This article is mainly about the related introduction of the power supply pfc circuit, and focuses on the detailed explanation of the cause and solution of the power supply pfc circuit failure.
LCD TV power supply pfc circuitMost of the LCD color TV power supply boards use built-in power supply, which is mainly composed of mains input and anti-interference circuit, rectifier filter circuit, CPU power supply, secondary power supply, PFC circuit, PWM main power supply, voltage stabilization control circuit, over-voltage, over-current and over-heat protection, and on standby It is composed of control and other circuits, which is controlled by the mainboard CPU to turn on/standby, and only 5V auxiliary power output is available during standby.
The working process of the LCD color TV power supply board: After power on, the mains rectification and filtering will generate a 300V DC voltage. First, the auxiliary power circuit will be supplied with power, and the auxiliary power supply will work first, and output 5V voltage to supply power to the mainboard CPU; the CPU will output control after receiving the boot command The signal ON/OFF enables the PFC circuit and PWM main power supply on the power board to start working; after the PFC circuit works, the 300V after the mains rectification and filtering is corrected, and the voltage is increased to about 370~410V, and then the main Power supply and auxiliary power supply; after the PWM pulse oscillation main power supply works, several DC voltages such as 12V, 18V and 24V are output from the secondary side of the transformer to supply power to the load circuit. Among them, the 12V and 18V voltages mainly supply power to the signal processing circuit and sound power amplifier circuit of the motherboard; the 24V voltage mainly supplies power to the backlight circuit (high-voltage board).
Compared with the switching power supply of CRT color TV, the switching power supply of LCD color TV is much more complicated in circuit composition, technical content, and circuit principle. It adopts many new technologies and circuits, mainly adding PFC circuit and high-power power supply board. Half-bridge push-pull output circuit.
1. PFC circuit
The traditional switching power supply is directly filtered by a large-capacity capacitor after rectification, as shown in Figure 13a, to provide a smooth DC voltage for the load circuit. The large-capacity filter capacitor is equivalent to the most direct load of the bridge rectifier circuit, so its load is capacitive, the current is 90° ahead, the AC voltage and current phases are inconsistent, the maximum current and the maximum voltage do not appear at the same time, so The calculation of power also needs to be multiplied by the power factor of a circuit, that is, P=UICOSψ. It can be seen that increasing the power factor can improve the utilization rate of grid energy.
For this reason, most of the LCD color TV power supply boards are added with a PFC circuit between the back end of the mains bridge rectifier circuit and the front end of the power filter capacitor, so that the phases of the voltage and current supplied to the switching power supply are corrected to the same phase, which not only improves the power supply The power factor, after being corrected by the PFC circuit, can reduce the pollution of electrical appliances to the grid voltage and the circuit itself, which is electromagnetic compatibility
PFC circuits are divided into two types: active PFC and passive PFC. The active PFC circuit is composed of more electronic components, and the cost is relatively high. A PFC chopper circuit is added at the back end of the bridge rectifier circuit and the front end of the electrolytic filter capacitor, as shown in Figure 13b. It chops the pulsating current after bridge rectification into several small current waveforms, so that the envelope of the entire current waveform is in the same phase as the voltage waveform to achieve the same phase of the voltage and current waveforms. The active PFC circuit has a correction effect Well, it is widely used in LCD color TV and plasma color TV power supply circuits. The passive PFC circuit mainly adds a large PFC inductor before the capacitor filtering after the bridge rectification, and uses the characteristic of the inductor voltage to lead 90° to compensate for the characteristic of the electrolytic filter capacitor current leading 90°. The actual effect is slightly worse, but it is only used in In individual small LCD TVs.
Figure 14 is the PFC circuit of Sanyo 32CA828 LCD color TV power supply + backlight board, in which IC901 PFC controller adopts FAN6961 SZ, and high-power field effect switch Q901 and transformer energy storage inductor L906, rectifier filter circuit D902, C920, FB903, 0925 And other external components, form a parallel PFC circuit.
After power on, the AC 220V mains voltage is input to the two-stage filter circuit composed of C901, L901, C902, C903, C904, L902 through the fuse F901, which is used to filter out high-frequency interference pulses in the mains and prevent the TV The internal pulses pollute the mains grid. After AC 220V filters out high-frequency interference signals, after BD901 full bridge rectification and C910 filtering, it outputs a pulsating voltage of about 100Hz, which is sent to the PFC circuit. One way is to charge the PFC filter capacitors C920, C925 through D904 to form a voltage of 300V, and the standby state is the auxiliary power supply; one way is added to the D pole of the switching tube Q901 through the energy storage inductor L906.
After turning on the remote control, the power-on/off control circuit outputs VCC voltage. When it is applied to pin 8 of IC901, its internal circuit starts to work, and the PWM pulse is output from pin 7 to the G pole of Q901 through R932, R913, D905, so that Q901 is turned on. The 100Hz pulsating voltage output by the BD901 full-bridge rectification passes through the primary winding 1/2-5//6 of L906, the DS pole of the switch Q901, and R916 to form a loop to the ground. During the on-time of Q901, L906 stores energy and generates induced electromotive force in the primary winding 1/2-5//6, the polarity of which is left positive and right negative, and D902 is cut off.
When Q901 is turned off, the polarity of the electromotive force generated in the primary winding 1/2-5//6 of L906 is reversed, and superimposed with the 100Hz pulsating voltage output by the full bridge rectification, and then rectified by D902, filtered by C920, FB903, and C925, A boosted voltage B+ of 390V is formed to supply power to the main and auxiliary power supplies and the backlight boost output circuit.
The PFC output voltage is sent to the sampling voltage divider circuit of the PFC circuit through the current limiting resistor R801. When the B+ voltage output by the PFC circuit rises, the sampling voltage divided by R920～R922 and R923//R919 also rises, passing IC901's 1 The pin makes the internal error amplifier operate and adjusts the PWM pulse output from pin 7 to reduce the duty cycle of the pulse added to the G pole of Q901, thereby shortening the on-time of Q901, and the PFC circuit output B+ voltage drops. Voltage stabilization control function. When the B+ voltage output by the PFC circuit drops, the above process is reversed, and it also plays a role of voltage stabilization control.
Pin 5 of IC901 is the input terminal of the zero-crossing detector. The induced voltage on the secondary side of the energy storage inductor L906 is sent to pin 5 through R911 and C912 to control the phase of the internal oscillator circuit so that the duty cycle of the pulse modulation signal output from pin 7 follows The 100Hz voltage waveform signal is changed to achieve the same phase of the voltage waveform and the current waveform, preventing Q901 from being damaged when the input voltage is close to 0V and Q901 is in the conducting state when the peak and valley of the pulse comes.
The 4 feet of IC901 are the overcurrent protection input terminal. When Q901 will be severely overcurrent, the voltage drop formed at both ends of its S pole resistor R916 will increase, and the voltage on pin 4 of IC901 will also increase through R915, so that the overcurrent protection function inside the IC will act and the output of pin 7 will be cut off. Signal to form over-current protection. When the resistance value of R916 increases, it will cause the over-current protection function to malfunction and make the B+ voltage behave as 300V.
Solution to the failure of the pfc circuit of the LCD TV power supplyAccording to the analysis of the repair case, it is found that the PFC circuit of some LCD TV power supply boards is low due to the output PFC voltage.
Cause some untimely failures to appear, and the maintenance methods for such failures are now explained.
Trouble phenomenon: Unscheduled power-on, remote control does not power-on after long-term standby, unscheduled automatic shutdown, unscheduled black screen, etc.
Power board number: 4162, 5024, 2289, 1673, 1185, 1901, etc.
Involved models: LED40K16X3D, LED40K11P, LED40K16P, LED42T39AK, LED42K100, LED42K200,
LED42K300, LED42H310, LED46K01P, LED46T39AK, LED37K01, LED37K11,
LED37K16, LED50K660X3D, LED50EC660X3D, LED55T36GP, TLM37E29,
TLM37V68, TLM37V88P, TLM40V66PK, TLM40V68P, TLM40V68PK, TLM40V69P,
TLM42V67PK, TLM42V68PK, TLM42V86PK, TLM42P69GP, TLM42E58P, etc.
Key voltage: PFC output voltage should be above 380V, if it is lower than 370V, it is usually 340V-360V, which is very easy to output
This type of failure occurs.
The cause of the failure: The voltage input of the sampling feedback terminal of PFC chip 1 pin is too high, and the chip voltage stabilizing circuit reduces the excitation output PWM
The duty cycle of the signal causes the PFC output voltage to decrease, which affects the normal operation of the main power circuit.
The output voltage of the source circuit is abnormal.
Fault: PFC chip 1 pin peripheral sampling circuit has leakage (the adhesive leakage of the upper voltage divider sampling resistor)
Fault auxiliary judgment conditions:
1. Weather influence: The frequency of faults increases in humid climate.
2. Power-on conditions: the remote control does not power on in standby mode, and it can be turned on after turning off the main power supply.
Maintenance treatment method:
1. Preventive treatment
For humid climate areas, if there are many such faults, it is recommended to open the cover to repair the products of the above mentioned models, including
For non-power failure or replacement of a new power board, moisture-proof treatment of the PFC voltage and circuit is required to avoid subsequent occurrences
Untimely failure. The specific plan is as follows:
First, dry the area where the sampling resistors involved are located to remove moisture, and then coat them with 708 silica gel.
A resistor to prevent the influence of moisture.
2. Maintainability treatment
For malfunctioning TVs with the above phenomenon, it is recommended to measure the output voltage of the power supply circuit during maintenance, especially
For power boards with PFC circuits, if the PFC output voltage is lower than 375V, the PFC voltage sampling circuit must be
Overhaul processing. For this type of failure, it is mostly caused by the leakage of the PFC sampling circuit. The common problem is that the PFC chip is connected to pin 1
The adhesive leakage of the upper divider sampling chip resistor (510K or 1M) causes irregular faults. The specific plan is as follows:
Solder down the several SMD resistors involved, and clean the back glue (red glue) used in the SMD production process under the resistor, PCB
The adhesive on the backing is also cleaned up, and the resistance is measured and then soldered back. (Red glue is easy to absorb moisture, causing the impedance at both ends of the resistor to drop.
Cause the sampling voltage change), start measuring the PFC voltage and confirm that it is restored to above 380V.
The overhaul plan of the LCD TV power supply pfc circuit1. What is a PFC circuit? To put it plainly, the PFC circuit is to increase the +300V voltage after the bridge stack rectification to +375V----+400V. This is also the first point of the difference between the power supply of LCD TV and CRT TV. The second point of difference is that the secondary voltage is lower than that of CRT. The other parts are the same as the principle of ordinary switching power supply. The measured voltage across the large filter capacitor 330U/450V is +375V---+400V, which indicates that the power factor correction circuit is working normally; if the measured voltage across the capacitor is +300V, it means that the PFC circuit is not working, and the main check is for PFC oscillation integrated circuit. ! i$ M8 c& H. r3
2. When repairing the LCD power supply, first confirm the status of the fuse, the fuse is intact, usually the switch tube in the PFC correction circuit has not failed. Then measure whether there is a short circuit between the large electrolytic capacitor and the ground, and there is a charging resistance of more than tens of kiloohms, indicating that the power supply has not broken down. If the fuse is damaged, the first one should check the PFC correction circuit switch tube, and the second one should check the auxiliary power IC.
3. General output voltages of +5V, +12V, +24V below 40 inches; general output voltages of +5V, +12V, +18V, +24V above 40 inches. Among them, +5 V is the standby voltage, +12V is for the digital board, +18V is for the sound, and +24 V is for the backlight board. In practice maintenance, as long as each group of power boards with the same voltage and power can be replaced.
4. The power supply board can be removed from the TV for independent maintenance. When repairing, only need to short-circuit the switch control circuit transistors C and E (or connect a resistance of about 1.5K to the +5V output terminal of the auxiliary power supply). The machine is in the power-on state, and each voltage has an output. In the switching power supply of some LCD color TVs, only the +12V or +24V output terminal has a certain power load, the main switching power supply can be in normal working state. So you can connect a 36 V bulb of an electric bicycle to the +24V output terminal as a fake load (or connect a motorcycle bulb as a fake load at the +12V output terminal). ! ~) ^/ @+g,
5. After the LCD power supply is powered on, the auxiliary power supply works first and outputs +5V voltage to the CPU on the digital board. At this time, the whole machine is in the standby state. When the "standby" button is pressed, the CPU outputs the power-on level, and the PFC circuit works first, after converting the +300V pulsating DC voltage into a normal DC voltage (+380V), then the pulse width oscillator of the main switching power supply starts to work , Then the secondary output of the main switching transformer +12V, +24V voltage, the whole machine enters the normal working state. /
6. Protection circuit. In addition to the common spike absorption protection circuit in the LCD color TV switching power supply, there are also overvoltage and overload protection circuits for +24V, +12V and +5V voltage. The protection circuit mostly uses four operational amplifiers. LM324, quad voltage comparator LM339, dual voltage comparator LM393 or dual operational amplifier LM358. The over-current and over-voltage protection circuit can be disconnected and not used during maintenance. If the voltage returns to normal, it means that the protection circuit is caused. At this time, it is necessary to disconnect step by step which way is working. Then perform repairs. ) M7 c6 w4 M: s(L'? 4 {; Y
7. Before turning on the machine, confirm whether there are any explosive parts or capacitor bulging. If so, replace and measure all related components. It is recommended that when testing the machine after replacing all damaged components, it is best to remove the fuse of the original machine and connect a 220V/100W bulb, which can effectively prevent parts from being blown up again.
8. The output current of the main switch voltage of +24V or +12 V is relatively large, which requires higher rectifier diodes. Generally, low-dropout high-power Schottky diodes are used, which cannot be replaced by ordinary rectifier diodes. In addition, after the load is connected, the voltage rises instead, which is mostly caused by poor power filtering.
9. The load capacity of the power supply is poor. First, check whether the PFC voltage is normal (380V). If it is normal, the problem lies in the thick film of the power supply. It is usually caused by the poor load capacity of the thick film of the power supply. Please pay attention to this point.
10. On the power board, the heat sink with the ** triangle mark and the circuit under the heat sink are all thermal grounds. Strict direct contact with your hands! Note that any testing equipment cannot be directly connected between hot and cold ground.
Concluding remarksThis is the end of the related introduction about the LCD TV power supply pfc circuit. If there are any deficiencies, please correct me.
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