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Class D Switching Power Amplifiers and Power D/A conversion

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Old 5th November 2009, 06:48 PM   #21
Pafi is offline Pafi  Hungary
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As far as the higher Rds and Qrr of the higher voltage part, it seems you have two choices. Either put up with higher switching and/or conduction losses or put up with it being fragile.
...or choose BCA topology, or disable body diode with series schottky and antiparallel FRED.
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Old 5th November 2009, 10:02 PM   #22
fredos is offline fredos  Canada
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Cascode Output device... That work very well, get ride of switching lose and recovery issue..I have a design with +/-240V supply with STP62NS04Z as switching device, APT6011 (X2) as cascode device and IXYS DSEE30-12 as recovery diode....The cascode device use the +15V of each mosfet driver, so the switching device never see more than 11V between their D-S. That help a lot the miller capacitance in the same time...Just be carefull to dont switch ''too fast'', you should keep in mind the Dv/Dt of the cascoded device...Or use snubber over the switching device!
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Old 6th November 2009, 03:32 PM   #23
DjLeco is offline DjLeco  Romania
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Fredos, some schematics about you discuss,will be helpfull and appreciated!
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Old 6th November 2009, 07:30 PM   #24
fredos is offline fredos  Canada
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Hope it say all...
Attached Images
File Type: jpg Cascode1.jpg (73.5 KB, 4044 views)
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Old 6th November 2009, 08:29 PM   #25
Pafi is offline Pafi  Hungary
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Are C1 and C2 connected to the right place?
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Old 6th November 2009, 08:31 PM   #26
fredos is offline fredos  Canada
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Yep, to keep the cascode effect...need to be in the 10uf range!
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Old 6th November 2009, 09:05 PM   #27
Pafi is offline Pafi  Hungary
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Are you really sure? 10uF of Cgs is not a good omen! Not to mention that they will never be discharged, so Q4 and Q2 will be open forever after once opened.

Last edited by Pafi; 6th November 2009 at 09:08 PM.
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Old 6th November 2009, 09:15 PM   #28
fredos is offline fredos  Canada
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They are the cascode device...The switching is done by Q1 and Q3...See Q2 and Q4 as ''voltage regulator'' for Q1 and Q3...When Q1 or Q3 open, they ''disconnect'' source of Q2 and Q4 and the leg act as a ''normal'' single mosfet switch. In that configuration the switching '' speed'' are dertemined by the switching device and they only see at worst 12V between their D-S junction. The recovery diode of the switching device and the cascoded devices are in serie, so due to the hight voltage drop, nearly all recovery current flow trought D4 and D5, wich should be fast type..I wish that these simple explaination is clear.
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Old 6th November 2009, 09:19 PM   #29
fredos is offline fredos  Canada
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By the way, Q2 and Q4 dont need to be a fast switching device, just big high voltage and high current fet...
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Old 6th November 2009, 09:38 PM   #30
Eva is offline Eva  Spain
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I think the concept is good, but in the circuit diagram D1, D3, C1 and C2 are not in the right place. I have done the same for SMPS but with a bipolar transistor as the high voltage device (and a current transformer to feed the base).

Also, I think MOSFET choice is not good. How about PSMN1R7-30YL for the low voltage transistor, SPW47N60C3 for the high voltage one and STTH30R06 or FFP30S60S for the diode?

But the price paid for low reverse recovery charge in the diode are high diode conduction losses... And with SPW47N60CFD with only 2uC of body diode charge at 46A (25ºC, 100A/us) and the ability to bypass part of diode current with Rds-on, the advantage is not that clear... Ultrafast body diodes are not that bad.

btw: Why APT parts look like absolute junk on datasheets? They are overpriced yet easily outperformed by anything recent from IR or Infineon.
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