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: 30mag2017 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 peakpeak 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] <[email protected]>
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: 28mag2017 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 peakpeak square wave.
>> The fundamental sine part of that is 4/pi * 180 = 229V pkpk
>> 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 pkpk = 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] <[email protected]>
>> 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.
>> >
>> >
>> >
>> >
>>
>>
>>
>>
>>
>
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