RE: Recent s.s.t.c. work

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "BNJ" <firebee@xxxxxxxxxxxxxxx>

>From a quick perusal of your schematic, I suggest you may have some
fundamental architectural and device issues if the actual hardware conforms
with the circuit diagram. While the points below don't directly answer your
questions, they are issues that need to be sorted before you go any further
and are probably significant contributors to your observed hardware
behaviour.

First: Q2/Q3 are an emitter follower pair and need their bases to be driven
with an input that swings substantially across the full range of required
IGBT gate swing (neglecting Q2/Q3 base emitter drops). While Q1 will
actively pull the drive to Q2/Q3 high, there is nothing to actively pull it
low shown in your schematic other than R4/R5 and this will be relatively
slow given their values. Hence Q2/Q3 would spend undesirable time in a
quasi-linear state in a high to low transition. Not good !

Second: give some more thought to the drive level required for Q2/Q3. They
will be driving short duration high current pulses into/out of the IGBT
gate, possibly 10's of amps peak, given your driver supply levels and the
value of R6 and the IGBT internal gate resistance. The current gain of the
D44H8 and D45H8 can be as low as 40 with a collector current of only 4A (and
will be even less with higher collector current). The drive to Q2/Q3 needs
to be something that is fast, can swing nearly the full +/-supply range and
can drive short term peak currents of amps into/out of Q2/Q3 bases
respectively. This is one of the challenges in trying to use a bipolar
emitter follower complementary pair and why many of us go for MOSFETs
instead.

Third: The Vce Absolute max of Q2 and Q3 is 60V (from data sheets I looked
up). You are at that rating if you are driving with +/-30V supplies. That's
asking for trouble. The Vce max of Q1 is 30V and Vcb is 50V (from a data
sheet) and this also represents marginal design at best! I didn't look up
the specs on the CYN17-3 but it is worth checking as well. Depending on the
transistor models in your simulator, these device characteristics might not
be well modelled and giving you a less than a true view of actual circuit
behaviour.

I commend your efforts so far, including your attempt at using opto couplers
as an isolation method. I have two variants of brick drivers designs, one
using complementary MOSFETs and gate drive transformers and another using
high speed optocouplers and complementary MOSFETs, although the latter is
substantially more complex than your present design approach.

Regards

BNJ

-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Tuesday, 28 March 2006 7:53 AM
To: tesla@xxxxxxxxxx
Subject: Re: Recent s.s.t.c. work


Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

To anyone giving any thought to this:  I'm just seeing, with further
perusal of the simulation, that a) I still have a phase-shift problem
thru the loop (causing non-z.c. switching); but more to the immediate
point, b) while the drive to the IGBT gate looks clean, there is a
significant (simulation) oscillation occurring in the currents of Q2
and D8!  A burst of several hundred mA peak, occupying most of
every-other half-cycle (of the simulated spark-burst of
cycles).  Frequency, ~17 MHz.  So...that may be what's happening in
the hardware.  I'll be trying to find out why that is, in the simulation
first.

KCH

Tesla list wrote:
>Original poster: "K. C. Herrick" <mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx>
>
>Here's a bit of a puzzle; perhaps someone can offer a clue toward
>solving it.  I post
>
><http://www.hot-streamer.com/temp/tch-drvr.jpg>http://www.hot-streamer.com/
temp/tch-drvr.jpg
>
>
>, the schematic of my IGBT driver.  I briefly repeat how it is to
>work: Input excitation is present all the time, keeping  C1 & C2
>charged via R1, D1 & D2.  Negative-polarity input signal, via D7, Q3
>& R6, keeps the IGBT's gate at ~-27V (with R5 holding it there
>between half-cycles).  Q1 is kept off between spark events by A1 being on.
>
>During each burst of excitation to turn on the IGBT, A1 is turned
>off.  R3 turns Q1 on during positive half-cycles of the input.  D6 &
>Q1 pass the positive-going signal to Q2/Q3 and their output drives
>the IGBT, between ~+ and - 27V (perhaps ~26, with the Q2 & Q3 b-e drops).
>
>I've incorporated D8 & D9 to soak up any voltage overshoot that
>might occur at the gates; one or the other is to conduct at an
>overshoot and clamp it to the voltage of C1 or C2.  I would have
>expected neither D8 nor D9 normally to conduct.
>
>Testing so far with no H.V. applied to the IGBTs, I see good
>IGBT-gate waveshapes: rise/fall of n.g.t. 200 ns, -27 to
>+27V.  Smooth as can be with just the barest ringing top & bottom
>using a 100 MHz scope & probe.  But here's the rub:  With less than
>10% duty cycle, on vs. off, Q2 >>and D8!<< rather quickly become
>hot.  Why Q3 does not become at least equally as warm as Q2, and why
>D8 warms up >>at all<<, I cannot fathom.  Anyone have a thought on it?
>
>Ken Herrick
>
>Tesla list wrote:
>>Original poster: "K. C. Herrick" <mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx>
>>
>>Hello Steve-
>>
>>No, I decided to do away with that, on the basis of an additional
>>simulation.  The simulation showed that I could get a proper phase
>>shift--yielding the correct phase for feedback--by taking feedback
>>from a 1-turn loop around the primary rather than from a current
>>transformer.  So that's what I've implemented.  I still have the
>>phase-shift assy and if this is a total failure I will resurrect
>>that--provided that my energy for this Tesla stuff continues to
>>hold out to a sufficient degree.
>>
>>In the next day or so I'll bit the bullet & crank up the juice bit
>>by bit.  And I do have the camera handy!  In fact, as I've
>>mentioned before, I can now take scope photos with it.  I just hope
>>there'll be some good photos to take.
>>
>>Ken
>>
>>Tesla list wrote:
>>>Original poster: "Steve Ward"
>>><mailto:steve.ward@xxxxxxxxx><steve.ward@xxxxxxxxx>
>>>
>>>Hi Ken,
>>>
>>>Will you be employing your delay register on this next attempt to try
>>>and compensate for delays?  I hope that works as expected.  Be sure to
>>>have a camera handy to take pictures of the sparks!
>>>
>>>Good luck, let us know how it turns out.
>>>
>>>Steve
>>>
>>>On 3/24/06, Tesla list <mailto:tesla@xxxxxxxxxx><tesla@xxxxxxxxxx> wrote:
>>> > Original poster: "K. C. Herrick"
<mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx>
>>> >
>>> > I see that my last posting on this topic was in December.  In the
>>> > interim I've been slothful to a fault (altho happily busy for 3 of
>>> > the weeks entertaining our Most Perfect Granddaughter, 2 1/2,
>>> > visiting from Berlin).  But I have now gotten to the stage where I'm
>>> > ready to put the H.V. to it once again.  I've checked the gate
>>> > waveforms & all 4 appear OK.  Next, it's... turn up the variac &
>>> > watch for the smoke.  It's only trepidation, accumulated from years &
>>> > years of such practice, that keeps me from doing it today; So perhaps
>>> > I'll first just sit & think about it for a while...
>>> >
>>> > Someone asked, after I reported my last failure (death of an IGBT
>>> > brick), what might have caused it, and at the time I didn't
>>> > know.  But while rebuilding the drivers, I discovered that I had
>>> > positioned a wire-wrap pin, in one of the gate circuits, so that,
>>> > when I fastened the board down above the mains capacitors, the pin
>>> > pressed against one of the capacitor terminals--hidden from view, of
>>> > course.  I didn't locate the source of the resultant smoke until I
>>> > started rebuilding.
>>> >
>>> > As I've already reported, I utilize NPN/PNP emitter-follower
>>> > driver-pairs for each of the 4 H-bridge IGBTs, transformer-driven,
>>> > with the 4 transformer signals always applied (from my "pilot
>>> > oscillator")--and now rebuilt with opto-isolators acting to gate-on
>>> > drive to the NPNs during the spark-event times.  That way, all 4 gate
>>> > voltages are kept at -28 or so between sparks by the continuous drive
>>> > from the PNPs.  As before, the continuous transformer signal also
>>> > serves to keep the + and - drive-supply electrolytics charged up.
>>> >
>>> > So stay tuned, so to speak--& don't be startled by smoke seen coming
>>> > from the vicinity of California; it'll only be me once again.
>>> >
>>> > Ken Herrick
>>> >
>>> >
>>> >
>>> >
>>> >
>>>
>>
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