SCR (Silicon-Controled Rectifier)

The thyristor is a 4 layers semiconductor device which acts as an almost ideal switch.

The thyristor`s symbol and structure are:
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Looking at the previous picture: A = anode, G = gate C = K = cathode
Basic Operation
To understand the thrystor operación, the following picture shows an equivalent circuit
When we applied a current IG to the terminal G (base terminal of transistor Q1 and colector terminal of transistor Q2), two currents appears:IC2 and IB1, but IC2 = IB1.
IB1 is the base current of transistor Q1 and causes that a collector current flows (IC1).
IC1 in turn feeds the base of the transistor Q2 (IB2), causing more colector current in Q2 (IC2). IC2 is the same current IB1 that flows on the base of Q1, so.....
This regenerative process is repeateded until Q1 and Q2 saturate causing the thrystor to fire.
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The parameters of the SCR are:
- VRDM: Maximum reverse voltage (VG = 0)
- VFOM: Maximum direct voltage (VG = 0)
- IF: Maximum allowed direct current
- PG: Maximum dissipation of power between gate and cathode.
- TGV-IGT: Maximum voltage or current required in the gate (G) for priming (REVIZAR)
- IH: Minimum anode current required to keep the thyristor priming
- Dv / dt: Maximum voltage variation without producing priming.
- Di / dt: Maximum accepted current variation before destroying the thyristor.
Note: dv / dt, di / dt: See parameters of SCR in thyristor DC

Characteristic curve
The picture below shows the relation between the switching voltage and the gate current.

When the thyristor is reverse biased behaves like a common diode (see the leakage current feature).
The thyristor in its direct bias region also behaves like a common diode. (When the thyristor has already been activated (On)). See points D and E.

For high Gate current (IG) values (see point C), the anode-cathode voltage is lower (VC).
If IG decreases, the anode-cathode voltage increases. (see points B and A, and anode-cathode voltage VA and VB).
In conclusion, if we reduce the gate current IG, the anode-cathode voltage will tend to increase before the SCR goes ON
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