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I have a SMPS capable of outputting 33A at 12V and I would like to make it have a variable output with full current potential, or close to it.

Now,

  1. Is it possible to do it with a PWM-controlled MOSFET with a filter at the output?
  2. Is Arduino with its 490Hz/980Hz enough, or would higher PWM frequency values (either through changing Arduino timers prescale factor or with 555 timer) be necessary in order to simplify the output filter?
  3. Could the output be made stable enough to power microcontrollers at 5V without a linear voltage stabilizer in between?

If the above solution is impractical, what else is an option except a buck or buck-boost converter?

Edit: added the link.

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  • \$\begingroup\$ Please provide a link to the SMPS. \$\endgroup\$ – gbulmer Sep 25 '14 at 18:15
  • \$\begingroup\$ I'd use a synchronous buck converter with hefty push-pull mosfets - should be able to supply up to 11.9Volts and all the way down to below 0.5 volts. Minimal feedback is one advantage of this method because the pulse width inherently defines the output average voltage level largely irrespective of output current. \$\endgroup\$ – Andy aka Sep 25 '14 at 18:52
  • \$\begingroup\$ If the "filter" is a series inductor then it IS a buck converter. 980 Hz is enough if inductor is large enough - BUT higher will make life easier. A simple linear, super low dropout regulator at the end would make life easier. \$\endgroup\$ – Russell McMahon Sep 26 '14 at 0:01
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In short no (without linear VR type losses or with only passive type loads).

What you describe with an inductor in the output filter is a buck converter. Without an inductor, you'll only be able to chop the power up which is useful for circuits that don't mind the chopped power (think LEDs). Microcontrollers don't like that kind of power though.

If you don't want to use an inductor in a buck converter you'll be burning as much power as a linear VR.

Why avoid a buck converter when it's ideal for what you want. Also, if you decide to try and make a chopper circuit work, ensure you're using feedback back to the arduino to regulate the output voltage. What you've just described though is an inductorless buck converter, so I don't follow why you don't just add the inductor to make it a buck converter?

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  • \$\begingroup\$ My first thought was using a buck converter, but I have had problems locating inductors rated for 30A, as well as a general increase in price for the circuit. So I thought if maybe there was another way to go by. \$\endgroup\$ – Aureus Daelin Sep 25 '14 at 19:15
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    \$\begingroup\$ @AureusDaelin linear regulator with a massive heatsink then if you want to go cheap. Another alternative depending upon application may be to linearly regulate down to 5V for your ucontroller and then have it drive switches that use the 12V high current power supply. Lastly, it may be cheaper and I know it's more common to use multiphase buck converters for high current capability. You could buy a bunch of smaller inductors and each one would add a phase to your SMPS. This decreases filtering need at the output, but increases complexity. \$\endgroup\$ – horta Sep 25 '14 at 19:46
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Nearly all SMPS have a feedback terminal buried inside them that is set to be some fraction of the output. The SMPS controller's goal is to keep that terminal at a fixed voltage, and by doing so, the output voltage settles at the desired value. Change the feedback, and you change the output voltage.

Now to actually find that terminal and modify the circuit, that's a different issue.

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    \$\begingroup\$ It's not likely all that easy. A significantly different voltage may lead to different dynamics, which would not be what the existing supply compensation is tuned for. In one direction my might just get sluggish response, which might be OK, but in the other direction you could get instability. \$\endgroup\$ – Olin Lathrop Sep 25 '14 at 18:39

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