DC-DC converter connection question

[xrunner]

Maybe this is a dumb question but ...

I want to power a DC-DC converter from a +13.8V supply. The output of it will be adjusted to about 7V to 8V, but - the output will be used as a negative voltage to bias a circuit in a radio, the radio being powered from the same +13.8V supply. So, to have a negative V to the bias circuit, I'll have to ground the positive output of the DC-DC converter, which will obviously then be connected to the input negative supply to the converter, again the same supply the radio is using. I am assuming that won't damage the converter - am I correct?

[jeroen79]

It depends. Will you be buying or DIYing?

If you are buying an existing design you have to check how it is designed.
Some will have a floating output, others will have one of the inputs tied to one of the outputs. (often a common negative)

If you are building something yourself then you make the outputs floating or tie positive out to negative in.

[ovnr]

A simple buck converter will die. You need an inverting converter which already gives a negative output voltage by default, or an isolated DC/DC converter.

It sounds like you want a ready-made brick, in which case here are some negative-output converters, and here are some isolated converters where you can safely ground the positive output.

[xrunner]

Good info thanks guys. Exactly what I needed to know. 

[Someone]

I'll have to ground the positive output of the DC-DC converter, which will obviously then be connected to the input negative supply to the converter, again the same supply the radio is using. I am assuming that won't damage the converter - am I correct?

You can use an inverting or isolated output power supply which will directly create the negative supply, or use a buck regulator with some trickery by connecting its output to the GND as you describe:
http://www.vishay.com/docs/76946/creatingnegativeoutputvoltage.pdf
http://www.ti.com/lit/an/snva022e/snva022e.pdf