[TenTec] (no subject)

[Dr. Gerald N. Johnson]

On Sun, 2007-09-23 at 17:12 -0700, Kevin Purcell wrote:
> 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.

True for excessive power situations.
> 
> This is not an over-voltage (punch through) failure which would  
> happen in the RF cycle timescale.

Its the punch through failure that I'm saying can happen in 1/2 RF
cycle, not a thermal failure.
> 
> 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.
> 

Seems to. Depends on the thermal time constant of the transistor package
as well as the heatsink. Buying robust transistors can improve their
lifetime, whether from excess RF voltage or excess collector current.
Using more than the minimal heatsink can significantly lengthen the
heating time constant.

> On Sept 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
> 
> 

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer