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Author Topic: Beginner class D design  (Read 2053 times)

Offline OgreVorbis

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Re: Beginner class D design
« Reply #60 on: February 28, 2019, 1451 UTC »
Using the NCP drivers which are super fast BTW, I've never bothered with any dead time adjustment and driven the gates directly from both O/P pins, DC coupled with NO RES.

Mind you I have been using the GaN fets, again super fast, but previously used SiC and they were fine only having to use 15V rather than 6V as the driving Vcc.

Again I'd point you toward the article on AMFONE, there's even better FET drivers now with an RF isolated barrier (what next!) but the NCP jobbies are fine.

Str.

I looked at that amfone link, but it lead me to an image. I managed to get the thread from the URL, but I didn't find anything in the thread discussing a better driver than the NCP. What is the name of it?


On my new board I need to make sure that I have enough space for my output transformer. Can I use only 4 cores for 8 fets? Would I just need to alter the windings to compensate or will it overheat or not match? If not, I asked before, but can I stack the cores on top of each other? This should be the last question I have before I can begin designing. It will really change my layout depending on the answer to this.

Offline Stretchyman

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Re: Beginner class D design
« Reply #61 on: February 28, 2019, 1708 UTC »
Can't find the exact post but look for, VE3ELQ.

The NCP A ones are fine anyway.

Regards stacking the cores I gave up with the binoculars and stack 2 T200-2's, they be fine.

Str.
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Offline redhat

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Re: Beginner class D design
« Reply #62 on: March 01, 2019, 0411 UTC »
I would leave a small amount of R in series with the gates for two reasons;

1. it helps dampen any tendency of the network and other strays to ring (Better drive waveforms).

2. should a transistor failure occur, it prevents the output of the driver from going complete short circuit.  I have lost a few drivers due to transistor failure,  No more once a little R was added.  I'm using three 10 ohm 1206 resistor in parallel to make a wider low L resistor, and also spread the dissipation.

+-RH
« Last Edit: March 02, 2019, 2028 UTC by redhat »
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Offline Stretchyman

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Re: Beginner class D design
« Reply #63 on: March 01, 2019, 0728 UTC »
Leaving space on a PCB is no bad thing!

I must admit I've lost 1 or 2 drivers in the past when thing went a bit west and the O/P's went S/C.


BTW I've tried to find any mention of the alternative drivers but it would appear that the post has been removed.

Stick to the NCP's.

Do read up on ALL the post on AMFONE concentrating on the posts by Nigel.

Personally I'd stick to the toroidal O/P tran and make sure the windings are opposed as shown in the design on there.

By careful choice of series L/C components (low Q?) you end up with a kinda class D/E thing where the efficiency is around 93-95% and the B/W around 1MHz.

A good compromise I think....

Str.
« Last Edit: March 01, 2019, 0806 UTC by Stretchyman »
'It's better to give than receive' so why RX when you can TX!

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Offline OgreVorbis

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Re: Beginner class D design
« Reply #64 on: March 07, 2019, 1703 UTC »
I just had a thought. Is there a reason why there is always PWM to the drain of the mosfets? Why not apply PWM into the drivers instead and eliminate a separate PWM board. Why is it not done this way?

Offline redhat

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Re: Beginner class D design
« Reply #65 on: March 07, 2019, 1742 UTC »
Per my previous (two posts back) reply, device limitations in regard to switching speed is the main reason.  To get 99% modulation depth, the switching devices would have to be capable of switching at 100X carrier frequency, hence at 7 MHz, the devices would have to produce something resembling a square wave at 700 MHz.

There are ways around this problem.  Harris series combines a bunch (48?) PA modules, arranged in binary order, big steps, and little steps.  Their drive signals are switched on and off according to how much instantaneous power is required for the envelope power as dictated by the incoming audio.  Nautel at this point is conventional PWM with DSP based AM/AM and AM/PM correction, similar to the smaller DAX series transmitters from Harris/Gates/Who ever they are this week.

My attitude is, if it were possible and financially viable, it would already be in a commercial product.

+-RH
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Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com