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Messages - OgreVorbis

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16
The RF Workbench / Re: Beginner class D design
« on: June 12, 2019, 2122 UTC »
I have resumed work on the project. It is very near completion. I just need a good heatsink for the FETs and I'm searching for one now. I want a cooling aggregate type.

I still don't know what to do to the transformer when I add another pair of FETs.

Do I change nothing?
Do I need to add another winding on the secondary?
Do I need to add more than four cores for eight FETs?

17
So I'm interested in finding out if there is anyone under 30 y.o. on this forum. It seems to me like the only people interested in this hobby are older (maybe 50-60).
If you are younger, how did you get into the hobby?

For me this all started with an interest in computers which then moved to electronics and then to radio.
I was somewhat interested in radio when I was only 15 y.o. but I didn't have the skills required to do anything really interesting.

The part where I think I differ from most people who are into radio is that I am not into it to make contacts or to do other ham-like activities and I am not into it even to play music (but that's part of it). There is just a huge excitement I get from putting out the waves and knowing that it's going very far. The physics of it just makes me really happy. Am I the only one who's like this?

18
The RF Workbench / Re: Beginner class D design
« on: April 15, 2019, 1734 UTC »

To drive a High side and Low side Devices I guess? Or tie them together for single FET. Just use a dual Inverter from the O/P of your Osc. Sine in and square out, inverted and double inverted to drive either side.

Str.

Sorry, I don't understand the term "high/low side device". Does this mean the devices on one side of a push-pull amplifier (like what I'm building)? So then I don't need an inverter. Just use the NCPs?

19
The RF Workbench / Re: Beginner class D design
« on: April 15, 2019, 1119 UTC »
See last two posts.

I realized after my design that it looks like the NCP drivers have an inverter built-in. The truth table in the datasheet makes it look like it won't work for this purpose though, but I don't know why else they would have such a feature. Is this to use in a push-pull amplifier with one square wave input? If so, then I don't even need the inverter :P

20
The RF Workbench / Re: Beginner class D design
« 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.

21
The RF Workbench / Re: Beginner class D design
« 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.


22
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.

23
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?

24
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.

25
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.

26
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.

27
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.

28
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?

29
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.

30
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.

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