The silicon controlled rectifier (SCR) is part of the family of 4-layer
silicon devices. To comprehend its structure, let's revisit the
structure of a diode.
A diode consists of a PN junction, with the P
channel containing mostly positive charges and the N channel mostly
negative charges. Similarly, the 4-layer device has a PNPN structure, comparable two BJT transistors. The first transistor is PNP-type, with the
anode connected to the emitter, and the second one is NPN-type, with the
cathode connected to the emitter. The following image shows the difference between diode and the 4-layers device.
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| (a) Diode structure, (b) 4-layers device structure |
Now, let's explore the two-transistor analogy. As mentioned earlier,
we can simplify the PNPN structure into two BJT transistors: PNP and
NPN. In this setup, the anode of the device corresponds to the emitter
of the PNP transistor. The gate of the PNP transistor is connected to
the collector of the NPN transistor, while the collector of the PNP
transistor is linked to the gate of the NPN transistor. Finally, the
emitter of the NPN transistor corresponds to the device's cathode. The SCR, or Silicon Controlled Rectifier, features a gate terminal
connected to the base of the NPN transistor and the collector of the PNP
transistor, as
illustrated in the following image.
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| SCR internal structure |
Now, following our comprehension of the internal structure of the SCR,
let's explore the I-V characteristic curve. The 4-layer device
remains off if the forward current (IF) is less than the holding current
(IH). To initiate device conduction, the forward voltage (VF) must be
increased until it reaches the forward-breakover voltage (VBR). At this point, the forward current (IF) equals the switching current (IS),
activating the internal transistor and causing a subsequent drop in
voltage across the device, rendering it active (ON). Furthermore, the
device can also be triggered by augmenting the reverse voltage (VR);
however, once it attains the maximum reverse break-over voltage, it
cannot return to its normal state.
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| 4-layer device characteristic curve |
The SCR adheres to the same IV curve; however, it possesses the capability to be triggered through the gate. Activating the SCR merely requires sending a pulse through the gate. If
the forward current exceeds the holding current, the device remains ON. To turn-off the device, we can use either the current interruption method, wherein the forward current is either cutoff or reduced until it diminishes below the holding current threshold. Alternatively, the forced commutation method can be utilized by directing current through the SCR in reverse, thereby causing the net current to drop below the holding current. The following image illustrates the current interruption method and the forced commutation method. In figure (a) the device get activating by sending a pulse through the SCR gate, and to turn-off the deice we need to close the switch. By closing the switch we reducing the forward current to fall below the holding current causing the device to turn off. The second figure (b) the device turn-off by inducing a revirce current through the SCR.
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| (a) SCR turn on using current interruption (b) SCR turn-off using forced commutation |
In further discussions we will discuss about the applications of SCR, I hope you find this useful.