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Class D Switching Power Amplifiers and Power D/A conversion

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Old 1st June 2005, 12:53 AM   #131
Kenshin is offline Kenshin  China
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ONsemi's NTP3055 and NTP2955 are N channel and P channel MOSFETs optimized for bridge use. They are immune to (so-called) reverse recovery dv/dt problems, and have betted RDS(on) matching than 540/9540. The price is slightly higher than 540/9540 in China (3.5 Yuan v.s. 2.5 Yuan).

Have any person tried that in a class D ?
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Old 1st June 2005, 11:28 AM   #132
sovadk is offline sovadk  Denmark
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NTP2955 is only rated to Vbr=60V, which should never be reached. Also the Ron i high. Cg is low however, which gives faster switching = lees distortion and less f*C loss.
NTP3055 I can't find.
Anywhy why do you want to match the fets and what do you mean by that?
At a given switching frequency the trade off between Ron and Cg is not the same for P-ch and N-ch Fets. Matching for equal loss in both transistors is difficult. P-ch Fets have 3 times worse carateristics than N-ch. Aming for a fully N-ch design is best. Modern N-fets like FDD3682 and IRF6665 perform outstanding.
I've always thought that it would be impossible to design a fast descrete high side N-ch driver and make it work in a Half-bridge, but it turns out that it can be done. Actually it's not tant difficult.

Fuji also knows how to make good semiconductors: 2SK3598-01 100Vbr 20A 22nC 62mOhm. Also their SMD TFP package is very nice.
TPCA8006-H (100Vbr 67mOhm 12nC) or TPCA8007-H (100V 47mOhm 15nC) from Toshiba are also a wise choice.
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Old 1st June 2005, 11:54 AM   #133
bcarso is offline bcarso  United States
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IMO the tricky part is having the motion of the upper FET "aid" the turnon/turnoff rather than fight it.

Apparently there are some fairly significant patents on some of the schemes for high side drive, embodied in integrated driver parts.

It is appealing though to do the drive in discrete form if the spurious coupling can be kept under control, since it should be possible to reduce jitter, noise, and signal self-heating shifts and thereby reduce open-loop noise and distortion.

I built a tight-layout discrete driver, but using medium-sized leaded parts like TO92 case transistors it became clear that unwanted couplings just couldn't be adequately controlled. The project was about to migrate to an SMD carefully thought-out layout when I was diverted to other activities.
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Old 1st June 2005, 12:28 PM   #134
sovadk is offline sovadk  Denmark
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Can you post a schematic of you driver stage. I would very much like to see how you have done it.
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Old 1st June 2005, 01:17 PM   #135
bcarso is offline bcarso  United States
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I would like to. Unfortunately the schematics were left behind at my last consulting client's offices and I don't think I preserved a copy. I ended up deciding it wasn't the best way to go anyway, and started in on another approach. If time permits I will pursue it and post.
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Old 5th June 2005, 04:11 PM   #136
rinox is offline rinox  Italy
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Very good project... but i'm thinking.... may be is better use two N devices instead of complementary power devices... I think it should solve a lot of problems.
Another way.... we know that there are a lot of diff in capacitance between the same complimentary couple, so why don' t try to use a smaller P channel device like 9520 that have smallest capacitance?
This is only an idea, I think to remember I saw this solution in some commercial amps, but I' m not sure... I will try to make some searches.
By the way...... B R A V O (good boy) !!!!!
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Old 25th November 2005, 10:13 PM   #137
GK00 is offline GK00  United States
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Hello, I have recently recieved a project in which I have to build a fully discrete class D amp. Here are the specs that I need:

50 watts into an 8 ohm load
Input impedance to be 50 Kohms
Frequency response - DC -- 100 Khz +- 2 db
Output impedance such that the damping factor is 100 or more.
1 volt into the input will give full output


I'm thinking the schematic mentioned earlier would be a good place to start but if anyone knows if it can do those things, please let me know. I won't have access to PSpice until monday so I can't test it yet.

Also any tips on how to proceed would be greatly appriciated.
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Old 25th November 2005, 10:27 PM   #138
bcarso is offline bcarso  United States
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That signal bandwidth is a tall order unless you use something like Bruno Putzey's UcD approach, where the output filter is an intrinsic part of the functionality of the modulator.

I don't have time to go into detail but maybe someone else can point you to some more material about it.
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Old 26th November 2005, 10:32 PM   #139
sovadk is offline sovadk  Denmark
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It's not a problem to reduce the order of the outputfilter (which i 2 for a LC filter). In UCD it's done by adding a differentiated version of the outputfilter back to the summing point (done with the feedback capasitor) and making the filter a part of the modulator. UCD has a 1. order responce.

You can take contol of the LC filter and make it load independent by makin PD feedback.
If you want to reduce the order of the filter by one, you can do what I've done in the schematic below.
I haven't added any values, but the concept is right. This dosn't only apply to switching amplifiers.
Click the image to open in full size.
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