EP0674469A2 - Ballast for operating a AC low pressure discharge lamp in a vehicle - Google Patents

Abstract

The circuit has a counter-clock voltage converter with a transformer (T) which has 2 primary windings (Wp1,Wp2), with differing numbers of winding turns (Np1,Np2). The primary windings are coupled to a common potential (UB) in alternation via respective switches (S1,S2), the gas discharge lamp (GEL) connected in series with a capacitor (C) in the transformer secondary circuit. Pref. the switches are controlled via a common timing component (ZB), the potential coupled to a common tap-off terminal (A) between the primary windings via a coupling inductance (L).

Description

The invention relates to a ballast for operating an AC low-pressure gas discharge lamp in a motor vehicle with a current-fed push-pull converter, which has a transformer and which has two circuit breakers, which alternately switch two primary-side transformer windings against a common potential and in the secondary circuit of the Transformer a capacitor and a low pressure gas discharge lamp are connected in series.

When AC low-pressure gas discharge lamps are operated, an inhomogeneous light distribution is particularly noticeable in the case of fluorescent tubes filled with noble gases, which is formed in particular as a stratification of luminous and non-luminous regions in the longitudinal direction of the tubes.

In order to reduce this effect, it is known to superimpose a direct current on the lamp alternating voltage. The direct current source required for this in the secondary circuit of the voltage converter causes additional circuitry and costs.

It is also known that even slight asymmetries in the curve shapes of the lamp alternating current significantly reduce this effect.

Particularly for low-pressure gas discharge lamps, which are to be used in motor vehicles, the ballast is aimed at a circuit which is particularly simple in terms of circuitry because of the resulting small size and the considerable cost savings in large numbers.

It is therefore the object of the present invention a ballast for a
To create AC low-pressure gas discharge lamp, which eliminates the negative effect described above and is characterized by a particularly simple and inexpensive structure.

This object is achieved in that the primary-side transformer windings have different numbers of turns.

A particularly advantageous embodiment of a ballast according to the invention will be shown below with reference to the drawing.

To operate one
AC low-pressure gas discharge lamp (GEL) in a motor vehicle requires a voltage converter which converts the direct voltage (UB) of the vehicle electrical system into an alternating voltage suitable for operating the low-pressure gas discharge lamp (GEL). Such a voltage converter can be designed particularly simply, as shown here, as a current-fed push-pull flow converter. This initially consists of a transformer (T) with two transformer windings (WP1, WP2) on the primary side, which are electrically connected in a common connection point (A). In an alternative, particularly simple embodiment, the two transformer windings (WP1, WP2) can be replaced by a single winding with a tap.

The two transformer windings (WP1, WP2) are alternately energized via two circuit breakers (S1, S2). In order to enable particularly simple control of the circuit breakers (S1, S2), these switch the transformer windings (WP1, WP2) against a common potential; in the exemplary embodiment shown in the figure, this is the ground potential of the vehicle electrical system voltage (UB).

It is particularly advantageous to carry out a temporally symmetrical energization of both transformer windings (WP1, WP2) by means of the circuit breakers (S1, S2), which are usually designed as semiconductor switches. This makes the control of the circuit breakers (S1, S2) particularly simple since they are carried out by the same time-determining components (ZB).

With push-pull converters of this type, control that is both symmetrical in terms of time and current is usually selected in order to prevent saturation effects from DC components in the primary windings (WP1, WP2), which can lead to high currents or even destruction of the circuit breakers (S1, S2).

In order to rule out such saturation effects even when the primary windings are not symmetrically energized, it is known to couple the input current via a coupling inductance (L) into the common connection point (A) of both primary windings (WP1, WP2). The coupling inductance (L) ensures equalization of different currents during the two clock phases.

A low-pressure gas discharge lamp (GEL) and a capacitor (C) are connected in series with the secondary winding (WS) of the transformer (T). The impedance of the capacitor (C) ensures current limitation during operation of the low-pressure gas discharge lamp (GEL).

To prevent the inhomogeneous luminance distribution along the discharge tube described at the outset, the aim is to apply an alternating current to the low-pressure gas discharge lamp (GEL), in which the positive and the negative current half-wave have different curve shapes, but the total current has no resulting direct current component.

This corresponds to the requirement for different curve shapes but the same area (amount of the time integral of the current) for both current half-waves.

Both requirements are met in a surprisingly simple way by the fact that the two primary-side windings (WP1, WP2) of the transformer (T) have different numbers of turns (NP1, NP2), or if the transformer (T) only has one tap (A) Provided winding, the number of turns before and behind the tap are different.

As a result of the mutual energization of the two primary windings (WP1, WP2), two differently shaped current half-waves are induced in the secondary winding (WS) due to the different number of turns.

The requirement for equal area of the current half-waves is met by the capacitor (C), which prevents the flow of a DC component in the secondary circuit (WS-C-GEL).

In the primary circuit, DC saturation of the primary windings is effectively prevented by the coupling inductance (L).

A remarkable advantage of the ballast designed in this way is that the circuit measures according to the invention, namely the transformer windings (WP1, WP2) on the primary side, with different numbers of turns, can be implemented almost cost-neutral.

Reference list Ballast for operating an AC low-pressure gas discharge lamp in a motor vehicle

UB

Vehicle electrical system voltage

L

Coupling inductance

T

transformer

WP1, WP2

Primary windings (of transformer T)

NP1, NP2

Number of turns (of the primary windings WP1, WP2)

WS

Secondary winding (of transformer T)

S1, S2

Circuit breaker

Eg

time-determining components

C.

capacitor

GEL

Low pressure gas discharge lamp

Claims (3)

Ballast to operate a
AC low-pressure gas discharge lamp in a motor vehicle with a current-fed push-pull converter which has a transformer (T) and which has two circuit breakers (S1, S2) which mutually switch two primary-side transformer windings (WS1, WS2) against a common potential and at a capacitor (C) and a low-pressure gas discharge lamp (GEL) are connected in series in the secondary circuit of the transformer (T), characterized in that the primary-side transformer windings (WS1, WS2) have different numbers of turns (NP1, NP2). Ballast according to Claim 1, characterized in that the transformer (T) has only one winding provided with a tap instead of two windings (WS1, WS2) on the primary side. Ballast according to claim 1, characterized in that the two circuit breakers (S1, S2) are controlled by the same components (ZB) which determine the switching time.

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