The AM Forum

I decided to run some numbers and see how my single FET CW rig performed on the efficiency scale.  It uses a Chinese DDS VFO to drive an IXYS 614 chip, and then an FQA11N90 FET.  Output core uses a 1:2 ratio with 48 volts on the drain.

11% duty cycle:  38 watts output with 85% efficiency.  SWR rises to 1.35:1 here.

21% duty cycle:  59 watts output with 66% efficiency.  SWR is lowest here at 1.25:1.

40% duty cycle:  55 watts output with LOTS of current being drawn.  I'm too scared to hold the key down too long to get numbers for fear of blowing the FET.

So, my question is where should I run it?  I can run at 11% key down forever, but 59 watts is nicer.  On QSO's, I do notice a fair amount of heat coming from the FET, but it takes it.

Jon
KA1TDQ

Hmmm..... why only 48 volts?  Using 11N90's rated at 900 volts, how about 11% duty and raise the voltage until it says uncle?

On CW, a few hundred volts is good for a few hundred watts out, no?   Better efficiency too.

For example, I was thinking that a standard isolation transformer (120V : 120V) with a Variac control  would easily double your drain voltage and give you X 4 RF power output.   (using 85% efficiency)

Be sure you have enough drive and the gate is well saturated. Play with the output core turns ratio a little for best eff and output power when raising the drain voltage.

T

You know, you're right. I didn't think about that.  I have an isolation transformer that would work nicely. I'll build a power supply this weekend and see what I get for power output. Thanks!

Jon

Ok, so I built a test power supply and got it up to around 90 volts.  Power output went to over 100 watts, but the SWR went crazy high too (Around 2.5:1).  My antenna is pretty much resonant as well.  I'm guessing that I need to change the turns ratio on the output core?  Right now it has 1 turn on the primary (DC going to drain) and 2 turns on the secondary (ground to filter).  

I also noted that as voltage went higher, drain current stayed pretty much the same.

Jon

Good going, Jon.

I would think that the voltage and current should both double to give X4 power increase when the match is right. So, yes, play around with the turns ratio looking for the better match and power peak.

I built a pre-driver for my 4-1000A rig using two 11N90s in parallel  with 130V. I think I ended up with 1 turn on the primary and 4 turns on the secondary or something like that. It was fairly critical to find the sweet spot, so play around.

Be sure there is plenty of drive saturation too. This is a class C or D amp, yes?

T

Well, I'm hoping it's class D.  The last time I looked at this with a scope, I couldn't get a repeatable waveform going into the gate.  It works though, and 85% efficiency is nothing to shake a stick at.  

If I find that the sweet spot is anything other than 120 vdc, when I build the permanent power supply I'll use just a common household wall mount light dimmer to set the voltage.  It's a lot smaller and chassis mountable than a Variac.  

I'll re-do the core with 4 turns on the secondary and see what that yields.  It's always easier to remove turns than add them later.

Jon

Long story, but something ended up frying.  I went back to what I had before with 2 turns on the secondary, but with just a single core instead.  The picture is attached.  I also ended up replacing the FQA11N90 with an IRF510.  The IXDD614 got replaced with a new one and I did a better soldering job with this one.

Result:  I don't believe it because it doesn't add up.  The wattmeter shows 80 watts out with an SWR of 1.45:1.  Drain current is 1.22 amps.  I'm using a switching 48 VDC power supply that I got from Amazon.  If you multiply the current and voltage together, the maximum power output could possibly be is 57.6 watts.  

When I measure drain voltage un-keyed, I get 48 volts.  When I key down, it jumps up to 85 volts.  Is that just RF following the DC path down the cable past the decoupling capacitor?  Or is the RF messing with the regulation on the power supply and I really am getting 85 volts?  

Anyway, the IRF510 doesn't get hot to speak of.  I had a fairly long QSO with a guy in San Diego, and everything seemed to work fine.  He said that my homebrew rig sounded good with no drift or chirp.  Of course Sherlock, I'm using a DDS VFO and FETs.   8)

Jon

Ok, I have some better numbers...

I placed a .047uF capacitor across the input terminals for the 48 vdc and re-took the readings.  Things are much better now with the RF staying out of the DC path.

I'm getting 46.0 VDC key-down with 1.42 amps drain current.  That works out to 65.3 watts ideally.  The wattmeter says 85 watts, but that's not possible.  The IRF510 starts to get warm 5 seconds into key down, but not hot at all.  Of course, I have a monster heat sink too.

Jon