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Headroom - Was Re: [TCML] IGBT paralleling
"Headroom" is a term that's been around for ages.
Simply, its just another term for margin (with respect to some performance parameter).
Dan
http://www.easternvoltageresearch.com
DRSSTC Kits, Books, and Plasma Speakers
Who came up with this term "headroom"?,,Wikipedia has not.
D C Cox?
Joe in Texas
On Wed, 14 Jul 2010 12:36 -0500, "Steve Ward" <steve.ward@xxxxxxxxx>
wrote:
> 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-i
> > > gb
> > > > >>>>> 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.
> > > > >>>>>>>>>
> > > > >>>>>>>>> _______________________________________________
> > > > >>>>>>>>> Tesla mailing list
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