F5X -- the EUVL Approach - Page 78 - diyAudio
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Old 27th August 2012, 04:27 PM   #771
Dansk is offline Dansk  Denmark
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Ok. Now we may have reached my point of misunderstanding!...

You say that the 24V headroom is doing nothing but spending electricity.
I assumed that this headroom would be available as class A/B-amplification. Is that not the case?
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Old 27th August 2012, 05:46 PM   #772
EUVL is offline EUVL  Europe
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Your normal bias is 32V 1A per Mosfet. i.e. 32W.
This is as much as I would dissipate continuously on a TO247 Mosfet.

Assume a 8 ohm speaker at with +/-28V output in balanced mode.
Maximum current is, in theory, 7A.

I'll let you calculate the dissipation of the Mosfets in Class AB based on that.
It will be less than 32x7 = 224W, but it will mostly likely be a lot more than 32W.

If you are happy to take that risk, please go ahead. Don't let me discourage you.




Patrick
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Old 27th August 2012, 09:47 PM   #773
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you can get 50W pr fet. BUT, then you have to know what your doing. regards to heatsinks.
so the fets are spread across the sinks for maximum effect. and thats a whole lot of math ifself
in general i agree with Evul on a 32W max pr fet.
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Old 28th August 2012, 04:21 AM   #774
EUVL is offline EUVL  Europe
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If you want to know more about Class A thermal design, I wrote an article for Jan Didden's Linear Audio.
It is in Volume 3, but as with all my articles, will be available for free download 6 months later.

Thanks to Jan's generosity.


Patrick
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Old 30th August 2012, 09:25 AM   #775
NicMac is offline NicMac  Italy
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Default Why degenerating the MOSFETs?

I am trying to learn/understand why the MOSFETs in the F5X are degenerated and looking around I have found multiple explanations for using/not-using MOSFET degeneration....

Not being an EE it is difficult for me to understand which of these claims are true and which of these forms the rationale for the implementation of MOSFET degeneration in the F5X.

Claimed effects of degeneration:
1) Make the gain of the circuit more linear.
2) Boosts the output impedance.
3) Provides series feedback.
4) Reduces 3rd order products.
5) Transforms input impedance to be real instead of imaginary.
6) Increase NP-matching accuracy.
7) Decreases the usable output swing.
8) Increases noise.

Any opinions?

Cheers,

Nic
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Old 30th August 2012, 06:58 PM   #776
EUVL is offline EUVL  Europe
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We don't parallel MOSFETs in F5x (standard).
So the 0R22 is not for current balancing.

But look at the curve of Id vs Vgs at 25C in the datasheet.
And check what the current would be at say 75~100C Junction temp, keeping Vgs constant.

Now add the 0R22, keeping the voltage across Vgs+0R22 constant with temperature.
Check how much the current changes at 75~100C now.

And please come back and tell the complete story.
Thanks in advance.




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Old 31st August 2012, 09:04 AM   #777
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by EUVL View Post
And please come back and tell the complete story.
From data sheet:
@25˚C a 2SK1530 will conduct 2A, when Vgs is about 2.3V
@125˚C, with the same Vgs, the device will conduct roughly 3A

If we add in the 0R22 this will drop the voltage seen by the FET by E = I*R or 2-3A*0R22 = 0.44-0.66V
@ Vgs = 2.3V - 0.44V = 1.9V the FET will conduct 2A @ 125˚C

Let me see if I got it wright:
At constant Vgs the current conducted by the FET increases with temperature.
The source resistor lower the Vgs seen by the FET proportionally to the conducted current.
I.e. the source resistor provide negative current feedback, possibly preventing thermal run away.
If the resistor has a positive tempco it will also provide a thermal negative feedback on the current.

I am sure this is not the hole story, but the only piece I think I understand.

Did I pass the exam?
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Old 31st August 2012, 05:41 PM   #778
EUVL is offline EUVL  Europe
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Yes, you passed the exam.


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Old 31st August 2012, 09:42 PM   #779
AndrewT is offline AndrewT  Scotland
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and the negative feedback effect of the source resistor works for Vertical mosFETs and for Lateral mosFETs and for BJTs. The NFB is not confined to just vertical mosFETs that want to runaway into oblivion.
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Old 4th September 2012, 07:53 AM   #780
qusp is offline qusp  Australia
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Quote:
Originally Posted by EUVL View Post
If you want to know more about Class A thermal design, I wrote an article for Jan Didden's Linear Audio.
It is in Volume 3, but as with all my articles, will be available for free download 6 months later.

Thanks to Jan's generosity.


Patrick
and your own
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