Got this laptop in for service. It won't show a charge light or turn on. PQ2,PQ3 AO4433 are part of the power on circuit. I see:
PF1: 19.1v
PQ3-Drain 19.1v
PQ3-Source 19v
PQ3-Gate 19v
PQ2-Source 19v
PQ2-Gate 19v
PQ2-Drain 0v to board power
Power is passing through PQ3 just fine. Power is not passing through PQ2. So I attach my bench supply to PR2. At about 1v PR2 draws about 1.5 amps. Something is shorted. Off goes the bench supply and I remove the CPU since it can look like a short then start to test capacitors. Some tantalums around CAP15 appear to be shorted but the diode voltage drop of 0.025 to 0.050 is is too high so it's probably the supply to the Northbridge U28. Many ceramic caps near and including (but not limited to) PC108, PC138, PC146, PC150 appear shorted with the same diode voltage drop of 0.005 as PR2. Mosfets PQ27 and PQ28 have the same 0.025 voltage drop as the nearby caps so are probably not shorted.
These shorted parts must be part of the same power plane. So I attach the bench power supply to one of the shorted caps and turn up the power until about 1.2 amps is drawn. I feel around to all the caps I remember to show shorted and any close semiconductor and notice that PC139 is burning hot. PC139 has a visible flaw which looks like a scratch from my probes. I probed many caps and none of them have scratches so the flaw must be internal. Off PC139 comes and now the short is gone.
The board still won't power up. Adapter connected and the board does nothing when power is pressed. 0.68v is supplied to the many MOSFETS waiting to operate. 5v and 3.3v supplies from PU13 are not powered. 12v to the battery terminals makes the LEDs blink twice green orange when power is pressed. 12v is supplied to many MOSFETS waiting to operate. 5v and 3.3v supplies from PU13 are powered.
The adapter power needs to get through PQ2 to PR2 to power up the system but it stops between PQ3 and PQ2. The vicinity of the TI BQ24751 battery charge manager seems to be the only place where the 19v between PQ3 and PQ2 shows up so I analyze that circuit first. Many of the important components on the BQ24751 datasheet sample schematic can be identified by continuity testing to the gates.
PQ3-Q1 Adapter PVCC Enable (passive only), ACDRV
PQ2-Q2 Battery PVCC Enable, ACDRV through PR9 (PQ3-gate is tied to PQ2-gate through PR9)
PQ1-Q3 BATDRV
PQ18-Q4 HIDRV
PQ19-Q5 LODRV
PD14-D1 BTEST REGEN
PR2-SYSTEM powers many of the main MOSFETS around the board
A dual power battery/adapter circuit has some special requirements:
* The BQ24751 must be powered from the battery or the adapter entirely through passive components.
* SYSTEM must be powered by the adapter or the battery, preferably the adapter. It can't be powered by both at the same time or the adapter would charge the battery unregulated. Since Q2 and Q3 are controlled by the BQ24751, it must choose where SYSTEM power comes from.
How the BQ24751 gets power and how adapter power gets to SYSTEM is not obvious until you notice the normally invisible components. In normal MOSFET circuits the reverse diode is always reversed biased so it can be ignored. In this circuit the reverse diodes do most of the work. For Q1 and Q3, the reverse diodes do all the work. The MOSFET part of Q1 can only backfeed power into the adapter which is impossible because the adapter is either not attached or has a higher voltage than anything the battery can provide so the MOSFET essentially does nothing on or off. Q1 could easily be a diode. The MOSFET of Q3 can only feed unregulated SYSTEM power into the battery so it should never be turned on except in the unlikely event that the BQ24751 wants to jump start the battery circuit. Q3-MOSFET can be turned on when powered from the battery since the battery voltage would always be a bit higher than the SYSTEM voltage.
The adapter power passes through the reverse diode of Q1 and powers the BQ24751-PVCC. The battery power passes through the reverse diodes of Q3 and Q2 to power the BQ24751-PVCC. The BQ24751 gets the highest available supply entirely through passive components and switches immediately when the adapter is inserted or withdrawn. The power supply diodes are opposed so neither circuit can power the other with the MOSFETs off.
In a P channel MOSFET power flows against the arrow in reverse of the protection diode. A high gate blocks the MOSFET. A low gate makes the MOSFET conduct.
When the BQ24751 wants to run SYSTEM from the adapter it brings BATFET-Q3-gate high to block unregulated battery charging and ACDRV-Q2-gate low so Q2-MOSFET powers SYSTEM from the adapter. Then the BQ24751 can charge the battery from SYSTEM power through Q4,Q5. While running from the adapter the SYSTEM voltage is always higher than the battery voltage so the reverse diode in Q3 does not conduct.
When the adapter is disconnected the reverse diode in Q3 conducts immediately and maintains power to SYSTEM. Then the reverse diode in Q2 conducts immediately keeping the power on for BQ24751-PVCC. The BQ24751 is designed to handle sudden supply voltage changes and all of the PWM circuits must quickly ramp up as the SYSTEM voltage drops from 19v to 11v or less.
Now I can debug the circuit. With + on drain and - on source all the reverse diodes work with about 0.500 voltage drop. With - on drain and + on source all MOSFETS do not conduct which is good. I cross -drain to gate. PQ1 and PQ3 conduct. PQ2 does not conduct.
I plug in the adapter and I see that PQ3-gate and PQ2-gate are high so both are turned off. SYSTEM is 0.68v which is unexpected power leaking through some component. If I ground PQ2-gate SYSTEM power at PR2 jumps to 18v so PQ2 works. The BQ24751 has turned PQ2 off/high and it needs to be on/low because the adapter is connected. With PQ2 shorted the laptop refuses to power up and just blinks LED1-Power-Green LED2-Battery-Yellow. This means it knows there's something wrong and is refusing the power SYSTEM through PQ2.
PVCC 19v from the power adapter shows up several places in the vicinity of PQ4 but no component stands out as defective when comparing voltages with the adapter to voltages with the bench supply hooked to the battery terminals. I don't see any other component that could be damaged by a shorted capacitor in SYSTEM so I'm giving up.
For no good reason I happened to leave the CMOS battery unplugged all day. Plug it in. Up it powers. Laptop works fine. I didn't see that one coming.
PF1: 19.1v
PQ3-Drain 19.1v
PQ3-Source 19v
PQ3-Gate 19v
PQ2-Source 19v
PQ2-Gate 19v
PQ2-Drain 0v to board power
Power is passing through PQ3 just fine. Power is not passing through PQ2. So I attach my bench supply to PR2. At about 1v PR2 draws about 1.5 amps. Something is shorted. Off goes the bench supply and I remove the CPU since it can look like a short then start to test capacitors. Some tantalums around CAP15 appear to be shorted but the diode voltage drop of 0.025 to 0.050 is is too high so it's probably the supply to the Northbridge U28. Many ceramic caps near and including (but not limited to) PC108, PC138, PC146, PC150 appear shorted with the same diode voltage drop of 0.005 as PR2. Mosfets PQ27 and PQ28 have the same 0.025 voltage drop as the nearby caps so are probably not shorted.
These shorted parts must be part of the same power plane. So I attach the bench power supply to one of the shorted caps and turn up the power until about 1.2 amps is drawn. I feel around to all the caps I remember to show shorted and any close semiconductor and notice that PC139 is burning hot. PC139 has a visible flaw which looks like a scratch from my probes. I probed many caps and none of them have scratches so the flaw must be internal. Off PC139 comes and now the short is gone.
The board still won't power up. Adapter connected and the board does nothing when power is pressed. 0.68v is supplied to the many MOSFETS waiting to operate. 5v and 3.3v supplies from PU13 are not powered. 12v to the battery terminals makes the LEDs blink twice green orange when power is pressed. 12v is supplied to many MOSFETS waiting to operate. 5v and 3.3v supplies from PU13 are powered.
The adapter power needs to get through PQ2 to PR2 to power up the system but it stops between PQ3 and PQ2. The vicinity of the TI BQ24751 battery charge manager seems to be the only place where the 19v between PQ3 and PQ2 shows up so I analyze that circuit first. Many of the important components on the BQ24751 datasheet sample schematic can be identified by continuity testing to the gates.
PQ3-Q1 Adapter PVCC Enable (passive only), ACDRV
PQ2-Q2 Battery PVCC Enable, ACDRV through PR9 (PQ3-gate is tied to PQ2-gate through PR9)
PQ1-Q3 BATDRV
PQ18-Q4 HIDRV
PQ19-Q5 LODRV
PD14-D1 BTEST REGEN
PR2-SYSTEM powers many of the main MOSFETS around the board
A dual power battery/adapter circuit has some special requirements:
* The BQ24751 must be powered from the battery or the adapter entirely through passive components.
* SYSTEM must be powered by the adapter or the battery, preferably the adapter. It can't be powered by both at the same time or the adapter would charge the battery unregulated. Since Q2 and Q3 are controlled by the BQ24751, it must choose where SYSTEM power comes from.
How the BQ24751 gets power and how adapter power gets to SYSTEM is not obvious until you notice the normally invisible components. In normal MOSFET circuits the reverse diode is always reversed biased so it can be ignored. In this circuit the reverse diodes do most of the work. For Q1 and Q3, the reverse diodes do all the work. The MOSFET part of Q1 can only backfeed power into the adapter which is impossible because the adapter is either not attached or has a higher voltage than anything the battery can provide so the MOSFET essentially does nothing on or off. Q1 could easily be a diode. The MOSFET of Q3 can only feed unregulated SYSTEM power into the battery so it should never be turned on except in the unlikely event that the BQ24751 wants to jump start the battery circuit. Q3-MOSFET can be turned on when powered from the battery since the battery voltage would always be a bit higher than the SYSTEM voltage.
The adapter power passes through the reverse diode of Q1 and powers the BQ24751-PVCC. The battery power passes through the reverse diodes of Q3 and Q2 to power the BQ24751-PVCC. The BQ24751 gets the highest available supply entirely through passive components and switches immediately when the adapter is inserted or withdrawn. The power supply diodes are opposed so neither circuit can power the other with the MOSFETs off.
In a P channel MOSFET power flows against the arrow in reverse of the protection diode. A high gate blocks the MOSFET. A low gate makes the MOSFET conduct.
When the BQ24751 wants to run SYSTEM from the adapter it brings BATFET-Q3-gate high to block unregulated battery charging and ACDRV-Q2-gate low so Q2-MOSFET powers SYSTEM from the adapter. Then the BQ24751 can charge the battery from SYSTEM power through Q4,Q5. While running from the adapter the SYSTEM voltage is always higher than the battery voltage so the reverse diode in Q3 does not conduct.
When the adapter is disconnected the reverse diode in Q3 conducts immediately and maintains power to SYSTEM. Then the reverse diode in Q2 conducts immediately keeping the power on for BQ24751-PVCC. The BQ24751 is designed to handle sudden supply voltage changes and all of the PWM circuits must quickly ramp up as the SYSTEM voltage drops from 19v to 11v or less.
Now I can debug the circuit. With + on drain and - on source all the reverse diodes work with about 0.500 voltage drop. With - on drain and + on source all MOSFETS do not conduct which is good. I cross -drain to gate. PQ1 and PQ3 conduct. PQ2 does not conduct.
I plug in the adapter and I see that PQ3-gate and PQ2-gate are high so both are turned off. SYSTEM is 0.68v which is unexpected power leaking through some component. If I ground PQ2-gate SYSTEM power at PR2 jumps to 18v so PQ2 works. The BQ24751 has turned PQ2 off/high and it needs to be on/low because the adapter is connected. With PQ2 shorted the laptop refuses to power up and just blinks LED1-Power-Green LED2-Battery-Yellow. This means it knows there's something wrong and is refusing the power SYSTEM through PQ2.
PVCC 19v from the power adapter shows up several places in the vicinity of PQ4 but no component stands out as defective when comparing voltages with the adapter to voltages with the bench supply hooked to the battery terminals. I don't see any other component that could be damaged by a shorted capacitor in SYSTEM so I'm giving up.
For no good reason I happened to leave the CMOS battery unplugged all day. Plug it in. Up it powers. Laptop works fine. I didn't see that one coming.
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