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Author Topic: Beginner class D design  (Read 2052 times)

Offline OgreVorbis

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Beginner class D design
« on: November 12, 2018, 0104 UTC »
As I am building the transmitter discussed in this thread, I decided to document the progress on my blog. If you're looking to build your own class D, check it out. There is a massive archive of PDFs and schematics in there as well as an in depth explanation. I am not as experienced as some here, but I always update any inaccuracies. I also approach the topic from a novice's perspective.

http://dosaidsoft.com/wp/category/radio/class-d/

Original message:

I am trying to gain a better understanding of a class D transmitter, and while I do understand most of the concepts, I need a little clarification.

So I am probably going to start with this design: http://www.w1vd.com/40M375WclassDRev2.0.pdf
Yes, I know it's old. And I think there might be some parts left out. It looks like there is an unlabeled resistor at the gate of each FET.

First thing I am not clear on is if the exciter needs to be biphasic in order to make a class D or E amp. You can see in the schematic I linked that it has phase 1 and 2 input and it says 12V, but how many amps does it need (on the input)? Can I build half of the amp and use a normal single output DDS module? I am not sure how to make a two phase exciter. I think class D is more broadband, so I want to go with that. I'm still not even exactly sure what the difference is between D and E despite them being described so often. I never find a clear difference.

So when I have the amp and exciter built, I think I'll start trying to drive it with a mod transformer instead of PWM just to start because it's easier. So how many ohms do I need to match right at the drain of the FETs? Is it 50? No, probably not because that's after the matching right? So if I use a generic audio amp, what transformer ratio do I need? I think I'll use a microwave oven transformer. I have one that has all coils removed, so it's just a bare core I can wrap around.

If I need a two phase exciter. Where can I find one? I do know about the classeradio.com guy, but he only sells the PWM and not the exciter. He didn't respond anyway. Probably because I'm not ham.

And, yes, Stretchyman, I know you're here too. Your transmitter is tempting, but I don't want to just buy it without understanding how it works because that gets annoying in the long run.

Would you mind explaining a little to me? Mostly on how your DDS module works (I assume you use DDS). Is it a two phase thing or is it essentially just a 9850 and a PIC on a board - like what you can get on eBay? I would really like to see the schematics and also if you can clarify how the exciter module works. If you'd like a small payment for your schematics, I'd be willing. I'd just buy the whole thing if you can tell me how to change it for more bandwidth.

If I was building it myself I'd have used Class D for more bandwidth, so it's not perfect from my standpoint. What about your amp makes it Class E and could it be changed to D? I've seen a class D 100W amp that was using two SiC devices and it works 3-8 MHz. What makes yours only 200 Khz range?

I know that's a lot of questions and I am being very critical of the design. I know it is better than most others out there given that it's using the newest GaN and drivers. I hope you understand I'm am trying to learn w/o just buying stuff and wiring it up. That's too easy for this hobby and it makes it boring.
« Last Edit: February 25, 2019, 1842 UTC by OgreVorbis »

Offline redhat

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Re: Beginner class D design
« Reply #1 on: November 12, 2018, 0200 UTC »
First off, Class D and D/E or E are not the same.  Class D/E or E amps are inherently narrow band affairs, because they operate correctly only over a small range of frequencies, usually less than 5%.  Class D amplifiers operate over a wider range, but have a highly distorted output that must be filtered before being radiated.  Class D amplifiers also become much less efficient and harder to drive beyond 3 MHz, which is why most designs beyond 80 meters are Class E or similar.

As to the W1VD stuff, his designs are similar to Class E.  Note the resonant tank between the two halves of the amplifier.  This tank network is responsible for the narrow band characteristic of the amplifier, but also allows it to output a much cleaner spectrum than a class D amplifier.  The unmarked 'resistors' in the schematic between the gates and the fet drivers are actually striplines.  I've found that driving the fets through a low ohm resistor, usually around 3.3 ohms, dampens the gate network and reduces overshoots.  The drive current required will depend on what fets you want to use, and also what drivers.  There are a lot of variables here, but at 40 meters, plan on 400mA per driver/fet combination when driving a fet with 1000pF input capacity with 15V.  In practice this is low, 18-20V is more common with SiC devices, but with regular mosfets 12V is fine.  I use IXDD614 drivers with C2M0160120D SiC's and seem to be getting around 93% efficiency when everything is happy, about 87% DC to RF efficiency with the PWM modulator.  I also got rid of the 4:1 output balun in W1VD's design and chose to use a simple 1:1 ac coupled balun on the output to keep the current levels more reasonable, at the expense of higher required B+ and drain voltage ratings.  I also borrowed the phase splitter circuit from a Nautel NX series transmitter to derive the push pull drive from a single ended source.

Are you sure you wouldn't rather just buy something to get started?

+-RH
Somewhere under the stars...
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Offline Stretchyman

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Re: Beginner class D design
« Reply #2 on: November 12, 2018, 0746 UTC »
Good to see the interest....

Would you mind explaining a little to me? Mostly on how your DDS module works (I assume you use DDS). Is it a two phase thing or is it essentially just a 9850 and a PIC on a board

Yes, just a PIC driving an AD9850 using the I/Q O/Ps driving SOIC FET drivers driving GaN FETs


if you can tell me how to change it for more bandwidth.

Er, no... it's class E and therefore narrow banded, 200KHz is loads anyway!


What about your amp makes it Class E and could it be changed to D? I've seen a class D 100W amp that was using two SiC devices and it works 3-8 MHz. What makes yours only 200 Khz range?

Same as above...Please point us towards the design you mention......


BTW, stay away from using some rewound transformer, PWM is simple, I can send you a PCB, 1 inch square, will mod ANY voltage and upto 10A, you'll just need to make a PWM filter, 2 caps and 2 toroids.

Simples....!

Str.

p.s. I'm not selling or offering the schematics, the design is simple as and the same as ALL the other class E designs out there, I just use modern devices, not those crappy 11N90's they keep wibbling on about, they'd rather use valves anyway, bless 'em!
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Offline CoolAM Radio

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Re: Beginner class D design
« Reply #3 on: November 12, 2018, 1801 UTC »
For anyone interested - Information/Schematic Diagrams & more!
posted on my "archive.org" pages!

View Online-/ or Download

https://archive.org/download/Transmitters-Antennas/TX%20TRANSMITTERS/


André
CoolAM Radio - Shortwave
http://jingleproductions.coolam.nl
   Transmitters Do It On Air!!!
::::::::::::::::::::::::::::::::::::::

Offline OgreVorbis

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Re: Beginner class D design
« Reply #4 on: November 14, 2018, 1737 UTC »
Thanks for your responses.

OK, so I've made some progress reading some more Class D/E schematics I could find out there. I came to find that the FAT5 from shortwaveradio.co.uk has a huge PDF with a ton of info. None of the other sites/documents I could find were nearly as thorough.

So I've compiled some info and made a schematic of my own from the bits I could find.
It is linked at the bottom. Please let me know if you see any errors. Some things I'm not clear on:

The capacitors on the drains - are they correct?
Not sure why they used 8V regulators, wouldn't 12V be better and more standard? The datasheet on these drivers also shows a slight improvement in switching speed with more volts.
I want to keep it simple to test it, so I want to try and use the old fashioned mod transformer and audio amp. I should probably know this, but: What turns ratio would I need?

https://drive.google.com/open?id=1mbt60zDB50rt65chP51dezb1KN28foh5

Offline redhat

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Re: Beginner class D design
« Reply #5 on: November 14, 2018, 1804 UTC »
The topology to me seems odd, but then again I've been stuck in my own little world the last few years.  How much power are you aiming for?  One of the problems with class D is the ridiculous supply voltages required to get any kind of decent power.  This is where (in my mind) CMCD/class E shines; the peak voltage across the drains is Pi times supply volts.  To get the same voltage swing requires a 16:1 output transformer, or 4x more supply.  I need around 79Vdc to achieve ~550W carrier.  To achieve the same thing with class D you will need 250Vdc.  If your output transformer is wound 1:16, only 62V is required.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Stretchyman

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Re: Beginner class D design
« Reply #6 on: November 14, 2018, 1946 UTC »
RH is right. It will however 'work'.

So build it and let us know how you get on.

Str.
'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


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Offline OgreVorbis

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Re: Beginner class D design
« Reply #7 on: November 17, 2018, 1259 UTC »
So I've made some progress. I decided to just make a pro PCB instead of trying to make a dead bug. The PCBs from china are cheap enough anyway.

So here is what I made. It is based on the schematic I posted earlier, but I added spaces for 4 extra FETs. Let me know what you think or if you see any errors.

I created the board in Sprint Layout which I would highly recommend. CAD software is overly complicated for these types of designs imo (and I don't know how to use it :).

I'm curious to see how well this works compared to class E. I know it won't be quite as good, but I'm more interested in bandwidth than obtaining max power from each device.

https://drive.google.com/open?id=1p_pzBloYqnlwnSdilyqJNou5V1moF8OQ

I decided to get two sets of mosfets: C2M0160120D and C2M0280120D

One has less capacitance, but lower current handling. As I understand it, the lower the capacitance, the higher the potential switching speed. I'm unsure of what is better though, more amps, or less capacitance. I find that the higher amp devices have more capacitance. So what is the higher priority? Amps could make up for the greater cap, but the less caps will make it faster. Not sure what's better.

Offline Stretchyman

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Re: Beginner class D design
« Reply #8 on: November 17, 2018, 1350 UTC »
PCB is missing a lot of holes!

Lower C devices are easier to drive but have higher on resistance.

Take your pick. The SiC devices are OK, however GaN are far superior but(5X)more $.

Below 5MHz everything is fairly easy but at 7MHz things aren't as efficient. Most of the designs will work upto 14MHz (with GaN) but losses are higher and generating good square wave drive problematic.

Good luck on your quest. However as mentioned D/E is the best method. You can cover 200-500KHz without retuning and that's enough to cover any band segment.

Str.
« Last Edit: November 17, 2018, 1530 UTC by Stretchyman »
'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


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Offline redhat

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Re: Beginner class D design
« Reply #9 on: November 17, 2018, 1644 UTC »
1 driver per fet will be required above 3 mhz in class D.  Low RDSon is required for class D as well, I would stick to the lower R fets.  I also don't like the thin and windy traced on the drains.  This should be a large plane, not a trace.  You will have a lot of ringing problems due to the stray inductance.  Areas like this I also make sure there is no copper on the back side to create stray capacitance which will load the output unnecessarily.  Same thing with the gate traces, large and fat equals lower inductance and better waveforms.

Little things like that make a BIG difference!

+-RH
« Last Edit: November 17, 2018, 1649 UTC by redhat »
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline OgreVorbis

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Re: Beginner class D design
« Reply #10 on: November 17, 2018, 1726 UTC »
1 driver per fet will be required above 3 mhz in class D.  Low RDSon is required for class D as well, I would stick to the lower R fets.  I also don't like the thin and windy traced on the drains.  This should be a large plane, not a trace.  You will have a lot of ringing problems due to the stray inductance.  Areas like this I also make sure there is no copper on the back side to create stray capacitance which will load the output unnecessarily.  Same thing with the gate traces, large and fat equals lower inductance and better waveforms.

Little things like that make a BIG difference!

+-RH

OK, that makes sense. I don't think I can fix the drain traces though. There's just not enough space on there. I think I'll attach them to a raised piece of copper clad board. I've seen this somewhere before. It shouldn't be a deal killer though, right? I mean at these frequencies, I wouldn't expect a trace like that to have any level of inductance that would cause problems. Most inductors at these frequencies use toroids, so a little loop shouldn't do much. That's just speculation though. I'm new to this and you do know more, so...

In terms of the 1 FET per driver. I did already know that, but then I saw this guy demoing his class D transmitter. It has 4 FETs and two drivers and makes 200W carrier on 40m. His 2 FET version made 100W carrier, so it doesn't seem like the drivers are limiting it. I'm also using the TC4452 and he is the lower amp TC4422.
https://www.youtube.com/watch?v=QnM2Uum-_Cs

Offline redhat

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Re: Beginner class D design
« Reply #11 on: November 17, 2018, 1735 UTC »
Something about this doesn't look right.  He's using a MW slug on 40 meters (so much for calibration).  Also, everywhere I've read said that using wire in output transformers is problematic above 80 meters due to circuit strays and coax should be used instead.

You would think a little L here and there wouldn't hurt, but it can create voltage spikes and transients that can kill your fets and other components.  From scratch its taken me about three years to get to where my TX is now, and there were a lot of little details along the way that no one seemed to publish or know.

That's one of the reasons I just can't believe that the frankstein stuff over on the classeradio site actually works.  None of this stuff is plug and play, and I have to agree with Stretchy, a lot of the secret sauce to this is the layout design.

Without seeing his design in action and looking at waveforms and such, its hard to know what is really going on.  I suspect the efficiency is nothing to write home about, and who knows what kind of reliability your going to have.

+-RH
« Last Edit: November 17, 2018, 1736 UTC by redhat »
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline OgreVorbis

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Re: Beginner class D design
« Reply #12 on: November 17, 2018, 1751 UTC »
Hearing what you know, I'd be interested to see some pictures of your design. Do you have any?

If not, then maybe you could give me a general idea of the layout. Are you using PCBs? How do you make connections (other than short as possible)? etc. Or maybe just some tips or things that you did wrong during the process.

I'd really appreciate it, but I understand if you want to keep it a secret.

Offline redhat

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Re: Beginner class D design
« Reply #13 on: November 17, 2018, 1920 UTC »
I have no problem releasing schemo's and pics, but this probably isn't the best venue for it, for reasons of liability.

Shoot me an email, and I'll show you what I'm working on.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Stretchyman

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Re: Beginner class D design
« Reply #14 on: November 17, 2018, 2003 UTC »
Only took me two years!

I had a lot of other designs to copy and a PCB layout to prove. That took a few goes too!

Like the man said, big fat traces.

Str.

p.s. ping me a mail toooo. Can show you the layout etc...
« Last Edit: November 17, 2018, 2016 UTC by Stretchyman »
'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


                                              ;)