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Author Topic: Beginner class D design  (Read 5326 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.
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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?
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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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Offline OgreVorbis

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Re: Beginner class D design
« Reply #66 on: March 25, 2019, 1846 UTC »
I'm on my way with the updated design and I have a few questions.

I have this audio amplifier: https://www.parts-express.com/wondom-aa-ab31242-1x600w-class-d-audio-amplifier-board-(t-amp-technology)--320-3346

It is a class D design and I was originally going to use it with a transformer, but I think I may be able to use it directly. What do you think?

Here is the datasheet for the IC it is using: https://www.ti.com/lit/ds/symlink/tas5630.pdf

So I have to check if the ground is isolated, right?
Then I need to apply whatever voltage I need for the carrier at the ground?

Is this how it should be done?

My other question is about the inverter circuit. I don't have too much experience with logic circuits, but I know I probably need one of these: SN74HC132N, SN74AHCT14N, or CD74HCU04E
Which should I use? I am going to use it along with a crystal oscillator to generate the inverted wave.
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Offline redhat

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Re: Beginner class D design
« Reply #67 on: March 26, 2019, 1244 UTC »
The amp may work direct without a transformer if the protection circuitry is primitive enough to ignore DC imbalance on the driver pairs, something that unfortunately can only be found through trial and error.  Output power will be square law proportionate to input voltage, so I would be sure that the limitation input voltage of the modulator (40V) is enough to achieve the PEP you are aiming for.

To avoid any possibility of cross conduction (caused by logic gate propagation delays), I would use a 74LS86 configured as a push pull converter to get your drive.

Stretchy, where are these links you keep talking about?

+-RH
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Offline Josh

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Re: Beginner class D design
« Reply #68 on: March 26, 2019, 1813 UTC »
I love the simplicity of rc coupling compared to buffers or xformers and so on, but sometimes the added complexity of xformers and buffers expiates many sins in the most effective if not efficient manner.
Conveniently located near Vincennes Indiana.

Offline Stretchyman

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Re: Beginner class D design
« Reply #69 on: March 27, 2019, 0717 UTC »
The links have gone but the original design work is by VE3ELQ, so just check for posts by him.

Modulator wise I tried one of those Amps and drove a MT , if caught fire!

Not sure what you're up to with the logic inverter?

Pray tell?

Str.

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Offline redhat

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Re: Beginner class D design
« Reply #70 on: March 27, 2019, 1424 UTC »


+-RH
Somewhere under the stars...
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Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Transmitter Man

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Re: Beginner class D design
« Reply #71 on: April 13, 2019, 1332 UTC »
OV,

Here is Nigel's (VE3ELQ) response to my question re driver chips;

"The best RF deck FET drivers I have tried by a wide margin are the NCP81074A.  With a Tr/Tf of 4 ns, matched delays, and 10A of source/sink they are both super fast and capable of driving higher gate C FETs. No problem with the 150pf SIC FETs at 7.3 mhz or the newer GaN FETs.  They are inexpensive and small so I recommend 1 driver per FET up nice and close to keep the gate lead as short as practicable. They work great.

https://www.onsemi.com/pub/Collateral/NCP81074-D.PDF"

Do you have any spare boards to sell although I believe you'll need to re-do them should you decide to use the above drivers. So cheap now I'd gladly contribute to any updated board.

I do like the look of your 8 device boards.

Transmitter Man


Offline Stretchyman

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Re: Beginner class D design
« Reply #72 on: April 14, 2019, 1548 UTC »
Yep, thems the ones! They drive the GaN FETs with ease and are OK (get a bit warm) for the SiC devices. Makes for a greatly simplified design.

Cardinal CPP Osc - NCP Driver- FET O/P- LPF.
'It's better to give than receive' so why RX when you can TX!

                            Buy one from me, NOW!

Great discounts on ALL my transmitters if purchased via HFUnderground


                                              ;)

Offline OgreVorbis

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Re: Beginner class D design
« Reply #73 on: April 14, 2019, 2347 UTC »
OV,

Here is Nigel's (VE3ELQ) response to my question re driver chips;

"The best RF deck FET drivers I have tried by a wide margin are the NCP81074A.  With a Tr/Tf of 4 ns, matched delays, and 10A of source/sink they are both super fast and capable of driving higher gate C FETs. No problem with the 150pf SIC FETs at 7.3 mhz or the newer GaN FETs.  They are inexpensive and small so I recommend 1 driver per FET up nice and close to keep the gate lead as short as practicable. They work great.

https://www.onsemi.com/pub/Collateral/NCP81074-D.PDF"

Do you have any spare boards to sell although I believe you'll need to re-do them should you decide to use the above drivers. So cheap now I'd gladly contribute to any updated board.

I do like the look of your 8 device boards.

Transmitter Man

My new updated board should be here for experimentation in a few weeks.
RH: I tried that logic circuit that you posted. I'm assuming this is to prevent cross conduction. Well, I tried it with the 74LS86 and a 74HC04E (didn't have the 7406) and it doesn't seem any different than my simple circuit with 74AHCT14N, so I stuck with that for this design. It does cross for a couple ns at a really low level, so I think it's OK.

« Last Edit: April 14, 2019, 2352 UTC by OgreVorbis »
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Offline OgreVorbis

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Re: Beginner class D design
« Reply #74 on: April 14, 2019, 2359 UTC »
Do you have any spare boards to sell although I believe you'll need to re-do them should you decide to use the above drivers. So cheap now I'd gladly contribute to any updated board.

I forgot to answer the question. Yes, I do have 4 more of the old PCBs left if you want one. I don't have time to sell a parts kit right now though. Maybe when I finish development and I'm at a good stopping point.
I've been looking to sell them anyway. They work perfectly up to 5 MHz, but no higher. This new design you see above should do 10 MHz easily, I hope. . . I can also easily swap the MOSFET pins to make it work with GaN devices in the same package. If it works, then I will also sell a GaN version of the board. The cheapest GaN to work with it costs about $20 each and the SiC are only $3.50.
« Last Edit: April 15, 2019, 1043 UTC by OgreVorbis »
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