1, the LCD Power Bank, the secondary power supply first work, the output +5 V voltage to the digital board CPU, this machine is in standby mode. When the "standby" button, the CPU output boot level, PFC circuit first work, + 300V pulsating DC voltage into a normal DC voltage (+ 380V), then the main switching power supply pulse width oscillator began to work , Then the main switch transformer secondary output +12 V, +24 V voltage, the machine into the normal working condition.
2, what is the PFC circuit? PFC circuit that white is the bridge after the rectifier +300 V voltage increased to +375V ---- +400 V. This is the TV LCD Power Bank and CRT TV power of the difference between the first point, the difference is that the secondary voltage is lower than the CRT, the other places with the same general principle of switching power supply are the same. Measured the filter capacitor 330U / 450V voltage is +375V --- +400 V, then the power factor correction circuit is working properly; if the measured voltage across the capacitor +300 V, indicating that the PFC circuit is not working, the main check PFC oscillation integrated circuit.
3, maintenance of LCD Power Bank, the first to confirm the state of the fuse, the fuse is intact, usually PFC correction circuit switch tube, etc. did not fail. And then measure the large electrolytic capacitor to the ground whether there is a short circuit, there are dozens of ohm above the charge resistance, indicating that the power is not breakdown. If the fuse is damaged, the first one should check the PFC correction circuit switch and the second to check the secondary power IC.
4, 40 inches below the general output + 5V, +12 V, +24 V three groups of voltage; 40 inches above the general output +5 V, +12 V, +18 V, +24 V four groups of voltage. Which +5 V for the standby voltage, +12 V for the digital board, +18 V for sound, +24 V for the backlight board. In the practice of maintenance, as long as the voltage of each group, the power of the same power board can be replaced.
5, the power board can be removed from the TV on the independent maintenance, maintenance only need to switch the circuit control circuit transistor C, E short (or a resistance of about 1.5K and the secondary LCD Power Bank + 5V output connected), the whole Machine is in the boot state, the voltage is output. In some LCD TV switching power supply, only +12 V or +24 V output with a certain power load, the main switching power supply to the normal working condition. So at the +24 V output you can take an electric bike's 36 V bulb for a dummy load (or a + 6V output with a motorcycle light bulb).
6, protection circuit, in the LCD color TV switching LCD Power Bank, in addition to a common spike absorption protection circuit, but also in the +24 V, +12 V and +5 V voltage overvoltage, overload protection circuit, the protection circuit using four operational amplifiers LM324, four-voltage comparator LM339, dual-voltage comparator LM393 or dual op amp LM358. Over-current over-voltage protection circuit, in the maintenance can be removed from the unused, if the voltage back to normal, indicating that the protection circuit caused by this time to step off which is the way to work. And then repair.