Title: Shorted lamp detection in backlight system
Abstract: A power conversion circuit senses an output voltage to detect shorted lamp conditions in a backlight system. The power conversion circuit can drive at least one fluorescent lamp. A voltage sensing feedback circuit, such as a capacitive voltage divider or a resistive voltage divider, senses the output voltage at an output of the power conversion circuit and generates a voltage feedback signal for a shorted lamp detector. The shorted lamp detector reliably detects a shorted lamp condition of one fluorescent lamp in a multi-lamp configuration or detects a short circuit condition of the output voltage line coupling the output voltage of the power conversion circuit to the fluorescent lamps.
Patent Number: 6,870,330 Issued on 03/22/2005 to Choi
| Inventors:
|
Choi; Hwangsoo (Fullerton, CA)
|
| Assignee:
|
Microsemi Corporation (Irvine, CA)
|
| Appl. No.:
|
400326 |
| Filed:
|
March 26, 2003 |
| Current U.S. Class: |
315/307; 315/291; 315/224 |
| Intern'l Class: |
G05F 001//00 |
| Field of Search: |
315/307,291,297,274,276,279,282,224,219
|
References Cited [Referenced By]
U.S. Patent Documents
| 4745339 | May., 1988 | Izawa et al.
| |
| 5420779 | May., 1995 | Payne.
| |
| 5485059 | Jan., 1996 | Yamashita et al. | 315/307.
|
| 5629588 | May., 1997 | Oda et al.
| |
| 5635799 | Jun., 1997 | Hesterman.
| |
| 5663613 | Sep., 1997 | Yamashita et al.
| |
| 5710489 | Jan., 1998 | Nilssen.
| |
| 5751115 | May., 1998 | Jayaraman et al.
| |
| 5751120 | May., 1998 | Zeitler et al.
| |
| 5777439 | Jul., 1998 | Hua.
| |
| 5808422 | Sep., 1998 | Venkitasubrahmanian et al.
| |
| 5883473 | Mar., 1999 | Li et al.
| |
| 5910709 | Jun., 1999 | Stevanovic et al.
| |
| 5930126 | Jul., 1999 | Griffin et al.
| |
| 6211625 | Apr., 2001 | Nilssen.
| |
| 6259615 | Jul., 2001 | Lin.
| |
| 6331755 | Dec., 2001 | Ribarich et al.
| |
| 6340870 | Jan., 2002 | Yamashita et al.
| |
| 6710555 | Mar., 2004 | Terada et al. | 315/291.
|
| 2004/0051473 | Mar., 2004 | Jales et al. | 315/276.
|
Primary Examiner: Vo; Tuyet Thi
Attorney, Agent or Firm: Knobbe, Martens, Olson & Bear, LLP.
Claims
What is claimed is:
1. A power conversion circuit with shorted lamp detection for driving at
least one fluorescent lamp, the circuit comprising:
an inverter configured to receive a substantially direct current input
voltage and to generate an alternating current lamp voltage to drive the
fluorescent lamps; and
a shorted lamp detector configured to monitor the alternating current lamp
voltage and to generate a feedback voltage with an amplitude proportional
to the amplitude of the lame voltage, wherein the shorted lamp detector
produces periodic pulses if the amplitude of the feedback voltage is above
a predefined threshold to indicate normal operations and produces a
substantially direct current voltage if the amplitude of the feedback
voltage is below the predefined threshold to detect a shorted lamp
condition.
2. The power conversion circuit of claim 1, wherein the inverter comprises:
a primary network configured to receive the substantially direct current
input voltage;
a controller configured to output driving signals to the primary network to
generate an alternating current signal in the primary network; and
a secondary network coupled to the primary network and configured to output
the alternating current lamp voltage.
3. The power conversion circuit of claim 2, wherein the controller is
disabled when the shorted lamp condition lasts longer than a predetermined
duration.
4. A power conversion circuit with shorted lame detection for driving at
least one fluorescent lamp, the circuit comprising:
an inverter configured to receive a substantially direct current input
voltage and to generate an alternating current lamp voltage to drive the
fluorescent lamps, wherein the inverter comprises:
a primary network configured to receive the substantially direct current
input voltage;
a controller configured to output driving signals to the primary network to
generate an alternating current signal in the primary network; and
a secondary network coupled to the primary network and configured to output
the alternating current lamb voltage;
a shorted lamp detector configured to monitor the alternating current lamp
voltage to detect a shorted lamp condition; and
a voltage sensing feedback circuit coupled to the output of the secondary
network to sense the alternating current lamp voltage and to generate a
voltage feedback signal with an amplitude proportional to the amplitude of
the alternating current lamp voltage for the shorted lamp detector.
5. The power conversion circuit of claim 4, wherein the voltage sensing
feedback circuit is a capacitive voltage divider.
6. The power conversion-circuit of claim 4, wherein the voltage sensing
feedback circuit is a resistive voltage divider.
7. The power conversion circuit of claim 4, wherein the shorted lamp
detector comprises:
an open collector comparator coupled to the output of the voltage sensing
feedback circuit;
a holding capacitor coupled to the output of the open collector comparator;
a pull-up resistor coupled to the output of the open collector comparator;
and
a reference comparator coupled to the output of the open collector
comparator.
8. The power conversion circuit of claim 4, wherein the shorted lamp
detector comprises:
a high voltage detector coupled to the output of the voltage sensing
feedback circuit;
a conditioning circuit coupled to the output of the high voltage detector;
and
a threshold detector coupled to the output of the conditioning circuit.
9. The power conversion circuit of claim 8, wherein the high voltage
detector is a single transistor amplifier.
10. The power conversion circuit of claim 8, wherein the conditioning
circuit comprises:
a half-wave rectifier;
a timing resistor; and
a charging capacitor.
11. The power conversion circuit of claim 8, wherein the threshold detector
is a comparator.
12. A method for detecting a shorted lamp condition in a backlight system,
the method comprising the acts of:
sensing a lamp voltage provided by an inverter to drive at least one
fluorescent lamp in the backlight system;
generating a feedback voltage with an amplitude proportional to the
amplitude of the lamp voltage;
generating periodic pulses if the amplitude of the feedback voltage is
above a predefined threshold indicative of normal operations; and
generating a substantially direct current voltage if the amplitude of the
feedback voltage is below the predefined threshold indicative of the
shorted lamp condition.
13. The method of claim 12 further comprising the act of disabling the
inverter when the shorted lamp condition lasts longer than a predetermined
duration.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power conversion circuit for driving
fluorescent lamps in a backlight system, and more particularly relates to
a lamp inverter for improved detection of a shorted lamp condition in the
backlight system.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is
required but the power required to generate the light is limited. One
particular type of fluorescent lamp is a cold cathode fluorescent lamp
(CCFL). CCFLs are used for back lighting or edge lighting of liquid
crystal displays (LCDs). LCDs are typically used in notebook computers,
web browsers, automotive and industrial instrumentations, and
entertainment systems. Each LCD typically uses multiple CCFLs.
CCFL tubes typically contain a gas, such as argon, xenon, or the like,
along with a small amount of mercury. After an initial ignition stage and
the formation of plasma, current flows through the tube. The current
causes the generation of ultraviolet light. The ultraviolet light strikes
a phosphorescent material that coats the inner wall of the tube to cause
the phosphorescent material to emit visible light.
A power conversion circuit (e.g., an inverter) is generally used for
driving one or more CCFLs. The power conversion circuit accepts a direct
current (DC) input voltage and provides an alternating current (AC) output
voltage to the CCFLs. The brightness (or the light intensity) of the CCFLs
is controlled by controlling the current (i.e., the lamp current) through
the CCFLs. For example, the CCFLs can be dimmed or brightened by
decreasing or increasing the average lamp current.
CCFLs are susceptible to defects or damage, which can cause short circuit
conditions that may damage the power conversion circuit. The power
conversion circuit is typically difficult and expensive to replace after
installation. Thus, shorted lamp protection is generally provided to
protect the power conversion circuit during a shorted lamp condition. The
impedance of an operable CCFL is typically between 80 kilohms and 100
kilohms. The shorted lamp condition occurs when the impedance across the
CCFL is significantly lower (e.g., less than 2 kilohms). This shorted lamp
condition is typically detected by sensing the lamp current. For example,
a sensing transformer or a sensing resistor can be coupled in series with
the CCFLs to sense the lamp current and to provide a feedback signal to
the power conversion circuit. The power conversion circuit may shut down
when the average lamp current becomes excessive, which indicates a shorted
lamp condition.
One problem with sensing the lamp current to detect the shorted lamp
condition is that some shorted lamp conditions may not be reliably
detected, especially when the power conversion circuit drives multiple
CCFLs. For example, the lamp current may only increase 20%-30% when one
CCFL is shorted in a multiple CCFL configuration. The 20%-30% increase may
be within the range of operating lamp currents for increasing the
intensity of the CCFLs and may not trigger the shorted lamp protection.
Furthermore, the sensing transformer used in some applications has a
current limit which can impede the detection of the shorted lamp
condition. In addition, lamp current sensing does not sense a short
circuit condition at the output of the power conversion circuit, which can
be caused by improper installation of the power conversion circuit or the
CCFLs.
SUMMARY OF THE INVENTION
One aspect of embodiments in accordance with the present invention is a
backlight system that senses an output voltage (or a lamp voltage) to
detect a shorted lamp condition. The backlight system senses a decrease in
the output voltage resulting from the shorted lamp condition. The
backlight system reliably detects a short circuit condition of one lamp in
a multi-lamp parallel configuration.
In one embodiment, the power conversion circuit includes a controller, a
primary network, a secondary network, a voltage sensing feedback circuit,
and a shorted lamp detector. Input power is provided to the controller and
to the primary network. The controller provides driving signals to the
primary network. The secondary network is coupled to the primary network
and produces the output voltage to drive the CCFL. The voltage sensing
feedback circuit is coupled to the secondary network to sense the output
voltage and to generate a voltage feedback signal for the shorted lamp
detector. The shorted lamp detector outputs a disable signal to the
controller to shut down the power conversion circuit when the shorted lamp
condition is detected.
In one embodiment, the voltage sensing feedback circuit uses a voltage
divider (e.g., a capacitive voltage divider or a resistive voltage
divider) to generate the voltage feedback signal. During normal
operations, the output voltage is an AC signal with a typical lamp voltage
amplitude (e.g., a root-mean-square (rms) value in the range of 1-2
kilovolts) and a typical lamp operating frequency (e.g., 30-100
kilohertz). The voltage divider reduces the amplitude of the output
voltage proportionately to a detectable level. For example, the element
values of the voltage divider can be chosen such that the amplitude of the
voltage feedback signal is one-thousandth of the amplitude of the output
voltage. Thus, the rms amplitude of the voltage feedback signal is
approximately one-thousandth of the output voltage (e.g., in the range of
1-2 volts) during normal operations. During the shorted lamp condition,
the amplitude of the output voltage is relatively low or close to zero.
Correspondingly, the amplitude of the voltage feedback signal is close to
zero during the shorted lamp condition.
In one embodiment, the shorted lamp detector includes a high voltage
detector, a conditioning circuit, and a threshold detector. The voltage
feedback signal is provided to the high voltage detector. The high voltage
detector outputs periodic pulses during normal operations. For example,
the voltage feedback signal is an AC signal with sufficient amplitude
(e.g., greater than 0.7 volts) to cause the high voltage detector to
generate periodic pulses of the same frequency and fixed amplitude during
normal operations. During the shorted lamp condition, the amplitude of the
voltage feedback signal is close to zero and is insufficient to cause the
high voltage detector to generate periodic pulses. Thus, the high voltage
detector outputs substantially zero volt during the shorted lamp
condition.
The output of the high voltage detector is coupled to the conditioning
circuit. The conditioning circuit outputs a substantially DC voltage of a
first level when periodic pulses are present at the high voltage detector
output. The output of the conditioning circuit transitions to a
substantially DC voltage of a second level when the periodic pulses stop.
The output of the conditioning circuit is coupled to the threshold
detector. The threshold detector compares the output of the conditioning
circuit with a predefined reference voltage to detect shorted lamp
conditions. The threshold circuit outputs a signal to disable the power
conversion circuit when a shorted lamp condition is detected. In one
embodiment, the output of the threshold detector is coupled to the
controller of the power conversion circuit.
In one embodiment, an intermittent shorted lamp condition does not affect
the operation of the power conversion circuit. The power conversion
circuit may not be harmed by intermittent shorted lamp conditions that
last less than a predetermined duration (e.g., one second). Thus, the
power conversion circuit is not disabled as a result of the intermittent
shorted lamp condition.
During the intermittent shorted lamp condition, the periodic pulses at the
output of the high voltage detector are absent for less than the
predetermined duration. The rate at which the output voltage of the
conditioning circuit transitions from the first level to the second level
is controlled so that the absence of periodic pulses for less than the
predetermined duration does not trigger the threshold detector to output a
disable signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a power conversion circuit according to one
embodiment of the present invention.
FIG. 2 is a schematic diagram of one embodiment of the power conversion
circuit shown in FIG. 1.
FIG. 3 is a schematic diagram of one embodiment of a shorted lamp detector
shown in FIG. 2.
FIG. 4 illustrates timing diagrams that show the waveforms of various
signals in the shorted lamp detector of FIG. 3.
FIG. 5 is a schematic of an alternative embodiment of the shorted lamp
detector.
FIG. 6 illustrates an application of the shorted lamp detector in a power
conversion circuit with floating outputs.
FIG. 7 illustrates an application of the shorted lamp detector in a power
conversion circuit for driving multiple fluorescent lamps.
FIG. 8 illustrates an alternative application of the shorted lamp detector
in a power conversion circuit for driving multiple fluorescent lamps.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments in accordance with aspects of the present invention
will be described hereinafter with reference to the drawings.
FIG. 1 is a block diagram of a power conversion circuit according to one
embodiment of the present invention. The power conversion circuit (or the
lamp inverter) converts a substantially DC input voltage (V-IN) into an AC
output voltage (V-OUT) to drive a CCFL 112 in a backlight system. An AC
current (or a lamp current) flows through the CCFL 112 to provide
illumination in an electronic device 104, such as, for example, a flat
panel display, a personal digital assistant, a palm top computer, a
scanner, a facsimile machine, a copier, or the like.
The power conversion circuit includes a controller 108, a primary network
100, a secondary network 102, a voltage sensing feedback circuit 106 and a
shorted lamp detector 110. The input voltage is provided to the controller
108 and to the primary network 100. The primary network 100 is controlled
by driving signals provided by the controller 108. The secondary network
102 is coupled to the primary network 100 and produces the output voltage
(or the lamp voltage) to drive the CCFL 112. The voltage sensing feedback
circuit 106 is coupled to the secondary network 102 and generates a
voltage feedback signal indicative of the lamp voltage for the shorted
lamp detector 110. The shorted lamp detector 110 outputs a disable signal
(DISABLE) to the controller 108 when a shorted lamp condition is detected.
The output voltage is an AC signal with an effective (e.g., rms) typical
lamp voltage amplitude (e.g., in the range of 1-2 kilovolts) during normal
operations. When a shorted lamp condition occurs, the level of the output
voltage is significantly lower (e.g., less than 100 volts rms). The
voltage sensing feedback circuit 106 senses the output voltage and
provides a voltage feedback signal proportional to the output voltage to
the shorted lamp detector 110. The shorted lamp detector 110 outputs the
disable signal when the output voltage has been significantly lower than
the normal operating level for at least a predetermined duration
indicating a non-intermittent shorted lamp condition.
FIG. 2 is a schematic diagram of one embodiment of the power conversion
circuit shown in FIG. 1. The power conversion circuit includes a direct
drive controller 208 and a direct drive primary network 210. Other types
of controllers and primary networks are possible. The direct drive
controller 208 and the direct drive primary network 210 are provided as
examples. The direct drive primary network 210 is controlled by two
driving signals (A and B) provided by the direct drive controller 208 and
works with the secondary network 102 to provide the output voltage (V-OUT)
to one or more parallel connected CCFLs shown as CCFLs 220(1)-220(n)
(collectively referred to as the CCFLs 220). The voltage sensing feedback
circuit 106 is coupled to the output of the secondary network 102 and in
parallel with the CCFLs 220. The output of the voltage sensing feedback
circuit 106 is provided to the shorted lamp detector 110, which outputs a
disable signal (DISABLE) to the direct drive controller 208 when a shorted
lamp condition is detected.
In one embodiment, the direct drive primary network 210 includes a first
switching transistor 200, a second switching transistor 202, and a primary
winding of a transformer 204. The input voltage is provided to a
center-tap of the primary winding of the transformer 204. The switching
transistors 200, 202 are coupled to respective opposite terminals of the
primary winding of the transformer 204 to alternately switch the
respective terminals to ground. For example, the first switching
transistor 200 is an n-type field-effect transistor (N-FET) with a drain
terminal coupled to a first terminal of the primary winding of the
transformer 204 and with a source terminal coupled to ground. The second
switching transistor 202 is an N-FET with a drain terminal coupled to a
second terminal of the primary winding of the transformer 204 and with a
source terminal coupled to ground. The switching transistors 200, 202 are
controlled by the respective driving signals (A, B), which are coupled to
gate terminals of the respective switching transistors 200, 202.
An AC signal (or a transformer drive signal) on the primary winding results
from alternating conduction by the switching transistors 200, 202. Other
configurations to couple the input voltage and switching transistors to
the transformer 204 are possible to produce the transformer drive signal.
The transformer drive signal is magnetically coupled to a secondary
winding of the transformer 206 in the secondary network 102, which also
includes a DC blocking capacitor 206. A first terminal of the secondary
winding of the transformer 204 is coupled to ground while a second
terminal of the secondary winding is coupled to a first terminal of the DC
blocking capacitor 206. The second terminal of the DC blocking capacitor
206 is coupled to the output of the secondary network 102, which provides
the output voltage (or the lamp voltage) to drive the CCFLs 220.
In one embodiment, the voltage sensing feedback circuit 106 is a voltage
divider. The voltage sensing feedback circuit 106 includes dividing
elements (e.g., resistors or capacitors) 222, 224. The first dividing
element 222 is coupled between the output of the secondary network 102 and
a common node. The second dividing 224 is coupled between the common node
and ground. The voltage at the common node is the voltage feedback signal
(Vfb), which has an amplitude that is proportional to the amplitude of the
output voltage.
During normal operations, the output voltage is a relatively high voltage
(e.g., thousands of volts) AC signal. The voltage divider of the voltage
sensing feedback circuit 106 reduces the amplitude of the output voltage
proportionately to a detectable level. For example, the voltage divider is
designed with an approximate ratio of 1000:1. In one embodiment, the
voltage divider is a capacitive voltage divider with a first capacitor
having a capacitor value of approximately 2.0 picofarads and a second
capacitor having a capacitor value of approximately 2.2 nanofarads to
produce a scaled version of the output voltage. The resulting amplitude of
the voltage feedback signal is approximately one-thousandth the amplitude
of the output voltage (e.g., several volts) and can be processed by
relatively low voltage electronics. During shorted lamp conditions, the
amplitude of the output voltage is a less than a hundred volts.
Correspondingly, the amplitude of the voltage feedback signal is less than
one-hundredth of a volt or close to zero.
In one embodiment, the dividing elements 222, 224 can be discrete
components or can be fabricated on a printed circuit board (PCB). The PCB
can include other components of the power conversion circuit. In one
embodiment, the first dividing element 222 is fabricated on the PCB while
the second capacitor 224 is a discrete component.
In one embodiment, the shorted lamp detector 110 includes a high voltage
detector 216, a conditioning circuit 214 and a threshold detector 212. The
voltage feedback signal is provided to the high voltage detector 216,
which outputs periodic pulses when the amplitude of the voltage feedback
signal is above a voltage threshold indicating normal operations. The high
voltage detector 216 outputs no pulses (or a DC voltage) when the
amplitude of the voltage feedback signal is below a voltage threshold
indicating a shorted lamp condition.
The output (Vls) of the high voltage detector 216 is provided to the
conditioning circuit 214. The conditioning circuit 214 tracks periodic
pulses from the high voltage detector 216. The conditioning circuit 214
outputs a substantially DC voltage at a first level with the presence of
periodic pulses and transitions to a substantially DC output voltage at a
second level with the absence of periodic pulses for more than a
predetermined duration. The absence of periodic pulses for less than the
predetermined duration indicates an intermittent shorted lamp condition
that does not affect operation of the power conversion circuit.
The output voltage (Vph) of the conditioning circuit 214 is provided to the
threshold detector 212. The threshold detector 212 compares the output of
the conditioning circuit 214 with a predefined reference voltage. The
threshold detector 212 outputs a disable signal when the output of the
conditioning circuit 214 crosses the predefined reference voltage that
indicates a non-intermittent shorted lamp condition. In one embodiment,
the threshold detector 212 outputs the disable signal to the direct drive
controller 208.
FIG. 3 is a schematic diagram of one embodiment of the shorted lamp
detector 110 shown in FIG. 2. The high voltage detector 216 detects a high
voltage signal and converts the high voltage signal to pulses. The
conditioning circuit 214 conditions the pulses to a DC level with a
predetermined time constant. The threshold detector 212 is a comparator
circuit. In one embodiment, the high voltage detector 216 is single
transistor amplifier that includes an AC coupling capacitor 300, a base
resistor 302, a collector resistor 306, and an NPN transistor 304. The AC
coupling capacitor 300 couples the voltage feedback signal (Vfb) to a base
terminal of the NPN transistor 304. The base resistor 302 is coupled
between a power source Vcc (e.g., 5 volts) and the base terminal of the
NPN transistor 304. The collector resistor 306 is coupled between the
power source Vcc and a collector terminal of the NPN transistor 304. An
emitter terminal of the NPN transistor 304 is coupled to ground.
The collector terminal of the NPN transistor 304 provides the output (Vls)
of the high voltage detector 216. During normal operations, the voltage
feedback signal (Vfb) is an AC signal with sufficient amplitude (e.g.,
greater than 0.7 volt) to generate periodic pulses at the output (Vls) of
the high voltage detector 216. The voltage feedback signal causes the NPN
transistor 304 to alternately turn on and turn off during normal
operations. When the NPN transistor 304 is on, the collector terminal of
the NPN transistor 304 is coupled to ground. When the NPN transistor 304
is off, the voltage at the collector terminal of the NPN transistor 304
rises to the level of the power source (Vcc). Thus, the high voltage
detector 216 outputs periodic pulses with voltage levels alternating
between ground and Vcc during normal operations.
During shorted lamp conditions, the amplitude of the voltage feedback
signal (Vfb) is close to zero. The base resistor 302 sets up the bias of
the NPN transistor 304 to be on. Thus, the collector terminal of the NPN
transistor 304 is coupled to ground and the high voltage detector 216
outputs a substantially DC signal at approximately zero during shorted
lamp conditions.
In one embodiment, the conditioning circuit 214 is a half-wave rectifier
with a timing conditioning circuit. The conditioning circuit 214 includes
a rectifier diode 308, a timing resistor 310 and a charging capacitor 312.
The rectifier diode 308 is coupled between an input terminal and an output
terminal of the conditioning circuit 214. An anode of the rectifier diode
308 is coupled to the input terminal, and a cathode of the rectifier diode
308 is coupled to the output terminal. The timing resistor 310 and the
charging capacitor 312 are coupled in parallel between the output terminal
of the conditioning circuit 214 and ground.
During normal operations, the periodic pulses of the output (Vls) from the
high voltage detector 216 pass through the rectifier diode 308 to charge
the charging capacitor 312. The conditioning circuit 214 produces an
output voltage (Vod) that has a level that is relatively steady and that
corresponds to the peak voltage of the periodic pulses of the output (Vls)
during normal operations. During shorted lamp conditions, the output (Vls)
of the high voltage detector 216 is coupled to ground and has no effect on
the rectifier diode 308. The charging capacitor 312 discharges through the
timing resistor 310 during shorted lamp conditions, and the output voltage
(Vod) of the conditioning circuit 214 decreases to approximately zero at a
rate determined by the timing resistor 310.
In one embodiment, the comparator circuit 212 includes a comparator 314.
The output (Vod) of the conditioning circuit 214 is provided to an
inverting (-) terminal of the comparator 314, and a reference voltage
(Vref) is provided to a non-inverting (+) terminal of the comparator 314.
The output of the comparator 314 is the output (DISABLE) of the shorted
lamp detector 110. During normal operations, the level of the output (Vod)
the conditioning circuit 214 is greater than the reference voltage, and
the comparator 314 causes the DISABLE output of the shorted lamp detector
110 to be low (i.e., inactive). During shorted lamp conditions, the output
level of the conditioning circuit 214 is less than the reference voltage
(or approximately zero), and the comparator 314 causes the DISABLE output
of the shorted lamp detector 110 to be high (i.e., active) to indicate the
detection of a shorted lamp condition. The power conversion circuit may be
disabled (or shut down) when the shorted lamp condition is detected. The
output (Vod) of the conditioning circuit 214 can be alternately provided
to the non-inverting (+) terminal of the comparator 314 with the reference
voltage (Vref) provided to the inverting (-) terminal of the comparator
314. Then, the DISABLE output has an opposite logic associated with active
or inactive states.
In one embodiment, the rate at which the output voltage of the conditioning
circuit 214 transitions from the peak voltage to zero is controlled so
that the power conversion circuit is not disabled as a result of
intermittent shorted lamp conditions. For example, intermittent shorted
lamp conditions may be shorted lamp conditions that last less than a
predetermined duration (e.g., one second). The periodic pulses at the
output of the high voltage detector 216 are absent for less than the
predetermined duration during the intermittent shorted lamp conditions.
The output of the conditioning circuit 214 begins to discharge during the
absence of the periodic pulses from the output voltage (Vls) of the high
voltage detector 216. The value of the timing resistor 310 is chosen to
set the discharge rate of the output voltage (Vod) of the conditioning
circuit 214 such that the transition from the peak voltage to a level
corresponding to the reference voltage of the comparator 314 is
approximately equal to or is greater than the predetermined duration
corresponding to the intermittent shorted lamp condition. Thus, the
comparator 314 is not triggered by the absence of periodic pulses for less
than the predetermined duration corresponding to intermittent shorted lamp
conditions.
FIG. 4 illustrates timing diagrams that show the waveforms of various
signals in the shorted lamp detector 110 of FIG. 3. A graph 400 represents
the voltage feedback signal (Vfb) provided by the voltage sensing feedback
circuit 106 to the shorted lamp detector 110. A graph 402 represents the
detected signal voltage (Vls) at the output of the high voltage detector
216. A graph 404 represents the output voltage (Vod) of the conditioning
circuit 214. A graph 406 represents the DISABLE output signal of the
shorted lamp detector 110.
As illustrated by the graphs 400 and 420, during normal operations, the
voltage feedback signal (Vfb) is substantially an AC signal with
sufficient amplitude such that the high voltage detector 216 generates
periodic pulses in response. For example, normal operations occur during
intervals T0-T1, T1-T2, T2-T3 and T4-T5. The high voltage detector 216
outputs periodic pulses during the intervals T0-T1, T1-T2, T2-T3 and T4-T5
with transitions corresponding to transitions of the voltage feedback
signal across a voltage threshold (VBE). The output of the high voltage
detector 216 is high (e.g., approximately 5 volts) when the voltage
feedback signal is lower than the voltage threshold (e.g., during the
interval T0-T1). The output of the high voltage detector 216 is low (e.g.,
approximately zero volt) when the voltage feedback signal is higher than
the voltage threshold (e.g., during the interval T1-T2). During shorted
lamp conditions, the voltage feedback signal is substantially a DC signal,
and the high voltage detector 216 outputs a substantially DC signal (e.g.,
approximately zero volt) in response. For example, shorted lamp conditions
occur during the interval T3-T4 and during the interval T5-T6.
The output voltage of the conditioning circuit 214 follows the periodic
pulses from the high voltage detector 216 and maintains a substantially
constant level corresponding to the peak voltage of the periodic pulses
during normal operations. For example, the output voltage of the
conditioning circuit 214 increases with each cycle of the periodic pulses
until the peak voltage of the periodic pulses is reached and thereafter
holds the peak voltage during intervals T0-T1, T1-T2, T2-T3 and T4-T5. The
output voltage of the conditioning circuit 214 decreases to approximately
zero at a predetermined rate during shorted lamp conditions. For example,
the output voltage of the conditioning circuit 214 decreases during the
interval T3-T4 and during the interval T5-T6.
As illustrated by graph 406, the DISABLE output of the shorted lamp
detector 110 is low (i.e., inactive) during normal operations (e.g.,
during the intervals T0-T1, T1-T2, T2-T3 and T4-T5) and intermittent
shorted lamp condition (e.g., during the interval T3-T4). The output of
the shorted lamp detector 110 is low when the output voltage of the
conditioning circuit 214 is greater than the reference voltage (Vref)
after the start up of the power conversion circuit.
As shown by the graph 404, during normal operations, the output voltage
(Vod) of the conditioning circuit 214 is substantially a DC voltage
corresponding to the peak voltage of periodic pulses from the high voltage
detector 216 which is greater than the reference voltage (Vref). During
intermittent shorted lamp conditions, the output voltage (Vod) of the
conditioning circuit 214 decreases. However, the rate of decrease in the
output voltage (Vod) is controlled such that the output voltage (Vod)
continues to be greater than the reference voltage within a predetermined
duration which defines the maximum duration of any intermittent shorted
lamp condition.
The DISABLE output of the shorted lamp detector 110 is high (i.e., active)
when shorted lamp conditions last longer than the predetermined duration
corresponding to the intermittent shorted lamp condition (e.g., after the
time T6 at the end of the interval T5-T6). Shorted lamp conditions lasting
longer than the predetermined duration (e.g., a condition lasting
throughout the interval T5-T6) causes the output (Vod) of the conditioning
circuit 214 to fall below the reference voltage, and the shorted lamp
detector 110 outputs an active DISABLE signal to indicate the detection of
a non-intermittent shorted lamp condition.
FIG. 5 is a schematic of an alternative embodiment 110' of the shorted lamp
detector 110. The shorted lamp detector 110' includes an AC coupling
capacitor 500, a signal sensing resistor (R1) 510, an open collector (or
an open drain) comparator 502, a holding capacitor 504, a pull-up resistor
506, and a reference comparator 508. The voltage feedback signal (Vfb)
from the voltage sensing feedback circuit 106 is provided to the open
collector comparator 502 via the series AC coupling capacitor 500 and the
signal sensing resistor 510 coupled between an input of the open collector
comparator 502 and ground. The open collector comparator 502 compares the
voltage feedback signal with a threshold voltage (Vth). An output of the
open collector comparator 502 is coupled to a common node. The holding
capacitor 504 is coupled between the common node and ground. The pull-up
resistor 506 is coupled between the common node and a power source (Vcc).
The common node is also coupled to a non-inverting (+) terminal of the
reference comparator 508. A reference voltage (Vref) is coupled to an
inverting (-) terminal of the reference comparator 508. The reference
comparator generates the DISABLE output for the shorted lamp detector 110.
In one embodiment in accordance with FIG. 5, the open collector comparator
502 actively pulls the common node down to a relatively low voltage (e.g.,
approximately ground) when the voltage feedback signal is above the
threshold voltage. The open collector comparator 502 is inactive when the
voltage feedback signal is below the threshold voltage, and the pull-up
resistor 506 supplies current to increase the voltage on the common node.
During normal operations, the voltage feedback signal is a periodic
voltage that fluctuates above and below the threshold voltage. Thus, the
open collector comparator 502 periodically grounds the common node during
normal operations. The periodic grounding of the common node drains any
charges stored in the holding capacitor 504, and the common node maintains
a relatively low voltage which is less than the reference voltage. As a
result, the output of the reference comparator is low (i.e., inactive) to
indicate that a shorted lamp condition has not been detected.
During shorted lamp conditions, the voltage feedback signal is less than
the threshold voltage, and the open collector comparator 502 is inactive.
The power source charges the holding capacitor 504 via the pull-up
resistor 506. The common node reaches a voltage that is approximately the
level of the power source, which is greater than the reference voltage. As
a result, the output of the reference comparator 508 is high (i.e.,
active) to indicate that a shorted lamp condition has been detected.
FIG. 6 illustrates an application of the shorted lamp detector 110 in a
power conversion circuit with floating outputs. The power conversion
circuit includes DC blocking (or AC coupling) capacitors 602, 604 coupled
in series with respective output terminals of a secondary winding 600 of a
transformer in a secondary network to generate a floating output voltage
(V-OUT) across the CCFL 112. A partial circuit of the power conversion
circuit illustrating the secondary network and a voltage sensing feedback
circuit is shown for clarity.
The voltage feedback signal (Vfb) for the shorted lamp detector 110 is
derived from the voltage sensing feedback circuit which includes two
voltage dividers (e.g., two capacitive voltage dividers or two resistive
voltage dividers) coupled in series across the floating output voltage (or
the lamp voltage). Two capacitive voltage dividers are illustrated as
examples. For example, a first capacitor 208 and a second capacitor 210
are coupled in series between a first terminal of the floating output
voltage and ground to form a first capacitive voltage divider. A third
capacitor 606 and a fourth capacitor 698 are coupled in series between
ground and a second terminal of the floating output voltage to form a
second capacitive voltage divider., The voltage feedback signal is taken
from the common node connecting the first capacitor 208 and the second
capacitor 210.
FIG. 7 illustrates a configuration for detecting shorted lamp conditions
using a single detection point in a power conversion circuit for driving
multiple fluorescent lamps. A secondary winding 704 of a transformer in a
secondary network of the power conversion circuit provides an output
voltage (V-OUT) to commonly connected input terminals of a plurality of DC
blocking capacitors shown as DC blocking capacitors 700(1)-700(n)
(collectively referred to as the DC blocking capacitors 700). A plurality
of CCFLs shown as CCFLs 702(1)-702(n) (collectively referred to as the
CCFLs 702) are coupled between respective output terminals of the DC
blocking capacitors 700 and ground. A high voltage divider (e.g., a
resistive voltage divider or a capacitive voltage divider) is coupled
across the secondary winding 704 to sense the output voltage and to
generate a voltage feedback signal (Vfb) for a shorted lamp detector 110.
A capacitive voltage divider is illustrated as an example. For example, a
first capacitor 706 and a second capacitor 708 are coupled in series
between the output voltage and ground. The voltage feedback signal is
derived from the common node connecting the first capacitor 706 and the
second capacitor 708.
In one embodiment, the power conversion circuit advantageously employs
direct drive topology. For example, the power conversion circuit uses a
direct drive controller and a direct drive primary network to generate the
output voltage across the secondary winding 704 of the transformer in the
secondary network. The values of the DC blocking capacitors 700 are
relatively large (e.g., 100 picofarads-1,000 picofarads) which allows for
the detection of shorted lamp conditions among the plurality of CCFLs 702
using one voltage feedback signal.
FIG. 8 illustrates an alternate configuration for detecting shorted lamp
conditions using multiple detection points in a power conversion circuit
for driving multiple fluorescent lamps. A secondary winding 804 of a
transformer in a secondary network of the power conversion circuit
provides an output voltage (V-OUT) to commonly connected input terminals
of a plurality of DC blocking capacitors shown as DC blocking capacitors
806(1)-806(n) (collectively referred to as the DC blocking capacitors
806). A plurality of CCFLs shown as CCFLs 702(1)-702(n) (collectively
referred to as the CCFLs 702) are coupled between respective output
terminals of the DC blocking capacitors 806 and ground. A plurality of
voltage dividers shown as voltage dividers 800(1)-800(n) (collectively
referred to as the voltage dividers 800) are coupled in parallel with the
respective CCFLs 702 to sense the voltages across the respective CCFLs 702
and to generate respective voltage feedback signals Vf(1)-Vf(n). The
voltage feedback signals are provided to respective shorted lamp detectors
shown as shorted lamp detectors 802(1)-802(n) (collectively referred to as
the shorted lamp detectors 802). The shorted lamp detectors 802 provide
respective outputs, DISABLE(1)-DISABLE(n), to indicate shorted lamp
conditions for the respective CCFLs 702.
In one embodiment, the power conversion circuit employs Royer oscillator
inverter architecture, and the DC blocking capacitors 806 are relatively
small (e.g., approximately 10 picofarads). Shorted lamp conditions are
reliably detected by sensing the voltages across each of the CCFLs 702.
Although described above in connection with CCFLs, it should be understood
that a similar apparatus and method can be used to drive fluorescent lamps
having filaments, neon lamps, and the like.
The presently disclosed embodiments are to be considered in all respect as
illustrative and not restrictive. The scope of the invention being
indicated by the append claims, rather than the foregoing description, and
all changes which comes within the meaning and ranges of equivalency of
the claims are therefore, intended to be embrace therein.
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