# DC-to-DC conversion for solar, off the shelf or custom?

I am working on a small solar power system. The critical problem right now for me is how to do the DC-to-DC conversion. Essentially, the problem is that you have an array of large 12V lead-acid batteries and need to power a wide variety of different DC devices which typically require voltages like 3, 5, -5 and 9. Also, you may have 12V devices but need a regulated supply so even for hooking up the batteries to a 12V device is not trivial.

Problem #1: confusing multiplicity of circuit possibilities

The main off-the-shelf options seem to be either buck converters or DC switchers. One problem is that there are many different ways to design such devices and what you can buy is often a black box, so you often don't know what kind of circuit you are buying. The manufacturers claimed efficiency is often much different than the real efficiency you actually get when you plug it in and measure it. Therefore, it is hard to know without a lot of trial and error which products might be good or bad.

Problem #2: compromise circuits

A lot of switcher products accept a range of input voltages, thus they may be "compromise designs" that sacrifice efficiency for flexibility. In my case I have just ONE input voltage: 12V, so I don't need a chip that accepts a range of input voltages. So, if I a buy switcher chips I may be unnecessarily losing efficiency to get flexibility I don't need.

Problem #3: single-input single-output designs

Virtually all commercial products are not designed to output a range of different output voltages; they take one input and generate one output. Therefore, to use off-the-shelf components, you have to use a whole bunch of different independent converters, which is bulky and probably a lot more inefficient than an integrated design.

Custom Design

To avoid these problems, the alternative is to make a custom design: a board that takes a large 12V input, divides and regulates it to a range of different smaller voltages according to what is wanted. So, for example, the board could have 6x 3V outputs, 8x 5V outputs, 5x 9V outputs, etc, whatever I want. The challenge is that making such a board, especially one that would have good efficiency is potentially extremely complicated. For example, I have been reading the book "GaN Transistors for Efficient Power Conversion" by Alex Lidow and "Switch-Mode Power Supplies" by Christophe Basso and it is evident that making a such a board could be extremely complicated and take a long time, at least for me. Although one alternative I guess would be to pay an expert to design it.

There is a halfway solution here and that is to make a custom board, but to use switchers, which come in SMD packages. You just plaster a bunch of switchers on the board, so you solve the bulky multiplicity of devices problem, but the design is not really integrated--it's just a large number of switchers in the same box.

So, what would be a good solution here? If I am trying to get >95% efficiency, is cobbling together commercial products "good enough" (Option 1) or should I look at a multi-switcher box (Option 2) or go whole hog and try to make a custom board (Option 3)?

• I´d say that the first thing to think about (if you haven´t already) is a decent power controller. After that, DC-DC converters are so cheap these days, I wouldn´t bother building anything myself. Jul 22 '16 at 12:34

Off-the-shelf products are used when you just want something to do a job, and don't need to know what kind of circuit it uses. If it does the job then it's good.

If a product doesn't meet its claimed specs then it may be fraudulent, or you don't understand the specs.

Everything is a compromise. You are using Lead-acid batteries - that's a compromise.

A 12V lead-acid battery isn't 'just' 12V, its voltage may vary from over 14V at full charge down to 11V or lower when discharged. A converter which is optimized for this range may work slightly better than one which is designed to handle higher voltage. Or it might not, depending on the particular circuit and components used in each converter.

Off-the-shelf converters are usually single output because that's what most customers want - and there's no profit in having your shelves filled up with products that people don't want!

A multi output converter is only good for a specific application that needs a particular combination of output voltages and currents, eg. a PC motherboard. They often have a single switching circuit that uses a transformer to generate the different output voltages. As a result there may be some interaction between outputs. This is a compromise, but works for certain applications.

Custom Design

If off-the-shelf components are not satisfactory then a custom design is the only solution. But that has its own compromises, so before you decide to go that way consider whether you really need it, what the tradeoffs are, and what difference it will make to the whole system.

Don't get hung up on converter efficiency without considering other components in the system. Why are you using lead-acid batteries? Why do you need multiple output voltages? How much capacity do you really need, and what are you willing to do to get it?

Solar systems generally have very poor efficiency because the panels themselves are not efficient - but that doesn't matter so long as you get the power you need. Variation in solar output due to weather conditions is so great that worrying about a bit of extra loss in the converters is pointless. If you are so close to the limit that it makes a difference, you need more capacity!

So, what would be a good solution here? If I am trying to get >95% efficiency, is cobbling together commercial products "good enough" (Option 1) or should I look at a multi-switcher box (Option 2) or go whole hog and try to make a custom board (Option 3)?

First drop the requirement for >95% efficiency. Then decide whether the effort of producing a custom board is worth it. Complex switching converter designs are not easy to get right, so unless you have experience in this area it might be better to just use proven commercial products.