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Alpha Nirvana 39w 8ohm Class A Amp
Alpha Nirvana 39w 8ohm Class A Amp
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Old 29th June 2020, 11:46 PM   #1361
caglarm is offline caglarm  Turkey
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Click the image to open in full size.

Are you sure that the directions of these diodes are correct?
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Old 30th June 2020, 12:16 AM   #1362
andyr is offline andyr  Australia
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Quote:
Originally Posted by caglarm View Post

Are you sure that the directions of these diodes are correct?
Yes, it seemed strange to me - but I just connected up the diodes the way they are shown on the PCB ... and the red LED goes on at clipping.

Andy
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Old 30th June 2020, 12:20 AM   #1363
AKSA is offline AKSA  Australia
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The diodes are correct. They conduct only if the gate drive is higher than 4V Over the positive rail.
Gives us a cleaner clip recovery.

HD
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Old 30th June 2020, 02:17 AM   #1364
jhofland is offline jhofland  United States
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Alpha Nirvana 39w 8ohm Class A Amp
Hugh,
Forgive me if I'm off base. The key to understanding how the gate might go higher than the positive rail has everything to do with bootstrap capacitor C131. If there is a relatively abrupt increase in output voltage that same increase will be seen at the positive side of C131 while the voltage across the cap won't change much.
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Old 30th June 2020, 02:36 AM   #1365
AKSA is offline AKSA  Australia
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HI Jan,
The drive at the gate can exceed the positive rail, but the upper nmos won't turn on until the source gets within about two volts off the rail. At that point, the gate would be 2.5V above the rail, and to prevent the nmos turning off because of dropping Vds, better to put in these diodes to drain drive to the positive rail and turn off the nmos at its gain, allowing recovering when gate potention drops and gate turns on the channel as drive falls.

At least, that is my understanding? Is this wrong?

Beautiful build of the AN you are doing, looks like a DartZeel!!

Hugh
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Old 30th June 2020, 11:19 PM   #1366
caglarm is offline caglarm  Turkey
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Quote:
Originally Posted by AKSA View Post
gate drive is higher than 4V Over the positive rail.

HD

Can this ever be possible?
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Old 30th June 2020, 11:37 PM   #1367
jhofland is offline jhofland  United States
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Alpha Nirvana 39w 8ohm Class A Amp
Read the post just before AKSA's. If you understand what C131 does when the output rapidly rises you can realize that it is possible to drive the gate above the rail voltage.
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Old 1st July 2020, 02:32 AM   #1368
AKSA is offline AKSA  Australia
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Yes, Caglarm, a bootstrap means that when the cap is charged at idle it will hold around 15V across it. Since the negative side (on the positive rail bootstrap) is connected to the output, if it rises to +24V (peak voltage wrt ground) then the bootstrap cap will drive up the voltage at its positive electrode because it holds a lot of energy and can overcome the positive rail to boostrap resistor, 1.5K in this case.

So, can see a positive rail at the bottom of the bootstrap rising ABOVE the upper rail, it is quite normal and a useful way to drive a mosfet right up to its limits on the rail.

Hugh
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Old 1st July 2020, 08:41 PM   #1369
caglarm is offline caglarm  Turkey
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Quote:
Originally Posted by AKSA View Post
Yes, Caglarm, a bootstrap means that when the cap is charged at idle it will hold around 15V across it. Since the negative side (on the positive rail bootstrap) is connected to the output, if it rises to +24V (peak voltage wrt ground) then the bootstrap cap will drive up the voltage at its positive electrode because it holds a lot of energy and can overcome the positive rail to boostrap resistor, 1.5K in this case.

So, can see a positive rail at the bottom of the bootstrap rising ABOVE the upper rail, it is quite normal and a useful way to drive a mosfet right up to its limits on the rail.

Hugh
Thank you. I understood properly now .
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