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Old 13th February 2012, 02:47 PM   #1
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Default F5 turbo problems

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
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Old 13th February 2012, 03:11 PM   #2
sg2 is offline sg2  France
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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-

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stéphane
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Old 13th February 2012, 03:30 PM   #3
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15V rails ?
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Old 13th February 2012, 04:09 PM   #4
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Quote:
Originally Posted by sg2 View Post
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
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Old 13th February 2012, 04:36 PM   #5
sg2 is offline sg2  France
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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
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Old 13th February 2012, 05:39 PM   #6
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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
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Old 13th February 2012, 05:55 PM   #7
pidesd is online now pidesd  Canada
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maybe raise supply voltage?
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Old 13th February 2012, 05:56 PM   #8
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Aka don't let Vds go down too much ?

(max drive capacitance + max transconductance )
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Old 13th February 2012, 07:34 PM   #9
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I think the amplifier is trying to tell you not to clip it at 100 KHz.

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Old 13th February 2012, 07:39 PM   #10
AndrewT is offline AndrewT  Scotland
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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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