dV/dt triggering of a SCR(Thyristor)

[Nandika]

Hello Friends,

Following quote is copied from Wikipedia(Link:http://en.wikipedia.org/wiki/Silicon-co ... _rectifier)
I have learned something about SCR(Thyristor).In normally we can apply gate threshold current(=Latching current) to turn on the SCR.
In following statement says dV/dt method for turn on a SCR.
I cant understand this method.It's unclear.

If the applied voltage increases rapidly enough, capacitive coupling may induce enough charge into the gate to trigger the device into the "on" state; this is referred to as "dv/dt triggering." This is usually prevented by limiting the rate of voltage rise across the device, perhaps by using a snubber. "dv/dt triggering" may not switch the SCR into full conduction rapidly, and the partially triggered SCR may dissipate more power than is usual, possibly harming the device.

What is this " applied voltage increases rapidly " means?
What is "capacitive coupling"?

Please anyone help me to understand this way.

Thank in advance...

[SevenZero]

SCR.png (3.25 KiB) Viewed 7951 times

dv/dt Triggering of a SCR will be performed as below.

- When the device is forward biased, J1 and J3 are forward biased, J2 is reverse biased.
- Junction J2 behaves as a capacitor, due to the charges existing across the junction.
- If voltage across the device is V, the charge by Q and capacitance by C then,
ic = dQ/dt
Q = CV
ic = d(CV) / dt
= C. dV/dt + V. dC/dt
as dC/dt = 0

ic = C.dV/dt

- Therefore when the rate of change of voltage across the device becomes large, the device may turn ON, even if the voltage across the device is small.

[Nandika]

Thanks ZevenZero..

I understood it simply according to your answer.But,This has big theory behind.
Equation manipulation is not clear.It's my problem,I dont know calculus
I must learn it.

dV/dt triggering is clear.

[SemiconductorCat]

Thanks ZevenZero..

I understood it simply according to your answer.But,This has big theory behind.
Equation manipulation is not clear.It's my problem,I dont know calculus
I must learn it.

dV/dt triggering is clear.

you mean this,
>> ic = d(CV) / dt
= C. dV/dt + V. dC/dt

it's simple product rule know.
http://en.wikipedia.org/wiki/Product_rule
here there's no partial differential equations involved. IT's simply the product rule.
And SevenZero , please be kind to label what is Ic there.


however something called that 'dv/dt driver circuit' may not the easy to understand either.
Anyway I heard that you alread told that's not what you looking for.

[Nandika]

Thanks SemiconductorCat,

This system (dV/dt triggring) has semiconductor level explanation.I have an idea about this capacitor/capacitance which generated J2 junction.
I think,I have small idea about semiconductor level explanation.