Why 50VDC???

[aloyolla]

Hi guys!
I saw many schematics in this site and the voltage used in the FETs in general is 50VDC.

Why people use this voltage since the FETs as a breakdown voltage in 900VDC like FQA11N90???

Thanks.

Arlindo - PY2-LOY
aloyolla@gmail.com
Brasil - São Paulo - Campinas.

[steve_qix]

The 45 volt carrier DC voltage was chosen to leave a good safety factor for the MOSFETs in the RF amplifier.

Everything has to be designed around PEAK voltages not DC voltages, so:

If your transmitter operates at 45 VDC at carrier, and your modulator will deliver 200% positive modulation, that's 135V peak.

The RF amplifier operating at 135V (in other words, under full positive modulation) in class E will generate RF peaks of about 475 volts IF EVERYTHING IS WORKING PERFECTLY and tuned correctly.

If the transmitter is mis-tuned, or if the antenna falls down or disconnects or a T/R relay fails, etc. etc. the peak voltage will go much higher.

So, that's why 45 volts is specified, to give plenty of head room between the MOSFET breakdown voltage and the operating peak voltage.

The reliability is GREATLY enhanced.

[aloyolla]

Great Steve, worderfull explanation.

I forgot the peak modulation.

Thanks again.

LOY.

[K9ACT]

What happened to the "quote" type reply option?

Anyway, I also forgot or didn't think about the peak voltage problem when I got into this. My original intent was to modulate a big tube with this approach. With tubes you add the audio to the carrier and get lots of bang for the buck.

To get the same power level using series modulation (if that is the correct term) one needs two to four times the plate voltage to take advantage of the high mod levels.

For 100% modulation I would have to run my carrier at half the normal level or double the plate voltage to maintain the same output level.

I think I finally got this right but if I missed something let me know.

Jack

[steve_qix]

What happened to the "quote" type reply option?

Anyway, I also forgot or didn't think about the peak voltage problem when I got into this. My original intent was to modulate a big tube with this approach. With tubes you add the audio to the carrier and get lots of bang for the buck.

To get the same power level using series modulation (if that is the correct term) one needs two to four times the plate voltage to take advantage of the high mod levels.

For 100% modulation I would have to run my carrier at half the normal level or double the plate voltage to maintain the same output level.

I think I finally got this right but if I missed something let me know.

Jack

Well, sort of The voltages are the same under high level modulation (plate, drain, etc), regardless of the method one uses to get there. With a transformer coupled modulator, the modulator AC voltage is added to, or subtracted from, the DC voltage. So, under 100% modulation, you have 2x the DC. Under 200% modulation you have 3x the DC. For a tube, say an 813, operating at 2000 VDC at carrier, the peak voltage for 200% positive modulation would be 6000 volts.

It's the same voltage with series modulation. If you wanted your series modulator to be able to modulate your 813 running at 2000V to 200% positive, you would need a 6000V power supply. There would be 4000 volts across the modulator and 2000 across the final, at carrier. At full positive modulation, you'd get 6000 volts across the RF amplifier, and 0 volts drop across the modulator.

So, the voltages are the same, regardless. The difference is with series modulation, you need to have the highest voltage available all of the time, and the modulator "bucks" (or drops) the voltage. WIth the transformer, the voltages are added. The method the modulator uses to "drop" or "buck" the high voltage determines the efficiency of the modulator. WIth an analog series modulator, the efficiency is much lower than obtainable with a pulse width modulator.

Does this help or make it worse?

Regards,

Steve

[K9ACT]

Does this help or make it worse?

Not quite sure but I think it confirms my notion that I would need a 6kv supply to do legal limit with tubes and I do not have any desire to do that. Tubes are fun but 6kv is not fun.

My original idea was sparked by a friend who purchased a Continental 150kw SW bcst transmitter that uses power supply/PDM modulator modules in 600V bricks that added up to what ever power was wanted. This seemed like a neat idea but even if we could work out the modules in a reasonably economical design, we still have the need for very unfriendly voltages to do what we can do with 2kv over even 100v is solid state.

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