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cascode switching

K

kell

Jan 1, 1970
0
Carrying your logic one step further, do we not come across a second
mode that might have destroyed the transistor:

Once the voltage produced by the inductive current collapse peaks, it
starts back down through zero on the negative swing of the ringing
wave form. At the point where the voltage at the collector drops 0.7
volts negative (referenced to its other end at B+), the collector-base
junction of the transistor becomes forward biased through the 10 Ohm
resistor. The collector-base junction becomes a 0.7 volt voltage clamp
for all the stored energy with only a 10 Ohm resistor for a load.
It would explain why this particular topology failed where the others
didn't.

I'd be interested to hear your response.

Rick

I think it's the initial spike that fried the base collector junction.
I put a cap across the coil as Jim suggested and the circuit operates
now, though I haven't put it through thousands of cycles. As someone
here mentioned already, I think the cap slowing down the dV/dt at the
collector gave it time to sweep the charge out of the base and turn off
quick enough so it would withstand the continued rise of the voltage.
Of course yes it rings, even more with the cap on it I guess. I
suppose it's possible the base-collector diode is getting a baker's
dozen millijoules pushed through it on the backswing. I hadn't thought
about that, but maybe it won't hurt. I don't have a scope. I'll just
have to see how things go out on the road!
 
K

kelvin_cool_ohm

Jan 1, 1970
0
I think it's the initial spike that fried the base collector junction.
I put a cap across the coil as Jim suggested and the circuit operates
now, though I haven't put it through thousands of cycles. As someone
here mentioned already, I think the cap slowing down the dV/dt at the
collector gave it time to sweep the charge out of the base and turn off
quick enough so it would withstand the continued rise of the voltage.
Of course yes it rings, even more with the cap on it I guess. I
suppose it's possible the base-collector diode is getting a baker's
dozen millijoules pushed through it on the backswing. I hadn't thought
about that, but maybe it won't hurt. I don't have a scope. I'll just
have to see how things go out on the road!
Thanks for the follow-up. I was intrigued. Now I've learned something
new.

Thanks also to John Popelish.

Rick
 
K

Ken Smith

Jan 1, 1970
0
I tried firing a car ignition coil by connecting
a 400 volt transistor in cascode, like this:

+12 v
|
|
+--+-------+
| |
| |
10R )
| ignition)
| coil )
| )
| _______)
| c/
+--| MJE13009
e\
|
|
points
|
|
gnd

I've built circuts sort of like this. It didn't involve points. Several
things can be going on and many of them are bad. I'll take them sort of
in order.

At the instant of the points closing, the fact that the coil has a largish
stray capacitance makes a current spike. Check that the 10R limits the
base current enough to keep the transistor safe.

At the instant the points open the transistor is saturated. It will take
it a little time to stop conducting. During that time, the emitter goes
above 12V and the E-B junction can break down.

Just as the transistor is actually turning off, there is a large collector
bas voltage and a collector current flowing. This can lead to 2nd break
down issues.

When (and if) the spark happens on the secondary, a burst of high
frequency stuff is produced on the primary. If the collector to emitter
capacitance is enough, this can also breakdown the e-b junction. I don't
think the auto-ignition coil make a high enough frequency for this to
happen.

When the primary rebounds, it will likely swing well below the +12V. This
will forwards bias the collector junction and flow a current. Check that
the 10R is enough to keep that current within bounds.

The same rebound caused current will pump carriers into the base so that
when the collector flies up again, there is another recovery time.

I suggest you try:

Place a diode in series with the collector so that the current can't
reverse.

Add a Baker clamp to the transistor.

Hook a string of 4 diodes in series from the transistors emitter to the
+12V.
 
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