Re: Bulk Order 15kW SSTC Boards

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: a a <hermantoothrot2000-at-yahoo-dot-com> 

Hi,

I understand the whole switching loss thing, and how faster switching is 
better, but I thought it wouldn't be a big deal. I haven't actually run the 
numbers, so I could be wrong, but my guess was that when soft switching at 
a reletively low frequency, the switching loss wouldn't be huge, and that a 
50% increase in switching loss would be worth the benefits of using an IGBT.

Tesla list <tesla-at-pupman-dot-com> wrote:
Original poster: "Jim Lux"

switching speed has a significance over the max rate at which you can toggle
it. While switching, the device is dissipating a LOT more power than when
fully on or off. Slowing the switching speed may radically increase the
total power consumption.

Consider an example: 100 kHz frequency (i.e. switching twice in 10
microseconds).. If you switch in 1 microsecond, you'll be in the "linear"
mode for 10% of the time. For an idealized trapezoidal switch, the
dissipation is a parabola waveform, peaking half way through when the source
and load impedances are equal. (If the load is, say, 1 ohm, and you're
running 1 Volt, the peak dissipation is at 0.5 V output, when the pass
device is dissipating 0.25 W and the load is dissipating 0.25W (half a volt
-at- half an amp)) Making an (incorrect, but i! t's late) approximation that the
average power dissipation will be half that, we'll dissipate 1/8W during the
switch interval, and nothing during the rest... Our 1 W (peak) load will
dissipate 1/8*10% = 1/80W in switching losses

Now, use a device that switches 10 times faster, so the switching duration
is now 1%... Now our device dissipates 1/8*1% in switching losses... if
you're talking kilowatts, it adds up quick..

This was a very simple analysis for an on/off type switching. If you're
switching an Hbridge, then it's a bit trickier (the total power dissipation
has two humps, one as one device turns off, the other as the other device
turns on)


This kind of thing might be insignificant if you were building, say, a 2kHz
PWM inverter to run an electric motor... but, when your switching rate
starts to get close to the transition time, you're getting awfully close to
Class A or Class B type power dissipations...
----- Original ! Message -----
From: "Tesla list"
To:
Sent: Wednesday, September 24, 2003 8:03 PM
Subject: RE: Bulk Order 15kW SSTC Boards


 > Original poster: jimmy hynes
 >
 > Hi,
 > The IGBT is only slightly slower though. What about an IGBT like
 > HGTG30N60A4D? That one's rise +
 > fall time is shorter, but has a little more delay. It seems like the
 > difference in switching speed
 > is minimal.
 >
 >
 > --- Tesla list wrote:
 > > Original poster: "Mccauley, Daniel H"
 > >
 > >
 > > IGBT's are still slow devices. Also, at 100kHz, you would typically
 > > only run this device at about 5-8A current.
 > > Even so, even though I may build a coil to run at 100kHz (which is a
 > > relatively big coil for a SSTC), others might
 > > want to run at 300kHz etc...
 > >
 > > I'm no longer building the 15kW boards due to lack of orders and
minimum
 > > quantities etc..., but am about completed
 > > the redesign of the PlasmaSonic II SSTC which is designed for up to
 > > 10kW.
 > >
 > > Dan
 > >
 > >
 > >
 > > > Hi Dan,
 > > >
 > > > Why are you going to use IXFN44N50 MOSFETs? They cost $17.42
 > > > each, while a
 > > > IRG4PF50W IGBT costs
 > > > $4.43 from the same place. The IGBT is rated for 900 volts
 > > > instead of 500,
 > > > 51 amps instead of 44,
 > > > and also has less gate capacitance.
 > > >
 > > > They do take about 100ns (40%)longer to switch, but if you're
 > > > going to make
 > > > a big coil the
 > > > frequency should be down anyway, right?
 > >
 > >
 >
 >
 > =====
 > Jimmy
 >
 >