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Old 8th June 2009, 02:10 PM   #1
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Default UM10155 from NXP

Hello!

I'm an electronics student, and I'm currently trying to learn about switching amplifiers, namely the phase-shift controlled self-oscillation amplifier, UM10155.

I tried to mathematically quantify the variation of switching frequency along different input values, but as it seems to me, I would need to consider different harmonics, affected differently by the feedback branch.

Is there any way to quantify it?


Second, what's the better way to decrease out-of-band (>20kHz) noise? Increase zero-input sampling frequency?


How should the frequency change with the input? Is it good to let the sampling frequency change a lot? From delta-sigma modulation, it seems crucial, but what about UcD?


Finally, I would like to know why does UcD uses an oversampling rate as low as 20? Why doesn't it work at higher rates?


Best Regards,

Manuel Duarte
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Old 9th June 2009, 06:59 AM   #2
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It would be very useful for you to get a SPICE simulation program to help you understand the workings of a UCD-style amplifier. As I'm sure you realise, the various elements in the feedback loop interact with each other in complex ways, and being able to see of effects of component value changes in SPICE will prove to be informative.

If you have got good quality inductors and low-inductance capacitors in the output filter, then you should not have any out-of band noise problems. Good pcb layout helps here of course.

Becuase the PWM mark-to-space ration produced by the self-oscillating modulator in a UCD design is not clocked, it doesn't suffer from the quantisation errors that a clocked design running at a low frequency would have. You can make a UCD amp run at 1MHz if you really want to, but don't expect the audio performance to be any better than one running at 350kHz.
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Old 17th June 2009, 04:03 AM   #3
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Thank you.

What kind of output inductors should be used in this type of amplifier? toroidal or selenoidal?


I have it's spice model to play with, but making the math first, of course
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Old 17th June 2009, 06:53 AM   #4
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It is advisable to use output inductors with a closed magnetic path. I have had good results from toroidal output inductors using the precision-gapped ferrite toroids from Ferroxcube, but these cores are difficult to obtain. I know that Eva has used dust-iron toroids with good results, but you do need to use toroids intended for operation at the 300kHz or so that your amplifier is likely to be working at.

For the amplifier I designed as my current project for my employer, I use RM cores in 3F3 material which work very well and have very low flux leakage if you use the gapped ones. (The gap is on the internal leg). It is difficult to buy gapped ones in small quantities, but for the first prototypes I used ungapped cores with small pieces of an old credit card cut to provide the spacer to gap the two halves. (These were cores in N87 material bought from RS components in the UK). Gapping the cores in this way (to give an AL of approximately 160nH) does increase the external flux due to the leakage from the gaps in the outside legs, but it is ok for building a prototype.

I recommend that you download the Ferroxcube data sheet called "Soft Ferrites and Accessories" which has useful nomographs to help you use the correct size of core for the inductance and current that you will be using in the amplifier.
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Old 19th June 2009, 02:54 PM   #5
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Quote:
Originally posted by monaural
Thank you.

What kind of output inductors should be used in this type of amplifier? toroidal or selenoidal?


I have it's spice model to play with, but making the math first, of course
Hi Duarte,

I use toroidal 'micrometals T130-2' or T106-2, they have very good characteristics for class d, you can buy this cores cheap on ebay.

micrometals Inductor Design Software:

http://www.micrometals.com/downloads...Jan12_2008.exe
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Old 19th June 2009, 03:32 PM   #6
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Well, I was definitely not thinking about assembling the inductor myself, but buying one, insted. Essentially because a lot of parameters may change, and I prefer to use an inductor whose parameters are fairly known.

I found a power-inductor (SER2918H-223KL) from coilcraft, which has very good specs: power efficient (2.9mOhm), and it's wideband (SRF=15MHz)

http://www.coilcraft.com/ser2900.cfm
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Old 19th June 2009, 03:42 PM   #7
smms73 is offline smms73  Portugal
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picture of the T130-2 inductance versus current.

Click the image to open in full size.

picture of the T130-26 inductance versus current.(not suitable for class d)

Click the image to open in full size.


This two cores have the same shape and size but have different core material.
The first core is suitable for class d (-2 material), the second is not, because his inductance changes with current.
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Old 19th June 2009, 04:08 PM   #8
smms73 is offline smms73  Portugal
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Quote:
Originally posted by monaural
Well, I was definitely not thinking about assembling the inductor myself, but buying one, insted. Essentially because a lot of parameters may change, and I prefer to use an inductor whose parameters are fairly known.

I found a power-inductor (SER2918H-223KL) from coilcraft, which has very good specs: power efficient (2.9mOhm), and it's wideband (SRF=15MHz)

http://www.coilcraft.com/ser2900.cfm
It seems a good choice, very linear until 10amps. Where do you buy the coil and how much?

The parameters for core materials dont change more that the commercial coils, and with the software from micrometal is very simple to design your one coil with the characteristics that you need.
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Old 2nd November 2009, 02:40 AM   #9
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Quote:
Originally Posted by monaural View Post
I'm an electronics student, and I'm currently trying to learn about switching amplifiers, namely the phase-shift controlled self-oscillation amplifier, UM10155.

I tried to mathematically quantify the variation of switching frequency along different input values, but as it seems to me, I would need to consider different harmonics, affected differently by the feedback branch.

Is there any way to quantify it?
Hi monaural,

Did you get anywhere with your modelling? I've been trying to get my head around it and fail every time In fact, why does the switching frequency change at all? Isn't it set by the phase lag of the output filter, the comparator propagation delay and the phase lead of the feedback network, none of which vary with signal amplitude?
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Old 2nd November 2009, 06:38 PM   #10
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Quote:
Originally Posted by smms73 View Post
It seems a good choice, very linear until 10amps. Where do you buy the coil and how much?

The parameters for core materials dont change more that the commercial coils, and with the software from micrometal is very simple to design your one coil with the characteristics that you need.
What i meant was that I have no way to measure a DIY coil parameters.
Anyway, I bought those from coilcraft.


Quote:
Originally Posted by HarryDymond
Did you get anywhere with your modelling? I've been trying to get my head around it and fail every time In fact, why does the switching frequency change at all? Isn't it set by the phase lag of the output filter, the comparator propagation delay and the phase lead of the feedback network, none of which vary with signal amplitude?
Actually I did arrive at some conclusions.

First of all, the system is non-linear, so, obviously, Barkhausen criterion can't be used.

Second: oscillation frequency depends on output signal (so, therefore, on input signal)

Third: I couldn't get any analytical solution to express oscillation frequency in terms of the input signal (it's trivial to derive a numerical solution to the problem, but its also useless, at least for me)

If i find something, i will post here.
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