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BJT under surge condition

J

Jack// ani

Jan 1, 1970
0
Hi all,

I'm reading power electronics...can't understand why this happens
so with a BJT.

Under surge condition Gate turn-off thyristor(GTOs) goes into deeper
saturation due to regenerative action. On the other hand, a bipolar
transistor tends to come out of saturation.

Please explain me.

Thanks
 
J

John Popelish

Jan 1, 1970
0
Jack// ani said:
Hi all,

I'm reading power electronics...can't understand why this happens
so with a BJT.

Under surge condition Gate turn-off thyristor(GTOs) goes into deeper
saturation due to regenerative action. On the other hand, a bipolar
transistor tends to come out of saturation.

Please explain me.

Thanks

Do you have a data sheet of part number documenting this property?
The only reason I can imagine is one involving temperature rise.
 
W

Winfield Hill

Jan 1, 1970
0
John Popelish wrote...
Do you have a data sheet of part number documenting this property?
The only reason I can imagine is one involving temperature rise.

"Deeper into saturation?" If deeper means a lower saturation voltage,
then of course not. But if it means effectively delivering more drive-
current from the load, to handle the increased load current, then yes.
Yawn.
 
T

Terry Given

Jan 1, 1970
0
Jack// ani said:
Hi all,

I'm reading power electronics...can't understand why this happens
so with a BJT.

Under surge condition Gate turn-off thyristor(GTOs) goes into deeper
saturation due to regenerative action. On the other hand, a bipolar
transistor tends to come out of saturation.

Please explain me.

Thanks

To quote an un-named lecturer, "some semblence of understanding is
required". This question belongs in SEB, not here. But the answer is
very simple. The "T" in GTO stands for Thyristor, These have positive
feedback maintaining base drive, so as load current goes up, base
current goes up too.

In a BJT, base current is normally supplied by an external circuit, and
as such is fixed(*). as load current goes up, the BJT runs out of gain,
so Vce increases (all the way up to the supply voltage in many cases).

(*) a proportional base drive circuit (a-la Severns) will of course
handle an overload without "de-saturating", as the base current is a
constant(ish) portion of the load current. Until the transistor
disappears in a puff of smoke :)

Cheers
Terry
 
K

Ken Smith

Jan 1, 1970
0
Do you have a data sheet of part number documenting this property?
The only reason I can imagine is one involving temperature rise.

IGBTs are a lot like MOS gated SCRs. In the N channel device, there is an
N channel MOSFET and a wanted PNP bipolar. There is also an unwanted NPN
structure in there. This NPN has a very low HFE and has a low resistance
path from its base to emitter but at high currents, it does start to add
some base drive to the PNP.

This slows the turn off a bit.
 
K

Ken Smith

Jan 1, 1970
0
John Popelish said:
Do you have a data sheet of part number documenting this property?
The only reason I can imagine is one involving temperature rise.

(I hit a wrong button and posted a partly completed copy)

Going "deeper into saturation" at high currents is very common in things
like SCRs triacs and IGBTs

The all have 4 layers of silicon that if you look at it looks like an NON
hooked to a PNP like this:


-----
e!
\!
PNP !-----
/! !c
c! !/
------! NPN
!\
!e
-----

In the SCR and triac, the positive feedback is desired. In the IGBT it is
really unwanted.

In the N channel IGBT, there is an N channel MOSFET and a wanted PNP
bipolar. There is also an unwanted NPN structure in there. This NPN has
a very low HFE and has a low resistance path from its base to emitter but
at high currents, it does start to add some base drive to the PNP.

This slows the turn off when the current just before turn off is higher.
 
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