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Old 29th June 2005, 03:09 PM   #11
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Bruno

What do you think about making the dead-time variable and setting it for minimum idle current ?

Regards

Charles
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Old 29th June 2005, 03:19 PM   #12
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Quote:
Originally posted by phase_accurate
What do you think about making the dead-time variable and setting it for minimum idle current ?
The 400W modules have a pot to set idle current. Setting idle current to minimum (around 35mA) results in a ridiculous amount of distortion. Setting it at 60mA produces good THD performance. The increase in idle losses corresponds to the energy in snubbers and parasitics that is not recovered by the inductor, so still no cross-conduction. The increased idle losses are a small premium for the better performance.
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Old 29th June 2005, 03:27 PM   #13
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So you would recommend to start at minimum idle-current and then decrease dead-time until THD is smallest ?

What about ageing and thermal runaway ?

Regards

Charles
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Old 29th June 2005, 03:39 PM   #14
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Quote:
Originally posted by phase_accurate
So you would recommend to start at minimum idle-current and then decrease dead-time until THD is smallest ?
I wouldn't recommend doing anything with this potentiometer. It's set at the factory for 60 to 70mA.

Quote:
Originally posted by phase_accurate
What about ageing and thermal runaway ?
Aging is not a significant factor. Temperature coefficients across the circuit are matched.
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Old 29th June 2005, 03:43 PM   #15
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Quote:
I wouldn't recommend doing anything with this potentiometer. It's set at the factory for 60 to 70mA.
Sorry for being unspecific.
I wondered how to optimally set dead-time in general - not on an industrially manufactured module - since there are some people developing their own class-d amps .....

Regards

Charles
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Old 29th June 2005, 04:15 PM   #16
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Quote:
Originally posted by phase_accurate
Sorry for being unspecific.
I wondered how to optimally set dead-time in general - not on an industrially manufactured module - since there are some people developing their own class-d amps .....
My only problem is - Feind hört mit. I could tell you all the details in private, but not in a forum which is ostensibly visited by prospective competitors. The basic answer is: for low THD, as short as possible without causing shoot-through. For low idle current: precisely as long as the resonant transition.
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Old 29th June 2005, 04:44 PM   #17
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Telling this in private can be dangerous as well since you'l never know what job someone is going to apply for some day !

I just wanted to know what (simple) procedure you would suggest if someone uses adjustable deadtime on an amp (which is quite eays to implement) to set said deadtime.

On an amp I once made we just set it for lowest idle current which was not the best choice obviously. Since I don't own this one anymore I can't play around with it now. But I assume we'd have achieved better THD with the same circuit by optimising deadtime than we actually did.

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Charles
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Old 29th June 2005, 07:41 PM   #18
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Well you can measure idle current while twiddling the dead time control. When dead time becomes negative, this is immediately obvious from a sudden rise in idle current.
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Old 29th June 2005, 10:01 PM   #19
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Hi,

If the mosfet could be considered ON when the plateau region is over (gate voltage now climbing sharply again) could it not be considered OFF at the beginning of the plateau region?

I realise some current may and likely is flowing at this point, and how much is likely to vary, but there's still full voltage across the device...

I'd think that's actually the point you'd be concerned with, when the voltage starts dropping across it (the start of the plateau region), is when your output node actually switches, and so would be at the end of deadtime.

I'd noticed before that instantaneous output current can shift the plateau region as well, some data sheets show this directly in the gate charge waveforms.

We aren't talking about any particular designs here just good techniques so nothing should be top secret? Of course I see things differently from my end of things.

I still think temperature is a prime indicator if you're on the right track or not, not everyone has the 100Ghz scopes.
Regards,
Chris
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Old 29th June 2005, 11:09 PM   #20
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Quote:
Originally posted by classd4sure
If the mosfet could be considered ON when the plateau region is over (gate voltage now climbing sharply again) could it not be considered OFF at the beginning of the plateau region?
NO! The drain voltage may not have started coming down yet, but the FET is already conducting. It may only be considered OFF when Vgs<Vth.
Quote:
Originally posted by classd4sure
I realise some current may and likely is flowing at this point, and how much is likely to vary, but there's still full voltage across the device...
Precisely. If the output stage is delivering, say 10A and you're on the "hard" transition, plateau starts only at Id=10A. This is not quite what you'd call "off".
Quote:
Originally posted by classd4sure
I'd think that's actually the point you'd be concerned with, when the voltage starts dropping across it (the start of the plateau region), is when your output node actually switches, and so would be at the end of deadtime.
Hmmmm... I've often dreamt about making an adaptive control circuit that made for negative dead time so it always crosses over precisely at Id=Iout. Hugely interesting, both in terms of distortion and efficiency, but "fairly" impractical.
Quote:
Originally posted by classd4sure
I'd noticed before that instantaneous output current can shift the plateau region as well, some data sheets show this directly in the gate charge waveforms.
This is quite obvious. As long as the FET is transitioning (Vds is moving down), the miller effect will insure that Vgs cannot move beyond the point corresponding with drain current.
It's quite simple, isn't it? If Vds>>0, Id is determined only by Vgs. If Vgs were higher, either Vds should be 0 or there should be shoot-through preventing Vds from coming down.
Quote:
Originally posted by classd4sure
We aren't talking about any particular designs here just good techniques so nothing should be top secret? Of course I see things differently from my end of things.
As I explained in private earlier, I try to point people in the right direction, without giving recipes. Recipes do not constitute "understanding". Understanding comes only through study, experimentation, and at times a friendly hint from others.
There are too many people around who try to cream recipes from forums such as these in order to get into the market with their "own" designs quickly, without feeling the need to understand what they're doing. I don't feel any need to give these people a cheap ride by writing a "how-to" guide.

However, there are a few folks out here who themselves work and study hard to gain a proper in-depth understanding. There, it makes little sense to withold information. You yourself, for instance, will find out these things in due time anyway. So then I think it's worth to give nudges in the right direction, knowing full well that these hints won't be understood by people simply looking for a quick design help.
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