U.S. patent number RE44,133 [Application Number 12/617,924] was granted by the patent office on 2013-04-09 for fixed operating frequency inverter for cold cathode fluorescent lamp having strike frequency adjusted by voltage to current phase relationship.
This patent grant is currently assigned to Monolithic Power Systems, Inc.. The grantee listed for this patent is James C. Moyer, Timothy J. Rust. Invention is credited to James C. Moyer, Timothy J. Rust.
United States Patent |
RE44,133 |
Moyer , et al. |
April 9, 2013 |
Fixed operating frequency inverter for cold cathode fluorescent
lamp having strike frequency adjusted by voltage to current phase
relationship
Abstract
A method of driving a lamp that uses a DC to AC inverter that is
connected to a primary winding of a transformer is disclosed. The
inverter frequency is variable, and in one embodiment, may be
controlled by a voltage controlled oscillator. Circuitry is
included that monitors the phase relationship between a voltage
across a secondary of the transformer and a current through the
primary of the transformer. The circuitry monitors the phase
relationship and adjusts the inverter frequency, such as by
adjusting voltage controlled oscillator, so that the phase
relationship is maintained at a predetermined relationship.
Inventors: |
Moyer; James C. (San Jose,
CA), Rust; Timothy J. (Fremont, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Moyer; James C.
Rust; Timothy J. |
San Jose
Fremont |
CA
CA |
US
US |
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|
Assignee: |
Monolithic Power Systems, Inc.
(San Jose, CA)
|
Family
ID: |
34314057 |
Appl.
No.: |
12/617,924 |
Filed: |
November 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10677612 |
Oct 2, 2003 |
6919694 |
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Reissue of: |
11060237 |
Feb 16, 2005 |
7294974 |
Nov 13, 2007 |
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Current U.S.
Class: |
315/291; 315/224;
315/276; 315/209R; 315/307; 315/DIG.7; 315/194 |
Current CPC
Class: |
H05B
41/2828 (20130101); Y10S 315/05 (20130101); Y10S
315/07 (20130101) |
Current International
Class: |
G05F
1/00 (20060101) |
Field of
Search: |
;315/209R,194,142,224,244,291,276,307,DIG.5,DIG.7
;363/16,17,24,41,132 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1296542 |
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Mar 2003 |
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EP |
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1296542 |
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Mar 2003 |
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EP |
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05-100552 |
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Apr 1993 |
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JP |
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Other References
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Andreycak, Bill, Phase Shifted, Zero Voltage Transition Design
Considerations and the UC3875 PWM Controller, Application Note,
Unitrode Corporation, U-136A (1999). cited by applicant .
Balogh, Laszio; The New UC3879 Phase-Shifted PWM Controller
Simplifies the Design of Zero Voltage Transition Full-Bridge
Converters; Unitrode Corporation, 1999. cited by applicant .
Chen, W. et al; A Comparative Study of a Class of Full Bridge
Zero-Voltage Switched PWM Converters; Virginia Polytechnic
Institutes and State University; Blacksburg, Virginia, 1995. cited
by applicant .
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Electronic Ballast Applications," IEEE Transactions on Industrial
Electronics, vol. 41, No. 4, Aug. 1994. cited by applicant .
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Seminar, SEM800, Unitrode Corp. (Sep. 1991). cited by applicant
.
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Institute and State University; Blacksburg, Virginia, 1990. cited
by applicant .
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Development Center; Schenectady, NY, Apr. 1989. cited by applicant
.
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Width Modulation with Phase-Shifted Control," Topic 5, Power Supply
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.
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Simplifies the Design of ZVS Resonant Inverters and DC/DC Power
Supplies," APEC 1995 Conf. Proceedings, pp. 794-701, vol. 2, Issue
0. cited by applicant .
Pressman, Abraham I.; Switching Power Supply Design; McGraw-Hill
1991. cited by applicant .
Redl, R., et al., "Optimum ZVS Full-Bridge DC/DC Convert er with
PWM Phase- Shift Control : Analysis , Design Considerations, and
Experimental Results," IEEE APEC, pp. 159-165, 1994. cited by
applicant .
Steigerwald, Robert L., "A Comparison of Half-Bridge Resonant
Converter Topologies," IEEE Transactions on Power Electronics, vol.
3, No. 2, Apr. 1998. cited by applicant .
Sum, K. Kit (editor); Recent Developments in Resonant Power
Conversion; Intertec Communications Press; Summit Electronics,
Inc., Highland, Illinois, 1988. cited by applicant .
Takano, H, et al. "Feasible Characteristic Evaluations of Resonant
PWM Inverter-Link DC-DC High-Power Converters using High-Voltage
Transformer Parasitic Components," Proceedings of Applied Power
Electronics Conference and Exposition (Dallas, TX), pp. 913-919,
Mar. 1995. cited by applicant .
Vlatkovic, V., et al., Small-Signal Analysis of the Phase-Shifted
PWM Converter, IEEE Transactions on Power Electronics, vol. 7, No.
1, Jan. 1992, pp. 128-135. cited by applicant .
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Edition, p. 760; Newnes, 1999. cited by applicant .
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Sons, Inc. pp. 322-324, 1997. cited by applicant .
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2004-029258. Japan Patent Office. Dispatched Jul. 24, 2006. 2
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Coaton, J.R. and A.M. Marsden, Lamps and Lighting, Joh Wiley &
Sons, Inc. pp. 322-324, 1999. cited by applicant.
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Primary Examiner: Philogene; Haiss
Attorney, Agent or Firm: Perkins Coie, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION(S)
This Application is .Iadd.a reissue application of U.S. patent
application Ser. No. 11/060,237, filed Feb. 16, 2005, which is
.Iaddend.a continuation of U.S. patent application Ser. No.
10/677,612, filed on Oct. 2, 2003, now U.S. Pat. No. 6,919,694
.Iadd.each of .Iaddend.which is hereby incorporated by reference in
its entirety.
Claims
The invention claimed is:
1. A method of driving a lamp that uses a DC to AC inverter that is
connected to a winding of a transformer comprising: (a) monitoring
a phase relationship between a voltage across said winding of said
transformer and a current through said winding of said transformer;
and (b) keeping said phase relationship between said voltage across
said winding of said transformer and said current through said
winding of said transformer at substantially a predetermined
relationship.
2. The method of claim 1, wherein said voltage across said winding
of said transformer is substantially in phase with said current
through said winding of said transformer.
3. The method of claim 1, wherein during .Iadd.an .Iaddend.ignition
of said lamp, .[.the.]. .Iadd.an .Iaddend.operating frequency of
said inverter is increased .[.by maintaining.]. .Iadd.in order to
maintain .Iaddend.said predetermined relationship between said
voltage across said winding and said current through said
winding.
4. The method of claim 2, wherein said voltage across said winding
of said transformer is maintained substantially in phase .Iadd.with
said current through said winding .Iaddend.by using .[.the.].
.Iadd.a .Iaddend.zero-crossing information of said current .[.in.].
.Iadd.through .Iaddend.said winding.
5. An apparatus for driving a lamp comprising a transformer having
a primary and a secondary; .[.means for converting.]. .Iadd.a
circuit configured to convert .Iaddend.a DC power into AC power
.[.and operating at a frequency, the means for converting driving
the primary.]..Iadd., wherein the circuit is configured to operate
at a frequency and drive the primary .Iaddend.of said transformer;
means for .Iadd.monitoring .Iaddend.phase .[.comparison that
monitors a phase relationship.]. between a voltage across said
primary of said transformer and a current through said primary of
said transformer; and means for .Iadd.adjusting .Iaddend.frequency
.[.control that adjusts the frequency.]. of said .[.means for
converting.]. .Iadd.circuit .Iaddend.such that .[.said.]. .Iadd.a
.Iaddend.phase relationship between said voltage across said
primary of said transformer and said current through said primary
of said transformer is maintained at substantially a predetermined
relationship.
6. The apparatus of claim 5, further including a voltage controlled
oscillator that is responsive to said means for .Iadd.adjusting
.Iaddend.frequency .[.control.]. and .Iadd.configured .Iaddend.to
output an oscillation used by said .[.means for converting.].
.Iadd.circuit .Iaddend.to generate said frequency.
7. The apparatus of claim 5, wherein .[.said means for phase
comparison and said means for frequency control operate to maintain
said phase relationship as being substantially in phase.].
.Iadd.said voltage across said winding of said transformer is
maintained at substantially in phase with said current through said
winding of said transformer.Iaddend..
8. The apparatus of claim 5 wherein said means for phase
.[.comparison.]. further includes a zero-crossing detector for
monitoring said current through said primary.
.Iadd.9. A method of driving a lamp that uses a DC to AC inverter
that is coupled to a transformer having a primary and a secondary,
the method comprising: (a) determining a phase relationship between
a voltage across either the primary or the secondary of the
transformer and a current through either the primary or the
secondary of the transformer; and (b) keeping the phase
relationship between the voltage across either the primary or the
secondary of the transformer and the current through either the
primary or the secondary of the transformer at a substantially
predetermined relationship..Iaddend.
.Iadd.10. The method of claim 9, wherein the voltage across either
the primary or the secondary of the transformer is substantially in
phase with the current through either the primary or the secondary
of the transformer..Iaddend.
.Iadd.11. The method of claim 9, wherein during an ignition of the
lamp, an operating frequency of the inverter is increased in order
to maintain the substantially predetermined relationship between
the voltage across either the primary or the secondary of the
transformer and the current through either the primary or the
secondary of the transformer..Iaddend.
.Iadd.12. The method of claim 10, wherein the voltage across either
the primary or the secondary of the transformer is maintained
substantially in phase with the current through either the primary
or the secondary of the transformer by using a zero-crossing
information of the current through either the primary or the
secondary of the transformer..Iaddend.
.Iadd.13. The method of claim 11, wherein the operating frequency
is increased to a strike frequency..Iaddend.
.Iadd.14. A method of striking a lamp comprising: determining a
phase relationship between a voltage across a transformer and a
current through the transformer; and sweeping a frequency of a
signal applied to the transformer to a strike frequency based on
the phase relationship between the voltage across the transformer
and the current through the transformer..Iaddend.
.Iadd.15. The method of claim 14, wherein determining the phase
relationship between the voltage across the transformer and the
current through the transformer comprises: determining a phase
relationship between a voltage across either a primary or a
secondary of the transformer and a current through either a primary
or a secondary of the transformer..Iaddend.
.Iadd.16. The method of claim 15, wherein sweeping the frequency of
the signal applied to the transformer to the strike frequency based
on the phase relationship between the voltage across the
transformer and the current through the transformer comprises:
sweeping the frequency of the signal applied to the primary of the
transformer to a strike frequency when the voltage across either
the primary or the secondary of the transformer is not
substantially in phase with the current through either the primary
or the secondary of the transformer..Iaddend.
.Iadd.17. The method of claim 16 further comprising: changing from
the strike frequency to a normal operating frequency..Iaddend.
.Iadd.18. The method of claim 17, wherein changing from the strike
frequency to the normal operating frequency occurs when the voltage
across either the primary or the secondary of the transformer is
substantially in phase with the current through either the primary
or the secondary of the transformer..Iaddend.
.Iadd.19. A method of converting a DC input voltage to an AC signal
for driving a lamp, comprising: controllably switching the DC input
voltage ON and OFF to generate an AC signal; monitoring two or more
signals, wherein the two or more signals comprise at least a drive
voltage signal and a drive current signal; determining a phase
relationship between the drive voltage signal and the drive current
signal to generate a control signal; and controlling a frequency of
the AC signal based on the control signal..Iaddend.
.Iadd.20. The method of claim 19, wherein the two or more signals
further comprise either a lamp current signal or a lamp voltage
signal..Iaddend.
.Iadd.21. The method of claim 20 further comprising: controlling
the frequency of the AC signal based on either the lamp current
signal or the lamp voltage signal..Iaddend.
.Iadd.22. The method of claim 19, wherein controlling a frequency
of the AC signal based on the control signal comprises: sweeping a
frequency of the AC signal to a strike frequency when the control
signal indicates the phase relationship between the drive voltage
signal and the drive current signal is not substantially in phase;
and changing from the strike frequency to a normal operating
frequency when the control signal indicates the phase relationship
between the drive voltage signal and the drive current signal is
substantially in phase..Iaddend.
.Iadd.23. A method comprising: receiving a DC voltage; generating
one or more switching signals; generating an AC signal from the DC
voltage based on the switching signals; monitoring two or more
signals, wherein the two or more signals comprise at least a drive
voltage signal and a drive current signal; determining a phase
relationship between the drive voltage signal and the drive current
signal to generate a control signal; and sweeping a frequency of
the AC signal based on the control signal..Iaddend.
.Iadd.24. The method of claim 23, wherein determining a phase
relationship between the drive voltage and the drive current signal
to generate the control signal comprises: determining a phase
relationship between the drive voltage and a zero crossing of the
drive current signal to generate a control signal..Iaddend.
.Iadd.25. The method of claim 23, wherein sweeping a frequency of
the AC signal comprises: increasing the frequency to a strike
frequency when the control signal indicates the phase relationship
is not substantially in phase..Iaddend.
.Iadd.26. The method of claim 25, further comprising: changing the
frequency from the strike frequency to a normal operating frequency
when the control signal indicates the phase relationship is
substantially in phase..Iaddend.
.Iadd.27. An inverter circuit for driving a lamp, the circuit
comprising: one or more switches to controllably switch an input
voltage ON and OFF to generate an AC signal; and a control circuit
configured to: monitor a drive voltage signal and a drive current
signal, determine a phase relationship between the drive voltage
signal and the drive current signal to generate a control signal,
and control a frequency of the AC signal based on the control
signal..Iaddend.
.Iadd.28. The inverter circuit of claim 27, wherein the control
circuit is further configured to determine a phase relationship
between the drive voltage signal and a zero crossing of the drive
current signal to generate the control signal..Iaddend.
.Iadd.29. The inverter circuit of claim 27, wherein the control
circuit is further configured to increase the frequency of the AC
signal to a strike frequency when the control signal indicates the
phase relationship is not substantially in phase..Iaddend.
.Iadd.30. The inverter circuit of claim 29, wherein the control
circuit is further configured to change the frequency from the
strike frequency to a normal operating frequency when the control
signal indicates the phase relationship is substantially in
phase..Iaddend.
.Iadd.31. An integrated inverter controller comprising: a first
circuit configured to monitor at least two or more signals, wherein
the two or more signals comprise at least a drive voltage signal
and a drive current signal; a second circuit configured to
determine a phase relationship between the drive voltage signal and
the drive current signal to generate a control signal; a third
circuit configured to control the frequency of an AC signal based
on the control signal..Iaddend.
.Iadd.32. The integrated inverter controller of claim 31, wherein
the third circuit is further configured to: sweep the frequency of
the AC signal when the control signal indicates the phrase
relationship is not substantially in phase..Iaddend.
.Iadd.33. The integrated inverter controller of claim 31, wherein
the AC signal is externally generated by controllably switching a
DC input voltage ON and OFF..Iaddend.
.Iadd.34. The integrated inverter controller of claim 31, wherein
the third circuit is further configured to increase the frequency
of the AC signal to a strike frequency when the control signal
indicates the phase relationship is not substantially in
phase..Iaddend.
.Iadd.35. The integrated inverter controller of claim 34, wherein
the third circuit is further configured to change the frequency
from the strike frequency to a normal operating frequency when the
control signal indicates the phase relationship is substantially in
phase..Iaddend.
.Iadd.36. The integrated inverter controller of claim 31, wherein
the two or signals further comprise either a lamp voltage signal or
a lamp current signal..Iaddend.
.Iadd.37. An inverter controller for driving a lamp, the controller
comprising: a first circuit configured to monitor at least two or
more signals, wherein the two or more signals comprise at least a
drive voltage signal and a drive current signal; a second circuit
configured to determine a phase relationship between the drive
voltage signal and the drive current signal to generate a control
signal; a third circuit configured to control the frequency of an
AC signal based on the control signal..Iaddend.
.Iadd.38. The inverter controller of claim 37, wherein the third
circuit is further configured to sweep the frequency of the AC
signal when the control signal indicates the phase relationship is
not substantially in phase..Iaddend.
.Iadd.39. The inverter controller of claim 37, wherein the AC
signal is externally generated by controllably switching a DC input
voltage ON and OFF..Iaddend.
.Iadd.40. The inverter controller of claim 37, wherein the third
circuit is further configured to increase the frequency of the AC
signal to a strike frequency when the control signal indicates the
phase relationship is not substantially in phase..Iaddend.
.Iadd.41. The inverter controller of claim 40, wherein the third
circuit is further configured to change the frequency from the
strike frequency to a normal operating frequency when the control
signal indicates the phase relationship is substantially in
phase..Iaddend.
.Iadd.42. The inverter controller of claim 41, wherein the two or
signals further comprise either a lamp voltage signal or a lamp
current signal..Iaddend.
Description
FIELD OF THE INVENTION
The present invention relates to discharge lighting and, in
particular, to efficiently supplying electrical power for igniting
of a discharge lamp by sweeping to a strike frequency based on the
phase relationship between the current and the voltage in the
load.
BACKGROUND OF THE INVENTION
A discharge lamp, such as a cold cathode fluorescent lamp (CCFL),
has terminal voltage characteristics that vary depending upon the
immediate history and the frequency of a stimulus (AC signal)
applied to the lamp. Until the CCFL is "struck" or ignited, the
lamp will not conduct a current with an applied terminal voltage
that is less than the strike voltage. Once an electrical arc is
struck inside the CCFL, the terminal voltage may fall to a run
voltage that is approximately 1/3 of the strike voltage over a
relatively wide range of input currents. When the CCFL is driven by
an AC signal at a relatively high frequency, the CCFL (once struck)
will not extinguish on each cycle and will exhibit a positive
resistance terminal characteristic. Since the CCFL efficiency
improves at relatively higher frequencies, the CCFL is usually
driven by AC signals having frequencies that range from 50
Kilohertz to 100 Kilohertz.
Driving a CCFL with a relatively high frequency square-shaped AC
signal will produce the maximum useful lifetime for the lamp.
However, since the square shape of an AC signal may cause
significant interference with other circuits in the vicinity of the
circuitry driving the CCFL, the lamp is typically driven with an AC
signal that has a less than optimal shape such as a sine-shaped AC
signal.
Most small CCFLs are used in battery powered systems, e.g.,
notebook computers and personal digital assistants. The system
battery supplies a direct current (DC) voltage ranging from 7 to 20
Volts with a nominal value of about 12V to an input of a DC to AC
inverter. A common technique for converting a relatively low DC
input voltage to a higher AC output voltage is to chop up the DC
input signal with power switches, filter out the harmonic signals
produced by the chopping, and output a relatively clean sine-shaped
AC signal. The voltage of the AC signal is stepped up with a
transformer to a relatively high voltage, e.g., from 12 to 1500
Volts. The power switches may be bipolar junction transistors (BJT)
or field effect transistors (MOSFET). Also, the transistors may be
discrete or integrated into the same package as the control
circuitry for the DC to AC converter.
In some prior art inverters, the inverter is a fixed frequency
inverter that sweeps to the strike frequency based on sensing the
current from the lamp. However, this approach may not be able to
generate a high enough voltage to ignite a lamp. Alternatively,
this approach may not be effective in mass produced devices or may
miss resonance.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same becomes
better understood by reference to the following detailed
description, when taken in conjunction to with the accompanying
drawings, wherein:
FIG. 1 is a schematic illustration of a tank circuit used for
driving a cold cathode fluorescent lamp (CCFL).
FIG. 2 is an equivalent circuit of the tank circuit of FIG. 1.
FIG. 3 shows the steady state response curve of the tank circuit as
a function of frequency for a loaded and unloaded condition.
FIG. 4 is a prior art circuit used to modify the operating
frequency based upon the magnitude of the CCFL current.
FIGS. 5A-5C are waveforms illustrating the principles of the
present invention.
FIG. 6 is a voltage-controlled oscillator control logic used to
control the operating resonant frequency of the present
invention.
FIG. 7 shows a full bridge output stage that may be used in the
present invention.
FIG. 8 shows a half bridge output stage that may be used in the
present invention.
FIG. 9 shows a push-pull output stage that may be used in the
present invention.
FIG. 10 shows a circuit that uses multiple feedback paths to
independently optimize resonant frequency control and lamp current
and voltage control.
DETAILED DESCRIPTION OF THE INVENTION
As noted above, inverters for driving a CCFL typically comprise a
DC to AC converter, a filter circuit, and a transformer. Examples
of such circuits are shown in U.S. Pat. No. 6,114,814 to Shannon et
al., assigned to the assignee of the present invention and herein
incorporated by reference in its entirety. In addition, other prior
art inverter circuits, such as a constant frequency half-bridge
(CFHB) circuit or a inductive-mode half-bridge (IMHB) circuit, may
be used to drive a CCFL. The present invention may be used in
conjunction with any of these inverter circuits, as well as other
inverter circuits. The disclosure herein teaches a method and
apparatus for striking and supplying electrical power to a
discharge lamp, such as a cold cathode fluorescent lamp (CCFL).
According to the present invention, the inverter will "sweep" to
the strike frequency. Thus, a "fixed frequency" CCFL inverter that
sweeps to strike frequency based on the phase relationship between
the current and the voltage in the load is next described. The
decision to sweep is independent of the feedback parameters from
the lamp.
FIG. 1 shows a typical tank circuit that is used to drive a CCFL
load. The tank circuit includes a driving voltage generator, such
as a full bridge inverter, that can drive the primary of a
transformer through a primary coupling capacitor C.sub.p. The CCFL
lamp is connected across the terminals of the secondary of the
transformer (also referred to as the transformer secondary). Also
connected across the terminals of the transformer secondary is a
capacitive voltage divider and a parasitic and/or stray capacitance
included in C.sub.s. The circuit of FIG. 1 can be simplified into
the equivalent circuit shown in FIG. 2 during operation in the
frequency range of interest. The primary coupling capacitor C.sub.p
and the transformer leakage inductance L.sub.lk, in practice,
determine the resonant frequency of the tank after the lamp has
been struck. The lamp is represented as a resistance
R.sub.lamp.
Note that the transformer's magnetizing inductance is typically
greater than ten times the leakage inductance in a well-designed,
ungapped transformer. Therefore, the current through the
magnetizing inductance (not shown) can be neglected to the first
order. Further, after the lamp has been struck, the equivalent
resistance of the lamp is typically one-third of the reactance of
C.sub.s, so that most of the secondary current flows through the
lamp (R.sub.lamp) and not through C.sub.s. Note that both the lamp
resistance and the secondary capacitance are shown transformed to
the primary in FIG. 2.
Turning to FIG. 3, the response of the tank circuit with the lamp
conducting is shown as the lower curve 301. If the lamp is not
conducting (either because it has not yet been struck or because it
has been broken), there is practically no load on the tank circuit
and the response is approximately represented by the upper curve
303. The parameter A is used generically in FIG. 3 to represent the
magnitude of the response of the tank circuit.
Notice that the unloaded resonant frequency (where the curve hits
its peak) of the tank circuit is higher than the loaded resonant
frequency because all of the secondary current flows through
C.sub.s when the lamp is not conducting. The equivalent tuning
capacitance is the series combination of C.sub.p and C.sub.s.
From the lower curve 301, the operating frequency of an inverter
should be tuned to point A in FIG. 3 for the highest efficiency
after the lamp has been struck. Unfortunately, it is often not
possible to generate enough voltage across the secondary of the
transformer at this same operating frequency A (same as point B in
FIG. 3) to guarantee that the lamp will strike. Therefore, it is
necessary to increase the frequency at which the lamp is struck in
order to guarantee an adequate strike voltage across the lamp to
ignite the lamp. Thus, there are two problems that must be
addressed. First, the unloaded resonant frequency of the tank
circuit must be found in order to strike the lamp. Second, and
relatedly, the control circuitry must be able to determine when to
search for the strike frequency.
In the prior art, the decision to change the operating frequency
was based upon the magnitude of the lamp current. As shown in FIG.
4, a comparator is used to decide whether the lamp current is less
than or greater than a preset threshold. If the lamp current is
less than the threshold, the signal from the output of the
comparator causes the control circuitry to raise the operating
frequency according to some predetermined strategy in an attempt to
strike the lamp. However, there are several problems with this
approach that may complicate the strategy to strike the lamp and
make the startup sequence of the lamp awkward.
For example, if the threshold of the comparator is set too high, it
may not be possible to use analog dimming of the lamp. In this
case, a lamp current that is less than the threshold would cause
the control circuitry to decide that the lamp had extinguished or
broken and it would try to correct accordingly even though no fault
had occurred. Another pitfall of a high comparator threshold is
that the power available at the strike frequency may not be
sufficient to raise the lamp current above the threshold. This
could hang the control circuitry in a state where it continues to
try to strike the lamp at the strike frequency even though the lamp
is already conducting. Thus, the control strategy would have to
account for these possibilities and somehow circumvent these
pitfalls.
In the alternative, if the threshold of the comparator is set too
low, this may trigger falsely. For example, this may happen because
the lamp and its wiring have a small amount of stray capacitive
coupling between the high and low ends of the lamp. If the current
through the stray capacitance is high enough to cross the low
comparative threshold, the control circuitry would be fooled into
thinking the lamp had already struck and would try to switch to run
mode even though the lamp was not conducting. In such a situation,
it would be difficult to strike the lamp.
With respect to finding the unloaded resonant frequency, the prior
art approaches suggests measuring the unloaded resonant frequency
and then tuning the open lamp operating frequency accordingly using
an auxiliary resistor. Other approaches use a scanning technique
that seems to adapt to normal component variations across the
production spread.
Independent Frequency and Loop Control
In accordance with the present invention, the inverter operating
frequency is controlled independently from the regulation loops. In
particular, the operating frequency is determined by a fixed
frequency oscillator for normal operation after the lamp has
ignited. Alternatively, the operating frequency can be locked to an
external synchronization clock during normal operation. However,
when the lamp is not conducting (either because it is broken or
because it has not yet ignited), the operating frequency is swept
higher in order to ensure adequate voltage at the output of the
inverter module to strike the lamp.
In accordance with the present invention, the inverter operating
frequency "tries" to run at a predetermined fixed frequency.
However, if it is determined that the output current and voltage
are out of phase by more than a threshold magnitude, then the
"fixed" frequency control is overridden and the operating frequency
is adjusted to bring the current and voltage substantially into
phase. The idea of keeping the voltage and current in phase is
taught in our U.S. Pat. No. 6,114,814 in the context of optimizing
switch efficiency. However, it has been found in the present
invention that maintaining the correct phase relationship may also
be used for generating enough voltage to strike the lamp.
Hardware Implementation
When the driving inverter is operating normally at point A of FIG.
3, the current across the primary winding of the transformer and
the driving voltage have the relationship shown in FIG. 5A. Note
that the waveforms in FIGS. 5A-5C assume that the driver is a
pulse-width-modulated (PWM) full bridge. Nevertheless, the idea
shown can be implemented with a PWM half bridge or a push-pull
output stage as well. As seen in FIG. 5A, the voltage and current
are substantially in phase. This is the criterion for setting the
"fixed" operating frequency while driving a full load (the lamp is
at maximum brightness).
Now consider what happens if the inverter continues to operate at
the fixed frequency with a non-conducting lamp. This corresponds to
point B in FIG. 3. This results in the waveforms shown in FIG. 5B.
Because the operating point is significantly lower than the
resonant frequency of the tank circuit, the load (lamp) at the
driver appears to be capacitive and the current across the primary
winding of the transformer leads the driving voltage. In this
condition, it may not be possible to generate the specified strike
voltage given the variations in inductor and capacitor Q. Note that
in FIG. 5B, the loop has increased the pulse width of the output
wave form in an attempt to force current through the lamp.
In order to guarantee a sufficient strike voltage, it is necessary
to raise the operating point (i.e., frequency) to near the open
lamp (unloaded) resonant frequency of the tank circuit. In other
words, it is preferable to move the operating point to near point C
of FIG. 3.
The waveforms for the operating point C are shown in FIG. 5C. The
criterion for this case is that the current through the primary
winding and the driving voltage are once again substantially in
phase. The technique for ensuring this is to drive the frequency
higher until the trailing edge of the voltage wave form is
substantially synchronous with the falling zero crossing of the
current in the primary winding. Note that the lamp voltage
regulator has narrowed the output pulse width because very little
power is required to maintain strike voltage across the lamp when
the driving voltage and the current across the primary winding are
both in phase.
There are several advantages to using the technique of maintaining
the voltage and current in phase instead of switching modes when
the feedback lamp current falls below a particular threshold as
taught in the prior art. First, the unloaded resonant frequency of
the tank circuit can be easily found and the strike frequency is
close enough to resonance to ensure plenty of open-lamp voltage.
Because the trailing edge of the driving voltage and the falling
zero crossing of the current across the primary winding are
essentially coincidental, the frequency is constrained to the
capacitive side (low side) of the resonant peak and can not hop
over the peak of the upper curve 303 and run away on the high
side.
Another benefit is that, as soon as the lamp starts to dissipate
power, the response curve of the tank circuit starts to change. The
resonant peak starts moving down in frequency. In other words, the
upper curve 303 slowly morphs into the lower curve 301 as you move
from the unloaded condition to the loaded condition. Since the
frequency controller tries to keep the operation on the capacitive
side of resonance, the operating frequency starts sliding lower
even before there is noticeable current in the lamp. Thus, the
operating frequency remains nearly optimal throughout the start-up
transient and moves towards the "fixed" operating frequency as
early as possible. In other words, there is no need to detect the
lamp current before leaving open lamp mode and approaching steady
state run mode.
Variations on Independent Loop and Frequency Control
The phase of the output stage current may be measured at different
points. In some embodiments, the voltage phase is determined by the
output switch timing. The current may be measured in the output
transistors as taught in U.S. Pat. No. 6,114,814. Alternatively, in
the case where the output topology is a half-bridge, the voltage
phase may be determined by the output switch timing and the current
may be measured at the cold end of the transformer primary. Still
alternatively, in the case where the output topology is a push-pull
circuit driving a center-tapped transformer, the current may be
measured across the on-resistance of the power switches.
The Solution
In general, the operating frequency is generated by a
voltage-controlled oscillator (VCO). Alternatively, the operating
frequency may be current controlled. Thus, the abbreviation VCO/ICO
is used herein to identify both of these posibilities. The control
input of the VCO/ICO is normally driven all the way to the low
frequency of its control range or the VCO/ICO is synchronized to an
external reference clock. This is the normal frequency after the
lamp has been struck. The frequency is swept up higher when the
falling zero crossing of the current flowing through the primary
winding occurs in the second half of the driving voltage pulse.
Small errors in setting the normal open frequency can be tolerated
by the system because the loaded Q can be very low (Q=1), which
means the phase difference between voltage and current changes very
slowly with frequency.
If the lamp has not ignited (or has extinguished or has been
broken), operating at the normal frequency in a system adjusted as
described above will cause the phase of the current waveform to
lead the voltage significantly (capacitive load). This is evidence
that the operating frequency is far removed from the resonant
frequency of the tank. Depending on the quality of the components
comprising the tank, it may not be possible to obtain adequate
voltage on the secondary to guarantee that the lamp would
strike.
According to the present invention, a simple Boolean expression
that compares the phase lag of the output voltage with the
zero-crossing of the output current provides an error correction
signal to the control node of the VCO/ICO. The VCO/ICO can then be
"swept up" in frequency until the voltage and current are once more
substantially in phase. In this manner, there is sufficient gain in
the tank to ensure striking the lamp. Once the lamp strikes, the
output voltage no longer lags the output current and the VCO/ICO
sweeps down to its normal operating frequency.
One example of the control logic for a VCO and pulsed current
source are shown in FIG. 6. As seen, the pulsed current source C1
drives the VCO control node and is much larger in magnitude
(typically greater than ten times) than the weak current sink C2.
The ratio of the magnitudes of the current sink and the pulsed
current source determines the phase error allowed by the frequency
control loop. If it is desired to lock the operating frequency to
an external clock, then the weak current sink in FIG. 6 would
represent the maximum current available from the phase locked loop
phase comparator block. The circuit of FIG. 6 includes Boolean
logic that operates as a phase comparator.
The zero-crossing detector for the current flowing in the primary
winding can be configured in many different ways for the various
driver-staged topologies. For example, in the case of a full bridge
output stage, as seen in FIG. 7, the primary current can be sensed
across the R.sub.dson of the switches in the bridge. The R.sub.dson
in this example is measured across switches 2 and 4 of FIG. 7 to
sense the primary current. Alternatively, in the case of a half
bridge, output stage that the current in the primary winding can be
sensed in the return leg of the primary winding as seen in FIG. 8
across R.sub.psense. Finally, with appropriate blanking as seen in
FIG. 9, the primary current can be sensed across the R.sub.dson of
the switches in a push-pull output stage. The R.sub.dson in the
example of FIG. 9 is measured across switches 1 and 2 to sense the
primary current.
By separating the functions of frequency control and lamp current
and voltage control, both strategies can be optimized
independently. For example, the circuit of FIG. 10 shows multiple
feedback paths through a common pulse-width modulator that controls
lamp current, open lamp voltage, and secondary current. Because all
three loops use the same compensation node and modulator, the
system moves smoothly from one mode to another without annoying
glitches and flashes that can occur when a loop is broken and the
compensation node for one parameter drifts off to an extreme of its
control range.
If it is desired to synchronize the operating frequency with an
external reference clock, the VCO control node can be driven with
the output of a phase comparator. Under normal operating conditions
with the lamp ignited, the oscillator would run near the low end of
its control range. To ignite the lamp, the same logic described
above overwhelms the output of the phase comparator and drives the
operating frequency up to the resonant frequency of the unloaded
tank.
As will be seen in further detail below, the lamp current, lamp
voltage, and secondary current are maintained by closed loops
independent of the operating frequency.
Configuration of Multiple Feedback Paths
It is typical in a CCFL inverter that other feedback paths are
present for various reasons. In one embodiment, the multiple
feedback paths converge on the same point to control various
physical parameters in the system.
For example, one important feedback parameter is lamp current or
lamp power. This is an important feedback path because it
determines what the lamp looks like to the user and it can affect
the lifetime of the lamp.
Minor feedback parameters monitor fault conditions such as
open/broken lamp (maximum lamp voltage) and secondary overcurrent
(shorted output). These loops are less critical than the main loop
because, by definition, the lamp is not making light.
In one embodiment, all of these various feedback paths converge at
the compensation (Comp) node. The advantage to this is that the
voltage at the Comp node is maintained in its active region and the
hand-off between the various control loops is smooth and
well-behaved. Note that, if one or more of the loops did not use
the common Comp node, then the Comp voltage is likely to wander off
to some arbitrary voltage while a minor feedback path is in
control. This would result in the feedback parameter that uses the
Comp node to possibly be in error when control returns to it
abruptly.
Variations on Multiple Feedback Paths
The multiple feedback path concept may be expanded to any
combination of several feedback parameters and ways of combining
them in any particular controller. The main feedback parameter can
be either lamp current sensed in a resistor or output power
computed and averaged as taught in our U.S. Pat. No. 6,114,814.
Minor feedback parameters usually include lamp voltage (either
balanced or unbalanced) in combination with some scheme of sensing
module output current. Note that the output current does not
necessarily return to the lamp current sense resistor--it may
dangerously pass from the high voltage side of the transformer
secondary through an unfortunate person and directly to ground.
Therefore, it is necessary to find a way to measure module output
current that is independent of sensing the lamp current.
In one embodiment, the current may be sensed in the transformer
secondary current. In other implementations, the current is sensed
in the transformer primary, measuring it in the output power
switches. The current in the secondary can be inferred from the
current in the primary. The short circuit current in the secondary
is very nearly the current in the primary divided by the turns
ratio.
Other parameters may be measured and fed back through the Comp
node. For example, light output from the lamp could be measured
with a photodiode and this parameter could "dither" the lamp
current or power to guarantee uniform light across the production
spread of panels, lamps, and modules.
The lamp current may be sensed using a full-wave sense amplifier as
described in our co-pending U.S. patent application Ser. No.
10/354,541 entitled "FULL WAVE SENSE AMPLIFIER AND DISCHARGE LAMP
INVERTER INCORPORATING THE SAME" filed Jan. 29, 2003 which is
hereby incorporated by reference in its entirety. Further, the
amplifiers and comparators at the Comp node may also use a
controlled-offset technique as described in our co-pending U.S.
patent application Ser. No. 10/656,087 entitled "CONTROLLED OFFSET
AMPLIFIER" filed Sep. 5, 2003 which is hereby incorporated by
reference in its entirety.
While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
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