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

To:	"[email protected]" <[email protected]>, [email protected]
Subject:	LF: Re: I: Fw: For today the FETs survived...
From:	Andy Talbot <[email protected]>
Date:	Sun, 28 May 2017 20:18:12 +0100
Cc:	[email protected]
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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] <[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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