The secrets of the LCD TV high voltage board circuit (inverter) structure

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) AC.

There are many types of high-voltage board circuits. Depending on the driving circuit, there are mainly the following configuration schemes.

1. "PWM control chip + Royer structure drive circuit" configuration scheme

1. The basic structure of the solution of "PWM control chip + Royer structure drive circuit"

Figure 1 shows the basic structure of the "PWM control chip + Royer structure drive circuit" solution.

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.

The ON/OFF in Figure 1 is the input terminal of the oscillator start/stop control signal. This control signal comes from the mainboard microcontroller (MCU). When the LCD color TV changes from standby mode to normal working mode, the MCU sends a start-up signal (high/low level change signal) to the oscillator. After receiving the signal, the oscillator starts to work and generates an oscillation signal with a frequency of 40~80kHz and sends it to the modulator. After being modulated with the PWM brightness adjustment signal sent by the MCU part inside the modulator, the PWM excitation pulse signal is output and sent to the DC conversion circuit, so that the DC conversion circuit generates a controllable DC voltage to power the power tube of the Royer structure drive circuit. The power tube and the peripheral capacitor c1 and the transformer winding L1 (equivalent to the inductor) form a self-excited oscillation circuit. The generated oscillation signal is coupled by power amplification and boost transformer boost, and high-frequency AC high voltage is output to light up the backlight tube.

Figure 1 shows the basic structure of the "PWM control chip + Royer structure drive circuit" solution.

In order to protect the lamp, it is necessary to set up overcurrent and overvoltage protection circuits. The overcurrent protection detection signal is obtained from the sampling resistor R connected in series to the backlight lamp and transmitted to the drive control chip. The overvoltage protection detection signal is obtained from L3 and also transmitted to the drive control chip. When the output voltage and the working current of the backlight lamp are abnormal, the drive control chip controls the modulator to stop output, thereby playing a protective role.

When adjusting the brightness, the brightness control signal is added to the driver control chip. By changing the duty cycle of the PWM pulse output by the driver control chip, the DC voltage output by the DC converter is changed, which changes the voltage applied to the driver output tube, that is, changes the oscillation amplitude of the self-excited 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 multiple backlight tubes need to be driven, they must be driven by corresponding multiple step-up transformer output circuits and corresponding excitation circuits.

2. Actual circuit analysis

In the high-voltage board circuit using "PWM control chip + Royer structure drive circuit", the PWM control IC mainly uses TL1451, BA9741, BA9743, SP9741, BI3101, BI3102, TL494, KA7500, etc. The following takes the "TL1451 + Royer structure drive circuit" high-voltage board circuit as an example for introduction, and the relevant circuit is shown in Figure 2.

Figure 2 "TL1451+Royer structure drive circuit" high voltage board circuit

TL1451 is a PWM control chip, which is widely used in switching power supplies and inverter circuits. The chip consists of a reference power supply, an oscillator, an error amplifier, a timer and a PWM comparator. TL1451 can be used to form various switching power supplies and control systems, which can not only simplify the switching power supply and control system, make it easier to maintain and reduce costs, but more importantly, it can reduce the failure rate of the system and improve the reliability of the system equipment operation.

TL1451 is a dual-channel drive control circuit that can output two PWM control pulses, which are controlled by two drive circuits respectively. Each drive circuit can drive two CCFLs to work. TL1451 adapts to a wide power supply voltage range and can work under 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, other chips with basically the same internal circuits and pin functions as TL1451 include BA9741, SP9741, etc.

Figure 3 TL1451 internal circuit diagram

Table 1 TL1451 pin functions

(1) Control circuit

The control circuit consists of the 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 the power supply terminal 9 of U201 (TL1451) through the turned-on Q202. After TL1451 is powered, its internal reference voltage source works first and outputs a 2.5V reference voltage. This reference voltage is not only supplied to the TL1451 internal circuit, but also output through pin 16 to supply the external circuit of the chip as a reference voltage. Then, TL1451 starts the internal oscillation circuit to start working, and the oscillation frequency is determined by the size of the timing resistor R204 and timing capacitor C208 connected to pins 1 and 2. After the oscillation circuit works, an oscillation pulse is generated and added to PWM comparator 1 and PWM comparator 2. After transformation and shaping, PWM pulses are output from pins 7 and 10 to go to 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, L202. Their function is to convert the input 12V DC voltage into a controllable DC voltage to supply power to the power output tubes (Q209, Q210 and Q211, Q212). Since the working principles of the two circuits are the same, the following only analyzes the working conditions of one circuit (the one outputted at pin 10 of TL1451).

The PWM excitation pulse output from the 10th pin of U201 (TL1451) is amplified by the push-pull circuit of the totem pole composed of Q205 and Q207, coupled by R216 and C211, and added to the gate of the P-channel field effect switch tube Q203, so that the switch tube Q203 works in the switching state. When Q203 is turned on, the 12v voltage is added to the collectors of the power output tubes Q209 and Q210 respectively through the S and D poles of the field effect tube Q203, the inductor L201, and the 4~5 and 4~2 windings of the step-up transformer PT201, to supply power to Q209 and Q210; during the cut-off period of Q203, since the current in the inductor cannot change suddenly, L201 generates a pulse voltage with positive right and negative left 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, the freewheeling diode D201, and the left end of L201, releasing energy and continuing to power the output tubes Q209 and Q210.

From the above analysis, it can be seen that this is a switching step-down DC-DC converter.

(3) Driving circuit

The driving circuit (two circuits in total) is used to generate AC high voltage that meets the requirements to drive the CCFL to work. It is mainly composed of driving output tubes (Q209, Q210 and Q211, Q212), step-up transformers (PT201 and PT202), etc. The following takes one of the circuits (Q209, Q210, RT201) as an example for introduction.

As can be seen from Figure 2, the circuit composed of components such as Q209, Q210, and RT201 is a typical Royer structure drive circuit, that is, a self-excited multi-resonant oscillator. The circuit relies on the correct connection of the transformer primary side and the same-name end of the feedback winding to meet the phase condition of self-excited oscillation, that is, to meet the positive feedback condition. The amplitude condition is met by firstly reasonably selecting the circuit parameters so that the amplifier can establish a suitable static operating point, and secondly by changing the number of turns of the feedback winding, or the degree of coupling between it and the primary winding, 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 is induced into high voltage by transformer PT201, and supplies power to CCFL through C215, C216 and connector CN202. Because the oscillation waveform of the transformer-coupled self-excited oscillation circuit is a standard sine wave, it just meets the power supply requirements of CCFL, so the design of the final circuit can be simplified.

(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, the following analysis will only take the brightness control signal of pin 13 as an example.

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 at pin 13 of TL1451 changes → the duty cycle of the output pulse at pin 10 of TL1451 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 the CCFL changes, thereby achieving the purpose of adjusting the brightness.

(5) Protection circuit

① Overvoltage protection circuit: When the voltage output by Q203 is too high due to some unexpected reasons, the Zener diode D203 breaks down, and the voltage is divided by R220 and R222, so that the voltage added to pin 11 of TL1451 rises, and the internal circuit controls pin 10 to stop outputting PWM pulses, thereby achieving the purpose of protection.

Similarly, when the voltage output by Q204 is too high due to some unexpected reasons, the Zener diode D204 breaks down, and the voltage is divided by R221 and R223, causing the voltage at pin 6 of TL1451 to rise. The internal circuit controls pin 7 to stop outputting PWM pulses, thereby achieving the purpose of protection.

② Undervoltage protection circuit: When the system is just powered on or the TL1451 supply voltage is less than 3.6V due to unexpected reasons, its output drive transistor is likely to be damaged due to poor conduction. Therefore, an undervoltage protection circuit (UVLO) is set inside the TL1451.

After the undervoltage protection circuit is activated, the PWM pulses output from pins 7 and 10 will be cut off, thereby achieving the purpose of protection.

③ Overcurrent protection circuit: The overcurrent protection circuit is used to protect CCFL from aging or damage due to excessive current. The following is an example of CN202. After the high voltage generated by PT201 passes through the CCFL connected to CN202, an AC voltage that varies with the working 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 the 14th foot of TL1451; if the working current of CCFL is too large, the voltage at the 14th foot will increase a lot. When it reaches a certain value, it will be controlled by the internal processing of TL1451 to stop outputting PWM pulses at the 10th foot, thereby achieving the purpose of protection.

④ Balance protection circuit: There is a voltage comparator inside the 5th and 12th pins of TL1451. The voltage comparator has two in-phase input terminals and one inverting input terminal. The voltage of the inverting input terminal is half (1.25V) of the reference voltage (2.5V). The two in-phase input terminals are connected to the output terminals of error amplifier 1 and error amplifier 2 respectively. Therefore, the voltage comparator can detect the size of the output voltage 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. The output voltage triggers the timing circuit, so that 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.

2. "PWM control chip + push-pull structure drive circuit" configuration scheme

1. The basic structure of the solution consisting of "PWM control chip + push-pull structure drive circuit"

The basic structure of the "PWM control chip + push-pull structure drive circuit" scheme is very simple, as shown in Figure 4. The push-pull driver only uses two N-channel power field effect transistors V1 and V2, and connects the neutral tap of the step-up transformer T to the positive power supply Vcc. The two power tubes V1 and V2 work alternately and output an AC voltage. Since the power transistors share a common ground, the drive control circuit is simple; in addition, since the transformer has a certain leakage inductance, it can limit the short-circuit current, thereby improving the reliability of the circuit.

For the push-pull structure driving circuit, the variation range of the DC power supply Vcc is required to be small, otherwise, the efficiency of the driving circuit will be reduced. Therefore, the push-pull structure is not suitable for notebook computers, but it is very ideal for LCD monitors and LCD color TVs because the inverter DC power supply voltage is usually stable within ±20%.

When the circuit is working, under the control of the PWM control chip, the two switch tubes V1 and V2 in the push-pull circuit are turned on alternately, and AC voltages with opposite phases are formed at both ends of the primary windings L1 and L2. Changing the duty cycle of the switch pulses input to V1 and V2 can change the on and off time of V1 and V2, thereby changing the energy storage of the transformer and changing the output voltage value. It should be noted that when V1 and V2 are turned on at the same time, it is equivalent to the primary winding of the transformer being short-circuited, so the two switch tubes should be avoided from being turned on at the same time.

Figure 4 Basic structure of the "PWM control chip + push-pull structure drive circuit" solution

2. Actual circuit analysis

In the high-voltage board circuit using "PWM control chip + push-pull structure drive circuit", the PWM control chip mainly uses OZ9RR, etc. The following analysis takes the "OZ9RR + push-pull structure drive circuit" high-voltage board circuit as an example, and the relevant circuit is shown in Figure 5.

Figure 5 "OZ9RR+ push-pull structure drive circuit" high voltage board circuit

OZ9RR is a backlight high-voltage inverter PWM control chip produced by OZMicro, with the following features: constant operating frequency, and the operating frequency can be synchronized by external signals; built-in synchronous PWM lamp brightness control circuit, wide brightness control range; built-in intelligent lamp ignition and normal working state control circuit; equipped with lamp open circuit and overvoltage protection function; can support multi-lamp mode operation. The internal circuit block diagram of OZ9RR is shown in Figure 6, and the pin functions are shown in Table 2.

Table 2 OZ9RR pin functions

Figure 6 OZ9RR internal circuit diagram

(1) Control circuit

The control circuit consists of the PWM control chip U1 (OZ9RR) and its peripheral components.

The Vdd voltage (5V) generated by the power supply circuit is added to the power supply terminal 6 pin of OZ9RR after current limiting by R5, providing OZ9RR with the voltage required for operation.

When the lamp needs to be lit, the EN signal of the high-voltage board input port (from the mainboard MCU) is at a low level (0~1V), which controls the N-channel field effect transistor Q1 to be cut off, and then controls pin 1 of OZ9RR to a high level (3~5V).

When OZ9RR receives power at pin 6 and a high-level signal at pin 1, the internal oscillation circuit starts to work, and its oscillation frequency is determined by the size of the timing capacitors C9 and C11 connected to pin 2. After the oscillation circuit works, it generates oscillation pulses, which are added to the internal logic control circuit and the drive circuit. After transformation and shaping, PWM pulses are output from pins 5 to 4 to drive the drive circuit to work.

(2) Driving circuit

The driving circuit is used to generate AC high voltage that meets the requirements to drive CCFL to work. The driving circuit consists of dual driving tubes U2, step-up transformer T1, etc. This is a push-pull circuit structure with zero voltage switching. When working, Vin (12V) output by the power supply circuit is added to the drain of the two field effect tubes V1 and V2 in U2 through the 2~1 winding and 2~5 winding of the step-up transformer T1; the driving pulses generated by the 5th and 4th pins of OZ9RR are added to the gates of V1 and V2 in U2 respectively. Under the action of the driving pulse, the two switch tubes V1 and V2 in U2 are alternately turned on, and symmetrical switch tube driving pulses are output. After being stepped up by the step-up transformer, a voltage and current similar to a sine wave are generated to ignite the backlight tube.

(3) Brightness adjustment circuit

Pin 7 of OZ9RR is a dual-function terminal for brightness control and voltage detection of step-up transformer. When brightness adjustment is required, the brightness control signal DIM generated by the microcontroller is divided by R1 and R2 and isolated by D1, and then added to pin 7 of OZ9RR. After being processed by the internal circuit, the duty cycle of the driving pulse output from pins 5-4 is controlled to achieve the purpose of brightness control.

The DIM input port of the high-voltage board inputs a continuously adjustable DC control voltage, and the control voltage range is 0.5~3.6V, 0.5V corresponds to the lowest brightness, and 3.6V corresponds to the highest brightness.

(4) Protection circuit

① Undervoltage protection circuit: Pin 6 of OZ9RR is a 5V power supply terminal. There is also an undervoltage protection circuit inside pin 6. When the power supply voltage is lower than 3.8V, the undervoltage protection circuit will be activated and OZ9RR will control pins 5~4 to stop outputting drive pulses.

②Soft start protection circuit: Pin 1 of OZ9RR is a multi-function pin. In addition to being used to introduce the EN control voltage, it is also connected to the soft start timing capacitor C5 to play the role of soft start timing. After OZ9RR works, the circuit inside pin 1 charges C5. As the voltage across C5 increases, the drive pulse output by OZ9RR controls the switch tube to provide more energy to the step-up transformer. The use of the soft start circuit can prevent excessive impact current from being generated when the backlight is initially working.

③ Current stabilization circuit: The current stabilization circuit is used to protect CCFL from aging or damage due to excessive current. R12 on the secondary side of the step-up transformer is an overcurrent detection resistor. The voltage across R12 changes with the operating current. The larger the current, the higher the voltage across R12. This voltage is filtered by C12 and added to the 8th foot of OZ9RR as the current detection terminal.

During the backlight tube ignition stage (start-up period), the high-voltage power supply needs to provide a higher frequency ignition voltage. Generally speaking, the ignition frequency is about 1.3 times the normal working frequency, which is determined by the external fixed capacitor on pin 2 of OZ9RR.

The ignition time of OZ9RR is set to 2 s. If the 8-pin lamp current detection terminal cannot detect the lamp current after 2 s, OZ9RR will stop working.

After the backlight tube is ignited, the tube enters the normal working stage. OZ9RR detects the tube current through pin 8 and stabilizes the tube current through the control circuit. The reference voltage of pin 8 is about 1.25V. The driving voltage frequency of the tube during normal working is also determined by the timing capacitor of pin 2.

In addition, if the operating current of CCFL is too large, the voltage of pin 8 of OZ9RR will increase a lot. When it reaches a certain value, the internal processing of OZ9RR will control pins 5~4 to stop outputting driving pulses, thus achieving the purpose of protection.

④ Overvoltage protection circuit: The overvoltage protection circuit in OZ9RR can prevent the secondary side of the lamp boost transformer from generating excessively high voltage under abnormal conditions and damaging the boost transformer. During the startup phase, the 7-pin voltage detection/brightness control terminal detects the secondary voltage of the boost transformer. When it reaches 3V, OZ9RR will no longer increase the output voltage and enter the stable output voltage stage.

⑤ Lamp open circuit protection circuit: If the lamp is in poor contact with the lamp holder, the lamp is removed, or the lamp is damaged, OZ9RR will automatically cut off the drive pulse output from pins 5 to 4 to achieve the purpose of protection.

3. "PWM control chip + full-bridge structure drive circuit" configuration scheme

1. The basic structure of the solution is "PWM control chip + full-bridge structure drive circuit"

The "PWM control chip + full-bridge structure drive circuit" configuration is most suitable for applications with a very wide DC power supply voltage, so almost all laptops use the full-bridge method. In laptops, the DC power supply of the inverter comes directly from the main DC power supply of the system, and its range is usually 7V (low battery voltage) ~ 21V (AC adapter). In addition, this configuration is also widely used in LCD color TVs and LCD monitors.

The full-bridge structure drive circuit is generally composed of four field effect tubes or four transistors. According to the types of field effect tubes or transistors, there are two main structural forms of this structure. One is to use four N-channel field effect tubes; the other is to use two N-channel field effect tubes and two P-channel field effect tubes.

(1) The full-bridge drive circuit uses four N-channel field effect transistors

The full-bridge drive circuit uses four N-channel field effect transistors as shown in Figure 7.

Figure 7 Full-bridge drive circuit using four N-channel field effect transistors

When the circuit is working, under the control of the drive control Ic, V1 and V4 are turned on at the same time, V2 and V3 are turned on at the same time, and when V1 and V4 are turned on, V2 and V3 are turned off. In other words, V1, V4 and V2, V3 are turned on alternately to form an AC voltage on the primary side of the transformer. By changing the duty cycle of the switching pulse, the on and off time of V1, V4 and V2, V3 can be changed, thereby changing the energy storage of the transformer and changing the output voltage value.

It should be noted that if the conduction time of V1, V4 is asymmetric with that of V2, V3, the AC voltage on the primary side of the transformer will contain a DC component, which will generate a large DC component on the secondary side of the transformer, causing magnetic circuit saturation. Therefore, the full-bridge circuit should avoid the generation of DC voltage components, and a capacitor can also be connected in series in the primary circuit to block the DC current.

(2) The full-bridge drive circuit uses two N-channel and two P-channel field effect transistors

The full-bridge drive circuit uses two N-channel and two P-channel field effect transistors as shown in Figure 8.

Figure 8 Full-bridge drive circuit using two N-channel and two P-channel field effect transistors

When the circuit is working, under the control of the drive control IC, V4 and V1 are turned on at the same time, V2 and V3 are turned on at the same time, and when V4 and V1 are turned on, V2 and V3 are cut off. In other words, V4, V1 and V2, V3 are turned on alternately, so that an AC voltage is formed on the primary side of the transformer.

In the "PWM control chip + full-bridge structure drive circuit" configuration scheme, PWM control chips commonly used are OZ960, OZ970, OZ9910, BIT3105, BIT3106, MPS1010B, MP1026, MP1029, MP1038, BD9883, BD9884, etc.

2. "OZ960+ full-bridge structure drive circuit" high-voltage board circuit

The high voltage board circuit composed of "OZ960 + full-bridge structure drive circuit" is shown in Figure 9.

Figure 9 "OZ960+ full-bridge structure drive circuit" high-voltage board circuit

OZ960 is a backlight high-voltage inverter PWM control chip with the following features: high efficiency, zero voltage switching; support for a wide input voltage range; constant operating frequency; wide dimming range; soft start function; built-in light start protection and overvoltage protection, etc. The internal circuit diagram of OZ960 is shown in Figure 10, and the pin functions are shown in Table 3.

Figure 10 OZ960 internal circuit diagram

Table 3 OZ960 pin functions

(1) Drive control circuit

The drive control circuit consists of U901 (OZ960) and its peripheral components.

The Vdd voltage (usually 5V) generated by the switching power supply is limited by R904 and added to the power supply terminal 5 of OZ960 to provide the voltage required for OZ960 to work.

When the LCD color TV needs to be lit, the ON/OFF signal output by the microcontroller is high level, and through R903, the voltage added to pin 3 of OZ960 is high level (voltage above 1.5V is high level).

OZ960 gets power supply at pin 5 and high level signal at pin 3, then the internal oscillator circuit starts to work, and the oscillation frequency is determined by the value of timing resistor R908 and timing capacitor C912 connected to pins 17 and 18. After the oscillator circuit works, it generates oscillation pulses, which are added to the internal zero voltage switching phase shift control circuit and drive circuit, and after transformation and shaping, PWM pulses are output from pins 19, 20, 12, and 11 to the full bridge drive circuit.

(2) Full-bridge drive circuit

The full-bridge drive circuit is used to generate AC high voltage that meets the requirements to drive the CCFL to work, and is composed of Q904, Q905, Q906, Q907, T901, etc. This is a full-bridge circuit structure with zero voltage switching. Vcc (usually 12V) voltage is added to the source of Q904 and Q906, and the source of Q905 and Q907 is grounded. Under the control of the drive pulse output by OZ960 (whose waveform is shown in Figure 11), Q904 and Q907 are turned on at the same time, Q905 and Q906 are turned on at the same time, and when Q904 and Q907 are turned on, Q905 and Q906 are turned off. In other words, Q904, Q907 and Q905, Q906 are turned on alternately, and symmetrical switch tube drive pulses are output. Through the resonant circuit composed of C915, C916, C917, C918, step-up transformer T901 and backlight tube, a voltage and current approximately in the shape of a sine wave are generated to ignite the backlight tube.

Figure 11 Drive pulse waveform output by OZ960

(3) Brightness adjustment circuit

Pin 14 of OZ960 is the brightness control terminal. When the brightness needs to be adjusted, the brightness control voltage generated by the microcontroller is divided by R906 and R907 and added to pin 14. After being processed by the internal circuit, the duty cycle of the drive pulse output by OZ960 is controlled to achieve the purpose of brightness control.

(4) Protection circuit

①Soft start protection circuit: The 4-pin soft start terminal of OZ960 is connected to an external soft start capacitor C904, which plays the role of soft start timing. After OZ960 works, the circuit inside the 4-pin charges C904. As the voltage across C904 increases, the drive pulse output by OZ960 controls the energy provided by the drive tube to the high-voltage transformer and gradually increases. The soft start circuit can prevent excessive impact current from being generated when the backlight is initially working.

② Overvoltage protection circuit: The overvoltage protection circuit in OZ960 can prevent the secondary side of the high-voltage transformer of the lamp from generating excessively high voltage under abnormal conditions, thereby damaging the high-voltage transformer and the lamp. In the circuit, the high voltage generated by the secondary side of T901 is divided by R930, R932 and R931, R933, and then added to pin 2 as a sampling voltage through D909 and D910. In the startup phase, pin 2 detects the secondary voltage of the high-voltage transformer. When the voltage of pin 2 reaches 2V, OZ960 will no longer increase the output voltage, but enter the stage of stabilizing the output voltage.

③ Overcurrent protection circuit: The overcurrent protection circuit is used to protect CCFL from aging or damage due to excessive current.

In the circuit, R936 and R937 are overcurrent detection resistors. The voltage across R936 and R937 changes with the working current. The larger the current, the higher the voltage across R936 and R937. This voltage is added to the 9th pin of OZ960 through D912 and D914 as the current detection terminal, and the lamp current is stabilized through the internal control circuit. If the working current of CCFL is too large, the voltage of the 9th pin will increase a lot. When the voltage of the 9th pin reaches 1.25V, it will be controlled by the internal processing of OZ960 to stop outputting the driving pulse, so as to achieve the purpose of protection.

3. "BIT3105+ full-bridge structure drive circuit" high-voltage board circuit

The high voltage board circuit composed of "BIT3105 + full bridge structure drive circuit" is shown in Figure 12. BIT3105 is a PWM control chip, and its internal circuit block diagram is shown in Figure 13. The pin functions are shown in Table 4.

Figure 12 "BIT3105+ full-bridge structure drive circuit" high-voltage board circuit

Figure 13 BIT3105 internal circuit block diagram

Table 4 BIT3105 pin functions

(1) Drive control circuit

The drive control circuit consists of U1 (BIT3105) and its peripheral components.

When the LCD backlight needs to be turned on, the ON/OFF signal output by the microcontroller is high level, which is added to the b pole of Q2 through R25, controlling Q2 to turn on, and its collector outputs low level, which in turn turns Q1 on; thus, the 5V voltage is added to the 13th and 18th pins of BIT3105 through the turned-on Q1, and the internal oscillation circuit of BIT3105 starts to work. The oscillation frequency is determined by the timing resistor and timing capacitor value connected to the 5th and 7th pins. After the oscillation circuit works, an oscillation pulse is generated, which is added to the internal drive circuit after frequency division, and is output from the 9th to 12th pins after transformation and shaping to the full-bridge drive circuit.

(2) Full-bridge drive circuit

The full-bridge drive circuit is used to generate AC high voltage that meets the requirements to drive the CCFL to work. It is composed of components such as U2, U3, T1, T2, etc., among which T1 and T2 are high-voltage transformers, U2 and U3 are composite field-effect transistors, containing two MOS tubes (one P-channel MOS tube and one N-channel MOS tube).

The oscillation pulse generated by the internal oscillation circuit of BIT3105 outputs a P-channel MOS drive signal from pins 11 to 12 of BIT3105, which is sent to pin 4 of the drive circuit U2 and U3. After being amplified by the internal PMOS tubes of U2 and U3, it is output from pins 5 and 6 of U2 and U3. On the other hand, an N-channel MOS drive signal is output from pins 9 to 10 of BIT3105, which is sent to pin 2 of the drive circuit U2 and U3. After being amplified by the internal NMOS tubes of U2 and U3, it is output from pins 7 to 8 of U2 and U3.

Driven by the driving pulse, the PMOS and NMOS tubes inside U2 and U3 are alternately turned on and off, and pulse signals are output from pins 5 to 8 of U2 and U3. They are added to the primary windings of T1 and T2 through C14 to C16. After being transformed by T1 and T2, high voltage is output at the secondary windings of T1 and T3 transformers.

The high voltage output from the secondary side of transformer T1 enters lamp 1 and lamp 2 through CN1 and CN2, lighting the lamps. In addition, the current output from pin 2 of CN2 forms a loop through R21 and R22 to the ground, and generates a feedback voltage at the upper end of R21 and R22, which is fed back to the inverting input terminal of the internal amplifier of pin 1 of BIT3105 through D6 and R7, automatically stabilizing the working state of the internal amplifier of BIT3105.

The high voltage output from the secondary of transformer T2 enters lamp 3 and lamp 4 through CN3 and CN4, lighting the lamps. In addition, the current output from pin 2 of CN4 forms a loop through R19 to the ground, and generates a feedback voltage at the upper end of R19, which is fed back to the inverting input terminal of the internal amplifier at pin 1 of BIT3105 through D5 and R7, automatically stabilizing the working state of the internal amplifier of BIT3105.

(3) Brightness adjustment circuit

R1, R2, R3, C10 and the internal circuit of BIT3105 together form the lamp brightness control circuit. When the brightness of the lamp needs to be controlled, the PWM control voltage ADJ is sent from the mainboard, divided by R1 and R2, filtered by C10 and limited by R3, and then added to the 1st pin of BIT3105. After being processed by the internal circuit of BIT3105, the duty cycle of the driving pulse output by BIT3105 is controlled to achieve the purpose of brightness control.

(4) Current protection circuit

The current detection circuit of lamp 1 and lamp 2 on CN1 and CN2 is composed of D1, R23, C18, R17 and the 4-pin internal circuit of BIT3105.

When lamp 1 and lamp 2 are lit, a detection voltage will be formed at the upper end of R23, and this voltage will be sent to pin 4 of BIT3105 through R17. When the current of lamp 1 or lamp 2 decreases due to some reason, the voltage obtained at the upper end of R23 will drop, causing the voltage at pin 4 of BIT3105 to drop. When it drops below 0.3V, pins 9 to 12 stop outputting drive pulses, and the circuit is in protection state.

The current detection circuit of lamp 3 and lamp 4 on CN3 and CN4 is composed of D2, R14, R15, R16, Q3, Q4 and the internal circuit of pin 4 of BIT3105.

When lamps 3 and 4 are lit, a detection voltage will be formed at the upper end of R14. When the current of lamps 3 and 4 decreases due to some reason, the voltage obtained at the upper end of R14 drops, the gate voltage of Q3 drops, and the drain voltage rises, thereby controlling the leakage voltage of Q4 to drop and sending it to pin 4 of BIT3105. When the voltage at pin 4 drops below 0.3V, pins 9 to 12 stop outputting drive pulses, and the circuit is in a protection state.

4. "BIT3106 + full-bridge structure drive circuit" high-voltage board circuit

The high voltage board circuit composed of "BIT3106 + full bridge structure drive circuit" is shown in Figure 14. BIT3106 is a PWM control chip, and its internal circuit is equivalent to two BIT3105 composites, as shown in Figure 15. The pin functions of BIT3106 are shown in Table 5.

Figure 14 "BIT3106 + full-bridge structure drive circuit" high-voltage board circuit

Figure 15 BIT3106 internal circuit block diagram

Table 5 BIT3106 pin functions

(1) Drive control circuit

The drive control circuit is composed of U1 (BIT3106) and its peripheral components. When the lamp needs to be lit, the ON/OFF signal output by the microcontroller is high level, which controls Q1 to be turned on, and its collector outputs low level, which in turn turns Q2 on. Then the Vin voltage input from pins 1 and 2 of CN1 is added to pins 6 and 12 of BIT3106 through R14 and the turned-on Q2, and the internal oscillation circuit of BIT3106 starts to work. The oscillation frequency is determined by the timing resistor and timing capacitor value connected to pins 8 and 9. After the oscillation circuit works, an oscillation pulse is generated, which is added to the internal drive circuit after frequency division, and output from pins 13 to 16 after transformation and shaping to the full-bridge drive circuit.

(2) Full-bridge drive circuit

The full-bridge drive circuit is used to generate AC high voltage that meets the requirements and drive CCFL to work. The drive circuit is composed of Q7A~Q10A, U2A, U3A, T1A~T3A and Q78~Q10B, U2B, U3B, T1B~T3B and other components. Among them, T1A~T3A, T1B~T3B are high-voltage transformers; U2A, U3A, U2B, U3B are all composite field-effect transistors, that is, they are composed of two MOS tubes, one is a P-channel MOS tube, and the other is an N-channel MOS tube.

The oscillation pulse generated by the internal oscillation circuit of BIT3106 is processed and outputs P-channel MOS drive signals from pins 17, 16, 14, and 13 of BIT3106, and N-channel MOS drive signals from pins 15 and 16 of BIT3106 to drive the A and B drive circuits. Since the two drive circuits are the same, only the A drive circuit is used as an example for explanation.

The driving signal output from pin 18 of BIT3106 is amplified by Q4A, push-pull buffered by Q10A and Q8A, added to pin 4 of U3A through R21A, amplified by the internal PMOS tube, and output from pins 5 to 6 of U3A; the signal output from pin 16 of BIT3106 is sent to pin 2 of U3A through R22A, amplified by the internal NMOS tube, and output from pins 7 to 8 of U3A; the signal output from pin 17 of BIT3106 is amplified by Q3A, push-pull buffered by Q9A and Q7A, added to pin 4 of U2A through R18A, amplified by the internal PMOS tube, and output from pins 5 to 6 of U2A; the driving signal output from pin 15 of BIT3106 is sent to pin 2 of U2A through R19A, amplified by the internal NMOS tube, and output from pins 7 to 8 of U2A.

Driven by the driving pulse, the MOS tubes inside U2A and U3A are alternately turned on and off, and pulse signals are output from pins 5 to 8 of U2A and U3A, which are added to the primary windings of T1A to T3A through C10A, C11A, and C24A. After being transformed by T1A to T3A, high voltage is output at the secondary windings of the T1A to T3A transformers.

The high voltage output from the secondary of transformer T1A enters group A lamp 1 through pin 1 of CN5, and the current is output from pin 3 of CN5, forming a loop through R24A and R25A to the ground, and group A lamp 1 is lit. To ensure the stability of the backlight brightness, the voltage generated at the upper end of R25A is used as a negative feedback signal and fed back to the inverting input end of the internal amplifier at pin 29 of BIT3106, automatically stabilizing the working state of the internal amplifier of BIT3106.

The high voltage output from transformer T2A enters group A lamp 2 through pin 2 of CN5, and the current is output from pin 4 of CN5, forming a loop through R24B and R25B to the ground, and group A lamp 2 is lit. To ensure the stability of the backlight brightness, the voltage generated at the upper end of R25B is used as a negative feedback signal and fed back to the inverting input end of the internal amplifier at pin 29 of BIT3106, automatically stabilizing the working state of the internal amplifier of BIT3106.

The high voltage output from transformer T3A enters group A lamp 3 through pin 1 of CN4, and the current is output from pin 2 of CN4, forming a loop through R24C and R25C to the ground, and group A lamp 3 is lit. To ensure the stability of the backlight brightness, the voltage generated at the upper end of R25C is used as a negative feedback signal and fed back to the inverting input end of the internal amplifier at pin 29 of BIT3106, automatically stabilizing the working state of the internal amplifier of BIT3106.

(3) Brightness adjustment circuit

R1, R3, R40, D2A, D2B, R38, and R39 together form the brightness control circuit of the A and B lamp units. When the brightness of the lamp needs to be controlled, the PWM control voltage ADJ sent from the mainboard is input from the 4th pin of CN1, and after voltage division by R1 and R3, filtering by C23 and current limiting by R40, it is added to the 29th pin and 2nd pin of BIT3106 by D2A, R38 and D2B, R39 respectively. After being processed by the internal circuit of BIT3106, the purpose of brightness control is achieved by controlling the duty cycle of the driving pulse output by BIT3106.

(4) Protection circuit

① Current protection circuit: The overcurrent protection circuit of the three lamps in group A is composed of D3A, D3B, D3C, Q5A, Q5B, Q5C and the internal circuit of pin 27 of BIT3106.

After the A-group lamp 1 connected to the 1 and 3 pins of CN5 is lit, a detection voltage will be formed on the upper ends of R24A and R25A, which will be divided by D3A, R33A and R32A and sent to the gate of Q5A; after the A-group lamp 2 connected to the 2 and 4 pins of CN5 is lit, a detection voltage will be formed on the upper ends of R24B and R25B, which will be divided by D3B, R33B and R32B and sent to the gate of Q5B; after the A-group lamp 3 connected to the 1 and 2 pins of CN4 is lit, a detection voltage will be formed on the upper ends of R24C and R25C, which will be divided by D3C, R33C and R32C and sent to the gate of Q5C.

Q5A, Q5B, and Q5C together form a series current detection circuit. When the current of the three lamps in group A or one of them decreases for some reason, the voltage obtained at the upper end of R25A, R25B, and R25C decreases, and the current of the series current detection circuit composed of Q5A, Q5B, and Q5C decreases. The gate voltage of Q6A increases, and its conduction degree increases. The D-pole voltage of Q6A decreases and is sent to the 27th pin of BIT3106. When the voltage of the 27th pin drops to 0.3V, the pulses output from the 17th and 18th pins are cut off, and the circuit is in a protection state.

The structure and working principle of the lamp current detection protection circuit of group B are exactly the same as those of group A. Therefore, among the three lamps in group A or group B, as long as the current of any lamp drops or the lamp is open, the corresponding current detection circuit will be activated and protected.

② Overvoltage protection circuit: The overvoltage protection circuit is mainly used to detect whether the high voltage output by the transformer increases abnormally.

BIT3106 has two overvoltage detection ports, namely pin 5 and pin 26 of BIT3106. Pin 26 is used to detect the high voltage output of T1A, T2A, and T3A. Pin 5 is used to detect the high voltage output of T1B, T2B, and T3B. The following takes the high voltage protection circuit of group A as an example.

The AC high voltage output by T1A is divided by C30 and C31, and then rectified by D4 to form the first voltage; the AC high voltage output by T2A is divided by C33 and C34, and then rectified by D5 to form the second voltage; the AC high voltage output by T3A is divided by C37 and C38, and then rectified by D6 to form the third voltage. The three voltages are divided by R12A and R23A and filtered by C12A before being sent to the 26th pin of BIT3106. When the high voltage output by T1A, T2A, and T3A at the same time or any group of secondary sides increases for some reason, the voltage of the 26th pin of BIT3106 will increase. When it is higher than 2V, after being processed by the internal circuit of BIT3106, the 17th and 18th pins will be controlled to stop outputting drive pulses, thereby achieving the purpose of overvoltage protection.

4. "PWM control chip + half-bridge structure drive circuit" configuration scheme

Compared with the full-bridge structure, the biggest advantage of the half-bridge structure driving circuit is that each channel uses two fewer MOS field effect transistors, as shown in Figure 16. However, it requires a transformer with a higher transformation ratio, which will increase the cost of the transformer.

Figure 16 Schematic diagram of half-bridge structure drive circuit

When the circuit is working, under the control of the driver control IC, switch pulses are output from the Vg1 and Vg2 terminals to control V1 and V2 to conduct alternately, so that an AC voltage is formed on the primary side of the transformer. By changing the duty cycle of the switch pulse, the conduction and cut-off time of V1 and V2 can be changed, thereby changing the energy storage of the transformer and the output voltage value.

In LCD color TVs, inverter circuits using a half-bridge structure are relatively rare, so no examples will be given here for analysis.