48V Battery Voltage Regulator

Question

I initially designed a circuit that would take a 12V battery as its power supply and power up a comparator circuit that would turn on/off a 12V fan based on temperature. This 12V battery could have been anywhere between 11.58-14.4V due to dis/charge so I regulated it with an LM7812, no worries. Comparator circuit uses LM358, NTC thermistor and a BJT for turning fan on/off.

Now my requirements have changed: The circuit will be supplied from a 48V battery that will have a voltage range of 43.2-67.2V. The load is now 48V (x4 12V computer fans in series). I have no idea how to regulate the power supply that is this high. Everything I look at, e.g. LM2576HV only goes up to 63V and Vout possibility is only ~15V.

I'm thinking of powering the comparator circuit with a 9V battery so that I don't have to drastically change all my components.

My questions are: 1. Do you know of an IC that can take this kind of Vin (43.2-67.2V) to have a Vout of 48V? Is there a simple alternative solution to this? 2. Is the 9V to power the comparator circuit a good idea? Will have to adjust the resistors so the LM358 opamp can turn on/off at values I want and pretty sure it can be supplied by 9V. 3. With the increased supply and load, will my current BJT be able to handle the change. It's a BDX53C and the load will be 1.11A.

Thank you.

Answer 1

Firstly, your use of a 7812 voltage regulator for you initial circuit may have seemed fine when you tested it but the 7812 needs at least 14V (or above) for it to provide a regulated output of 12V - if you check what the output is when the input drops significantly below 14V you will see that I'm correct. At 11.58V, it's likely that your "regulated" output is less than 11V.

I'm saying this because you have a misconception as to how linear voltage regulators work and you are in danger of believing you can apply this to higher voltage supplies like the 48V in your question.

You have a desired input voltage range of 43.2V to 67.2V and your required output is 48V. Without compromising your intended requirements, the only way this can be achieved is with a buck-boost switching regulator.

In addition to this, powering fans in series will likely destroy one fan - it will go open circuit (or burning) and then nothing will work. To simplify this, consider using a switching buck regulator that converts your 43.2-67.2V to +12V, then power your fans in parallel.

Maybe try one of these: -

I'm assuming you don't need more than 1A at 12V output as per your original design.

EDIT A higher output version is this and now, you will have to use external transistors on the device: -

I would also seriously consider the LTC3810. Here's a picture and also see figure 19 in the data sheet: -

Answer 2

I dunno, can you run those fans in series? That might not be such a hot idea. And what if one fails such that current no longer flows through it - then all of them shut down. You may want to consider putting them in parallel and using a '48V' - to 12V DC-DC to make them go. I just had to spec something like this from Vicor, which has modules that run in inputs over the range of 35-75VDC in the '48V' range. There are likely more cost effective options; you'll want to consider other requirements when selecting a converter.

Answer 3

You could consider using a linear regulator similar to the 7812 that can handle those voltages. It's an inefficient way of doing it, compared to switching regulator, but it is comparatively simpler to get going quickly. It would depend on how much loss you are willing to tolerate.

The TL783, with connections similar to the LM317, could be a regulator you could consider using if efficiency isn't an issue. Make sure you include some reasonably heavy heat sinking and use resistors with sufficient power dissipation ratings. Note that regulators like the TL783 and the now obsolete VB408 with essentially identical connections have a not insignificant minimum load requirement (though most switching regulators will also impose that condition)