Electric lamp and discharge devices: systems – Pulsating or a.c. supply – Discharge control discharge device load
Reexamination Certificate
2002-09-18
2004-08-24
Vo, Tuyet T. (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
Discharge control discharge device load
C315S282000, C315S287000, C315S279000
Reexamination Certificate
active
06781324
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a ballast for at least one electric incandescent lamp, having at least two switching elements that are coupled to one another with the formation of a first tie point, a drive circuit with at least one output for driving the at least two switching elements with the aid of a drive signal that is characterized by a pulse duty factor and an operating frequency, and with an input for a dimming signal, and a power transformer whose primary side is coupled to the first tie point, it being possible to connect the at least one electric incandescent lamp to the secondary side. It also relates to a method for operating a ballast for at least one electric incandescent lamp, in which the ballast comprises at least two switching elements that are coupled to one another with the formation of a first tie point, a drive circuit with at least one output for driving the at least two switching elements with the aid of a drive signal that is characterized by a pulse duty factor and an operating frequency, and with an input for a dimming signal, and a power transformer whose primary side is coupled to the first tie point, it being possible to connect the at least one electric incandescent lamp to the secondary side, the first step in the method being to apply a dimming signal to the input of the drive circuit, after which the drive circuit generates a drive signal as a function of the dimming signal.
BACKGROUND ART
The present invention relates to the dimming of electric incandescent lamps that are driven via what is termed an electronic transformer. In order to control such an electronic transformer via a digital or analog interface, it is necessary to be able to set the voltage at its output terminals over a wide voltage range. It is necessary in this case to be able, in particular, to set the voltage very accurately such that the light flux characteristic prescribed for dimming in what is termed the DALI Standard is observed, and thus all lamps of a group of electronic transformers appear equally bright even in the dimmed state. That is to say, different electronic transformers that drive different incandescent lamps must provide the same output voltage as precisely as possible in response to a dimming signal. This is necessary, since brightness differences are visible with particular effectiveness precisely in the dimmed state. A pulse-width method (asymmetric duty cycle) in which the power switches switch alternately in the half bridge is particularly advantageous for the output voltage setting required for dimming. In this case, the switch-on time of one switch is lengthened at the expense of the switch-on time of the other switch. The operating frequency remains constant in this method, and only the switch-on conditions of the switches change.
DISCLOSURE OF THE INVENTION
Although
FIG. 1
shows an exemplary embodiment of the ballast according to the invention, it can also be used to describe the prior art: two power switches are denoted in
FIG. 1
by T
1
and T
2
, and are driven by a drive circuit
10
that is coupled, for its part, to the output of a dimmer
12
, the dimmer
12
applying a dimming signal U
d
to the drive circuit
10
. The supply voltage of the two switching elements T
1
and T
2
is what is termed the intermediate circuit voltage U
z
, which is provided here across a capacitor C
z
. The tie point VP
1
of the two switching elements T
1
and T
2
is connected to the primary side L
1
of a transformer Ü. The other terminal of the primary side L
1
of the transformer Ü is connected to the tie point of two coupling capacitors C
k1
, C
k2
that are connected with the other electrode to the capacitor C
z
. At least one incandescent lamp LA
1
, LA
2
is connected to the secondary side L
2
of the transformer Ü.
FIG. 2
shows the profile of the voltage U
1
on the primary side L
1
of the transformer Ü, the pulse duty factor between the switch-on time t
2
of the top switch T
2
and the switch-on time t
1
of the lower switch T
1
being varied such that t
1
<t
2
.
The root-mean-square value of the secondary-side output voltage U
2eff
is calculated as:
U
2
eff
=
U
z
N
·
d
·
(
1
-
d
)
.
Here, d stands for the pulse duty factor, that is to say d=t
1
/T, wherein T=t
1
+t
2
denotes the period of the primary-side voltage U
1
across the power transformer Ü, the reciprocal of which corresponds to the operating frequency f
s
. As already mentioned, U
z
corresponds to the intermediate circuit voltage, and N to the number of turns per unit length of the power transformer Ü. The range 0 to 0.5 is permissible for d. It is usual in the case of dimming to dim down to a minimum value of approximately 1% of the maximum light flux. Very small pulse duty factors in the range of d=0.03 to 0.05 are achieved in this case. The switch-on time t
1
is very small in this case. Consequently, the spectrum of the output voltage U2 is shifted drastically in the direction of higher frequencies. The voltage at the output terminals is affected thereby only to a small extent, the more so as the additional voltage drop across the transformer leakage inductance can be compensated by appropriate switching measures known from the prior art.
However, a particular disadvantage arises in the case of the use of a ballast to operate a plurality of incandescent lamps. Specifically, the shifting of the spectrum gives rise to a substantial inductive voltage drop across the supply leads to the incandescent lamps. This voltage drop can be up to several volts in the case of small pulse duty factors d and at customary operating frequencies from 30 kHz to 50 kHz. If a plurality of incandescent lamps in parallel connection are then operated by a ballast, they therefore burn, depending on the design of the lamp supply lead, that is to say, in particular, in accordance with the inductance of the lamp supply lead, which is determined essentially by the length of the lamp supply lead, with a clearly visible difference in brightness. With reference to the mode of operation mentioned first, in which individual incandescent lamps are driven by dedicated transformers, this likewise holds when these are dimmed jointly via a common digital bus.
It is therefore the object of the present invention to develop a generic ballast and the generic method in such a way that differences in brightness during operation of a plurality of incandescent lamps by such a ballast and during operation of various incandescent lamps having various electronic transformers, which are jointly dimmed via a common bus are avoided to a very large extent.
The above object is achieved according to a first aspect by virtue of the fact that the drive circuit is designed to vary the pulse duty factor and the operating frequency of the drive signal as a function of the dimming signal.
In accordance with a further aspect, the object is also achieved by a generic method in which in the second step the drive signal is generated by varying the operating frequency and the pulse duty factor as a function of the dimming signal.
This measure permits a substantial reduction in the operating frequency, as a result of which it is possible to considerably reduce or even entirely avoid a shifting of the spectrum of the output voltage during dimming. Furthermore, losses in the case of generic circuits are proportional to the operating frequency. The lowering of the operating frequency therefore results in fewer losses in the case of the invention. A further advantage with regard to the electromagnetic compatibility arises from the fact that fewer high-frequency components occur on the supply leads to the electric incandescent lamps. In addition, it is possible to select a more cost-effective timer for implementing the pulses, since the generation of narrow pulses is more difficult to implement technically, and the pulses can be selected to be wider in the case of the present invention. Consequently, the ballasts according to the invention can be implemented very cost-effectively
Patent-Treuhand-Gesellschaft fur elektrische Gluhlampen mbH
Vo Tuyet T.
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