Problem: Stepping up voltage of a high frequency sine wave = 50 KHz to 100 KHz range.

Inside SMPS circuits I find a donut shaped transformer that I think is used to step down the voltage from the high voltage circuit plus isolate it from mains. I have read that SMPS oscillators work at frequencies in 10s to 100s of Kilocycles. From what I read transformers can be used in either direction but not equally well.

So I have a sine wave generated around 50 to 100 KHz and 0 to 5 volts can I simply use the round transformer with the connectors reversed to step up this voltage to whatever ratio that the transformer was originally stepping down?

Even if it is not a perfect solution will it work at all and what part will not work? What can I do to make it work? I do not want to use a amplifier because the amplifiers I can get are all up to 20 KHz and I do not have a 20 voltage or higher and a + - power supply or SMPS that the amplifiers need. Oh and I have many broken SMPS units I can take apart.

Thank you.

  • 3
    \$\begingroup\$ Donut shaped transformer = Toroidal transformer. But I've heard experienced engineers refer to them as "the donut" or "the heavy donut thing" :-) \$\endgroup\$ Commented Nov 2, 2012 at 21:18
  • \$\begingroup\$ I have just been told by a friend "Most SMPS topologies use flyback type of conversion, where these magnetic material is expected to store energy in charge cycle and transfer it to secondary during discharge decay cycle. With SMPS technology Magnetic core saturation is prevented in large by small charge time and current limit technique. With sine wave, overall core area requirement may be more to prevent core saturation and linear energy transfer. Also efficiency with sine wave transfer using SMPS ferrite core tend to be 20 to 30 %." So which is correct please, that I can use it or cannot? \$\endgroup\$ Commented Nov 3, 2012 at 10:30

2 Answers 2


A transformer will work in either direction. In each case, the open circuit voltage out the secondary will the that put into the primary times the number of secondary turns divided by the number of primary turns. The only difference is the two windings flip primary and secondary roles when you use the transformer in opposite directions.

You need to look carefully at the impedance the transformer primary presents to the driving circuit, regardles of which direction the transformer is used in. Without load on the secondary, the primary will look just like a inductor. As the secondary is loaded, the primary will look lower impedance. For a ideal transformer, the impedance on the secondary will be reflected on the primary divided by the square of the turns ratio. For example, if the primary has 100 turns and the secondary 200, the turns ratio is 2:1. The open circuit voltage will be multiplied by the turns ratio (it will be 2x of the primary at the secondary), and the impedance tied to the secondary will appear as 1/4 that on the primary.

As long as your source can drive whatever winding you choose as the primary and it does not overload the transformer, it will basically work. Of course there will be losses, but getting into that gets a lot more complicated.


Yes a transformer is a reversible, passive magnetic component. As you orient the part to scale voltage up also the current available to output is scaled down by same ratio N and impedance ratio is N squared.

Parasitic capacitance will create a natural frequency for resonance, which you can also tune with external caps if you are driving a high impedance load.

This is a common method for generating high voltage, 20~50KHz freq. signals.

What do you intend to drive it with? Piezo devices prefer this method, such as for ultrasound.

  • \$\begingroup\$ I want to drive a langevin piezo transducer 48 KHz 50 watts 8 ohms resonance 5000 PF but I only want to run it at 30 watts. what to check on toroidal transformer and what changes to make? what ratio to prefer? \$\endgroup\$ Commented Nov 3, 2012 at 4:19
  • \$\begingroup\$ @Richman Is this an appropriate approach for my question as well? If so, please use it to make an answer there and I'll upvote it. Thanks. \$\endgroup\$ Commented Nov 3, 2012 at 4:39
  • \$\begingroup\$ It would help if you knew the B coercivity value and maximum current rating. Can you test inductance at various load currents? Piezo theory is a bit intense for this question, but is same as all crystals. I suspect you want to use transformer for impedance matching in parallel or series mode. 1st steps up voltage, latter steps up current. \$\endgroup\$ Commented Nov 3, 2012 at 18:09
  • \$\begingroup\$ i suggest parallel mode for piezo loaded by air and series mode if loaded by fluid. Piezo devices including, quartz, ceramic or any crystal, when resonating in preferred axis will exhibit two resonant modes, namely; low velocity/high pressure/ low Z (series mode) or high velocity/low pressure/high Z (parallel mode). Both equivalent circuits are shown in link below. The electrical responses of each are shown below. [1]: i.sstatic.net/5bf8T.gif [2]: i.sstatic.net/UUyfy.gif \$\endgroup\$ Commented Nov 3, 2012 at 23:39
  • \$\begingroup\$ I can test inductance at various current ranges. I am trying to use low impedance resonance. I can also get the transducer type in the question by @AnindoGhosh. \$\endgroup\$ Commented Nov 4, 2012 at 5:29

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