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.

[tony359]

I started looking into this laptop and tried o familiarise myself with the power circuit.

I think I found the issue but I'd like to understand what I am seeing - also because I'd like to make a repair video!

I think I have a short and when the charger IC tries to turn on the first mosfet, it detects the short and stops immediately. I only tried with a meter but I can see some pulses on the gate of PQ89001 for 7 times, then it stops.

Looking at the manual for the charger IC, I see a mention of the 7 tries but I am not sure I understand what's happening.

The manual says that the charger IC will try to turn on the ACFET (the first mosfet after the adaptor?) and "if the voltage between source and gate is still less than 5.7V, it turns off ACFET". This would repeat 7 times indeed.
I understand that I should read 25V on the gate of the mosfet (19V + 6V) but what matters is the 6V the charger IC is sending to turn on the mosfet.



But then I don't understand the logic. What is the condition which will cause Vg - Vs to be less than 5.7V if the charger IC is sending 6V to it?

The manual is here: https://www.ti.com/lit/ds/symlink/bq...=1761019169338
The part I am talking about is on page 16-17

Thanks so far for the help!

[tony359]

I'm going to update myself.
I think I am mixing up "faults" here. What I quoted above is a fault where the mosfet refuses to turn on - for example the mosfet is shorted and when 6V are provided, they also appear on the Source so Vg-Vs = 0 and the charge IC says "stop".


but I kept reading the document and I found this


So the IC is sensing the short (via current sensing resistors) and it's applying the very same "7 tries" behaviour.

Do I see this correctly?

[m1ch43lzm]

The X421EA schematics may help, at least for the charging circuit (it's totally different board, but ASUS appears to have copy/pasted the same charging circuit with minor changes if any, the component labels look the same), page 58, it says SN2867RUYR for the charging IC PU8900 but it's equivalent to BQ24780S
https://www.badcaps.net/forum/troubl...tics-asus-x421

Measure resistance to GND after the current sense resistor PRS8901, AC_BAT_SYS, that's your main power rail, it can be either the battery voltage, or the voltage from the AC adapter, if you have a short there anything can cause it, it could be as simple as a shorted cap, or it can be an IC or a high side MOSFET (hopefully not), you may want to check this video about voltage injection: https://www.youtube.com/watch?v=wp1dPG9_N1w
You want to set the voltage on your bench PSU to max 1v, and check with thermal camera what component gets hot