# Piezo scheme, heating transistor

1. ## 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.

2. ## 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.

3. ## 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.

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4. ## 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?

5. ## Re: Piezo scheme, heating transistor

The voltage levels don't make sense to me.

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6. ## Re: Piezo scheme, heating transistor

Invalid attachments.

7. ## 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:

feedback signal:

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8. ## 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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9. ## 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.

10. ## 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

11. ## Re: Piezo scheme, heating transistor

Originally Posted by KlausST
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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12. ## 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

13. ## 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?

14. ## 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

15. ## Re: Piezo scheme, heating transistor

Originally Posted by KlausST
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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