Power sensing circuit

Question

Hi All,

I am with EE back-ground and recently I had question in interview regarding “How to sense/know when device needs power?”.

So, basically I had a good interview on different topics and one of them was MOSFET.
I answered mostly all questions properly until this question comes up.
Interviewer asked me: “If you are making power supply for a laptop and when laptop goes in to sleep that time your supply should not provide full power (may be negligible). And when you on the laptop or started working on it at that time your supply should provide required power.”
How can you design circuit which sense/detects when device needs power?

Can any one of you help me on this? I would really appreciated any input.

Thank you,

pj

dougw · Answer

It sounds like you need to start with a sensor that detects when the computer needs power and when it doesn't. Many laptops have a switch that senses when the display/lid closes and generates a signal to turn off the display and go to sleep. Many computers have a timer that senses that no activity has occurred for a preset period and when the timer times out the computer goes into screensaver mode or sleep mode. These often wake up when a keyboard key is pushed, or the mouse is moved. The shutdown or sleep process is usually controlled by the CPU, so the shutdown signal needs to go to the CPU, which controls at least part of the power supply. The wakeup signal could go to the CPU if it has a built-in capability to convert this into a wakeup sequence, or it could directly activate power to parts of the circuit that were turned off. In that scenario, the CPU would get involved after waking up to keep power applied until a sleep cycle is required. There are lots of possible triggers and lots of potential power up and down sequences, including a simple power switch.

jw0752 · Answer

Hi Pujan,

I am not an expert but it seems to me that this may have been a trick question. If the electronics of the computer are making the decision to go into sleep or low power mode does the power supply really have to do anything? The power supply only reacts to load demand and if the electronics has lowered the demand and therefore the load the power supply will see a higher impedance and provide less power. Most computer power supplies are switch mode and if they are line powered saving energy may not be an issue. If I have misunderstood or if I am wrong about the computer controls actually changing something in a power supplied it would be great to find out. The engineers on this site will quickly set me straight if I am off and we can both get some new ideas.

John

aswinvenu · Answer

Hi Pujan, I think there is a misconception here. Power modes are achieved not by limiting the supply current from the supply but by reducing the conception from the load. Let me elaborate it for you. In your case you wanted to achieve sleep mode. Then CPU gives commands to most of the peripheral controllers to go to sleep mode after that CPU itself will go into a state (no clocking) where it wait for an external interrupt to wakeup. So its doesn't work like, say, right now your system draws 3Amps and you wanted to achieve low power mode and you limit the current from the source to 0.1Amps "No"! But power supplies in computers are very intelligent they are called PMIC ( Power Management Integrated Circuits) They will take one input and generate multiple rails of LDO(Low Dropout Regulators) and DCDC Converters.This chip will be hooked to an I2C bus or some other communication bus to the CPU. All the peripherals will be powered from this IC. CPU will have the absolute control over all the functionality of these ICs. CPU can shutdown any rails using I2C commands. In your case you can use a MOSFET to turn on or off the rails to a peripheral and there by achieve low power. Regards, Aswin

Power sensing circuit

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