Konka's 32-inch LCD TV has a black screen. After disassembling the power, observe that the backlight is not lit. Test the LED boost filter capacitor 250V / 22UF voltage is 98V, equal to its supply voltage. The BOOST boost circuit is not working properly, and the fault has nothing to do with the LED strip. Test LED driver block N701OZ9902CGN (16-pin) 1-pin UVLS, input voltage undervoltage protection detection pin voltage is about 3.8V, 2-pin VCC working voltage input pin is 12V, 3-pin ENA chip on/off control input pin is 4V Left and right, both are normal and the main chip has issued a backlight turn-on command. After the power is turned off, the BOOST boost circuit is detected. It is quickly found that the LED boost diode VD751SB2200 is broken down. After replacing the VD751, the test machine is OK. After discovering many cases, it is estimated to be a common problem.
The following is a partial circuit diagram of the actually drawn LED boost circuit.
The BOOST boost circuit is mainly composed of V701, L701, L703, VD751, C753, R715, R714 and other components, similar to the PFC circuit in the switching power supply, L701, L703 is the energy storage inductor.
When the PWM driving pulse of the 15-pin output of the LED backlight driving integrated block N701 is high level, the switching transistor V7014N25 is turned on, and the 98V voltage passes through the L701-L703-V701-R714/R715-ground to form a current loop. At this time, L701, The left + right-voltage is formed in L703, the diode VD751 is turned off, and the energy is stored in L701, L703. When the PWM drive pulse of the 15-pin output of the N701 is low, V701 is turned off. Since the current at both ends of the inductor cannot be abrupt, the induced electromotive force polarity in L701 and L703 is reversed, that is, left-right +, and input 98V. The voltage superposition turns VD751 on and charges C753, during which L701, L703 release energy outward. The measured voltage on the C753 250V/22UF capacitor is about 156V.
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