Samsung 32-inch LCD screens built 16 lamp, with the screen matching the lamp driver circuit board model. The lamp driver circuit consists of two BD9884 and 8 units of power output circuit full-bridge architecture, as shown in the figure. Power output tubes containing n-channel and Sp8M3 p-Channel MOSFET modules. Two SP8M3 modules and output high voltage transformer t-a circuit full-bridge output schema. Ll transformer primary winding power output modules, secondary cold cathode fluorescent tube high voltage winding L2, L3 induced voltage as the secondary low-voltage winding voltage detection part of the sampled voltage is sent to the BD9884FV.


, Signal flow and brief introduction of working principle

When the CPU on the number pad issue "backlight on" instructions, oscillators began to work on the backlight driving Board, generating frequency oscillation signal lOOkHz, into the modulator, signals from the CPU PWM brightness modulation, modulation l0OkHz excited oscillation of output intermittent signal, into the power output circuit, output high pressure and light the backlight lamp.

PWM changes the output pulse width modulation signal, so as to achieve the objective of changing the backlight brightness, after the backlight lamp lighted, L2, c and CCFL combination makes high-voltage sinusoidal waveform shape (low Q value series resonance), electric capacity c volume resistance and the inductance L2 to the backlight lamp current-limiting function.

Series sampling in the backlight lamp pressure drop on the resistor r as the backlight lamp operation state test signals, sent to the protection circuit (10393); L3 sampling the output voltage signal is also transmitted to the protection circuit, when the output voltage and the backlight lamp operating current when an exception occurs, protecting the circuit control, modulator stop losing m.

Since Samsung 32-inch screen is 16 back-lit lamps, due to the backlight lamp can not simply connected in parallel or in series, so must be equipped with appropriate 16 high voltage output transformer and corresponding excitation circuit. BD9884FV has two excitation output (26), (27) script output, (23), (24) foot output. Each incentive to lose m provided two full-bridge power circuit with excitation signal, each set of full-bridge power output to two high voltage transformer output drive voltage (lit two cold cathode fluorescent tubes), so that every piece of BD9884FV can drive 8 lamps, driven by two BD9884FV a total of 16 lamp.

Second, introduction to ICS BD9884FV and power output module SP8M3

BD9884FV is a Rohm (Roma) company specializes in high voltage driving circuit for LCD backlight lamp one of the series of integrated circuit design (suitable for different screens and circuit selection form BD9882~BD9886 series). PIN function and the measured voltage is shown in the table below. The IC supports multiple lamp large-screen liquid crystal display backlighting lamp high voltage driving circuit for each BD9884FV can be supported to 8 tube driven.

SP8M3 is a combination of n-channel +P channel power amplifier MOSFET modules, internal block diagram as shown in the figure on the right, the module uses SS0P-B28 package (surface mount), is small in size, power, and resistance is small, good symmetry and so on. SP8M3 internal n-Channel MOSFET tube's main parameters are: VDDS=30V,ID-7A;P Channel MOS-FET the main parameter is: VDDS=-30V,ID=-5A.
Third, high-voltage excitation circuit analysis

Samsung 32-inch LCD using two BD9884FV complete incentive to 16 tubes backlight drivers, circuits is more complex, in order to facilitate the understanding of the high-voltage drive circuit, first introduced a two-lamp driving circuit of the basic programme consisting of BD9884FV (as shown in above figure).

Two channels at the same time (16) foot entered the on/off button a start signal and cyclophosphamide foot input control signal PWM brightness control. ② ⑤ feet first channel of the output signal, black, black foot second channel of the output signal.


Ql, Q2, Tl, and Cl, CCFL1, Rl first channel excitation circuit ⑩ feet is the t-channel backlight lamps: as sampled feedback input, ⑩ feet is the output sampling feedback for high voltage input, when an exception is when the output voltage is abnormal or lamp t, immediately stop the output pulse, play a protective role.

Ql, Q2 full-bridge architecture power output circuits, equivalent circuit as shown in the figure on the right. BD9884FV designed to support half-bridge architecture power output mode, in this circuit adds components such as Q507, Q508, so that it will have features that support full-bridge power output schema. Output circuit consists of Tl, C1, CCFL1 and R1 series resonant circuit, a low Q values after startup, 24V power supply plus the backlight lamp driver circuit on the Board immediately, the voltage directly on the power output module Q1~Q4, the other way, after buck, voltage regulator 6V and added it to the BD9884FV (28) feet as VCC voltage.

At CPU sent to of boot on/off signal into (16) feet, BD9884FV internal oscillator began work, produced lO0kHz square signal into modulation device, on CPU to BD9884FV ① feet of PWM brightness control signal for modulation, and zoom, last from (26), and (27) feet output incentive signal, added to full bridge schema power output circuit Ql, and Q2 of two only n ditch road MOS tube of gate (Gl) BU; from right figure equivalent circuit in the can see, Q1, Q2 four MOS line of full-bridge architecture is composed of four legs, from (26) feet, (27) foot output signals, respectively, added to the Ql, Q2 power module power n-Channel MOS on the tube, making it take turns conducting.

Large flow of signals through the L1, after Tl boost added to the backlight lamp and its light.

T1 L3, Cl and CCFL1 a low of series resonant circuit Q value, when excited resonance frequency and the oscillation frequency is the same as a sinusoidal output waveform correction. After the CCFL1 lamp light, the Tl of inductive reactance and capacitive reactance of C1 to the lamp current-limiting function. Sampling resistor R1 works for CCFL1 lamp current, the voltage lamp operation State is reflected correctly. Dang lamp work exception Shi, lamp current produced changes, in Rl Shang produced of pressure drop Ui also corresponding changes, the lamp t for current sampling voltage Ui feedback to BD9884FV of (18) feet, control oscillation incentive circuit stop work (in more lamp of LCD screen in the, dang a a only lamp appeared fault or started performance has differences Shi, that will appeared lamp cannot started points bright or bright about that out fault).

Tl L2 for winding output voltage over voltage, under voltage sampling, sampling voltage Uv feedback oscillation, control IC BD9884FV ⑩ feet, the reflected light of the sampled voltage Uv lamp high voltage output is normal or not. When the circuit when there is a fault that caused the abnormal voltage, by ⑩ the inside of the foot control circuit controlling the oscillation circuit stops working.

BD9884FV (26), (27) feet to inspire two sets of output channels full-bridge architecture power output H{circuit, as shown in the following figure. Which, Q1, and Q2 for a group, Q3, and Q4 for a group, this two group of incentive entered end parallel Hou received Yu BD9884FV of (26), and (27) feet, that BD9884FV of a incentive output channel support two group power output circuit, by Q1, and Q2 composition of way output circuit, in output end connection two only high pressure output transformer, and support two only backlight lamp, such each road channel can to support 4 only backlight lamp, A BD9884FV of two channels to support 8 lamp.

Two parallel BD9884FV using a control signal control, which can support up to 16 back-lit lamp in this scenario requires two BD9884FV four-channel output H{PWM signal phase shifts of 90 degrees, four tube rolling power blackouts, power supply, more uniform brightness and minimize disruption. To this end, part joining up two BD9884FV II ~ ⑥ feet correspond to conduct phase-shift control.
Four, the protection circuit analysis

Backlight lamp drive circuit enter light lamp power, and points bright backlight lamp, and requirements whole screen brightness uniform, and stable, in actual application in the, due to power and the lamp of characteristics by temperature, factors effect, will caused glow brightness not stable at, requirements backlight lamp high pressure drive circuit to has automatically regulator, and stability flow function addition, due to LCD screen for more lamp system, dang and a only backlight lamp damaged or performance bad Shi, the lamp not bright or brightness very low, is LCD screen brightness not uniform, even appeared dark district, this is not allows of, Requested a protective shut down backlight driver circuit.


BU said, to solve the problem, backlight set up on automatic detection of high voltage driving circuit output voltage, automatically detect lamp current and stable voltage and current control circuit. When a backlight lamp is damaged or bad H{dark area, the defective lamp current or current is extremely small, measurement and control circuits to control, control the backlight lamp high voltage driving circuit stops working, black screen appears.

The backlight lamp drive circuit voltage, and current stable control and the automatically Yu measuring protection circuit diagrams as Shang figure by shows, and high pressure transformer of L3 is output voltage of sampling winding, resistance r is lamp current sampling resistance L3 of sampling voltage after voltage feedback circuit added to BD9884FV of voltage feedback entered end ⑩ feet, r Shang of sampling voltage Ui (by D502, and C1 rectifier filter, reflect lamp work current lost small) after current feedback circuit added to BD9884FV of current feedback entered end Krispy Kreme feet, After the two feedback voltage into the 13D9884FV and cyclophosphamide plus with feet to the brightness of the PWM signal to the PWM circuit brightness apoptosis, complete brightness brightness control and stability.

Meanwhile, r on the sampled voltage to control circuits a IC502 (10393) compared with the reference voltage, when the lamp aging, damage, sampling sharp change in voltage, comparator, and output control voltage into the I3D9884FV (17) feet, cause the oscillator to stop working, the entire circuit stops working.

Following the first channel, for example, details the working principle of protection, specific circuit as shown in the figure.

1. Voltage and current feedback circuit

Tl, L2, R553, R554, D510 ⑩ pin voltage feedback circuit and BD9884FV. If the output voltage instability due to some reason, L2 Uv also changes the output voltage, the voltage after the R553, R554 points sampling, the D510 BD9884FV ⑩ foot.

Rl, D502, C1, R537, R538 ⑨ pin current feedback circuits and BD9884FV. After the lamps lit if changes in current due to temperature change of brightness caused instability, changes in currents of pressure drop in the sampling resistor of Ui changes, the D502, Cl rectified and filtered, after R5.37, R538 voltage sampling of the voltage, the D502 Krispy Kreme is added to BD9884FV feet.

Voltage and current feedback circuit of feedback signal into the BD9884FV modulation circuits, from ① footed PWM intensity control signals into the circuit, completed the brightness control of the interaction and stability control of voltage and current to the lamp.

2, the abnormal lamp current protection control circuit

The circuit consists of sampling circuits, benchmark circuits and control the output consists of two parts. Among them, the sampling circuit formed by Q105, R540, D530, sampled voltage is taken from the Ui. When the lamp is working properly,. Q105 base Ui flow, Q105 increased collector current Ie, Q105, saturated hydraulic conductivity, and collector voltage Ue close () v, D530 cutoff. When the lamp is damaged or aging, Ui is very little or no voltage, Q105 collector current Ie is very small, then increased collector voltage Ue, voltage when the voltage is higher than the 1C502 II foot, D530 breakover, D530 JC502 benchmark circuits are added to this voltage input ② feet.

Baseline comparator comparator 10393, high control precision and control adjustable threshold, equivalent circuit as shown in the figure on the right. IC502 ③ feet is the reference voltage input, (2) feet is the current sampled voltage inputs, ① foot control signal output, R571, R572 voltage determines the size of reference voltage.

When ② ③ feet high, feet for low level output pin ① feet high; when the pins and aim at low level, pin-II high voltage, output pins ① for low level. While working in a normal, due to the sampling circuit was brought low (voltages less than 1V), plus the IC502 II foot, free foot voltage from R571, R572 (1) kN) pressure setting of 3V, ② pin voltage is less than third foot voltage, at which point ① feet high level output. When the backlight lamp is damaged, it delivers high sampling circuit (6V), voltage of ② pin voltage is greater than the third foot, ① feet low level output at this time.

BD9884FV (17) feet to protect the control inputs, connected with the IC502 ① feet. When the BD9884FV is working correctly, (17) feet voltage 1~1.5V (set by R529, R530); when the backlight lamp fails, IC502 ① feet low, BD9884FV (17) feet low level voltages less than 1V, BD9884FV under the control of the internal circuit, stops the excitation pulse output.

Because of the way big-screen LCD lamp, so the circuits BU a sampling circuit is set for each lamp, after blocking diode at the output of multiple sampling circuit (D530, D830, etc) in a benchmark comparison on the control end of the circuit (IC502 II feet), as shown in the following figure. Multiple lamps in working, as long as there is any one lamp not working properly, its elevated Ue is by isolating the diode increases in benchmark circuits, circuit action immediately.