DC/DC Converter

Different pieces of equipment often require different voltages to power them.

In these cases it’s most common to design the battery at the voltage rating to handle the bulk of the requirements and then use a DC/DC Converter to step that voltage up or down to provide a secondary feed for the ‘orphaned’ equipment
that needs a different voltage.

This allows legacy 12V devices to be connected to a power system built around a newer 48V configuration.

1) How do I size a DC/DC Converter?

Firstly, look for a unit that has the correct DC/DC conversion.

Most converters list their voltage rating in the order of source/load. So a "48/12" converter will take a source feed from a 48V battery and convert it to a 12V supply for 12V loads.

Once you've chosen the voltage conversion you need, then look for a unit with the loud outlet current value to suit your loads. This could be determined by adding all the maximum demand ratings of all your 12V loads (plus a buffer factor of course), but it should also be chosen while considering the ratings of any circuit fuses or switches protecting the loads. It is usually best if the DC/DC Converter can deliver 'a bit more' current that the switchgear will trip at. As always, ensure you consider cable ratings & voltage drop in your sizings!

And lastly you need to consider inlet/outlet isolation.

• Non-Isolated models have a common negative (ground) between the inlet and outlet sides. This can be convenient for vehicle applications that use common grounding to the vehicle chassis. They can also be smaller, lighter, cheaper and more efficient.
• Isolated models have full galvanic isolation between the inlet and outlet circuits. In some applications this can provide a higher level of safety. The galvanic separation means the unit can assist in breaking ground loops and minimise noise for sensitive applications. It also provides a floating reference voltage for the outlet.

In most cases either type may be used.

2) When would it be a good idea to use a same/same voltage converter?

Some loads, particularly communications devices such as modems, can have a very limited operating window.

On the other hand, most solar systems will see the battery voltage fluctuate significantly between 'depleted' and 'under charge' states.

For example, a 12V battery could go down to 10.5V at a heavily discharged condition, then the next day it could be up around 14.5V while being recharged. In cold weather the charge voltage could be as high as 15V or even 16V.

If you need to use a load device that doesn't have the 'tolerance range' to handle that fluctuation, then a DC/DC Converter is the best way to 'smooth' the voltage so as not to damage the load device.

3) Can multiple converters be 'stacked' to deliver more current?

In most cases, yes, though it depends on the individual manufacturer.

4) Can different types of converters be used in one system?

Usually.

If you have a 48V system but need both a 12V AND a 24V feed for devices that cannot accept the primary 48V system's power, then it's normal to use a 48/12 and a 48/24 converter to supply the lower voltage circuits.

5) Why do the examples always 'step-down' the voltage? There are models that can 'step-up' too!

Lower voltages mean more losses due to voltage drop as well as higher current and heavier cable. In the interests of system efficiency I always prefer to design a system with a higher voltage battery and then use Converters to step it down as needed.

However, so long as the wiring & protection hardware is appropriately sized, starting from a lower voltage and stepping up is perfectly fine.

In fact, if the majority of the loads are best operated at 12V then it's usually more efficient and reliable to design the battery at 12V (despite the voltage drop and cable gauge complications) and use step-up Converters to run the 'left over' 24V or 48V devices.