Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2000-09-08
2002-03-19
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S307000, C315S219000, C315SDIG004, C315S119000, C361S091100
Reexamination Certificate
active
06359391
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present disclosure relates generally to backlight inverter circuitry for controlling a load. More specifically, the present disclosure relates to a system for providing overvoltage protection during a pulse width modulation (PWM) dimming of a liquid crystal display (LCD) backlight inverter.
2. Background of the Related Art
Overvoltage protection, which may include open circuit protection, is generally required for a backlight inverter circuit controlling a load, such as lamps, to provide safe and effective operation. When one or more lamps are not connected to the outputs of a ballast of the LCD backlight inverter, there is a huge voltage, i.e., an overvoltage condition occurs, across the ballast outputs, if some kind of protection method or system is not implemented. The overvoltage condition results in having a higher output voltage at the outputs of the ballast than a nominal output voltage when the lamps are connected to the outputs of the ballast. During the overvoltage condition, one can be injured if contact is made with the ballast outputs. Further, the overvoltage condition can damage the components of the ballast, and/or cause the ballast to run into an unexpected state, and eventually cause the ballast to be damaged.
Overvoltage protection in lamp driving circuits is described in U.S. Pat. Nos. 5,680,017, 6,011,360 and 6,084,361, the contents of which are hereby incorporated herein by reference.
In the present state of the art, as shown by the prior art backlight inverter of
FIG. 1
, when an overvoltage condition is detected, i.e., when the output voltage (which is proportional to an input current to controller U
2
) of the ballast circuit to the lamps is greater than a threshold voltage as compared by a comparator within the controller U
2
, a one shot timer is activated by an output of the comparator to time a one shot time interval.
The internal components of the controller U
2
are described in detail in U.S. Pat. No. 5,680,017. Accordingly, the comparator used to detect an overvoltage condition may include comparator
421
which can detect a minimum overvoltage condition (OV′), comparator
424
which can detect a maximum overvoltage condition (OVMAX), or comparator
427
which can detect a “panic” overvoltage condition (OVPANIC). The input current is fed to pin VL of the controller U
2
.
With continued reference to U.S. Pat. No. 5,680,017 and additionally to U.S. Pat. No. 6,011,360, the one shot timer may include an external capacitor, i.e., outside controller U
2
, which is connected to the third pin designated by CP, and an external resistor which is connected to the 12th pin designated by Rref. If these components and circuitry are used with the controller U
2
of the backlight inverter of
FIG. 1
, for example, and show that the overvoltage condition exists at the end of the one shot time, the backlight inverter circuit of
FIG. 1
activates overvoltage protection. When overvoltage protection is triggered during full light output, the circuit is placed into a standby mode, and the output to lamps J
1
, J
2
drops to zero.
As a side note, when short circuit protection is triggered by the backlight inverter of
FIG. 1
, a power stage module PSM is shut down by cutting off the supply voltage chip_Vdd to the controller U
2
, and accordingly, the output of control signals S
1
and S
2
to the power stage module PSM and PWM dimming logic circuitry U
3
, respectively. The oscillation of signal S
1
is controlled by at least a LampOn signal received by the controller U
2
from the PWM dimming logic circuitry U
3
. The signals S
3
and S
1
drive a low and a high side, respectively, of a power switch within the power stage module PSM.
Signal S
2
causes the PWM dimming logic circuitry U
3
to generate and transmit signal S
3
to the power stage module PSM to operate the power stage module PSM. Once the controller U
2
stops operating, signals S
1
and S
3
stop being generated. Then, the output voltage to lamps J
1
, J
2
drops to zero.
The one shot timer is necessary to avoid shut down due to a fault trigger, where there could be a spike or a large transient output voltage for a very short time, even when the lamps are connected to the ballast outputs. In the case of a fault trigger, the overvoltage protection is not activated at the end of the one shot timer, because it is very unlikely a fault condition will occur at the beginning and end of the one shot timer, if the lamps are connected to the ballast outputs.
A disadvantage of this conventional overvoltage protection scheme arises when pulse width modulation (PWM) is used to dim an LCD backlight inverter as shown by
FIG. 1
, i.e., the circuitry used to control the load or the lamps. Dimming is achieved by turning on and off the lamps with a fixed frequency, and the dimming level, i.e., the amount of dimming, is determined according to the duty cycle of the PWM signal generated by the PWM signal generator GEN and fed to the PWM dimming logic circuitry U
3
. If the fixed frequency is approximately 170 Hz, one is not able to sense the lamp discontinuously turning on and off (i.e., flickering) and dimming is perceived with the average light output.
When the one shot timer is used for overvoltage protection, the duration of the one shot timer, e.g., 10 msec to one second, is typically much longer than the period of the PWM signal, e.g., approximately 5-6 msec. Since the lamps are discontinuously on and off due to the fixed frequency during PWM dimming of the LCD backlight inverter, even when the lamps are removed or disconnected from the ballast outputs B
1
, B
2
, it is impossible to reliably detect a fault condition at the end of the one shot timer, due to a different dimming level, operating frequency, etc. This problem is further described below with reference to
FIGS. 2A-2C
.
FIG. 2A
is block diagram of a prior art overvoltage protection system designated generally by reference numeral
10
. In this system, output sensing “OutputSensing”) and threshold “Threshold”) voltages are inputted into a comparator
12
within a controller (not shown), which may be similar to controller U
2
. The output sensing voltage corresponds to the ballast voltage output level. If the output sensing voltage is greater than the threshold voltage, an activation signal is transmitted to a one shot timer
14
and a logic circuit
16
within the controller. The activation signal activates the one shot timer
14
and the one shot timer
14
begins to time a predetermined one shot time interval. At the end of the predetermined one shot time interval, if the output sensing voltage is still greater than the threshold voltage, the controller stops oscillating and transmits signals SIG
1
, SIG
2
to turn off at least one power switch within a power stage module (not shown), which may be similar to the power stage module PSM of FIG.
1
.
FIG. 2B
illustrates a timing diagram during activation of the one shot timer
14
of
FIG. 2A
The diagram shows the PWM signal and the corresponding output sensing voltage which is assumed to be higher than the threshold voltage. A high stop timer signal (signaling the beginning of the one shot timer interval) “StopTimer”) is generated at time A by the one shot timer
14
. When the output sensing voltage is high, i.e., at points A and C during a high PWM signal, the lamp is intended to turn on, and when the output sensing voltage is low, i.e., at points B and D during a low PWM signal, the lamp is intended to turn off.
FIG. 2C
illustrates a timing diagram depicting the time when the one shot timer
14
is deactivated, i.e., at the end of the predetermined one shot time interval.
A problem thus can arise at the end of the predetermined one shot time interval as exemplified in
FIGS. 2B and 2C
. As it can be seen from
FIG. 2C
, when the one shot timer
14
is off, i.e., at the end of the predetermined one shot time interval, the output voltage across the ballast outputs of the lamp is low, e.g., when the one shot time interval expires within time
Alemu Ephrem
Philips Electronics North America Corporation
Wong Don
LandOfFree
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