Transmitter problem

Welcome and feel free to ask questions about class E, mosfet's, and AM.
You must be registered to post.

Transmitter problem

Postby Gilly » Tue Aug 14, 2007 8:03 am

Hi everyone

Have just completed a Single FET Design TX for 75M using 2 IRFP450s in parallel.
Its Digitally driven with a VFO and the Mosfet drivers are (3) UCC37321s ICs.
The digital part goes well with great waveforms on the Gates of the FETs ( 12.5V ).
But that is where it all ends.

The transformer is made up of 4 FB43-1020s with a 1:2 turns ratio.. The tuning capacitor is 500pf and the loading cap is appox 2500pf.The inductor is 14 turns on a Red Toriod 2" in diag..
The Shunt capacitors are Silver Mica and at the moment is 500pf.
The TX is stable with no parastics, the transformer is cold and the tuning of the output circuit is smooth..

My problem is that the efficiency is only at around 50%.
I cannot get a Softlanding or backporch effects on the Drain waveforms.
Also the Drain voltage on the low power setting of 21V is 90Volts and 200V on the high setting of 43V.
Have tried different values of Shunt capacitance ranging from 220pf to 900pf with noticable effects.
Another thing is that the Low time of the Drain Waveform is shorter than the Rise Time.
The waveform does not have much ringing and is clean looking.

Any ideas...

Thanks


Wayne
VK3AIY
Gilly
Class E Enthusiast
 
Posts: 167
Joined: Tue May 15, 2007 6:59 pm
Location: Mt.Eliza Victoria Australia

Postby frank carcia » Tue Aug 14, 2007 1:04 pm

Try removing a turn or two from the output incuctor. Q may be too high.
swag
frank carcia
Senior Contributer
 
Posts: 765
Joined: Fri Jan 31, 2003 10:48 am
Location: Enfield Ct. / Niantic Ct.

Postby blaine » Wed Aug 15, 2007 5:30 pm

whats your drain voltage and what is the rating on the FETs?
how much drive do they require on the gates? is 12.5v enough?
if they are paralleled are you driving them in the same phase?
I am thinking your gate C must be fairly low with only 2 FETS it shouldnt be too hard to drive them. Can you see on the gate waveform where the FETs turn on?
wish I was more familiar with those, maybe Steve has some input.
Blaine N1GTU
"The box said that I needed to have Windows 98 or better...
so I installed Linux." --- LinuxNewbie.org
User avatar
blaine
Site Admin
 
Posts: 98
Joined: Fri Jan 31, 2003 1:13 am
Location: CT

Postby Gilly » Thu Aug 16, 2007 2:54 am

Hi Frank

Have tried adding turns and removing etc: with no difference but have checked the Output circuit again with an my MFJ259 and it needed more C in the loading capacitor ( now has 3000pf )and will test again tonight and advise how it goes...

Thanks Blaine for your comments....
The specifations of the IRFP450's are:
VDSS 500V
RDS .4Ohms
ID 14A

Have 2 voltages for the TX
Low power or tuneup of 21V
High when all goes well of 42V
The 2 FETs are in parallel ( using the Single FET design ) and require just 1 phase. They are driven using 3 UCC37321.
The Gate Waveforms with and without Drain Voltage is great really.
You can see the Step when the Capacitance kicks in and climbs up to 12.5 and levels off with a fast discharge etc:
Not sure how to send photos....will see on the forum and send pictures etc:

Thanks for all the comments and will advise how I go...

Are just starting to build Steves Class H Modulator...
Why is Resistor R40 listed as (a) and (b)....can I combine the 2 of 2.4K resistors as 1 of 4.8K ???


Thanks


Wayne
Gilly
Class E Enthusiast
 
Posts: 167
Joined: Tue May 15, 2007 6:59 pm
Location: Mt.Eliza Victoria Australia

Postby kf1z » Thu Aug 16, 2007 7:00 am

Hi Wayne.....

Let's see.....

You said you checked the output circuit with an MFJ259 and you needed more load C..... remember, you're not looking for resonance there (somewhat close, but not spot on).

The IRFP450 input capacitance is somewhere near 2600pf each so that's probably not a problem..


I wonder how much current you see when you tune it up?


Also curious on your vfo/driver set-up? are you using Steve's design?

Certainly not an expert, but after some of my recent testing, sounds like maybe the problem is in the driver....
Sounds like your "on" time is a little short...?

On my rig the voltage I see on the gates is more like 16 volts at peak..
(with 12 volts at the drivers)

That's why I'm curious of your vfo/driver set-up.....
User avatar
kf1z
Senior Contributer
 
Posts: 558
Joined: Sat Mar 18, 2006 7:07 am
Location: Ely, NV.

Postby frank carcia » Thu Aug 16, 2007 7:14 am

3000 pf of loading C sounds a bit high. My 22 FET rig runs about 1850 pf with about 400 or so pf series tune. My inductor is a bit hih making the Q a bit high so the l needs to be reduced a bit.
I would think with 2 FETs the drain Z is fairly high, you might also try a 1:1 turns ratio. gfz
frank carcia
Senior Contributer
 
Posts: 765
Joined: Fri Jan 31, 2003 10:48 am
Location: Enfield Ct. / Niantic Ct.

Postby kf1z » Thu Aug 16, 2007 7:41 am

Have you measured the output inductor's actual value?

You said 14 turns on a red core.....


Well if that red core is a type 2 iron....

Then I think the inductance is around 2.8uH


Is that about right?
User avatar
kf1z
Senior Contributer
 
Posts: 558
Joined: Sat Mar 18, 2006 7:07 am
Location: Ely, NV.

Postby frank carcia » Thu Aug 16, 2007 8:03 am

2.8 sounds way low for 75M that could be why the cap vlaues are so high.
frank carcia
Senior Contributer
 
Posts: 765
Joined: Fri Jan 31, 2003 10:48 am
Location: Enfield Ct. / Niantic Ct.

Postby Gilly » Thu Aug 16, 2007 8:38 am

Guess what!

Its working!!

This stuff is tricky.

I thought that the transformer would have to be a least 1:2 to get the Z up for the Output circuit etc:
I did what you suggested Frank and made the transformer 1:1 and now I have backporch and flat landing available....maybe not the best waveform but quite exceptable without too much ringing etc:

Also the drain waveform is now showing 3 - 3.5 times ( 150V ) the supply voltage of 42V. Before it was over 200V....

To be honest have know idea how much current can be drawn from these things...it seems OK @ 4amps with 40V as I just left it going and walked away for about 10 minutes and it did not blowup!!!

Its caused my poor oil filled 50Watt Dummy load grief when I came back so its putting out quite a bit of power as my power meter only goes to 100watts and its off the scale....the FETs are maybe warm...

Looks like there are some messages so will submit this and reply


Thanks


Wayne
Gilly
Class E Enthusiast
 
Posts: 167
Joined: Tue May 15, 2007 6:59 pm
Location: Mt.Eliza Victoria Australia

Postby frank carcia » Thu Aug 16, 2007 9:08 am

50 watts carrier per device is a conservative number for 75 meters.
positive peak 150%
frank carcia
Senior Contributer
 
Posts: 765
Joined: Fri Jan 31, 2003 10:48 am
Location: Enfield Ct. / Niantic Ct.

Postby Gilly » Thu Aug 16, 2007 9:10 am

Thanks for all the comments as I probably would not have got this working if not for the Forum...
Not many people in Australia are using Class E so this forum has been great..

The toroid came out of my junk box...it seemed about right so gave it a go.
But it gets HOT.

The VFO is Steve design but made it for 14Mhz and divide it by 4 for 75M..
Originally this TX was for 40M but underestimated the digital drive for that frequency as my 3 UCC37321 drivers just could not manage it and so divided it by 4 and off it went on 75M.
The VFO is small and in a thick metal box and is really very stable..
Made the band spread narrow so can spot in really well.

Now that the TX is under control will increase the Inductance of the coil and try again and see how it goes...

But its going well now thats the main thing and in Australia the legal power is only 100watts so efficiency is not all that important as the TX is putting out more than what is required!!
Will send in pictures to show what it looks like..

Now for the Class H Modulator..

Thanks


Wayne
Gilly
Class E Enthusiast
 
Posts: 167
Joined: Tue May 15, 2007 6:59 pm
Location: Mt.Eliza Victoria Australia

Postby VK3KRI » Sat Aug 18, 2007 6:27 pm

Gilly wrote:The toroid came out of my junk box...it seemed about right so gave it a go.
But it gets HOT.


I think you'll have trouble with nearly any powedered iron toroid in that position due to the voltage across it and the relativly small number of turns. A pity as toroid could make for a nice small RF package. Maybe an air - core toroid ?
Ian VK3KRI
VK3KRI
Member
 
Posts: 44
Joined: Fri Jan 31, 2003 8:32 pm
Location: Melbourne, Australia

Postby Gilly » Sun Aug 19, 2007 8:21 am

Hi Ian

Thanks for your comments.
Tried a power level of 50Watts and the Coil still gets quite hot..
Originally the TX was for 40M and just tried using the Toroid as an experiment really.....as there was not much room around the output circuit for an air wound coil for 80M..
Will replace it with a Air Coil...it will stick up a bit high though but it does not matter.
Are starting Steves Class H modulator.
Have designed the board and hope to have it all finished within a month.
Are enjoying this Class E stuff ... have never built anything like this before and its good to learn something new...
Hope to catch you on air one day...

See you


Wayne
Gilly
Class E Enthusiast
 
Posts: 167
Joined: Tue May 15, 2007 6:59 pm
Location: Mt.Eliza Victoria Australia


Return to Class E Discussion

Who is online

Users browsing this forum: No registered users and 1 guest

cron