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.
"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.
--
Thanks,
- Win
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.
-- -- kensmith@rahul.net forging knowledge
(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.
-- -- kensmith@rahul.net forging knowledge
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
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