Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2001-05-07
2002-05-28
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S017000
Reexamination Certificate
active
06396722
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a DC to AC power converter circuit. More particularly, the present invention provides a high efficiency controller circuit that regulates power delivered to a load using a zero-voltage-switching technique. General utility for the present invention is found as a circuit for driving one or more Cold Cathode Fluorescent Lamps (CCFLs), however, those skilled in the art will recognize that the present invention can be utilized with any load where high efficiency and precise power control is required.
DESCRIPTION OF RELATED ART
FIG. 1
depicts a convention CCFL power supply system
10
. The system broadly includes a power supply
12
, a CCFL driving circuit
16
, a controller
14
, a feedback loop
18
, and one or more lamps CCFL associated with an LCD panel
20
. Power supply
12
supplies a DC voltage to circuit
16
, and is controlled by controller
14
, through transistor Q
3
. Circuit
16
is a self-resonating circuit, known as a Royer circuit. Essentially, circuit
16
is a self-oscillating dc to ac converter, whose resonant frequency is set by L
1
and C
1
, and N
1
-N
4
designate transformer windings and number of turns of the windings. In operation, transistors Q
1
and Q
2
alternately conduct and switch the input voltage across windings N
1
and N
2
, respectively. If Q
1
is conducting, the input voltage is placed across winding N
1
. Voltages with corresponding polarity will be placed across the other windings. The induced voltage in N
4
makes the base of Q
2
positive, and Q
1
conducts with very little voltage drop between the collector and emitter. The induced voltage at N
4
also holds Q
2
at cutoff. Q
1
conducts until the flux in the core of TX
1
reaches saturation.
Upon saturation, the collector of Q
1
rises rapidly (to a value determined by the base circuit), and the induced voltages in the transformer decrease rapidly. Q
1
is pulled further out of saturation, and V
CE
rises, causing the voltage across N
1
to further decrease. The loss in base drive causes Q
1
to turn off, which in turn causes the flux in the core to fall back slightly and induces a current in N
4
to turn on Q
2
. The induced voltage in N
4
keeps Q
1
conducting in saturation until the core saturates in the opposite direction, and a similar reversed operation takes place to complete the switching cycle.
Although the inverter circuit
16
is composed of relatively few components, its proper operation depends on complex interactions of nonlinearities of the transistors and the transformer. In addition, variations in C
1
, Q
1
and Q
2
(typically, 35% tolerance) do not permit the circuit
16
to be adapted for parallel transformer arrangements, since any duplication of the circuit
16
will produce additional, undesirable operating frequencies, which may resonate at certain harmonics. When applied to a CCFL load, this circuit produces a “beat” effect in the CCFLs, which is both noticeable and undesirable. Even if the tolerances are closely matched, because circuit
16
operates in self-resonant mode, the beat effects cannot be removed, as any duplication of the circuit will have its own unique operating frequency.
Some other driving systems can be found in U.S. Pat. Nos. 5,430,641; 5,619,402; 5,615,093; 5,818,172. Each of these references suffers from low efficiency, two-stage power conversion, variable-frequency operation, and/or load dependence. Additionally, when the load includes CCFL(s) and assemblies, parasitic capacitances are introduced, which affects the impedance of the CCFL itself. In order to effectively design a circuit for proper operation, the circuit must be designed to include consideration of the parasitic impedances for driving the CCFL load. Such efforts are not only time-consuming and expensive, but it is also difficult to yield an optimal converter design when dealing with various loads. Therefore, there is a need to overcome these drawbacks and provide a circuit solution that features high efficiency, reliable ignition of CCFLs, load-independent power regulation and single frequency power conversion.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an optimized system for driving a load, obtains an optimal operation for various LCD panel loads, thereby improving the reliability of the system.
Broadly defined, the present invention provides A DC/AC converter circuit for controllably delivering power to a load, comprising an input voltage source; a first plurality of overlapping switches and a second plurality of overlapping switches being selectively coupled to said voltage source, the first plurality of overlapping switches defining a first conduction path, the second plurality of overlapping switches defining a second conduction path. A pulse generator is provided to generate a pulse signal. Drive circuitry receives the pulse signal and controls the conduction state of the first and second plurality of switches. A transformer is provided having a primary side and a secondary side, the primary side is selectively coupled to the voltage source in an alternating fashion through the first conduction path and, alternately, through the second conduction path. A load is coupled to the secondary side of the transformer. A feedback loop circuit is provided between the load and the drive circuitry that supplies a feedback signal indicative of power being supplied to the load. The drive circuitry alternates the conduction state of the first and second plurality of switches, and the overlap time of the switches in the first plurality of switches, and the overlap time of the switches in the second plurality of switches, to couple the voltage source to the primary side based at least in part on the feedback signal and the pulse signal.
The drive circuitry is constructed to generate a first complimentary pulse signal from the pulse signal, and a ramp signal from the pulse signal. The pulse signal is supplied to a first one of the first plurality of switches to control the conduction state thereof, and the ramp signal is compared with at least the feedback signal to generate a second pulse signal, where a controllable conduction overlap condition exists between the conduction state of the first and second switches of the first plurality of switches. The second pulse signal is supplied to a second one of the first plurality of switches and controlling the conduction state thereof. The drive circuitry further generates a second complimentary pulse signal based on the second pulse signal, wherein said first and second complimentary pulse signals control the conduction state of a first and second ones of the second plurality of switches, respectively. Likewise, a controllable conduction overlap condition exists between the conduction state of the first and second switches of the second plurality of switches.
In method form, the present invention provides a method for controlling a zero-voltage switching circuit to deliver power to a load comprising the steps of supplying a DC voltage source; coupling a first and second transistor defining a first conduction path and a third and fourth transistor defining a second conduction path to the voltage source and a primary side of a transformer; generating a pulse signal to having a predetermined pulse width; coupling a load to a secondary side of said transformer; generating a feedback signal from the load; and controlling the feedback signal and the pulse signal to determine the conduction state of said first, second, third and fourth transistors.
In the first embodiment, the present invention provides a converter circuit for delivering power to a CCFL load, which includes a voltage source, a transformer having a primary side and a secondary side, a first pair of switches and a second pair of switches defining a first and second conduction path, respectively, between the voltage source and the primary side, a CCFL load circuit coupled to the secondary side,a pulse generator generating a pulse signal, a feedback circuit coupled to the load generating a feedback
Grossman Tucker Perreault & Pfleger PLLC
Micro International Limited
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