Message-ID: <FFAE74DDEB784DFC90D800821A4414DD@gnat>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <15c68b2d2ed.marcocadeddu@tin.it>
Date: Fri, 2 Jun 2017 13:36:37 +0100
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Subject: LF: Re: I: Fw: For today the FETs survived...
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Status: O

Hi Marco I dont know where you are putting the caps but I am assuming they 
are passing a high RF corrent. I found that the old style (valve) high 
voltage caps in my Class E test rig changed capacitance permanently during 
and after a run (capacitance reduced). I assumed that the current was fusing 
the foil connection to the lead-out wires. When using "pulse rated" 
capacitors as used in SMPSUs I had no further capacitance change problems. 
The ones I have are RIFA PHE 428 2000v from RS Components. I think Farnell 
do a WIMA equivalent.

Best Wishes
Alan
G3NYK

----- Original Message ----- 
From: <marcocadeddu@tin.it>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, June 02, 2017 1:06 PM
Subject: R: Re: Re: Re: LF: Re: I: Fw: For today the FETs survived...


now my thoughts are even more confused....
Hi Andy, me again....
after a long night of meditation, I remembered I had a bounch of old
style mica capacitators 1000pF 1000V..
I put
9 of them in parallel to get the closer value to the 8800pF I had
before.
Of course the resonance moved a bit and now is on 134.5 kHz with 3dB
bandwidth of 9 kHz (Q=15 I know is a bit to high..) so carried out some
tests on 134 kHz (of course on the dummyload) all without guard circuit
1st test xfmr 7T/19T (Ae197mm² R50 mat T38): 2Wout @10Vdc; 6,5
Wout@20Vdc; 13,7Wout@30Vdc stable.. @40Vdc after a quick peak the power
slowly goes down till below 1W!
2nd test I had still on hands the previous xfmr 5T/12T (Ae197mm² same
core as 1st test) so was worth to make a trial... same trend :-( for
Vdc> 30V after a first peak... it goes down.
I had another core available with different material (N30, Ae 154mm²
R58) so I prepared a new xfmr with this core 7T/14T:
1,4Wout@10Vdc; 4,6Wout@20Vdc; 9,2Wout@30Vdc .... at 40Vdc a short burst
with almost 15Wout and then down down down... :-((((((

I noticed a difference in comparison to your project: you connected the
+ and - rails to ground via 10nF and here I used 100nF caps could it
explain this behaviour?

73 Marco

----Messaggio originale----
Da: marcocadeddu@tin.it
Data: 1-giu-2017 22.20
A: <rsgb_lf_group@blacksheep.org>
Ogg: R: Re: Re: Re: LF: Re: I: Fw: For today the FETs survived...

that was also my thought.. but they run at room temperature: the
resonance is obtained with 4 polyesther 2200 pF 2000 Vdc caps. the
classic boxes 25x15x5mm

Marco

----
Messaggio originale----
Da: andy.g4jnt@gmail.com
Data: 1-giu-2017 22.03
A: <rsgb_lf_group@blacksheep.org>
Ogg: Re: Re: Re: LF: Re: I: Fw: For today the FETs survived...

If the conditions change as the PA is operating, it looks like
capacitors
heating up and changing its value.   I can't think of any other
component
that will change with dissipation / heating.  What type of capacitor
are
you using in the tank?

Andy


On 1 June 2017 at 19:59, marcocadeddu@tin.it <marcocadeddu@tin.it>
wrote:

> anybody has good ideas for a replacement hobby?? fishing? growing
> flowers?
>
> Andy: the suggestion of try without the guard circuit revealed that
> some effects in this area are present.
>
> I left the guard coil in place (I'm a bit lazy...) and disconnected
> simply the 2 wires from the rectifier bridge.
> The output improve of about 4dB with Vdd from 10 to 30V, the output
is
> stable up to 20Vdd, on 30V it show a peak of 16W, then the output
> starts to decrease till a couple of watts :-( this happens also at 40
> and 50V.
> The resonance of the output LC (with the guard coil open) shifts from
> 137 to 140 kHz and the bandwidth decreases from 20kHz to 10 kHz.
>
> I need a long weekend of meditation...
> 73 Marco IK1HSS
>
>
> ----Messaggio originale----
> Da: marcocadeddu@tin.it
> Data: 31-mag-2017 21.05
> A: <rsgb_lf_group@blacksheep.org>
> Ogg: R: Re: Re: LF: Re: I: Fw: For today the FETs survived...
>
> Andy.. you are almost better than online help desks :-D
>
> yes the guard circuit is on place but no current is flowing toward
the
> PA, testing disconnetting it needs just to warm up the iron ;-)
>
> the PS should
> provide enough juice for 1200W input and the IRF460A are rated for
20A
> @ 25°C (13A @ 100°C) so.. I admit it would like to give a try ;-)
>
> I don't guess the core is saturating specially at this power level
> where rms is only 22V, the core is 50mm OD and has 195,7 mm²Ae: if I
> am
> not totally wrong B should be < 0,03T @50V with 7 turns on the
primary
>
> Will tru to disconnect the guard circuit just in case the squirrel is
> running in its cage ;-)
>
> Thanks again for assistence
>
> Marco IK1HSS
>
>
>
> ----Messaggio originale----
> Da: andy.g4jnt@gmail.com
> Data: 31-mag-2017 20.39
> A: <rsgb_lf_group@blacksheep.org>
> Cc: <dead.fets@gmail.com>
> Ogg: Re: Re: LF: Re: I: Fw: For today the FETs survived...
>
> 7:19 turns (assuming 50R output)  means you have a load resistance of
> 6.8
> ohms which for 180V DC (81 V RSM fund sine) is nearly 1kW  I don't
> think
> you really mean to go that extreme do you?    13 ohms is more
> realistic.
>
> As for the tank resonance changing as power increases, that is very
> wrong.
> I wonder if the transformer is saturating.   Not sure of your core
Ae,
> but
> lets assume 200mm square, a core of about 16mm diameter.
>
> V = 4.44.F.N.A.B    Plugging in 137kHz 7 turns, 200 mm^2 and a Bmax
of
> 0.1
> that suggests 85V RMS.
> Which is exactlyly what you have.  I suggest more primary turns .
> Before
> a transformer ratio of 1:2 was suggested, for Rload = 13 ohms
>
> Is the guard circuit in place ?   Don't forget, it has to be
> customised
> to
> you exact currents and coil Q.  Get teh PA operating to its proper
> settings
> foirst - that you can do at low voltage power, it scales perfectly.
> Only
> when it it working properly can you add and set up the guard circuit.
>
> When I did teh 700W PA, I had a complete workign (albeit unreliable)
> unit
> before even thinking of teh guard circuitry.
>
> Andy
>
>
>
>
>
> On 31 May 2017 at 18:50, marcocadeddu@tin.it <marcocadeddu@tin.it>
> wrote:
>
> > Hi Andy... me again...
> >
> > I was so curious to see what could happen thatI had a very quick
> > dinner and connected all, but...
> >
> > now the output xfmr has 7T/19T here my
> > readings/calculations:
> > (see attached picture)
> > again the power increase from 10 to 30Vcc then from 30 to 50Vcc
after
> > an initial burst it start to fall down..
> > I checked also the resonance of the LC: till 30Vcc is tuned on 137
> kHz
> > with a 3dB bandwidth of 20 kHz, when I move to 40 and 50Vcc the
> > "maximum" output shifts to 165 kHz...
> >
> > mumble mumble
> >
> > I tempted to have roasted FETs for dessert and see what happens at
> > 180V!
> >
> > Marco, IK1HSS
> >
> >
> > ----Messaggio originale----
> > Da: andy.g4jnt@gmail.com
> > Data: 30-mag-2017 23.50
> > A: <rsgb_lf_group@blacksheep.org>
> > Cc: <dead.fets@gmail.com>
> > Ogg: Re: LF: Re: I: Fw: For today the FETs survived...
> >
> > I've just looked again at the circuit diagram you sent - on there
the
> > values are different from your statement in the email.  It shows
> > primary 5
> > turns, secondary 12 turns so a load resistance in the order of 9
ohms
> > which
> > is rather low if you are intending a Vdd of 180V - but closer to
the
> > ideal
> > Rl
> >
> > The tank components have a reactance of 130 ohms which is too high
a
> Q
> > is
> > used with that 9 ohms Rload, You should be aiming for a Q in the
> region
> > of
> > 6.
> >
> > Even with the optimum load R of 13 ohms described last time for 500
> > Watts
> > from 180V rail the resulting Q of 10 is a bit too high - you will
end
> > up
> > with high voltage and critical tuning
> >
> > Andy  G4JNT
> >
> > On 29 May 2017 at 19:07, Andy Talbot <andy.g4jnt@gmail.com> wrote:
> >
> > > Yes.
> > > As you'll see in my original write up, I originally forgot that
the
> > peak
> > > of the fundamental sine component of a square wave is GREATER
than
> > the peak
> > > by a factor of 4 / pi and initially my PA delivered a lot more
> power
> > (1.6
> > > times) than it was supposed to.
> > >
> > > So if the square wave has a peak value of 1, its fundamental sine
> > > component has a peak value of 4/pi or around 1.27.  The RMS of
the
> > > resulting sine  is SQRT(2) less than this giving a Peak square to
> RMS-
> > sine
> > > ratio of  0.9..   If you specifye peak-peak of the square wave, a
> > further
> > > factor of 2 applies, leading to the 0.45 ratio described before.
> > >
> > > Incidentally, this same ratio appears in that equation for  flux
> in
> a
> > > magnetic code,   V = 4.44.F.N.A.B
> > > The magic number 4.44  is actually SQRT(2) * pi     and comes
about
> > from
> > > the same sort of sine to square transform.
> > >
> > > Andy
> > >
> > > On 29 May 2017 at 18:48, marcocadeddu@tin.it <marcocadeddu@tin.
it>
> > wrote:
> > >
> > >>
> > >> uhuh... a slightly silly misleading assumption... Vdc are the
same
> > of
> > >> Vrms before FETs make their work!
> > >>
> > >> Thank you Andy for pointing out it!!
> > >> With this approach calculation changes a bit and probably with
the
> > >> right Xfmr  the PA can give higher satisfaction :-)
> > >>
> > >> Hopefully the FETs will survive and this time I'm ready to
burnout
> > the
> > >> antenna hi
> > >>
> > >> Will keep you both updated, thank you once more Andy
> > >>
> > >> 73 Marco, IK1HSS
> > >> ----Messaggio originale----
> > >> Da: andy.g4jnt@gmail.com
> > >> Data: 28-mag-2017 21.18
> > >> A: "marcocadeddu@tin.it"<marcocadeddu@tin.it>,
> > >> <rsgb_lf_group@blacksheep.org>
> > >> Cc: <dead.fets@gmail.com>
> > >> Ogg: LF: Re: I: Fw: For today the FETs survived...
> > >>
> > >> First thing I noticed is that your turns ratio on the output
> > >> transformer
> > >> doesn't look right.
> > >> You quote "* ... with primary winding of 15 turns and secondary
of
> > 12
> > >> turns...*"
> > >>
> > >> 180V DC in a half bridge is 180V peak-peak square wave.
> > >> The fundamental sine part of that is  4/pi * 180 = 229V pk-pk
> > >> so is 229V /[2.SQRT(2)] = 81V RMS
> > >>
> > >> To a good approximation RMS(fund) from a half bridge is Vrms
> (fund)
> =
> > >> 0.45VDC
> > >>
> > >> For 500 Watts out, Rload =  81 ^ 2 / 500 =  13 ohms
> > >>
> > >> So to match to 50 ohms you need a turns ratio of SQRT(50/13) =
> 1.9:
> > >> 1     so
> > >> call it 2:1  Keeping 12 turns on the  secondary means you need 6
> > turns
> > >> on
> > >> the primary
> > >>
> > >> When operating at reduced voltage, the power out will vary
exactly
> > as
> > >> the
> > >> square of the voltage.
> > >> Recalculating from first principles for a 12V supply:
> > >>
> > >> 12V  DC = 12V pk-pk = 12 / [2.SQRT(2)] * 4/pi = 5.4V RMS
> > (fundamental)
> > >> in 13 ohms should give 5.4^2/13 = 2.2 Watts
> > >>
> > >> check using ratio of voltages, squared :
> > >>
> > >> (12V/180V) ^ 2 * 500W = 2.2 Watts which is the same as above.
> > >> QED
> > >>
> > >> Your 15:12 ratio result sin a load impedance of (15/12)^2 * 50 =
> 78
> > >> ohms
> > >>
> > >> At 40V DC == 18V RMS(fund) that will give 18^2/78 = 4.1 watts
> which
> > is
> > >> actually LESS that you are seeing - the 2* discrepancy is odd,
but
> > the
> > >> low
> > >> power is in the area of what you measured..
> > >>
> > >> Andy  G4JNT
> > >>
> > >>
> > >>
> > >> On 28 May 2017 at 19:34, marcocadeddu@tin.it <marcocadeddu@tin.
it>
> > >> wrote:
> > >>
> > >> > Hi Chris,
> > >> >
> > >> > I tried to post this message on the reflector but apparently I
> had
> > no
> > >> > success..
> > >> > As promised I keep you updated but as you can read in the
> > >> > attachment the first trials were not enocouraging...
> > >> > Andy, may I ask you to read my report? your interpretation and
> > >> > suggestion are welcome!
> > >> >
> > >> > 73, Marco IK1HSS
> > >> >
> > >> >
> > >> > -----Original message-----
> > >> >
> > >> > From: "marcocadeddu@tin.it" marcocadeddu@tin.it
> > >> > Date: Sun, 28 May 2017 17:01:33 +0200
> > >> > To: rsgb_lf_group@blacksheep.org
> > >> > Subject: For today the FETs survived...
> > >> >
> > >> > Hi LF,
> > >> >
> > >> > hope that also the toroids of Chris survived!
> > >> > My FETs survived, but they are not working as expected :-(
> > >> > Attached the report on my attempt to duplicate the half bridge
> of
> > >> > Andy..
> > >> > Has anyone suggestions before I try to cook all connecting to
> the
> > >> > 180Vdc supply?
> > >> >
> > >> > Thank you
> > >> > 73 Marco IK1HSS
> > >> >
> > >> >
> > >> > --
> > >> > This message has been scanned by E.F.A. Project and is
believed
> to
> > be
> > >> > clean.
> > >> >
> > >> >
> > >> >
> > >> >
> > >> >
> > >> >
> > >> >
> > >> > ---------- Forwarded message ----------
> > >> > From: "marcocadeddu@tin.it" <marcocadeddu@tin.it>
> > >> > To: <rsgb_lf_group@blacksheep.org>
> > >> > Cc:
> > >> > Bcc:
> > >> > Date: Sun, 28 May 2017 17:01:33 +0200 (CEST)
> > >> > Subject: For today the FETs survived...
> > >> > Hi LF,
> > >> >
> > >> > hope that also the toroids of Chris survived!
> > >> > My FETs survived, but they are not working as expected :-(
> > >> > Attached the report on my attempt to duplicate the half bridge
> of
> > >> > Andy..
> > >> > Has anyone suggestions before I try to cook all connecting to
> the
> > >> > 180Vdc supply?
> > >> >
> > >> > Thank you
> > >> > 73 Marco IK1HSS
> > >> >
> > >> >
> > >> > --
> > >> > This message has been scanned by E.F.A. Project and is
believed
> to
> > be
> > >> > clean.
> > >> >
> > >> >
> > >> >
> > >> >
> > >>
> > >>
> > >>
> > >>
> > >>
> > >
> >
> >
> >
>
>
>
>
>
>
>
>
>