Off topic (experience with switcher design)

scotta

Hi,

I have a project that must run off 24 volts. From this
24 volt supply I need 5 volts to run IO and 3.3 volts
for the Propeller.

With a 24 volt supply the 5 volt linear regulator must
drop 19 volts and dissipates about 7.6 watts with a
400mA load.

This raises the temperature inside the (plastic) enclosure
to 110F with 76F ambient temperatures. The project
will be installed at a paper mill where the temperature
will exceed 100F. It's going to melt.........

Here are the questions:
Have any of you worked with National's Simple Switchers, or the clones ?
How important is inductor,diode and capacitor selection ?
Is PCB layout that critical (given I follow the guidelines) ?
Will it latch up and destroy my circuit ?

Thanks !

Leon

A switcher is the answer. The Nat Semi chips are quite easy to use, but you need to select the parts carefully and pay a lot of attention to your PCB layout, to avoid EMC problems.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

scotta

Is coupling between the inductor and the feedback trace that
big of an issue ?

BTW: I this is my first run in with a switcher and I kid you not,
I have never used an inductor in 20 years [noparse]:)[/noparse]

Paul Baker

Inductor placement is always an issue when placing a switcher near the design. However it is less of an issue with digital. In layout it is best to place the inductor along the edge of the board.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

scotta

Thanks Leon and Paul.

There is a board mounted LCD display. How far away do
I need to keep the steel LCD frame front the inductor ?

Can I use a cylindrical inductor if I mount it 1 inch away ?

Scott

Paul Baker

Steel frame shouldn't be an issue, they are typically grounded and wont be affected, it's signal lines near the inductor that cause problems.

BTW if EMI (electromagnetic interference) is a concern with the inductor, using a toroidal inductor is best because the·EM lines flow through the ferrite with much less concentration in free space.

But I would try the circuit out first and only if you see a problem with inductive pickup, try a toroid.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 9/3/2008 6:16:23 PM GMT

scotta

I just found a toroid made by Alfamag in digikey. They
have a surface mount and through hole in basically
the same package. All in a .5 by .5 inch package.

I feel much better.

Praxis

Here is a circuit, PCB example and picture using a LM2596 switching regulator.

This example the regulator is a variable output type but you can get fixed 5 volt versions (R1,R2 & C4 not required).

The inductor is a "sealed" type.

The parts as shown in the give a 5 volt output for a 8 to 35 volt input and the inductor shown is for 3 amp output, you can select one that is suitable for your current requirements as well as the diode (D1) which in this case is a 3A Shottky.

All these parts can be got from either or RS or Farnell.

Cheers

scotta

I just fired up a MC34063.

Its powering the circuit just fine off of 24 volts and
its not even close to being warm.

All on a breadboard on top of it (cringe).

Thanks everyone.

Paul Baker

Hi scotta, I'm moving your post over to the sandbox since you admit it is off topic.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.