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Power consumption circuit

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Newbie level 5
Sep 25, 2010
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Hello Friends,

I am preparing for interview related to Power Electronics Engineer position and I would like to know some ideas or answer/inputs on power consumption circuit.
In the past, couple of times, interviewer asked me very simple question about power consumption in laptop/mac/tablet/surface. Question was like this: "which circuit one should design or use to figure out how much power is required to run micro-processor? (in other words, If it is in sleep mode or not using fully then which circuit will identify that and tell micro-processor?)
I hope I have explained it correctly.

I am looking at circuit level answer. (I know there are PMIC chips which can do this but in interview, they check circuit level knowledge).

Thank you and highly appreciated your inputs. :)

Hi Klaus,

Thanks for replying on this.
Basically, my question is how do I (circuit) decide whether to turn off power partially when it is not utilizing?
I think, there are multiple ways to know at a given time how much power uC is needed based on applications running.
I am looking to give answer with proper explanation and that is why I need EXPRT advise.


The power supply neither does decide this nor does it specially have to know this.
It's the microcontroller / processor that reduces it's consumption.
Usually this is controlled by software.
There are several ways to reduce power consumption. Reduce or switch OFF clock, power down periferals...


Thanks Klaus for the quick reply.
Actually, you are correct that it can be done though software in micro-processor. BUT in past interview, they asked me solution based on circuit. (I am not sure but something like Dynamic Voltage Scaling which will measure how much voltage/power is needed at a given time and that circuit's output will tell micro-processor and it does what you mentioned (reduced clock speed, frequency, power down some rails etc..)

I know it is not real world answer but that is what they ask and check in interview.
Apologize if it is annoying.
Thanks. :)


I'm not sure if I understsnd the concept...
Isn't it a question of demand and supply.

You say..
which will measure how much voltage/power is needed at a given time
My understanding is different.
I see it like a diesel engine and an electric crane. The diesel engine does not need to know beforehand how much stones the crane has to lift.
The stones create a demand of power ... and the job of the diesel engine is to deliver this power. No need to measure, no need for a special crane motor, no need for communication, no need fo intelligence.
The diesel engine just needs to keep frequency and voltage constsnt.

The same is true for the power supply of a processor. It needs to keep the voltage constant...but the processor draws the current that then results in power.

For sure you may put in some intelligence ... but this usually is software controlled.

An example:
When you don't move the mouse of a PC for a longer time, then the software detects this, and may switch OFF your monitor.

Correct, when we don't move mouse for some predefine time, it consider an ideal time and software detects that and that will tell micro-processor accordingly. I am with you till this. (what is next after processor receives command?. Do it send some instructions to some circuit (like PMIC) and then not supply power and go to sleep mode?

If I am not wrong, i understood top-level functionality (like your mouse example) but there should be some circuit which will turn-off partially (to save battery) after micro-processor received indication from software.

Really appreciate your inputs/explanation with examples and time.


In case of the monitor:
If I'm not mistaken the processor switches OFF the sync / data signals to the monitor. The monitor detects this and switches OFF the signal oricessing. This saves power.
Additionally it switches OFF the backlight of a TFT. Here indeed the backlight power supply is involved.

I'm not familiar with nowadays processors power saving modes.
But I rather guess that it switches OFF some functions, like data processing, so it switches OFF the demand, not the supply.

Did you do internet search on power saving modes of modern processors?

Yes, I tried googling but majority results are going towards PMIC (which I know is ultimate answer but PMIC handles/controls lot of other stuffs too).
Only reason I have posted here to get detail circuit level answer through Power Electronics Engineers/experts or power domain engineers.

Thank you Klaus for all your inputs. BTW, do you have any reference which I can try for this?


I´m a bit confused now.
I´m still unsure what you or the interviewer is after. Please clarify.
None of your posts use the words "measure" and "control" (besides post #9 and PMIC function)

So are you talking about power control or power measurement?
And is it focussed on electrical power? (post#1) (or processing power(post#3), WiFi range, generated heat, battery life..)
Is it focussed on processor_internal circuitry ... or external periferals (onboard like WiFi, or off board, like Monitor and HDD)


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