>1. The problem regarding the break-down voltage of transistors or fets
is >really very simple to get around. In the off-line switch mode power
supply >industry this problem was solved many years ago by the simple
expedient of >connecting 2 transistors in series to double the break-down
rating, exactly >the same as putting several zeners in series to achieve a
higher voltage. By >utilising a low cost IC known as a "High-side /
Low-side" driver the top >transistor base / gate is operated at the required
higher voltage (half-rail)and >the bottom device is simply treated as a
single transistor shunt stabiliser. >The two transistors are connected in
series and the bases / gates are >effectively controlled in parallel, except
the top transistor has the higher >required voltage to ensure correct
operation. High-side driver IC's are >available with up to 600V operating
capabilities and cost a couple of $'s. The >circuit operates as the classic
shunt stabiliser, in my opinion the only >suitable solution for a tetrode
screen supply.
Still doesn't necessarily get around second breakdown. The problem is that
bipolar transistors can have a lot of dissipation or a lot of volts, but
there are fundamental difficulties in having both at the same time. It's
possible that insulated base devices might manage it.
The idea of a current limited (or constant current) source driving a shunt
regulator may not be the only way to go, but it seems to me the most
sensible.
73
Peter G3RZP
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