Talbot Andrew wrote:
snip
> In the early days of 73kHz I tried this approach, but the high Q
> output network was too prone to destroying devices when mistuned, and
> particularly when the antenna went off resonance. The ultimate design
> was a half bridge circuit running off 340 Volt rectified mains, output
> transformer and C/L tank using an air wound inductor rather than
> conventional Pi filter. For a time it delivered 700 Watts into a
> dummy load of multiple 150W light bulbs quite happily - until I keyed
> the circuit a few times. Remember what the resistance of light bulbs
> are when cold and the current surges when switched on. There is a lot
> of energy in 1000uF of capacitors charged to 340V. Never did like CW
> ! After blowing four IRF450 fets spectacularly plus two bridge
> driver chips I decided this was rather dangerous and kept with
> conventional class B designs from then on.
I also experimented with a bridge circuit (a modification of a surplus
SMPS, but not using rectified mains). I had a "blow up" and it took out
all four MOSFETs. In earlier days for LF transmitting I tried using a
DC coupled Clas AB audio amplifier but with replacement "fast" bipolar
transistors, and when that blew it took out four transistors (including
drivers) and some time probing around to confirm the extent of failed
transistors in the DC coupled circuit.
Of the several n+n pushpull LF amplifiers I have made, using transformer
coupling for input and output, when there has been a failure it is
usually one transistor breaking down, so it is easy to diagose and
relatively cheap to restore. Using suitable varistors across each side
of the pushpull stage is a prudent precaution to protect against
transient spikes, and at LF the capacitance of the varistor is
negligible.
73, Bob ZL2CA
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