Fixed 5V buck converter?

[jwallis]

Hi,
Background: I'm making a project for the car and need a very reliable 5V @ .5A with very short spikes up to 3A.

Most buck converters seem variable dependent on the input (meaning after you set the screw in what I assume is a variable resistor, if the input V changes, the output V changes).

I found one "fixed" 5V buck converter on amazon, but it is tiny and of questionable quality:
https://www.amazon.com/Regulator-DROK-Converter-Step-Down-Transformer/dp/B0758ZTS61/ref=sr_1_5

And the next best thing I can find is something like
https://www.amazon.com/SeeedStudio-CPT-C5-Converter-Switch-Rectification/dp/B01I1Q3A92/ref=sr_1_1

Which is several times the price.  I might use a typical buck converter + linear regulator to get a consistent 5V.  I'd rather find a high quality fixed 8-16V in 5V out buck converter...

Any help is appreciated!

[Wawa]

Cigarette lighter style USB chargers are (should be) made for the car environment.
Leo..

[Riva]

Most buck converters seem variable dependent on the input (meaning after you set the screw in what I assume is a variable resistor, if the input V changes, the output V changes).

This is not the case, these buck converters have adjustment but it is to set the output voltage. Once set, different input voltages will still give the same output voltage (assuming the input voltage is higher than the output).
Fixed output buck converters just have a preset output voltage that cannot be easily adjusted.

[Paul__B]

Most buck converters seem variable dependent on the input (meaning after you set the screw in what I assume is a variable resistor, if the input V changes, the output V changes).

How do you know that?

Clearly, if the output voltage changes as the input voltage does, it is not functioning as a regulator!

[MarkT]

Hi,
Background: I'm making a project for the car and need a very reliable 5V @ .5A with very short spikes up to 3A.

Not quite enough information.  How short are the longest spikes?  What is the worse-case duty cycle?

[jwallis]

How do you know that?

This is not the case

Ugh, thank you both.  I would *swear* I determined this experimentally using different power sources and a voltmeter.  I think I was originally using a wall wart and maybe it was of low quality.  Wasted most of a day looking for and buying parts.  Just verified your correctness with a car battery and a 9V battery.

Thanks again.  I guess it's good to be wrong sometimes, since this definitely makes my life easier : )

[wvmarle]

Like linear regulators, most buck converters need a minimum drop-out, typically 1-2V. So for 5V output you need 6-7V or more on the input.
There are buck/boost converters which can take both higher and lower voltages on the input.

[gilshultz]

I would look at a SEPIC converter (Buck Boost). None of the buck converters I have worked with including the $1.00 ones regulate and hold the output voltage within a few millivolts over its input rating. Many have a 3A rating which would be adequate for what you are trying to do.  The one you reference is a nice unit but it did not show its ratings.  Be sure to look at Load_Dump requirements if you want very reliable system.  Depending on the magnitude of your spike adding a bulk capacitor may be all you need.
Good Luck & Have Fun!
Gil