FSP107-2PS01 two-in-one power supply board circuit analysis

FSP107-2PS01 two-in-one power supply assembly is mainly used with LG 26-inch screens to directly drive CCFL lamps, as shown in Figure 1. If used with screens from other manufacturers (such as Samsung, AU, etc.), only those with a balance board can be used, otherwise it cannot be used.

Figure 1 FSP107-2PS01 two-in-one power supply

1. Analysis of the principle of power supply

The power supply of this component adopts the combination of FAN6961+STR-W6252, and its circuit is shown in Figure 2.

Figure 2 Power supply circuit diagram using FAN6961+STR-W6252

The power supply outputs two sets of voltages: one is the 12V voltage, which powers the accompanying audio integrated circuit, upper screen power supply circuit, etc., and after voltage doubling, it powers the integrated high-frequency head circuit of the medium amplifier; the other is the 5VSTB (1A) voltage, which powers the standby part and small signal processing part of the mainboard.

1. Filtering and anti-interference circuit

The anti-interference circuit is composed of Fl01, RV101, TH101, C102-C105, Rl01-R103, L102, and L103. Its function is to enhance the electromagnetic compatibility of the TV. The circuit is bidirectional. On the one hand, it can suppress high-frequency interference from entering the TV to ensure the normal operation of the TV. On the other hand, it can suppress the high-frequency interference generated by the switching power supply from entering the power grid and interfering with other electrical equipment.

L101 and L102 are common mode chokes, which are two independent coils wound on the same magnetic ring, with the same number of turns and opposite winding directions. The magnetic flux generated in the magnetic ring cancels each other out, and the magnetic core will not saturate. They mainly suppress common mode interference. The larger the inductance value, the better the low-frequency interference suppression effect. Capacitors C102 and C103 mainly suppress the interference between the live wire and the neutral wire. The larger the capacitance value, the better the low-frequency interference suppression effect.

2. Power Factor Correction (PFC) Circuit

The power factor correction (PFC) circuit uses the new power factor correction controller FAN6961 (IC120) produced by Siemens, which is packaged with 8 pins and can accurately adjust the output DC voltage to achieve the purpose of power factor correction. The power supply voltage of this IC can be as high as 25V, the startup current is less than 25μA, the operating current can be less than 6mA, and it can perform zero current detection and cycle-by-cycle current limiting. Its pin functions and measured voltages are shown in Table 1.

Table 1FAN6961 pin functions and measured voltages

After the second startup, the ① pin (STB) of the plug-in CN201 receives a high level from the motherboard, Q262 is turned on, the optocoupler PC700 is turned on, Q700 is turned on, and the +15.07V voltage outputted from its e-pole is added to the ⑧ pin of IC120. After IC120 is powered on, the internal circuit starts to work, and the MOSFET drive signal is outputted from the ⑦ pin of IC120, which is added to the gate of Q120 after being excited by Q121, so that it works in the switching state. The +300V DC voltage is added to the D pole of the switch tube Q120 through the PFC inductor L120. Due to the energy storage function of L120, the oscillating switch pulse is rectified by D120, and a DC voltage of about 400V is obtained on C610.

The pin ① of IC120 is connected to the inverting input terminal of the voltage amplifier, and the non-inverting input terminal is connected to the 2.5V reference voltage. The PFC output voltage is divided by resistors R120~R123 and R124 and connected to the pin ① of IC120 to detect the PFC output voltage.

Pin ② of 1C120 is the output terminal of the internal voltage amplifier. Through the feedback integration network (capacitors C124, C123 and resistor R133) (1) the startup of the switching power supply, the 4CXN voltage generated by the oscillation PFC circuit is added to the drain of the internal MOS tube of U601 through the windings ① to ④ of the transformer T600, and the capacitor C606 connected to the pin ④ is charged through the internal soft start circuit. When the voltage across the capacitor C606 rises to 8.9V, U601 starts to oscillate, and the ① of T600 is turned on. -④ winding has current flowing through it, at this time T600③-⑤ winding generates induced electromotive force through mutual inductance, after D700 rectification and C700 filtering, 18.69V voltage (18.83V in standby mode) is obtained, and after D701 rectification, C606 filtering, ZD602 voltage stabilization, it is added to U601④ foot, forming secondary startup power supply; on the other hand, this voltage is added to Q700, and after Q700 control, it is added to IC120⑧ foot, providing working voltage for PFC oscillation IC. When U601 obtains a stable startup power supply inside, the internal oscillator continues to oscillate, and electromotive force will be generated on T600 secondary ⑥-⑩ winding, ⑦-⑨ winding and ⑧-⑩ winding.

The purpose is to effectively suppress the second harmonic in the input rectified voltage ripple. In order to stabilize the output voltage, the output signal of the voltage amplifier is directly input to the multiplier to obtain the signal of the current detection comparator.

IC120 (3) pin is the frequency adjustment input terminal, and is connected to an external resistor-capacitor network R127 and C128 to adjust the frequency of the internal sawtooth wave generator.

Pin ④ of IC120 is the overcurrent protection detection input pin of the switch tube, which is connected to the current comparator inside the IC. R125 is the sampling resistor.

3. Power supply circuit composed of STR-W6252

The power supply circuit is mainly composed of switching transformer T600, integrated block U601 (STR-W6252) and related circuits.

After the second power-on, this part of the circuit keeps working and provides a 5VSTB voltage to the control system; after the second power-on, the power supply outputs 12V voltage for the whole machine. The U601 pin function and measured voltage are shown in Table 2.

Table 2 U601 pin functions and measured voltages

(1) Startup and oscillation of switching power supply

The 4CXN voltage generated by the PFC circuit is added to the drain of the MOS tube inside U601 through the ① to ④ windings of the transformer T600, and is charged to the external capacitor C606 at the ④ pin through the internal soft-start circuit. When the voltage across the capacitor C606 rises to 8.9V, U601 starts to oscillate, and current flows through the ①-④ windings of T600. At this time, the ③-⑤ windings of T600 generate an induced electromotive force through mutual inductance, which is rectified by D700 and filtered by C700 to obtain a voltage of 18.69V (18.83V in standby mode), which is added to the ④ pin of U601 after being rectified by D701, filtered by C606, and stabilized by ZD602, forming a secondary startup power supply; on the other hand, the voltage is added to Q700, and then added to the ⑧ pin of IC120 after being controlled by Q700, providing working voltage for the PFC oscillation IC. When U601 obtains a stable starting power supply, the internal oscillator continues to oscillate, and electromotive force will be generated on the T600 secondary windings ⑥-⑩, ⑦-⑨ and ⑧-⑩.

(2) Secondary rectifier filter circuit

The electromotive force generated in the secondary windings ⑦-⑨ of T600 is rectified by D210 and filtered by C211, L210, and C213 to output 5VSTB voltage, which is then processed and output from the ⑦ pin of the socket CN2Ⅲ for use by the motherboard microcontroller system. The electromotive force generated in the secondary windings ⑥-⑩ of T600 is rectified by D230 and filtered by C230 to output 18V voltage.

This voltage is added to the e-pole of Q221 as a switching voltage of 12V.

The electromotive force generated in the secondary windings of T6CK (⑧-⑩) is rectified by D220 and filtered by C221, and the output voltage is added to the D pole of Q220. The G pole of Q220 is controlled by the on/standby signal. After the second startup, the ① pin (STB) of CN201 gets a high level from the mainboard, Q262 is turned on, Q261 is turned on, and the 18V voltage generated by the rectification of D230 and the filtering of C230 is added to the G pole of Q220 after being stabilized by Q211 in series. Q220 is turned on, and a 12V voltage is generated under the control of the precision device IC220. After filtering by C223, it is output from the ③ and ④ pins of CN201 and supplied to the accompanying sound circuit on the mainboard.

(3) Voltage stabilization circuit

The +5V and +12V voltages are divided by resistors R251, R252 and R254 respectively, and then added to the control electrode of the three-terminal precision voltage regulator IC250. When the output voltage increases, the voltage of the control electrode also increases, and the voltage of the K electrode decreases, the current output from the second pin of PC600 increases, the light emission of the light-emitting diode in the optocoupler PC600 increases, the conduction of its internal photosensitive transistor increases, the voltage of the sixth pin of U601 increases, and the oscillation circuit inside U601 reduces the duty cycle of the output pulse, thereby reducing the output voltage and achieving the purpose of voltage stabilization.

(4) Overvoltage protection circuit

When the +5VSTB and +12V voltages increase a lot, ZD261 and ZD260 avalanche breakdown, IC260 turns on, the voltage at point B goes directly to ground, and the 12V voltage is cut off, resulting in no output, thereby effectively protecting the subsequent circuits.

2. Analysis and maintenance of each unit circuit of the inverter part

The control chip of the inverter circuit of this machine adopts LX16921DW (U301), as shown in Figure 3.

Figure 3 Inverter circuit diagram using LX16921DW (U301)

The pin functions and measured voltages of LX16921DW are shown in Table 3.

Table 3 Pin functions and measured voltages of LX16921DW

1. Oscillation start

The power supply and enable control circuit of U301 consists of Q302, Q301, and ZD301. The backlight on/off control signal (ONIOFF) is added to the base of Q301, Q301 is turned on, Q302 is turned on, and the 12V voltage is stepped down by R302 and added to the emitter of Q302. After passing through the voltage stabilizing diode ZD301, a stable 5V voltage is generated and added to the (20) and 6) pins of U301.

2.PWM pulse width modulation and output

The inverter circuit in this component provides five selectable dimming modes, see Table 4. This circuit uses the external digital PWM control mode. The brightness control signal (V_PWM) sent from the CPU on the main signal processing board is added to the U301 9th pin, and the 7th pin inputs a fixed level VDDA.

The VDDA voltage is generated by an internal low dropout (LDO) regulator at pin ①.

After comparing the brightness control signal (V_PWM) input from the 9th pin of U301 with the voltage on the triangular wave oscillation frequency adjustment capacitor connected to the 3rd pin, the low-frequency PWM brightness control signal is output and sent to the in-phase terminal of the current error amplifier, and the current detection signal input from the (14th pin) and added to the inverting terminal of the current error amplifier is modulated. The modulated pulse is sent to the timing logic and error detector, and the oscillation pulse is modulated into a discontinuous excitation oscillation pulse. After amplification by the internal drive circuit, two groups of excitation drive signals are output: one group outputs the A-channel excitation signal from the (19th pin) and the B-channel excitation signal from the (18th pin); the second group outputs the C-channel excitation signal from the (16th pin of U301 and the D-channel excitation signal from the (15th pin). This scheme only uses one group of drive signals, and the second group of drive signals is not used and is connected to the ground through resistors R311 and R312.

3. High voltage excitation drive circuit

U301 (19) and (18) pins are the drive pulse output terminals (AOUT and BOUT respectively), which output high and low levels alternately. After being amplified by the drive circuit composed of the previous half-bridge structure, they are sent to the dual MOSFET tube and the high-voltage transformer to generate a high-voltage excitation signal to drive the lamp to emit light.

The excitation signal (DC level of about 0.058V) output from pin (19) of U301 is amplified by the Q351~Q353 pre-stage and coupled to pin (10) of T350 through capacitor C350; the excitation signal (DC level of about (J.(J56V)) output from pin (18) of U301 is amplified by the Q361~Q363 pre-stage and directly connected to pin (6) of T350. Under the action of the high and low levels alternately output from pins (19) and (18) of U301, the subsequent MOS tube is alternately turned on and off. When Q352 and Q363 are turned on, Q362 and Q353 are turned off, and the current in the primary of transformer T350 flows from From top to bottom; when Q352 and Q363 are turned off, Q362 and Q353 are turned on, and the current in the primary of T350 flows from bottom to top. Through the isolation of T350, a high-frequency square wave voltage is generated at the secondary of T350 and added to the G poles of N-channel MOS tubes Q400 and Q410. Under the switching action of Q400 and Q410, the high-frequency square wave voltage is further increased and added to the primary of the step-up transformer T420. Then, through the coupling of transformer T420, a high-frequency square wave with a higher voltage is induced at the secondary, and a sine wave is generated under the action of the LC resonant circuit composed of the transformer leakage inductance and the loop capacitor, thereby driving the cold cathode lamp to emit light.

When the high-frequency square wave sensed by the secondary of T402 jumps from a low level to a high level, the output waveform slowly rises to the maximum due to the inhibitory effect of the leakage inductance; when the high-frequency square wave sensed by the secondary of T402 jumps from a high level to a low level, the output waveform slowly drops to the minimum due to the inhibitory effect of the leakage inductance, and in this way the square wave becomes a sine wave and is added to both ends of the lamp tube.

At the moment of power-on, the voltage can reach 1500V; during normal operation, the output is an AC voltage of about 500V.

4. Protection circuit

(1) Overcurrent detection input

Connect an external resistor R320 to ground at the pin (11) of U301. A full-wave AC voltage proportional to the current of the secondary coil of the high-voltage transformer is generated across R320 and compared with the 2V reference voltage of the internal digital comparator at the pin (11) of U301. If the peak voltage is greater than 2V, the comparator turns off the output to prevent false protection at the moment of power on; if the voltage at this pin continues to be greater than 2V, U301 will turn off the pulse output at pins (19) and (18).

(2) Overvoltage detection input

The voltage input at pin ⑩ of U301 is compared with the 3.2V reference voltage of the internal digital comparator to generate a peak voltage with an amplitude higher than 3.2V. This voltage generates a digital logic pulse with a frequency range of 30kHz~500kHz according to the pulse signal.

The sampling current of the step-up transformer T420 is divided into two paths. One path is coupled through C421 and C424, and after being rectified by D323 and D325, a sampling voltage is formed on resistors R337 and R338, which is added to the negative end of the diode in D322; the other path is coupled through C423 and C427, and after being rectified by D327, a sampling voltage is formed on resistors R324 and R323, which is added to pin ⑤ of U302 (LM358).

When the output voltage of T420 is too high due to some reasons, D322 avalanche breakdown occurs, and after being rectified by D321, it is added to U302 pin ③. Compared with the fixed voltage input at pin ②, a voltage of about 5V is output from U302 pin ①, and then it is stepped down by D303 and added to U301 (13) pin, and at the same time it is added to the gate of the state detection circuit Q320. On the other hand, the sampling voltage added to U302 pin ⑤ is compared and amplified with the fixed voltage input at U302 pin ⑥ by the internal comparison, and a voltage of about 5V is output from U302 pin ⑦, and then it is stepped down by D320 and added to U301 (13) pin.

(3) Current detection input

The input voltage of U301 (14) pin is compared with the 1V reference voltage of the internal digital comparator to generate a pulse with an amplitude greater than 1V (during the lighting mode, the current sensing input is disabled), which controls the number of pulses sent to the oscillator by the digital logic pulse generator, thereby changing the operating frequency of the oscillator. The input voltage of U301 (14) pin is proportional to the lamp current.

The sampling current of T420 is divided into two paths. One path is coupled through C421 and C424, rectified by D323 and D325, and then added to the gate of Q305 through D306. The other path is rectified by D328, divided by R316 and R319, and added to the U301 (14) pin after current limiting by R315.

(4) Inverter fault protection

Pin U301 (14) is the current detection input terminal. To remove the protection, just disconnect the external resistor R315. Pin U301 (13) is the overvoltage detection input terminal. To remove the protection, just disconnect the external resistors D320 and D303. Pin U301 (11) is the overcurrent detection input terminal. To remove the protection, just ground the pin.