How to judge the quality of the power IC chip in the circuit?

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How to judge the quality of the power IC chip in the circuit? [Copy link]

How to accurately judge the quality of the power IC chip in the circuit in daily circuit maintenance work is an important part of repairing TV, audio and video equipment. If the judgment is inaccurate, not only will a lot of energy be spent, but the key point is that the fault in the integrated circuit still exists. Therefore, it is a compulsory course for every repairer to make a correct judgment on the integrated circuit. Let Liwei Electronics focus on power management IC to briefly explain how to judge whether the power IC chip in the integrated circuit is normal. 1. First of all, you must understand the purpose, internal structure principle, main electrical characteristics, etc. of the IC in the circuit, and analyze the internal electrical schematic diagram when necessary. In addition to these, if there are DC voltages, waveforms, and forward and reverse DC resistance values of each pin to ground, then it provides more favorable conditions for judgment before inspection. 2. Then determine the location according to the fault phenomenon, and then find the faulty component according to the location. Sometimes multiple judgment methods are needed to prove whether the device is indeed damaged. 3. There are generally two methods for checking and judging the IC in the circuit: one is offline judgment, that is, the judgment that the IC in the circuit is not soldered into the printed circuit board. This method is very difficult to determine the quality of the IC in the circuit without special instruments and equipment. Generally, the DC resistance method can be used to measure the forward and reverse resistance values between each pin corresponding to the ground pin, and compare it with the intact integrated circuit. The replacement method can also be used to insert the suspicious integrated circuit into the position of the same model integrated circuit of the normal device to determine its quality. Of course, if conditions permit, the integrated circuit tester can be used to quantitatively test the main parameters, so that the use is more guaranteed.

4. There is also online inspection and judgment, that is, the judgment method of the integrated circuit connected to the printed circuit board. Online judgment is the most practical method for repairing integrated circuits in television, audio, and video equipment. The following is an explanation of several situations:

DC working voltage measurement method: It is mainly to measure the DC working voltage value of each pin to the ground ; and then compare it with the nominal value to judge the quality of the integrated circuit. Using voltage measurement to judge the quality of the integrated circuit is one of the most commonly used methods in maintenance, but it is important to distinguish non-fault voltage errors. When measuring the DC working voltage of each pin of the integrated circuit, if the voltage of an individual pin does not match the voltage value marked in the schematic diagram or maintenance technical data, do not rush to conclude that the integrated circuit is damaged. You should first eliminate the following factors before making a determination.

1)Whether the nominal voltage provided is reliable, because the values marked on some manuals, schematics and other materials are quite different from the actual voltage, and sometimes even wrong. At this time, you should find more relevant information for comparison, and if necessary, analyze the internal schematics and peripheral circuits and then perform theoretical calculations or estimates to prove whether the voltage is wrong.

2)It is necessary to distinguish the nature of the nominal voltage provided and what kind of working state the voltage belongs to. Because the individual pins of the integrated circuit change significantly with the injection signal, you can change the position of the band or the recording and playback switch at this time, and then observe whether the voltage is normal. If the latter is normal, it means that the nominal voltage belongs to a certain working voltage, and this working voltage refers to a certain condition, that is, the working state of the test is different, and the measured voltage is also different.

3)Pay attention to the pin voltage changes caused by variable components in the peripheral circuit. When the measured voltage does not match the nominal voltage, it may be because the individual pins or the peripheral circuits related to the pins are connected to a variable resistance potentiometer or switch(such as volume potentiometer, brightness, contrast, video, fast forward, fast reverse, record and play switch, audio amplitude modulation switch, etc.). The pin voltage will be significantly different due to the different positions of these potentiometers and switches. Therefore, when a pin voltage does not match, consider the position change of the pin or the potentiometer and switch associated with the pin. You can turn or pull the pin to see if the pin voltage is close to the nominal value.

4)Prevent errors caused by measurement. Errors may be caused by different internal resistances of the multimeter head or different DC voltage ranges. In general, the DC voltage marked is tested when the internal resistance of the test instrument is greater than 20KΩ/V. When a multimeter with an internal resistance less than 20KΩ/V is used for testing, the measured result will be lower than the originally marked voltage. In addition, it should be noted that the voltage measured on different voltage ranges will be different, especially when using a large range, the reading deviation will have a more significant impact. 5) When the measured voltage of a certain pin is inconsistent with the normal value, the quality of the IC should be judged based on whether the pin voltage has a significant impact on the normal operation of the IC and the corresponding changes in other pin voltages. 6) If the voltage of each pin of IC is normal, it is generally considered that IC is normal; if the voltage of some pins of IC is abnormal, we should start from the point where the deviation from normal value is the largest and check whether the peripheral components are faulty. If there is no fault, IC is likely to be damaged.5pt]7) For dynamic receiving devices, such as televisions, the voltages of each pin of the IC are different when there is a signal or not. If you find that the voltage of the pin that should not change changes greatly, and the voltage that should change with the signal size and the different positions of the adjustable components does not change, it can be determined that the IC is damaged.

8) For devices with multiple working modes, such as video recorders, the voltages of each pin of the IC are also different in different working modes.

The above points are that when there is no fault in the circuit, the measured results are different from the nominal value due to some reasons. Therefore, in general, a test condition should be specified when testing the DC voltage or DC resistance of the integrated circuit, especially when it is recorded as actual experience data. Usually, each potentiometer is rotated to the mechanical middle position, and the signal source uses a standard signal under a certain field strength. Of course, if the position of each function switch can be recorded, it will be more representative. If the measured voltage of individual pins still does not meet the nominal value after excluding the above factors, further analysis of the reasons is required, but there are only two possibilities. One is caused by the fault of the integrated circuit itself; the other is caused by the peripheral circuit of the integrated circuit. Distinguishing these two sources of faults is also the key to repairing integrated circuit home appliances.

AC working voltage measurement method: In order to grasp the changes of ICAC signal, you can use a multimeter with a dB jack to make an approximate measurement of the ICAC working voltage. When testing, the multimeter is set to the AC voltage block, and the positive probe is inserted into the dB jack; for multimeters without a dB jack, it is necessary to connect a 0.1~0.5uF DC blocking capacitor in series with the positive probe. This method is suitable for ICs with relatively low operating frequencies, such as the video amplifier stage and field scanning circuit of a television. Since these circuits have different inherent frequencies and waveforms, the measured data are approximate values, or are regarded as presence or absence.

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Online inspection determines whether the integrated circuit needs to be powered on for testing. Not everyone can measure it. Comparative measurement is often used.

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Maintenance is sometimes more troublesome than design