Note: Descriptions are shown in the official language in which they were submitted.
6 1 7
FLUORESCE~T TUBE DRIYER AND LIG~T NG SYSTE~
BACKGROUND OF T~E INVENTION
This in~ention relates to a fluorescent t~be driver
and a fluorescent lighting system including such a dri~er.
The invention is concerned with a dri~er working from
a direct voltage supply, prefera~ly but not essentially a low
voltage supply (less than 50 volts) and including an
oscillator circuit producing a relatively high frequency
oscillation (15 to 50 KHz preferably 25 to 50 KHz3 and
including a tran f~rmer supplying arc ourrent to the
fluorescent tube. Known cirCuitC operating from a s~irect
voltage supply at a high frequency are described for example
in UK Patent Speoifications 1010208, 1308578,
2126810, 2246034r 2244608, 2212g95 and 2095930. However
these operate by controlling the current to the driver by
varying the pulse width or the fre~uency of the drive signal.
US-A-4723098 which is regarded as the closest prior
art describes a driver circuit with a closed loop control
system but this adjusts the frequency of the drive signal
which inevitably varies the arc curren~ crest factor.
US-A-4862040 also describes a driver circuit in which
the frequency of the drive signal is varied.
~ he present invention particularly aims to proYide a
driver capable of operating at high efficie~cy and which is
particularly suited for use in heat conscious and energy
sa~ing ~n~ironments. Additionally the invention aims to
produce a driver which will promote optimum tube life.
SummarY of the Invention
T~e in~ention is particularly concerned with a
fluore-~cent tube driver arranged to be supplied by a direct
voltage source tparticularly a voltage source less than 50
volts, preferably 24 Yolts nominal) and having a tube driver
output in the range 10 to 100 watts of arc power (preferably
AhtENOED SHEET
2141617
r ~ ~ r
! ~~a
10 to 40 watts) with a frequency of oscillation in the range
to 50 KHz ~preferably 32 KHz nominal), the driYer
comprising an arc waveform trans~ormer, the primary winding
of which is supplied with a variab~e dri~e current derived
from the low ~oltage source, and the secondary winding of
which is adapted to be connected to the electrodes of a
:.
.~
AbNDED Sl !EET
WO94/04011 21 416 17 2 PCT~GB93/01618
~luorescen~ tube, such tha~ in use an altern~ting tube arc
current will flow throuah the secondary winding and tube.
The driver of the present invention is of the above type and
has a number of novel and ad~antageous features which may be
claimed independently or in any combination. These are as
follows:
A closed loop control sys~em is provided for
maintaining substantially constant the tube arc current by
varying the amplitude of the arc current waveform. This
?referably comprises means for measuring the arc current
flowing through the secondary winding and tube to produce a
~easured signal representing actual arc current supply, means
for generating 2 reference signal representing desired arc
c~rrent, comparator means for comparing the measureà signal
and the reference signal and producing an error signal, and a
current adjus~ment means responsive to the error signal for
deriving the drive current such as to maintain the tube arc
current constant.
A tube arc current ~onstantly held at the right level
j extends the tube life. Insufficient arc current results in a
lack of anode fall voltage across the electrodes resulting in
insufficient electrode temperature, which in turn results in
much reduced electrode life and hence reduced tube life due
to inefficient operation of the electrodes. Excessive arc
current resul~s in electrode over dissipation and hence
reduced electrode life.
The closed loop arc current control system utilises a
true current servo system rather than a pulse width
modulation system. This has the advantage that the arc wave
form duty cycle is constant which in turn results in a
constant arc current crest factor irrespective of the degree
of correction introduced by the error amplifier. The net
gain is that tube life variation due to varying crest factor
is eliminated.
The arc current crest factor is the ratio of peak to
~MS arc current. The greater this figure, the greater is the
stress to the tube electrodes during tube operation. Stress
21~1~17
WO94/04011 3 PCT/GB93/01618
to the electrodes results in loss of electron emissive
material from the electrode surface which is deposite~ on the
tube wall and is-observed as tube l'end blackening". In a
preferred form of the present invention a crest current
factor of l.3 to l.5 is achieved, dependent upon the tube
type.
The driver is arranged to produce a fully symmetrical
tube arc wave form. This prevents the phenomena of mercury
vapour migration within ~he tube. Mercury vapour migration
ultimately results in light output reduction at one end of
the tube necessitating tube replacement. A fully symmetrical
arc wave form prevents mercury vapou- migration and
ultimately results in increased tube life. -~
The closed loop arc current control system of the `~
invention holds the arc current constant over a wide range of
DC supply voltages (typically 22 to 33 volts DC for a nominal
24 volt supply) . `
As the tube volt-ampere characteristic is only -
temperature dependent, it is inherent that holding tube arc
current constant results in constant tube arc power and hence
~ constant light output at a given temperature. Thus the
i operating arc power and the light output can be selected to
suit the individual application.
For example, in an energy saving, heat dissipation
conscious application of the driver of the invention, the
selected ~alue of arc current may be selected such that the
tube light output is a percentage, less than lO0, (preferably
80 to 90%~ of rated output e.g. 87 per cent. This allows one
to choose a required combination of tube radiated heat, tube
light output and unit power consumption. By selecting and
controlling the tube arc current one contols the tube arc
power. However the arc current selected should fall within
the tube preferred range.
The driver is particularly designed for temperature
sensitive situations, for example where the tube is to light
shelving, confined spaces or refrigeration units.
Independently of the closed loop control therefore the
WO94/04011 2141~17 PCT/GB93/01618
invention concerns a driver which is arranged to run a tube
at tube arc power of 80 to 90% of the tube rated power and at
a constant arc current crest factor below l.7 and preferably
in the range l.5 to l.3.
The means for providin~ ~the reference signal
compriseS a resistive, divide~derived, voltage from a
reference voltage. Selection of~the appropriate divider is
via a jumper link system. The tap position by which the tube
is connected to the secondary winding of the arc transformer
is preferably also selected by a jumper link selection
system. By these means a wide variety of tube types and
styles can be used and the exactly right conditions for each
can be selected in seconds. In order to adjust the driver
for the selected tube type, two header selections must be
made. The first selection is used to select one of a number
(six to eight for example) of resistor 2airs which determine
the tube arc current~ The second selection is used to select
one of a number of taps from the transformer secondary
driving the tube arc. This second selection i5 necessary in
order to ensure compliance with the tube operating arc
voltage. With this arrangement the design is capable of
driving tubes within the range lO to 40 watts and all th~t is
required to tailor the system to a specific tube is the
correct selection of the arc current resistor pair and the
c~rrect selection of the transformer tap position.
The arc current flow circuit includes no dissipating
device forming an arc current limiting element and this leads
to higher efficiency. Efficiencies in the range 80 to 85 per
cent are normally achieved with the circuit described.
The drive voltage adjustment means comprises a low
differential current source having a voltage output V2 and a
switched mode power supply unit, arranged to receive the
supply voltage V and having an output voltage Vl arranged to
be supplied to the low differential current supply unit. The
voltages V2 and Vl are measured by the switched mode supply
unit which is arranged to hold the current source
differential V2 minus Vl constant. The current source is
2141617 ~
PCT/GB93J01618
-, WO94/04011
~rogrammed by the error slgnal and the switched mode supply
unit is programmed from the current source. The switched
mode power supply unit again gives high efficiency of
operation. The switched mode approach also allows the unit a
wide operating voltage range of 22 to 33 the volts DC for a
nominal input voltage of 24 volts.
The high efficiency enables the use of the driver in
confined spaces where excessive heat generation would be a
~roblem and in energy saving situations.
A strike oscillator is provided for providing a
strike voltage, higher than the arc drive voltage, to the
tube arc at start up. Additionally a heater circuit is
provided for providlng a current through the electrodes to
heat these for a short period before each strike up attempt.
For example a strike up attempt may consist of electrode
?reheating for a period e.g. approximately l.5 seconds
followed by the application of a high vol~age strike pulse
across the tube arc. According to another feature of the
invention means are provided for detecting whether or not the
tube strikes. If the tube strikes the strike oscillator is
disabled and the arc transformer driver enabled and the
closed loop arc current control will become operative. If
the tube fails to strike the arc transformer driver continues
to drive but will be inhibited after a brief pause period of
or example approximately 2.5 seconds. The strike sequence
will then be reinitiated, that is the preheating followed by
the application of strike voltage is repeated. Means are
provided for repeating this sequence for a predetermined
number of strike attempts, for example 6 to 8, following
which the attempt sequence will be terminated. If striking
does not occur during this sequence the unit will shut down
and remove potential from the tube terminals. A shut down
would, for example, result with a worn out tube or if the
tube were absent. A shut down may be reset by removal and
reapplication of power.
Should a tube be removed during operation or become
extinguished for any other reason then the system is arranged
- . : - . : - . ...... :.. -
WO94~04011 2 1 ~1 6 1~ 6 PCT/GB93/01618
to re-enter restrike mode wnereby the strike attempt sequence
will restart. If the tube is replaced within the
predetermined duration (e.g. 25 seconds) then the tube will
be restruck. If the tube is not replaced the system will
shut down. This feature enables carefu;l tube changing whilst
the system is running provided that one is aware of the
potential hazards involved.
~rief Description of ~he Drawings
One embodiment of fluorescent tube driver and
lighting system, in accordance with the invention, will now
be described, by way of example only, with reference to the
accompanying drawings of which:-
Figure l is a bloc~ circuit diagram of a driver andtu~e,
Figure 2 shows a plot of arc current and arc voltage
provided by the driver of Figure l,
Figure 3 show details of the circuitry of Figure l
and is divided into parts 3a to 3f which fit together as seen
in Figure 3,
Figure 4 shows the bobbin of the arc transformer of
Figure l, and
Figure 5 shows the bobbin of the electrode
transformer of Figure l.
Detailed Description of one Embodiment
. _ . .
Referring to the ~lock diagram of ~igure l, a hot
cathode fluorescent tube 12 has electrodes 13, 14 and is
arranged to be dri~en from a 24 volt nominal DC supply lS as
the power source. In order ~o preheat the electrodes before
stri~e up, sespective electrodes 13, 14 are connected in
series with secondary windings 16, 17 of an electrode heating
transformer 18, the primary winding 20 of which has an
intermediate part l9 thereof connected to the DC supply 15.
The ends of the primary winding are connected to an electrode
~141617 ~
WO94/04011 7 PCT/GB93/01618
tranformer push/pull driver 21.
A tube arc waveform transformer 22 has a primary
winding 23 connected across an arc ~ransformer push/pull
driver 24 and has a secondary winding 25 one end 26 of which
is connected via line ~7 and a resistor Rs to the electrode
14 to provide the tube arc current supply. An arc
transformer jumper tap selection unit 28 connects a selected
tap off point of the secondary winding 25 via line 30 to the
electrode 13.
Lines 31, 32 connected across the resistor Rs provide
an input measure of the arc current wave form to the input of
an RMS-DC convertor 33, the output of which, on line 34,
provides a DC voltage signal (representing the measured RMS
ar~ c~rrent which is normally in the range 230 to 285 mA) to
one lnput of an integrating error am~lifier 35. The output
from this circuit is a DC voltage proportional to the RMS
tube arc current on a l:l basis ie one volt DC equals one amp
RMS of arc current. A reference voltage source 36 provides a
reference voltage of 15 volts and this is connected via a
resistor 37 to a jumper switch RMS arc current selector
header unit 38 with the connection between resistor 37 and
unit 38 connected to the other input of the integrating error
amplifier 35. The RMS arc current selection header unit 38
comprises a plurality of resistors in parallel connected to
earth and the jumper switch allows one of these dividers to
be tapped thus providing a selected RMS arc current programme
voltage (on the basis of one volt equals one amp) to the
other input of the integrating error amplifier. The output
(error signal) from the amplifier 35 is connected on line 40
to a programme input 39 of a low differential current source
unit 41, the current output of which is connected by line 42
and diode 43 to an interme~iate connection of the primary
winding 23 of the arc transformer 22. This current I is the
driYe current to the arc transformer (l~2 to 2 amps). A
swit~hed mode voltage source unit 44 has an input voltage V
(nominally 24 volts) on line 45 from the DC supply l5 and an
output voltage Vl on line 46 which is supplied as an input to
wo 94~04011 2 1 4 1 6 17 8 PCT/GB93/U1618 ~;
the low differential current source unit 41. The voltages on
lines 46 and 42 are fed back to the voltage source unit 44
which is arranged to hold the current source differential Vl
minus V2 constant.
A strike oscillator 47 is!connected via diode 48 and
line 49 to the midpoint of th~;primary winding 23 of the
transformer 22. A strike capa~ tor Cs is connected between
the line 49 and earth and is~~arranged to be charged up by the
strike oscillator.
A system control unit 50 includes a multi-strike
attempt timing circuit 51 and a "give up" circuit 52, each
connected to receive a start up signal on line 53 from a
power up reset/start switch unit 54. The timing circuit 51
has a phase l output on line 55A to enable the electrode
transformer driver 21 and the strike oscillator 47, and a
?hase 2 output on line 55B to enable the arc transformer
driver 24. The give up circuit 5~ has an o~tput on line 56
arranged to give a shut down signal to the circuits 51, 21
and 24. The output of the error amplifier 35 is also
connected-on line 57 to an input of the multi-strike attempt
timing circuit 51 and the give up circuit 52 to indicate
whether or not the tube has been struck. A short circuit
overload shut down circuit 58 is connected across the lines
3l, 32 and arranged to detect a short circuit and when so
detected to supply a shut down signal to the programme input
of the low differential current source 41.
As indicated at the top of Figure l the circuit DC
supply 15 is derived from an original DC supply 60 via a
reverse polarity over current protection unit 61 and a
filtering unit 62.
The system operates as follows. On system power up,
the power up reset generator 54 issues a reset/start pulse to
the system controller 50.
Initially, the system controller enters phase one
which involves enabling the strike oscillator and enabling
the ~ube-end electrode pre-heat system. The strike
oscillator is a simple flyback convertor which is used to
, .:,, ,,,, . . , ... , ... ... , . , , . , . ~ .. . . ... . . .
WO94/04011 2 1 4 1 6 1 7 PCT/GBg3/01618
charge the strike capacitor, Cs~ to approximately 91 volts
which is later stepped up by the arc transformer to generate
a high tension tu~e strike potential.
The electrode pre-heat system consists of a push-pull
step down transformer configuration generating two
galvanically isolated electrode pre-heat voltages.
During phase one which is of approximately l.5
seconds duration, the main arc transformer push-pull drive
system is disabled.
During phase two, the strike oscillator and th~
electrode pre-heat system are disabled and the arc
transformer push-pull drive is enabled. Initially, drive to
the arc transformer results in the voltage across Cs being
stepped up by the transformer ratio. This results in the
generation of a high potential across the tube arc resulting
ln a tube strike. The result is a decaying alternating
voltage across the tube arc as the strike capacitor
discharges.
During initial strike period, a current flow is
established into the tube arc which has the effect of
activating the sys~em arc current control loop.
Current flowing to the arc results in an alternating
arc current wave form across Rs, a measure of which is passed
to the RMS to DC convertor 33, the outpu~ from which is a DC
voltage proportional to the RMS tube arc current. The
voltage from the RMS output is ~ompared in the error
amplifier 35, to a selected programme voltage (at input 39)
representing the desired tube RMS arc current. The amplifier
output (error signal) on line 40 is utilised to programme the
linear low differential current source 41, the resulting
output from which is fed to the arc transformer primary
winding.
The current flow in the primary winding is stepped
down by the transformer ratio to produce an arc current
which, in a matter of milliseconds, is adjusted to the value
set at the error amplifier programme input 39, by nature of
the arc current feedback taken from Rs (as described). This
WO94/04011 10 PCT/GB93/01618
I
?rovides a closed 'oop servo control system maintaining
substantially constant the t~be arc current by varying the
amplitude of the arc curr~ent waveform.
The net effect is that the tube strike is initiated
by the discharge of the strike capacitor but maintained by
the arc cuxrent control loop. By nature of the fact that the
tube volt/ampere characteristic is fixed, the desired arc
current results in a fixed arc power.
If the tube arc fails to strike on application of the
stri.~e ~oltage, then a signal from the output of the error
amplifier, on line 57, which signal is indicative of a strike
failure, is utilised to force the system control circuit to
re-e~ter the phase l of the strike procedure and thus restart
the strike sequence.
Repetitive strike failure is measured by the give up
circuitry 52, which after a predetermined period
corresponding to a predetermined number of failures commands
a system shut down on line 56, whereby both the electrode and
arc drive systems are disabled. This action will occur after
approximately 6 to 8 strike attempts or approximately 25
seconds.
The arc transformer programmable current source 41 is
of t inear design and is designed to operate at low
differential voltages to attain low dissipation. As the
output voltage V2 from this current source will be
determined by the tube arc voltage as set by the transformer
ratio (V2 is typically about 18 volts), and the current
source differential will be hundreds of millivolts tVl minus
V2 is typically about 0.38 volts), the switched mode power
supply 44 is employed to efficiently reduce the 22 to 33 volt
~nominal 24 volts) supply 15 to the voltage attained at Vl.
The switched mode power supply unit monitors the current
source differential voltage ~Vl-V2) and holds this ~alue
constant irrespective of the voltage of V2 dictated by the
tube.
~ t is important to realise that the current source
41 is programmed by the error amplifier 35 and the switched
214161~
WO94/04011 ll PCT/GB93/01618
mode ~ower supply 44 is programmed from the curren~ source in
that order.
The ratio- of the arc transformer is selec~ed such
that Vp is placed in ~he optimum operating range of 13 to 18
volts.
, The arc transformer push-pull driver 24 is chosen to
produce a 90 to 95 per cent duty cycle, symmetrical arc
voltage wave form of a frequency 32 KHz nominal. An example
of an arc voltage wave form and resulting arc current wave
form plot~ed against time, as achieved by the Figure l
circ~it is shown in Figure 2. The fixed duty cycle of 90 to
95 per cent, attains the low arc current crest factor of
typically l~3 to l.5 dependent on the tube type. The wave
form has sharply defined on off characteristics.
The electrode drive push-pull system results in the
production of two electrode drive voltages which a~ain are
symmetrical and of frequency 36 KHz nominal.
This system is designed to cope with a wide range of
standard "off the shelf" tubes and for this purpose it has
j the ability to deliver tube arc powers of between lO and 40
watts and is quickly configurable to the desired tube style
I by the appropriate placement of the two printed circuit style
jumper links 38 and 28 which are used to select tube arc
current and tube arc voltage. For example, eight dividers
: may be provided for generating the arc current prsgramme
~oltages required for six popular tubes with provision for
two spares and four arc transformer tap sele~tion positions
are provided in the unit 28. By utilisation of the jumper
link selection sys~em, it is possible to select the tube type
within seconds. The first selection is used to select one of
the number of resistor pairs in the unit 38 which determine
the tube arc current. The second selection is used to select
one of a number of taps (for example four) from the
transformer driving tube arc in the system 28. This second
selection is necessary in order to ensure compliance with the
tube operating arc voltage. In the circuit described six
tubes are catered for with two spare uncommitted tube types.
¦ W094/0401l 2 1 41 6 17 12 PCT/GB93/01618
The six tubes are 18 watt two foot (61cms), 30 watt three
foot (91.4cms), 36 watt four foot (121.9cms), 36wp1, 38 watt
three foot six inches (106.7cms) and 40wpl. It should be
emphasised however that the design is capable of dri~ing any
tubes within the range 10 to 40 watts~nd is by no means
limited to the above types. ;;~.
Figure 3 shows the implement~ation of the described
system which may be related to the block diagram of Figure 1
by reference to the following circuit section descriptions.
Section 1 Switched mode voltage source
Section 2 Low differential current source
Section 3 Integrating Error ~mplifier
Section 4 ~MS to the DC con~ertor
Section 5 System control
Section 6 RMS arc current selection header and resistors
Section 7 Arc transformer push-pull driver
- Section 8 Strike oscillator
Section 9 Electrode transformer push-pull dri~er
Section 10 Electrode transformer
Section 11 Ar~ transformer
Section 12 Arc transformer tap selection
Section 13 Power-up reset circuit
Section 14 Short circuit/overload shut-down
- With reference to Fiyure 3, the 24 volt nominal DC
supply is applied to Jl-l(+VE) and Jl-3(0V). Dl provides
protection from reverse connection of the 24 volt supply.
The four terminal connection to the tube is made at J2. One
electrode should be connected to J2-1 and J2-2 whilst the
second electrode is connected to J2-3 and J2-4. The
connections and types and values of the parts are shown on
the Figure.
~-! Section 1 of Figure 3 forms an efficient step-down switched mode volta~e source and Section 2 forms a low
j differential linear current source. The function of the
¦ switching regulator is to hold the current source
differential voltage, TP14 to TP15, constant at 380mV thus
maintaining low current source dissipation. The switched
:~; WO94/~011 21~1617 PCT/GB93/01618
mode voltage source operates at approximately 72RHz and is
based around a pulse width modulation control device t U4 type
Tp494 .
A voltage of 380mV greater than the voltage at TPl5
is generated at- U4 pin 2 by the networ~ R27, Dll, DlO, Q4,
R23, U3, R24, R25. This voltage is utilised to program the
switched mode supply to issue an output voltage at TPl4 of
TPl5 + 380mV thus maintaining 380mV differential from TPl4 to
TPl5.
The controlled pulse width output from U4 pin 8 and
ll drives the main switching mosfet Ql9 via buffer stage Q5
and Q6. Ql9 forms the switching element of a flybac~ step-
: down configuration which consists of a flyback inductor L2, aback emf catch diode Dl5 and a storage capacitor C30.
. A voltage waveform representative of the tube arc
current waveform and derived from R85 is fed to the RMS to DC
convertor (Section 4) via the sense + and sense - lines. The
convertor is not a true RMS to DC conversion circuit but a
,quasi design which operates sastisfactorily provided that, as
. in this case, the input waveshape and duty cycle are fixed.
The convertor operates by first of all buffering the signal
via voltage follower U7 and removing front edge waveform
overshoot via low pass filter R55/Cl9. The resulting
waveform is then applied to the peak detector consisting of
,U6/Dl2/R37/C22 and R59 which results in a voltage being
developed across C22 which is representative of the peak arc
'current. A voltage representative of the RMS arc current is
then generated by passing the peak voltage via a buffer amp
(USA) to a potential divider R33/R34 which attenuates the'
,signal by the arc,waveform duty cycle:factor (95% or X 0.95)
resulting in a voltage representative of the RMS arc current
at TP5.
- The integrating error amplifier (Section 3) compares
the voltage representing achieved~RMS arc current at TP5 with
a ~oltage representing the required RMS current which is
formed by the RMS arc current selection header system
(Section 6).' The header HDl allows the selection of any one
2141617
WO94/0401l 14 PCT/GB93/01618
from eight possible potential dividers.
Both the achieved RMS arc current voltage and the
re~uired RMS arc current voltage utilise a scaling f ~ctor of
1 volt DC = 1 Amp RMS of arc cur~ent.
The output from the error a~p~lsfier, USD pin 14, is
used to program the current source (Section 2) which in turn
issues a current to the arc trans,former primary which is
stepped down by the arc transforme~ to form the arc current.
Section 7 is the arc transformer push-pull driver
st~ge. U9 (an IP494) is a pulse width modulator driver IC
but configured to deliver a fixed push-pull duty cycle of
95%. The 95~ duty cycle resulting in 5~ dead time when
neither push or pull drivers are active, ensures that there
is no possibility of both push and pull drivers being
energised simultaneously due to drive signal o~erlap. The
push-pull signals originating from U9 pins 9 and 10 are
buffered via Q15/16/17/18 and passed to the drive mosfets Q21
and Q22 which drive the two arc transformer primary windings.
Note that the two driving mosfets Q21 and QZ2 dri~e the
primary windings in opposing phase to generate the push-pull
operatsng mode. U9 operates at a frequency of 62 XHz as
determined by l.l/R73 C24 which due to the push-pull drive
nature ! results in an arc waveform of half this value ie
31KHz.
The electrode transformer push-pull drivers (Section
9) is of similar design to the arc transformer push-pull
dri~er stage. This stage drives the electrode transformer
~Section 10) for approximately 1.5 seconds during the strike
sequence after which it is inhibited. The operating
frequency is 74KHz as determined by l.l/R71, ~23 which
! results in an electrode waveform frequency of 37KHz.
- Section 8 represents the strike oscillator the
purpose of which is to charge strike capacitor C32 to
approximately 91V. The oscillator is a simple flyback
configuration with Ll forming the flyback inductor and
operates at 65 to 85KHz.
The complete system is controlled and synchronised by
WO94/04011 21 41~17 PCT/GB93/01618
the system controller (Section 5) which is reset on power-up
by the pow~r-up reset circuit (Section 13).
The two phases of the strike sequence are represented
- by the logic state of the P/S & R line (Prime, strike and
run). With P/S & R in the high state, Phase 1 of the strike
sequence is initiated with the electrode transformer push-
I pull driver enabled via DPlC, the strike oscillator enabled
due to Q11 being in the "off" state and the arc transformer
push-pull driver being disabled due also to Qll being in the
"off" state.
With P/S & R in the low state, Phase 2 of the strike
sequence is initiated where the electrode transformer push-
pull driver is disabled via DPlC, the strike oscillator is
disabled due to Qll being in the "on" state and the arc
transformer driver is enabled due also to Qll being in the
i "on" state.
¦ By utilising a signal from the output error amplifier
! derived by Z2, R12, R16 and Q3, the control circuit is able
to detect a tube strike failure and thus time a period by
time constant R17/C8 during which multi-strike attempts are
made via repeated cycling of Phase 1 and Phase 2 strike
procedures. Failure to strike within this period results in
a system shut-down by U2A pin 2 switching low into the "give
up " state shutting down both electrode and arc drive
waveform generators.
Values for the arc current set resistors R40 to R47
are selected such that six tubes may be catered for with two
spare uncommitted tube ~ypes. The six tubes used in this
example are 18W 2 foot (61cms), 30W 3 foot ~91.4cms), 36W 4
foot (121.9cms), 36WPL, 38W 31f2 foot (106.7 cms) and 40WPL.
The value of arc current selected for each tube
results in a tube arc power which is 82% that of rated tube
arc power. This provides a highly desirable combination of
tube radiated heat, tube light output and system power
consumption. The arc current may however be tailored to the
requirements of the individual.
A suitable arc transformer design for the above tube
21 416 1~ `, ;` . .;; ~
W094/0401l 16 PCT/GB93/01618
range is given in Figure 4. The table below shows the
required transformer ~ap selectlons made on the arc
transformer tape selection header and the corresponding RMS
arc current values selected via the RMS arc current selection
header resulting in 82% of rated arc power.
On comparative test run using a driver D in
accordance with the invention as disclosed herein and a 36W
rated tube, run at a total tube power of 29.5W, as compared
with a known system C run at 50Hz mains voltage and at the
36W rated arc power, with ambient temperature of 21C to
22.5C, gave the following results.
Total Tube Power Light Output Tl T2 T3 Hottest
% of Mains Component
.
Temperature
C 36W 100 4319.3 19.3 ~ 53.0
(Ballast~
D 29.5W 87 2415.2 15.2 17.0
(Heatsink)
,.
Where Tl, T2, T3 are all temperatures in degrees centigrade
! above ambient temperature respectively taken at-the tube
electrodes r one quarter of the tube length from the
electrodes, and midpoint of the tube.
.
_ .
Tube TypeRMS Arc Current fsrRequired Transformer
82% Rated Arc Power Tap Pin Number
. . '
18W 6lcms 25OmA 10
30W 91.4cms250mA 11
36W 121.9cms275mA 11
36WPL 285mA 11
38W 106.7cms285mA 11
40WPL 230mA 12
214161q'-''
W094/04011 17 PCT/GB93/01618
A design for ~he electrode transformer is shown in
Figure 5.
Both transformers are ~ased around Philips RMlO
formers. In order to ob~ain a high arc transformer operating
efficiency, a low lo~s ferrite material should be utilised.
As the arc transformer operates at 32KHz, a suitable low loss
ferrite at this frequency is either Philips 3C85 ungapped or
Siemens N41 ungapped. The design shown in Figure 4 operates
at 170 to 230mT (Milli-Teslas) which ensures freedom from
magnetic saturation. A similar srade of ferrite may be
utilised for the electrode transformer.
Figure 4 shows the transformer bobbin and pins for
forming the arc transformer 22. The windings are not shown
but all the primaries are wound in the same direction and all
the secondaries are wound in the same direction. The bobbin
is a Philips 43220Q21-34060 with 12 pins. The windings are
as follows:
First primary, -12 turns, 0.75 millimetre diameter ECW
Second primary, 12 turns, 0.75 millimetre diameter ECW
First secondary, 48 turns, 0.375 millimetre diameter ECW
Second secondary, 12 turns, 0.375 millimetre diameter ECW
'- Third secondary, 24 turns, 0.375 millimetre diameter ECW
Fourth secondary, 26 turns, 0.375 millimetre diameter ECW.
The first primary starts at pin l and finishes at pin
3, the second primary starts at pin 4 and finishes at pin 6,
the first secondary starts at pin 7 and finishes at pin 9,
the secondary starts at pin 9 and finishes at pin lO, the
third secondary starts a~ pin lO and finishes at pin ll and
the fourth secondary starts at pin ll and finishes at pin 12.
The primaries must be put on to the bobbin first and
interwinding tape should be used between the primaries and
secondaries and between secondary one and secondary two and
over the outside.
Figure 5 shows the transformer bobbin and pins for
the electrode ~ransformer 17. The bobbin is a Philips 4322-
021-34730 with 12 pins and all windings are in the same
wo 94/04011 18 PCI`/GB93/01618
direction and made fr..o~ ,0.-4 millimetre diameter wixe.
Winding l s~arts at pin 10, has 48 turns and finishes
at pin 3~ .
Winding 2 starts at pin 12, has 48 turns and finishe
at pin 1.
Winding 3 starts at pin 4, has 12 turns and finishes
at pin 9.
Winding 4 s~arts at pin 6, has 12 turns and finishes
at Din 7.
Insulating tape rated at 80C continuous should be
used between each winding and on the external winding.
- It should be emphasised that the design shown is
capable of dri~ing any tube in the range 10 to 40 watts and
is by no means limited to the above selection. It can be
adapted to operate tubes of higher voltage. All that is
required to tailor the system to a specific tube is the
correct selection of the RMS arc current program voltage and
correct design of the arc transformer winding.
If the arc transformer is required to be- modified in
order to comply with a speci~ic tube not mentioned, then one
should retain the same primary winding as Figure 4, but
select the secondary winding such that TP7 (the primary tap)
r~ is placed r at approximately 17.5 volts with the RMS tube
voltage at the desired value.
If VT = RMS voltage of tube at desired tube power
D = Du~y Cycle of drive = 0.95
VTP7 Voltage at TP7 = 17O5V
NS = Number of secondary turns
Np - Number of primary turns = 12
Then NS = VTNp x 1
VTp7 D
i
If the RMS arc current is required to be modified to
a value not available from the existing values, then a spare
i,
2141617
W094/04011 l9 PCT/GB93/01618
resistor/header ~osi~ion should be utilised to set the
desired arc current progr~m voltage ie R41 and the value for
R4l calculated as follows:
If R4l Value of R4l
V = Voltage representing RMS arG current
1 VDC = lV R~S of arc current
R6l Value of R6l = 2Ok
R4l 6l
(15/V-l)
While one embodiment of the system has been described
in detail for use with a low voltage 24 volt nominal source,
the concepts can be applied to a system run from other
voltages or from any conventional mains voltage source
converted to produce a DC voltage supply, and with -`
appropriate change of component values.
