DC-DC Converter for USB Hub

[rbroberts]

I have a USB hub which wants a 5V, 3A supply. Naturally, one is supplied as an AC-DC brick, but I'm an amateur astronomer and am often in the field with a DC supply (deep-cycle marine battery) for my power supply and prefer to avoid doing DC-AC-DC for efficiency. I haven't found any commercial bricks for this and am wondering if building one is as simple as buying a module, sticking it in a project box with a heat sink and wiring it up with appropriate plugs.

I was looking at this module: http://www.powerstream.com/DC3.htm in particular the one labeled PST-SU-12-5.

TIA,

roland

[sofaspud]

Yes, it's that easy. The module may even be efficient enough that a heatsink isn't necessary. For
$80 it ought to be. Though not as efficient and only slightly less simple, you could look into a
circuit using a voltage regulator such as the LM350. There are lots of voltage regulator types and
you may want to look into using one. Virtually all are easy to implement into a circuit.
5V, 3A indicates a 6-port hub. What the hub actually requires is 0.5A for each USB device plugged
into it. i.e. If you will only be using 4 ports on the hub you only need a 5V, 2A power supply. Worth
considering because the cost will vary proportional to the power requirements.

[rbroberts]

Well, they also have a 15W version which is about 1/2 that price, as well as the one from DigiKey at http://search.digikey.com/scripts/DkSea ... 02-1247-ND.

For some imaging setups, I am using all 6 ports, although I from the actual use cases they are almost certainly not really drawing their full 0.5A all at once. Hmm, that makes me think your idea of using an LM350 might be worth trying first since it's cheap. I was worrying about using a linear device due to power dissipation, but most of the time only 3 devices will be active at once, two of which are cameras and have their own power, the other "big" pair are USB-RS232 devices.

Thanks,

roland

[sofaspud]

The downside of using a linear regulator like the LM350 is that it will dissipate power whether any port
devices are active or not. I mentioned it as an option, but whether it is a practical solution depends on
the size of the battery and how long it must supply juice before a recharge. It's possible that maybe half
the power gets wasted as heat. It kinda sounds like it would be worth the trouble to set up your rig and
measure the USB maximum DC current draw. I've used those "metal can" DC-DC modules and they are slick
and enticing. But they ain't cheap.

[MrAl]

Hi,


As some of the other posts mentioned, using a linear regulator is also a possibility but the power
dissipation may be a problem unless the input and output voltage differential is not too high.

If the requirement is a 5v output and the input is 12v and the output current is 1 amp then
the input power is 12 watts and the output power is 5 watts, making the system only
about 42 percent efficient which is very low.
On the other hand for the same requirements and a 6v input (instead of 12) then the input
power is 6 watts and the output power is 5 watts making the system about 83 percent efficient,
which isnt that bad at all.
It depends then on what the input voltage is. If the input voltage is 12v then the system would
be much better off using a dc to dc step down converter, which can easily be 80 percent efficient.
It the input is only 6v then a linear will work just the same.

There isnt that much power used during times when all the loads are off however, as the quiescent
current for either power supply should be fairly low.

National Semiconductor makes a line of "Simple Switchers" that would allow you to create a buck
converter with only a few parts that could handle 3 amps. You may want to look into those designs.

[haklesup]

National Semiconductor makes a line of "Simple Switchers" that would allow you to create a buck converter with only a few parts that could handle 3 amps. You may want to look into those designs.

I was thinking along the same lines. To answer, I was playing around with the webbench design tool which helps you select one of the ICs and recommends the parts to use to make such a converter. In many cases you can buy an evaluation board for about $30 and the parts to put on it for another $10 or less. In many cases you can sample the IC. In almost all cases the parts are easy to get from DigiKey or Mouser etc including the eval board

While the webench (google that) tool is fast to create a particular design, you will need to spend some time flipping through some of the datasheets to select the best one for you (and there will be more than several that qualify). In many cases all you will need is the IC, a couple capacitors and resistors a zener and a coil. Efficiencies in the 80% to 95% range are possible.

All in all, sometimes there is too much choice with this route

[MrAl]

National Semiconductor makes a line of "Simple Switchers" that would allow you to create a buck converter with only a few parts that could handle 3 amps. You may want to look into those designs.

I was thinking along the same lines. To answer, I was playing around with the webbench design tool which helps you select one of the ICs and recommends the parts to use to make such a converter. In many cases you can buy an evaluation board for about $30 and the parts to put on it for another $10 or less. In many cases you can sample the IC. In almost all cases the parts are easy to get from DigiKey or Mouser etc including the eval board

While the webench (google that) tool is fast to create a particular design, you will need to spend some time flipping through some of the datasheets to select the best one for you (and there will be more than several that qualify). In many cases all you will need is the IC, a couple capacitors and resistors a zener and a coil. Efficiencies in the 80% to 95% range are possible.

All in all, sometimes there is too much choice with this route

Hi,

A zener? Which design is that?

I was thinking:
1. IC chip
2. Schottky
3. input cap
4. output cap
5. 5 amp inductor
6. two resistors if IC is ADJ type

[Engineer3104]

This regulator might do what you need

http://www.dimensionengineering.com/DE-SWADJ3.htm

I've used several of their products. They seem almost bullet proof

[MrAl]

Hi,

Yeah me too. I even used one for a Li-ion charger after a little mod to the circuit.
I even used cheap electrolytics for the output caps

[haklesup]

A zener? Which design is that?

Careless me, that probably was a schottky diode. I was flipping through datasheets fairly quickly and the symbols are a little similar

[MrAl]

Hi hackle,


Oh, that makes sense now

BTW i have used cheap 3 amp Schottky's for the 3 amp regulators, but i think the design
really calls for a 4 or 5 amp device. I havent gone through any rigorous analysis with
that in mind yet however. Maybe i'll take a quick look at that...

[jollyrgr]

I understand you DON'T want to go DC AC DC. But let me throw some economics in for you. The device you posted the link to is $80. Inside it goes DC to AC and back to DC and is basically a switching regulator. So why not just use a switching power supply? For about $30 USD you can get a mini-ITX form factor power supply. These are commonly used by "Carputer" builders. Here is one example with specs. This link is to a China supplier but has the specs.
http://www.alibaba.com/product-gs/22579 ... Power.html
(This board can be found on sites like eBay. For example eBay item number: 270420363841)

A linear voltage regulator CAN be made using a regulator such as a 7805 and a pass transistor. But this will waste battery engergy. If that is not a concern, here is a page describing such a project.
http://www.rason.org/Projects/vreg/vreg.htm

While you don't like the 12VDC to 120VAC inverters this would be a cheaper path still. A 75W 12V to 120VAC inverter goes for about $10USD. They even make some of these with USB connectors and 5VDC outputs. (eBay item number: 170355853963)

While your own suggestion will work, I think spending $80 on a regulator is a bit much. And that does not include case, PC board, cabling etc. The automobile "carputer" ITX power supply would not be too bad and if you get the right model can easily supply 5V @ 5A.

I'd go with the linear regulator and pass transistor or the 12V to 120VAC inverter if I were doing this project.