I went to design a 40V 2A power supply to run 3 stepper motor 3A.
i used L298 to deal with motor, the max volt can drive is 50V with 2A " i get this from datasheet L298 is this right? ".
so i went to design this power supply with regulator .
The problem is : if i design a 40v 5a power supply and connect it with 3 L298 and 3 stepper , if one of this stepper run only so the current will be 4 - 5A and will burn the L298 and the stepper .
is this right?
No. Where do you get the '4-5A' figure? You have stated that each stepper is a 3A type.
If each stepper is rated at 3A and you can be sure only one will be driven at any time then a 5A supply (which means the maximum current it can provide is 5A) will happily provide just 3A when one motor is on. If all 3 steppers may be on at the same tiime then your supply needs to be rated at > 9A.
For a 40V 10A supply your best bet is a SMPS. It will be cheaper and more efficient than a linear type.
The LTSpice simulation program (freeware) is particularly suited to designing SMPS.
BTW, if the L298 is rated at 2A max it is unsuitable for directly driving a stepper which requires 3A; additional buffering, or a different driver chip, would be needed.
Your first circuit will give a capacitor voltage of ~ 55V !
Will 25000uF be enough? The ripple voltage will be considerable if you're drawing 9A. I think a voltage regulator would be necessary with that arrangement.
But as I said, a SMPS would be better.
Edit: Re the second circuit, see my post #7 doubts about the L298 suitability for this project.