EP1784062A1 - Electronic ballast circuit and adjustment method - Google Patents

Abstract

An electronic cut-in unit for operating a fluorescent lamp comprises a half-bridge inverter (Q1,Q2), a voltage divider (R1,R2) in parallel to the inverter input with the lamp (L) between the inverter output and the divider. The touch ratio of the inverter can be altered so that a direct current can be passed to the lamp at a desired level. An independent claim is also included for an operating process for the above unit.

Description

Technical area

The present invention relates to an electric ballast for operating and / or dimming a fluorescent lamp with an inverter and a parallel to the input of the inverter voltage divider, wherein between the tap of the voltage divider and the output of the inverter, the fluorescent lamp is switchable. Moreover, the present invention relates to a corresponding method for adjusting an electronic ballast for the supply of a fluorescent lamp with the AC of an inverter and the DC generated by means of a voltage divider.

State of the art

Dimmed fluorescent lamps under certain circumstances form disc discharges. They express themselves by alternately bright and dark areas in the lamp of a few centimeters in size. The disc discharges can be eliminated by a small DC current, which is sent through the lamp in addition to the high-frequency alternating current. A related circuit is in the patent US 5 034 660 shown.

The DC current needed to compensate for disc discharges is usually generated by a voltage divider and flows through the lamp and the lamp inductor into the half-bridge of the inverter. Such a circuit is from the document EP 0 422 255 A1 known. It has been shown that larger diameter (T8) fluorescent lamps require significantly more DC power to eliminate disc discharges than smaller diameter lamps (T4, T5). On the other hand, it has also been found that too much DC affects the stability in the lower dimming range (by 1% of the nominal luminous flux), especially for the thin lamps. That's why in particular ECGs (electronic ballasts) with lamp detection can adjust the DC current to the diameter of the currently operated lamp.

Another problem associated with DC injection is that DC voltage which is applied to the lamp during preheating of the lamp - that is, as long as the lamp does not burn. This DC voltage can cause the prescribed limits for the RMS value and the crest factor of the open circuit or open circuit voltage across the lamp during preheating to be exceeded.

This problem has so far been countered by a compromise dimensioning. However, it had to be accepted that fluorescent lamps with a larger diameter tend to disc discharges and fluorescent lamps with a smaller diameter are choppy in the cold. In addition, this compromise dimensioning exceeds the threshold values for effective value and crest factor of the empty voltage in some lamps.

Presentation of the invention

The object of the present invention is therefore to propose an electronic ballast with which fluorescent lamps of different diameters can be better operated. In addition, a corresponding method for setting an electronic ballast is to be provided.

According to the invention this object is achieved by an electronic ballast for operating and / or dimming a fluorescent lamp with an inverter and a parallel to the input of the inverter voltage divider, wherein between the tap of the voltage divider and the output of the inverter, the fluorescent lamp is switchable, and wherein the duty cycle of the inverter is variable, so that a DC current through the fluorescent lamp with the duty cycle can be set to a desired value.

In addition, according to the present invention, there is provided a method of adjusting an electronic ballast for supplying a fluorescent lamp with the AC of a half-bridge inverter and a DC current generated by means of a voltage divider by adjusting a duty ratio of the Inverter such that the direct current through the fluorescent lamp reaches a desired value.

In this way, the direct current which is necessary for the operation of the fluorescent lamp can advantageously be varied almost as desired without great effort, without components having to be exchanged. In particular, disk discharges in lamps of larger diameter and flicker phenomena in lamps of smaller diameter can be virtually completely prevented.

Preferably, the inverter of the electronic ballast has a microcontroller with which the duty cycle is adjustable. Thus, the duty cycle can be adjusted by software.

The duty cycle should be able to be varied within a range of 30% / 70% to 70% / 30%. Duty ratios outside this range make the dimensioning of the load circuit more difficult.

According to a likewise preferred embodiment, the voltage divider is dimensioned so that at a duty cycle of 50% / 50%, half of the DC current required for the fluorescent lamp to be used flows through the fluorescent lamp. Thus, the duty cycle compared to the ratio 50% / 50% is not too much detuned.

Brief description of the drawings

The present invention will now be explained in more detail with reference to the accompanying drawing, which shows a schematic diagram of an electronic ballast according to the invention.

Preferred embodiment of the invention

The embodiment described in more detail below represents a preferred embodiment of the present invention.

According to the circuit diagram shown in the figure, a fluorescent lamp L is supplied with direct current via a voltage divider R1 / R2, which divides a DC link voltage Uzw in the corresponding ratio and provides this divided voltage at its tap. At the tap a pole of the fluorescent lamp L is connected.

In addition to the resistor R2 of the voltage divider, a capacitor CK is also closed to ground.

The DC link voltage Uzw also serves as an input voltage for an inverter, which is realized here by a half-bridge consisting of a series connection of two transistors Q1 and Q2. The transistors Q 1 and Q 2 are controlled by a microcontroller μC in a desired duty cycle.

At the node between the transistors Q1 and Q2 a lamp inductor LD is connected. On the other hand, this is connected to the second pole of the fluorescent lamp and a capacitor CZ, which is also grounded.

The DC current generated by the voltage divider R1 / R2 flows through the fluorescent lamp L and the lamp inductor LD in the half-bridge Q1 / Q2. The divider ratio and the internal resistance of the voltage divider R1 / R2 determine the size of the direct current and its course over the dimming range. The duty cycle of the half-bridge Q1 / Q2 is usually 50% / 50%, so that at the node between the transistors Q1 and Q2 in the time average half the DC link voltage Uzw / 2 is applied.

According to the invention, the DC current is also adjusted by changing the DC component of the half-bridge voltage at the node between the transistors Q1 and Q2 via the duty cycle of the half-bridge control. This duty cycle can be easily adjusted via the software in the case of an ECG with lamp detection, in which a microcontroller μC controls the half-bridge Q1 / Q2. The hardware of the circuit does not change. Conveniently, however, the resistors R1 and R2 are dimensioned differently than in the case of a rigid duty cycle.

In the following two dimensioning examples are given. A circuit with a fixed duty cycle of 50% / 50% is chosen as the starting point. With a dimensioning of R1 = 150 kΩ, R2 = 820 kΩ and Uzw = 400 V, a maximum direct current of 789 μA and an empty voltage of 138 V result for the selected duty cycle.

According to the present invention, the DC current is adjusted by the divider ratio and / or the duty cycle. Example 1 below shows the extreme case that the direct current is set solely via the duty cycle. The maximum direct current should again be 789 μA and the empty voltage should be a maximum of 138 V. Since the DC current is adjusted by the duty ratio alone, the resistors R1 and R2 are the same to choose large, namely R1 = R2 = 258 kΩ. In order to achieve the mentioned DC current, a duty cycle of 15, 5% / 84.5% is now required. Since the resistors R1 and R2 are the same size, the DC and the drain voltage at a duty cycle 50% / 50% decrease to 0. However, the selected lamp requires a maximum direct current of 789 μA and thus the mentioned duty cycle of 15.5% / 84.5%, which corresponds to a very strong detuning. This strong detuning complicates the dimensioning of the load circuit LD, CZ and CK, since the load circuit must be completely recirculated each time a transistor Q1, Q2 is switched on in order to avoid switching losses and radio interference.

The following example 2 shows a more favorable dimensioning. Here, the resistors R1 and R2 are chosen so that at a duty cycle 50% / 50%, only half of the DC current required for the lamp flows through the lamp. The other half of the DC current is generated by a detuning of the duty cycle. In this case, the direct current of 789 μA is achieved by the resistors R1 = 192 kΩ and R2 = 393 kΩ with a duty cycle of 32.7% / 67.3%. This means that there is a much weaker detuning so that the load circuit can be dimensioned more easily. When the duty cycle is reversed (67.3% / 32.7%), then the DC and the empty voltage disappear. Intermediate values of the direct current can be set as desired via the pulse duty factor. Thus, a dimmable electronic ballast with variable DC component of the lamp current can be realized without further ado.

Claims (7)

Electric ballast for operating a fluorescent lamp with a half - bridge inverter (Q1, Q2) and a voltage divider (R1, R2) connected in parallel with the input of the inverter, the fluorescent lamp (L) being switchable between the tap of the voltage divider (R1, R2) and the output of the inverter (Q1, Q2), characterized in that - The duty cycle of the inverter (Q1, Q2) is variable, so that a direct current through the fluorescent lamp (L) with the duty cycle can be set to a desired value. Electronic ballast according to claim 1, wherein the inverter (Q1, Q2) has a microcontroller (μC), with which the duty cycle is adjustable. The electronic ballast of claim 1 or 2, wherein the duty cycle is variable in a range of 30% / 70% to 70% / 30%. Electronic ballast according to one of the preceding claims, wherein the voltage divider is dimensioned such that at a duty cycle of 50% / 50% half of the required for the fluorescent lamp to be used (L) direct current flows through the fluorescent lamp. Method for adjusting an electronic ballast for supplying a fluorescent lamp (L) with the alternating current of a half-bridge inverter (Q1, Q2) and with a direct current generated by means of a voltage divider (R1, R2),
marked by - Setting a duty cycle of the inverter (Q1, Q2) such that the direct current through the fluorescent lamp (L) reaches a desired value. The method of claim 5, wherein the duty cycle is variable in a range of 30% / 70% to 70% / 30%. The method of claim 5 or 6, wherein at a duty cycle of 50% / 50% half of the total dc current is generated by the fluorescent lamp from the inverter and the voltage divider is dimensioned accordingly.

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