Beginner class D design

[TheRelayStation]

Refering to the title of the post I don't see what relevance repurposing a ham radio has at all?

the relevance is about getting on the air and broadcasting, re-read my post.

Time and patience, Yes, plugging in a ham radio, anyone can do that.

again, you need to re-read my post, there is a bit more involved than that.

We're talking about design, which you make clear you have absolutely no idea of.

repairing, re-purposing and modifying existing equipment requires the thought of "design", does it not ?

[TheRelayStation]

Another great point then?

I think not.

Str.

in my opinion, it seems most people are against the idea of modifying existing equipment for the purposes of high fidelity AM broadcasting, like it is some sort of "sin" to discuss it.
i dont really see how it should be treated any differently, if you dont possess the knowledge to build one from scratch, how can you understand how to modify and re-purpose it then ?

[redhat]

Difference of philosophy I suppose, but I wouldn't call it a 'sin'.  Modify to your hearts content, do what makes sense to you.  I would rather start with something that is designed for full duty cycle (my FT857 sure isn't, heatsink is WAY too small).  Most ham stuff Is designed with a price point in mind, as is all commercially available stuff.  To that extent, a lot of corners are cut, and I do agree the older gear had a lot more meat to it than the modern flimsy electronics we know as ham gear.  The heatsink alone in my prototype transmitter weighs more than most solid state ham transceivers I've seen.

+-RH

[Stretchyman]

Thanks RH, sanity restored, somewhat!

Lets stick to Class D design, shall we...?

Str.

[OgreVorbis]

It doesn't make sense to try to modify something that has an antiquated and overly complicated design. Using the modern technologies of DDS and class E or D doesn't seem to be a part of commercial ham radios. Maybe I'm wrong because I don't own any ham gear, but that's what it looks like to me.

That aside, the PCBs for my project have shipped and the FETs are already here. I will keep this thread updated with the progress when I begin the build.

I also have two new questions:

1. I tested a modulation transformer on a small class D (not the one I'm building). I've encountered a problem with the transformer. It is a 1:1 115V toroidal transformer. I don't know the VA, but it weighs about 15 kgs. I've read of using toroidal transformers as modulation transformers before and it looks like there are hams doing this and that it works well. Now we get to the problem: How do they not overload the audio amplifier when the ohms drops so low at the low end of the audio spectrum? If I look at the resistance at 100 Hz, it is 1 ohm and at 1 KHz, it is 6 ohm. So how can I get any bass into my signal without overloading the audio amplifier. Do I just need to filter the bass a certain amount?

2. Can I stack ferrite cores on the output transformer? For example, if I have 8 FETs, can I have four cores on the bottom and four on the top and then run the wire through all of them similar to if they were lined up? In the designs I've looked at, it seems like there is always one core per FET. Is this for power handling reasons or is it because the matching needs to change for each additional FET? (ie could I get away with four cores on an 8 FET amp, given that the power level is not too high or is it for matching?)

[redhat]

1. I tested a modulation transformer on a small class D (not the one I'm building). I've encountered a problem with the transformer. It is a 1:1 115V toroidal transformer. I don't know the VA, but it weighs about 15 kgs. I've read of using toroidal transformers as modulation transformers before and it looks like there are hams doing this and that it works well. Now we get to the problem: How do they not overload the audio amplifier when the ohms drops so low at the low end of the audio spectrum? If I look at the resistance at 100 Hz, it is 1 ohm and at 1 KHz, it is 6 ohm. So how can I get any bass into my signal without overloading the audio amplifier. Do I just need to filter the bass a certain amount?

You may want to take a look at this paper as it covers some of the problems of using transformer modulation at the few hundred watt level.
http://amfone.net/Amforum/index.php?action=dlattach;topic=30798.0;attach=31360

2. Can I stack ferrite cores on the output transformer? For example, if I have 8 FETs, can I have four cores on the bottom and four on the top and then run the wire through all of them similar to if they were lined up? In the designs I've looked at, it seems like there is always one core per FET. Is this for power handling reasons or is it because the matching needs to change for each additional FET? (ie could I get away with four cores on an 8 FET amp, given that the power level is not too high or is it for matching?)

Does..not...compute.  I would need to see a picture.  The choice of core, how many turns, ect are design considerations made and influenced by power level and frequency mostly.  Maybe your talking about the huge output transformer in the 'QIX design?  I've found that making a large binoccular core out of four 1020 cable beads seems to be good enough until you get to the kilowatt level.  In class D you will have more harmonic power to contend with and it would be best to make your output transformer a little overkill to make sure you've got enough headroom to handle the higher than normal harmonic currents.

+-RH

[OgreVorbis]

You may want to take a look at this paper as it covers some of the problems of using transformer modulation at the few hundred watt level.
http://amfone.net/Amforum/index.php?action=dlattach;topic=30798.0;attach=31360

+-RH

I've already read that article. It gave me most of the info I have now about the transformer, but I'm still not sure why the impedance is so low at low frequencies and how that can be rectified 

I've discovered that the problem is not the actual transformer. If I connect an 8 ohm load to the transformer, I can see a pretty flat load on the other winding (not perfect, but no where near as bad as when it's connected to the TX). What determines the resistance here and why is it not flat? I tried changing the cap next to the RF choke, but it made no difference.

I know this is probably hard to determine without knowing the actual circuit. Also, this is a preliminary question and I'm not even sure it will be a problem when I get the amp I'm building, but it could be because the designs are very similar.

[Stretchyman]

Iron saturates at low frequencies, HPF your audio @100Hz.

I'd understand PWM if I were you, it's so simple you'll wonder why you ever used any other method.

My modulator probably weighs about 2oz.

The filter would weigh a bit but only 2-3lb.

You're building Class D RF so why not Class D audio too?

Str 

[redhat]

How are you measuring the impedance?  Stretchy is right, as frequency decreases, inductive reactance will go down, more so if the transformer is not tape wound with hypersil type steel.

+-RH

[Josh]

1. I tested a modulation transformer on a small class D (not the one I'm building). I've encountered a problem with the transformer. It is a 1:1 115V toroidal transformer. I don't know the VA, but it weighs about 15 kgs. I've read of using toroidal transformers as modulation transformers before and it looks like there are hams doing this and that it works well. Now we get to the problem: How do they not overload the audio amplifier when the ohms drops so low at the low end of the audio spectrum? If I look at the resistance at 100 Hz, it is 1 ohm and at 1 KHz, it is 6 ohm. So how can I get any bass into my signal without overloading the audio amplifier. Do I just need to filter the bass a certain amount?

You may want to take a look at this paper as it covers some of the problems of using transformer modulation at the few hundred watt level.
http://amfone.net/Amforum/index.php?action=dlattach;topic=30798.0;attach=31360

2. Can I stack ferrite cores on the output transformer? For example, if I have 8 FETs, can I have four cores on the bottom and four on the top and then run the wire through all of them similar to if they were lined up? In the designs I've looked at, it seems like there is always one core per FET. Is this for power handling reasons or is it because the matching needs to change for each additional FET? (ie could I get away with four cores on an 8 FET amp, given that the power level is not too high or is it for matching?)

Does..not...compute.  I would need to see a picture.  The choice of core, how many turns, ect are design considerations made and influenced by power level and frequency mostly.  Maybe your talking about the huge output transformer in the 'QIX design?  I've found that making a large binoccular core out of four 1020 cable beads seems to be good enough until you get to the kilowatt level.  In class D you will have more harmonic power to contend with and it would be best to make your output transformer a little overkill to make sure you've got enough headroom to handle the higher than normal harmonic currents.

+-RH

Just stack em till you get the temp range you want. You're watching the temp of the core right?

[OgreVorbis]

Iron saturates at low frequencies, HPF your audio @100Hz.

I'd understand PWM if I were you, it's so simple you'll wonder why you ever used any other method.

My modulator probably weighs about 2oz.

The filter would weigh a bit but only 2-3lb.

You're building Class D RF so why not Class D audio too?

Str 

I can understand that the iron core might be saturating, but this doesn't seem like the case. When an 8 ohm load is on the transformer, it's pretty flat down to 100 Hz, but NOT when it's connected to the transmitter.

Let me know when you have a PWM board ready and I'll order it. It's nice to make use of old things around the shop though and that's why I'm testing the transformer first as I have several of them laying around. BTW, my audio amp is a class D (still less efficient than PWM though).

How are you measuring the impedance?  Stretchy is right, as frequency decreases, inductive reactance will go down, more so if the transformer is not tape wound with hypersil type steel.

+-RH

I am using an LCR meter that has resistance measurements at 100Hz, 120Hz, 1KHz and 10KHz. And I don't doubt the measurements because the amp shuts itself down unless the bass is equalized in the audio. When hooked up to an 8 ohm load, this is no longer the case. So the problem is only with the TX connected.

The HPF idea seems like not a bad one. I already tried that with an equalizer, but maybe I need more aggressive filtering like with a real HPF. That would probably work, but it seems like I'm dodging the real problem.

[redhat]

You could add some R in series with the transformer and see if that helps.  I suppose you could add a capacitor as well to get a similar HPF effect, albeit not as sharp.  Does the problem go away if you back off the modulation some?

+-RH

[Stretchyman]

Cap on the I/P using the I/P Z and the Xc of the cap as an HPF, works for me.

Str.

[OgreVorbis]

OK, I'm back at this after a little break.

I've got my 8 SIC FET board mounted and populated with 4 x C2M0280120D and 2 x TC4452 drivers.
I am using a balun with 4 x FB 43 1020 (someone suggested the FB 61 1020 instead)
1 turn primary, 2 turns secondary
My output circuit is incredibly simple. It is just the balun right now. That's it  . I've seen a guy pull off matching like this before, so I think I just need to change my core type. And yes, I'd prefer it this way because I want broadband as the highest priority.

So I did my first test today and this is what I got:
140W @ 40V, 7MHz, 6.8A
200W @ 40V, 3MHz, 7.1A

So not good efficiency, but I am happy that it works at all as this is my first build.

I have my drivers at 8V right now. The two changes that I plan to make are change the cores to type 61 and increase the driver voltage. The max driver voltage for the TC4452 is 18V. What do you think I should set it to? Do you have any other suggestions?

I also have some C2M0160120D in stock that I could try.

[redhat]

In order to get clean switching, you really need to hit the gates of SiC fets with 15V or greater.  18-20V seems about optimal, any less and RDSon comes up pretty quick and will deteriorate your efficiency and cause excessive heating.

Also, are your power measurements before or after the lowpass filter?  Without a filter, power is likely to read substantially higher due to large amounts of harmonics, which on conventional class D is going to be quite high.

Not sure what your output transformer looks like, but hopefully the primary is made out of large-ish copper tuning to reduce Q and therefore excessive ringing due to circuit strays.  I would try and do something similar to what is found on solid state VHF PA's where some semirigid line is bent into U shapes and the shields soldered together.  This will also help maintain impedances and keep ringing down.

+-RH