Lenovo ThinkPad L450 notebook cannot be charged and repaired

Lenovo ThinkPad L450 notebook cannot be charged and repaired.

Lenovo ThinkPad L450 notebook can’t be charged. After measuring the motherboard, it is found that 20V is only 19.4V The investigation focuses on these two protective isolation tubes.

[Machine model] Lenovo ThinkPad L450

[Malfunction] The notebook cannot be charged.

[maintenance process]

When I received Lenovo ThinkPad L450, the fault described that the notebook did not charge. If the battery is used, it functions normally, and the battery has been replaced by itself. The plug-in test found that there was more than 20 currents, and there was really no charging action after the battery was installed. First, I tested my own battery, and found that there was a current of 600 mA, but my battery was not fully charged, so I had to get a charging current of more than 1A, which was determined to be a motherboard problem. Disassemble the machine, test again, and find that the single battery is in normal use.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 1

When measuring the motherboard and charging abnormality, first test whether the common point voltage is normal. It was found that 20V was only 19.4V when measured.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 2

It can be judged that the fault range is basically that the protection isolation is not fully working, which causes the back-stage abnormality. Focus on these two protective isolation pipes.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 3

First, measure that the S-pole conduction voltage of the first tube PQ20 is 20.2V, which is consistent with the voltage of the power supply.

The re-measurement control signal is also 20.2V normal.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 4

The 20.6V of the control electrode of PQ21 is a little low.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 5

Power off after voltage measurement, thinking of testing again, unplugging the plug and powering on again, 5A heavy current! Is there a big short circuit in the common point of the motherboard? When the power is cut off again, the resistance value of common point is more than 600, and there is no short circuit.

It’s still normal to plug in the battery and start it up. Check that the adapter current is 600 mA again, and it’s not short-circuited. This month, no less than four protective isolation tubes have been repaired, and the fault is also that the battery is normal, so the adapter is short-circuited. It seems that this is also a breakdown of protection and isolation!

Next, measure whether the resistance of the protective isolation tube is normal. First, the reverse resistance of PQ21 is infinite, and the forward resistance of 560 is normal. For accurate disassembly measurement, the protective isolation tubes are all N-channel tubes, which are turned on at a high level. The positive and negative probes and control electrodes are applied with positive voltage, and the working conduction is normal, and the reverse voltage is also turned off. Then, measure another PQ20.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 6

When measured on the motherboard, it is found that the resistance value is zero, and it also conducts in both directions. If this tube is broken, remove it and replace it. After dismantling and measuring again, it was found that the bidirectional conduction controller was out of control.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 7

The board is disassembled, repaired and plugged in for testing. The voltage at the common point is directly measured to 20.3V, which is consistent with the adapter voltage.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 8

Installation, battery test, plug-in current rose by 1.67A perfectly and it is still rising steadily.

Lenovo ThinkPad L450 notebook cannot be charged and repaired Figure 9

Enter PE for charging test, and it has also been shown as the power supply to turn on the charging. In order to avoid false charging, it is better to measure whether the control voltage of the charging tube is normal.

Lenovo ThinkPad L450 notebook cannot be charged for maintenance Figure 10

It is found that there is also a control voltage of 14,3V, and it seems that it is really charged.

The customer’s battery is hungry, and there is no electricity at all. After startup and shutdown for charging test, the battery has not been completely scrapped, and it can be fully charged. Until the end of this repair.