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Date: Thu, 15 Feb 2018 20:16:45 +0100
From: mhx AT iae DOT nl
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] (Off-topic) Question wrt driving HV piezo transducer
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On 2018-02-15 17:23, Richard Rasker (rasker AT linetec DOT nl) [via 
geda-user AT delorie DOT com] wrote:
> Hello,
> 
> My apologies for asking a non-gEDA/PCB related question here, but I've
> been asked to look into a project with a piezo transducer for distance
> measurements. I was supplied with a transducer for research and
> testing (the Airmar ARK41, f0 ~= 40kHz, drive = 1800Vpp max).
> 
> The main thing here is that I'm not all that knowledgeable about
> HV-transformers, and that there's not too much to be found on the
> Internet, either in terms of schematic designs or in terms of
> suitable/available parts. As we're talking about high voltage and a
> rather pricey piece of hardware (some $350, I believe), I'd like to be
> a bit better prepared than just build some stuff and blast away, with
> possible catastrophic (read: expensive) failure.
> 
> I was thinking about obtaining or making a ferrite-core transformer
> with a 1:100 turns ratio, and driving this with a half H-MOSFET bridge
> from an 18V (adjustable) DC power supply. Can someone point out some
> existing designs? Or are there people here who are willing to discuss
> some design ideas? For instance, I'm wondering if a resonant setup
> (i.e. with a secondary HV capacitor in the nF range) is a good idea or
> not, how to estimate a damping resistor in the output and quite a few
> more questions. I do have some experience with old TV flyback style HV
> generators, but this is a little different.

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.

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.

I would start with 450V dc (double rectified line) because the 
transformer
will be a lot easier. 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.

Oh, and buy ear-muffs :-)

-marcel