ASUS X550 Laptop no AC detected/no charging - Page 1
Forum section: electronics / repair

author: ubis
author_rank: Newbie
timestamp: June 06, 2019, 06:33:48 pm
content: Hi there,I'm trying to fix an X550 laptop which doesn't detect that AC is plugged in and it doesn't charge the battery, nor turn on when battery is taken out.Quick backstory:Laptop received reversed polarity from the AC jack. Laptop was able to turn on with battery only and after plugging in the original charger it shown that it's charging, although it didn't charge at all.I didn't had schematic back then and I did a quick look to notice that power comes in to PQ8801 but doesn't come out of PQ8802. I thought great, it burned those MOSFET's so I bypassed them and I thought this was it. I was wrong. Also I thought that it burned the charging chip PU8800 because it didn't turned on the FET's once they were replaced.Later I replaced the charging chip PU8800 and I did have battery plugged in and after playing a bit with the laptop I left it alone only to notice after that it's completely off. Connected to the lab power supply and noticed enormous amount of current, 2-3A. I forgot that I had bypassed these FET's PQ8801 and PQ8802 when I replaced the charging chip. Once again, a quick look and now it was dead short somewhere. Low side charging FET PQ8805 was shorted. I think high side was shorted or it wasn't, I can't remember now, but I replaced PQ8805, PQ8804 and the charging chip again.Laptop still doesn't turn on when there's only AC connected, only with connected battery. Yet, the charging LED is on, Windows is showing that it's charging, although it shows 0% current capacity.Now for the past few days I'm been trying to troubleshoot why that charger doesn't turn on the FET's.I think the charger chip is fine. When only AC is connected, I can see 2.61V on ACDET pin, 6.1V on REGN pin, 19V on VCC pin. There is no 0V on ACOK.In the charger datasheet:QuoteSo I have ACDET, I have VCC(UVLO is about 3.5V and SRN is 0V), which should make ACOK present, and in fact it does. But because charging chip disconnects power from the AC quickly, there's no +3VA pullup on ACOK pin. I have tested this by plugging in battery and unplugging AC, ACOK was pulled down as it should, by internal FET, it's not pulled down when AC is plugged in.This is about REGN pin, which is present as well:Code:Linear regulator output. REGN is the output of the 6-V linear regulator supplied from VCC. The LDO is active whenREGN 16 voltage on ACDET pin is above 0.6 V and voltage on VCC is above UVLO. Connect a 1-µF ceramic capacitor fromREGN to GND.REGN's output is 6.1V, I have ACDET and VCC as stated.Next what I have tried is removing PQ8802 and leave just PQ8801, to see if chip will turn on the FET. It did. Although voltage on the gate is 25V and main AC / chip's VCC is at 19V, I suppose there's some sort of boost converter or something inside the chip. Once I solder back PQ8802 the chip won't turn on these FET's anymore.This is why I believe charging chip is fine. Which led to possible chip's protection mode:Code:The bq24735 has a unique short-circuit protection feature. Its cycle-by-cycle current monitoring feature isachieved through monitoring the voltage drop across RDS(on) of the MOSFETs after a certain amount of blankingtime. In case of MOSFET short or inductor short circuit, the overcurrent condition is sensed by two comparatorsand two counters will be triggered. After seven times of short circuit events, the charger will be latched off andACFET and RBFET are turned off to disconnect adapter from system. BATFET is turned on to connect batterypack to system. To reset the charger from latch-off status, the IC VCC pin must be pulled below UVLO or theACDET pin must be pulled below 0.6 V. This can be achieved by removing the adapter and shutting down theoperation system. The low-side MOSFET short circuit voltage drop threshold can be adjusted through SMBuscommand. ChargeOption() bit  = 0, 1 sets the low-side threshold to 135 mV and 230 mV, respectively. Thehigh-side MOSFET short circuit voltage drop threshold can be adjusted through SMBus command.ChargeOption() bit  = 0, 1 disables the function and sets the threshold to 750 mV, respectively. During boostfunction, if the low-side MOSFET short-circuit protection threshold is used for cycle-by-cycle current limiting, thecharger will not latch up.I have tried replacing PQ8803, PQ8804 and PQ8805 and even tried without them, without success.Below I have attached scope image with what is happening on gate pins of PQ8801 and PQ8802 when plugging in AC. It doesn't cycle, after this, gate is pulled down unless I reconnect the AC and get the same result.Also A/D_DOCK_IN doesn't come anywhere else, the charging chip is controlling whenever it should fed AC_BAT_SYS or not.Now I'm kinda lost, not sure what I should check now...