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Title: Progress on my 100 Watt Transmitter Post by: N4LTA on June 30, 2021, 08:46:47 PM Finally got some time to put together a board and wind a new transformer.
The board control circuits seem to work fine, in fact the Tx enable circuit just might work for CW. That will require some testing. I fed the board a 5 volt 7770 KHz signal and powered itwith my 50 volt variable power supply. The unfiltered output looks really good and should not be difficult to filter. The output measured 135 watts on my wattmeter on the 200 Watt scale. Not being a sine wave likely makes the meter a bit off, but likely in the ballpark. Power to the MOSFETs was about 52 volts at 2.1 amperes. or about 110 watts input. I verified the voltage with an accurate bench meter but not the current which was measured on the power supply. Anyway - I suspect that I am in the ballpark of 100 watts out with a filtered sine wave. The output transformer is almost exactly as Rod's with 6 turns on each side of the primary with a center tap. The windings are opposed as Brad suggested. The secondary has 12 turns so that I could remove some turns as necessary. The resistance of the output stage is higher than expected at 25 Ohms and linear with respect to voltage. The digital scope (200MHz) shows nothing unusual on the output waveform. I didn't look with my 400 MHz analog scope. It usually shows things I don't want to see. I really need to get this to about 12 ohms or so and build the PWM filter and output filter. I am thinking I need to lower the resistance in the primary to increase drain current. I should be able to do this by reducing the turns ratio or taking turns off the secondary. I will try to take two turns of the secondary. The phase splitter is Rods design with a few small changes. The deck has a 5 volt and an adjustable 0-16 volt higher current power supply. The adjustable supply is for the driver chips. MOSFETs are Cree C3M0350120D 1200v. Drivers are NCP81074A Good news is the small heat sink barely gets warm and the 100 Watt dummy load get very warm. Any ideas or comments are welcome. Pat N4LTA Title: Re: Progress on my 100 Watt Transmitter Post by: vk3alk on July 02, 2021, 11:11:04 PM Your progress looks very good Pat and efficiency appears to be high..... AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
Figures are great but the sure thing is a coldish heatsink and a hot dummy load..... ;D Yes play around with the turns ratio maybe more the secondary then the primary to lower the loading value... I changed mine to a load of appox 7 ohms... Be interested to see how the PWM works too... Wayne |