WO1997030569A1

WO1997030569A1 - In-rush surge limiting electronic ballast

Info

Publication number: WO1997030569A1
Application number: PCT/US1997/002017
Authority: WO
Inventors: William L. Keith; Patrick J. Keegan; Peter W. Shackle; Michael W. Bandel; Bruce R. Rhodes; Ronald J. Bezdon
Original assignee: Energy Savings, Inc.
Priority date: 1996-02-13
Filing date: 1997-02-11
Publication date: 1997-08-21

Abstract

An electronic ballast (40) includes a filter capacitor (26) coupled to the output of a rectifier circuit (25) and a surge limiter (41, 43) connected in series with the filter capacitor for limiting current through the filter capacitor when power is first applied to the ballast. The surge limiter includes a resistor (41) and an SCR (43) connected in parallel between the filter capacitor and a common rail (56). The SCR is triggered by a signal derived from voltages within the ballast. A diode (49), connected anti-parallel with the SCR, by-passes the resistor when current is drawn from the filter capacitor.

Description

IN-RUSH SURGE LIMITING ELECTRONIC BALLAST

BACKGROUND OF THE INVENTION

Field of invention

This invention relates to electronic ballasts for fluorescent lamps and, in particular, to an electronic ballast with reduced surge current when turned on.

Prior art

A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.

A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 hz. )

An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 khz. Since a frequency much higher than 50-60 hz. is used, the inductors in an electronic ballast can be much smaller than the inductor in a magnetic ballast.

Converting from alternating current to direct current is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. Some ballasts include a boost circuit between the rectifier and the filter capacitor for increasing the voltage to the lamp. The filter capacitor has a large capacitance, on the order of 50 μf, and represents a low impedance to the AC line voltage. When an electronic ballast is first turned on, there is typically a large current surge through the rectifier to the filter capacitor, charging the capacitor. When the filter capacitor has charged to approximately the nominal voltage of the AC power line, the current has decreased substantially.

The initial current surge, or in-rush current, requires that the rectifiers used in the full wave bridge have a high current rating and the rectifiers are, therefore, physically large and expensive. If a large number of ballasts are on the same branch circuit, e.g. in a department store, the current surge can be significant and cause problems with automatic switches used in energy conservation systems, such as motion detectors or lighting level controllers.

Many electronic ballast use what is known as a "flyback" boost circuit in which the energy stored in an inductor is supplied to the filter capacitor as small pulses of current at high voltage, utilizing the ^δ δ_t characteristic of an inductor. U.S. patent 3,265,930

(Powell) discloses such a ballast. An inductor between the bridge rectifier and the filter capacitor in a ballast does not limit surge current because the inductor is relatively small, on the order of 2 millihenries, and has a DC resistance of only a few ohms.

It is known in the art to control in-rush current for an incandescent lamp. U.S. Patent 4,855,649 (Masaki) discloses adding a resistor in series with an incandescent lamp to reduce in-rush current. A bi-directional switch is connected in parallel with the resistor to bypass the resistor after a brief delay. Additional circuitry provides a trigger signal to the bi-directional switch, which must be triggered each half cycle of the AC line voltage.

PCT Application Number PCT/IB95/00467 discloses an in- rush current limiting circuit for electronic ballasts in which a resistor is connected between the source of current and a filter capacitor. This embodiment of the circuit is undesirable because it reduces the efficiency of the electronic ballast. In another embodiment of the circuit, the resistor is by-passed by a semiconductor switching element. The switching element is difficult to control because one is operating on the high voltage side of the storage capacitor. Simply using a voltage divider does not overcome the problem. For example, if a component has a turn-on voltage of 3 volts ± 0.1 volt, the high voltage can vary ± 7 volts on a 220 volt system. This broad range affects the timing of the switching element and, therefore, the efficiency of the circuit.

In view of the foregoing, it is therefore an object of the invention to provide an efficient, low cost surge limiter for an electronic ballast.

Another object of the invention is to provide a surge limiter using as few components as possible.

A further object of the invention is to provide a surge limiter for an electronic ballast without affecting the normal operation of the ballast, e.g. without affecting the efficiency of the ballast.

SUMMARY OF THE INVENTION

The foregoing objects are achieved in this invention in which an electronic ballast includes a filter capacitor coupled to the output of a rectifier circuit and a surge limiter connected in series with the filter capacitor for limiting current through the filter capacitor when power is first applied to the ballast. The surge limiter includes a resistor and an SCR connected in parallel. The SCR is triggered by a signal derived from voltages within the ballast. A diode, connected anti-parallel with the SCR, by-passes the resistor when current is drawn from the filter capacitor. If the ballast includes a boost circuit providing a low voltage, it is preferred to obtain the trigger signal for the SCR from the low voltage, thereby minimizing the cost of components and simplifying timing.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram of the basic components of an electronic ballast;

FIG. 2 is a schematic of the converter portion of an electronic ballast constructed in accordance with the prior art; FIG. 3 is a schematic of the converter portion of an electronic ballast constructed in accordance with the invention; and

FIG. 4 is a schematic of an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the major components of an electronic ballast constructed in accordance with the prior art. Converter 12 rectifies the alternating current, producing a direct current which charges filter capacitor 14. Filter capacitor 14 provides the DC power for inverter 16. One or more fluorescent lamps are coupled to output lines 19 of inverter 16. The invention relates to storage capacitor 14 and can be used with any combination of converter and inverter. For example, converter 12 can be a simple rectifier bridge or include a boost circuit, a buck- boost circuit, or a buck circuit. Inverter 16 can be a push-pull inverter, a bridge inverter, or a half-bridge inverter. The output of inverter 16 can be series resonant, parallel loaded, or transformer coupled

FIG. 2 is a schematic of a converter such as could be used for converter 12 in FIG. 1. In converter 20, bridge rectifier 21 has the DC diagonal thereof connected to filter capacitor 26 by way of inductor 23 and diode 25. Inductor 23 is part of boost circuit 24 and has the current therethrough switched by transistor Qi. Each time that transistor Qi shuts off, a high voltage pulse of current is applied to capacitors 26. A pair of capacitors connected in series is illustrated to correspond to the physical implementation of the circuit. A single capacitor could be used instead. Capacitor 29 provides some noise suppression on the input to the boost circuit and has a significantly lower value of capacitance than capacitors 26.

Within boost circuit 24, inductor 27 is magnetically coupled to inductor 23 for driving transistor Qi at a high frequency. Inductor 31 is also magnetically coupled to inductor 23 and has the output thereof coupled through diode 33 to filter capacitor 35. The output of capacitor 35 is a low voltage, e.g. approximately twenty volts, for powering other semiconductor devices in the ballast.

FIG. 3 illustrates a ballast constructed in accordance with the invention in which temporary, current limiting means is included in the ballast for limiting in-rush current. In accordance with the invention, resistor 41 is connected to common rail 56 in series with filter capacitors 26 and SCR 43 is connected in parallel with resistor 41. The gate of SCR 43 is coupled to a voltage divider including resistors 45 and 46, coupled between low voltage rail 48 and common rail 56. In a preferred embodiment of the invention, resistor 41 has a resistance of 10-100 ohms and, therefore, limits the in-rush current to 1-10 amperes, assuming a line voltage of 110 volts.

When power is applied to converter 40, boost circuit 24 begins operating and capacitor 35 charges to approximately twelve volts. The voltage on rail 48 is divided and applied to the gate of SCR 43. Approximately thirty milliseconds after power is first applied to converter 40, SCR 43 fires, bypassing resistor 41. SCR 43 remains conductive for as long as power is applied to converter 40. That is, SCR 43 need only be triggered once each time power is applied to a ballast incorporating the surge limiter of the invention.

Diode 49 is poled oppositely to SCR 43 and bypasses resistor 41 when current is drawn from capacitors 26. SCR 43 conducts the charging current to common rail 56 through capacitors 26 and diode 49 conducts the discharge current through the capacitors to an inverter (not shown in FIG. 3) . Because the surge limiter is connected to common rail 56 and in series with capacitors 26, the semiconductors can be relatively small and inexpensive. Also, voltage sensing and, therefore, timing is more accurate when operating at low voltages. The surge limiter does not limit the low voltage current through the boost circuit (the current through transistor Qi for charging inductor 23 (FIG. 2)), enabling the boost circuit to function normally even when in-rush current is being limited. Thus, a surge limiter in accordance with the invention has minimal effect on the operation of the ballast, uses a minimal number of components, and the components are relatively inexpensive. The trigger signal for SCR 43 can be derived from anywhere else in a ballast, i.e. from the converter or the inverter. FIG. 4 illustrates an example of the invention wherein the trigger signal for SCR 43 is obtained from the high voltage rail rather than from a low voltage rail. Specifically, a voltage divider including resistors 51 and 53 is coupled between high voltage rail 55 and common rail 56. The circuit of FIG. 4 operates in the same manner as circuit of FIG. 3 in that circuit delays existing within the ballast are relied upon to provide a delay in triggering SCR 43.

Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, SCR 43 can be replaced with a power MOS transistor or other solid state switch.

Claims

What is claimed is:

1. An electronic ballast for a gas discharge lamp, said ballast comprising: a rectifier circuit for converting alternating current into direct current; a filter capacitor coupled to the output of said rectifier circuit and charged by said direct current; an inverter coupled to said filter capacitor and having an output for connection to a gas discharge lamp, said inverter converting direct current from said filter capacitor into high frequency alternating current at said output; a surge limiter connected in series between a common rail and said filter capacitor for limiting current through said filter capacitor when power is first applied to said ballast.

2. The electronic ballast as set forth in claim 1 wherein said surge limiter includes: a first resistor coupled in series with said filter capacitor and having a first terminal connected to said common rail; and an SCR coupled in parallel with said first resistor.

3. The electronic ballast as set forth in claim 2 wherein said surge limiter further includes: a diode coupled in parallel with said first resistor, wherein said diode and said SCR are oppositely poled.

4. The electronic ballast as set forth in claim 1 wherein said ballast includes: a boost circuit coupled to the output of said rectifier circuit for augmenting said direct current; wherein said boost circuit is coupled to the gate of said SCR for triggering the SCR after said filter capacitor is charged initially.