Abstract:
A push-pull driver for powering fluorescent lamps in a backlight system includes a transformer with three primary windings to realize the advantages of both a push-pull switching topology and a full-bridge switching topology. The first and the second primary windings alternately conduct currents in opposite polarities to generate an alternating current signal to power one or more lamps coupled to a secondary winding of the transformer. The third primary winding is short-circuited to preserve energy stored in the transformer in a null state when both the first and the second primary windings are not conducting.

Description:
CLAIM FOR PRIORITY 
     This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/591,264, filed on Jul. 26, 2004, and entitled “System and Method for Driving CCFL Backlights Using a Push-Pull Inverter and a Transformer with Three Primary Windings,” the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention generally relates to a driver circuit in a backlight system for powering fluorescent lamps, and more particularly, relates to a driver circuit that combines the advantages of a push-pull switching topology and a full-bridge switching topology. 
     2. Description of the Related Art 
     In liquid crystal display (LCD) applications, backlight is needed to illuminate a screen to make a visible display. A number of conventional inverter topologies (e.g., active clamping forward, phase-shifted full-bridge, resonant full-bridge, asymmetric half-bridge, push-pull, etc.) facilitate zero voltage or zero current switching to minimize switching stresses and losses. Among these conventional inverter topologies, the full-bridge topology and the push-pull topology are acceptable for cold cathode fluorescent lamp (CCFL) inverter applications because of their capability to produce symmetric lamp current waveforms. 
     Both the conventional full-bridge topology and the conventional push-pull topology have advantages and disadvantages for the CCFL inverter applications. The conventional full-bridge topology has an ability to control circuit behavior at all times. For example, a short circuit can be placed across a primary winding of a transformer in the conventional full-bridge topology when drive voltage is not applied to the primary winding. The conventional full-bridge topology advantageously preserves stored energy in the transformer or in inductor-capacitor (or tank) circuits. 
     In contrast, the conventional push-pull topology sometimes looses direct control of circuit behavior. For example, an open circuit is created within positive and negative power supply limits at primary windings of a transformer in the conventional push-pull topology when drive voltage is not applied to the primary windings. The conventional push-pull topology allows stored energy in the transformer and any tank circuits to leak back into primary winding circuits, often creating voltage spikes across switching transistors coupled to the primary windings. The cycle of energy storage and loss repeats for each cycle of the drive voltage. However, the conventional push-pull topology advantageously requires fewer driving control signals than the full-bridge topology, introduces less power loss in a power-delivering path and has fewer components. 
     The conventional full-bridge topology, on the other hand, generally has more complicated driving circuitry and is less power efficient. For example, the conventional full-bridge topology drives a set of upper switches and a set of lower switches. The upper switches and the lower switches often use different levels of gate drive control signals. In addition, the on-resistance of the upper switches appears as an I 2 R power loss in the power-delivering path. 
     SUMMARY 
     In one embodiment, the present invention proposes a push-pull driver with null-short feature that has advantages of both a conventional full-bridge topology and a conventional push-pull topology. For example, the push-pull driver with null-short feature restores control of circuit behavior when a drive voltage is inactive (or power is not being delivered to a load) without complicating driving control signals or introducing additional losses in a power delivery path. The push-pull driver with null-short feature advantageously allows the use of a push-pull controller to maintain benefits of the conventional push-pull topology while realizing the benefits of the conventional full-bridge topology. In other words, the push-pull controller appears to a transformer and its secondary winding in the push-pull driver with null-short feature as though it is a full-bridge controller. 
     In one embodiment, a push-pull driver (or inverter) includes a transformer with three primary windings and four semiconductor switches (or switching transistors). The transformer and the semiconductor switches are arranged in a push-pull switching topology. For example, the first semiconductor switch is coupled between a first terminal of the first primary winding and a reference node. The second semiconductor switch is coupled between a second terminal of the second primary winding and the reference node. A power supply (or voltage source) is coupled to a second terminal of the first primary winding and a first terminal of the second primary winding. In one embodiment, a current feedback circuit (e.g., a sensing resistor) is coupled between the reference node and ground to detect current levels in the first and the second primary windings. 
     The first and the second primary windings are configured to deliver power in alternating polarities (or phases) to a load (e.g., a lamp) coupled across a secondary winding of the transformer. For example, the first semiconductor switch and the second semiconductor switch alternately (or periodically) conduct to generate an alternating current (AC) signal across the secondary winding of the transformer. Power is delivered in a first polarity to the load when the first semiconductor switch is active, and power is delivered in a second (or opposite) polarity to the load when the second semiconductor switch is active. In one embodiment, the load includes at least one fluorescent lamp (or CCFL) for backlighting a display panel (e.g., a LCD). 
     The third semiconductor switch and the fourth semiconductor switch are respectively coupled between opposite terminals of the third primary winding and a common voltage (or regulated voltage). The third and the fourth semiconductor switches are active (or on) when the first and the second semiconductor switches are both inactive (or off). Thus, the third primary winding is configured to be short-circuited when power is not delivered to the load. Shorting the third primary winding advantageously freezes (or substantially maintains) the flux state of the transformer core and minimizes losses (or improves power efficiency). 
     In one embodiment, the three primary windings are tri-filar windings or wound side-by-side in a single layer on a bobbin. The first and the second primary windings have approximately the same number of turns. The first and the second primary windings can be part of one primary winding with a center-tap for coupling to the power supply and opposite terminals for coupling to the first semiconductor switch and the second semiconductor switch respectively. In one embodiment, the three primary windings have approximately the same number of turns (e.g., 17). 
     In one embodiment, the first and the second semiconductor switches are N-type transistors (e.g., N-type field-effect-transistors or bipolar junction transistors) while the third and the fourth semiconductor switches are P-type transistors. In alternate embodiments, the first and the second semiconductor switches are P-type transistors while the third and the fourth semiconductor switches are N-type transistors. The four semiconductor switches can be advantageously controlled by a push-pull controller that outputs two driving signals. For example, the first driving signal controls the first and the third semiconductor switches while the second driving signal controls the second and the fourth semiconductor switches. 
     For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These drawings and the associated description herein are provided to illustrate embodiments and are not intended to be limiting. 
         FIG. 1  illustrates one embodiment of a push-pull driver with null-short feature. 
         FIG. 2  illustrates another embodiment of a push-pull driver with null-short feature and connections to a push-pull controller. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Although particular embodiments are described herein, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, will be apparent to those of ordinary skill in the art. 
       FIG. 1  illustrates one embodiment of a push-pull driver with null-short feature. The push-pull driver (or inverter) includes a transformer  100  with a first primary winding  104 , a second primary winding  102  and a third primary winding  106 . A first terminal of the second primary winding  102  and a second terminal of the first primary winding  104  are commonly connected to a power supply (VS 1 ). A lamp load  110  is coupled across a secondary winding  108  of the transformer  100 . The lamp load  110  can include one or more CCFLs in a backlight system for LCD applications. 
     The push-pull driver also includes four semiconductor switches (or switching transistors)  112 ,  114 ,  116 ,  118  coupled to the transformer  100 . The four semiconductor switches  112 ,  114 ,  116 ,  118  can be P-type or N-type transistors (e.g., bipolar junction transistors or field-effect-transistors). In the embodiment shown in  FIG. 1 , the first and the second semiconductor switches  112 ,  114  are N-type metal-oxide-semiconductor field-effect-transistors (N-MOSFETs) while the third and the fourth semiconductor switches  116 ,  118  are P-MOSFETs. The first and the second semiconductor switches  112 ,  114  contribute to losses in power delivered to the lamp load  110 . Although P-MOSFETs can be used to implement the first and the second semiconductor switches  112 ,  114 , N-MOSFETs typically have lower on-resistance to reduce power loss. The third and the fourth semiconductor switches  116 ,  118  conduct magnetizing current and do not contribute to power loss. 
     The first semiconductor switch (Q 1 )  112  has a drain terminal coupled to a first terminal of the first primary winding  104  and a source terminal coupled to a reference node. The second semiconductor switch (Q 2 )  114  has a drain terminal coupled to a second terminal of the second primary winding  102  and a source terminal coupled to the reference node. In the embodiment shown in  FIG. 1 , a sensing resistor (RS)  120  is coupled between the reference node and ground for detecting current levels in the first primary winding  104  and the second primary winding  102 . The third semiconductor switch (Q 3 )  116  has a drain terminal coupled to a first terminal of the third primary winding  106  and a source terminal coupled to a common voltage (VS 2 ). The fourth semiconductor switch (Q 4 )  118  has a drain terminal coupled to a second terminal of the third primary winding  106  and a source terminal coupled to the common voltage. 
     A first driving signal (A) is coupled to gate terminals of the first semiconductor switch  112  and the third semiconductor switch  116 . A second driving signal (B) is coupled to gate terminals of the second semiconductor switch  114  and the fourth semiconductor switch  118 . The first driving signal and the second driving signal are periodically active to generate an AC signal (e.g., lamp signal) to power the lamp load  110 . For example, the first driving signal is active (or logic high) for a first duration to turn on the first semiconductor switch  112 . Current flows in the first primary winding  104  when the first semiconductor switch  112  is on and a corresponding current flows in a first direction (or polarity) in the secondary winding  108 . The second driving signal is active for a second duration to turn on the second semiconductor switch  114 . Current flows in the second primary winding  102  when the second semiconductor switch  114  is on and a corresponding current flows in a second direction in the secondary winding  108 . 
     The active states of the first driving signal and the second driving signal do not overlap. When the first driving signal is inactive (or logic low), the third semiconductor switch  116  is active (or on) and couples the first terminal of the third primary winding  106  to the common voltage. When the second driving signal is inactive, the fourth semiconductor switch  118  is on and couples the second terminal of the third primary winding  106  to the common voltage. Thus, when both the first driving signal and the second driving signal are inactive, the third primary winding  106  is effectively short-circuited and conducts a magnetizing current. Shorting the third primary winding  106  advantageously freezes (or substantially maintains) the flux state of the transformer core during a null state when neither the first semiconductor switch  112  nor the second semiconductor switch  114  are active to deliver power (or pulse of energy) to the lamp load  110 . Shorting the third primary winding  106  during the null state advantageously minimizes losses and improves power efficiency. Although the embodiment shown in  FIG. 1  uses two semiconductor switches  116 ,  118  controlled by two driving signals (A, B) to short the third primary winding  106 , other configurations are possible to short the third primary winding  106  during the null state. 
     The first primary winding  104  and the second primary winding  102  have approximately the same number of turns. The third primary winding  106  is configured to conduct magnetizing current and can have an arbitrary number of turns. In one embodiment, the three primary windings  102 ,  104 ,  106  are tri-filar windings or wound side-by-side in a single layer on a bobbin with approximately the same number of turns (e.g., 17). The first and the second primary windings (or power windings)  104 ,  102  can be part of one primary winding with a center-tap for coupling to the power supply and opposite terminals for coupling to the first semiconductor switch  112  and the second semiconductor switch  114  respectively. The power supply can be a direct current (DC) voltage source (e.g., a battery) with a range of amplitudes (e.g., from approximately 10–20 volts). 
       FIG. 2  illustrates another embodiment of a push-pull driver with null-short feature and connections to a push-pull controller  200 . The push-pull driver shown in  FIG. 2  is substantially similar to the push-pull driver shown in  FIG. 1  with an additional filter resistor (R 2 )  202 , a filter capacitor (C 1 )  204  and the push-pull controller  200 . The transformer  100  and connections of the primary windings  102 ,  104 ,  106  to the semiconductor switches  112 ,  114 ,  116 ,  118  are schematically equivalent to the embodiment shown in  FIG. 1 . The primary windings  102 ,  104 ,  106 , however, are drawn to show the first primary winding  104  and the second primary winding  102  as a center-tap primary winding. 
     The filter resistor  202  is coupled between the reference node and a first terminal of the filter capacitor  204 . A second terminal of the filter capacitor  204  is coupled to ground. The voltage across the filter capacitor  204  is provided to current sense inputs (CS+, CS−) of the push-pull controller  200 . The voltage across the filter capacitor  204  provides an indication of an average current level conducted by the first and the second primary windings  104 ,  102  which is used to control power delivered to the lamp load  110  (or brightness of the lamp load  110 ). For example, the active durations of the first and the second driving signals can be increased to increase power (or brightness) for the lamp load  110  or decreased to decrease power for the lamp load  110 . The push-pull controller  200  outputs two gate drive control signals (Aout, Bout) corresponding to the first driving signal and the second driving signal. In one embodiment, the push-pull controller  200  is powered by a regulated voltage (Vin) that has approximately the same voltage (e.g., 10 volts) as the common voltage (VS 2 ). 
     The push-pull driver with null-short feature described above improves power efficiency to prolong battery life while saving circuit board space which can be used for other functions (e.g., ambient light control). Similar to a conventional push-pull topology, the gate drive control signals are simple and power loss of one semiconductor switch (e.g., an N-MOSFET) appears in the power-delivering path. Similar to a conventional full-bridge topology, a short circuit is placed across a primary winding of a transformer when power is not applied to the transformer to preserve energy stored in the transformer or any resonant tank circuits. The push-pull controller  200  of the push-pull driver with null-short feature advantageously maintains direct control of the transformer  100  when both the first and the second semiconductor switches  112 ,  114  are inactive. In other words, the push-pull driver with null-short feature allows a push-pull controller  220  to appear as a full-bridge controller to the core and secondary side of the transformer  100 . 
     Various embodiments have been described above. Although described with reference to these specific embodiments, the descriptions are intended to be illustrative and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.