Re: Re: Re: LF: Re: I: Fw: For today the FETs survived...

[Andy Talbot <andy.g4jnt@gmail.com>]

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.
> >> >
> >> >
> >> >
> >> >
> >>
> >>
> >>
> >>
> >>
> >
>
>
>