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: <[email protected]>
To: <[email protected]>
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: [email protected]
Data: 1-giu-2017 22.20
A: <[email protected]>
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: [email protected]
Data: 1-giu-2017 22.03
A: <[email protected]>
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, [email protected] <[email protected]>
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: [email protected]
Data: 31-mag-2017 21.05
A: <[email protected]>
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: [email protected]
Data: 31-mag-2017 20.39
A: <[email protected]>
Cc: <[email protected]>
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, [email protected] <[email protected]>
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: [email protected]
> Data: 30-mag-2017 23.50
> A: <[email protected]>
> Cc: <[email protected]>
> 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 <[email protected]> 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, [email protected] <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: [email protected]
> >> Data: 28-mag-2017 21.18
> >> A: "[email protected]"<[email protected]>,
> >> <[email protected]>
> >> Cc: <[email protected]>
> >> 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, [email protected] <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: "[email protected]" [email protected]
> >> > Date: Sun, 28 May 2017 17:01:33 +0200
> >> > To: [email protected]
> >> > 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: "[email protected]" <[email protected]>
> >> > To: <[email protected]>
> >> > 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.
> >> >
> >> >
> >> >
> >> >
> >>
> >>
> >>
> >>
> >>
> >
>
>
>
| 