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Balanced Monoblock F4s with very high Bias
Balanced Monoblock F4s with very high Bias
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Old 6th December 2018, 09:55 AM   #31
MASantos is offline MASantos  Portugal
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Regarding the 5U case dissipation, I had one with an F4 running 30V rails and about 0.6A per mosfet. So I had about 108W per side being dissipated. temperature was a nice 45 on the sinks with about 25 room air temperature.
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Old 6th December 2018, 12:13 PM   #32
syracuze is online now syracuze  United Kingdom
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Balanced Monoblock F4s with very high Bias
Quote:
Originally Posted by MASantos View Post
Regarding the 5U case dissipation, I had one with an F4 running 30V rails and about 0.6A per mosfet. So I had about 108W per side being dissipated. temperature was a nice 45 on the sinks with about 25 room air temperature.
Thanks a lot MASantos!

Have you also tried a bit higher bias?
With 30V rails, moving from 0.6A to 0.8A bias (33% increase) would end up having 144W dissipation per side (as opposed to current 108W)

With that 33% increase, 20 degrees C temperature difference in heatsinks over ambient would increase to 20X1.33 = 26-27 degrees C; which with a 25 C room temperature, ends up being 51-52 C heatsinks.

I wonder whether you have tried and it did not work out or you haven't tried at all.
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Old 6th December 2018, 12:18 PM   #33
syracuze is online now syracuze  United Kingdom
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Balanced Monoblock F4s with very high Bias
Quote:
Originally Posted by indra1 View Post
Transconductance of IRF mosfets (square law device) is proportional to square root of drain current.
I usually estimate transconductance at intended bias by :
Code:
gfs(Id1) ~ gfs(Id2) * sqrt(Id1/Id2)

where :
gfs(Id1)  transconductance at intended drain current
Id1       intended drain current
gfs(Id2)  datasheet transconductance at cited drain current
Id2       datasheet cited drain current
Estimating from Vishay datasheets :
Code:
             Datasheet           Estimate
           gfs   @Id(A)   gfs @1.3A   gfs @2A
N-ch
IRFP140    9.8    19        2.56       3.18
IRFP150   13      25        2.96       3.67
IRFP240    6.9    12        2.27       2.82
IRFP244    6.7     9        2.54       3.15
P-ch
IRFP9140   6.2    13        1.96       2.43
IRFP9240   4.2     7.2      1.78       2.21
Please note that the estimate is based on Vishay minimum gfs value, no description of typical value. Spread of value is pretty wide, a maximum gfs device can have double the transconductance of a minimum gfs device.

Would this mean IRFP 9140 is actually a better match for IRFP 240 (compared to IRFP 9240 that is commonly used)


Disclaimer = I am not really deep in this topic at all, the notion of transconductance is beyond my paygrade
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Old 6th December 2018, 01:20 PM   #34
Zen Mod is offline Zen Mod  Serbia
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Balanced Monoblock F4s with very high Bias
Babelfish M25 kits are having IRFP240+IRFP9140

first read about that issue and solution from ilimzn , kudos
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Old 6th December 2018, 03:35 PM   #35
indra1 is offline indra1  Indonesia
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Quote:
Originally Posted by syracuze View Post
Would this mean IRFP 9140 is actually a better match for IRFP 240 ...
Minor enough difference for me to put on the side, spread of variation is wider. You need curve tracer plot for a good match.
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