DC to DC Voltage stepup

[CrazyBillyBob]

I'm trying to see if it's feasible to find (worst case make) a 12Vdc to 20Vdc converter that will handle 2.5Amps constant. The 20V out needs to be pretty clean too as it's feeding a computer (laptop). The 12V source will be an automotive environment so noise and voltage spikes abound! Is this a crazy Idea, or do able. (plan b is to replace laptop with a Pico ITX, and 7" touch screen).

Thanks!
Bruce

[Bigglez]

I'm trying to see if it's feasible to find (worst case make) a 12Vdc to 20Vdc converter that will handle 2.5Amps constant. The 20V out needs to be pretty clean too as it's feeding a computer (laptop). The 12V source will be an automotive environment so noise and voltage spikes abound! Is this a crazy Idea, or do able. (plan b is to replace laptop with a Pico ITX, and 7" touch screen).

Yes, no problem. Fifty watts is well within a simple
flyback boost design. Assuming the automobile
ground is also the laptop ground, the converter
will use an off-the-shelf inductor (no coil winding
required). Controller ICs for this task abound.

I'm surprised that a commercial product doesn't exist
(or have you not searched?). Police cars and UPS
trucks or similar often have computer displays
in the vehicle.

[CeaSaR]

If this is truly for a laptop, why not just use a 100 watt inverter and run
the standard charger/ps through there. Just remember to place a cutoff
switch in line with the 12v so as not to run down your battery. The Black
and Decker 100w inverter I have runs both my and my son's laptop
without a hickup.

Just food for thought.

CeaSaR

[MrAl]

Hi there,


A typical design for a 12v to 20v converter using the LM3478
would go something like this:

Vin: 10 to 14vdc
Vout: 20vdc
Fo=500kHz
MOSFET: IRF7821 (peak switch current around 5 to 6 amps)
Inductor: 15uH, low Rs, like Coilcraft SER2915H-153KL
Schottky: 8A,40v
Cin,Cout: 22 to 50uf, low ESR

Schematic:





Alternate IC ideas:

LM3488
LM5022

[CrazyBillyBob]

If this is truly for a laptop, why not just use a 100 watt inverter and run
the standard charger/ps through there. Just remember to place a cutoff
switch in line with the 12v so as not to run down your battery. The Black
and Decker 100w inverter I have runs both my and my son's laptop
without a hickup.

Just food for thought.

CeaSaR

The issue with this is that we've seen 2 laptop die a painful death with this setup. Normally it's when the car is started and the laptop is on that it pops.
So the goal was to drop the inverter, plus the inverter to power brick is really inefficient.



MrAL
is there any extra cleaning or conditioning that would need to done on the output?

[MrAl]

Hi again,


More filtering on the output is probably not required for running
a computer, but if you are worried just add a small inductor like
2uH and another cap, but keep the feedback on the first two caps
shown in the schematic. I doubt this would be needed however.

I am assuming you have already built electronic circuits up in the
past too, and that you test the supply out before using it with your
computer.

[jollyrgr]

To do this for the knowledge of how to do this is one thing. Making a practical boost circuit for less than what you can buy is another. They make universal devices to do exactly what you want. My FIRST choice would be do do what CeaSar did as you can get 300W 12V to 120VAC inverters in the $20 range and these can be used for other purposes.

They also make "universal" laptop power supplies with adjustable output voltages. Here is just one example:
http://www.b2cpowershop.com/en/universa ... r-80w.html

Here it is at a cheaper price:
http://www.dealextreme.com/details.dx/sku.3438

Unless you have huge reserves of junkbox parts and lots of time you can't beat sub $15 supplies that are ready to go.

If cheap no name adapters concern you, you can buy a brand name device for five to six times as much that will do the same thing.

http://www.igo.com/productresults.asp?a ... 6600034-02

[Bigglez]

The issue with this is that we've seen 2 laptop die a painful death with this setup. Normally it's when the car is started and the laptop is on that it pops.
So the goal was to drop the inverter, plus the inverter to power brick is really inefficient.

Why not add your own circuitry to disable the inverter if
the vehicle battery voltage drops? (I know some, if not
all, inverters have this to prevent the inverter from draining
the battery, but they are slow to respond to glitches).

You may just need a transient over voltage device to
condition the inverter's output so it doesn't stress the
NB power supply.

Something very simple, perhaps a 120V AC relay in the
output of the inverter, which in turn is powered by the
12V DC voltage detector on the inverter's input.

It probably wont harm the NB which has battery power
when the AC fails. It probably wont harm the inverter
or the NB power adapter. You may add a timer to
stop the relay from chattering. The goal is to get the
NB off the inverter in case the inverter's power surges.

[CrazyBillyBob]

Thanks for all the info. I was looking in to this for a no electronic friend who asked me If I knew how he could do this, I didn't but was sure you all would point us in the right direction.... You Did! Based on cost he's going with on of the PSU's like those listed by jollyrgr.

Even though he didn't go with it MrAl I think your schematic is going into my drawing bin for use later!



Thanks again!

[MrAl]

Hi Crazy,

I think buying a ready made solution is better too unless the person
building this thing has a lot of experience with building and troubleshooting
electronic equipment.
Good luck, and maybe you can let us know how it worked out in the
end.