Re: LF: RE: QRP 1W /PA circuit

[Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de>]

Hi Rik,

The driver is a IXDD414CI which drives the 4 FETs by using a ferrite core (Al=4000, OD=20mm). Primary 4 turns in series with 2 Ohm and 10uF. Then there are 4 secondary widings with 5 turns each. Each FET has a 1 Ohm Gate resistor in front. This is to prevent overswing at turn on/off. Using a ICL7667, the gate resistors are not necessary (but the 7667 cannot drive the 4 FETs with sufficient switching times) but the IXDD414 is another world (i use it in a 300W PA at 10 MHz).
I can make a drawing if necessary.
BTW trying the ferrite core principle instead of the IR2113 (which has problems at 137 kHz and 4 nF input C on the H side, at least at high operating temperatures and after some hours of operation) is an idea of Markus/DF6NM.

The Gates are driven very securely by that method. Also it is much easier to configurate and only needs a single low-side driver for that "complex" arrangement. Another advantage is that once a FET is blown up, making a short cut between Gate and Source, it also prevents the other FETs to get switched on, so it may help to protect the other FETs. A poly switch fuse in series ( i like them ;-) ) helps to pretect the driver from overheating when operating on that short cut.

73, Stefan/DK7FC

Am 11.01.2012 15:27, schrieb Rik Strobbe:

Hi Stefan,

 

how do you drive the H-bridge amp ?

 

73, Rik  ON7YD - OR7T

 

---

Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens Stefan Schäfer [Stefan.Schaefer@iup.uni-heidelberg.de]
Verzonden: woensdag 11 januari 2012 13:34
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: LF: QRP 1W /PA circuit

LF,

Last night i shot my PA again :-( No replacement FETs were available so i had to stay quite during the night. Replacement parts are ordered now and will arrive tomorrow :-)

During that time i will run a LF beacon on 136.172 kHz running 1W TX power in DFCW-90, starting in the afternoon. If someone can receive that weak QRPP signal, then a report is very apprechiated :-)

Also i will do some optimisations on my recently posted PA output stage (http://dl.dropbox.com/u/19882028/LF/DK7FC_137kHz_TX_antenna_and_PA_schematic.jpg), just in case someone wants to build a similar design and is interested in general:

-R= 100 Ohm/2W (In series with C, across  Drain-Source)
-C= 220pF/WIMA FKP-1 2kV  (In series with R, across Drain-Source)
-MOSFET: The IRFP460 will be replaced by a SPW20N60C3 :-)

73, Stefan/DK7FC