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Better power MOSFET models in LTSpice
Better power MOSFET models in LTSpice
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Old 3rd January 2015, 09:26 PM   #41
lhquam is offline lhquam  United States
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Are the equations for the new Ksubthresh model available? I am curious about how the subthreshold behavior was merged with the VDMOS model. A major benefit of Kst is the ease with which it can be added to existing power FET models. The only change Bob Cordell made to his old IRFP240C and IRFP9240C models was the addition of ksubthes=xxx.
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Old 3rd January 2015, 09:40 PM   #42
keantoken is offline keantoken  United States
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That can work, but we're finding that there is some extra finesse needed to get the wingspread plots and harmonics accurate in some cases.

Mike never showed us the equations he used. You could try asking him yourself through the LTSpice support email address.
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Old 3rd January 2015, 09:47 PM   #43
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by lhquam View Post
Are the equations for the new Ksubthresh model available? I am curious about how the subthreshold behavior was merged with the VDMOS model. A major benefit of Kst is the ease with which it can be added to existing power FET models. The only change Bob Cordell made to his old IRFP240C and IRFP9240C models was the addition of ksubthes=xxx.
Hi Ihquam,

Yes, one gets pretty close by just adding ksubthres to a good existing VDMOS model.

These are the ksubthres models that I have been using. ksubthres was added with a value that provided a decent match to my EKV models for the IRFP240 and 9240.

* ksubthres models
*
* IRFP240Ckst VDMOS copyright Cordell Audio December 6, 2010
.model irfp240Ckst VDMOS(nchan Vto=4.0 Kp=4.8 Lambda=0.0032 Rs=0.01 Rd=0.1 Rds=1e7 Cgdmax=2600p Cgdmin=10p a=0.35 Cgs=1250p Cjo=3000p m=0.75 VJ=2.5 IS=4.0E-06 N=2.4 ksubthres=190m)
*
*
*
* IRFP9240Ckst VDMOS copyright Cordell Audio December 6, 2010
.model irfp9240Ckst VDMOS(pchan Vto=-3.76 Kp=9 Lambda=0.004 Rs=0.064 Rd=0.1 Rds=1e7 Cgdmax=1200p Cgdmin=15p a=0.26 Cgs=1130p Cjo=2070p m=0.68 VJ=2.5 IS=4.0E-06 N=2.4 ksubthres=107m)
*


Note that Ian and Keantoken are looking at some modeling approaches that may provide increased accuracy.

Cheers,
Bob
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Old 4th January 2015, 12:06 AM   #44
fas42 is offline fas42  Australia
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I'm playing with a nominally high performance circuit at the moment, using the IRFP pair, and using the ksubthres models I get a difference in simulated performance. The behaviour at crossover has varied, and this translates to a decrease in 2nd and 3rd, and an increase in much higher harmonics in the FFT - about 4dBs worth in the particular instance.
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Old 4th January 2015, 01:27 AM   #45
Bob Cordell is offline Bob Cordell  United States
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Originally Posted by fas42 View Post
I'm playing with a nominally high performance circuit at the moment, using the IRFP pair, and using the ksubthres models I get a difference in simulated performance. The behaviour at crossover has varied, and this translates to a decrease in 2nd and 3rd, and an increase in much higher harmonics in the FFT - about 4dBs worth in the particular instance.
Hi Frank,

What models were you using previously? The conventional VDMOS without ksubthres? Were these my models?

When I changed from conventional VDMOS models to EKV or VDMOSksubthres I did see a slight net increase in THD, but I don't remember a significant increase in high order hermonics.

Cheers,
Bob
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Old 4th January 2015, 02:45 AM   #46
fas42 is offline fas42  Australia
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Bob, yes, I was using your IRFP(9)240C models as is, and I did separate runs without ksubthres, and with, this being the only change between them.

The circuit uses strong local feedback in the output stage, and biasing to prevent the output devices switching off - I used the technique of subtracting the output of the two runs from each other in LTspice to see where the variation occurred, and this was a high frequency wriggle at crossover - 20kHz, near maximum power, into 2 ohms. This is a severe test of the circuit, but it performs well - I suspect the variation will be much less for more moderate situations, something I haven't looked at yet.

Cheers,
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Old 4th January 2015, 12:45 PM   #47
Bob Cordell is offline Bob Cordell  United States
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Originally Posted by fas42 View Post
Bob, yes, I was using your IRFP(9)240C models as is, and I did separate runs without ksubthres, and with, this being the only change between them.

The circuit uses strong local feedback in the output stage, and biasing to prevent the output devices switching off - I used the technique of subtracting the output of the two runs from each other in LTspice to see where the variation occurred, and this was a high frequency wriggle at crossover - 20kHz, near maximum power, into 2 ohms. This is a severe test of the circuit, but it performs well - I suspect the variation will be much less for more moderate situations, something I haven't looked at yet.

Cheers,
Hi fas42,

I have attached my EKV models for the 240 and 9240 in a zipped folder. These models are what I have been using as my baseline reference for evaluating the VDMOS ksubthreshold models. The test benches include Id vs Vgs, log Id vs Vgs, gm vs Id, wingspread, and THD.

These simulations are ready to run.

Wingspread and THD are done for bias currents of 150, 250, 350 and 450mA at 20V peak into 8 ohms. For each value of bias current, output dc offset was adjusted to be less than 1mV.

Source resistors in all cases were 0 ohms.

Cheers,
Bob
Attached Files
File Type: zip EKV 240_9240.zip (13.3 KB, 235 views)
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Old 4th January 2015, 10:03 PM   #48
fas42 is offline fas42  Australia
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That's excellent, Bob, thanks very much for putting that material up - very good for getting a feel for what's going on at a raw level, .
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Old 6th January 2015, 05:16 AM   #49
davidsrsb is offline davidsrsb  Malaysia
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I tried the SKA amplifier simulation with the EKV models and the crossover looks much more progressive (and realistic) than using the Cordell models. A strange side effect is that when injecting an ac current source into the output, the Cordell models cause a low level oscillation, while this is gone after a few us using the ksubthreshold version
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Old 6th January 2015, 11:12 AM   #50
Bob Cordell is offline Bob Cordell  United States
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Originally Posted by davidsrsb View Post
I tried the SKA amplifier simulation with the EKV models and the crossover looks much more progressive (and realistic) than using the Cordell models. A strange side effect is that when injecting an ac current source into the output, the Cordell models cause a low level oscillation, while this is gone after a few us using the ksubthreshold version
Hi david,

That's great to hear. I would not have thought that the VDMOS model without ksubthres would have caused an oscillation as long as the idle bias was high enough to provide a reasonable amount of transconductance from one output device or the other.

Cheers,
Bob
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