DC/DC Power Supply Help

Hi all, I am in need of a DC-DC Power supply schematic that outputs between 12.7 and 14.5VDC and an input that could be anywhere from 12VDC up to 36VDC. The crutch is this-I need it to put out at around 80 amps. Realistically, the supply will be running 80% of the time under 20amps, but there are occassions where it will need to draw upwards of 60amps continuous.

I found this schematic (attached), but it looks like it is only about 50% efficient assuming a 24v in and 12v out. Am I correct on this or should it be higher?

Does anyone know of any simple circuits for a supply that will work for me, or know of one that I can simply modify to get the power output needed? Any thoughts would be appreciated. To give you an idea of my experience level, I was an EE major for 2 years but never completed my schooling.

I don't need anything fancy, and really am trying to keep it as simple as possible and get 80% efficiency or better. Is this possible with BJT transistors, or will I need to use a switching supply or MOSFET design?

Thanks !

this is a link to the schematic I referenced: http://www.zen22142.zen.co.uk/Circuits/Power/3730.htm

Hi Mozencrath.
Welcome to the forum.
your link schematic is a large power supply, not a DC to DC converter, what your asking is possible to build, but would be an advanced project, given first the selection of voltages you want, and it would be a fairly large unit, given the current you want, retail units are few and far between with that current rating.

Your schematic is an AC to DC power supply, using multiple transistors to up the current.
So what you want is to take a DC source and make that a high current DC source, in there is a problem, i dont think you will get much more power from your DC source than it can deliver, so whats the high current driving, if you dont mind me asking ?.

Ive seen your schematic before on the web site its on, some good circuits on there.
Any way i think you need to clarify what you want to accomplish.

Dave is correct, if you have 12V input and want out 14.5V at 80A, then your input will need to deliver more then 80A@100% efficiency. You can not create power out of nothing, you can only convert quantities of power. ex. want more voltage, then you will need to decrease your current. want more current then you will need to decrease your voltage.

I would try to keep your input V greater then your output V and do a search for dc-dc buck converter. Right now I think you are a little confused on exactly what you need. Hopefully we can clear some of that up if you provide us with some details.

Yes the circuit technically is an AC to DC power supply, but I simply used a 24V battery pack instead of an AC transformer. I will be a bit more clear for you as well. As far as input, I do not need it to accept 12VDC to 36VDC, I mean I can input 12, 24 or 36VDC since my power source is 12V batteries....

What I am powering is actually my Prius' 12V system. The system is currently powered by an internal DC/DC Converter within the prius that converts the main 1.3kw battery (220V nom) into 13.3 - 14.1 VDC. It has a rated output of 100A, however the system will only ever draw 80 amps max on rare occassions. Typically the load is about 15-30 amps.

I mean, I was even thinking of trying to find a simple circuit that can take my 24V battery setup and just output ~13-14V. Can I do something like this without transistors? Just an LM317 to regulate the voltage, and a capacitor to smooth it out?

Your advice is GREATLY appreciated!

What do you have against the design that you need to change it? Essentially your gonna be creating the exact same thing. Why do you wish to reinvent the wheel?

Yes, but the LM317 is only designed for at it's max 1A, you will however get less then that. Also, VR's are very inefficient the further the input V is away form the regulated voltage.

Dropping 24V to 12V using a resistive circuit will give an efficiency of 50%. To get a higher efficiency, you will need to store the energy in an inductor and send it to the output. Pulse width modulation will be needed to do this and will enable a constant output voltage with varying load and input voltage.

Fets are better for switching circuits since they can have a very low resistance when turned fully on.

Do you have an example of a high current DC to DC that would utilize FET's?

I have nothing against the schematic I posted, but I can not deal with the hassle of trying to cool hundreds of watts of power dissipation from the outboard transistors. And, with only 50% efficiency, I would need almost double the battery capacity to run the system.

This should tell you more about what you need.

Such devices are already so common that it would probably be easier to buy rather than build one.

Your question may be better asked at an electric vehicle forum as there are many DIY types out there who would have needed something similar (although typically from a higher input voltage).

Here is one manufacturere of these devices.

Thanks for the links, but @ powerstream I would be looking at spending almost $1,000. My only option is to make my own. I work at an electronics supplier and we stock over 750,000 different parts obsolete and new, so my cost on parts would be essentially zero.

OK, you have 2 problems.

Firstly (and I didn't notice this before) your minimum input voltage is below your output voltage, and the maximum is above it. This means that you need something that is neither a buck or a boost regulator. One solution to this is a boost followed by a buck regulator (and this doubles the complexity).

The second is the power output. At these levels the design is very critical.

For these reasons, the design would be somewhat complex.

I am trying to think of a *simple* way to do it, and my best suggestion is to find a way to combine the output of several lower current devices. To do that is tricky, and the best I can suggest is that each regulator would need to be current limited and possibly isolated from the others by a diode.