basic buck converter question for input filtering

Thread Starter

mike _Jacobs

Joined Jun 9, 2021
223
I broke this question into a different thread as it is a different topic then my other buck converter question.

So in regards to a typical buck topology...

We have the high side FET if you want to call it that rapidly switching.
The current into the inductor while the switch is closed is rising linearly... of great.....

That means that when i look at the input current into a buck converter the input current looks like a saw tooth.
This cant be good for the source of whatever is supplying the buck converter.

In practical design how is that handled? The source providing voltage input to the buck would be seeing an oscillator current up and down and up and down. If the supply source was some kind of closed loop feedback voltage source i imagine that could cause major oscillatory and stability issues.
 

Ian0

Joined Aug 7, 2020
10,076
You're right. The input current to a buck regulator, and the output current of a boost regulator are both discontinuous, with lots of ripple current.
A large, low ESR capacitor across the input reduces the ripple.
 

MisterBill2

Joined Jan 23, 2018
19,051
Definitely low ESR capacitors across the input to supply the high current draw periods. And adequately sized wire in all of the segments of the circuits, including the return (GROUND) segments.
 

Ian0

Joined Aug 7, 2020
10,076
Layout is everything.
Most datasheets for buck converter ICs include a recommended layout. You'd need a very good reason to ignore it.
 

MisterBill2

Joined Jan 23, 2018
19,051
The complexities and considerations for a switching type of supply are why I get them from a competent source rather than design and build them.
 

Thread Starter

mike _Jacobs

Joined Jun 9, 2021
223
Im surprised to hear that. I know it is sensitive but people build converters on breadboards all the time without issue.
Perhaps not with the most ideal results but they are also not catastrophic failures.
 

MisterBill2

Joined Jan 23, 2018
19,051
The term "converter"applies to a huge realm of circuits that operate in a vast array of modes. Certainly some versions and designs do not require as optimized arrangements as others.
Aside from that, also consider that not everything presented on the internet is honestly presented. And a potion of what is shown is totally faked.
 
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