What is a Buck-Boost DC-DC converter

Thread Starter

Papabravo

Joined Feb 24, 2006
17,240
I have always understood that the term "buck-boost" referred to a particular topology of DC-DC converter that took a positive DC input voltage and created a negative DC output voltage. This term seems to also apply (loosely) to a situation where you can have a positive DC input voltage that can be either less than, equal to, or greater than the output voltage. I remember working on a SEPIC converter that had this property once upon a time, but I do not think it applies in this case.

Is this a case of the term "buck-boost" being overloaded (like a C++ operator) to actually refer to multiple things depending on context, or is there something else going on here?
 

Thread Starter

Papabravo

Joined Feb 24, 2006
17,240
Maybe it is just the collection of authors that I have chosen to consult.
The most recent one has been Basso, C., Switch-Mode Power Supplies
Are you saying there is a difference of opinion on the meaning of the term?
 

Thread Starter

Papabravo

Joined Feb 24, 2006
17,240
That is probably why I was confused. I have not had occasion to use or study the converter in Figure 18, but I can point to several sources that refer to Figure 33, as a buck-boost converter. Thanks for the illustration.
 

dl324

Joined Mar 30, 2015
13,505
That is probably why I was confused.
When I studied switching regulators, we didn't use the terms buck or boost. We always called them step down or step up. I don't recall studying a step up/down regulator. I think I started using buck and boost after joining this forum (before that, I hadn't designed or thought about a switching regulator since the early 80's).
 

Ian0

Joined Aug 7, 2020
3,754
I don't see how the term came about.
If you redraw the so-called buck-boost with the opposite polarity, it's obvious that it is just a flyback converter with the load returned to the positive supply instead of the negative supply.
There is another circuit with four switches which genuinely is a buck converter followed by a boost converter.
https://www.ti.com/lit/ds/symlink/lm5118.pdf?ts=1633410178778&ref_url=https%3A%2F%2Fwww.ti.com%2Ftool%2FTIDA-010086

I'm still puzzled where the term "buck" came from. A dollar? A male rabbit?
 

ronsimpson

Joined Oct 7, 2019
1,630
I'm still puzzled where the term "buck" came from
Buck, Verb: oppose or resist.
It is used to describe reducing the in coming voltage to a lower lever.
Boost, Verb: to increase
It is used to increase the voltage to greater than the input value.

There are several different ways to make a buck/boost power supply. Just this morning I was working on a power supply, Vin 4 to 6V Vout 5.0V. It switches from one mode to another at about Vin=5V. (maybe I should say it boost and bucks at the same time)
 

Ian0

Joined Aug 7, 2020
3,754
Buck, Verb: oppose or resist.
As in "to buck the trend"?
I always wondered. . . .
It switches from one mode to another at about Vin=5V
Is that the circuit with with four switches (usually 2 transistors and 2 diodes) with an inductor between them? so that it is either step down if the left side is switching or step up if the right side is switching (assuming left to right power flow)
 

tindel

Joined Sep 16, 2012
810
It's clear from this thread that there is no clear industry standard for naming these topologies, but I do find that Bob Mommano's book, Fundamentals of Power Supply Design, did a good job being distinct and succinct about the differences. His take:

Single inductor topologies:
Buck: Two switch voltage reducing switch regulator
Boost: Two switch voltage increasing switch regulator
Buck-Boost: Two switch voltage inverting switch regulator Sometimes called a flyback converter (not to be confused with the isolated flyback which uses two tightly coupled inductors)
Buck-OR-Boost: Four switch voltage increasing or reducing regulator combining both buck and boost topologies with a single inductor.

Unfortunately, I don't think the book is print any longer. It is a pretty good introductory book to switching power systems, but won't make you an expert. It lacks an index which I've found frustrating.
 

crutschow

Joined Mar 14, 2008
28,161
I've always taken the term to mean it can buck (reduce the input voltage) or boost (increase the input voltage) depending upon whether the input voltage is above or below the desired output voltage, all voltages being the same polarity.
 

ronsimpson

Joined Oct 7, 2019
1,630
Is that the circuit with with four switches (usually 2 transistors and 2 diodes) with an inductor between them? so that it is either step down if the left side is switching or step up if the right side is switching (assuming left to right power flow)
There are many different ways:
Here is an example.
If switch right is open (all the time), and switch left is switching, then it is a clear buck.
If switch left is closed (all the time), and switch right is switching, then it boosts.
One option is to run the supply in boost or in buck and switch from one to the other as needed.
1633803910268.png
Option two is to boost/buck at the same time. Close both switches and let power build up in L1. Then open both switches and the left side of L pushed down into ground and the right side jumps up and pushes into Cout and powers the load.

Option three is to do all three modes. 24Vin 12V out drive only the buck switch (left). 6Vin 12V out drive only the boost switch (right). When Vin is 15V to 9V, drive both switches.

More ways to do this but I am not writing a book today. lol RonS.
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
Of course, in the AC context, Buck-Boost generally means the addition of a simple mains transformer.
I often used this method when installing 200v Japanese equipment in a 240v N.A. environment. ;)
 

DickCappels

Joined Aug 21, 2008
7,954
It is good that this discussion is taking place. I have seen many examples of students using the term buck-boost to refer to any switching power supply that used an inductor.

To summarize my view, there are:
Buck,
Boost, and
Buck-boost.

Not sure whether Cuk belongs even on this list.
 

Ian0

Joined Aug 7, 2020
3,754
At least there is one vote (or two?) that Cuk is a buck-boost. Let's leave it in that category since it does both.
It's generally known as a Boost-Buck, as the Boost part comes first - it creates a voltage in the coupling capacitor greater than the supply voltage, then the same switching action makes that step down the voltage to the output.
A really nice circuit for those rare occasions when you need a negative supply.
If you run it at 50:50 mark-space and make sure it has enough load to keep it in continuous conduction you get a nice low-ripple negative supply of almost the same voltage as your positive supply, and it doesn't put noise on the positive supply.
You can also do some neat things with ripple cancellation by winding both inductors on the same core.
 

ronsimpson

Joined Oct 7, 2019
1,630
At least there is one vote (or two?) that Cuk is a buck-boost.
There are a number of Cuk versions.
1633869848490.png
Which I draw like this:
1633869957651.png
Because it creates a negative voltage I think it is not a boost or a buck, but inverting. I see the reason that (Vin is above and below Vout). Example: (Vin=+8 to +20, Vout-12). But I think that reason alone separates this Cuk from a Boost/Buck.

I see that, in the above pictures, S & L1 make a boost or flyback and D & L2 make a buck. I see the Boost/Buck pieces.
Vout does not lay in-between Vin-min and Vin-max, for that reason I vote against the Cuk.

Circuit below, coupled inductors 1:1, is a flyback and does not buck. Input can be above and/or below output.
1633870655375.png
 
Top