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TL494 Inverter CCFL LCD SchematicJPG




TL494 Inverter CCFL LCD SchematicJPG


TL494 Inverter CCFL LCD SchematicJPG




The TL494 is a pulse-width-modulation (PWM) control circuit that can be used to design various types of switching power supplies, such as inverters, converters, regulators, and chargers. One of the applications of the TL494 is to drive cold cathode fluorescent lamps (CCFLs) that are used as backlight sources for liquid crystal displays (LCDs). CCFLs require high voltage and high frequency alternating current (AC) to operate, which can be generated by a TL494-based inverter circuit. In this article, we will explain the basic principles and components of a TL494 inverter CCFL LCD schematic.


Principles of a TL494 inverter CCFL LCD schematic




A TL494 inverter CCFL LCD schematic consists of four main parts: a DC input source, a TL494 PWM controller, a transformer, and a CCFL driver. The following diagram shows a simplified schematic of a TL494 inverter CCFL LCD circuit:




TL494 Inverter CCFL LCD SchematicJPG




The DC input source provides a stable and regulated DC voltage to the circuit, which can be derived from a battery, an AC adapter, or another power supply. The TL494 PWM controller is the core of the circuit, which generates two complementary PWM signals with adjustable frequency and duty cycle. These PWM signals are fed to two MOSFETs that act as switches to control the current flow through the primary winding of the transformer. The transformer is used to step up the voltage from the DC input source to a high AC voltage suitable for driving the CCFLs. The transformer also provides electrical isolation between the input and output sides of the circuit. The CCFL driver is composed of two diodes, two capacitors, and two resistors that form a voltage doubler circuit. The voltage doubler circuit converts the high AC voltage from the secondary winding of the transformer to an even higher DC voltage that can ignite and sustain the CCFLs. The CCFLs are connected in series across the output terminals of the voltage doubler circuit.


Components of a TL494 inverter CCFL LCD schematic




The components of a TL494 inverter CCFL LCD schematic can vary depending on the design specifications and requirements, such as the input voltage, output power, switching frequency, efficiency, and protection features. However, some of the common components and their functions are as follows:



  • TL494 PWM controller: The TL494 is an integrated circuit that contains two error amplifiers, an oscillator, a dead-time control circuit, a pulse-steering flip-flop, a 5V reference regulator, and an output stage with two totem-pole drivers. The TL494 can be configured to operate in various modes, such as single-ended or push-pull, fixed or variable frequency, and duty cycle control or pulse width modulation. The TL494 has 16 pins that are connected to external components for setting the operating parameters and feedback signals. The following table summarizes the pin functions and connections of the TL494:






Pin number


Pin name


Pin function


Pin connection




1


C1


Error amplifier 1 compensation pin


A capacitor is connected between this pin and ground to set the gain and bandwidth of error amplifier 1




2


Rt


Oscillator timing resistor pin


A resistor is connected between this pin and Ct pin to set the oscillator frequency




3


Ct


Oscillator timing capacitor pin


A capacitor is connected between this pin and ground to set the oscillator frequency and duty cycle range




4


C2


Error amplifier 2 compensation pin


A capacitor is connected between this pin and ground to set the gain and bandwidth of error amplifier 2




5


Inv. input 2


Error amplifier 2 inverting input pin


A feedback signal is applied to this pin to control the output duty cycle




6


Non-inv. input 2


Error amplifier 2 non-inverting input pin


A reference voltage is applied to this pin to set the output duty cycle




7


Dead time control


Dead time control pin


A resistor or a potentiometer is connected between this pin and ground to set the dead time between the two output signals




8


Vcc


Supply voltage pin


A DC voltage between 7V and 40V is applied to this pin to power the TL494




9


Output 2


Output driver 2 pin


This pin delivers a PWM signal to drive the MOSFET connected to the upper half of the transformer primary winding




10


Output 1


Output driver 1 pin


This pin delivers a PWM signal to drive the MOSFET connected to the lower half of the transformer primary winding




11


Inv. input 1


Error amplifier 1 inverting input pin


A feedback signal is applied to this pin to control the output duty cycle in single-ended mode or the output voltage in push-pull mode




12


Non-inv. input 1


Error amplifier 1 non-inverting input pin


A reference voltage is applied to this pin to set the output duty cycle in single-ended mode or the output voltage in push-pull mode




13


GND


Ground pin


This pin is connected to the common ground of the circuit<


14 E Pulse-steering flip-flop enable pin This pin is used to enable or disable the pulse-steering flip-flop that determines the output mode (single-ended or push-pull) <


15 Vref Reference voltage output pin This pin provides a regulated 5V output that can be used as a reference voltage for the error amplifiers and other circuits <


  • 16 C Pulse-steering flip-flop control pin This pin is used to select the output mode (single-ended or push-pull) by applying a logic level (high or low) to it MOSFETs: The MOSFETs are used as switches to control the current flow through the transformer primary winding. The MOSFETs should have low on-resistance, high breakdown voltage, fast switching speed, and adequate heat dissipation. The MOSFETs are driven by the PWM signals from the TL494 output drivers, which are amplified by a gate driver circuit if necessary. The gate driver circuit can be composed of transistors, resistors, diodes, and capacitors that provide sufficient gate voltage and current for the MOSFETs. The gate driver circuit also provides protection features such as overcurrent, overvoltage, and short-circuit protection.

  • Transformer: The transformer is used to step up the voltage from the DC input source to a high AC voltage suitable for driving the CCFLs. The transformer should have a high turns ratio, low leakage inductance, low core loss, and high insulation. The transformer can be designed using various types of cores, such as ferrite, iron, or toroidal cores. The transformer can also be customized according to the desired output power, frequency, and waveform. The transformer should be matched with the CCFLs in terms of impedance, current, and voltage.CCFL driver: The CCFL driver is composed of two diodes, two capacitors, and two resistors that form a voltage doubler circuit. The voltage doubler circuit converts the high AC voltage from the secondary winding of the transformer to an even higher DC voltage that can ignite and sustain the CCFLs. The CCFL driver should have low power loss, high efficiency, and stable output voltage. The CCFL driver should also provide protection features such as overvoltage, overcurrent, and open-circuit protection.



  • CCFLs: The CCFLs are cold cathode fluorescent lamps that are used as backlight sources for liquid crystal displays (LCDs). CCFLs consist of glass tubes filled with mercury vapor and electrodes at both ends. When a high voltage is applied to the electrodes, the mercury vapor emits ultraviolet (UV) light, which excites a phosphor coating on the inner wall of the tube. The phosphor coating then emits visible light of different colors depending on its composition. CCFLs have advantages such as high brightness, long lifespan, low power consumption, and wide color gamut. However, CCFLs also have disadvantages such as high voltage requirement, mercury pollution, and flickering.



Conclusion




In this article, we have explained the basic principles and components of a TL494 inverter CCFL LCD schematic. A TL494 inverter CCFL LCD schematic is a switching power supply circuit that can generate a high AC voltage to drive CCFLs for LCD backlighting. The circuit co


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