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Author Topic: "RF and molecular bond breaking Kanzius style"  (Read 102629 times)

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Hi Grum,

nicely spotted,  i did not see that.

Guess its because of the extra peaks on that signal, so the scope gets confused.

Regards   Itsu
   

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OK,  4x 22 Ohm parallel (5.5 Ohm) from driver output to ground.

First screenshot overall signal at 13.5MHz, looking good:
yellow: input from FG
Blue:   output from driver to ground

Next 2 screenshots are zoomed in on the rise / fall times of 4.5MHz, then 13.5MHz (look at the increased delay on 13.5MHz)
Also looking good,  the 4x 22 Ohm resistors getting hot fast.
Looks like the IXIS driver can rise in 7ns with almost 3A flowing through it (I assume 15V VCC).  That is pretty quick.
The phase shift is caused by the driver's propagation delay (30ns) but this delay is not the same as the rise time.

To further characterize this driver's performance under heavy load you can use one 1/4W 1.5Ω carbon resistor to stress the driver even more, but to prevent this resistor from burning up you should use very sparse pulses (low duty cycle).

If you had your versatile Rigol generator, you'd set it up like this:

1) Set CH1 of your signal generator to the Square waveform (Period: 100ns, StartPhase: 0º, Duty Cycle: 50%)
2) Set the Burst mode of CH1 to (Type: N_Cycle, Cycles: 1, Burst Period: 20ms., StartPhase: 359.999º, Source: Internal, Delay: 0 ).

This will set your signal generator to output a 100ns positive pulse every 20 milliseconds (very low duty cycle) that will not burn up the resistor even if there is more than 10A flowing through it.

If you want to test the fall time under load (sinking performance) jest connect the load resistor to VCC.

Of course MOSFETs present capacitive loads to the driver, which is a little different because at the beginning of the pulse capacitors behave like a 0Ω resistor (short) and after 5 RC constants they behave almost like infinite resistance (open).

This is further complicated by the Miller effect, but we won't go there for the driver performance characterization alone.
One remedy for the Miller effect is cascode configuration.
« Last Edit: 2015-06-19, 17:41:57 by verpies »
   

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Next test with only 1 MOSFET attached,  no drain voltage applied.
Again yellow input FG, blue gate signal  <snip>  on 13.5MHz.
So we see a dramatic difference compared with the signals of only the 5.5 Ohm resistor attached.


Yes, that is a difference between resistive load and capacitive load.  It is classic and that is the way it is supposed to look with this RC constant.

The only discrepancy from ideal capacitance charging curve are those squiggles that I marked in red circles.  They could represent reflections or some phenomena inside the MOSFET - I wonder which one.  Do these squiggles remain if you connect the driver's output to a lone pulse capacitor of comparable capacitance, instead to the MOSFET's gate ?

Anyway, to square this signal some more (now you have a distorted triangle waveform) you'd need to hit this gate capacitance with higher voltage and current.  
This is difficult and risks gate oxide puncturing if you allow the voltage across this capacitance to exceed 20V, but you can get away with charging it with a higher voltage without damage if you stop charging before the voltage across this capacitance exceeds 20V.  This is advanced stuff.
   

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but somehow the MOSFETs (capacitance?) cause the driver to misbehave.
Yes, it looks like the driver cannot handle that much gate capacitance and the resulting spikes case your oscilloscope to misread the pulse frequency.

I don't know how expensive these IXIS drivers are, but I suggest using one driver per one MOSFET instead of one driver per two MOSFETs. 
If you feel very adventurous, you could try two paralleled drivers per one MOSFET.
   

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Thanks verpies,

great analysis once again.
That capacitor test looks a good one to do.
Increasing the drivers Vcc to 24V is possible, however at 1.8A it will exceed its power level specs by over 100%
Using 1 driver / MOSFET is possible, they are not that expensive and i still have a spare one.

Here 2 short video's, first one shows how i did the last tests,
the second one showing that on 4.5MHz all is still working very fine (200W output on the Bird Wattmeter).

https://www.youtube.com/watch?v=n5kpIdr4cWk&feature=youtu.be

https://www.youtube.com/watch?v=K-DrchtPjPI&feature=youtu.be

Regards Itsu
   

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2.9nF (3x MKP caps parallel) on the output of the driver alone, first screenshot at 4.5MHz, second on 13.5MHz:


Itsu
   

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Thanks verpies,

great analysis once again.
That capacitor test looks a good one to do.
Increasing the drivers Vcc to 24V is possible, however at 1.8A it will exceed its power level specs by over 100%
Using 1 driver / MOSFET is possible, they are not that expensive and i still have a spare one.

Here 2 short video's, first one shows how i did the last tests,
the second one showing that on 4.5MHz all is still working very fine (200W output on the Bird Wattmeter).

https://www.youtube.com/watch?v=n5kpIdr4cWk&feature=youtu.be

https://www.youtube.com/watch?v=K-DrchtPjPI&feature=youtu.be

Regards Itsu


Wow, like your Bird watt meter $$$$$ :D

Yes I would say those mosfets don't like the higher frequency for some reason.

200w @ 42v is very good. Normally if you double the voltage you will quadruple the output (800w) :D

I would work with the 4.52Mhz as the third harmonic down, not sure it is not also industrial like 13.56 X 2 = 27.12MHz  13.56/3 = 4.52MHz. shared with the Ham bands as some are ;)

Verpies also from me , great analysis, I was thinking of something about the capacitance of the fets at that frequency for some reason, changing to a different brand (IRF's  or the like) might be interesting if there are some knocking around.

regards

Mike 8)


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I'm wrong it is 6.78MHz, the other is correct

regards

Mike 8)


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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
Arthur Schopenhauer, Philosopher, 1788-1860

As a general rule, the most successful person in life is the person that has the best information.
   

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2.9nF (3x MKP caps parallel) on the output of the driver alone, first screenshot at 4.5MHz, second on 13.5MHz:
Nooooo!  Why is the output more distorted at lower frequency than the higher ????

Could it be that longer switching period gives the chance for a 14MHz LC ringing to occur?  
How long are the wires between the driver and the 2.9nF cap ? ...and the loop area of these wires?
« Last Edit: 2015-06-19, 21:37:42 by verpies »
   

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Wow, like your Bird watt meter $$$$$ :D

Yes I would say those mosfets don't like the higher frequency for some reason.

200w @ 42v is very good. Normally if you double the voltage you will quadruple the output (800w) :D

I would work with the 4.52Mhz as the third harmonic down, not sure it is not also industrial like 13.56 X 2 = 27.12MHz  13.56/3 = 4.52MHz. shared with the Ham bands as some are ;)

Verpies also from me , great analysis, I was thinking of something about the capacitance of the fets at that frequency for some reason, changing to a different brand (IRF's  or the like) might be interesting if there are some knocking around.

regards

Mike 8)

Mike,   i always wanted to own a Bird 43 Wattmeter, and just ran into this bargain, could not resist.

I tried 82V on the drains, not really  quadrupling the output, but more then 500W on 4.5Mhz, see picture.
The dummyload went up in flames right away, so could not tune  :(

I have 2   DE275X2-102N06A coming, also a bargain on Ebay, so could try them in parallel mode.


Regards Itsu
   

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Nooooo!  Why is the output more distorted at lower frequency than the higher ????

Could it be that longer switching period gives the chance for a 2.5MHz LC ringing to occur?  
How long are the wires between the driver and the 2.9nF cap ? ...and the loop area of these wires?


Yes,  good question.

I had 1 cm length of wires running from the caps and another 2cm of distance from ground point to ground driver making a 4cm loop, shorter is hardly possible.
I could reduce the grounding point distance somewhat.

Itsu
   

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Quote
To further characterize this driver's performance under heavy load you can use one 1/4W 1.5Ω carbon resistor to stress the driver even more, but to prevent this resistor from burning up you should use very sparse pulses (low duty cycle).

If you had your versatile Rigol generator, you'd set it up like this:

1) Set CH1 of your signal generator to the Square waveform (Period: 100ns, StartPhase: 0º, Duty Cycle: 50%)
2) Set the Burst mode of CH1 to (Type: N_Cycle, Cycles: 1, Burst Period: 20ms., StartPhase: 359.999º, Source: Internal, Delay: 0 ).

This will set your signal generator to output a 100ns positive pulse every 20 milliseconds (very low duty cycle) that will not burn up the resistor even if there is more than 10A flowing through it.

Ok,  tried that,  but the result was not what i would expected see first screenshot.

So i tried without the burst mode setting, see second screenshot.

Regards Itsu
   

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I tried the cap test again, now with a single 2.2nF ceramic capacitor almost directly soldered on the output / ground lead of the driver (0.5cm leads)
Result is the same:

Regards Itsu
   

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Ok,  tried that,  but the result was not what i would expected see first screenshot.
So I immediately can see that I made an arithmetic mistake and forgot about the 50% duty cycle setting.  I should have written:
"This will set your signal generator to output a 50ns positive pulse every 20 milliseconds"

Nonetheless, the scopeshot shows that the driver has no problems sourcing currents close to the 8A mark (assuming 12V VCC)

So i tried without the burst mode setting, see second screenshot.
...and your 1.5Ω 1.4W resistor did not vaporize ?!
   

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I tried the cap test again, now with a single 2.2nF ceramic capacitor almost directly soldered on the output / ground lead of the driver (0.5cm leads)
Result is the same:
I don't think so.  If you look at the LC ringing frequency then it was approximately ~14MHz before and now it is >28MHz (35ns period)
Over 200% change is a significant one and supports the thesis that the capacitor loop is ringing because you decreased this loop and the ringing frequency increased significantly (more than can be explained by the -24% decrease in capacitance which is under a radical in LC frequency formula).

Zoom and check my scoposcopy in case I made a mistake.

« Last Edit: 2015-06-19, 22:02:50 by verpies »
   

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Quote
...and your 1.5Ω 1.4W resistor did not vaporize ?!

No,  but in 1 second it had no change i guess.


Quote
Zoom and check my scoposcopy in case I made a mistake.

Right,  i see,   i was able to measure the last ringing, the first one is much lower in frequency, so indeed the cap is ringing on the lower frequency.

Itsu
   

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No,  but in 1 second it had no change i guess.
Amazing!  
Average Power = DutyCycle * V2 / R
50% * 12V2 / 1.5Ω = 48W
...pretty good for a 1/4W resistor even for 1s.

Maybe some overload protection kicked in inside the driver and the current became limited.
   

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Probably, it does come with an "enable" pin to be used for shutting off the output, however i have hard wired this pin to Vcc  :o

Itsu

   

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So somehow either the driver (IXDD414ci) or the MOSFETs (FQA11N90c) or the combination is not suitable for 13.56MHz.
Research via Google confirms no designs to be available for 13.56MHz with those components, so not sure why i thought it would work.
I have an IXRFD630 driver and DE275X2-102N06A MOSFET on order which should be able to handle 13.56MHz for lateron tests   


I will use for now the 4.5MHz module i have also because my FG will be away for repairs, so will be using my Chinese FG (till 5MHz)

Things to do:

# further RF sealing the amplifier and (new) dummy load
# build a (regulated) 100V / 5A PS
# looking into the modulation part (audio amp. modulating the drain voltage)
# how to deliver the RF to the target (via antenna's / air capacitors. ??)

Thanks all for your help up till now, please continue to contribute / replicate whenever you can.


Regards Itsu
« Last Edit: 2015-06-20, 21:54:07 by Itsu »
   

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Itsu

where haver you ordered your parts from in Europe? that dual mosfet will handle over a Kw :D but can't find a supplier

regards

Mike 8)


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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
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As a general rule, the most successful person in life is the person that has the best information.
   

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Hi Mike,


i use this website to find parts:   http://www.findchips.com/

Not specific Europe, but Farnell and Digi-key operate also in Europe and deliver very fast.
I would order the  IXRFD630  driver from digi-key.

Regards Itsu
   

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Hi Mike,


i use this website to find parts:   http://www.findchips.com/

Not specific Europe, but Farnell and Digi-key operate also in Europe and deliver very fast.
I would order the  IXRFD630  driver from digi-key.

Regards Itsu

yes the driver is no problem, thanks

Mike 8)


---------------------------
"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
Arthur Schopenhauer, Philosopher, 1788-1860

As a general rule, the most successful person in life is the person that has the best information.
   

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Received my  IXRFD630 driver and DE275X2-102N06A MOSFET.
Also my 50 Ohm 100W Globar resistor arrived, so finally can start building the new KW dummy load and the 13.56MHz amp.

The weather is very nice overhere though, so don't expect anything any soon  ;D


Regards itsu
   

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Received my  IXRFD630 driver and DE275X2-102N06A MOSFET.
Expensive little bugger!  $50
   

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Expensive little bugger!  $50

Yep, but a great performer :D ;D :P

regards

Mike 8)


---------------------------
"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident."
Arthur Schopenhauer, Philosopher, 1788-1860

As a general rule, the most successful person in life is the person that has the best information.
   
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