Transformer Selection

The CCFL lamp manufacturer supplies the following lamp characteristics:

1. Strike voltage (Vstrike)

2. Running voltage (Vrun)

3. Frequency of operation (Fres)

4. Power (W)

5. Current (Ilamp)

The first step is to select the transformer according to the power

requirement from the catalog.

The second step is to decide the termination.

Use this formula to find the turn ratio needed to obtain the strike voltage of

the lamp.

TR = Turns ratio

Vin min = Battery voltage

Cold Cathode Fluorescent Lamps (CCFLs) are used to illuminate Liquid

Crystal Displays (LCDs). The LCD display is used in laptop computers, gas

pumps, automobiles, test equipment, PDAs and medical instruments.

CCFLs are small, efficient and inexpensive. The lamp must be driven by a

specialized power supply. High sinusoidal AC voltage is needed to start the

lamps, but once started, the voltage drops to a lower level. CCFL circuits

are usually powered from a low voltage DC source of 5-12V. The DC to AC

power supply needs a transformer to change low DC input voltage to high

sinusoidal AC voltage.

The Cooper Bussmann

designed to work with inexpensive Royer class self-oscillating circuits. The

Royer circuit works with input voltage from 2.5 to 20Vdc and is capable of

producing 90% efficiency above 5Vdc input.

Royer Diagram

A current-fed, push-pull topology is commonly used to power the CCFL

transformer. This topology accommodates a wide input voltage and

consists of a resonant push-pull stage, a Pulse-Width-Modulated (PWM)

buck-derived control stage and a high-voltage secondary stage. The push-

pull stage consists of transistors Q2 and Q3 to drive the center-tapped

transformer T1. The transistors are driven 180° out of phase at 50% duty

cycle with an auxiliary winding on the transformer. A resonant tank is

formed between the primary inductance of the transformer and a low-

loss, external resonant bulk capacitor C1. The resonant tank provides a

sinusoidal voltage to the transformer’s primary winding and sets the

system’s operating frequency.

The high voltage at the secondary of transformer is used to ignite and

operate the lamp. Since the ignition or “strike voltage” is higher than the

operating voltage, a high voltage ballast capacitor C2 is required to allow

a voltage difference between the transformer secondary and the lamp. To

minimize lamp stress and improve efficiency, the striking voltage

waveforms should be sinusoidal.

CCFL Transformer Application Note

TR

V

V

in

strike

×

×

=

2

min

π