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Messages - OgreVorbis

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76
The RF Workbench / Re: Beginner class D design
« 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.

77
The RF Workbench / Re: Beginner class D design
« 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?

78
The RF Workbench / Re: Beginner class D design
« 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.

79
The RF Workbench / Re: Beginner class D design
« on: February 26, 2019, 2014 UTC »
How are you introducing 'dead time' into the phases of the RF drive?  At 7MHz, you will need about 40% duty cycle on each phase to prevent cross conduction (shoot through).  This could be one of the reasons for the relatively poor efficiency on 7 MHz.

+-RH

Does that mean 40% on, 60% off, or the opposite? And it's not relative to each other, it is just on time vs off time for each phase individually?
I want to make sure I did this right. I have a feeling I did it backwards.

80
The RF Workbench / Re: Beginner class D design
« on: February 26, 2019, 1959 UTC »
Alright, well I popped two fets, but I learned some things. . .

So I set the 40% 60% duty cycle. It improved a bit (the RF watts went up to 460W from 410W and the efficiency went to 80% up from 74%), so I changed it a bit more and then they popped immediately. I wasn't able to even read the amps. I have to change the duty cycle for each wave individually. Is having them not exactly the same or overlapping a huge problem? I got a new high power current limiting supply, so I'll use that in the future. I am not 100% sure that it was 40 60, but it should be pretty close. My scope doesn't have any fancy stuff. I just had to read the ns/division and do the calculation. I am surprised how little of a change beyond the 40 60 did it.

Anyway, here is the data I gathered:
The DDS exciter is making a decent looking square wave. It has 7ns of rise time regardless of frequency (from 3 to 8 MHz it stays at 7ns). And . . . the drivers have a 20ns rise time (again - regardless of frequency). The fall time on the drivers, does however, seem to get even slightly worse with increased frequency (goes up to like 25ns). So I think the drivers are the culprit. I didn't do the exact math, but I'm thinking 20ns is probably too much for 7 MHz. Is this correct?

If so, then I'm on to making a new PCB. I am going to use the drivers stretchy suggested and put them as close as possible to the fets. I gave the datasheet a quick look and the pinout looks confusing. I am not sure how to hook them up. I addition to the new drivers I am also going to have a custom crystal on the main PCB with an inverter to generate the second wave. It's either going to be the SN74AHCT14N or SN74HC132N for the inverter.

81
The RF Workbench / Re: Beginner class D design
« on: February 25, 2019, 1651 UTC »
How are you introducing 'dead time' into the phases of the RF drive?  At 7MHz, you will need about 40% duty cycle on each phase to prevent cross conduction (shoot through).  This could be one of the reasons for the relatively poor efficiency on 7 MHz.

+-RH

I think it is 40% 60%, but I am using a pre-made DDS module (made for this purpose). I checked it on the scope a while back to just see what voltage it was putting out, but I didn't look at duty cycle. I really need to check it again to verify. I am also going to check the outputs of the TC4452 to make sure they're staying square.

So I am having correspondence with a knowledgeable guy via email. He tells me that I should use 20V (in which case I'd have to upgrade to a different driver and change my PCB). Are any of you using the TC4452 or TC44xx on 40 meters? Could the voltage or the driver itself be the problem? My suspicion is the guy is just being nitpicky about the 20V and it doesn't really matter. (As I said earlier, I am using 18V now.)

In addition, I tried changing the cores to type 61 1020. I seem to be getting an erroneous reading from my bird meter now. It says I'm putting out 800W instead of 600W with the old core at 3 MHz even though my power supply is only drawing 700W. At 7MHz the power and the efficiency did not change.

82
The RF Workbench / Re: Beginner class D design
« on: February 24, 2019, 1957 UTC »
OK, so I've increased the voltage from 8V up to 18V. Here is what I am getting now:

3MHz 13.8V 4.0A 50W H=5.2W 90.6%
7MHz 13.8V 3.4A 32W H=14.9W 68.2%
3MHz 28.1V 7.5A 200W H=10.7W 94.9%
7MHz 28.1V 6.67A 140W H=47.4W 74.7%
3MHz 48V 14A 640W H=32W 95.2%
7MHz 48V 12.7A 410W H=198W 67.4%

I also briefly tested 5MHz, but my power supply is rated at 600W, so I was really pushing it and didn't get proper measurements.
It made 600W at 5MHz, so clearly it doesn't drop off linearly. It is also possible that my 640W reading it limited by the power supply. I have a 1500W supply on the way.

I have yet to change the core material and that is my next trial. It is amazing how much better it is operating with the 18V.
Despite the success, I still haven't met my goal of 7MHz. Any ideas of other changes I could make? What is usually the weakest link here?

83
The RF Workbench / Re: Beginner class D design
« on: February 19, 2019, 1711 UTC »
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

OK, thanks! It is really invaluable having your help, no one around me in my day to day could answer, so I appreciate it.

I am measuring after the lowpass filter. It won't fully work on the 3MHz though which might be contributing to the higher number there.
The wire I am using I believe is 12 AWG silicone stranded wire. It is not a coax. I do have some RG-402 though.
What type of ferrite cores do you use? I suspect my main problem is the core type.

Another thing I am wondering for the future: I have four 1020 cores arranged in a binocular form. My board has space for eight fets even though now I am using only four. When I add the additional ferrite cores I won't have space, so can I stack them so I have four on the bottom and four on top. And then loop the wire through all of them (not sure if I would need to cross over to the opposite top side). I have never seen it done this way.

84
The RF Workbench / Re: Beginner class D design
« on: February 19, 2019, 1233 UTC »
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  :P. 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.

85
The RF Workbench / Re: Beginner class D design
« on: November 30, 2018, 0244 UTC »
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.

86
The RF Workbench / Re: Beginner class D design
« on: November 28, 2018, 1036 UTC »

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  :P

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.

87
The RF Workbench / Re: Beginner class D design
« on: November 28, 2018, 0139 UTC »
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?)

88
The RF Workbench / Re: Beginner class D design
« 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.

89
The RF Workbench / Re: Beginner class D design
« 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

90
The RF Workbench / Re: Beginner class D design
« 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.

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