I have a circuit designed to run a electro piezotransformer at resonance frequency of around 80kHz. As a controller was used microcontroller Attiny 25, which by measuring feedback current generates PWM with the resonant frequency to drive the piezotransformer. Operating voltage is 4V and current around 1.5-1.6 A. The filter inductance is overheating due to the saturation and output current drops significantly, till piezo stops operating. Before was used 4.7 uH inductance with saturating current 4A, now I tried 6.8 uH @ 3.6 A from Murata, but the result is the same. What could be done to overcome that problem? Any help is very appreciated. Thank you very much in advance.enter image description here Thanks again for the response. I have another question regarding output voltage and current of the driver. Voltage measured on the input of the piezo has a square wave shape on the positive part and sine wave form on the negative side. But the current shows very strange shape and frequency about 160 kHz instead of 80 kHz. Please, I need your help to understand the current shape reasons and to calculate the output power of the driver. It is very important for me. Thanks in advance.[The blue one is a voltage, and yellow is a current[][2]2

  • \$\begingroup\$ You would do better to post that edit as a new question, then refer back to this one since it provides background information. If the answer thaty Tony Stewart gave you to this question was helpful, then please consider accepting it so that this question will be marked as solved (and Tony gets his 15 points.) \$\endgroup\$
    – JRE
    Oct 12 '16 at 7:59
  • \$\begingroup\$ From your oscillogram, you exceed 4 A and thus saturate the inductor. This is your first problem to solve! \$\endgroup\$
    – winny
    Oct 14 '16 at 10:33

Isat must be avoided by current limit sensing cycle by cycle to avoid thermal runaway problem. As L is rated at -10% for Imax, it continues to drop rapidly for increasing current, which in turn reduces output voltage and thus demands more current and L spirals down.

Your circuit filters out Imax peaks with a LPF so you must regulate to average current at a lower value, such that Ipk does not exceed rated current. Increasing L tends to reduce Imax limit for same size.

enter image description here

Consider output power you need and what current required at 3.3V Choose 10A choke like this http://www.coilcraft.com/xal1010.cfm

  • \$\begingroup\$ I would choose a low L autotransformer to step up voltage instead. \$\endgroup\$ Sep 17 '16 at 20:52
  • \$\begingroup\$ Thanks a lot for very detailed answer and for suggestions. But if I use an inductor with 4.5A saturation current and my peak current less than 2A, why is it getting hot? May be the problem is in the reactive power excess and I have to perform better power factor correction by adding an additional capacitance in order to reduce the phase shift between voltage and current? Thanks... \$\endgroup\$
    – sergoy
    Sep 18 '16 at 13:04
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    \$\begingroup\$ Did you measure Ipk or I avg? and ensure ripple f is < SRF If DCR< 0.1Ohm 4.5A is 2W. So DCR is the gating property. The 4.7uH is only 32 mOhm. \$\endgroup\$ Sep 18 '16 at 15:17
  • \$\begingroup\$ @sergoy Please include your oscillogram of that 2 A peak current. \$\endgroup\$
    – winny
    Oct 12 '16 at 7:40

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