Note: Descriptions are shown in the official language in which they were submitted.
~LLAST CIRCUITS FOR ÇAS DISCHARGE LAMPS
This invention relates to ballast circuits for ga~
discharge lamps. In particular the invention relates to
ballast circuits which draw a low harmonic content input current
from an AC supply whilst operating a gas discharge lamp at n
higher frequency than that of the supply.
One such balla6t circuit is ~hown in U.K. Patent No.
2124042B. The circuits described in this patent are 60 called
capacitive charge pump circuits including a reservoir capacitor
connected acro6s the outputs of a full wave rectifier which is
in turn connected to an AC supply, the reservoir capacitor being
shunted by a series arrangement of two ;witching devices. A
discharge path is provided from the reservoir capacitorr through
an output load compri~ing a series resonant circuit con~tituted
by an inductor and a parallel arrangement of a discharge lamp
and a resonating capacitor connected across the cathodes of the
lamp, 80 as to periodically charge a control or charge pump
capacitor, this lowering the load voltage and drawing current
from the rectified supply. The re6ervoir capacitor i~
6ubsequently recharged by current flowing from the inductor at
times defined by the alternate switching of the two :witching
devices. The circuit is arranged 80 that the voltage across the
reservoir capacitor is always greater than the peak of the mains
supply.
Thus in operation of this circuit current and energy can be
taken from the mains at all parts of the mains cycle re6ulting
in a low harmonic content waveform being drawn from the 6upply.
It will be 6een that the effectiveness of 6uch a charge
pump circuit is dependent on the reservoir capacitor voltage,
and the amount of circulating current in the parallel
arrangement of the lamp and resonating capacitor. The amount
of thi1 circulating current is determined by the value of the
resonating capacitor and the operating current of the lamp. As
the resonating capacitor is connected across the lamp cathodes,
it provides cathode heating current. Thus the value of the
resonating capacitor is limited by the maximum current with
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which the cathode~ can be drlven without long term damage by
over heating, this causing a consequential limitation on the
amount of circulating current po~ible, and thuc the amount of
charge which can be pumped.
It is possible to place an additional capacitor across the
lamp thus providing a parallel current path to the cathode
circuit in order to increase the circulating current without an
accompanying increase in cathode current. Such an arrangement
creates problems however in that in normal operation the
switching devices will operate at a frequency higher than that
of the output resonant circuit constituted by the inductor,
lamp, resonating capacitor and additional capacitor. If the
lamp is removed, or a cathode breaks during operation of the
lamp, the remaining resonant circuit comprising the inductor and
additional capacitor will have a higher resonant frequency than
that of the original resonant circuit. Consequently the
remaining resonant circuit may be instantaneously at or below
resonant frequency. This situation may lead to damage to the
switching devices due to over current or capacitive switching.
Furthermore a large voltage may be left across the lamp
terminals thus creating a safety hazard. It is also the case
that without the additional capacitor the resonant circuit is
broken if the lamp is removed or a cathode is broken; this
safety feature is lost if an additional capacitor is used.
It is an ob~ect of the present invention to provlde an
improved ballast circuit for a discharge lamp.
According to the present invention there is provided a
ballast circuit for a discharge lamp, the ballast circuit
comprising:
a load circuit including the primary winding of a high
frequency transformer, the transformer further including a
secondary winding for connection across a discharge lamp;
a reservoir capacitive means effective to supply charge to
the load circuit;
and a capacitive charge pump circuit effective to transfer
charge from a charge pump capacitive means to the reservoir
capacitive means and to the load circuit, in operation, said
primary winding being effective to drive the capacitive charge
pump circuit.
In a circuit provided in accordance with the present
invention the transformer provides voltage isolation of the lamp
from the AC supply. Furthermore, the primary inductance,
inter-winding inductance and turr,s ratio of the transformer can
be ad~u6ted 80 as to determine the effective impedance of the
load circuit. A ballast circuit provided in accordance with the
present invention can be arranged such that, in operation, once
the lamp has 6truck and is of low impedance the voltage across
the re6ervolr capacitive means is instantaneously always at
least as great as the voltage produced by the rectified AC
supply.
The load circuit may include a series resonant circuit.
Advantageou~ly a resonating capacitive means is provided for
connection across said secondary windlng, whereby, in use, said
resonating capacitive means is connected to said secondary
winding via the lamp cathodes of said a discharge lamp, said
resonating capacitive means having a capacitance which is of a
value such that, in operation, said re~onating capacitive means
resonates with the interwinding inductance of the transformer in
order to strike and ballast said a discharge lamp.
Thus, by use of a circuit in accordance with the invention
the primary inductance of the transformer and associated
components within the resonant circuit may be ad~usted to
provide the necessary circulating current 80 as to obtain the
required supply input current waveform, but whilst maintaining
suitable heating current through the lamp cathode. The removal
of the lamp will reduce the resonant frequency of the output
resonant circuit, the transformer providing the additional
safety feature of electrical isolation of the lamp from the
input mains supply.
Ballast circuits provided in accordance with the invention
will now be de~cribed, by way of example only, with reference to
the accompanying drawings in which:
Figure 1 is a schematic circuit diagram of a ballast
circuit provided in accordance with the present invention;
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F~gures 2 and 3 are schematic circuit diagrams of ballast
clrcuits being adaptations of the ballast circuit of Flgure l;
Figure 4 is a schematic circuit diagram of ~ ballast
circuit including a boost inductor and not provided in
accordance with the present invention;
and Figure 5 i8 a 6chematic circuit diagram of another
ballast circuit including a control circuit and provided in
accordance with the pre6ent invention.
Referring to Figure 1, a ballast circuit, indicated
generally as 1, is connected via respective positive and
negative supply rails 3, 5 to the outputs of a full wave dlode
br1dge rectifier circuit 7 which is, in turn connected across an
AC supply 9. A radio frequency interference filter 11 is
connected across the supply on the AC side of the rectifier
circuit 7.
A series arrangement of capacltors Cl, C2 are connected
across the rails 3, 5, each capacitor Cl, C2 being shunted by a
respective diode Dl, D2. A series resonant circuit comprising
a capacitor C3 and the primary winding of a single wire wound
ballast transformer Tl is connected to the node between the
capacitors Cl, C2. A fluorescent lamp 13 is connected across
the secondary T2 of the transformer Tl, a resonating capacitor
C4 being connected across the lamp cathodes.
The series resonant circuit C3, Tl is also connected to the
node between two high frequency switching arrangements Ql, Q2
connected across the rails 3, 5, each arrangement Ql, Q2 being
shunted by a respective free wheel diode D5, D6. Each
switching Arrangement Ql, Q2 is powered by a respective further
secondary winding coupled to the primary winding of the
transformer Tl. A reservoir capacitor C5 is connected across
the rails 3, 5.
Thus in use of the circuit the capacitor C3 together with
the inter-winding inductance of Tl acts as the ballasting
impedance of the lamp 13, and resonates with the inductance of
the primary winding of the transformer Tl. Drive signals are
derived from the transformer Tl to switch the switches Ql, Q2
alternately, the radio frequency interference filter 11 being
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effective to prevent high frequency signals from being
transmitted to and from the mains supply 9. The capacitor C2
acts as a charge pump capacitor. Thus when Q2 switches on, C2
charges from the mains. When Q2 aubsequently switches off and
Ql switches on, part of the charge of C2 ifi transferred via Tl
to the reservoir capacitor C5. Diodes D3, D4 connected in the
rail 3 are effective to allow the charge pump action to transfer
charge from the capacitor C2 to the reservoir capacitor C5, the
voltage swing at the node between Cl and C2 providing the charge
pump swing voltage. Diodes Dl and D2 are effective to clamp
the voltage~ on Cl and C2.
It will be seen that the value of the reservoir capacitor
C5 will affect the operation of the circuit. When the value of
C5 is large, the voltage across the capacitor C5 will remain
substantially constant thus giving a smooth, unmodulated lamp
arc current. The charge pump action will however be less
efficient a6 the difference in voltage between the instantaneous
mains voltage near zero crossover, and the voltage on the
reservoir capacitor C5 will be large. If, however, the value
of C5 is smaller, the ripple voltage on C5 will be higher,
leading to a 100 Hz modulation of the lamp arc current although
the charge pump action will be more efficient. It is found
that a compromise between acceptable lamp current modulation and
input current waveform shape may be reached.
It will be appreciated that if the lamp 13, and
consequently the resonating capacitor C4 is removed from the
circuit, or a cathode breaks during operation of the lamp, the
effective resonant frequency of the resonant circuit will be
reduced. Hence there is no danger of the circuit operating at
or below reAonance.
A second particular circuit will now be described by way of
a further example and reference to figure 2, this being an
adaption of the fir6t example. Accordingly like parts will be
designat~d by like references. A diode, D7, is included in the
negative supply rail being effective in conjunction with diode
D4 and capacitors C2 and C7 to draw two pulses of current from
the rectified supply during each high frequency cycle. C2 and
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C7 are known as charge pump capacltors whose value 18 determlned
by the requlred power to be drawn from the supply and the
frequency of operatlon of the lnverter. Capacitor~ Cl, C6
provide a current path from the capacltive pumplng node N, the
~unction of Cl, C2, C6, C7, Dl, D2 and Tl, to the supply rails
of the reservoir capacltor C5 at all times. The capacitors Cl,
C6 are normally 6maller than the charge pump capacltors C2, C7,
often a factor ln the reglon 2 to 10; the value depends on the
requlred level of current to flow in the load when the supply
voltage is low eg near zero crossover as at this time the level
of current flow in the charge pump capacitors is low. Diodes Dl
and n2 ensure that capacitors C7 and C2 cannot charge to a
voltage greater than the instantaneous rectified mains voltage,
their connection to either the anode or cathode of diodes D4 and
lS D7 does not substantially affect the operation of the circuit.
A series resonant circuit comprising of Tl and C4 i8 used to
strike and ballast one (or more) discharge lamps, C4 being
effective to resonate with the interwinding inductance, or
leakage reactance of Tl. The switches Ql and Q2 constitute a
half bridge inverter and are switched at high frequency,
typically in the range 20kHz to 150kHz, either by signals
generated directly from the resonant circuit or from an
alternative source.
Thus by use of this circuit in accordance with the
invention the turns ratio, inter-winding inductance and primary
inductance of the transformer Tl may be ad~usted in order to
determine the effective impedance of the ballast circuit between
the inverter and charge pumping capacitor network whilst
maintaining correct cathode and lamp current and maintaining the
feature that when the lamp is removed or a cathode is broken the
re60nant circuit is also broken. It is advantageous in such
case~ when the resonant circuit is broken that the primary
inductance of the transformer be high, for a 240 Volt 70 Watt
circuit operated at 50 kHz this would be above lOmH, this being
effective to ensure that little current flows via the capacitive
charge pmping node N and as a consequence that the voltage
across the reservoir capacitor C5 does not rise above the peak
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of the rectlfied supply voltage.
It i8 a feature of both the first and second examples that
a serie~ resonant circuit is placed between the output of an
inverter and a charge pump capacitor network. Such circuits
when operating at a frequency near resonance provide a low
impedance path irrespective of the lamp impedance and therefore
draw significant power from the supply at such times. This
gives operational difficulties when the lamp load is of high
impedance, for example before the lamp has struck, in that the
voltage generated across the reservoir capacitor can become
unacceptably high and lead to the 6elf-destruction of the
circuit. This difficulty can be overcome by the use of a charge
pump disabling network which senses and is activated by the
overvoltage condition, however this adds to circuit complexity
and cost.
A third particular circuit will now be described with
reference to figure 3. This circuit is a development of the
principle of u6ing a transformer Tl as shown in Figures 1 and 2
and accordingly like parts are designated by like references.
~owever there is no resonating capacitor on the secondary T2 of
the transformer across the lamp 13. The circuit inherently
copes with the fault condition of a deactivated lamp as well as
missing lamp or broken cathode conditions without the need of a
over-voltage protection circuit as in the fault condition no
resonant circuit or significant load are present which would
cause effective pumping action and the rail voltage to rise.
The ballasting of the lamp 13 is achieved solely by the turns
ratio of the transformer together with the transformer
inter-winding inductance. The striking of the lamp iB achieved
by the voltage step-up generated by the tran6former together
with the application of cathode heating provided by windings T3
coupled closely to the secondary winding of the transformer.
Since there is no resonant circuit and the primary
inductance of Tl is high there is no low impedance path between
the output of the inverter and the charge pumping node N until
the lamp has struck. This event is co-incident with the
consumption of power by the lamp, and consequently there is no
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unavoidable overvoltage condition and no protection circuit i8
required.
It ~hould be appreciated in such a circuit that a slight
resonance effect may occur due to the self-capacitance of the
secondary winding of the transformer. It could be advantageous
to swamp this self-capacitance using a swamping capacitor (shown
in Figure 3 in dotted line C9) in order to ensure consistent
operational behaviour. ~owever the swamping capacitor would be
80 small as not to interfere with the above described circuit
behaviour.
Returning now to the general case in which a transformer
ballast is used to drive a capacitive charge pumping node.
It is a further feature of the transformer that voltage
isolation i6 provided between the lamp and the supply, thi6 can
be of advantage in terms of reducing the shock hazard from the
lamp or by the connection of an earthed starting aid directly to
the secondary winding.
The use of a transformer as a lamp ballasting circuit
allows the impedance between the inverter output and capacitive
charge pumping node to be lower than is practicable with the
conventional non-transformer series resonant circuit. This
enables the capacitor charge pump network to be dimensioned and
operated in such a manner 80 as to draw sufficient current from
the supply to maintain the voltage across the reservoir
capacitor above that of the rectified supply at all times and
providing supply current harmonic control without the need to
add circuit elements such as an inductor in the output rail of
the bridge rectifier. A circuit incorporating an inductor in
the output rail is shown in figure 4 and is de6cribed in more
detail later.
There are two possible modes of operation of the general
capacitor charge pump and transformer circuit provided in
accordance with the present invention.
Mode 1
During normal operation with the lamp(s) in circuit the
impedance of the transformer circuit is low enough to allow the
charge pump capacitors to charge substantially to the
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instantaneous rectified ma~ns voltage and to substantially
discharge during each hlgh frequency cycle throughout each
supply cycle. If the ~witching frequency is constant throughout
the supply frequency cycle a unity power factor waveform (one
with no or very low harmonic content) will be drawn. In thi~
mode of operation an increase in 6witching frequency will result
in an increase of input power and hence an increase in the
voltage across the re6ervoir capacitor. The energy drawn from
the mains in this mode of operation is given by the following
formula:-
P = fCVm Vm
where P = input power (Watts)
f = operating frequency (Hz)
C = value of charge pump capac~tors C2 + C7
Vm = rms voltage of supply voltage
Accordingly, for a required circuit arrangement, the
capacitances of the charge pump capacitors C2, C7 can be
determined from thig formula.
Mode 2
During normal operation with the lamp(s) in circuit the
impedance of the transformer circuit is low enough to allow the
charge pump capacitors to charge substantially to the
instantaneous rectified mains voltage and to ~ubstantially
discharge during each high frequency cycle. ~owever the
impedance of the transformer circuit is sufficiently high enoughthat this charging and discharging occurs only during a portion
of the supply cycle when the rectlfied 6upply voltage is below
some value, less than its peak. In this mode of operation the
current drawn from the ~upply will contain some harmonic content
but low in level and can be below levels set out in
international standards. This mode of operation is such that a
decrease in frequency will result in the charge pump capacitors
being charged to the instantaneous rectified ~upply voltage and
discharged for a larger part of the supply frequency cycle, the
input power being increased and the harmonic content of the
6upply current waveform being decreased together with the
characteristic increase of voltage across the re~ervoir
capacitor. For a given load power and inverter operating
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frequency both the capacitance of the charge pump capacltors and
the impedance of the transformer circuit feeding back to the
capacitive charge pump node will be higher than in a circuit
operated in mode 1.
Using a self oscillating inverter circuit it is generally
difficult to achieve satisfactory operation of the circuit in
either of the modes described above. In a self-oscillating
circuit the switching frequency of the inverter is controlled by
the current flowing in the resonant circuit; it is not
generally po6sible to control the voltage across the reservoir
capacitor by this means; it is also generally difficult to
arrange that switching takes place at optimum times throughout
the supply cycle. Following the sw$tching of the inverter the
charge pump capacitor6 C2, C7 will charge from the supply until
clamped by diodes Dl or D2. If the inverter does not 6witch at
this point power will continue to be consumed by the lamp load
but no further power will be drawn from the supply in that half
high frequency cycle. Accordingly, in order to optimise the
drawing of power from the mains in accordance with operational
modes 1 and 2 described it i~ neces~ary to switch before, at or
shortly after the times when diodes Dl or D2 clamp the voltage
across the charge pump capacitors C2, C7; thia is not
necessarily co-incidental with the natural switching point of a
self oscillating circuit. Generally both of these difficulties
(control of capacitive smoothing means voltage and switching
point) can be addressed by the inclusion of a boost inductor
LB added in series to the output of the bridge rectifler.
Figure 4 shows a circuit which includes a boost inductor LB.
The circuit includes components X' similar to those components
X in the circuits of Figures I to 3 and these are referenced as
indicated. Figure 4 also shows the resulting additional current
path. The inductor LB acts principally to conduct charge in a
direct path from the rectified supply to the reservoir capacitor
C5' and this compensates for the inefficient capacitive charge
pumping. Limited voltage regulation is achieved by the
mechanism whereby the boost inductor LB is discharged
according to the amount by which the voltage across the
reservoir capacitor C5'exceeds that of the rectified supply
voltage.
These problems can be overcome by the use of a control
circuit and driven inverter together with the transformer
circuit as described. It is possible to avoid the use of a
boost inductor and if a non-resonant ballast i8 also u~ed then a
highly cost effective ballast can be produced. The cost of
control circuits are likely to fall with the advancement of
semiconductor technology whereas the price of inductive
components and capacitors are unlikely to fall in the future.
A fourth particular circuit which is an example of such a
ballast is shown in figure 5. Agaln, l~ke part~ to those of
Figures 1 to 3 are designated by like references. In this
example the driven inverter is created using MOSFETS Ql, Q2
which are driven from a voltage controlled oscillator 20 via a
voltage transformer 22. Whilst it will be appreciated that
there are ~everal ways in which such a circuit might be
controlled, for example to regulate lamp power or lamp current,
it is particularly beneficial to regulate the voltage across the
reservoir capacitor C5 since this can be used to ensure that the
said voltage i8 maintained above the rectified supply during all
normal operating modes w~thout rising to voltages which might
over-stress components.
It is possible to dimension and operate such circuits
according to mode 1 or mode 2. This particular example operates
in mode 2 and is controlled by regulating the voltage across the
reservoir capacitor C5 to be a multiple of the rectified supply
voltage; whilst being simple to implement this control achieves
good power regulation against variation in 6upply voltage. The
control loop is implemented by sensing as depicted in figure 5.
Using node 'a' as a O Volt reference the voltage at node 'b'
shall be denoted Vs, the voltage at node 'c' shall be denoted
Vcs, the voltage across the reservoir capacitor being denoted
Vc. From observation it will be appreciated that Vs repre~ents
the rectified supply voltage and that Vcs represents a voltage
which switches between the rectified supply voltage and the
voltage across the reservoir capacitor at the high frequency
switchlng speed. Provided the high frequency has a symmetric
duty cycle the time averaged equivalent voltage of Vcs i8 given
by
Vcs = (Vc + Vs) / 2
The control circuit uses resistor chains Rl, R2; R3t R4 to
generate respectively two signals as follows:
V+ = kl (Vc + Vs)
V~ = k2 V6
where kl and k2 are constants determined by the resistor chains.
A differential amplifier 24 generates an output signal, Vo,
which is of the form
Vo = K3 ( Vc - k4 V8 )
where k3 and X4 are constants derived from kl, k2 and the
gain of the amplifier, ie Vo i6 proportional to the error of the
reservoir capacitor voltage being a fixed multiple (k4) of the
rectified supply voltage.
The voltage to frequency converter 20 is drlven by Vo and
has a response such that the output frequency increases with
Vo. Time constants which are effective to stabilise the control
loop and to time average the signals V+, V- and Vo are included
by capacitive means C10, Cll in the amplifier stage.
Figure 5 also shows that a low voltage supply for the
control circuit can be generated from a winding T4 coupled
closely to the primary of the transformer Tl. It will be
appreciated that a low voltage regulator and start - up circuit
and features such as implementing a different control mode
during the lamp striking phase could be added by a person
knowledgeable in the art. It is clear that the reservoir
capacitor voltage can be readily derived from the Vcs signal.
It should be noted that, for the purposes of minimising the
level of high frequency interference which is conducted onto the
6upply, it is advantageous to arrange that the capacitive charge
pumping network be fully symmetrical, in this case that C2
should be the same value as C7 and that Cl should be the same
value as C6; this can simplify and reduce the cost of the
necessary Radio Frequency Interference filter 11. To reduce
further the si~e of the RFl filter before the bridge rectifier a
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small capacitor, shown in Figures 2 as C8, (typically lOOnF) to
act as a hf bypass can be connected across the output of the
brid~e rectifier 7.
