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Rob Dingen 13th February 2012 03:47 PM

F5 turbo problems
 
1 Attachment(s)
Hi

I build the F5 turbo with low rails +/- 15V for driving a low impedance ribbon speaker.
Everything looks fine bandwidth 750Khz -3dB and I get about 35A from the output in this configuration.
I use 4 sets mosfets from IR the IRFP240 and IRFP9240 and 0,235 Ohm source resistors.
The fronted is the same as the F5 with 100 Ohm parallel in the feedback loop.
Now the problem if I feed the amplifier 100Khz and on the output the voltage rise above 20Vtt then the output collapse and the fets drawing lots of current about 10A each.
It doesn't mind if I load the output with a resistor or not.
What could be the problem.
I tried capacitors on the feedback resistors to reduce the bandwidth to 100Khz use input filter and 1N from drain to gate.
Nothing helps.

Rob

sg2 13th February 2012 04:11 PM

Hi,

Unability of the front end to supply enough current to properly swing the output mosfet's gate capacitance at such high freq ? this leading to what class D people call shoot through, ie N and P channel mosfets conducting too much at the same time, ie shorting V+ to V-

Regards,
--
stéphane

jacco vermeulen 13th February 2012 04:30 PM

15V rails ? :clown:

Rob Dingen 13th February 2012 05:09 PM

Quote:

Originally Posted by sg2 (Post 2904726)
Hi,

Unability of the front end to supply enough current to properly swing the output mosfet's gate capacitance at such high freq ? this leading to what class D people call shoot through, ie N and P channel mosfets conducting too much at the same time, ie shorting V+ to V-

Regards,
--
stéphane

Hi stéphane

Should I leave it like that and pay no attention to it or can it cause problems.

Rob

sg2 13th February 2012 05:36 PM

It could also be due to different things

But if my take is right then no it cant stay like that, you will burn your output stage sooner or later

Rob Dingen 13th February 2012 06:39 PM

Ok then a lot of people building the F5 turbo will have problems.
Hope that someone else has a bright idea how to tackle this problem.

Rob

pidesd 13th February 2012 06:55 PM

maybe raise supply voltage?

jacco vermeulen 13th February 2012 06:56 PM

Aka don't let Vds go down too much ?

(max drive capacitance + max transconductance )

Nelson Pass 13th February 2012 08:34 PM

I think the amplifier is trying to tell you not to clip it at 100 KHz.

:cool:

AndrewT 13th February 2012 08:39 PM

Is 20Vtt the same as 20Vpp?

If so, then an output of 20Vpp should not be clipping in an amp fed from +-15Vdc when there is insignificant load current.


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