Mail Archives: geda-user/2018/02/15/17:03:44
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; 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
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