[R-390] Re: R-390 Voltage Regulator

[Bob Camp]

Hi

Depending on just how nasty you line voltage is you may have to use 
some fairly fancy parts for your series pass devices. Normal bipolar 
transistors suffer from secondary breakdown. This limits just how high 
in voltage you can go with them. This can give you a bit of a surprise 
when your "200 volt" parts turn out to only be good to 100 volts.

Since you have some filtering ahead of the regulator you won't see the 
8:1 type of spikes that are on a normal AC line. Anything much below a 
2:1 surge is asking for trouble. There are a variety of simple faults 
that will put 240 on your 120 line for a fraction of a second to maybe 
a second. That gets you up to something in the vicinity of 600 volts on 
the input to the regulator.

Obviously 200 ma at a 300 volt drop is going to have some power 
associated with it. A reasonable heat sink has a time constant long 
enough to handle a surge so that part of it probably won't be an issue.

The second thing you need to worry about is a short circuit on the 
output of the regulator. Unless you have some very exotic fuses the 
transistor will work better as a fuse than the fuse will. The math is 
about the same as for a over voltage. The transistors have about three 
hundred volts on them and the current is who knows what. If you have a 
current limiter in the design you get maybe 300 ma. If it folds back 
you might have 30 or 40 ma. With no limiter you depend on the line fuse 
and the current might be an amp or so.

We don't short circuit the output of our supplies on a regular basis. 
Line transients don't come along every morning at 8:32. Both things do 
happen and a fire as a result is not a reasonable outcome. A fire that 
totals the radio is especially not a good thing.

Since we really don't know all the numbers exactly derating is normally 
used to give the design some "margin". If we want to do it according to 
the original design numbers on the radio we would cut all the ratings 
on the transistors in half. That means finding a part that will handle 
600 volts and quite a bit of current.

A mosfet sounds like the most likely device to use. Coming up with a 
good high voltage driver that will survive the normal faults may be a 
bit exciting.

I have one 390 not an A and a whole box full of 6082's .....

	Take Care!

		Bob Camp
		KB8TQ