DELL 14-7447 notebook no booting™ 3801 is repaired without power on.

[machine model] DELL 14-7447 notebook

[motherboard version number] Quanta AM7

[Failure] Notebook no booting is not powered on.

[maintenance process]

Received a DELL 14-7447 notebook, and the fault description notebook no booting is not powered on. Disassembly detection,Diode value is normal at each key point of the main board. Power-on test. It is found that 19V is normally generated when 19V is connected, but it is not generated when 3V 5V is actually measured. EN is normal, EN is normal, and the peripheral faults of EC are basically eliminated first.

The oscilloscope stabbed to the output of 3V Inductor, and immediately looked at the wave pattern, and found that the wave pattern is as follows.

Maintenance of DELL 14-7447 notebook no booting without power on Figure 1

Two problems can be seen through the wave pattern:

1. 3V overvoltage Current protection overvoltage Current protection point is exactly 3.87V or so. It meets the OVP Current protection threshold of the chip by 117.5%(3.3V*1.175=3.8775V)

2.VCC has been produced normally and can maintain hundreds of Ms. If the OVP action caused by the power supply circuit itself, it should be soft start+overvoltage delay Current protection, and there will be action in time. The soft start time is 0.91MS or so. Over-voltage delay time is only 0.5US, which is only 1MS or so. And or so power supply can maintain so many MSS, basically excluding Current protection power supply itself.

The preliminary judgment is that after the load Switch is turned on, the current control loop of the 3V power supply detects that the load current is too large, and the over-voltage of VCC is caused by the long opening time of the upper tube when the upper tube is turned on for a long time within the OC threshold time.

Immediately, all the rear stages of Switch converted by 3V were detected, and it was found that VCC such as 3VRUN did not have Short circuit. During the actual measurement, it was also found that EC was not working properly. I haven’t received the elevation of S3 yet, and I haven’t opened the rear Switch at all. Can’t find the problem, blind test, test thisMeasure thatS3# didn’t play well before, so it took a big test. I didn’t find any.

I can’t find the problem. I studied it carefully. Look at the wave form of 3V overvoltage. It’s not that the overvoltage of switch took almost 2 MS in an instant. There is a big slope, which means that it is not a 3V direct Short circuit voltage, but probably short circuit voltage. This short circuit voltage is actually easy to check, and the load capable of short circuit voltage must meet two conditions. That is, it meets the requirements at the same time. There are short circuit sources with more than 3V and 3V,short circuit. atnotebookIs to meet the requirements of 3V and 5V.

There is basically only one chip that meets the conditions of VCC. It’s the USB VCC chip (5V VCC is rarely used in notebooks). It’s true that the problem was found when U25 was measured immediately. The USB_RIGHT_EN# of 4 pin has obvious abnormal wave pattern as shown below.

Maintenance of DELL 14-7447 notebook no booting without power on Figure 2

But this signal is not directly connected to 3V, but to EC. Is it through the logic circuit inside EC? Such as pulling up to 3V? In order to confirm, the 3V is stable after specially disassembling the EC, and the judgment is 5V short circuit to USB_RIGHT_EN#. Then, because the EC uses 3.3V VCC, it is pulled up by the EC or other logic circuits. String to 3V, causing 3V overvoltage. It is not the short circuit of 3V 5V in U25, but the 5V VCC and EN short circuit in U25, which caused the 3V abnormality through the logic circuit in EC. But the ultimate reason is U25. Immediately replace U25 as shown in the figure below. After the replacement is completed, power on directly and trot all the way.short circuit, the maintenance is complete. Troubleshooting, the maintenance is over.

Maintenance of DELL 14-7447 notebook no booting without power on Figure 3