Hello.
Some of you mentioned that Trr (body diode reverse recovery time) should be low, but TO247 devices have Trr>200ns in most cases.
Can this be ammended by the addition of a ultrafast diode in parallel?
I have seen some designs, some as Tripath:
(http://www.tripath.com/downloads/RB-TA0105.pdf , see pag. 14)
...that use a "small" 4A ultrafast diode (MUR420) in parallel with the body diode, with much lower Trr (35ns or so). Is this in order to speed up recovery time?
What should be the current rating of the diode?
So, can I choose a mosfet with all parameters OK except for a slightly large Trr and then add two ultrafast diodes in parallel to improve situation?
Thanks!
Some of you mentioned that Trr (body diode reverse recovery time) should be low, but TO247 devices have Trr>200ns in most cases.
Can this be ammended by the addition of a ultrafast diode in parallel?
I have seen some designs, some as Tripath:
(http://www.tripath.com/downloads/RB-TA0105.pdf , see pag. 14)
...that use a "small" 4A ultrafast diode (MUR420) in parallel with the body diode, with much lower Trr (35ns or so). Is this in order to speed up recovery time?
What should be the current rating of the diode?
So, can I choose a mosfet with all parameters OK except for a slightly large Trr and then add two ultrafast diodes in parallel to improve situation?
Thanks!
Simply paralleling a Fast Recovery Diode with the mosfet isn't going to give you any benefit, because the Forward drop of Diode is greater than Forward Diode of Body Diode of mosfet....for a given amount of freewheeling current conduction..
Unless you use a Series Schottky to achieve Bulletproof solution as in Crest CD3000...
or a Centretapped inductor in some cases as in Crest LT1200....[but not that much robust]
Unless you use a Series Schottky to achieve Bulletproof solution as in Crest CD3000...
or a Centretapped inductor in some cases as in Crest LT1200....[but not that much robust]
Mmmm. interesting...
What problems will a high Trr (240ns or so) cause? Shoot-through? And will it be noticeable with no signal? I mean, if I have shoot-through will I notice that the mosfets get warm with no signal, right?
And what about the V/ns specification, is it related to Trr?
Thanks and sorry for so many questions
What problems will a high Trr (240ns or so) cause? Shoot-through? And will it be noticeable with no signal? I mean, if I have shoot-through will I notice that the mosfets get warm with no signal, right?
And what about the V/ns specification, is it related to Trr?
Thanks and sorry for so many questions
Pierre said:
Aren't you the same guy who made postfilterfeedback class-d[with Charles help] with complementary LM319 comparators....now it seems that you donot understand the concept of Freewheeling currents in switching circuits....How this could be.........Now I am confused.....
With high Trr, the Freewheeling current doesnot have quick discharge interval thereby increases the shoothrough, which leds to the Mosfet breakdown due to avalanche condition.....
Yes, I am the same guy... 
But wanted to clarify some concepts before trying to modify my old prototype for higher power and use larger mosfets.
The Tripath schematics have confused me a bit, sorry. Would like to hear them justify the use of that two diodes, perhaps we're missing some detail...
But wanted to clarify some concepts before trying to modify my old prototype for higher power and use larger mosfets.
The Tripath schematics have confused me a bit, sorry. Would like to hear them justify the use of that two diodes, perhaps we're missing some detail...
If the forward drop of body diode is greater than forward diode of anti parallel fast recovery diode than its good, but this never happens unless one uses a schottky instead of fast recovery diode.......but higher voltage schottkys have higher forward voltage drop...
In order to overcome this drawback, another solution is using schottky's in series with mosfets to stop the body diode conduction phenomena and than the anti-parallel connected fast recovery diode takes care of freewheeling current very easily and shootthrough current is reduced and mosfets are saved from premature failure......
In order to overcome this drawback, another solution is using schottky's in series with mosfets to stop the body diode conduction phenomena and than the anti-parallel connected fast recovery diode takes care of freewheeling current very easily and shootthrough current is reduced and mosfets are saved from premature failure......
Pierre,
Have a look at this Crest CD3000 output stage
Some useful links:
http://www-s.ti.com/sc/techlit/slup169.pdf
http://users.tkk.fi/~jwagner/tesla/SSTC/general-sstc-notes-mosfet-failure.htm
http://users.tkk.fi/~jwagner/tesla/SSTC/
Have a look at this Crest CD3000 output stage
Some useful links:
http://www-s.ti.com/sc/techlit/slup169.pdf
http://users.tkk.fi/~jwagner/tesla/SSTC/general-sstc-notes-mosfet-failure.htm
http://users.tkk.fi/~jwagner/tesla/SSTC/
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