Lenovo 340c-15iwl laptop can not be started for maintenance

Machine model: Lenovo 340c-15iwl

Fault phenomenon: the laptop can not be turned on

Maintenance process:
Colleague sent a Lenovo notebook 340c, the problem is no booting, plug in DC power supply, there is 0.01 current, press the power on button.If the keyboard can not be turned on or off, the keyboard of no booting is broken.
For the detection resistance pr5301 Measuring diode value in Current protection isolation, it is found that shorted to ground is directly Current heating test.Sure enough, a Capacitor is too hot to touch. Remove Capacitor and the motherboard is no longer Short circuit.I thought it should be OK, but I still stand by 0.01 no power on. It seems that it has to be repaired slowly.
First of all, pr5301 does not have 20V VCC, and then pq5301 and pq5302 are measured. The results show that the two tubes are not connected. Generally,Current protection isolation tubes are all P tubes. According to the drawing, these two tubes are n-tubes, and the g-pole of n-tubes can be connected only when the g-level is low-level, so they are not conductive.

Lenovo 340c-15iwl laptop can’t start maintenance Figure 1

Look at the drawing pq5301 and pq5302 are controlled by the 4 pin acdev of charging chip bq24780, and the 4 pin measured by bq24780 is also low level. It is analyzed that the drawing chip 4 pin must be VCC of 20V or so to connect pq5301 and pq5302.The condition of measuring charge and discharge chip is 28 pin 20V, 24 pin 6V and 6 pin 2.7V. According to the principle, if the condition meets 4 pin, there should be a 20V high-level output. Theoretically speaking, if the condition is satisfied, the chip will be broken if it doesn’t work. So we plan to change another chip for test.

If you look at the drawing, there is a pj5301 node. According to past experience, there is also a problem behind it, which leads to the failure of Current protection chip out of VCC.Disconnect pj5301 decisively, and then measure the g-level of pq5301 and pq5302. There is a high level of 24V, and the two tubes are also connected. It seems that the chip is good, or is it caused by the following problems.

However, no problem was found after typing diode value. After carefully looking at the drawings, several Capacitor and a pq5303 were found after pj5301 in the Current protection isolation circuit. After measuring pq5303, no problem was found. Think about it or remove pq5303, connect pj5301 node, power on the power source, and the current will rise instantaneously current of change, and then connect the screen to test Normally booting up.

Lenovo 340c-15iwl laptop can not be started maintenance Figure 2

It is added that the battery can be charged and discharged without pq5303 during the system test, but it is added for the stability and safety of the computer.

Lenovo 340c-15iwl laptop can’t start maintenance Figure 3

Conclusion: the main board Short circuit caused pq5303 soft breakdown, resulting in charging chip bq24780 Current protection, resulting in Current protection isolation circuit not conducting.In this case, if the chip is misjudged to be bad, it is easy to take a detour.

Lenovo 340c-15iwl notebook can not start maintenance Figure 4