Note: Descriptions are shown in the official language in which they were submitted.
B- '4 doftheiu~.ut;cn
The state of the art ballast uses high r ~ue~ y power switches. They must sustain at the same time the high voltage
needed to start the lamp and the high current needed during the steady state operation. This invention co .~ c 2 high
~e4uency separate DC sources, one providing the low current with high voltage during the start up and the other
providing high current with low voltage needed during the lamp steady state operation.
Highest lamp efficiency is achieved under cc --l ;i~ -c DC voltage. However DC wears out rapidly the lamp ele~ c So
the lamp must be powered by a ~yuul.e~ical AC waveform.
State of the art ballasts use AC high li~4uell~,y oscill~tionc Due to the "inertia" of the larnp's ionized gas, the high
~uell~iy is perceived by the laînp as - ~ "DC" energy. There are two types of high li~4uell~y ballasts; resonant
ones and low crest factor ones. The resonant type suffers of lamp limited efficiency and high laînp stress due to the
.. ~ ~'s high crest factor. The other type, while having low lamp crest waveforrns (squarewavwe), suffers from its own
low efficiency due to its switching losses.
The invention ~ . ;l~d here achieves better efficiency, less stress and less wear than the above by powering the lamp
with ~;c~ r;- A..1ly.10wer G~4ucn~,y s lu~c:wd~e pulses. Since this novel ballast does not use any AC coupling devices, such
as t ~ - or Cd,~Jdci~ , low r ~4ut:n~y does not affect the ballast size at all.
State of the art ballasts limit the lamp current with series devices such as Cd~dCIlul~. This counter-balances the negative
lamp , -~ - ~ However, the lamp current varies with the lamp ;~ ~tAi~e and in many cases also with the input
voltage. This leads to less than optimal working co Ji~ for the lamp.
This i-.~ ion cv~ pc current mode control of the PWM's, thus insuring precise, controllable and ~' -~~ ~~ ' '~ current
to the lamp with complete in~l~n-1f -~ e from the input voltage and lamp; ,. ,~ e and without the need of a series
device.
Fl,.IL ,l..ol~, c~ g the current fed topology with the current mode control, constant and precise current is achieved
. In C~ b;l~A~ n with precise timing of the full bridge switching this y,ua c~ es a high degree of current
y (AC) lt;~U~ ' - of the degree of ~llUIl~lly of the ballast.
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The idea of separate start-up is not new. For example it appears in the US patent 5,449,980. However it is of a ~ 'y
different approach than this invention. It counts on AC coupling elements such as l.A."~f... ,.- ~ b and series ca~ h Also
it seems that the start up voltage is limited to t~-vice the steady state voltage.
The idea of the low crest factor appeared recently in this ~eld. For example the US patent 5,446,347 clP~ a
ll r ' ' or llulA,dldd ~hl~v~dve as signal wdwfullll to power the lamp. However it uses a ~ different a~ oa.,L
in limiting the ballast internal ~1ic~irAtinn It does this by using a resonant circuitry that includes a l.A..~f~ ",~r and a
ca~d.;ilor. This hl~,nlion uses no Ir.~onAl-~e at all and reduces the losses by using low ;,vvilclullg rlt;~uenl,y in c~ n
with the current fed topology.
Summary of the I~
This ballast circuit operates from an input DC voltage. The DC voltage is geneldldd either by another power supply or by a
rectifier circuit fed from the AC line or by a Power Factor Corrected circuit fed from the AC line. It works as follows:
A full bridge operates at a fairly low rle~uen~,y (higher than 50Hz) with a duty cycle around 50%. It ploduces a balanced
AC wdv~r~,ll.. with a pl~.,.s~ly equal positive and negative time achieved by precise timing control of the full bridge
switches. This v~dv~;rull.. is the ballast output that is fed to the lamp. The bridge gets its input from two DC current sources
c~ t~ ;l in parallel. One current source pl~ ' ~ s high current but its voltage is limited to the input DC voltage. The
other current source can produce a high voltage on open circuit, such that a ~,,ch~pe lamp can ignite. However its current
capability is fairly low in C~ )A~ ;~OI~ to the other source. This low current source is bypassed by a small cal~dciloi in
parallel with a reverse blocking diode such that if the current demand is higher than its capability the current will be
provided by the other current source. One ell~ of the current sources is in the form of DC to DC high li~lu
converters opel~illg for example in the ~g~h~ L~. range of ~. ' g rl~
A variant of this invention is as follows: The 2 current sources are the same but the full bridge is split into 2 half bridges.
One half bridge (which drives one side of the lamp) is c~ P~d directly to the High current source. The other half
remains -- ~ as ~P.~. . ;bdd in the previous ~d~h
In this variant the lamp start-up occurs in an ab~.. - .1. ;.~Al manner but on the other hand there are ~ignificAnt
.,~. in efficiency since one half of the bridge uses low voltage c~ n~
BnefD~,...' O De3.,'. :'
Figure 1 shows an ~ of the invention using the full bridge a~roach
Figure 2 shows an e '- - ' of the invention using two half bridges a~lodcl
Detailed D~.'. ~ of the ~ ~. '
Both ~. ..1~1; " ,~ c~ mrri~e two high rl~uell~ collv~ working in current mode type, a reverse blocking diode and a
low r ~uen~ full bridge ( or two half bridges) as follows:
The boost collv~ ;l comrri~Ps Ql,Ll,Dl,PWMland Cl. The col.v~ller is internally current-limited to the ignition current
value( which is ~i~ifirAntly smaller than the steady state current). PWMl has over voltage protection since is fiP.P~k-
controlled to limit the output DC voltage to a high value (sl.ffi~iPnt to g - ~ ~ lamp ignition). Note that QlLlDl are
high rl~ . ~, high voltage, but low current c~
The Buck collv~llel C~ es q~"1'77T ,~, and PWM2. Its "~A. ;.. output voltage is limited by the input DC voltage.
PWM2 is fPedbA~k-controlled to limit the collvel It;l output current to the lamp steady-state current. Note that q2,d2,L2 are
high rl~u,r~, high current, but lowvoltage co...~ .Ix The collvc;llel operates in a current fed mode since it does not
have an output ca~ilol.
The blocking diode D3 isolates the ignition circuitry from the steady state circuitry. In other words, it isolates the high
voltage from the low voltage circuitry.
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The full bridge comprises switches Q3,Q4,Q5,Q6 and 50% clock of Fig 1 or switches q3,q4,Q5,Q6 and 50% clock of
Fig.2.Its purpose is to couple in a syl.lll.el.ical AC manner the lamp to the DC/DC converters(buck and boost). All
switches are u~ g at low frequency, such that the switching losses are minim~l Due to the position in the circuitry of
diode D3, only switches Q6 and Q5 have to sustain high voltage in the ~...1~1;....~l~1 shown in Fig 2
Diode d4 h~ mainly a protection role. It provides a non-dissipative cl~mpinlJ means of inductor L2 in the case of
il~t~,llu~t;d conduction through the output.
There are few observations to be made;
1) Capacitor Cl is needed to decouple the boost ac portion of switching current from the buck switching current.
Otherwise the current mode control of the buck converter is col I u,~Aed and might lead to instabilities.
2) The choke L2, via the bridge is practically in series with the output. It means that the lamp is fed with current rather
than voltage at frequencies higher than the feedback ~u~n~. On one hand this a~loach elimin~tes the cross c~
stress and dissipation of the bridge switches. On the other hand, in cc.~ ~bi 1~,.l ion with the PWM's current mode feedback
control, it ~,udl~ll~s that the lamp will work always in constant, controlled and ~ ~e current fed mode regardless of
the lamp type~ This is why such a ballast deserves to be qualified as an universal one.
