# -48V with buck-boost converter, common ground

Thread Starter

#### manubm

Joined Feb 25, 2016
7
I have a doubt about if I use a buck-boost converter with a -48v input to +12V ouput. Where is my common ground? I need to ground it!

Thank you

#### #12

Joined Nov 30, 2010
18,216
1) It depends on what is inside the converter. If the output is isolated, you can just connect whatever wire you want as "common".
2) You could think in terms of an inverting supply. Do you really need both boost and buck to get 12 volts out of -48 volts?

Thread Starter

#### manubm

Joined Feb 25, 2016
7
1) It depends on what is inside the converter. If the output is isolated, you can just connect whatever wire you want as "common".
2) You could think in terms of an inverting supply. Do you really need both boost and buck to get 12 volts out of -48 volts?
Do you mean that with this configuration i cant use a common earth between input and output?

Thread Starter

#### manubm

Joined Feb 25, 2016
7
Do you mean that with this configuration i cant use a common earth between input and output?
Sorry the output is not isolated.

#### AnalogKid

Joined Aug 1, 2013
8,814
What is the part number for the converter?

Thread Starter

#### manubm

Joined Feb 25, 2016
7
LM2576

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#### #12

Joined Nov 30, 2010
18,216
If the output is isolated, you can just connect whatever wire you want as "common".
Do you mean that with this configuration i cant use a common earth between input and output?
You did not show a, "configuration". You just drew a square and called it a buck/boost converter.
Now that you have said what is in the square, I think you have chosen the wrong chip.

#### AnalogKid

Joined Aug 1, 2013
8,814
The circuit in post #1 has 60 V max potential difference, -48 to +12. This is beyond the ratings for the standard 2675, and exactly equal to the max rating for the 2576HV, a dangerous thing to design in. What output current do you need? You might be better off buying a small fully isolated DC/DC converter that will do what you want.

ak

#### Dodgydave

Joined Jun 22, 2012
9,861
Note, This chip is a Down Converter not a booster, with common 0V ground.

Thread Starter

#### manubm

Joined Feb 25, 2016
7
My question is about the common Earth input-ouput.

I use the LM2576 to convert -48 to +12v (1A max in the ouput).

How I can Ground the ouput side?

Thanks.

#### AnalogKid

Joined Aug 1, 2013
8,814
There is a circuit trick for doing part of what you want with a buck converter, but it will not work with this chip. And even if it did, you cannot ground the output side.

ak

Thread Starter

#### manubm

Joined Feb 25, 2016
7
There is a circuit trick for doing part of what you want with a buck converter, but it will not work with this chip. And even if it did, you cannot ground the output side.

ak
Any suggestion chip (60v) to do that?

#### AnalogKid

Joined Aug 1, 2013
8,814
Post #7 - no.
Post #8 - no.
Post #11 - no.

ak

#### Dodgydave

Joined Jun 22, 2012
9,861
Manubm, you clearly are not reading these replies, your regulator is a Drop Down Positive converter, you need to use an isolated output type, or negative regulator.

Thread Starter

#### manubm

Joined Feb 25, 2016
7
I got it! I will try with a LM5160 Fly-Buck isolated output.

Thanks