Multi-Topology Piezoelectric Transformer Controller
Liquid crystal Display (LCD) enclosures and cold cathode fluorescent lamps (CCFLs) used in notebook computer and portable electronics Displays are becoming increasingly narrow, generating the need for a low profile CCFL power supply. Recent advances in single- and multi-layered piezoelectric ceramic transformers (PZT) have enabled the development of a new generation of efficient, size-reduced backlight converters. The UCC3975/6/7 family of 8-pin PZT controllers integrate the necessary circuitry for operating a PZT-based backlight supply using a flyback, half-bridge, or push-pull topology. The choice of power topology depends on application requirements such as input voltage, lamp voltage, and PZT gain.
A half-bridge PZT converter, using the UCC3976 is shown in Figure 1. External P- and N-Channel MOSFETs are driven out of phase at a fixed 50% duty cycle with anti-cross conduction circuitry provided by the controller. The half-bridge topology uses only a single magnetic component (LRES) reducing board area. As explained in the applications section of this datasheet, regulation of lamp current is achieved by controlling the operating frequency of the system.
The UCC3977 is designed to control a resonant push-pull topology as shown in Figure 2. This controller alternately drives external N-Channel MOSFETs at 50% duty cycle. The push-pull topology requires two external inductors (L1 and L2), but has the advantage of providing increased voltage across the piezoelectric transformer primary. In this case a small overlap is provided to the Gate drives, assuring an uninterrupted path for inductor current.
For piezoelectric transformer applications requiring additional gain, a resonant flyback topology CAN be implemented using the UCC3975. As shown in Figure 3, a magnetic transformer (T1) provides a stepped up voltage to the piezoelectric transformer primary. When compared to a traditional High-Voltage transformer used in a CCFL application, T1 is small and low profile because of its reduced turns ratio and voltage rating. In the resonant flyback application, a single Switch is driven at 50% duty cycle producing a half wave rectified sinusoid at the piezoelectric transformer primary.
By Texas Instruments
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