It is based on a eBay 150W step up converter (you can see it at the right).I use to convert the 12-13.8V to 19V to power my laptop.  I took out the 2 power elements and soldered as you can see in the picture. For 3A at the 19V the box is enough for cooling. If you need more cooling you can add one on the top of the box. I add an external LED 3mm with 1 Kohm resistor.