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Old 9th January 2005, 06:16 PM   #11
Pabo is offline Pabo  Sweden
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Kenshin

The total system bandwidth is allways kept at about 80-100kHz. The reason for creating a -1 slope in the output filter is that the phase margin allows for more feedback. Actually, TIM distorsion is less likely to occur if the output filter is faster.
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Old 10th January 2005, 06:27 AM   #12
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Kenshin is basically right.
But keep in mind that not every conventional amp's power bandwitdth is as high as it's small signal bandwidth. If you don't overdo it then there are not much problems to be expected. Bruno does this as well witch his UcD. The UcD 180 has an output filter with a cutoff-frequency of 30 kHz approx and the closed-loop bandwidth is 50 kHz.
This comes at the risk of of increased susceptibility to TIM but has the big advantage of getting a more or less load independant frequency response, because the NFB is now dominant.
If you want the best of both worlds then use a lowpass in front of your amp and make the whole combination behave like a bessel LPF.

If you add an LPF to the feedback just for getting rid of some RF hash then it would not touch touch the ICE patent as long as this filter's cutoff frequency is higher than the carrier frequency. It all depends on what you want to do.

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Old 10th January 2005, 07:00 AM   #13
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NFB from total output is a good idea,if no TIM--it's good for ballasting the resonance and reducing nonlinear distortion of the output LPF.

how could a -1 slope in the output filter decline the switching ripple sufficiently?
at 300KHz switching frequency,the amplitude of the open loop gain should be 1. Hence at 30KHz frequency,it will be 10. So it's hard to introduce an deep NFB. Another problem is that a hyerstis
modulator running at high ripple level will produce more nonlinear distortion from the RC integrator.
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Old 10th January 2005, 07:20 AM   #14
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The trick is to get a sifficient phase-marging AND still have an output filter of at least 2nd order AT ONCE !
That's what Pabo wants to achieveand that's also feasible with clever circuit topologies.

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Old 10th January 2005, 12:25 PM   #15
Kenshin is offline Kenshin  China
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The output filter is of order 2--this is OK.

but the feedback circuit- if add some positive phase shift in the NFB loop to get bigger phase margin,the ripple in feedback will also rise.
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Old 10th January 2005, 12:25 PM   #16
Kenshin is offline Kenshin  China
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ICE patent contains two switches operating at different phase.

would the design of creating positive phase shift in the feedback loop touch the ICE patent?
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Old 10th January 2005, 01:35 PM   #17
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Quote:
but the feedback circuit- if add some positive phase shift in the NFB loop to get bigger phase margin,the ripple in feedback will also rise.
Yes, it will be of the same height as if there was only a 1st order transfer function in total. This is the same as if feedback is used from the output stage directly and fed into a 1st order integrator.

Because the feedback thingie is not restricted to polyphase PWM in B&O's patent it does cover almost any PWM topology where the phase-lead is generated by a lowpass around an inner feedback loop.

It does however not cover any other topology that generates a phase-lead.

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Old 10th January 2005, 03:16 PM   #18
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Quote:
Originally posted by phase_accurate


Yes, it will be of the same height as if there was only a 1st order transfer function in total. This is the same as if feedback is used from the output stage directly and fed into a 1st order integrator.

Because the feedback thingie is not restricted to polyphase PWM in B&O's patent it does cover almost any PWM topology where the phase-lead is generated by a lowpass around an inner feedback loop.

It does however not cover any other topology that generates a phase-lead.

Regards

Charles
Would the UCD patent cover the other method?

Seems to be very little one can do without violating someones patent... kind of silly isn't it, as soon as you compare a sine to a triangle you've lost, yet, few of them actually invented anything.. at least there's free reign on negative feedback huh

Pabo I wasn't really sugesting you duplicate mueta, but they have a few techniques for measuring the current, if I remember right, their patent covers them all. I certainly agree with you, too many op amps!
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Old 10th January 2005, 03:44 PM   #19
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as soon as you compare a sine to a triangle you've lost,
I don't think so since the oldest patents of class da amps are already more than 25 years old and therefore obsolete !
And compensating for a pole as such is basic control theory, and might be successfully fought against unless the method is really elegant and new (like the aformentioned detail of ICE or the UcD patent).

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Charles
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Old 10th January 2005, 07:09 PM   #20
Pabo is offline Pabo  Sweden
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Quote:
ICE patent contains two switches operating at different phase.

would the design of creating positive phase shift in the feedback loop touch the ICE patent?
Kenshin

No, applying a zero in the feedback network is not included in this patent.
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