UCC2975, UCC2976, UCC2977

UCC3975, UCC3976, UCC3977

SLUS499A – NOVEMBER 2001 – REVISED JANUARY 2002

10

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APPLICATION INFORMATION

MECHANICAL

DISPLACEMENT

MECHANICAL

STRESS

0

0

VIN

VOUT

SUPPORTS

L = LENGTH

T = THICKNESS

h = HEIGHT

FORCE

FORCE

PRIMARY

SECONDARY

UDG–01076

Figure 7. Typical Longitudinal Mode Piezoelectric Transformer for CCFL Applications.

A typical multi-layer PZT with longitudinal mode geometry is shown in Figure 7, a single layer design would have

mechanical expansion and compression in the thickness direction (see Figure 6). This displacement on the

primary is transferred as a force in the longitudinal direction. Supports at

¼ and ¾ wavelength provide a means

for a standing wave to be generated at a resonant frequency as shown. Mechanical resonance occurs at

multiple standing wave frequencies (n) based on the transformer’s length and material velocity (v).

v

2

length

Voltage gain is a function of the PZT material coefficient g[

ω], the number of primary layers, the thickness of

the material and the overall length as follows:

V

length

layers

thickness

g[

w]

PZT electrical model

In order to predict PZT performance in a system, it is useful to develop an electrical circuit model. The model

shown in Figure 8 is often used to describe the behavior of a PZT near the fundamental resonant frequency.

Many PZT manufacturers will provide component values for the model based on measurements taken at

various frequencies and output loads.

(1)

(2)