Why Two MOSFETs? A look at the power management IC
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- เผยแพร่เมื่อ 5 ก.ย. 2024
- In this video I answer the question posed by one of my subscribers as to why there are two MOSFETs on the input. Please leave any comments below or contact me on therepairshare@gmail.com
Thanks for reply i am greatful
Another thing I would like you to explore is the difference between the first two MOSFETs in your Simplified Application Diagram and the second two MOSFETs. The first two MOSFETs protect against over-current and backflow conditions. The second two MOSFETs form a voltage "buck" regulator, with one MOSFET on the high side, and the other on the low side, connecting to ground. I am amazed that the BQ chip controls both pairs of MOSFETs. But since this is such a common application, I can see why the BQ chip was created, and is used is so many low-power devices. We see high-side/low-side pairs of MOSFETs in nearly all voltage reduction/regulation power circuits. Whenever we see a single IC, rather than a pair of MOSFETs, one only need look at the data sheet to see that the MOSFET pair has simply been combined into a single chip. Again, a very common application for low power devices. Anyway, bottom line -- I would love to see a discussion of the difference between the MOSFET pair on DC_IN versus the MOSFET pairs that are part of a voltage regulator circuit. Thanks!
One thing I have been confused about is why the two MOSFETs that form the inrush limiter have their source pins connected to each other. On the motherboard you repaired, DC_IN is connected to the drain pins of the first MOSFET, the source pins of both MOSFETs are connected to each other, and the drain pins of the second MOSFET is where the current emerges. For some reason, I had the belief that current in a DC circuit always flows from the voltage source to ground, and from source to drain. But in this example, current flows from drain to source, and then from source to drain. On many boards, these two MOSFETs are exactly the same IC, with the second one simply flipped. Your comment about the RBFET indicates that this reversal is to prevent current backflow. So these two MOSFETs, with the BQ chip, provide both inrush/overcurrent protection, as well as current backflow protection. Am I on the right track?
I tried to summarize your presentation, and to understand why the ACFET and RBFET are “reversed” and have their source pins connected to each other.
The ACFET, PQ3, has its drain pins connected to the 19.5V DC_JACK (+VA) line, and its source pins connected to the source pins of PQ4 (+DC_IN_SS). (+VA is connected to the ACDET pin of the BQ chip.)
"The ACFET allows the power management IC to control the voltage from the AC power adapter entering the motherboard. If the gate pin is high (which is usually around 25V), the 19V from the AC power adapter is allowed through the ACFET, and onto the motherboard."
The RBFET, PQ4, has its source pins connected to the source pins of PQ3 (+DC_IN_SS), and its drain pins connected to the current sense resistor (+DC_IN).
"The RBFET is the Reverse Blocking FET, which provides negative input voltage protection, and it prevents voltage from the battery feeding back to the DC_JACK."
You didn't mention the PD4 TVS diode, which protects the circuit from transients and over-voltage conditions.
Do you have a video here on how to check the ac drive or charging ic volts? If not, can you please explain how to check.
Just subscribed from Africa, Zambia
Do you have schematic? In many configurations the ACDRV pin of the power management IC connects to gates of two N Channel MOSFETS and should be about 25V in order to turn the N Channel MOSFETs on and allow the 20V to pass through to the current sense resistor.
@@therepairshare673 please make a video on that
sorry a bit oot, i wanna ask the output +vin is 9v too and it spread to the motherboard in 9v?