# How to get 12V from (automotive) 7V to 42V?

I'm trying to design a circuit that is supplied with a voltage between 7V to 30V (12V battery) and needs to output 12V @100mA.

How can I implement it? Do I have to use an DC-DC (step-up/boost) converter?

• Might be worth researching buck/boost converters
– user16324
Nov 29, 2013 at 10:14

Here are a couple of buck-boost converter offerings from Linear technology that would fit the bill. The second one is the preferred solution to me: -

Here is the LT page where you can enter your own parameters and it provides you with the chip options.

You basically need a switched mode power supply that is capable of both stepping down and stepping up the input voltage to 12V because your input voltage range is greater than and less than the required output voltage. Therefore a boost converter will not suffice; neither will a buck converter.

The simplest power electronic converter that could do this is the buck-boost converter: http://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter This converter produces a negative output voltage with respect to the input voltage. However, if the output can be floating, then this is not a problem. Just use the negative output of the converter your ground rail.

If you can't work with a buck boost, the next best bet is the flyback. This is similar to a buck boost, but can produce a positive output voltage because it is an isolated converter.

Another option is the SEPIC converter. However, this is more complicated to design and control since it uses two inductors.

To summarize, I would look at these converters in the following order:

• Buck-boost
• Flyback
• Sepic
• I don't think that the wiki article you listed is very good at explaining buck-boost topologies - it seems to imply that it naturally produces a negative voltage and this is not my experience. A buck-boost is a buck regulator (say) +5V from an input of 7V to 30V followed by a boost taking the +5V output from the buck to 12V. Nov 29, 2013 at 13:04
• @Andyaka What you are referring to is a two stage converter: i.e. buck followed by boost. With this arrangement you do get a positive output voltage. The link I posted is of a single stage buck-boost converter and it will definitely produce a negative output voltage. Nov 29, 2013 at 14:05

A SMPS (switched mode power supply) is a good choice here. (also called DC/DC)

Several typologies allow the input voltage to be either lower of higher than the output voltage. Have a look at:

• Flyback architectures
• SEPIC architectures
• And many others

If you are not comfortable with them, you can always do a multi-stage approach with the drawback of reducing the efficiency. E.G: The first stage lower the voltage to less than 7V and another stage will transform this 7 to 12V. Thus there is no overlapping problem.