Which is more noisy: Buck or boost

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
Hi Team,

This is a question asked to me in an interview.
The interviewer told Boost is more noisy because the switching mosfet is present between ground and Vin.
May i know your answer about this question.

Regards
HARi
 

MrSalts

Joined Apr 2, 2020
712
Hi Team,

This is a question asked to me in an interview.
The interviewer told Boost is more noisy because the switching mosfet is present between ground and Vin.
May i know your answer about this question.

Regards
HARi
Ha, I assume they suggested similar conditions? How did your interviewer define similar? Similar voltage increase, doubling vs half voltage change? Was the voltage delta the same in both cases? Or did your interviewer completely fail to define the conditions? Or did you fail to ask for conditions of each.
 

MrChips

Joined Oct 2, 2009
24,609
There is no valid answer.
There are two many variables, power, frequency, controller, components, filtering, etc.
I bet the interviewer does not know the correct answer.
 

Deleted member 115935

Joined Dec 31, 1969
0
Im afraid that kind of question is for two reasons in an interview
a) they are putting there boas forward, ánd wont hear anything else, as they KNOW they are rigth,
b) they are putting out a conjecture, and wondering if you ask questions , and how you tackle it

Unfortunately, the way to deal with one is the opposite of the other,
so you have to take a gamble on which way they are asking,

But IMHO, any interviewer that does that sort of question is not a good manager to work for.

I was once asked how many different types of flash there were,
were they asking NAND / NOR
or were they asking SPI or parallel
or were they asking leaded and none leaded
or ones in grey plastic and ones in ceramic ?

An example of a terrible question,
 

MrSalts

Joined Apr 2, 2020
712
I think questions like this are perfectly fine. Especially if the applicant is expected to be in front of a customer. Customers have all kinds of wild-ass theories and want to blame suppliers for everything. Hiring a tech service/application support engineer is a challenge because you need someone that can engage with a customer, ask appropriate questions, explain electronics/engineering/physics/life to the customer and make the customer understand that no product is perfect, especially when it is not used in an appropriate design, or that one of their idiot staff members did something wrong - all without making anyone feel stupid and keeping them as a customer.

Not all questions have binary answers. The fact that the hiring manager gave you a binary answer likely meant he wanted to move the interview along and not tell you why he asked the question, or that you failed his situational problem solving question.
 

Ian0

Joined Aug 7, 2020
3,754
Buck has continuous current on its output, Boost has continuous current on its input, Čuk has continuous current on both input and output.
So Buck makes most noise on its input supply, Boost makes most noise on its output supply, Čuk is the quietest.
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
Ha, I assume they suggested similar conditions? How did your interviewer define similar? Similar voltage increase, doubling vs half voltage change? Was the voltage delta the same in both cases? Or did your interviewer completely fail to define the conditions? Or did you fail to ask for conditions of each.
I did not ask any conditions.He just asked me "Among Buck and Boost which one has more noise at the output".
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
170
Buck has continuous current on its output, Boost has continuous current on its input, Čuk has continuous current on both input and output.
So Buck makes most noise on its input supply, Boost makes most noise on its output supply, Čuk is the quietest.
Hi Ian,

Thank you...
May I know how the current(Continuous and Discontinuous) related to noise.

Regards
Hari
 

Ian0

Joined Aug 7, 2020
3,754
Look where the current flows.
Buck regulator:
When the MOSFET it on, current flows from the power supply into the load through the inductor.
When the MOSFET is off, current flows from the inductor through the diode into the load.
Current is always flowing into the load.
Current only flows from the supply when the MOSFET is on.
So the power supply sees square-wave pulses of current, the current is either zero, or the full output current. Each pulse will pull down the power supply voltage by a small amount, and will probably cause ringing on the edges of the pulse.
The load current is a triangular wave varying from 10% above the load current to 10% below. There will be some variation in the output voltage, but not much.

In the boost regulator the opposite is true - Power supply current is triangular, varying by, say, ±10%
Output current is square, either zero or equal to (load current/duty cycle)
 
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