OK, so the project is finally completed and I’ve done some tests. Of course, this project will never be 100% complete. The transmitter does work and at a high efficiency of 95%, but only below 5MHz. It should work great for the 80 meter and 160 meter ham bands, but my goal of 40 meters (7MHz) has not yet been met.
I need to do more work to determine the weakest link. It is likely that it has to do with the drivers. I need to take some measurements with a scope to determine the problem. I will keep this post updated with the changes I make to get it working on 40m.
So far, there is one notable change I made that differs from the schematic I posted in my earlier post. I ended up needing 18V into the drivers rather than 8V. So I upgraded the power supply to 24V and used a 13.7V reg for the control board and then the two 18V regs for the drivers.
My efficiency is now as follows:
| Frequency | Voltage | Amps | RF Watts | Heat Watts | Efficiency |
|---|---|---|---|---|---|
| 3MHz | 13.8V | 4.0A | 50W | 5.2W | 90.6% |
| 7MHz | 13.8V | 3.4A | 32W | 14.9W | 68.2% |
| 3MHz | 28.1V | 7.5A | 200W | 10.7W | 94.9% |
| 7MHz | 28.1V | 6.67A | 140W | 47.4W | 74.7% |
| 3MHz | 48V | 14A | 640W | 32W | 95.2% |
| 7MHz | 48V | 12.7A | 410W | 198W | 67.4% |
Update 02/27/19
I’ve done more testing and have concluded that it is not possible to do more than 5 MHz (5.5 MHz tops) with this PCB. The TC4452 drivers are just not going to do it and I can’t change them out for something better as the ‘better’ ones use a different pinout. Looking on a scope I can see the rise time on the DDS module to be about 7ns regardless of frequency (3 – 8 MHz). However, looking at the output of the TC4452(s), it is about 20ns rise and 25ns fall time. I think this is too large for it to be efficient at 6 to 8 MHz. Rise time is critical for these types of amplifiers because the MOSFETs operate at the highest efficiency when they are fully on or off. If they are part way on (during the rise time), they generate a lot more heat.
Another thing I tested to try to improve the efficiency was changing the duty cycle to 40%/60% rather than the normal 50%/50%. This apparently reduces the chances of cross conduction (the time when both sets of MOSFETs are on at the same time). We don’t want both sides on at the same time because this is a push pull amplifier. It will result in poor efficiency and possibly popping a FET if there is too much cross conduction. After changing the duty cycle to 40/60 (ie 40% on time and 60% off time for each side respectively), the efficiency of the amp went up to 80% from the original 74% shown above. That is not nearly enough of a change to call it ‘working’ at 7 MHz though.
In order to get good efficiency at 7 – 8 MHz, I am going to have to redesign the board. Two alternative drivers that should work better at these frequencies are the IXDD614 and the NCP81074A. In addition to changing the drivers, I will use one driver per FET. The NCP only comes in a surface mount package, so I am a bit concerned with heatsinking, but I have seen this one used in a picture. I think it will work well.
So stay tuned. It will probably be a while until the next update here because I have to design and order the new PCB, but I’ll be sure to update on how that one is working at the higher frequencies.
NIce project; will follow your upcoming iterations in design.
Hi —
I’m also playing around with a Class D transmitter, using similar circuit topology. I think your efficiency issues at higher frequencies have not much to do with the drivers. Even the newer IXDD614 types don’t deliver more drive or operate “faster”, that is, in the sense they’d help get through the partially conducting region of the output FET’s turn on. Instead, you’ve limited the driver’s (whichever one you use) ability to charge the output FET’s gate capacity quickly by putting a 33 ohm current limiting resistor in the gate circuit. Try a test with them paralleled with a few ohms and see if high frequency efficiency changes…..
I will try that and thanks for the response!
It is a 3.3, not 33 ohm by the way.
Hey man, I just wanted to drop a note letting you know I appreciate your video on this, as well as the write up here. I’m very inexperienced with RF but am happy to learn. I started out with SDR’s grabbing images from the NOAA SATs, and learning how to replay 433mhz RF to turn on those RF outlets.
This stuff you’re doing is all over my head at this point, but finding stuff like this interests me, even if I only learn a little bit. That video on sending data with QAM was awesome – nice work.
Anyway, I know this isn’t related to your HF transmitter, but I wanted to just make sure you see someone out here appreciates the work you do.
Take care.
Hi,
I didn’t realize that I had to approve comments for them to appear. I would have responded much sooner if I knew there were comments.
Glad you appreciate it!!!
It’s available the pcb for testing?
73 by IU2MIR
Yes, the PCB is available.
Do you need the components? I have some MOSFET and drivers, but you will need to use your own components for the rest.
Let me know if you want to purchase the PCB or the PCB + MOSFET…
Send me email: pdmeyer07@gmail.com
Dear, I wrote in your email for to buy the PCB and some components, without answer. Can you check? The last time is 16 nov. 2019.
73, Paolo, IU2MIR
Hello,
Sorry, I checked it and I can’t find any email in my spam or inbox on that day. Can you please send an email again pdmeyer07@gmail.com
I really don’t know why I did not receive anything. Let me know what you need here if you still can not get through to my email.
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