I have a little problem, i been searching but i was unable to found (maybe do to my little knowlege on the matter) (and my little bit of english) a formula or a explanation to the following dilema:

i want to combine 2 power imputs being the same power suply, i mean 2 of the same wall converters (2 AC 110v 50-60hz to
DC 20v 4.5A), so im imagine that i could combine those 2 diferents power suplies to reach 20v 9A, while i use diodes to prevent the full power from coming back to the converters and fry them, and using a resistor (i dont know how many ohms or another thing i might need) to lower the 9A to 8.5A while keeping the 20V.

Thanks for everyone that read this.

 

Two things:

One is that the power supply sets the voltage and the load determines the current. The rules are that the power supply voltage must be in the range that the load requires and that the load not draw more current than the power supply is rated at.

To put two regulated power supplies in parallel is not easy because their output voltages are different and the one with the highest voltage setting will tend to supply most if not all of the current. I have not done this myself, so for now, I will only say that I would try low value resistors in series with the power supplied to help them share the current. The cost is that your regulation will probably suffer a little.

There are members here who have probably done this successfully, so please wait until they comment.

 

Getting two voltage supplies to play nice when put in parallel can be tricky, particularly if you want good voltage regulation.

Diodes can prevent one from backdriving the other, but you can still get into oscillation where the output current of the two swings over a wide range because neither knows how much it should be putting out and there's an infinite number of acceptable possibilities for them to wander amongst. This issues gets larger as the output impedance gets lower.

Small value ballast resistors can help.

Another strategy is to have one supply provide all of the current up to a certain point and then run at max output while the second supply starts providing the current above that. A conceptually simple way to do this is to put one diode in series with one output and two diodes in series with the other. Then the output to the load will be about Vs-Vd until you reach the max output capability of one supply, at which point it will drop to Vs-2Vd as the other supply starts picking up the slack. The complicating issue (aside from whether you can tolerate this range of voltage levels) is that you need a supply that will be happy delivering that max output that is pulling its output down by a diode drop without burning up or going into thermal shutdown. That might be tough with off-the-shelf wall warts.

A better option is to design a dual-input power supply that manages the load balancing between the two supplies. There may be even better options (even off-the-shelf solutions) using switched-mode techniques.

EDIT: Fix typos.

 

what if I use a dc to dc converter?

 

It is best to use a power supply rated at the required current. Otherwise, WBhan's suggestion of running one power supply at a slightly higher voltage with a lower current limit than the other may be the best solution in for a temporary setup. People tend not to design things that way because is it a very klugy solution.