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DC-AC INVERTER UNIT
DC/ AC Inverter Unit for Liquid Crystal Display (LCD) Panel
1. Introduction
Liquid crystal display (LCD) panels are used in various applications ranging from smaller portable electronic
equipment to larger fixed location units. Applications such as the display device for laptop PCs, word processors,
arcade game machines, pinball machines, video cameras, automobile navigation systems, and industrial machines.
The LCD panel itself cannot emit light. Therefore, a backlight system that supplies the light from behind is normally
required. The backlight system consists of a light emitting device that produces light, a conductor panel that
distributes the light to the entire LCD surface uniformly, and a power supply that drives the light emitting device.
Currently, the most commonly used light emitting device is a fluorescent tube called a cold cathode tube or CCFL.
The CCFL is called a cold cathode tube because even though the principle of illumination is the same as that of the
hot cathode tubes used by indoor lamps, this lamp does not require preheating of the filament. Also, the electrodes at
the end of the bulb stay at a low temperature while emitting light. A special power supply, inverter, which generates
approximately 1000 to 1500V AC is required to drive a CCFL. This inverter is a small power supply used to make this
CCFL illuminate, and is one of the important functional parts of a complete LCD display.
As LCD panels are used in various markets and applications, requirements for the backlight system have become
diversified as shown in table 1. In particular, the European computer display market is evolving from CRT displays to
the LCD displays, this is partly due to strict EMI emission regulations.
DC/AC Inverter characteristic applied to various
DC/AC Inverter characteristic
Market
Downsizing Efficiency Life Wide Dimming
View Finder ooo ooo o
Video Monitor oo ooo oo
Audio & Visual
LCD TV oo ooo oo
LCD Monitor oo oo ooo oo
Automotive Navigation oo oo ooo oo
Note PC oo ooo ooo oo
Computer
PCS ooo ooo oo oo
Communication Pager ooo oo oo
Game oo ooo oo
Others
Monitor oo oo ooo
Table 1
The balance of this application note will discuss the basic operation, features, and design precautions of the most
commonly used inverters. We will also discuss inverter requirements and selection methods for those planning to use
LCD panels.
In order to select the proper inverter unit, it is necessary to understand the features of the CCFL that is the load of
the inverter. For those users who use LCD panels for laptop PCs and first-time users of backlighting systems, the
fundamental features and basic operating principles of the CCFL and the inverter resonant push-pull circuit are
explained.
APPLICATION NOTE
PAGE 1
DC-AC INVERTER UNIT
DC/ AC Inverter Unit for Liquid Crystal Display (LCD) Panel
2.0 Characteristics of the cold cathode tube
The brightness and the electrical characteristics of the CCFL change depending on the ambient temperature. Specific
characteristics such as the length and diameter of the CCFL as well as the type and pressure of the gas, will also
affect these electrical characteristics. Therefore, it is necessary to check the characteristics of the specific CCFL that
you plan to use.
Discharge in the cold cathode tube
In a CCFL, the discharge starts when electrons and positive ions, which are accelerated by the high electric field,
collide with the surface of the cathode such that secondary electrons are emitted. After the initial discharge starts a
lower voltage is required to maintain the discharge. The CCFL emits ultraviolet light caused by the ejection of the
secondary electrons. This ultraviolet light strikes the fluorescent material painted on the tube surface and emits
visible light. Depending on the size of the LCD panel the typical CCFL diameter is 2 to 3mm and lengths vary from
about 50 to 280mm.
Discharge startup time and the tube voltage
The discharge starts when a high voltage of about 1000Vrms is applied between both electrodes of the CCFL. This
voltage is called the discharge starting voltage. (It is also sometimes referred to as the startup voltage or initial
voltage.)
When current flows through the CCFL, the impedance of the tube decreases and the voltage between the electrodes
of the CCFL drops rapidly. When current flows to a certain level, decline of the voltage stops and the CCFL shows an
almost constant voltage characteristic as shown in Figure 1. The voltage at this time is called the CCFL voltage and it
is approximately 300 to 700Vrms, depending on the type of the CCFL. Though this characteristic resembles that of
the zener diode, the CCFL has a negative constant voltage characteristic where the voltage drops when current
increases, whereas the voltage in a zener diode rises when current increases, giving it a positive constant voltage
characteristic.
CCFL Voltage vs. Current
Voltage (V)
Current (I)
Figure 1
In general, the starting voltage and the CCFL voltage tend to be higher under the following conditions:
zWhen the ambient temperature is low.
zWhen the diameter of the cold cathode tube is small.
zWhen the length of the CCFL is long.
APPLICATION NOTE
PAGE 2
DC-AC INVERTER UNIT
DC/ AC Inverter Unit for Liquid Crystal Display (LCD) Panel
In order to ensure that the CCFL starts, a starting voltage of 1000 to 1500Vrms, which is higher than the operating
voltage of the CCFL, is required as the inverter generates the open voltage (a voltage that the inverter generates
before the discharge starts).
CCFL current and brightness
In order to maintain the discharge after starting, it is necessary to keep current flowing. Even though the brightness
increases by increasing current to the CCFL, too much current may damage the electrodes and lead to a shorter
lifetime. While 2 to 7ma is commonly used for each CCFL, it is important to control the current at an appropriate
level. Reference should be made to the brightness verses rated current listed in the specification sheet of the CCFL.
Figure 2 is an example of CCFL brightness verses starting voltage and temperature. When the ambient temperature
is lower as shown in Figure 2, the brightness decreases as opposed to the rising starting voltage.
Lamp brightness and starting voltage vs. temperature
150
Brightness
100
Starting Voltage
(%)
50
0
0 20 40 60 80
Temperature(C)
Figure 2
Frequency
Generally, CCFL's are lit by alternating current (AC). This is because when direct current (DC) is used, the mercury
ions in the cold cathode tube are shifted, the intensity of the tube becomes unbalanced (catapholesis phenomenon),
and the life of the CCFL will be shorter. The AC frequency is typically 30 to 70 kHz. The higher the frequency, the
greater the light output. Due to current leakage through the wiring capacitance between the inverter and the CCFL,
light output to power efficiency may decrease. This is because not all of the output current from the inverter flows to
the CCFL. In addition, it is necessary to consider the interaction with the LCD refresh frequency to avoid flickers and
noise streaks that can occur from interference from the operating frequency of the LCD panel.
It is necessary to consult the CCFL specification regarding these and other characteristics. Other light characteristics
to consider are the color temperature and color spectrum of the individual lamp.
APPLICATION NOTE
PAGE 3
DC-AC INVERTER UNIT
DC/ AC Inverter Unit for Liquid Crystal Display (LCD) Panel
2.1 Fundamental operation
In order to select the appropriate inverter it is necessary to understand the basic theory of operation. When the
inverter is considered as a black box, there are seven parameters that can be given from outside as shown in Figure
3:
Input voltage VIN Input voltage to the inverter.
Input current IIN Input current to the inverter.
Output voltage VOUT CCFL voltage between the electrodes after the discharge has started.
Open voltage VOPEN Voltage necessary to start the discharge in the CCFL.
Equivalent load resistance RL Equivalent resistance obtained by dividing the CCFL voltage by the CCFL current.
Output current IOUT CCFL Current
Oscillator frequency f Frequency used when the CCFL is driven by alternating current.
Outline of parameters
I in
RL
V in V out, V open
f
(Lamp)
I out
Figure 3
DC/AC Inverter basic circuit
Figure 4
APPLICATION NOTE
PAGE 4
DC-AC INVERTER UNIT
DC/ AC Inverter Unit for Liquid Crystal Display (LCD) Panel
Figure 4 shows the basic inverter circuit. (Push-pull voltage resonant circuit) This circuit is suitable for CCFL's. This
circuit can generate a symmetric sine wave with a relatively simple design. The open circuit voltage necessary for
starting the CCFL is shown in the following formula when the primary and secondary winding ratio of the ideal
transformer is N and the ON voltage of the switching transistor Q1 or Q2 is VCESAT.
Open voltage (VOPEN) = 1.11 x N x (VIN - VCESAT)
Since there is no feedback from the output, the output voltage increases proportionally to the input voltage. The
relationship between the output voltage and the CCFL voltage and CCFL current are explained using the equivalent
circuit of the secondary of the transformer as shown in Figure 5.
Capacitor Co is inserted in series with the CCFL in this circuit. This capacitor Co allows the output voltage from the
transformer to be applied directly to both electrodes of the CCFL before the start of the discharge (when the
impedance of the CCFL is infinity and there is no apparent load).
Equivalent circuit of output
@
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