Hello, Lenovo Thinkpad E14 Gen.2 Type 20TA, GE4B0/GE5/B0 NM-D011 Rev. 1.0 board. Arrived in the following state: the device is always on and goes in a cycle: the "on" LED lights up, goes out after a few seconds and lights up again, cannot be turned off with the button. Inspection of the board is ok. The charger switches from 5V to 20V. I read that it seems to be a typical problem: the input MOSFETs QB5, QB6 do not open enough and get very hot due to the fact that TPS65994ADYBGR does not supply 28V to the gates. In my case, there were 21.4V on the QB5 gate, 28V on the QB6 gate, but the input MOSFETs do not heat up. The board is leaking, it seems to be approximately in the TPS area. The type-c jack is ok. The TPS strapping is ok. I changed the TPS (took from 2 sources), changed the input MOSFETs, no changes. The voltage of 21.4 V is not constant, but has a sawtooth shape, as if it is trying to reach 28 V and drops to 20 V. Consumption is 4-5 mA. Voltage and resistance on the TPS are ok, there is exchange on CC1 / CC2. I removed the capacitors that could theoretically affect (CB74, 75, 81) and the DB18 diode - no changes. Please tell me what else to look at, what could be leaking, are there really 3 TPS and all of them are defective? Thank you
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For each mosfet in the power path, test the resistance with no power to the board.
Measure:
source / drain
drain / gate
gate / drain
Post each measurement. Each should be hundreds of k ohms or higher else the mosfet may be defective. Test QB5 & QB6 for now.
The gate voltage on an N-channel mosfet must be boosted to a higher value than what is being passed between source / drain pins to enable. If you see 28v on QB6, I can only assume that QB5 should be similar unless QB5 is defective. -
Thank you.
Measured:
QB5
source / drain 1,08kohm
drain / gate 3mohm
gate / drain infinity
QB6
source / drain 1,08kohm
drain / gate 3,1mohm
gate / drain infinityComment
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The source / drain on each is too low. Flux and remove the mosfet(s). Test them again when they are on your workbench. Both may be defective.Comment
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Thank you, you have right, one of them (was new !!!) was defective, replaced both, now I have 28v at both gates, power consumption 8-12mAComment
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