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E.g. Is there a converter topology that can take a 12v input and convert it to -12 to +12v output with respect to GND?

For the record I'd really prefer something that doesn't use rare magnetics.

I could do it using a Buck converter and an H-Bridge on the output, but I'd really prefer a minimum cost/parts count solution.

EDIT: Following some great low-power suggestions, I'm actually looking at converting about 100 to 200 watts. It doesn't need isolation or both a positive and negative rail at the same time, just an output that can be adjusted from -12 to +12 volts. Regarding "rare" magnetics, some topologies use tapped inductors that have two windings of different gauges that often need to be customised specifically for each application. I'd prefer a solution that uses normal inductors or something that can be sourced fairly generically.

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  • \$\begingroup\$ would a transformer with a mid tap on the output coil do it for you? that's not exactly low part count I think. Nice question. \$\endgroup\$ – Vladimir Cravero May 28 '15 at 10:47
  • \$\begingroup\$ Do you mean like ICL7660A? \$\endgroup\$ – Roger Rowland May 28 '15 at 10:48
  • \$\begingroup\$ You already have 12V so do you only need -12, or are you looking for isolation and/or regulation? How many watts? \$\endgroup\$ – Spehro Pefhany May 28 '15 at 10:51
  • \$\begingroup\$ What power out requirements do you need? What do you mean by "rare" magnetics? \$\endgroup\$ – Andy aka May 28 '15 at 11:03
  • \$\begingroup\$ Vladimir, you've got me thinking.. I could create both + and - rails with a center tapped transformer, but I think I'd still need two extra FETs on the output to select the polarity making it 3 FETs total. I'm almost sure there's a solution using two FETs to drive a center tapped transformer... \$\endgroup\$ – Michael May 28 '15 at 11:21
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In theory a buck converter can produce any output voltage between the input and ground, while an inverting buck-boost converter can produce any output voltage below ground.


The relay approach

As the two topologies are very similar, you could switch between them using a DPDT relay. While not very elegant, this incurs only a modest increase in part count and board area. Unfortunately, you do need a custom controller for controlling this, but any simple microcontroller with an ADC would likely do just fine.

schematic

simulate this circuit – Schematic created using CircuitLab


Solid state

I later came up with a solid state (relayless) four quadrant variant, which can source and sink current at both positive and negative voltages, while maintaining the same ground reference.

schematic

simulate this circuit

Just like the relay circuit, this topology combines a buck converter with a buck-boost converter.

  • The benefits are instantaneous switching between step-down and inverting mode, the lack of moving parts, and the more compact size.

  • The main drawback I see is that the gate drivers of M3 and M4 must be able to swing their outputs below ground, while the digital control inputs are probably going to be between zero and a logic high (e.g. 3.3 V). This complicates the selection of gate driver chips.

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  • \$\begingroup\$ Thanks @jms, it's a shame it uses a relay but this is still the best answer I've seen. Very nice circuit! \$\endgroup\$ – Michael May 30 '15 at 23:34
  • \$\begingroup\$ @jms Cool Circuit .How would things be if the relay was implemented with solid state parts? \$\endgroup\$ – Autistic May 5 '17 at 20:58
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Yes, something like this exist.

I use the LT1054 switched capacitor voltage converter with good results. The current is limited to 100mA, but you can put multiples in parallel if you need more.

Check the data-sheet for examples. You'll likely be fine with the "basic inverter" configuration. The chip is quite flexible and comes in SO-8 and also in a DIY friendly DIP-8 package.

Also take a look at the ICL7660. It's a pin-compatible replacement. The data-sheet of the ICL7660 has some interesting examples that you won't find in the LT1054 data-sheet (for example voltage inversion and voltage doubling at the same time. I use this to get +9V and -5V out of a single 5V supply).

Only drawbacks are, that the output voltage can be a bit noisy and that the chip is a bit pricey for what it does.

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    \$\begingroup\$ Thanks Nils, this is a great suggestion for the information I gave, but despite my best efforts I neglected to mention I'd like to be able to handle about 100 to 200 watts so I think switched capacitor options are out. My bad. \$\endgroup\$ – Michael May 28 '15 at 11:01
  • \$\begingroup\$ Wow, that would be almost 17 ampere worst case. \$\endgroup\$ – Nils Pipenbrinck May 28 '15 at 12:32
  • \$\begingroup\$ Anyone said the magic words "isolated power supply"? \$\endgroup\$ – Gregory Kornblum May 30 '15 at 20:07

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