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I have an application where the load is a user-selectable component (an electric door strike) that comes in many flavors, with each flavor requiring a specific DC voltage, most commonly either 12V or 24V. I would prefer not to have separate parts for the different voltages, and so I would like to have a jumper-selectable 12V or 24V DC/DC converter on my board.

I have played around with TI's WBENCH Designer and it seems like getting an efficient DC/DC converter for a single voltage is a piece of cake, but how do I go about adapting one to produce different outputs depending on the jumper configuration?

My first thought was simply to use WBENCH Designer to produce two different designs based on the same TI chip and then "meld" them into a single circuit with some creative hackery. I'm reasonably confident this could be made to work, but I'm also pretty sure there would be some redundant components from going this route (particularly the large and relatively expensive inductors) and naturally I'd prefer to avoid that.

Oh and obviously I'm ignoring the possibility of using a voltage divider or simple dissipating regulator because I'd prefer not to waste power, but should I be? Is there a way to use one that isn't wasteful?

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3 Answers 3

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There's a number of ways you can go about it:

  1. Use an "adjustable" buck regulator and switch the adjustment resistors for different values through jumpers.

  2. Use multiple buck regulators to go from your input voltage to all the output voltages in one go (all running in parallel - only one of them active at a time)

  3. Use multiple buck regulators in a "tree" formation - one which for example goes from input voltage to say 24v, then one which takes the 24v to (for example) 18v, then another which takes the 18v to say 12v - or whatever voltage combinations you want.

Option 1 is probably the cheapest as you only need 1 regulator - picking your feedback resistors can be tricky though if you want precise voltages.

Option 2 is probably the most efficient if you want multiple voltages to be available at the same time.

Option 3 is most commonly used for a combination of buck and linear regulators - buck it down to a lower voltage then linear it down lower still to give a good clean output.

Oh, and by the very nature of the linear regulators there is no way to use them that isn't wasteful. They do have their place though as mentioned in option 3 (no switching ripple).

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  • \$\begingroup\$ I was already planning the "tree" option to also have a 5V supply. An installed unit will always need either 24V or 12V, never both, so using multiple regulators in steps to have both 24V and 12V available seems wasteful. The "adjustable" buck regulator is basically what I was describing as "melding" the two circuits designed in WBENCH Designer. If I do that, can I simply use the larger of the two inductors specified in the designs? \$\endgroup\$ Commented Dec 28, 2011 at 13:08
  • \$\begingroup\$ I have never used TI's switching products, so I cannot comment on that. I have only used the Maxim ones, and they just change the feedback resistance to give a different output voltage - the inductance and capacitance stay the same. \$\endgroup\$
    – Majenko
    Commented Dec 28, 2011 at 13:13
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Across a limited voltagerange (more than the 2:1 you'd need for 24V & 12V output) almost all that is required with many IC's is to change the output divider that feeds the "ref" or similar in.

eg if Ref requires a 1.25V input when the IC is "in regulation" then

  • for 12V you need a 12/1.25 divider = 8.6:1 so the resistors need to be in the ratio 7.6:1.

  • for 24V you need a 24/1.25 divider = 19.4:1 so the resistors need to be in the ratio 18.4:1.

The circuit below is for a venerable MC34063 but the principle is much the same for many ICs. Here the divider I mentioned above is formed by R2 and R1.

  • Vout = 1.25V x (R2/R1 + 1)

While a circuit may be less efficient when used across a range of output voltages rather than a single fixed one, it can be designed to be "acceptably efficient in most cases.

enter image description here

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The most flexible solution is to use a suitable MCU that senses the supply voltage or a jumper setting, and generates the correct output automatically, using a similar output stage to those used by switching PS devices. Microchip is one company that makes devices intended for such intelligent power supplies. You still need an inductor and MOSFET, or a couple of them, and some software has to be written, but it might be cost-effective.

Microchip has several relevant application notes, and free design assistance is available.

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  • \$\begingroup\$ Very interesting concept… I'm investigating the Microchip site to try and learn more. If the BOM cost can actually be lower with this approach I am happy to write a bit of software. \$\endgroup\$ Commented Dec 28, 2011 at 13:26
  • \$\begingroup\$ No matter what you do with the control circuitry, the issue of optimizing, duplicating, etc the power handling components would remain. \$\endgroup\$ Commented Dec 28, 2011 at 19:44

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