Troubleshooting of UC3842 laptop power adapter

Recently, I received a repair request for a laptop computer power adapter. This power adapter was a by-product (not an original accessory. It was a "three-no" product). Although the inspection and troubleshooting were not difficult, the cause of the failure was intriguing.

The power adapter was sent for repair because of no output voltage. When the AC power is turned on, the green indicator light does not light up. Check the AC input cable and there is no circuit breakers. It is determined that the internal components of the power supply are damaged. Use a "one" screwdriver to pry open the shell and check the circuit board. It is found that the fuse F1 in Figure 1 has been blown. It is suspected that the primary component has a breakdown current phenomenon. So the focus is on checking D4~D7, C7, and Q1. No abnormality is found, so F1 is replaced and the machine is powered on for testing, but the power supply still has no output. At this time, the two ends of the main filter capacitor C7 are measured +300V, which is normal. After the power is turned off, the two ends of C7 are still stored with high voltage, indicating that the switching power supply has not started. According to the internal schematic diagram of the control chip in Figure 2, it can be seen that the seventh foot of KA3842 is the starting end. When working normally, +300V is connected to the starting resistor R11 to start the control chip initially. This voltage must be above 16V (the starting current is about 0.5mA) to make the chip start normally. When KA3842 starts successfully, the pulse voltage of the ④ and ④ windings of the switching transformer is rectified by D1 and limited by R2. After being filtered by C5, a DC voltage of about 12V is obtained to replace the 16V starting voltage provided by the starting resistor R11 to power KA3842. When the power is on, the voltage of the ⑦ pin of KA3842 is 0V. For convenience, the author did not remove R11 for measurement (which caused unnecessary losses later), but only cut off the printed circuit of R11 and the ⑦ pin of KA3842. When the power is on, the voltage at both ends of C5 can rise to more than 16V, indicating that the starting resistors R11 and C5 are normal. After the power is turned off, the resistance of the multimeter is used to measure that KA3842 is suspended. The positive and negative resistance of the ⑦ pin to ground are both 75Ω. According to the internal schematic diagram of the device, this should be abnormal. It is very likely that the 35V voltage regulator between pin 7 and pin 5 has broken down, causing the startup voltage of R11 to short-circuit with the ground. Therefore, KA3842 was directly replaced with UC3842. After the printed circuit wiring of pin 7 was connected and powered on, the power indicator lighted up and then went out. Inspection found that the voltage regulator in UC3842 broke down again, just like KA3842. This is strange. The startup voltage is limited by R11, and the normal working voltage is also limited by R2. Moreover, the damage of the resistor will only increase the resistance or open the circuit, and generally will not cause a short circuit or a decrease in resistance. Then what caused the newly replaced internal voltage regulator of IC to break down again? To break down the diode inside the IC, the voltage must exceed the voltage regulation value of the tube. The second is a large current. However, after checking the printed circuit wiring design, no high voltage leakage to pin 7 was found. Could it be that the resistance of R11 is decreasing? Although it is impossible, I can't think of other factors.

When R11 was removed for inspection, it was found that although its resistance value was normal, a closer look revealed that there was a little burn mark on the first ring of the resistor color ring. When the printed board was checked, it was found that it was due to the compact arrangement of components on the printed board. R11 and R12 were both installed upright and close together. The pin of R12 touched the shell of R11. When the reverse peak high voltage on R12 broke through the insulation layer of the R11 shell, it was directly connected to the 7th pin of IC, causing its internal voltage regulator to break down and not work. This is equivalent to the negative electrode of D1 in the reverse peak absorption network directly touching the 7th pin of IC. The reverse peak high voltage and the starting voltage are superimposed on each other, and the voltage regulator in the IC can be broken down in an instant. Therefore, R11 was installed, and R12 was separated by an insulating material at the place where it touched. KA3842 was installed, and the power-on indicator light was lit normally. The output point output +20V was measured to be stable, and the switching power supply returned to normal.

Summary: After analysis, the cause of the failure is that the layout of the components of the unknown brand power supply is unreasonable. The two high-power resistors R11 and R12, which are welded vertically, are too close. When the power supply is accidentally vibrated or dropped, the two resistors collide due to inertia, causing the failure (which is also a common problem of this type of switching power supply). The brand model of this power supply is LI SHIN INTERNATIONAL ENTERPRISE CORP, model: 0335A2065. Users who have this power adapter should pay attention. I hope that readers will not choose unknown brands for cheapness when purchasing such key accessories, so as not to lose more than the small.

KA3842 pin function definition:

Pin function, definition

The internal error amplifier output terminal and the external Rc compensation network of pin ② can

①Determine the gain and frequency response characteristics of the error amplifier

②Feedback voltage input terminal, grounded in this circuit, control ① foot to achieve this function

③ Current detection terminal, detect the timing terminal of Q1 source current. External timing resistor and capacitor, the operating frequency of the oscillator is f=1.8

④(RTCT)

⑤ Grounding

⑥ Output terminal. Driving capacity is ±1A. Startup) Power supply terminal, if the startup voltage is lower than 16V, KA3842 will not work. Work

⑦ The rear limit voltage is 30V, the start threshold is 16V, and it will be shut down if it is lower than 10V

⑧ Reference voltage output terminal, outputs accurate +5V voltage, up to 5mA