Revealed: LCD color TV high-voltage board circuit (inverter)
composition
The high-voltage board circuit (inverter) is a DC-AC (direct
current-alternating current) converter, and its working process is the inversion
process of the switching power supply. The function of the switching power
supply is to convert the AC voltage of the mains power grid into a stable Vcc
voltage (12V or 24V), while the high-voltage board circuit is just the opposite.
It converts the Vcc voltage (12V or 24V) output by the switching power supply
into a high frequency ( 40~80kHz) high voltage (600~800V) alternating
current.
There are many types of high-voltage board circuits. Depending on the
driving circuit, there are mainly the following composition schemes.
1. PWM control chip + Royer structure drive circuit composition scheme
1. The basic structural form of the PWM control chip + Royer structure
drive circuit scheme
As can be seen from the figure, the high-voltage board circuit is mainly
composed of drive control circuit (oscillator, modulator), DC conversion
circuit, Royer structure drive circuit, voltage and current detection circuit,
CCFL, etc. In actual high-voltage boards, oscillators, modulators, and
protection circuits are often integrated together to form a small integrated
circuit, generally called a PWM control chip.
ON/OFF in Figure 1 is the oscillator start/stop control signal input
terminal. This control signal comes from the motherboard microcontroller (MCU).
When the LCD color TV changes from standby state to normal operating state, the
MCU sends a start-up signal (high/low level change signal) to the oscillator.
After receiving the signal, the oscillator starts Working, an oscillation signal
with a frequency of 40~80kHz is generated and sent to the modulator. After
modulation inside the modulator with the PWM brightness adjustment signal sent
from the MCU part, the PWM excitation pulse signal is output and sent to the DC
conversion circuit, causing the DC conversion circuit to generate The
controllable DC voltage supplies power to the Royer structure drive circuit
power tube. The power tube, peripheral capacitor c1 and transformer winding L1
(equivalent to the inductor) form a self-excited oscillation circuit. The
generated oscillation signal is boosted and coupled by the power amplification
and step-up transformer to output high-frequency AC high voltage to light up the
backlight tube.
Figure 1 The basic structural form of the PWM control chip + Royer
structure drive circuit scheme.
In order to protect the lamp, over-current and over-voltage protection
circuits need to be set up. The overcurrent protection detection signal is
obtained from the sampling resistor R connected in series on the backlight tube
and sent to the drive control chip. The overvoltage protection detection signal
is obtained from L3 and sent to the drive control chip. When the output voltage
and backlight operating current are abnormal, the drive control chip controls
the modulator to stop output, thus playing a protective role.
When adjusting the brightness, the brightness control signal is added to
the drive control chip. By changing the duty cycle of the PWM pulse output by
the drive control chip, the DC voltage output by the DC converter is changed,
which also changes the voltage applied to the drive output tube. The magnitude
of the voltage changes the oscillation amplitude of the self-oscillation,
thereby changing the signal amplitude output by the step-up transformer and the
high-voltage amplitude at both ends of the CCFL, thereby achieving the purpose
of adjusting the brightness.
This circuit can only drive one backlight tube. Since backlight tubes
cannot be used in parallel or series, if you need to drive multiple backlight
tubes, you must have multiple corresponding boost transformer output circuits
and matching excitation circuits. to drive.
2. Actual circuit analysis
In high-voltage board circuits that use PWM control chips + Royer structure
drive circuits, PWM control ICs mainly use TL1451, BA9741, BA9743, SP9741,
BI3101, BI3102, TL494, KA7500, etc. The following uses the TL1451+Royer
structure driver circuit high-voltage board circuit as
TL1451 is a PWM control chip that is widely used in switching power
supplies and inverter circuits. The chip consists of a reference power supply,
oscillator, error amplifier, timer and PWM comparator. TL1451 can be used to
form various switching power supplies. and control system, not only can the
switching power supply and control system be simplified, easy to repair, and
cost reduced, but more importantly, it can reduce the failure rate of the system
and improve the reliability of system equipment operation.
TL1451 is a dual-channel drive control circuit that can output two PWM
control pulses and control two drive circuits respectively. Each drive circuit
can drive two CCFLs. TL1451 adapts to a wide power supply voltage range and can
work on a single power supply of 3.6~40V. It has short circuit and low voltage
malfunction protection circuits. The internal circuit block diagram of TL1451 is
shown in Figure 3, and the pin functions are shown in Table 1. In addition,
chips that are basically the same as the internal circuit and pin functions of
TL1451 include BA9741, SP9741, etc.
Table 1TL1451 pin functions
(1) Control circuit
The control circuit consists of PWM control chip U201 (TL1451) and its
peripheral components.
When the lamp needs to be lit, the ON/OFF signal output by the
microcontroller is high level, and the control transistors Q201 and Q202 are
turned on. Therefore, the 12V DC voltage generated by the switching power supply
is added to U201 (TL1451) through the turned-on Q202. ) of the power supply
terminal 9 pin. After the TL1451 is powered, its internal reference voltage
source works first and outputs a 2.5V reference voltage. This reference voltage
not only supplies the TL1451 on-chip circuit, but also outputs through pin 16 to
supply the external circuit of the chip as a reference voltage. . Then, TL1451
starts the internal oscillation circuit and starts working. The oscillation
frequency is determined by the size of the external timing resistor R204 and
timing capacitor C208 connected to pins 1 and 2. After the oscillation circuit
works, an oscillation pulse is generated, which is added to PWM comparator 1 and
PWM comparator 2. After transformation and shaping, the PWM pulse is output from
pins 7 and 10 and goes to the two DC-DC conversion circuits.
(2) DC conversion circuit
There are two DC conversion circuits, which are composed of Q205, Q207,
Q203, D201, L201 and Q206, Q208, Q204, D202 and L202 respectively. Its function
is to convert the input 12V DC voltage into a controllable DC voltage for power
output. tubes (Q209, Q210 and Q211, Q212). Since the working principles of the
two channels are the same, only the working conditions of one of them (the one
output by pin 10 of TL1451) will be analyzed below.
The PWM excitation pulse output by pin 10 of U201 (TL1451) is push-pull
amplified by the totem pole circuit composed of Q205 and Q207, coupled by R216
and C211, and added to the gate of the P-channel field effect switch Q203, so
that the switch Q203 works in switch status. When Q203 is turned on, the 12v
voltage is added to the collectors of power output tubes Q209 and Q210
respectively through the S and D poles of the field effect transistor Q203, the
inductor L201, and the 4~5 and 4~2 windings of the step-up transformer PT201.
Q210 supplies power; during the cut-off period of Q203, since the current in the
inductor cannot mutate, L201 generates right positive and left negative pulse
voltages through self-inductance. Therefore, the positive voltage at the right
end of L201 forms a discharge circuit through the 4~5 and 4~2 windings of PT201,
the ce junction of the output tube Q209 or Q210, and the left end of the
freewheeling diode D201 and L201, releasing energy and continuing to supply
power to the output tubes Q209 and Q210. .
As can be seen from the above analysis, this is a switching step-down DC-DC
converter.
(3) Drive circuit
The drive circuit (two circuits in total) is used to generate AC high
voltage that meets the requirements and drive the CCFL to work. It is mainly
composed of drive output tubes (Q209, Q210 and Q211, Q212), step-up transformers
(PT201 and PT202), etc. The following is the following: All the way (Q209, Q210,
RT201) will be introduced as an example.
As can be seen from Figure 2, the circuit composed of Q209, Q210, RT201 and
other components is a typical Royer structure drive circuit, that is, a
self-excited multi-resonant oscillator. The circuit relies on the correct
connection of the primary side of the transformer and the same terminal of the
feedback winding to satisfy the phase condition of self-excited oscillation,
that is, to satisfy the positive feedback condition. To satisfy the amplitude
condition, firstly, the circuit parameters should be reasonably selected so that
the amplifier can establish a suitable static operating point. Secondly, the
number of turns of the feedback winding, or the degree of coupling between it
and the primary winding, should be changed to obtain a sufficiently strong
feedback amount. The amplitude stabilization effect is achieved by utilizing the
nonlinearity of the transistor.
The sine wave voltage generated by the self-excited oscillation circuit
induces high voltage through transformer PT201, which supplies power to CCFL
through C215, C216 and connector CN202. Because the oscillation waveform of the
transformer-coupled self-oscillation circuit is a standard sine wave, which is
exactly suitable for the power supply requirements of CCFL, it can simplify the
design of the final circuit.
(4) Brightness adjustment circuit
Pins 4 and 13 of U201 (TL1451) are the brightness control terminals. Since
the control processes of these two control signals are the same, only the
brightness control signal of pin 13 is used for analysis below.
When the brightness needs to be adjusted, the DIM control pulse output by
the microcontroller changes → the DC voltage generated after low-pass filtering
by R201 and C203 changes → the voltage of pin 13 of TL1451 changes → the duty of
the output pulse of pin 10 of TL1451 The ratio changes → the base voltage of
Q205 and Q207 changes → the gate voltage of Q203 changes → the supply voltage
output by Q203 changes → the amplitude of oscillation of Q209 and Q210 changes →
the high voltage output by PT201 changes → the voltage across CCFL changes to
achieve the purpose of adjusting brightness.
(5) Protection circuit
① Overvoltage protection circuit: When the voltage output by Q203 is too
high due to some unexpected reasons, the voltage regulator D203 breaks down, and
the voltage is divided by R220 and R222, causing the voltage of pin 11 of TL1451
to rise, and pin 10 is controlled through the internal circuit Stop outputting
PWM pulses to achieve protection purposes.
In the same way, when the voltage output by Q204 is too high due to some
unexpected reasons, the voltage regulator D204 breaks down, and the voltage is
divided by R221 and R223, causing the voltage of pin 6 of TL1451 to rise, and
the internal circuit controls pin 7 to stop outputting PWM. pulse to achieve the
purpose of protection.
②Under-voltage protection circuit: When the system has just been powered on
or when the supply voltage of TL1451 is less than 3.6V due to accidental
reasons, its output drive transistor is likely to be damaged due to poor
conduction. Therefore, TL1451 has an under-voltage protection circuit (UVLO)
internally.
After the under-voltage protection circuit is activated, the PWM pulses
output by pins 7 and 10 will be cut off to achieve the purpose of
protection.
③ Over-current protection circuit: The over-current protection circuit is
used to protect the CCFL from aging or damage due to excessive current. The
following takes CN202 as an example. After the high voltage generated by PT201
passes through the CCFL connected to CN202, an AC voltage that changes with the
operating current will be generated at both ends of R236. The greater the
current, the higher the voltage at both ends of R236. This voltage is rectified
by D207, filtered by R240 and C221, and then added to pin 14 of TL1451; if the
operating current of CCFL is too large, the voltage of pin 14 will increase a
lot. When it reaches a certain value, through internal processing of TL1451, pin
10 will be controlled to stop outputting PWM pulses, thereby achieving the
purpose of protection.
④ Balance protection circuit: There is a voltage comparator inside pins 5
and 12 of TL1451. The voltage comparator has two non-inverting input terminals
and one inverting input terminal. The voltage at the inverting input terminal is
half of the reference voltage (2.5V) ( 1.25V), the two non-inverting input
terminals are connected to the output terminals of error amplifier 1 and error
amplifier 2 respectively. Therefore, the voltage comparator can detect the
output voltages of the two error amplifiers. As long as one of them is higher
than half of the reference voltage (1.25V), the output of the voltage comparator
is high level, and the output voltage triggers the timing loop. As a result, the
reference voltage charges capacitor C207 through pin 15. When the voltage of
C207 reaches a certain value, the internal trigger is set to control pins 7 and
10 to stop outputting PWM pulses, thereby protecting the subsequent circuits and
equipment.
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