Is it possible to build a DIY 20 volt 90 watt 11A dc power supply?
Yes, this is possible. You should be able to buy ready made converters but since the circuit is simple, you will be easily able to build one if you want the experience. I have designed and build many such devices. If you are going to impose a load of 90 Watts to 220 Watts on a car battery, it would have to be for a short time, especially if the car engine is not running. This will depend on the age of the car, the size of the alternator and the capacity and condition of the battery. Consult the car manual to find the continuous rating of the cigarette lighter. Were you to overload the lighter cable, it could damage the other cables in contact with it.
Yes of course however you need the tools to do-it in an automobile it would be wise not to exceed 10A of load on the cigarette lighter as this is a heavy load on the automobile electrical circuit and it could cause a fire if you want 11A at 20V it means 220W that is about 20A from the plug because there will be some voltage drop and I am very generous at 11V! I think you need to revise your numbers and your goal also how much duty factor, a small automobile you would burn the alternator in a few weeks.
If it is 20v @ 11A the output wattage is 11 x 20 = 220W. Allowing for losses the input would be more like 300W, so 12V @ 25A. This is a lot to draw from the car battery, and also more than a car cigarette lighter socket is rated for (it is more like 10A). It probably needs to be regulated as the voltage range at the input is something like 10 to 15V. You are looking for a DC/DC converter rated at 12/13.8V input, 20V output and greater than 11A output. It could be that a 12 to 24V converter is around somewhere out there, rated at > 11A output meaning it is probably 300W. Nevertheless it would be possible to make one, with some knowledge of higher current construction techniques, and switch mode circuit design. I didn’t find such a supply in a quick search, probably because it is not all that practical for 12V input. You would be designing from scratch. See the Linear Technology website and you may find a design you can adapt. I did find 150W laptop supplies, these have an output current
