Download Inverter tests keep CCFLs shining

Inverter tests keep CCFLs shining
Tom Novitsky, VP Engineering and Bob Arnold, Engineering Manager, Endicott
Research Group, Endicott, NY
Endicott Research Group designs and manufactures DC-to-AC inverters used to
power backlights that use CCFLs (cold cathode fluorescent lamps) as a light
source. The company also manufactures drivers for backlights that use LEDs as
light sources. CCFL backlights have been the light source for LCD backlights for
years while LED backlights are just beginning use in applications that require
added reliability and brightness, although at higher cost.
Display applications require efficient inverters that can power a wide range of
displays with minimal input power. These requirements are especially important
with the ever increasing number of display products that use battery power,
which further requires the inverter to operate seamlessly over a wide range of
input voltage. Most of these displays utilize 5.7” to 15” diagonal LCDs,
backlighted with one or two CCFLs.
Using basic inverter design concepts, ERG engineers developed a low-profile
series of DC-AC inverters. These inverter cover a wide input input voltage range
(8 VDC to 18 VDC), open lamp detection, and they provide a wide dimming
range using onboard PWM (pulse width modulation) brightness control, and lamp
current regulation.
Inverter workings
The basic design of all ERG inverters involves a low-voltage oscillator that
converts a DC input, typically 5 V to 12 V, into a low-voltage AC waveform that
the a step-up transformer boosts to 1000 V rms to 2000 V rms, the voltage
needed to start a CCFL. This voltage is called a start or strike voltage.
Once the CCFL is lighted, the voltage and current required to maintain stable
light output drops to 500 to 1000 V rms and 5 mA rms to 6 mA rms. The lowvoltage oscillator may be a discrete component Royer’s oscillator or it may use
an IC to provide the required AC input to the transformer.
Maintaining stable LCD brightness requires that the inverter’s output current
remain stable over variations in input voltage and operating temperature. The
inverter monitors output current and actively adjusts transformer input to maintain
a 5% output current regulation. CCFL start or strike voltages vary widely with
temperature, increasing dramatically at lower temperatures (Figure 1). At startup,
the inverter monitors CCFL current while the strike voltage is increased to a
maximum of 1800 V rms. Once the CCFL lights, the inverter reduces the voltage
to the level required to maintain stable CCFL operation. If the CCFL is damaged
or if the connection to the CCFL is open, the SFW open lamp detection function
senses this and shuts down inverter operation pending fault correction.
Figure 1. Starting voltages for CCFL’s increase as temperature decreases.
Electrically, the CCFL load is a resistive load that varies not only among lamp
designs, but with temperature and packaging. Lamp size and electro chemistry
for displays of 5.7” to 15” size offer start voltages from 1000 V rms to 2000 V
rms with sustaining voltages in the 500 V rms to 1000 V rms range. Lamp
currents run from 5 mA to 6 mA rms. The start voltage increases with lamp age
and with decreasing temperature. The sustaining voltage and lamp current also
vary across the wide variety of CCFLs used in backlights. Additionally, variations
in CCFL packaging provide variations in capacitive losses across different LCD
assemblies that need to be considered.
Dimming or brightness control is also a key design consideration to provide
effective display function in night and daytime ambient light conditions. CCFL
brightness is controlled pulse width modulation (PWM) dimming wherein the
lamps are pulsed at a low frequency, typically 100 Hz to 200 Hz, and the duty
cycle for each cycle varies from zero to 100%. A duty cycle of 50% then will
provide display brightness approximately half that provided at a duty cycle of
100%. Figure 2 shows the lamp brightness versus current. Brightness stability at
short duty cycles is the typical challenge for inverter design so the SFW Series
design validation also included high dimming (low duty cycle) validation over
variations in input voltage and temperature.
Figure 2. A CCFL’s brightness is nearly linear with respect to current.
Production test
After ERG engineers verify an inverter design in the lab, the develop test
specifications for manufacturing. The manufacturing team tests all products for
proper function. The test data for each inverter, and there are thousands
produced every week, is recorded and saved with appropriate manufacturing
identification to provide a permanent record of the efficacy of each inverter
produced. This data is used by ERG to monitor process stability and to provide a
solid base of technical data on which to base product or process improvements.
The test station (Figure 3) uses a 110 K, 10 W resistor to simulate the load of a
CCFL to the inverter. A Fluke 45 DMM measures the input voltage while a
handheld Fluke 87V measures the current. On the output side, a Fluke
oscilloscope measures current and distortion through a Tektronix CT-2 current
probe. From the input and output measurements, a PC running custom software
calculates efficiency.
If an invert has PWM brightness control, the system makes the measurements at
full off, 50%, and full-on states. A 4-V power supply provides the control voltage
to the inverter under test. A 0-V input forces some inverters to set their PWM
controls to 100% duty cycle and 4-V for 0% while others use the opposite
settings. Technicians use the handheld DMM to measure the frequency of the
PWM signal at the inverter’s output. Test points on a test fixture or cable
harnesses provide access to the inverter’s input and output pins, depending on
the model. A 5-V or 12-V signal on the enable pin activates the inverter.
Figure 3. An automated test system measures an inverter’s input and output from
which ic calculates efficiency.
Manufacturing test stations also include custom software to provide automatic
input voltage regulation, inverter output measurement, and data recording for
each inverter. There are numerous test stations throughout the factory that
provide the manufacturing teams with a time efficient 100% test process.
Design challenges
Key challenges in designing a high voltage inverter include transformer design,
ground-plane integrity, and high voltage clearance. Transformer challenges
focused on meeting the 1800 V rms requirement, passing 2000 VDC Hipot (high
potential) testing and achieving the correct leakage inductance, which
determines the inverter oscillation frequency (55 KHz). The number of
transformer secondary sections, how they are wound, and the physical
dimensions all contribute to these challenges.
PCB layout for high voltage inverters involves maintaining the clearances
between high voltage traces and components required to insure against any high
voltage arcing across the PCB surface. Another design and layout challenges is
maintaining proper ground plane integrity is also key to providing regulated
current control over the expected input voltage and output load variations.