Dear group,
Since a long time i wanted to publish the circuit of my "big" VLF PA.
It is titled as a 600W PA since it has never handled more power so far,
due to the power limit of my /p generator. In fact i think it will
handle at least 1 kW, probably even 2 kW. As announced i will try a 2
kW generator in my next VLF experiment and i am not afraid to use this
PA for the first test.
The circuit is nothing new or very special. The mains voltage is
rectified and stabilized by a 2.2 mF capacitor. A H bridge using 4x
IRFP 460 FETs is switching at the desired frequency. To achieve an
accurate 50% duty cicle, a CMOS4013 is used in front of the
MOSFET driver. It is switched as a frequency divider (/2), thus the
input frequency has to be 2x the actually wanted frequency (17.94 kHz
> 8.97 kHz). The driver(s) are 2x IR2110. The operating
frequency is about 0.1...20 kHz (320 VDC) or 0.1...150 kHz (100
VDC). It is designed to work properly below 9 kHz. The input waveform
has to be a 12Vp rectangular waveform, ideally. The waveform is not
very critical, e.g. a sinusoidal or triangular waveform is no problem
as well.
A small transformer is used to generate a local 15V supply for the
drivers, ICs and LEDs.
There are several methods how the input signal can be applied. Either
directly (please use an audio transformer to decouple the PC from the
circuits ground which has mains potential!), as shown in the circuit or
via a fiber optic cable, as i do it. The fiber optic solution is a
special and (for some) strange method so i prefer to show the usual way
to do it...
If the mains is used to supply the PA (much lower output impedance), a
resistor has to be used to charge the 2.2 mF capacitor. A 100 W/230V
lamp might be used. If the capacitor is charged, this resistor should
be bypassed by a relay.
One has to take care about the output: There is no isolating
transformer used so the output has main potential! Thus, a
galvanic isolated coupling winding has to be used for applying the RF
to the loading coil. No ferrite output transformer is needed!
This PA is still using the first pair of FETs, there was no fault
so far. It was received in several countries that it is no problem
to transmit a one hour carrier at P > 500 W. Here, the heat sink
gets not even hand warm! So the efficiency can be assumed to be almost
100%.
The design can handle pretty much reactive power. It was no problem to
drive an antenna that is totally out of resonancy (kite lying on the
ground or wire suddenly disconnected due to strong pull).
The actual (active) output power can be regulated continuously by
adjusting the resonance.
So, if one of the new VLF TX stations want to change from an audio PA
to a high power switching mode PA, this well proven design may give
some ideas. Just ask if something is not understood.
See once again one of my very amateur like ugly hand drawings ;-) at http://www.iup.uni-heidelberg.de/schaefer_vlf/pic/DK7FC_600W_VLF_PA.pdf
Vy 73, Stefan/DK7FC
PS: Once again i explicit want to say that one has to take care about
the parts that are on mains potential! If you are not sure about the
handling, add an isolating transformer at the 230V AC input!
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