Thread: Question for Dima ! (Max 1771)

My circuit is very similar to bootstrapped mode schematics,except few changes:
1)bigger inductor-47 uH,should be capable of handling up to 2.5A.
2)Rsense - 20 mOhm (milli)5W.Don't use smaller than this,I tried first with 10mOhm,and blown the MOSFET.This brought me to use more powerful MOSFET,that can handle about 3A.Don't forget the heatsink!
If I remember correct,I get about 1.5A using 5v input,and about 2A with 12v input.But when I was approaching to 14-15V (simulating car voltage when engine is running),the MOSFET became VERY hot,and I was afraid it will blow while using for a long time.So I decided to stay with 5v input.
Why do you need so much current at 12v? If so,I think using flyback transformer would be better and more efficient.Using MAX1771 seems like using an inverter: like converting to 220v from 12v and then back to 12v,it is converting 12v to 5v and then back to 12v.And also,you wanna use 2 MAX787 and 4 Max1771,with coils,MOSFETS,and big heatsink...that's pretty many parts for 12v line!I think it will be cheaper and more efficient to double or even triple Sproggy's 12v section from his PSU.

Wouldn't it be easyer to just use 7812 ic's for the 12v?

agreed,might be a problem if u don't run the engine, but otherwise...

Hi Dima, Thanks for the help.
I will use other (better) parts.

Why do you need so much current at 12v?

It's not that I need it right now, But don't we all have a problem with to less current on the 12v line ?? I just want to make a psu which is powerfull enough, and will power the pc in future when I might upgrade it or something.
I think it's better to have enough juice to power it, than just too little, 'cause you have to do it all over again.
And, -correct me if i'm wrong- when I don't need all that current, it doesn't get generated, Right ?
Just want to play safe, so that I can add some new things in the future....

This might be a stupid question,
I know the MAX788 and 787 need headsinks,
But does the 1771 need one too ?
If so, how do you mount them on these ???
They're pretty flat without any room for an attachement or something

Anyway, thanks for the info..

Have the system running.. it's on my desk though.. don't want it in my car till i got a dc-dc

Greetz

Most systems needs about 3A max at 12v line,and just for short periods,for example,when spinnig up HD.Rest of the time,you will use about 1-1.5A max.
And MAX1771 don't need a heatsink,but the MOSFET do need.
4 MAX1771 will give you about 6A...this is too much for any system.Are you using Sproggy's PSU? If so,you can just add one or two MAX1771,it will be enough for you,untill some smart people here will teach us how to modify a desktop PSU

Hi,

I don't have the sproggy psu (I almost got all the parts, got the pcb ready, even got the coil, but national won't ship me an lm2587-12t )

So basically, 1 Max787 would work with 2 Max1771 ??? (this is cool, then I have some spare parts )

And since I have all these parts (max1771 also)
I thought of making a powerfull psu myself
(along with the datasheets samples, I'm no electronic guru (As you already know )

I'm just looking for alternatives, 'cause I think it will take a while before we all can modify our desktop psu's

Anyway, thanks for the info !!!
greetz !