I. INTRODUCTION Most of the popular 14- and 15-inch computer monitors use UC3842 (domestic model C3842) to complete functions such as startup, oscillation, voltage regulation, synchronous introduction, and drive. Together with the high-power MOS tube, it constitutes a radical Wide-range pulse-type regulated power supply. It is widely used in ENVISION, EC-1428, EC-1439, WANSTROW, CM-1428D, CM1431D, CM1435D, CM1439D, EMC, EM-1428, etc. At the same time, it also has reference to other color displays using UC3842 as power supply. value. Therefore, mastering the circuit principle of the power supply is of great significance for maintenance.
Second, the integrated circuit UC3842
1 UC3842 internal circuit structure shown in Figure 1.
2 î€ integrated block pin function shown in Figure 2.
3î€UC3842 electrical characteristic parameters Table 1 UC3842 DC voltage of each foot to the ground (working state)
Table 2 Recommended working conditions 3. Power circuit analysis is connected to the power supply. 220V AC is rectified by BD101. After C106 filtering, 300V pulsating DC is divided into three channels, as shown in Figure 3. The first path is added to Q101 via the primary of the switching transformer. Source; the second way, through the starting resistor R106 to provide the starting voltage for UC3842; the third way, through R130 to the IC101 overcurrent protection pin. When the R104 is open, the voltage will greatly exceed the protection voltage of 1V for protection; or the Q101 overcurrent will form an overcurrent protection when the voltage drop of 1V or more is formed on R104. When the UC3842 gets the startup voltage (the voltage must be higher than 16V, the circuit is activated), the IC101 internal circuit works. R106 and C112 form a delay circuit. Only when C112 is charged for 0î€5~1 seconds, the startup voltage of UC3842 will reach 16V. At this time, IC101 is started, and the square wave signal of 6-pin output is added to the gate of Q101 through R110. , so that Q101 works in the switch state. When Q101 is in the on state, +300V DC through the switching transformer T101 primary winding 1-3, Q101 source and drain, R104 to ground, electrical energy is converted into magnetic energy stored in the T101 primary winding; during Q101 cut-off, T101 secondary winding induction The voltage is rectified by a diode, and the capacitor filter supplies power to each load. The voltage induced by the 5-6 winding is rectified by D103, and the DC voltage formed by C109 filtering is used as the normal working voltage of IC101, and is also supplied as a sampling voltage to IC1011 and 2 for error amplification. D104 is an isolated working tube. The line sync signal is introduced into the IC1014 pin by C116 and R115, forcing the oscillation frequency of the internal oscillation circuit of IC101 to be synchronized with the line frequency.
Voltage regulation process: When the AC voltage rises or the load is reduced, the secondary output voltage will rise. The voltage induced by the 5-6 winding will be increased by the voltage divided by R113, VR101 and R114 and then applied to the IC1012 pin. High, through the IC101 internal logic operation circuit operation, the square wave duty cycle of the 6-pin output changes, the on-time of the switch Q101 becomes shorter, the cut-off time is extended, and the output voltage is lowered. On the other hand, if the AC voltage is lowered or the load becomes heavier, the on-time of Q101 becomes longer, the off-time becomes shorter, and the output voltage rises.
Synchronous oscillation: When the switching power supply starts working, it is in free oscillation state. When the line circuit starts working, the line synchronization signal is limited by C116 and R115, and D105 is limited. After D106 is shaped, a positive synchronous pulse signal is formed, and then coupled to C115 through C115. The IC1014 pin forces the IC101 internal oscillation to synchronize to the horizontal frequency, thus avoiding power supply interference with the image.
Fourth, the failure analysis [Example 1] failure phenomenon: EVENBION-1428 color display three no.
Analysis and maintenance: The Q101 source and each output voltage are both 0V. Test IC101, except that the voltage of pin 3 is high, the other is basically normal, and the 3 pin is the overcurrent protection terminal. When the voltage exceeds 1V, the protection is started. From the voltage analysis of the 3 pin, it is very likely that the load will be short-circuited, and the shutdown test tube ce knot has been hit. Wear a short circuit. Causes of line pipe breakdown: 1) The line reverse path capacitance becomes smaller, which makes the line reverse pulse increase, which exceeds the line pipe ce junction withstand voltage and penetrates the pipe; 2) the line load is too heavy for a long time, resulting in the line pipe Overheating damage; 3) The power supply voltage is too high, causing the reverse voltage to become high and breakdown the tube; 4) the quality of the line tube is poor. Remove the line tube, measure the power supply voltage, and find that each output voltage is too high, and adjust the output voltage of VR101. There is no change in the 6-foot square wave signal. Check the IC101 error amplification 1 and 2 feet. There is no problem in the quality of the external components. When adjusting VR101, the voltages of pins 1 and 2 are obviously changed, the error amplification input is normal, and the output voltage cannot be adjusted. From the inspection process combined with the working principle, it can be introduced that the fault should belong to IC101. After replacing IC101, adjust VR101, the output voltage changes normally, and the recovery tube is adjusted to 88~96V, and the test machine is normal.
Summary: Pipeline damage is nothing more than the above several cases, the high voltage in this case is the cause of damage to the pipe. Since the voltage is high and cannot be adjusted, IC101 should be mainly checked.
[Example 2] Fault phenomenon: When the power is turned off, the indicator diode is not lit.
Analysis and overhaul: When the power is turned off, the indicator diode is not lit, indicating that the power supply is not working. It is measured that there is no 300V voltage at both ends of C106, and it is found that the fuse is black and broken, and R104 and R105 are burnt black. Q101 breakdown, R104 blown due to Q101 breakdown, R105 burned out the cause is unknown, R105 end is connected with Q101S pole, the other end is connected with IC1013 foot, R105 may be caused by UC3842 damage. Replace all damaged components and everything is normal after booting.
Summary: The fuse burned must be caused by the rectifier bridge BD101, the filter capacitor C106 is faulty, the Q101 breakdown or the degaussing circuit is shorted. This fault is caused by the breakdown of Q101, which causes R104 to burn out, but the R105 burnout may be caused by UC3842 damage. Therefore, while replacing the above components, the UC3842 must be replaced at the same time, otherwise, large-area components will be burned again.
[Example 3] Fault phenomenon: When the power is turned off, the indicator diode is not lit.
Analysis and maintenance: The voltage at both ends of C106 is normal, the T101 secondary has no output, and the Q101G has no voltage (normally 2î€3V). At this time, the voltage of IC1017 is measured immediately, only 5V, which is obviously too low. The operating voltage of the UC3842 should be 10 to 30V. Shut down the R106 and measure it normally. Solder the connection between R106 and C106. Use a multimeter to measure the voltage between the lower end of R106 and the ground (R106 should be provoked at the lower end) and read about 85V. After the discharge, the R106 is welded, and the retest is still 5V. It is suspected that the UC3842 is broken. Take IC101 off and test the 7-pin and 5-pin forward-reverse resistance and the normal value is far from the normal value, only a few kΩ (normally, the black pen is connected to 7 feet, the red pen is 5k when it is 50kΩ; the red pen is connected to the 7-pin, When the black pen is connected to 5 feet, it is 150kΩ). It is suspected that the UC3842 is damaged. After replacement, the fault is eliminated.
Summary: The 7-pin internal of the UC3842 is a 36V regulator. When the regulator is damaged, the voltage drop will be severe or zero. The power supply voltage of this fault is seriously degraded, and the integrated block is hot. Generally, the internal block of the integrated block is short-circuited, and the fault will be eliminated after replacement.
[Example 4] Fault phenomenon: After the power is turned on, the indicator diode flashes on and off, and at the same time, the sound of "å±, å±" is emitted inside the machine.
Analysis and overhaul: After the power is turned on, the indicator diode turns on and off, indicating that the power supply starts to oscillate, but immediately stops vibrating. From the analysis of the working principle of the power supply, the vibration stop may be caused by overcurrent. The above analysis was confirmed by measuring the voltage across R104. The overcurrent caused by the load should be caused by a short circuit of the load, and each load is especially the object of the line circuit. First measure the tube ce knot and find that it has broken down. This fault is caused by poor quality of the pipe. After replacing the pipe, the main input of the power supply is normal, the grating size and brightness are normal (indicating that the reverse-pass capacitor is normal), and the fault is eliminated.
Summary: The cause of the fault is that the Q101 is over-current due to short-circuiting of the pipe, causing protection. When the voltage of the 3 pin is lowered, less than 1V, the 6-pin outputs a square wave, and the power supply starts to vibrate, but the overcurrent protection. In this way, the oscillation protection is repeated, and the above-mentioned indicator light is turned on and off.
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