The DC current rating for an IGBT is basically just the power
dissipated
(Vce sat @ Ic) times the thermal resistance from die to case,
keeping the
peak temperature < ~150C. At least several of the IGBTs i looked at
appeared to arrive at the current spec in this way.
I typically ignore the current rating and instead look at conduction
losses,
switching losses and thermal impedance, because honestly thats the
bottom
line. The amp rating is just some number that is almost never
applicable
to
the application.
As far as voltage headroom goes, operating at 50% of the part
rating is
quite good for most cases. Ive pushed some of my bridges all the
way up
to
their voltage rating (zero headroom), and that might work under
controlled
conditions. Ive also tested some older IGBTs for their ruggedness to
transients caused by hard switching of the IGBT or diode. It
seemed like
it
took very big and high energy transients before my CM300DY failed,
and
that
normal "switching noise" kind of stuff would not really bother it
much.
This suggests that running a higher bus voltage may be worth it.
Reliability data to settle disputes like this seems scarce.
I think a lot of coilers tend to neglect the thermal aspects of whats
going
on in the IGBT (among other things). Admittedly, its not a simple
task,
it
requires knowledge of some sort of SPICE program, and to extract the
thermal
time constants from the data-sheets plots of Zth vs time. Ive done
it
before to check the temperature peaks that i expect my IGBTs to
see, and
also to figure out the average die temperature after a minute or so
of
operation. I suspect thermal stress is a leading factor in the
failure
of
most DRSSTCs, particularly when someone tries to push 500A through a
TO-247
device... the thermal mass is much smaller than a brick, and you
dont see
people pushing bricks proportionally as hard.
To give the real numbers on the coil DC is talking about, it was
CM300s
operating at ~1800Apk playing 4-note polyphony. With no power factor
correction on the AC line, we were maxing the 100A meter installed
on the
variac. The RMS primary current is estimated to be (via pspice
simulation)
in the 200A range.
Steve
On Wed, Jul 14, 2010 at 9:10 AM, Michael Twieg <mdt24@xxxxxxxx>
wrote:
Literally no amount of circuitry can eliminate ringing and spikes
due to a
device's internal inductance. The actual collector of the device
is not
accessible to protection circuitry (unless you plan on tearing off
the case
and soldering directly to the die).
And "pushing" current is very different from pushing voltage. The
current
ratings for IGBTs are given under the assumption that they are
being hard
switched, which causes far more power dissipation than if they
were soft
switched. That's why the current ratings can be pushed so far for
tesla
coil bridges. It's not that the manufacturers are being
conservative with
their ratings (they have no incentive to do so), it's that we're not
operating them in a typical fashion.
Unlike the current ratings, the voltage ratings can't be pushed.
It's a
pretty hard limit which is independant of what load you're
driving. If you
try to operate your bus voltage above those ratings, they will
fail very
quickly. And any sane engineer will give at least 25% headroom on
the bus
voltage (much more if they don't have very good snubbers).
-Mike
On Tue, Jul 13, 2010 at 10:23 PM, DC Cox <resonance@xxxxxxxxxxxx>
wrote:
Proper circuit design eliminates the spikes and ringing problems.
Most IGBT circuits are "pushed" a bit. Steve Ward, at my open
house, was
pushing his CM600s with up to 80 Amps RMS at 220 V input.
Dr. Resonance
On Mon, Jul 12, 2010 at 4:48 PM, John Forcina <forcijo10@xxxxxxxxx>
wrote:
Apparently so. I would like to see how DC will be able to get
away
with
this with these mysterious IGBT's because if he claims they are
rated
at
2400V and 1650*1.414=2333.1v that gives almost zero headroom for
voltage
spikes ringing etc...
It is pretty much impossible to make a bus layout that can
accommodate
these
igbt's. Even with a very low inductance laminated layout the
sheer
slow
switching speed will create large switching spikes and will lead
to a
certain death to the igbt's. Not to mention, any decent power
engineer
will
know to de-rate the igbt's by several hundred volts to
accommodate for
this
and also because it's just good practice.
On Mon, Jul 12, 2010 at 1:12 PM, Drake Schutt <drake89@xxxxxxxxx>
wrote:
So DC you're saying that you raise AC voltage to 1.7kV before
rectification?
Sent from my iPhone
On Jul 12, 2010, at 9:08 AM, "Brian" <brianv@xxxxxxxx> wrote:
Hmmm Im a little confused, 2400 vdc IGBT is the handling
voltage of
the
device , once an IGBT is on...it is on, I am not sure what this
wasted
head
room is all about. Once the miller capacitance is overcome the
IGBT
is
considered on and now connects the rail voltages. Whether it
is big
voltage
or little voltage it don't matter. If you wish to drive them
hard at
full
rated with 2400Vdc then drive them hard if you wish not too then
don't.
I
am
not sure where the idea came in that there is a bunch of
wasted head
room
that has to be filled...maybe I am missing something in this
dialogue
somewhere...
-----Original Message-----
From: DC Cox [mailto:resonance@xxxxxxxxxxxx]
Sent: Sunday, July 11, 2010 6:27 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] IGBT paralleling
With a 2,400 VDC IGBT, running it at a line doubled 220 VAC
gives
2400
VDC
-
616 VDC --- over 1,784 Volts of wasted headroom that needs to be
filled,
hence the use of a power transformer to boost the AC input
from 220
to
around 1700 VAC. The headroom I'm referring to is similar to
your
audio
reference only in this case wasting AC power headroom on a large
IGBT
that
should be driven at higher potential to maximize coil output.
This is, of course, all not relevant with standard medium size
coils
using
rectified line drive (such as CM300 circuits) or line rectified
drive
with
a
voltage doubler circuit common with CM600 IGBTs.
D.C. Cox
On Sun, Jul 11, 2010 at 4:13 PM, Drake Schutt
<drake89@xxxxxxxxx>
wrote:
Dc- what do you mean when you refer to headroom in this post?
I'm
used to the term only in music production referring to dB.
Regards
Drake
On Jul 11, 2010, at 3:22 PM, DC Cox <resonance@xxxxxxxxxxxx>
wrote:With
CM300s or CM600s you can just double the 220 VAC line to get
642
VDC
for a good match.
If running, perhaps, a CM2400, you want to go up to near
2,400 VDC
on
the drive, so you end up using a 220/480 Volt 3 phase to get
up to
at
or near the 2,400 VDC rectified. This gives you better output
because you go from
642 VDC to 2400 VDC that is being switched into the primary
inductor.
Typically, with really large systems the only way to get from
220/440
VAC to produce the 2400 VDC drive max is to use a small 25
to 50
kVA
xmfr (surplus pole units).
Not a dual pole pig unit, just a 220/440 VAC 3 phase xmfr
delivering
around 1650 VAC before rectification.
The main point with large DRSSTC type coils is efficiency ---
eliminating all those losses in the heat & UV light
production in
the
spark gap, and obtaining quicker dI/dt rates.
As you pointed out small and medium size coils run just fine
without
using any pole xmfrs. Usually above 15-18 ft long sparks
the pole
xmfr boost helps out get to the higher potential of the larger
IGBTs
without wasting a lot of headroom.
Dr. Resonance
On Sun, Jul 11, 2010 at 2:14 PM, Gary Lau <glau1024@xxxxxxxxx>
wrote:
I'm going to bare my ignorance here. I thought that the whole
point
of
solid state TC's was that you don't need a multi-kilovolt
power
supply.
Are
pole pigs really used to power these? A _dual_ pig powered
magnifier???
Regards, Gary Lau
MA, USA
On Sun, Jul 11, 2010 at 12:16 PM, Scott Bogard <
sdbogard@xxxxxxxxx
wrote:
Hi John,
By chance is there a compiled list somewhere of "good
IGBTs"
that are used and those that aren't. Anyway back to the
original
question, is
it
possible to parallel them? Lets say for kicks I'm building a
dual
pig powered 30kVa magnifier with a LTR cap (I'm clearly
not, we
are
talking theoretical here.) Clearly the peak currents will
be
beyond any
reasonably
priced IGBT, is it possible to parallel lesser current
units to
handle
the
load, and what would that entail? Thanks.
Scott Bogard.
On 7/10/2010 5:09 PM, John Forcina wrote:
Those IGBT's seem far from ideal. The TO220 package is a
very
poor
choice
as far as thermal conductivity and the datasheet says it all
0.75C/W.
You
will not be able to remove enough heat from the surface of
the
IGBT
die
quick enough between current pulses and the device will
fail.
They do
not
have a internal anti-parallel diode so adding that externally
will
add
to
the final cost also. Not to mention doing that will add
additional
loop
area and stray inductance between units. One more thing
is the
switching
times are surprisingly slow for that small of a unit. td
(OFF)
96ns.
It's
not that slow however it does seem slow for that small of a
device.
I
have
seen much better overall performance from larger IGBT's.
My
suggestion
is
to just spend more money and use IGBT's that have been used
and
proven
to
work in Tesla Coils time and time again. There must be some
reason
that
we
all use them ;)
On Sat, Jul 10, 2010 at 4:38 PM, Scott Bogard<
sdbogard@xxxxxxxxx
wrote:
Interesting,
I am in the wee beginning stages of building my first
SISG,
and
as
such am in the market for IGBTs. I found these, which look
very
inexpensive
and have decent ratings.
http://www.newark.com/fairchild-semiconductor/hgtp12n60a4/single-
igb
t-600v-54a/dp/90B5642
My thinking is if heat is a problem or peak current, can I
just
parallel
them? at $1.50 a pop it seems infinitely better than 1 $18
IGBT
of
nearly
the same ratings... I didn't look at temperature or
package
information
yet, so maybe there is a problem there.
Scott Bogard.
On 7/10/2010 8:26 AM, McCauley, Daniel H wrote:
Scott,
The electric ratings may be the same or similar, but
you also
have to compare the mechanical ratings - in particular
the
thermal
ratings.
This
would be junction-to-case thermal impedances etc... The
expensive
IGBTs
that are commonly used in DRSSTCs are usually ISOBLOC type
packages
which
excellent thermal impedances. Compare this vs. a TO-247
package
of
the
same
die.
And when comparing a TO-247 package to an ISOBLOC, keep
in
mind
that
you
STILL NEED to add a thermal insulator between the TO-247 and
heatsink,
which
just makes the thermal impedance even worse. The
ISOBLOC (or
SOT-227)
doesn't require a thermal interface other a small
smidgeon of
thermal grease or a graphite pad.
Dan
http://www.easternvoltageresearch.com
DRSSTC, SSTC, Flyback, Plasma Speaker Kits
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:
tesla-bounces@xxxxxxxxxx]
On Behalf Of Scott Bogard
Sent: Friday, July 09, 2010 9:52 PM
To: Tesla Coil Mailing List
Subject: EXTERNAL: [TCML] IGBT paralleling
Greetings all,
So, after a bit of researching I've noticed there are
IGBTs
on Newark with exactly the same ratings as some of the
SSTC
approved
IGBTs,
but
at 1/10th the price. What makes these others so special
that
they are better, and if it is just a matter of peak
current
ratings, since
IGBTs
are
gate driven, can we just parallel a few to get the required
pulse current rating? I ask because I've not heard of
anybody
doing this, then
again
I've
only begun to research SSTC a little bit ago. Just musing.
Scott Bogard.
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