Note: Descriptions are shown in the official language in which they were submitted.
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Description
Starting circuit arrangement for starting a discharge lamp and
method for starting the discharge lamp
The invention relates to a starting circuit arrangement for
starting at least one discharge lamp by applying an electrical
starting voltage pulse to the discharge lamp, the starting
circuit arrangement having the following features: at least one
source circuit arrangement for providing an electrical primary
voltage pulse, at least one starting circuit for providing the
starting voltage pulse, and at least one inductive coupling
element for inductively coupling-in the primary voltage pulse
into the starting circuit for the purpose of generating the
starting voltage pulse. In addition to the starting circuit
arrangement, a method for starting a discharge lamp using the
starting circuit arrangement is specified.
A starting circuit arrangement of the type mentioned is in each,,
case known from EP 0 903 967 Al and EP 0 987 928 Al. When the
discharge lamp is started, a gas in a lamp arcing chamber of
the discharge lamp is ionized. The gas consists of, for
example, mercury vapor. An electrically conductive plasma
results. This plasma causes the discharge lamp to first
illuminate. In order to maintain this illumination and rapid
heating of inner electrodes of the lamp arcing chamber, the
discharge lamp is driven, for example, by a sinusoidal
alternating current (starting transfer current). A frequency of
this alternating current is, for example, 200 kHz. A starting
transfer voltage of the discharge lamp at room temperature is,
for example, from 150 V to 500 V. After a starting transfer
time in the seconds range (below one second to a few seconds),
the actual operating current can be impressed. The operating
current is sinusoidal or it has a square-wave form. An
operating voltage required for this purpose is, for example,
from 15 V to 225 V. From this phase on (after 1 min to 4 min),
the discharge lamp enters the desired, severely heated
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operating state with a high internal pressure in the lamp
arcing chamber, a very high luminous efficacy and a broadband
emission spectrum.
For starting purposes, the inner electrodes of a lamp arcing
chamber of the discharge lamp are driven by the starting
voltage pulse. An electrical flashover results which leads to
ionization of the gas. The starting voltage pulse is a set of a
large number of voltage pulses (voltage pulse train) . A pulse
repetition rate of the voltage pulses within a voltage pulse
train is from 1 MHz to 10 MHz. In order that the discharge lamp
is started, starting voltage pulses having a peak voltage in
the kV range are required. The starting voltage pulses are
therefore high-voltage pulse trains in the radiofrequency range
(high-voltage RF burst). The starting transfer voltage is
superimposed with these high-voltage pulse trains during the
starting process. The starting circuit arrangement is designed
such that superimposition and therefore starting of the
discharge lamp occurs at a maximum starting transfer voltage.
The known starting circuit arrangement essentially comprises a
source circuit arrangement, a starting circuit in the form of a
resonant circuit (starting resonant circuit, secondary resonant
circuit) and an inductive coupling element in the form of a
starting transformer. The discharge lamp is connected
electrically in parallel with the starting resonant circuit. In
the source circuit arrangement, the primary voltage pulse is
generated. With the aid of the starting transformer, the
primary voltage pulse is coupled-in into the starting resonant
circuit. The starting voltage pulse is produced in the starting
resonant circuit. This results in starting of the discharge
lamp. The components of the starting circuit arrangement, in
particular the starting transformer of the inductive coupling
element, are designed such that a magnification factor Q of the
starting resonant circuit results which is as high as possible.
The magnification factor Q is over 100.
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A starting circuit arrangement for starting a discharge lamp is
implemented, for example, in a so-called electronic ballast
(EB). The EB converts electrical energy from an available
system voltage such that the discharge lamp can be operated in
its optimum voltage, current and frequency range. For example,
the discharge lamp is a high-pressure lamp or an ultra-high-
pressure lamp, which are used as video and projection lamps
(VIP lamps).
At a relatively low lamp temperature (for example, room
temperature, approximately 20 C), a peak voltage for the
starting voltage pulse of several hundred to a few thousand
volts is sufficient for initiating the electrical flashover
between the inner electrodes of the lamp. The higher the lamp
temperature, the higher the peak voltage required for starting
the high-pressure or ultra-high-pressure lamp. In the case of a
VIP lamp, operating temperatures of from 950 C to 1050 C
generally occur. The starting voltage required for the
electrical flashover between the inner electrodes is extremely
high at these temperatures, since the gas in the lamp arcing
chamber has a severely electrically insulating effect owing to
a prevailing gas pressure. This results in the VIP lamp needing
to be cooled before it can be started again. The temperature of
the arcing chamber needs to be reduced to approximately 500 C
in the case of contemporary ballasts. Starting from a burner
operating temperature of the VIP lamp of approximately 1000 C,
cooling to 500 C lasts for approximately 30 seconds. Within the
cooling time of 30 seconds, restarting (hot restarting) of the
VIP lamp fails without any additional measures.
One object of the present invention is to specify an electrical
starting circuit arrangement which is suitable for starting a
discharge lamp even within the cooling time of the lamp.
In order to solve the object, the invention specifies a
starting circuit arrangement for starting at least one
discharge lamp by applying an electrical starting voltage pulse
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to the discharge lamp, the starting circuit arrangement having
the following features: at least one source circuit arrangement
for providing an electrical primary voltage pulse, at least one
starting circuit for providing the starting voltage pulse, and
at least one inductive coupling element for inductively
coupling-in the primary voltage pulse into the starting circuit
for the purpose of generating the starting voltage pulse. The
starting circuit arrangement is characterized by the fact that
the inductive coupling element has a transformation ratio for a
voltage transformation which is selected from the range of from
1/25 to 1/400, inclusive. The transformation ratio is
preferably selected from the range of from 1/40 to 1/200,
inclusive, and in particular from the range of from 1/40 to
1/70, inclusive.
In order to solve the object, the invention also specifies a
method for starting a discharge lamp by applying a starting
voltage pulse using the starting circuit arrangement as claimed
in one of the preceding claims, having the following method
steps: a) forming the starting resonant circuit with a
discharge lamp connected in parallel, and b) generating the
starting voltage pulse in the starting resonant circuit.
The discharge lamp or the inner electrodes of the discharge
lamp are connected to the starting resonant circuit, together
with further components. The starting circuit has all of the
reactive components required. Owing to the high transformation
ratio and the resonant voltage spike, which occurs either in
the starting circuit or in the coupling element, a
radiofrequency starting pulse with a very high peak voltage is
generated in the starting circuit. A starting voltage pulse
with a peak voltage of from 10 kV to 50 kV, inclusive, and in
particular with a peak voltage of from 15 kV to 25 kV,
inclusive, is preferably generated.
A radiofrequency voltage pulse with a pulse repetition rate in
the MHz range is generated. In one preferred refinement, a
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starting voltage pulse with a pulse repetition rate of from
0.5 MHz to 30 MHz, inclusive, and in particular of from 0.9 MHz
to 10 MHz, inclusive, is generated. Particularly good results
have been achieved, for example, at a pulse repetition rate of
1.5 MHz.
At this pulse repetition rate and at the high voltages, the
starting pulse duration of the starting voltage pulse can be
maintained for a short period of time. The starting pulse
duration is below 50 s. In one particular refinement, a
starting voltage pulse having a starting pulse duration from
the range of from 5 s to 30 s, inclusive, is generated. In
particular, starting pulse durations of below 20 s are
possible. These relatively short starting voltage pulses are
sufficient for making it possible to start the discharge lamp
even at high temperatures of the discharge lamp owing to the
high transformation ratio.
Frequent repeat starts make it possible to increase the
starting probability. In accordance with one further
refinement, the starting voltage pulse is therefore generated
at a repetition rate from the range of from 50 Hz to 10 kHz,
inclusive, and in particular from the range of from 100 Hz to
1 kHz, inclusive. At the repetition rate, a starting voltage
pulse is coupled-in into the starting circuit. At a relatively
high repetition rate, for example, 1 kHz, the probability of
starting being successful within a specific time interval is
increased.
The inductive coupling element has at least one starting
transformer. The starting transformer has a primary inductance
with at least one primary winding and a secondary inductance
with at least one secondary winding. The secondary inductance
of the starting transformer is part of the starting circuit.
The starting circuit may be in the form of a starting resonant
circuit. With the aid of the starting transformer, the primary
voltage pulse formed in the source circuit arrangement is
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coupled-in into the starting resonant circuit. For this
purpose, the starting transformer is preferably an RFHV
transformer having a ferromagnetic core (for example, ferrite
or ferrous powder core). The RFHV transformer is designed such
that it alone provides the high voltage transformation. The
RFHV transformer is therefore, for example, a transformer
having an output voltage of 25 kV. Using a starting transformer
with such a design, a markedly poorer magnification factor of
the starting resonant circuit is possible. The starting
resonant circuit has a magnification factor Q of below 100.
The inductive coupling element may have merely the
correspondingly designed starting transformer. In one
particular refinement, the inductive coupling element has at
least one coupling transformer. The starting transformer and
the coupling transformer are electrically connected to one
another such that, together, they form the inductive coupling
element. In one particular refinement, the starting transformer
and the coupling transformer are connected in series, for this
purpose. The primary inductance of the coupling transformer may
be part of the source circuit arrangement. The secondary
inductance of the coupling capacitor and the primary inductance
of the starting transformer are electrically connected. The
secondary inductance of the starting transformer is in turn
part of the starting resonant circuit.
The coupling transformer is used for voltage matching. This
means that the high transformation ratio of the inductive
coupling element is provided by the starting transformer
together with the coupling transformer. The transformation
ratio does not originate from the starting transformer alone.
The starting transformer therefore makes a lesser contribution.
This makes it possible for the secondary inductance and
therefore the secondary winding of the starting transformer to
be kept small. This is associated with the following particular
advantages: the nonreactive resistance of the secondary winding
is reduced by a low number of turns and therefore by a shorter
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wire length. The nonreactive resistance also has the operating
current of the discharge lamp flowing through it and therefore
leads to a permanent power loss which needs to be avoided.
Furthermore, the coupling capacitor results in further
decoupling of the starting resonant circuit and the source
circuit arrangement. A resonant voltage spike required for
forming the starting voltage pulse is therefore facilitated in
the starting resonant circuit.
With the explained combination of the starting transformer and
the coupling transformer, it is possible, with a favorable
design of the further components in the starting circuit, for
starting voltage pulses to be achieved with a peak voltage of
over 30 kV. In other words, this is the voltage which,
calculated from the positive maximum to the negative maximum,
has 60 kVpp (with the assumption that, in the present case, the
voltage is approximately sinusoidal).
In one particular refinement, the inductive coupling element
has at least one coupling resonant circuit. With the coupling
resonant circuit, a coupling transformer may be superfluous.
However, a coupling transformer is preferably provided, and the
coupling resonant circuit electrically connects the starting
transformer and the coupling transformer to one another. The
coupling resonant circuit is also referred to as a tank
resonant circuit. It is inserted between the coupling
transformer and the starting transformer such that it takes
over the resonance of the secondary winding of the starting
transformer. A resonant voltage spike, triggered by the primary
voltage pulse of the source circuit arrangement, occurs in the
coupling resonant circuit. On the other hand, the starting
circuit is not in the form of a starting resonant circuit. In
this case, no resonant voltage spike occurs. Only one starting
voltage pulse is generated at the inner electrodes of the
discharge lamp via the starting transformer. In this particular
refinement, too, the starting transformer does not provide the
transformation ratio for the entire inductive coupling element
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alone. The level of the transformation ratio depends to a very
great extent on the design of the tank circuit, which has a
self-transforming effect either owing to suitable capacitive or
inductive tapping or, in addition, owing to a coupling
transformer being connected upstream, as a result of its
contribution. For example, the tank resonant circuit develops a
voltage of from 2 kV to 10 kV. A moderate transformation for
the starting transformer is therefore possible. It is possible
to dispense with the coupling transformer depending on the type
of source circuit arrangement.
The source circuit arrangement has a suitable radiofrequency
switching element. The radiofrequency switching element has one
or more radiofrequency switching transistors. The
radiofrequency switching transistor is a (power) MOS transistor
and, in particular a Coo1MOS~ transistor or powerMESH (TM)
transistor or an FDmesh(T"') transistor or a silicon carbide FET
transistor.
The source circuit arrangement provides a high power for the
pulse duration. The source circuit arrangement is designed such
that an average primary power is between 300 W and 2 kW. This
results in a radiofrequency switching transistor used for
switching purposes carrying current pulses having a peak
current of between 10 A and 100 A. In this case, provision is
made for an efficiency to be sufficiently high. The cost of a
radiofrequency switching transistor depends to a very
considerable extent on its current-carrying capacity. The
higher the efficiency of the source circuit arrangement, the
lower current-carrying capacity can be selected for the
radiofrequency switching transistor(s).
In one particular refinement, the source circuit arrangement
for providing the primary voltage pulse has a radiofrequency
switching element with switching load relief. This in general
means that, at the time at which the switching element is
switched on, the applied voltage and the carried current are
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equal to or close to zero. In this manner, it is possible to
avoid a power loss peak which normally occurs during the
switching operation. A source circuit arrangement having a
particularly high efficiency results. One further important
advantage of the switching load relief is represented by the
avoidance of severe electromagnetic interference components
(EMC problem) at and far above the switching frequency.
The source circuit arrangement preferably has a topology which
is selected from the group consisting of class E, class D and
class DE. Class E and class DE are characterized by
particularly effective switching load relief. Owing to the high
switching load relief, an electrolyte capacitor, which is
generally required as the supply buffer store for the source
circuit arrangement, can be kept small. Furthermore, only a
radiofrequency switching transistor which only needs a limited
current-carrying capacity owing to the increased efficiency is
required for this switching stage. A relatively inexpensive
starting circuit arrangement therefore results. One further
advantage consists in the fact that an output voltage of the
switching stage, without any further matching, almost linearly
follows a DC voltage used to supply the switching stage. This
makes it possible to regulate the radiofrequency voltage via an
upstream voltage supply of the source circuit arrangement.
There are two types of class E: a parallel circuit or a series
circuit may be connected to the drain or collector terminal of
the transistor. The second solution is characterized by the
fact that it can be operated at very low supply voltages. It is
therefore possible to use a heavy-duty transistor having a
relatively low blocking capacity.
Very effective switching load relief is also achieved by a
radiofrequency switching stage of the class DE. This class is
based on a half bridge comprising two switching transistors.
The switching transistors used require a markedly lower
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dielectric strength than the switching transistor which is used
in class E.
The starting circuit arrangement is used in particular in EBs
for high-pressure discharge lamps and for ultra-high-pressure
discharge lamps, as are used in video and projection
technology. In the case of a high-pressure discharge lamp,
pressures of from 2 bar to 20 bar occur in the lamp arcing
chamber. In the case of ultra-high-pressure lamps, the
pressures fluctuate in the range from 100 bar to 200 bar. The
intention here is to achieve an emission spectrum which is as
broad as possible. A power for the VIP lamps is between 100 W
and 300 W, for example 120 W. Higher and lower powers are also
conceivable. Using the starting circuit it is possible to start
such discharge lamps even in the hot state during operation at
temperatures of over 500 C up to 1000 C.
In summary, the following essential advantages result with the
invention:
- The starting circuit arrangement makes it possible for a
discharge lamp to be restarted even at high temperatures
of over 500 C. Once the illumination of the discharge lamp
has been interrupted, it is not necessary for there to be
cooling for restarting. There is no delay in starting.
- With the invention it is possible to dispense with an
auxiliary starting electrode, which is often used for
maintaining the delay in starting which is conventional
nowadays.
The invention will be explained in more detail below with
reference to a plurality of exemplary embodiments and the
associated figures, in which:
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Figures 1, 2, 3A and 32 show circuit diagrams of various
exemplary embodiments of the starting circuit
arrangement.
Figure 4 shows the circuit diagram of a radiofrequency
switching stage of the class DE.
The starting circuit arrangement 1 for starting a discharge
lamp 2 by applying an electrical starting voltage pulse is
implemented in an EB for a high-pressure discharge lamp 2. The
high-pressure discharge lamp 2 is a VIP discharge lamp having a
power of 120 W. In alternative embodiments to this, the VIP
discharge lamp has a power of 100 W or 300 W.
The essential components of the starting circuit arrangement 1
are the source circuit arrangement 11 for providing the primary
voltage pulse, the starting circuit 12 for providing the
starting voltage pulse and the inductive coupling element 13
for inductively coupling-in the primary voltage pulse into the
starting circuit 12. The starting voltage pulse results from
the primary voltage pulse being coupled-in into the starting
circuit 12.
The inner electrodes 22 of the high-pressure discharge lamp 2
which are arranged in the lamp arcing chamber 21 are components
in the starting circuit 12. The starting voltage pulse
generated in the starting circuit 12 leads to an electrical
flashover between the inner electrodes 22. The gas in the
interior of the lamp arcing chamber 21 is ionized. The heating
plasma for the starting transfer is formed.
In order to maintain the heating plasma, the high-pressure
discharge lamp 2 is driven, via the voltage supply unit 122,
with a sinusoidal-like starting transfer voltage. The voltage
supply unit 122, which is integrated in the EB, makes available
a sinusoidal-like starting transfer voltage of between 150 V
and 500 V in the 100 kHz range.
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The source circuit arrangement 11 is supplied with a suitable
electrical DC voltage via the voltage supply unit 111. The
source circuit arrangement 11 has a radiofrequency switching
element 112. The radiofrequency switching element 112 has
switching load relief. This means that, at the time at which it
is switched on, the applied voltage and the carried current is
zero or close to zero. Components of the radiofrequency
switching element 112 are at least one radiofrequency switching
transistor 113 and at least one RF driver circuit 114. The
radiofrequency switching transistor 113 is driven by the RF
driver circuit 114. The radiofrequency switching transistor 113
is a CoolMOS~ transistor. In alternative refinements to this, a
powerMESH(TM) transistor, an FDmesh(TM' transistor or a silicon
carbide FET transistor are used. The RF driver circuit 114
provides a radiofrequency switching signal for the
radiofrequency switching transistor 113, which signal is
matched to the pulse repetition rate of the starting voltage
pulse to be achieved. This means that a primary voltage pulse
is generated in the source circuit arrangement 11 and has an
identical or very similar pulse repetition rate to that of the
starting voltage pulse in the starting circuit 12.
With the aid of the starting circuit arrangement 1, a
radiofrequency starting voltage pulse is generated for starting
the discharge lamp 2. A pulse repetition rate of the starting
voltage pulse, in a first embodiment, is approximately 1.5 MHz.
In a further embodiment, the pulse repetition rate is 4 MHz.
The peak voltage of the starting voltage pulse is 22 kV. This
corresponds to 44 kVpp. In a further embodiment, the peak
voltage of the starting voltage pulse is 30 kV (60 kVpp) . The
starting pulse duration is 20 s. In a further embodiment, the
starting pulse duration is 5 s.
The inductive coupling element 13 has a transformation ratio
for the voltage transformation of approximately 1/60. This high
transformation ratio makes it possible to achieve the
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radiofrequency voltage pulses with the high peak voltage. Owing
to the high peak voltage, it is possible to start the VIP high-
pressure discharge lamp at high burner surface temperatures of
over 500 C. It is not necessary to wait for the cooling phase
to elapse for successful restarting of the discharge lamp.
In order to achieve the high transformation ratio, three
examples are specified below:
Example 1:
The associated circuit diagram is reproduced in figure 1. The
inductive coupling element 13 merely comprises a starting
transformer 131. The starting transformer 131 produces the high
transformation ratio on its own. A primary inductance 1311 of
the starting transformer 131 is a component of the source
circuit arrangement 11. The secondary inductance 1312 of the
starting transformer 131 is a component of the starting circuit
12, which is in the form of a starting resonant circuit 121.
The primary voltage pulse is coupled-in into the starting
resonant circuit 121 via the primary inductance 121 and the
secondary inductance 1312 of the starting transformer 131.
The starting transformer 131 is an RFHV transformer having a
ferromagnetic core and corresponding numbers of turns of the
primary inductance 1311 and the secondary inductance 1312. The
starting resonant circuit 121 has a magnification factor Q of
far below 100.
The secondary inductance 1312 comprises two virtually identical
inductance elements having the same winding sense. These
inductance elements are combined with further components to
form a virtually symmetrical starting resonant circuit 121.
This makes it possible for the operating voltage and the
starting transfer voltage to be fed without any influence by
the high-voltage pulses for starting.
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The radiofrequency switching element 112 of the source circuit
arrangement 11 has a class E radiofrequency switching stage
(series topology).
Example 2:
The corresponding circuit diagram can be seen in figure 2. In
contrast to example 1, the inductive coupling element 13 has a
coupling transformer 132 in addition to the starting
transformer 131. The high transformation ratio is achieved by
the starting transformer 131 being coupled to the coupling
transformer 132. For this purpose, a primary inductance 1321 of
the coupling transformer 132 is a component in the source
circuit arrangement 11. The primary voltage pulse is coupled-in
into the starting resonant circuit 121 indirectly via the
secondary inductance 1322 of the coupling transformer 132 and
the primary inductance 1311 of the starting transformer 131. A
corresponding refinement of the coupling transformer 132
achieves a part-transformation ratio. The part-transformation
ratio of the starting transformer 131 can therefore be reduced.
The high transformation ratio of the entire inductive coupling
element 13 is maintained.
The radiofrequency switching element 112 of the source circuit
arrangement 11 likewise has a class E radiofrequency switching
stage, but with a parallel topology.
Example 3:
The associated circuit diagram is shown by figures 3A and 3B.
The starting circuit 12 is not in the form of a starting
resonant circuit. This means that the starting circuit 12,
apart from parasitic elements, cannot be caused to oscillate in
the frequency range of the starting voltage pulse.
In order to induce the starting voltage pulse in the starting
circuit 12, the inductive coupling element 13 has a tank
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resonant circuit (coupling resonant circuit) 133. The tank
resonant circuit 133 has a capacitive divider (figure 3A) . As
an alternative to this, the tank resonant circuit 133 is in the
form of a tank resonant circuit having a tapped coil (tapped
tank resonant circuit, figure 3B). This tank resonant circuit
133, in a development of the coupling element 13 in accordance
with the preceding example, is connected between the coupling
transformer 132 and the starting transformer 131. The primary
inductance 1311 of the starting transformer is a component in
the tank resonant circuit 133. Here, too, a part-transformation
ratio is taken over by the tank resonant circuit. The starting
transformer 131 can therefore manage with a smaller
transformation ratio.
In addition to the examples described, there is also a large
number of embodiments which result from corresponding
refinements of the source circuit arrangement 11, the starting
circuit 12 or the coupling element 13 and its components. For
example, the secondary inductance 1312 of the starting
transformer 131 has one part, in accordance with an embodiment
which is not illustrated. More than two components are likewise
conceivable. In a further embodiment, a radiofrequency
switching element 112 with a DE class switching stage is used
for the source circuit arrangement 11 as an alternative to the
radiofrequency switching element 112 with the E class switching
stage. A DE switching stage with a series circuit topology is
shown in figure 4. The radiofrequency switching stage of this
class has two radiofrequency switching transistors 113. Each of
the radiofrequency switching transistors can be driven by a
dedicated RF driver circuit 114.
