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From:	"Gene Heskett (gheskett AT shentel DOT net) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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To:	geda-user AT delorie DOT com
Subject:	Re: [geda-user] (Off-topic) Question wrt driving HV piezo transducer
Date:	Thu, 15 Feb 2018 19:39:27 -0500
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On Thursday 15 February 2018 17:02:28 Richard Rasker (rasker AT linetec DOT nl) 
[via geda-user AT delorie DOT com] wrote:

> Hi Marcel,
>
> > The piezo transducer will electrically look like a big capacitor and
> > with a 1:100 transformer you'll introduce a huge capacitive load on
> > the H-bridge
> > MOSFETs. At 40 kHz you're sure to destroy them with hard switching,
> > maybe already within the first few HF cycles.
>
> Yup, that's one of the problems alright. Transducer capacitance is
> somewhere in the 5 nF range, so that would transform to a primary load
> of 500 nF.
>
> However, this afternoon, the product dealer was kind enough to send me
> the schematic of a 500 Vpp development driver kit, featuring a
> center-tapped symmetrical push-pull transformer(*) circuit with two
> FDD16AN08 MOSFETs, each with a 1 uF capacitor across drain-source.
> These big DS-caps seem rather weird already.
> And on the secondary side, a 5.6 nF/1.5kV capacitor is effectively
> connected in parallel to the transducer, doubling the capacitive load
> to 10 nF. Also, there's  a 680 ohms damping(?) resistor on the
> secondary winding as well -- I wonder how one calculates the power
> requirement for that one; 

Carefully, and based on the peak voltage rating of that resistor, not so 
much its power rating. You must stay below that, else the resistor will 
be doing micro-arcing internally. Which will cause the value to drift 
low, increasing the power dissipation until the circuit fails.

For std carbon composition resistors you would be surprised at the max 
voltage, its quite low. Carbon film is only slightly better.

> the load is 500W during a burst, but that 
> lasts only a quarter of a millisecond. The resistor doesn't appear
> overly large, physically. The whole thing is powered with 12 volts,
> through a small SMD LM317 device with a 4700uF/16V low ESR buffer cap.
> Nothing unwieldy there.
>
> *: The transformer is 4+4:140 turns, so effectively that's a 1:35
> turns ratio.
>
> (I can't post the original schematic here for reasons of copyright,
> but I could create a gschem version for posting here -- that would be
> lightly less conspicuous... apart from what's described above, there's
> the driver circuitry, based on a humble 556 dual timer, driving the
> MOSFET gates in a way I haven't fully figured out yet -- it almost
> seems as if only one MOSFET gets the actual burst signal, whereas the
> other one merely gets the LF pulse rate, but not the burst frequency).
>
> > Your idea of adding a small inductor in the primary of the
> > transformer (or design the XFNR to have defined leakage) can
> > counteract that. The switching
> > frequency should be higher than the resonance frequency of leakage +
> > transformer and the piezo capacitance. Added benefit is that you
> > drive the
> > transducer with sinusoidal current. It will not like the harmonics
> > of a square wave, and it will probably foul up the distance
> > measurement.
>
> Yes, I also read something like this from the sparse information in
> the Internet. I definitely think that an extra series inductor is a
> good idea, probably something along the lines of a few uH.
>
> > I would start with 450V dc (double rectified line) because the
> > transformer
> > will be a lot easier.
>
> The whole thing is supposed to be powered by a 24VDC truck battery, so
> no line voltage available. Amps galore though :-)
>
> > You can look at datasheets for CFL / TL lamp drivers,
> > they are all using HV H-bridges and probably the notes will contain
> > some magnetics ideas, too.
>
> Ah, yes, that's something I hadn't thought of! Excellent idea!
>
> > Oh, and buy ear-muffs :-)
>
> I will -- although I think that for the time being, I'll use a $1
> 4700pF/6kV ceramic capacitor instead of the $350 real deal, until I
> got the whole thing ore or less figured out, calculated and under
> control. But I'll certainly keep your advice in mind when the time
> comes for live tone burst tests in my rather smallish workshop :-)
>
> Thanks for your helpful suggestions and other thoughts, this goes
> quite a long way pointing me in the right direction!
>
>
> Thanks again, best regards,
>
> Richard



-- 
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

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