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

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

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.
>
>
>
>