Backlight inverters for lcd panels

For the operation of an LCD panel, the light source whose luminous flux is transmitted through the structure is of paramount importanceliquid crystal, forms an image on the monitor screen.

For creating luminous flux are used fluorescent lamps cold cathode backlights (CCFL), which are located at the edges of the monitor (usually top and bottom) and, using frosted diffusing glass, evenly illuminate the entire surface of the LCD matrix.

The “ignition” of the lamps, as well as their power supply in operating mode, is provided by inverters. The inverter must ensure reliable starting of lamps with voltages above 1500 V and their stable operation for a long time at operating voltages from 600 to 1000 V. The lamps in LCD panels are connected using a capacitive circuit (see Fig. A1). The operating point of stable glow (PT - on the graph) is located on the line of intersection of the load straight line with the graph of the dependence of the discharge current on the voltage applied to the lamps. The inverter in the monitor creates conditions for a controlled glow discharge, and the operating point of the lamps is on the flat part of the curve, which makes it possible to achieve a constant glow for a long time and ensure effective brightness control.

The inverter performs the following functions:

*converts constant pressure(usually +12 V) to high-voltage AC;

*stabilizes the lamp current and, if necessary, regulates it;

*provides brightness adjustment;

*matches the output stage of the inverter with the input resistance of the lamps;

*Provides protection against short circuit and overload.

No matter how diverse the market may be modern inverters, the principles of their construction and operation are almost the same, which simplifies their repair.

Structural scheme inverter is shown in Fig. P2.

The standby mode and inverter switching block is made in in this case on keys Q1, Q2. The LCD panel takes some time to turn on, so the inverter also turns on 2...3 s after the panel switches to operating mode. ON/OFF voltage is supplied from the main board and the inverter enters operating mode. The same block ensures that the inverter is turned off when the LCD panel enters one of the energy saving modes. When a positive ON voltage (3...5 V) is supplied to the base of transistor Q1, a voltage of +12 V is supplied to the main circuit of the inverter - the brightness control unit and the PWM regulator.

The unit for monitoring and controlling the brightness of lamps and PWM (3 in Fig. P2) is made according to the circuit of an error amplifier (EA) and a PWM pulse shaper.

It receives the dimmer voltage from the main board of the monitor, after which this voltage is compared with the voltage feedback, and then an error signal is generated that controls the frequency of the PWM pulses. These pulses are used to control the DC/DC converter (1 in Fig. A2) and synchronize the operation of the converter-inverter. The amplitude of the pulses is constant and is determined by the supply voltage (+12 V), and their frequency depends on the brightness voltage and the threshold voltage level.

The DC/DC converter (1) provides constant (high) voltage, which is supplied to the autogenerator. This generator is turned on and controlled by PWM pulses from the control unit (3).

The level of the inverter's AC output voltage is determined by the parameters of the circuit elements, and its frequency is determined by the brightness control and the characteristics of the backlight lamps. The inverter converter is usually a self-excited generator. Both single-cycle and push-pull circuits can be used.

The protection unit (5 and 6) analyzes the level of voltage or current at the inverter output and generates feedback (OS) and overload voltages, which are supplied to the control unit (2) and PWM (3). If the value of one of these voltages (in case of short circuit, converter overload, reduced level supply voltage) exceeds the threshold value, the autogenerator stops operating.

As a rule, on the screen, the control unit, PWM and brightness control unit are combined in one chip. The converter is made on discrete elements with a load in the form of a pulse transformer, the additional winding of which is used to switch the trigger voltage.

All main inverter components are housed in SMD component housings.

There are a large number of modifications of inverters. The use of one type or another is determined by the type of LCD panel used in a given monitor, so inverters of the same type can be found from different manufacturers.

http://www. *****/shem/schematics. html? di=53303

General provisions

For the operation of an LCD panel, of paramount importance is the luminous flux of which, passed through the structure of the liquid crystal, forms an image on the monitor screen. To create a luminous flux, cold cathode fluorescent lamps (CCFLs) are used, which are located at the edges of the monitor (usually top and bottom) and, using frosted diffusing glass, evenly illuminate the entire surface of the LCD matrix. The “ignition” of the lamps, as well as their power supply in operating mode, is provided by inverters. The inverter must ensure reliable starting of lamps with voltages above 1500 V and their stable operation for a long time at operating voltages from 600 to 1000 V. The lamps in LCD monitors are connected using a capacitive circuit (see Fig. 1). The operating point of stable glow (PT - on the graph) is located on the line of intersection of the load straight line with the graph of the dependence of the discharge current on the voltage applied to the lamps. The inverter in the monitor creates conditions for a controlled glow discharge, and the operating point of the lamps is on the flat part of the curve, which makes it possible to achieve a constant glow for a long time and ensure effective brightness control.

Rice. 1. Graph of the position of the operating current of the stable glow of the lamps

The inverter performs the following functions:

converts DC voltage (usually +12 V) into high-voltage AC; stabilizes the lamp current and, if necessary, regulates it; provides brightness adjustment; matches the output stage of the inverter with the input resistance of the lamps; Provides short circuit and overload protection.

No matter how diverse the market for modern inverters is, the principles of their construction and operation are almost the same, which simplifies their repair.

The block diagram of the inverter is shown in Fig. 2. The block for standby mode and turning on the inverter is made in this case on keys Q1, Q2. The LCD monitor takes some time to turn on, so the inverter also turns on 2...3 s after the monitor is switched to operating mode. ON/OFF voltage is supplied from the main board and the inverter enters operating mode. The same block ensures that the inverter is turned off when the monitor switches to one of the energy saving modes. When a positive ON voltage (3...5 V) is supplied to the base of transistor Q1, +12V voltage is supplied to the main circuit of the inverter - the brightness control unit and the PWM regulator.

Rice. 2. Block diagram of the inverter

The unit for monitoring and controlling the brightness of lamps and PWM (3 in Fig. 2) is made according to the circuit of an error amplifier (EA) and a PWM pulse shaper. It receives the dimmer voltage from the main monitor board, after which this voltage is compared with the feedback voltage, and then an error signal is generated that controls the frequency of the PWM pulses. These pulses are used to control the DC/DC converter (1 in Fig. 2) and synchronize the operation of the inverter converter. The amplitude of the pulses is constant and is determined by the supply voltage (+12 V), and their frequency depends on the brightness voltage and the threshold voltage level.

The DC/DC converter provides constant (high) voltage, which is supplied to the autogenerator. This generator is turned on and controlled by PWM pulses from the control unit (3).

The protection unit (5 and 6) analyzes the level of voltage or current at the inverter output and generates feedback (OS) and overload voltages, which are supplied to the control unit (2) and PWM (3). If the value of one of these voltages (in the event of a short circuit, converter overload, low supply voltage level) exceeds the threshold value, the autogenerator stops operating.

All main inverter components are housed in SMD component housings.

Let's look at the most commonly used types of inverters, as well as their typical faults.

Inverter type PLCD2125207A from EMAX

This inverter is used in LCD monitors from Proview, Acer, AOC, BENQ and LG with a screen diagonal of no more than 15 inches. It is built according to a single-channel circuit with a minimum number of elements (Fig. 3). At an operating voltage of 700 V and a load current of 7 mA using two lamps, the maximum screen brightness is about 250 cd/m2. The starting output voltage of the inverter is 1650V, the protection response time is from 1 to 1.3 s. At idle, the output voltage is 1350 V. The greatest depth of brightness is achieved when the control voltage DIM (pin 4 of CON1 connector) changes from 0 (maximum brightness) to 5 V (minimum brightness). The inverter from SAMPO is made according to the same scheme.

Get full text

Rice. 3. Schematic diagram inverter PLCD2125207A

Description of the circuit diagram

+12 V voltage is supplied to the pin. 1 connector CON1 and through fuse F1 - to pin. 1-3 assemblies Q3 (source field effect transistor). The boost DC/DC converter is assembled using elements Q3-Q5, D1, D2, Q6. In operating mode, the resistance between the source and drain of transistor Q3 does not exceed 40 mOhm, while a current of up to 5 A is passed into the load. The converter is controlled by a brightness and PWM controller, which is made on a U1 chip of the TL5001 type (analogous to FP5001) from Feeling Tech. The main element of the controller is a comparator, in which the voltage of the sawtooth voltage generator (pin 7) is compared with the voltage of the control device, which in turn is determined by the relationship between the reference voltage of 1 V and the total feedback and brightness voltage (pin 4). The frequency of the sawtooth voltage of the internal generator (about 300 kHz) is determined by the value of resistor R6 (connected to pin 7 of U1). PWM pulses are taken from the output of the comparator (pin 1), which are supplied to the DC/DC converter circuit. The controller also provides short circuit and overload protection. If there is a short circuit at the inverter output, the voltage at the divider R17 R18 increases, it is rectified and supplied to the pin. 4 U1. If the voltage becomes 1.6 V, the controller protection circuit is activated. The protection response threshold is determined by the value of resistor R8. Capacitor C8 provides a “soft” start when starting the inverter or after the end of a short circuit. If short circuit lasts less than 1s (the time is determined by the capacitance of capacitor C7), then normal operation of the inverter continues. IN otherwise the inverter operation stops. To reliably start the converter, the protection response time is selected so as to be 10...15 times longer than the start and “ignition” time of the lamps. When the output stage is overloaded, the voltage at the right terminal of inductor L1 increases, the zener diode D2 begins to pass current, transistor Q6 opens and the response threshold of the protection circuit decreases. The converter is made according to the circuit of a half-bridge generator with self-excitation using transistors Q7, Q8 and transformer PT1. When the ON/OFF power-on voltage (3 V) is received from the main monitor board, transistor Q2 opens and power is supplied to controller U1 (+12 V to pin 2). PWM pulses with pin. 1 U1 through transistors Q3, Q4 goes to the gate of Q3, thereby starting the DC/DC converter. In turn, power is supplied from it to the autogenerator. After this, a high-voltage voltage appears on the secondary winding of transformer PT1. AC voltage, which goes to the backlight lamps. Winding 1-2 PT1 acts as a feedback generator. While the lamps are not turned on, the output voltage of the inverter rises to the starting voltage (1650V), and then the inverter goes into operating mode. If the lamps cannot be ignited (due to a break, “exhaustion”), spontaneous generation failure occurs.

Malfunctions of the PLCD2125207A inverter and how to eliminate them

Backlights do not turn on

Check the +12 V supply voltage at the pin. 2 U1. If it is not there, check fuse F1, transistors Q1, Q2. If fuse F1 is faulty, before replacing it, check transistors Q3, Q4, Q5 for a short circuit.

Then check the ENB or ON/OFF signal (pin 3 of CON1 connector) - its absence may be due to a malfunction of the monitor’s main board. This is checked in the following way: a control voltage of 3...5 V is supplied to the ON/OFF input from an independent power source or through a divider from a 12V source. If the lamps turn on, then the main board is faulty, otherwise the inverter is faulty.

If there is supply voltage and a turn-on signal, but the lamps do not light, then carry out an external inspection of the transformer PT1, capacitors C10, C11 and lamp connectors CON2, CON3, and replace the darkened and melted parts. If at the moment of switching on the pin. 11 transformers RT1 per a short time voltage pulses appear (the oscilloscope probe is connected through a divider in advance, before turning on the monitor), and the lamps do not light, then check the condition of the lamp contacts and the absence of mechanical damage on them. The lamps are removed from their seats, having first unscrewed the screw securing their housing to the matrix body, and, together with the metal housing in which they are installed, are removed evenly and without distortions. In some monitor models (Acer AL1513 and BENQ), the lamps are L-shaped and cover the LCD panel around the perimeter, and careless actions during dismantling can damage them. If the lamps are damaged or darkened (which indicates a loss of their properties), they are replaced. Lamps can only be replaced with ones of similar power and parameters, otherwise either the inverter will not be able to “ignite” them, or an arc discharge will occur, which will quickly damage the lamps.

The lamps turn on for a short time (about 1 second) and then turn off immediately

In this case, protection against short circuit or overload in the secondary circuits of the inverter is most likely triggered. Eliminate the reasons for the protection to operate, check the serviceability of the transformer PT1, capacitors C10 and C11 and the feedback circuit R17, R18, D3. They check the zener diode D2 and the transistor Q6, as well as the capacitor C8 and the divider R8 R9. If the voltage at the pin. 5 is less than 1 V, then replace the capacitor C7 (preferably with a tantalum one). If all of the above steps do not produce results, replace the U1 chip.

Get full text

Turning off the lamps may also be due to a failure of the converter generation. To diagnose this malfunction, instead of lamps, an equivalent load is connected to connectors CON2, CON3 - a resistor with a nominal value of 100 kOhm and a power of at least 10 W. A 10 ohm measuring resistor is connected in series with it. Devices are connected to it and the oscillation frequency is measured, which should be in the range from 54 kHz (at maximum brightness) to 46 kHz (at minimum brightness) and the load current from 6.8 to 7.8 mA. To monitor the output voltage, connect a voltmeter between pin 11 of transformer PT1 and the terminal of the load resistor. If the measured parameters do not correspond to the nominal, control the magnitude and stability of the supply voltage at inductor L1, and also check transistors Q7, Q8, C9. If, when the right (according to the diagram) diode of assembly D3 is disconnected from resistor R5, the screen lights up, then one of the lamps is faulty. Even with one working lamp, the image brightness is enough for the operator to work comfortably.

The screen flickers periodically and the brightness is unstable

Check the stability of the brightness voltage (DIM) on pin. 4 connectors CON1 and after resistor R3, having previously disabled feedback (resistor R5). If the control voltage at the connector is unstable, then the main board of the monitor is faulty (the test is carried out in all available modes of operation of the monitor and across the entire brightness range). If the voltage is unstable at the pin. 4 controllers U1, then check its mode by DC in accordance with table. 1, while the inverter must be in operating mode. The faulty microcircuit is replaced.

Table 1

Inverter status	Voltage at the terminals of the microcircuit U1, V

The inverter is on, but the lamps do not light up
The inverter is on, the lamps are lit

They check the stability and amplitude of oscillations of their own sawtooth pulse generator (pin 7), the signal swing should be from 0.7 to 1.3 V, and the frequency should be about 300 kHz. If the voltage is not stable, replace R6 or U1.

Instability of the inverter may be due to aging of the lamps or their damage (periodic loss of contact between the supply wires and the lamp terminals). To check this, as in the previous case, connect an equivalent load. If the inverter operates stably, then it is necessary to replace the lamps.

After some time (from several seconds to several minutes) the image disappears

The protection circuit is not working correctly. Check and, if necessary, replace capacitor C7 connected to the pin. 5 controllers, control the DC mode of controller U1 (see previous fault). Check the stability of the lamps by measuring the level of sawtooth pulses at the output of the feedback circuit, on the right anode D3 (swing about 5 V) with medium brightness set (50 units). If voltage surges occur, check the serviceability of the transformer and capacitors C9, C11. Finally, check the stability of the PWM controller circuit U1.

Inverter type DIVTL0144-D21 from SAMPO

The schematic diagram of this inverter is shown in Fig. 4. It is used to power the backlight lamps of 15-inch matrices from SUNGWUN, SAMSUNG, LG-PHILIPS, HITACHI, which are used in PROVIEW, ACER, BENQ, SAMSUNG, LG monitors. Operating voltage is 650 V at a load current of 7.5 mA (at maximum brightness) and 4.5 mA at minimum. The starting voltage (“ignition”) is 1900 V, the frequency of the supply voltage of the lamps is 55 kHz (at average brightness). The brightness control signal level ranges from 0 (maximum) to 5 V (minimum). The protection response time is 1…4 s.

Rice. 4. Schematic diagram of the DIVTL0144-D21 inverter

A U201 microcircuit of type BA9741 from ROHM (its analogue TL1451) is used as a controller and PWM. It is a two-channel controller, but in this case only one channel is used.

When the monitor is turned on, +12 V voltage is supplied to pin 1-3 of the Q203 transistor assembly (source of the field-effect transistor). When the monitor is turned on, the inverter ON/OFF start signal (+3 V) comes from the main board and opens transistors Q201, Q202. Thus, +12 V voltage is supplied to the pin. 9 controllers U201. After this, the internal sawtooth voltage generator begins to operate, the frequency of which is determined by the ratings of the elements R204 and C208 connected to the pin. 1 and 2 microcircuits. PWM pulses appear on pin 10 of the microcircuit, which are supplied to the gate of Q203 through an amplifier on transistors Q205, Q207. On the pin. 5-8 Q203 a constant voltage is generated, which is supplied to the autogenerator (on elements Q209, Q210, PT201). A sinusoidal voltage with a swing of 650 V and a frequency of 55 kHz (at the moment the lamps are “ignited” it reaches 1900 V) from the output of the converter through connectors CN201, CN202 is supplied to the backlight lamps. The elements D203, R220, R222 are used to generate a protection signal and a “soft” start. When the lamps are turned on, the energy consumption in the primary circuit of the inverter increases and the voltage at the output of the DC/DC converter (Q203, Q205, Q207) increases, the zener diode D203 begins to conduct current, and part of the voltage from the divider R220 R222 goes to pin 11 of the controller, thereby increasing the same threshold for the protection circuit to operate during startup.

Get full text

The stability and brightness of the lamps, as well as short-circuit protection, is ensured by a feedback circuit on elements D209, D205, R234, D207, C221. The feedback voltage is supplied to the pin. 14 microcircuits (direct input of the error amplifier), and the brightness voltage from the main monitor board (DIM) - to the inverse input of the control unit (pin 13), determining the frequency of PWM pulses at the controller output, and hence the output voltage level. At minimum brightness (DIM voltage is 5 V) it is 50 kHz, and at maximum (DIM voltage is zero) it is 60 kHz.

If the feedback voltage exceeds 1.6 V (pin 14 of the U201 chip), the protection circuit is turned on. If a short circuit in the load lasts less than 2 s (this is the time of charging capacitor C207 from the reference voltage +2.5 V - pin 15 of the microcircuit), the functionality of the inverter is restored, which ensures reliable starting of the lamps. If there is a long-term short circuit, the inverter turns off.

Malfunctions of the DIVTL0144-D21 inverter and methods for their elimination

Lamps don't light up

Check the presence of +12 V voltage on the pin. 1-3 Q203, serviceability of fuse F1 (installed on the main board of the monitor). If the fuse is faulty, then before installing a new one, check transistors Q201, Q202, as well as capacitors C201, C202, C225 for a short circuit.

Check the presence of ON/OFF voltage: when turning on the operating mode, it should be equal to 3V, and when turning off or switching to standby mode, it should be zero. If there is no control voltage, check the main board (the LCD monitor microcontroller controls the inverter switching on). If all of the above voltages are normal, and the PWM pulses are on the pin. 10 there is no V201 microcircuit, check zener diodes D203 and D201, transformer RT201 (can be determined by visual inspection by a darkened or melted case), capacitors C215, C216 and transistors Q209, Q210. If there is no short circuit, then check the serviceability and rating of capacitors C205 and C207. If the above elements are in good condition, replace the U201 controller. Note that the absence of illumination of the backlight lamps may be due to their breakage or mechanical failure.

Lamps turn on and off briefly

If the illumination persists for 2 s, then the feedback circuit is faulty. If, when disconnecting elements L201 and D207 from the circuit, pin. 7 of the U201 chip, PWM pulses appear, then either one of the backlight lamps or the feedback circuit is faulty. In this case, check the zener diode D203, diodes D205, D209, D207, capacitors C221, C219, and inductor L202. Monitor the voltage at the pin. 13 and 14 U201. In operating mode, the voltage at these pins should be the same (about 1 V - at average brightness). If the voltage at the pin. 14 is significantly lower than on pin. 13, then check diodes D205, D209 and lamps for open circuits. With a sharp increase in voltage at the pin. 14 U201 microcircuits (above the 1.6V level) check elements PT1, L202, C215, C216. If they are working, replace the U201 chip. When replacing it with an analogue (TL1451), check threshold voltage on pin 11 (1.6 V) and, if necessary, select the value of elements C205, R222. By selecting the values of elements R204, C208, the frequency of the sawtooth pulses is set: on the pin. 2 chips should be around 200 kHz.

The backlight turns off after some time (from several seconds to several minutes) after turning on the monitor

First, check capacitor C207 and resistor R207. Then check the serviceability of the contacts of the inverter and backlight lamps, capacitors C215, C216 (by replacement), transformer RT201, transistors Q209, Q210. Monitor the threshold voltage at the pin. 16 V201 (2.5V), if it is low or missing, replace the microcircuit. If the voltage at the pin. 12 is above 1.6V, check capacitor C208, otherwise also replace U201.

The brightness spontaneously changes (flashes) throughout the entire range or in individual monitor operating modes

If the malfunction appears only in certain resolution modes and in a certain brightness range, then the malfunction is related to the main board of the monitor (memory or LCD controller). If the brightness changes spontaneously in all modes, then the inverter is faulty. Check the brightness adjustment voltage (at pin 13 U201 - 1.3 V (at average brightness), but not higher than 1.6 V). If the voltage at the DIM contact is stable, and at the pin. 13 - no, replace the U201 chip. If the voltage at the pin. 14 is unstable or low (less than 0.3 V at minimum brightness), then instead of the lamps, connect an equivalent load - a resistor with a nominal value of 80 kOhm. If the defect persists, replace the U201 chip. If this replacement does not help, replace the lamps and also check the serviceability of their contacts. The voltage is measured at pin 12 of the U201 microcircuit; in operating mode it should be about 1.5V. If it is below this limit, check elements C209, R208.

Note. In inverters from other manufacturers (EMAX, TDK), made according to a similar circuit, but which use other components (except for the controller), instead of SI443 ® D9435, 2SC5706 ® 2SD2190, the voltage at the terminals of the U201 microcircuit can vary within ±0.3 V .

All about car tuning

Backlight inverters for lcd panels