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    Piezo scheme, heating transistor

    Hello, I am making a piezo feedback scheme (attached). I wonder why the transistor is heating up( it look like it starts cold and then in a while starts heating up, so maybe its something with capacitance)? I give +12V to scheme and amperage is jumping from 0.1 to 0.2A. The feedback signal is 110kHz. So my point is to make the transistor not to heat when this scheme is on. Any ideas why this is happening and how to prevent it, that all heat would go to piezo.Click image for larger version. 

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ID:	134342
    Last edited by Adiz; 9th December 2016 at 16:26.

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    Re: Piezo scheme, heating transistor

    I think your inductor is too small and is saturating. An online calculator shows that your L and C produce 407kHz, not 110kHz.



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    Re: Piezo scheme, heating transistor

    A self-oscillating transistor is not necessarily achieving high efficiency, you need to watch the waveforms.

    There are many possible problems, e.g. the collector voltage might rise to high so that transistor breakdown occurs, the oscillation frequency could also jump to a higher mode.



    •   Alt9th December 2016, 17:29

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    Re: Piezo scheme, heating transistor

    I scoped the feedback pin, so the signal is 113kHz. Attachment 134401
    And here is the signal on piezo Attachment 134402
    Is this enough to see the waveforms? or where I should scope?



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    Re: Piezo scheme, heating transistor

    The voltage levels don't make sense to me.



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    Re: Piezo scheme, heating transistor

    Invalid attachments.



    •   Alt12th December 2016, 16:09

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    Re: Piezo scheme, heating transistor

    Reattaching them (don't know why it doesn't work). Forgot to add that scope divider is 10:1
    piezo signal:
    Click image for larger version. 

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ID:	134430
    feedback signal:
    Click image for larger version. 

Name:	MpimKdP.png 
Views:	3 
Size:	63.5 KB 
ID:	134431



    •   Alt12th December 2016, 20:53

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    Re: Piezo scheme, heating transistor

    The mechanical resonance of the piezo is about 120kHz but the LC resonance is about 400kHz because the inductance is too low as I said before.


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    Re: Piezo scheme, heating transistor

    Hey again. So the transistor part is over, now I am going to add a step-up voltage regulator, so I could power everything from li-ion battery. I am using MIC2288:

    R1:40k, R2:4,7k, so I get ~11V output (for now its good, input voltage 3.7V).
    When I test this chip on a resistor, it looks quite stable output ( I added 104 capacitor to filter output). Now when I connect it with my piezo scheme, strange things happens, that I want to understand. First of all, piezo takes around 0.1A and with step-up whole schematics takes 0.4A, so I guess its quite normal, as I boost voltage around 3 times. The part I don't get is why the output signal is very unstable. I am adding gif of my osciloscope readings (can't make this gif work in forum post):
    http://i.imgur.com/3SYkGHi.gifv
    Also the step up heats up.



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    Re: Piezo scheme, heating transistor

    Hi,

    In case you build one or both circuits on a breadboard:
    I don't think it gives meaningful results, because a breadboard is not suitable for high frequency switching power applications.

    Klaus



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    Re: Piezo scheme, heating transistor

    Quote Originally Posted by KlausST View Post
    Hi,

    In case you build one or both circuits on a breadboard:
    I don't think it gives meaningful results, because a breadboard is not suitable for high frequency switching power applications.

    Klaus
    Hey, the step-up and piezo scheme are on 2 PCB and they both are connected on breadboard.



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    Re: Piezo scheme, heating transistor

    Hi,

    Hey, the step-up and piezo scheme are on 2 PCB and they both are connected on breadboard.
    --> should work.

    Klaus



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    Re: Piezo scheme, heating transistor

    Ok did some more testing. So everything was because of low supply voltage. Now I used 6V as a supply and signal got very clean and also whole scheme takes only 0.1A. What can be the theory why, with 3,7V supply voltage, this takes so much ampers and step-up gets hot? am I trying to boost too much? in that case, what parameter in datasheet shows how much can I boost?



    •   Alt28th December 2016, 13:01

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    Re: Piezo scheme, heating transistor

    Hi,

    I just see your picture of the PCBs.
    Are you sure you followed the design rules, especially "selecting capacitors", "selecting the diode" and "selecting the inductor".
    Just from the picture ... I don't think they are selected properly.

    Klaus

    - - - Updated - - -

    Hi,

    am I trying to boost too much? in that case, what parameter in datasheet shows how much can I boost?
    Are you referring to the MIC2288?

    The datasheet states this several times:
    * first page "general description" --> the MIC2288’s internal 34V/1A switch,
    * first page, "features": --> Output voltage adjustable to 34V
    * page 2, :overvoltage protection
    * page3, "absolute max ratings": switch voltage
    ...

    Klaus



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    Re: Piezo scheme, heating transistor

    Quote Originally Posted by KlausST View Post
    Hi,

    I just see your picture of the PCBs.
    Are you sure you followed the design rules, especially "selecting capacitors", "selecting the diode" and "selecting the inductor".
    Just from the picture ... I don't think they are selected properly.


    Klaus
    Yes components were selected by the design rules.



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