[TenTec] (no subject)

[Kevin Purcell]

The transistor "fry" because they heat up, their current gain  
increases and the collector current runs away.

<http://en.wikipedia.org/wiki/Thermal_runaway#Bipolar_transistors>

You aren't going to be able to heat them up in 17nS they are  
thermally too massive. The time constant in the power supply is not  
the determining factor. The thermal time constant of the system is  
the important issue.

This is not an over-voltage (punch through) failure which would  
happen in the RF cycle timescale.

I suspect (but don't know for sure) that this time constant is in the  
hundreds of milliseconds or possibly even longer. So a rapidly  
responding (but not too rapid) circuit breaker or fuse works as  
effective SWR protection.

On Sep 23, 2007, at 1:32 PM, Ken Brown wrote:

> 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.

--
73 DE N7WIM / G8UDP
Kevin Purcell
kevinpurcell at pobox.com