Forums

IGBT in Linear Mode

Started by Tim Wescott May 31, 2011
Any good articles in using IGBTs in linear mode?

With all due respect for the uselessness of SPICE when you can't do the 
work on paper -- does anyone know how good a SPICE model designed for an 
IGBT in switched service might work in linear mode?  Who tends to have 
better models for linear service?  Whether anyone has models 
specifically for IGBTs in linear mode?

TIA -- IGBTs seem to be enough of a corner case in the semiconductor 
world that it's hard to find definitive data about their operation 
beyond a cross-section of a generic chip, and some hand-waving arguments 
about how it works just like a MOSFET/BJT Darlington-ish pair.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
On Tue, 31 May 2011 08:36:46 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:

>Any good articles in using IGBTs in linear mode? > >With all due respect for the uselessness of SPICE when you can't do the >work on paper -- does anyone know how good a SPICE model designed for an >IGBT in switched service might work in linear mode? Who tends to have >better models for linear service? Whether anyone has models >specifically for IGBTs in linear mode? > >TIA -- IGBTs seem to be enough of a corner case in the semiconductor >world that it's hard to find definitive data about their operation >beyond a cross-section of a generic chip, and some hand-waving arguments >about how it works just like a MOSFET/BJT Darlington-ish pair.
www.irf.com/technical-info/appnotes/an-983.pdf Since an IGBT is a 4-layer device one would think that "linear mode" might be like tip-toeing to the edge of a cliff without a safety rope ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On May 31, 9:44=A0am, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:
> On Tue, 31 May 2011 08:36:46 -0700, Tim Wescott <t...@seemywebsite.com> > wrote: > > >Any good articles in using IGBTs in linear mode? > > >With all due respect for the uselessness of SPICE when you can't do the > >work on paper -- does anyone know how good a SPICE model designed for an > >IGBT in switched service might work in linear mode? =A0Who tends to have > >better models for linear service? =A0Whether anyone has models > >specifically for IGBTs in linear mode? > > >TIA -- IGBTs seem to be enough of a corner case in the semiconductor > >world that it's hard to find definitive data about their operation > >beyond a cross-section of a generic chip, and some hand-waving arguments > >about how it works just like a MOSFET/BJT Darlington-ish pair. > > www.irf.com/technical-info/appnotes/an-983.pdf > > Since an IGBT is a 4-layer device one would think that "linear mode" > might be like tip-toeing to the edge of a cliff without a safety rope > ;-)
Section 6: "1. Operation in short circuit. The current in the IGBT is limited by its gate voltage and transconductance and can reach values well in excess of 10 times its continuous rating. The level of hole current that flows underneath the N+ source contact can cause a drop across r=92b, large enough to turn on the NPN parasitic bipolar with possible latching. This is normally prevented by a reduction in r=92b, as mentioned in the previous section or by a reduction of the total device transconductance. Since this second technique increases conduction losses and reduces switching speed, two families of IGBTs have been made available by IR, one optimized for low conduction losses, the other for short circuit operation, as indicated in Section 9. "2. Inductive turn-off, sometimes referred to as "clamped IL." In an inductive turn-off the voltage swings from a few volts to the supply voltage with constant current and with no channel current. These conditions are different from those described in the previous section in so far as the load current is totally made up of holes flowing through r=92b. For this reason some manufacturers suggest the use of gate drive resistors to slow down the turn-off dv/dt and maintain some level of electron current, thereby avoiding a potential "dynamic latching" condition. IGBTs from International Rectifier can be operated at their maximum switching speed without any problem. Reasons to limit the switching speed should be external to the device (e.g., overshoots due to stray inductance), rather than internal. "3. Operation as a linear amplifier. Linear operation exercises the SOA of the IGBT in a combination of the two modes described above. No detailed characterization of IGBTs as linear amplifiers has been carried out by IR, given the limited use of IGBTs in this type of application." IOW, they didn't think it'd work so they didn't try it. OTOH: http://www.st.com/stonline/products/literature/an/3703.pdf Section 3.2, "Reducing dV/dt at turn-off" et seq has some tricks to give at least some of that cliff slightly less than infinite slope. Hey Tim, what's the required BW and signal fidelity? Why fight IGBTs' inability to do "real" linear operation; could PPM with averaging in the load work in your app? Mark L. Fergerson
On Tue, 31 May 2011 08:36:46 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:

>Any good articles in using IGBTs in linear mode? > >With all due respect for the uselessness of SPICE when you can't do the >work on paper -- does anyone know how good a SPICE model designed for an >IGBT in switched service might work in linear mode? Who tends to have >better models for linear service? Whether anyone has models >specifically for IGBTs in linear mode? > >TIA -- IGBTs seem to be enough of a corner case in the semiconductor >world that it's hard to find definitive data about their operation >beyond a cross-section of a generic chip, and some hand-waving arguments >about how it works just like a MOSFET/BJT Darlington-ish pair.
Don't know about IGBTs, but lots of high-power-rated mosfets will blow up at relatively low power dissipations in linear mode. We had to test a lot of them. ftp://66.117.156.8/ExFets.jpg John
On 05/31/2011 10:30 AM, nuny@bid.nes wrote:
> On May 31, 9:44 am, Jim Thompson<To-Email-Use-The-Envelope-I...@On-My- > Web-Site.com> wrote: >> On Tue, 31 May 2011 08:36:46 -0700, Tim Wescott<t...@seemywebsite.com> >> wrote: >> >>> Any good articles in using IGBTs in linear mode? >> >>> With all due respect for the uselessness of SPICE when you can't do the >>> work on paper -- does anyone know how good a SPICE model designed for an >>> IGBT in switched service might work in linear mode? Who tends to have >>> better models for linear service? Whether anyone has models >>> specifically for IGBTs in linear mode? >> >>> TIA -- IGBTs seem to be enough of a corner case in the semiconductor >>> world that it's hard to find definitive data about their operation >>> beyond a cross-section of a generic chip, and some hand-waving arguments >>> about how it works just like a MOSFET/BJT Darlington-ish pair. >> >> www.irf.com/technical-info/appnotes/an-983.pdf >> >> Since an IGBT is a 4-layer device one would think that "linear mode" >> might be like tip-toeing to the edge of a cliff without a safety rope >> ;-) > > Section 6: > > "1. Operation in short circuit. The current in the IGBT is limited > by its gate voltage and transconductance and can reach values well in > excess of 10 times its continuous rating. The level of hole current > that flows underneath the N+ source contact can cause a drop across > r&#2013266066;b, large enough to turn on the NPN parasitic bipolar with possible > latching. This is normally prevented by a reduction in r&#2013266066;b, as > mentioned in the previous section or by a reduction of the total > device transconductance. Since this second technique increases > conduction losses and reduces switching speed, two families of IGBTs > have been made available by IR, one optimized for low conduction > losses, the other for short circuit operation, as indicated in Section > 9. > > "2. Inductive turn-off, sometimes referred to as "clamped IL." In > an inductive turn-off the voltage swings from a few volts to the > supply voltage with constant current and with no channel current. > These conditions are different from those described in the previous > section in so far as the load current is totally made up of holes > flowing through r&#2013266066;b. For this reason some manufacturers suggest the > use of gate drive resistors to slow down the turn-off dv/dt and > maintain some level of electron current, thereby avoiding a potential > "dynamic latching" condition. IGBTs from International Rectifier can > be operated at their maximum switching speed without any problem. > Reasons to limit the switching speed should be external to the device > (e.g., overshoots due to stray inductance), rather than internal. > > "3. Operation as a linear amplifier. Linear operation exercises the > SOA of the IGBT in a combination of the two modes described above. No > detailed characterization of IGBTs as linear amplifiers has been > carried out by IR, given the limited use of IGBTs in this type of > application." > > IOW, they didn't think it'd work so they didn't try it. > > OTOH: > > http://www.st.com/stonline/products/literature/an/3703.pdf > > Section 3.2, "Reducing dV/dt at turn-off" et seq has some tricks to > give at least some of that cliff slightly less than infinite slope. > > Hey Tim, what's the required BW and signal fidelity? Why fight > IGBTs' inability to do "real" linear operation; could PPM with > averaging in the load work in your app?
I've just begun the dance with the customer, so I'm not sure of the required bandwidths, etc, although 10kHz is probably sufficient. PPM wouldn't work in this case (alas) -- it really is something where the pass device 'wants' to be working in linear mode. I'm not sure why they're interested in using IGBT -- I'll have to ask when the occasion comes up. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
On 05/31/2011 10:58 AM, John Larkin wrote:
> On Tue, 31 May 2011 08:36:46 -0700, Tim Wescott<tim@seemywebsite.com> > wrote: > >> Any good articles in using IGBTs in linear mode? >> >> With all due respect for the uselessness of SPICE when you can't do the >> work on paper -- does anyone know how good a SPICE model designed for an >> IGBT in switched service might work in linear mode? Who tends to have >> better models for linear service? Whether anyone has models >> specifically for IGBTs in linear mode? >> >> TIA -- IGBTs seem to be enough of a corner case in the semiconductor >> world that it's hard to find definitive data about their operation >> beyond a cross-section of a generic chip, and some hand-waving arguments >> about how it works just like a MOSFET/BJT Darlington-ish pair. > > Don't know about IGBTs, but lots of high-power-rated mosfets will blow > up at relatively low power dissipations in linear mode. We had to test > a lot of them. > > ftp://66.117.156.8/ExFets.jpg
The risk is undoubtedly there for IGBTs, as well. They're designed for high voltage * low current, or high current * low voltage, not middle^2. Were they blowing up at the expected power dissipation, or do they tend to blow up earlier than their maximum junction temperature & thermal conductivity would indicate? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
On Tue, 31 May 2011 11:06:58 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:

>On 05/31/2011 10:58 AM, John Larkin wrote: >> On Tue, 31 May 2011 08:36:46 -0700, Tim Wescott<tim@seemywebsite.com> >> wrote: >> >>> Any good articles in using IGBTs in linear mode? >>> >>> With all due respect for the uselessness of SPICE when you can't do the >>> work on paper -- does anyone know how good a SPICE model designed for an >>> IGBT in switched service might work in linear mode? Who tends to have >>> better models for linear service? Whether anyone has models >>> specifically for IGBTs in linear mode? >>> >>> TIA -- IGBTs seem to be enough of a corner case in the semiconductor >>> world that it's hard to find definitive data about their operation >>> beyond a cross-section of a generic chip, and some hand-waving arguments >>> about how it works just like a MOSFET/BJT Darlington-ish pair. >> >> Don't know about IGBTs, but lots of high-power-rated mosfets will blow >> up at relatively low power dissipations in linear mode. We had to test >> a lot of them. >> >> ftp://66.117.156.8/ExFets.jpg > >The risk is undoubtedly there for IGBTs, as well. They're designed for >high voltage * low current, or high current * low voltage, not middle^2. > >Were they blowing up at the expected power dissipation, or do they tend >to blow up earlier than their maximum junction temperature & thermal >conductivity would indicate?
We tested a bunch of mosfets rated for 300 watts continuous dissipation. They were bolted to a copper block, and pulsed at various power dissipations for 0.1 seconds at very low duty cycle. The application is small-bore MRI gradient drivers. I think we tested them at 200 volts D-S, can't recall exactly. Most of the ones we tried exploded at well below 300 watts. Power mosfets are apparently optimized for switchmode use, and get something like a second breakdown effect if made to dissipate lots of power at high voltages. Ixys makes some parts rated for high-power linear use. John
On Tue, 31 May 2011 11:06:58 -0700, Tim Wescott <tim@seemywebsite.com> wrote:

>On 05/31/2011 10:58 AM, John Larkin wrote: >> On Tue, 31 May 2011 08:36:46 -0700, Tim Wescott<tim@seemywebsite.com> >> wrote: >> >>> Any good articles in using IGBTs in linear mode? >>> >>> With all due respect for the uselessness of SPICE when you can't do the >>> work on paper -- does anyone know how good a SPICE model designed for an >>> IGBT in switched service might work in linear mode? Who tends to have >>> better models for linear service? Whether anyone has models >>> specifically for IGBTs in linear mode? >>> >>> TIA -- IGBTs seem to be enough of a corner case in the semiconductor >>> world that it's hard to find definitive data about their operation >>> beyond a cross-section of a generic chip, and some hand-waving arguments >>> about how it works just like a MOSFET/BJT Darlington-ish pair. >> >> Don't know about IGBTs, but lots of high-power-rated mosfets will blow >> up at relatively low power dissipations in linear mode. We had to test >> a lot of them. >> >> ftp://66.117.156.8/ExFets.jpg > >The risk is undoubtedly there for IGBTs, as well. They're designed for >high voltage * low current, or high current * low voltage, not middle^2. > >Were they blowing up at the expected power dissipation, or do they tend >to blow up earlier than their maximum junction temperature & thermal >conductivity would indicate?
Earlier, AIUI. A secondary breakdown/SOA (localized heating) sort of thing.
John Larkin wrote:
> On Tue, 31 May 2011 11:06:58 -0700, Tim Wescott <tim@seemywebsite.com> > wrote: > >> On 05/31/2011 10:58 AM, John Larkin wrote:
[...]
>>> ftp://66.117.156.8/ExFets.jpg >> The risk is undoubtedly there for IGBTs, as well. They're designed for >> high voltage * low current, or high current * low voltage, not middle^2. >> >> Were they blowing up at the expected power dissipation, or do they tend >> to blow up earlier than their maximum junction temperature & thermal >> conductivity would indicate? > > We tested a bunch of mosfets rated for 300 watts continuous > dissipation. They were bolted to a copper block, and pulsed at various > power dissipations for 0.1 seconds at very low duty cycle. The > application is small-bore MRI gradient drivers. I think we tested them > at 200 volts D-S, can't recall exactly. > > Most of the ones we tried exploded at well below 300 watts. Power > mosfets are apparently optimized for switchmode use, and get something > like a second breakdown effect if made to dissipate lots of power at > high voltages. > > Ixys makes some parts rated for high-power linear use. >
FETs should always be ok when not exceeding the SOA. Did you go outside of the SOA? 100msec is usually the lowest run or already considered DC. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
On Tue, 31 May 2011 17:29:35 -0700, Joerg <invalid@invalid.invalid>
wrote:

>John Larkin wrote: >> On Tue, 31 May 2011 11:06:58 -0700, Tim Wescott <tim@seemywebsite.com> >> wrote: >> >>> On 05/31/2011 10:58 AM, John Larkin wrote: > >[...] > >>>> ftp://66.117.156.8/ExFets.jpg >>> The risk is undoubtedly there for IGBTs, as well. They're designed for >>> high voltage * low current, or high current * low voltage, not middle^2. >>> >>> Were they blowing up at the expected power dissipation, or do they tend >>> to blow up earlier than their maximum junction temperature & thermal >>> conductivity would indicate? >> >> We tested a bunch of mosfets rated for 300 watts continuous >> dissipation. They were bolted to a copper block, and pulsed at various >> power dissipations for 0.1 seconds at very low duty cycle. The >> application is small-bore MRI gradient drivers. I think we tested them >> at 200 volts D-S, can't recall exactly. >> >> Most of the ones we tried exploded at well below 300 watts. Power >> mosfets are apparently optimized for switchmode use, and get something >> like a second breakdown effect if made to dissipate lots of power at >> high voltages. >> >> Ixys makes some parts rated for high-power linear use. >> > >FETs should always be ok when not exceeding the SOA. Did you go outside >of the SOA? 100msec is usually the lowest run or already considered DC.
Since they were rated for 300 watts continuous, we clearly weren't out of the SOA at less than 300 watts. I have seen some mosfet SOA curves that included voltage effects. I recall seeing a fet whose SOA was reduced by about 4:1 at higher voltages. For the fets we selected, we wound up estimating a thermal model. In the amplifier, ftp://jjlarkin.lmi.net/Amp.jpg we have a uP that digitizes rail voltages (up to +-200), amp output voltage, fet currents, and heatsink temperature. It runs a realtime simulation of junction temperatures and shuts things down at an estimated 140C Tj. That lets us push the fets pretty hard. But seriously, switchmode fets get fragile at higher voltages. The SOAs probably assume they are running close to saturation. [1] John [1] I use "saturation" in the bipolar sense, low Vds, ohmic region. Some people use the word to mean operating fets in the higher voltage, constant-current region.