulator; said switching regulator made deliberately nonresponsive to the frequency of the pulsating D.C. supplied by said rectification means so as not to compensate for the variation of input voltage at the pulsating frequency; said switching regulator output is filtered by a large capacitor to make up for the regulator's nonresponsivenesss to the variation in input voltage to said switching regulator caused by said pulsating D.C.; wherein said nonresponsiveness is accomplised by maintaining the on to off time ratio of the said switching regulator essentially constant over the full A.C. half cycle with any such ratio changes occurring over the average input voltage such that changes will not appreciably affect said ratio during any one half cycle of A.C. line power.

The switching regulator is driven by unfiltered rectified a.c. line power. The regulator utilizes an output filter capacitor which is effective at twice the a.c. line frequency. The switching duty cycle or rate is responsive to the regulator output voltage averaged over several halfcyclesofthea.c. linefrequen- cy. In this way, a unity power factor is achieved without third harmonic distortion.

Brief description of the drawings The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings in which: Figure 1 is an electrical block diagram of a power supply for a gas discharge lamp incorporating a d.c.

power supply according to the present invention; Figures 2A and 2B are electrical schematic diagrams of alternative switching regulator circuits that can be used in the d.c. power supply of Figure 1; and Figures 3A, 3B and 3Care graphs illustrating the effect on power factor of various types of switching regulator circuits.

Description of the preferred embodiment The following detailed description is of the best presently contemplated modes of carrying out the invention. This description is not to be taken in a limiting sence, but is made merely for the purpose of illustrating the general principles of the invention since the scope of the invention best is defined by the appended claims.

Operation characteristics attributed to forms of the invention first described also shall be attributed to forms later described, unless such characteristics obviously are unapplicable or unless specific exception is made.

Referring to Figure 1, a power supply 10 is used to energize a gas discharge lamp 11 which may be of the mercury vapor, metal-halide, sodium or fluorescent type. To this end, a.c. power from a source such as the 60 Hz, 120 volt power lines, is connected via a pair of terminals 1 2a, 1 2b to a bridge rectifier 13.

The unfiltered output of the bridge rectifier 13 is supplied via the lines 14 and 15 to a switching regulator 16. No filtering is used at the output of the bridge rectifier 13 to eliminate both (a) the lagging power factor and distortion which would result if an inductorwere used to filter the rectified a.c., and (b) the third harmonic distortion which would result if a capacitor were used.

The switching regulator 13 provides a substantially constant d.c. voltage to a pair of output lines 17, 18. A filter capacitor 19 connected across the output lines 17, 18 has a value sufficiently large so as to filter at twice the a.c. line frequency (e.g. at 120 Hz).

The duty cycle of the switching regulator 16 is controlled in response to the output voltage across the capacitor 19, and thereforwill depend on the average voltage level over several cycles of the rectified a.c. supplied to the input of the regulator 16.

This is in contradistinction to the typical prior art arrangement in which the switching regulator duty cycle is responsive to changes in the input line voltage and in which a small output filter capacitor is used. Such a capacitor filters effectively at the regulator switching frequency (typically 20k Hz) but not at twice the line frequency (e.g. at 120 Hz).

Therefor such a prior art regulator draws more current from the a.c. line during the low voltage portions of the a.c. cycle and less current during the high voltage portions. Such regulation produces a lagging power factor similar to that produced by inductive input filtering, and hence is undesirable.

As noted above, this is eliminated by the inventive arrangement in which the output filter capacitor 19 is sufficiently large so as to filter over several cycles of the supplied rectified a.c. voltage.

The regulated d.c. voltage on the lines 17, may be supplied to any load requiring direct current, however, in Figure 1 it is supplied to an inverter 20 which typically operates at a nominal frequency of 20k Hz. Thus the output of the inverter 20 is an a.c.

voltage square wave having a nominal 20k Hz frequency. This a.c. voltage is fed to the lamp 11 via a resonant network 21 which includes the lamp 11 as an element of a resonant circuit. A feedback path 22 carries a control signal that adjusts the frequency of the inverter 20 to correspond to the resonant frequency of the network 21. In this manner, a resistive load is seen by the inverter 20 regardless of the operating condition of the lamp 11, and sinusoidal voltage is provided to the lamp, resulting in improved lamp efficiency. The inverter switches at the current null points of the network 21 output sinusoidal voltage, since the inverter 20 operates at the network 21 resonantfrequncy. Switching losses are reduced since the current through the inverter switching transitors is at or near zero when switching occurs.Therefor less expensive transistors of lower rating can be used.

Dimming ofthe lamp 11 can be accomplished by controlling the voltage supplied by the regulator 16 to the inverter 20. To this, an external adjustment 23 is provided to control the duty cycle and hence the voltage output of the switching regulator 16. Alternatively, the regulator 16 duty cycle may be controlled by a photocell 14 positioned to sense the light level at a location illuminated by the lamp 11. In this way, if sunlight produces a high ambient light level at the location, this will be sensed by the photocell 24, resulting in dimming of the lamp 11. Energy is conserved while the desired illumination level is maintained.

In some applications it may be desirable to control the output to the lamp 11 in response to the current flowing to the lamp. This can be accomplished by providing a current sensing resistor 25 in series with one of the line 25, 26, to the lamp 11, as shown in Figure 1. A feedback line 26 is connected between the resistor 25 and the switching regulator 1 6'so as to control the output voltage cycle in response to the sensed output current to the lamp 11.

Figure 2A shows one type of switching regulator 1 6A that can be used as the regulator 16 in the power supply 10. A switching transistor 30 is turned on and off by an oscillator and drive circuit 31 at a rate (typicaly 30k Hz) that is above the audio range. When the transistor 30 is on, some energy is stored in an inductor 32 which maintains current during the off-time of the transistor 30 via a diode 33. A small RF filter capacitor 34 prevents RF signals, which may be generated by the switching transients of the transistor 30 from being conducted back to the a.c.

line.

In such a regulator 1 6A, the output voltage is a direct function of the input supply votage time the duty cycle or on-to-off time ratio of the transistor 30.

Since the output of the bridge rectifier 13 is unfiltered, the supply voltage to the switching regulator 16 varies between zero and the peak a.c. line voltage, at twice the input a.c. frequency. In a conventional switching regulator, the on-to-off time ratio normally is varied in response to the input supply voltage so that the on-time is greatest when the input voltage is least. As illustrated in Figure 3A, this results in maximum current flow when the input voltage is minimum. This corresponds to a negative or very lagging power factor, and is undesirable.

This effect can be overcome by reversing the conventional approach and (a) making the on-time of the switching transistor 30 a maximum when the input voltage is greatest, and (b) using an inductor 32 that is sufficiently large so as to maintain substantially the same current through the switching transistor 30 regardless of the input voltage. With this arrangement, the current waveform can be matched to the voltage waveform, by controlling the on-time at each portion of the a.c. line half cycle as illustrated in Figure 3B, thereby creating the effect of a resistive load or unit power factor.

An alternative approach is (a) to make the inductor large with respect to the switching frequency (e.g. 30 kHz) but small with respect to the a.c. line frequency (e.g., 60 Hz), and (b) to hold the duty cycle (on-to-off time ratio) of the switching transistor 30 constant over a complete half-cycle of the input a.c., but allowing it to vary only with gradual changes in the average input line voltage or required output voltage. In this case, the value of the output capacitor 19 must be sufficientto filter the strong, twice line frequency (e.g. 120 Hz) ripple that will be present at the regulator output. With this arrangement, the current waveform can be made very closeiy to correspond to the input voltage waveform, as illustrated in Figure 3C. The result is a unity power factor devoid of harmonic distortion.

A different form of switching regulator 16B, also usable as the resulator 16 in the power supply 10, is shown in Figure 2B. Here the inductor 35a is the primary winding of a ferrite core transformer 35, and is connected in serieswith the switching transistor 36 across the input lines 14, 15 from the bridge rectifier 13. With this arrangement, the inductor 35a is loaded and unloaded each time the transistor 36 is turned on and off atthe switching frequency. Output d.c. voltage is taken from the transformer secondary winding 35b via a diode 37. This circuit offers the advantage that should the switching transistor 36 become shorted for any reason, input current will not flow to the load.

In the circuit 16B, the output voltage may be regulated either by varying the switching frequency or the duty cycle. With variable frequency control the oscillator and drive circuit 38 advantageously includes a voltage controlled or like oscillator, the nominal frequency of which is controlled by theexternal adjustment 23, the photocell 24 or the feedback signal on the line 26. Here again, the capacitor 19 should be suficiently large so as to filter effectively at twice the a.c. line frequency. Changes in the filtered output voltage are used to vary the frequency of the oscillator and drive circuit 38 thereby to connect the switching frequency so as to achieve substantially constant output voltage.

The present application has been divided out of Application Nol 7915784, in which there is described and claimed a power supply for a gas discharge lamp comprising: a source of D.C. voltage; an inverter connected to receive D.C. voltage from said source comprising a pair of switching transistors for alternately switching D.C. voltage from said source; a resonant network whereby the output of the inverter as presented to the lamp will be sinusoidal in wave shape; said inverter includes a transfomer, the output of said transformer being connected to said resonant network which is in turn connected to the lamp; an oscillator connected to control the switching rate of said pair of switching transistors to switch said transistors at such time as the current flowing in or out of said resonant circuit shall be at or near zero; a feed-back means which determines that said resonant network is at resonance and that it presents a resistive load to said inverter; a frequency control means for said oscillator connected to the feedback means in such a manner as to control the frequency to maintain resonance in said resonant network.

Claims

1. A direct current power supply, comprising; a souce of D.C. voltage which includes rectification means for converting A.C. line power to pulsating D.C.; a switching regulator; connection means allowing the essentially unfiitered pulsating D.C. to be the source for said switching regulator; said switching regulator made deliberately nonresponsive to the frequency of the pulsating D.C. supplied by said rectification means so as not to compensate for the variation of input voltage at the pulsating frequency; said switching regulator output is filtered by a large capacitor to make up for the regulator's nonresponsiveness to the variation in input voltage to said switching regulator caused by said pulsating D.C.; wherein said nonresponsiveness is accomplished by maintaining the on to off time ratio of the said switching regulator essentially constant over the full A.C. half cycle with any such ratio changes occurring over the average input voltage such that changes will not appreciably affect said ratio during any one half cycle of A.C. line power.

2. A power supply according to claim 1 wherein said switching regulator comprises; a switching transistor connected in series with a storage inductor in series wifth a still further output filter capacitor, the output being taken from across said capacitor; a diode connected across the series combination of said storage inductor and said output capacitor; and, drive circuitry for said switching transistor deliberately made nonresponsive to changes in the input voltage at the pulsating D.C. frequency that are reflected at the output.

3. A power supply according to claim 1 wherein a small capacitor is inserted across the output of the rectifier means and thus the input of said switching regulator, said capacitor having an impedance high enough not to cause any filtering affects at the low line frequency but low enough to adequately filter the high frequency of the inverter; said capacitor averaging the high frequency switching pulses and thus producing in conjunction with the constant on to off ratio of the switching transistor a current wave form essentially in phase with the line voltage input.

4. A power supply according to claim 1 wherein said switching regulator comprises; a storage inductor type transformer; a primary winding of said transformer connected in series with a switching transistor; a secondary winding of said transformer polarized to discharge when said switching transitor is in the nonconductive mode by means of a diode connecting in series with said secondary winding; a drive for said switching transistor which is made nonresponsive to the frequency of the pulsating D.C.

supplied by said rectification means; and an output filter capacitor made large to make up for the regulators nonresponsiveness to the pulsating D.C.

supply.

4. A power supply according to claim 1 wherein said switching regulator comprises; a storage inductor type transformer; a primary winding of said transformer connected in series with a switching transistor; a secondary winding of said transformer polarized to discharge when said switching transistor is in the nonconductive mode by means of a diode connecting in series with said secondary - winding; a drive for said switching transistor made deliberately nonresponsive to the pusating D.C.

supplied by said rectification means; and and output filter capacitor made large to make up for the regulators nonresponsiveness to the pulsating D.C.

supply.

5. A power supply according to claim 4 wherein a small filter capacitor is connected across the input of said switching regulator to filter the high frequency only.

6. A power supply according to claim 1 wherein said switching regulator comprises; a switching transistor connected in series with a storage inductor large enough to maintain near constant current during the line frequency time of one half cycle in series with an output filter capacitor, the output being taken from across said capacitor; a diode connected across said storage inductor and output capacitor series combination; drive circuitry for said switching transistor deliberately made inversely responsive to the line frequency voltage changes to produce an average current equal in wave form to that of the line input voltage; and, a small filter capacitor connected across the in put to said switching regulator to filter the high frequency only.

Amendments to claims filed on 14 May 1982 Superseded claims 1,2 and 4 New or amended claims:

1. A direct current power supply, comprising; a source of D.C. voltage which includes rectification means for converting A.C. line powerto pulsating D.C.; a switching regulator; connection means allowing the essentially unfiltered pulsating D.C. to be the source for said switching regulator; said switching regulator being adapted to be nonresponsive to the frequency of the pulsating D.C. supplied by said rectification means so as not to compensate for the variation of input voltage at the pulsating frequency; said switching regulator output is filtered by a capacitor which is sufficiently large to compensate for the nonresponsiveness of the switching regulator to the variation in input voltage to said switching regulator caused by said pulsating D.C.; wherein said nonresponsiveness is accomplished by maintaining the on to off time ratio of the said switching regulator essentially constant over the full A.C. half cycle with any such ratio changes occurring over the average input voltage such that changes will not appreciably affect said ratio during any one half cycle of A.C. line power.

2. A power supply according to claim 1 wherein said switching regulator comprises; a switching transitor connected in series with a storage inductor in series with a still further output filter capacitor, the output being taken from across said capacitor; a diode connected across the series combination of said storage inductor and said output capacitor; and, drive circuitry for said switching transistor which is made nonresponsive to changes in the input voltage at the pulsating D.C. frequency that are reflected at the output.