For a project of mine, I'm trying to make a dual-sided SMPS that has independently variable positive and negative supply rails, current limiting, short circuit protection, etc etc...

My current plan of attack would to use a center-tapped transformer to convert wall power to about +/- 40VAC. I can easily rectify both and get about +/-28VDC. For the positive side supply I can easily use a voltage-controlled buck regulator.

My issue is that i don't know how to step down negative voltage for the minus side! i feel like a topology similar to a buck should work, but I'm not sure how to keep the transistors operating in the right modes.

I am considering using a buck-boost to get the negative output, but the controls for it are a lot higher :(

My question is basically, how could i create the variable, regulated negative voltage supply?


  • 1
    \$\begingroup\$ What current output are you looking for? \$\endgroup\$
    – Andy aka
    Commented Nov 25, 2014 at 8:29
  • \$\begingroup\$ I'd like 5A out. \$\endgroup\$ Commented Nov 25, 2014 at 16:18

2 Answers 2


In principle, you can make inverted buck converter; however, this will be difficult in real life since suitable parts are rare. You may want to try a different approach; generate negative output from the same positive input using inverting or Cuk converter topology. Read Jim Willams' "Switching Regulators for Poets" -> http://cds.linear.com/docs/en/application-note/an25fa.pdf if you desire more elegant solution.


The easiest solution is to separate the secondary winding into two parts. That is: end up with two separate windings.

How you do this depends on the transformer. Most E-I transformers are easy to work with - just follow the center-tap up under the outer-wrap of the transformer. Cut the outer wrap to expose where both wires from the windings are connected to the stranded wire leaving the transformer. Just un-solder / untwist those wires and connect them to two new wires.

Toroid transformers are more difficult to work with - but not impossible. I usually just unwrap the clear over-wrap until the winding connections are exposed. Same deal: just unsolder / untwist the center-tap connection and connect to separate lead wires.

This allows you to build two identical power supplies. This has the added benefit that you can use the supplies separately if you want or you can connect them in series and have a bipolar power supply.


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