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

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

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


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---------- 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.