Asus S410U no power: power on sequence on laptops?

[tony359]

HI all,

I have an Asus S410U which is dead, no signs of life. The PSU is outputting 19V and it works when under load with an 8OHM resistor.

On the motherboard, I have 19V in getting into the first mosfet and then nothing. I don't see obvious shorts though I haven't checked with thermal camera. I noticed that the rail going to the RAM reads 4 Ohm but that might be normal.

First thing is: how does a laptop generate the "standby voltage"? I'd imagine the 19V is being stepped down into a lower voltage for the standby logic to work. Is there a controller for that and how is that powered considering that when power is plugged in you only have 19V available?

The owner of the laptop claims that the laptop made a loud noise when a microphone was plugged into a connected USB audio interface. I didn't see any obvious damage on the board and I'm inclined to think it was just a coincidence - or maybe the speakers popping when power went off.

Are there schematics for this machine?

Thanks!
Tony

[mon2]

Follow post # 3 here:

https://www.badcaps.net/forum/troubl...987-asus-s410u

Is your unit the same? What are the markings on the DCin mosfets?

Is it QM3056 @ PQ8901?

Carefully measure the voltage to ground on each of the pins on the DCin mosfets:

source (1-2-3)
gate (4)
drain (5-6-7-8)

Most likely gate voltage is not ~25V which is required to power on (enable) the N-Channel mosfets used by the BQ charger ICs. Driven by the ACDRV pin on the charger IC.

[tony359]

Thanks, I found the boardview!

I'll open the laptop and let you know - meanwhile, for my own education, can you tell me how that works? 19V comes in and...? I've got an ATX PC in mind where the PSU is generating 5VSB. But there is no such thing on a laptop so what is the power on sequence?

Thanks so far!

[mon2]

Review the sticky above on charger circuits by piernov. It is a must read. Openly, most of the cases here are documented in his in depth articles.

[m1ch43lzm]

Thanks, I found the boardview!

I'll open the laptop and let you know - meanwhile, for my own education, can you tell me how that works? 19V comes in and...? I've got an ATX PC in mind where the PSU is generating 5VSB. But there is no such thing on a laptop so what is the power on sequence?

Thanks so far!

Start here: https://www.badcaps.net/forum/troubl...arging-circuit
The charging circuit either passes the 19V to the main power rail, or when the AC adapter is disconnected, it passes the battery voltage to the main rail
After you get either the 19v from charger, or the battery voltage at the main power rail (AC_BAT_SYS), you have a DC/DC converter (PU8701, RT8249CGQW) with 3.3V and 5V buck outputs, and auxiliary 3.3V LDO and 5V LDO outputs (the LDOs are always on, the buck outputs are enabled by the EC)

The 3.3V LDO powers the EC (embedded controller), some also call it SIO(Super I/O) or KBC (keyboard controller): the EC checks for the power button signal to turn on the remaining power rails, it also turns on the 3.3V and 5V buck outputs on standby

[tony359]

thanks - piernov post is amazing. I will study it with the board in front of me and report back 🙂

Cheers for now.