A riddle: HIGH output voltage ~30Hz-30kHz @ 120Vpp ac, LOW Iout<2mA @ Rload>250kOhm

[Margarine]

Hiya guys, my first post, here it goes.. >.<" .. ,

A maybe silly and slightly off-topic question for a audio-phile forum, but you guys may be my biggest chance for the right answere.. ..! Currently I'm helping out a student researcher who studdies neuro-electric processes, think of a TENSE unit or ab-toners and such.. In her case she needs a device capable of producing:

[1] Multiple transformer-seperated (bi-phase, NO common ground!) audio-range modulated channels, roughly 30Hz-30kHz.. Nerve-cells like to fire at 10-15kHz but we'd like a bit of headroom! She wants to experiment with different kind of waves, generated by software and a wide-range DAC.

[2] High output voltage somewhere round 120Vpp at verry low current coz anything above 2-5mA AC running trough the heart area may be lethal!

[3] The output signal MUST be absolutely cleaned from HF DC/DC-converter or class-D switching noise!

[4] The device should run from dc. 12V.. 12x1.2V= NiMh cells for safety and comfort, therefore I DON'T wanna pump like 25W into a sec. coil of a 9:230V ac. power transformer per channel as suggested by some; We only need those 2mA's ac. of output! So, .. Looking at the power consumption, it leaves me with a dillemma:

a. Should I use 2 (bi-phase) conventional high-power OpAmp's to drive the primary coil of the transformer, or..

b. Should I use a class-D/T type of driver such as TA2020 or VMA2015, does anyone knows if it's even possible to drive a transformer directly by one of these.. If so, I was thinking bout building the transformer myself using a ETD44 Ferrite Core / 2x6:130 turns, something small, PCB mounted but effective !

But here you skilled guys come in, the transformer-thing is where my skills start lackin', I'm usualy doing micro-controllers and PLC's.. @.@" .., so I'm able to read schematics and produce prof. quality multi-layer PCB's even, so, just haven't build a transformer yet.. *blush* ..

Anyone ANY suggestion? Greetzz.. Cindy!

PS: Awwwsome forum! Digged for a couple a days trough all the posts.. I've learned a LOT, thanks!!!

[jcx]

your requirements for base band xfmr does map pretty well onto audio - but audio xfmrs have the highest relative bandwidth of typical xmfr applications and therefore push the technology at both ends of the frequency range

when you're near the technology’s limits you probably need an expert - read everything on audio transformer manufacturer's sites, then ask them questions - they are used to custom designs

to reach the lower audio frequency limits cores are almost always high perm NiFe materials, coils still often need thousands of turns - not easy to do by hand when the wire size gets so small - and to push the high frequency response interleaved, sectioned winding techniques are used

a few audio specialty xmfr manufacturer's: jensen cinemag sowter lundahl


on the "cheap hack" end of the spectrum line power xmfrs have been used as tube amp outputs with claims of OK bandwidth when using toroidial types
for your application you should look at lowest standard VA - they are made down to 1-2 VA rating, and you want to use the lowest turns ratio for better bandwidth

because of Faraday's Law you want a 2x coil V rating xfmr if you're using it at 1/2 the frequency, it lowers core loss and excitation current a lot to take another ~2x in V derating - and fits available power xmfrs

in my old Digikey catalog I see a 1.6 VA toroid with 44 V sec, dual pri, wired 115 V (for bandwidth?, also try 230 for lower core loss) it should better match your supply and output requirement with bridged drive - and shouldn't require "25W" drive although buying, testing is the only way to determine the high frequency limit


efficiency may be hard to come by - I wouldn't expect an experimental rig to come close to optimums in all "corners" of design space
be willing to trade away any non-essential "optimum" when it moves the project forward faster

[Margarine]

@jcx: Thanks for your reply ,

For days now I'm diggin trough other ppl.'s research such as a Dutch Philips guy Ronald Dekker's page , who did a great tutorial on experimenting with various types of transformers driven by a switching-mode converter: ''An electronic 90V plate/anode battery'' and ''Flyback converters for dummies'', pretty simmilar to class-D.. , you (tube) guys, may find this intresting too, lotta background info!

Today I just fired up my iron, builded myself an induction test bench for osciloscope, collected anything transformer/coils I could find, some empty cores for some serious testing!

It seems you are right, I indeed need to limit my bandwidth somewhat, for now I'll be focussing on a straight line from ~1.. 20kHz, just had a convo with our project team, it shouldn't affect the results too much, also.. The wave-forms are generated by a microcontroler so.. When using a Motorla DSP I may be able to correct the problem-areas after whisle-trough selftest..

Looks like my project finaly took off.. Whoohoohh.. ^x^ ..!