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Old 23rd May 2010, 08:17 PM   #21
Pafi is offline Pafi  Hungary
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Quote:
Who would trade overall EMI performance for eliminating carrier residual only when there is no signal?
Eva!

BD modulation doesn't mean this! EMI performance can be identical or better then in case of AD (at fixed freq).

Quoted from Bruno:
"Output filter?
•Single-core
• no excitation current
• crossover distortion results"

Single core doesn't mean no excitation current. Differential (useful signal) and common mode inductance (excitation current) can be selected independently.
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Old 24th May 2010, 09:15 AM   #22
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Default Ecler uses Trilevel Class-BD modulation

http://www.ecler.com/images/download...tDTenglish.pdf
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Old 24th May 2010, 11:39 PM   #23
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Default BD modulation

This is what I was talking about, the output is quantized to one of three levels vs one of two levels for AD. This lowers EMI because the quantization noise (PWM output minus analog signal you want) is 6 db lower. But there is that inherant nonlinearity. And there is also a paper out there that claims that BD performs worst in inductive loads? True or not I don't know but if so it could be an issue with full bandwidth amps in the high frequency area driving driving more inductive loads.
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Old 25th May 2010, 03:58 AM   #24
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Quote:
Originally Posted by phase_accurate View Post
I think Kanwar is talking about what is described in Bruno's paper (which was mentioned some posts before) and not some three-state amp that is using an "open" state.

This has one advantage: The apparent switching frequency and also the actual sampling frequency is twice as high for a given carrier frequency when you use a carrier-based amp. Therefore you can use more loop gain.

Regards

Charles
Charles,
Do you think that reverse recovery in BD modulation is not that much drastic as in AD modulation?

Regards,
Kanwar
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Old 25th May 2010, 09:53 AM   #25
Pafi is offline Pafi  Hungary
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Quote:
This lowers EMI because the quantization noise (PWM output minus analog signal you want) is 6 db lower.
I don't think it's good to talk about quantization here, since PWM is basically analog. We can say carrier freq. component, or switching freq., component. Carrier residual is 6 dB lower, but this has nothing to do with EMI. EMI is generated by switching of the MOSFETs, and determined by supply voltage, schematic of switching stage, FET type, switching speed, snubbering, attachment method of heat sink, etc... Modulation scheme is unimportant.

Quote:
And there is also a paper out there that claims that BD performs worst in inductive loads? True or not I don't know...
Not. First of all, BD is a modulation. The load don't have any direct influence to the PWM modulator. If there were any indirect influence, then it would have been via output stage. Output topology can be different (BCA, or bridged), but if properly built, none of them care about the load current direction, so they don't care about inductive load. I tried both, especially with big (1 mH... 400 mH) almost pure inductive load, and there was no problem.
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Old 25th May 2010, 09:56 AM   #26
Pafi is offline Pafi  Hungary
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Kanwar!

For you too: modulation method and output topology are two things! Diode recovery depends on output topology, not modulation method!
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Old 25th May 2010, 10:02 AM   #27
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Default I agree............but

Quote:
Originally Posted by Pafi View Post
Kanwar!

For you too: modulation method and output topology are two things! Diode recovery depends on output topology, not modulation method!
Once Charles said about something w.r.t to reverse recovery in BD modulation thats why i asked...........
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Old 25th May 2010, 10:16 AM   #28
81bas is offline 81bas  Germany
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Quote:
Originally Posted by Pafi View Post
Modulation scheme is unimportant.
AD modulation has NO common mode ripple voltage and "common mode voltage is what radiated off cables".

BD modulation has ripple voltage in phase on both output wires, and at non zero modulation index the radiated frequency is doubled in compare to Fsw. So, having 500kHz Fsw, in BD modulation we will effectively radiate 1MHz stuff using long output cables.
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Old 25th May 2010, 10:18 AM   #29
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Quote:
Originally Posted by 81bas View Post
AD modulation has NO common mode ripple voltage and "common mode voltage is what radiated off cables".

BD modulation has ripple voltage in phase on both output wires, and at non zero modulation index the radiated frequency is doubled in compare to Fsw. So, having 500kHz Fsw, in BD modulation we will effectively radiate 1MHz stuff using long output cables.
A separate common mode coil is used by Ecler to reduce the effect of common-mode ripple floating on the outputs.


Floating rails and grounded bridge is another way of solving this inherent problem but it has its own disadvantages also.
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Old 25th May 2010, 10:53 AM   #30
Pafi is offline Pafi  Hungary
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Quote:
AD modulation has NO common mode ripple voltage and "common mode voltage is what radiated off cables".
I guess you mean: balanced (bridged) output has no common mode ripple. Yes. But 1 or 2 volts at 250...400 kHz is piece of cake. The switching transients are the real enemy! The frequency of common mode output ripple is the switching frequency (unless you use BCA topology). And - as Kanwar mentioned - it can be filtered out easily.

AD modulation in half bridge has the same common mode ripple, so I don't understand why would this be an issue.

Last edited by Pafi; 25th May 2010 at 11:03 AM.
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