Re: The OLTC II lives!

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

Original poster: David Sharpe <sccr4us-at-erols-dot-com> 

Hi Steve

Tesla list wrote:

 > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >
 >  ><<SNIPP>>
 >
 > I already use the "soft-start" SCRs to control the DC link voltage from 0 to
 > 600V, by changing the firing angle, lamp dimmer style. I have burnt out the
 > IGBT in the DC link breaker twice and am getting fed up with it, so I'm
 > going to leave it out. ...<<SNIPP>>

What is the failure mode of the IGBT?  It may be that you are exceeding the
Vce (SUS) max rating of the device.  This problem caused me to blow a
1200V, 50A totem pole module (sorry Terry F.   :^C ) on a DC motor
chopper I built.  If you slap a IGBT off  _too_fast_ , with inductance of
DC power supply (+ resonant choke) will "CHOKE" the IGBT.

Suggest strongly the use of an "Active Clamping" Strategy on the IGBT
especially since you are trying to rapidly interrupt current flow in a
predominantly inductive power path.  Below are some reference URL's:

  http://www.milimsys-dot-com/igbt/IGBT%20Papers/PCIM%20Nu%202003%20IGBT%20Traction%20ABB.pdf

ABB semiconductor discussion of 1.7kV NPT IGBT's and RBSOA testing.
The clamp as shown on page 5 is a suppressor diode (read HV zener) and
Schottky diode (a fast recovery HV diode will also work).  The IGBT
will turn off until VCE exceeds zener breakdown, then zener forward biases
IGBT.  Current linearly drops to zero when IGBT is off.  Yes the IGBT
will get hot eliminating the 0.5Li^2 stored bus energy, but it is only
performing this task _in_an _overcurrent/emergency_condition_ (i.e.
infrequently).  This also neatly _protects_ the IGBT from reverse
avalanche and exceeding VCE ratings.  This technique is used almost
universally for DC choppers, motor drives, and high power UPS
or inverter applications.

  http://www.semikron-dot-com/applica_help/e/3_6_3_2.pdf

Active clamping discussion by Semikron.  More info on sizing
components for clamping zeners/diodes, and detailed discussion
of operation and benefits.

The value, benefits and additional protection afforded by an IGBT
high speed circuit breaker are very worth while, you just need to
harden the power device and control the rate of cut off dynamically
which this circuit will automatically accomplish.  Also suggest you
consider through optoisolation having a means to monitor status
of IGBT (on or off), and a remote means to "close" or "open"
the circuit manually.  I am already including a pair of 200V/5W
zeners and high speed diodes in next spin of my DC chopper.

Best Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA

 > .<SNIP>
 >
 > As for the website, thanks. I learned a heck of a lot about power
 > electronics through building the OLTCs, so I published everything, to try
 > and egg other people on into playing with IGBTs, SCRs, and the like.
 >
 > Steve C.