Dr. Gerald N. Johnson wrote:
> TenTec PAs are relatively unique in being
> able to handle a great deal of SWR without frying instantly and so a
> simple DC current protecting is adequate whether fuse, fast circuit
> breaker, or power overcurrent shut down. In any of these the protection
> works in tens or 100s of milliseconds while without TenTec's circuit and
> thermal care the lifetime of the over voltaged RF power transistor may
> be half a cycle at 30 MHz, something like 17 nanoseconds.
>
I'm trying to understand this. What part of Ten-Tec's circuit keeps the
PA transistors from frying in a half cycle at 30 MHz? Looking at my Omni
VI PA schematic C25 a 33ufd capacitor on the PA board would hold enough
charge to supply the PA board for well over 17 nanoseconds, by my
calculations.
PA system load on the 12 VDC supply is roughly equal to 0.5 ohm if we
use the rough figures of 12 volts and 24 amperes. Time constant of 33
ufd and 0.5 ohms is about 16 microseconds. Or if in a high SWR condition
if the effective DC load resistance of the PA goes down to 0.05 ohms
there is still a full microsecond of charge available in that capacitor.
So even if the power supply current limiting, whatever method is used,
could shut down the supply in a nanosecond, there is enough charge in
C25 to keep the PA transistors running for over 17 nanoseconds. It looks
to me like other electrolytics in the bias circuitry would also keep
things going for longer than 17 nanoseconds, even if some fast shutdown
of the bias is attempted.
Is it just beefy PA transistors that keeps them from frying in a half
cycle at 30 MHz?
DE N6KB
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