Transformer Selection
The CCFL lamp manufacturer supplies the following lamp characteristics:
1. Strike voltage (V
strike
)
2. Running voltage (V
run
)
3. Frequency of operation (F
res
)
4. Power (W)
5. Current (I
lamp
)
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
V
in 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
®
Coiltronics
®
brand CCFL transformers are
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
