The silicon controlled rectifier SCR, is one of several power semiconductor devices along with Triacs (Triode AC’s), Diacs A Thyristors Two Transistor Analogy. Two Transistor model of Thyristor: The regenerative (latching) action of thyristor due to positive feedback can be explained using the two transistor model of the. SCR-Two Transistor analogy. Pradeep Chaudhary POWER Chopper drives-Two quadrant chopper drives ยท Electric drives-Three phase DC drives.

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The increase in values of both a 1 and a 2 would further increase the value of anode current I A which is a regenerative or positive feedback effect. The device has ideal states, i.

A thyristor can be considered as two complementary transistors. Under transient conditions, the capacitances of the pn junctions influence the characteristics of the thyristor. Connects Facebook Youtube Videos.

Silicon Controlled Rectifier SCR | Two Transistor Model | Operating Principle

Combining the two collector currents I C1 and I C2 yields But we cannot turn it OFF by control signal. This device has tow states i. That is why the name of thyristor consists of first four letters of thyratron tube and last five letters of transistor. Although there are many different members are available in thyristor family, but silicon controlled rectifiers are so widely used that as if thyristor and SCR become synonymous.

One being pnp and the other npn. It can block both forward and reverse voltage but can conduct only in one direction. An increase of I A which is an increase of I E1 would increase a 1 as shown in figure 4. Twoo capacitance of the pn junctions are shown in figure 4.


Anaolgy current gain a 1 varies with emitter current I E1 which is equal to I A ; and a 2 varies with emitter current I E2 which is equal to I k.

The value current then can only be controlled by external resistance of the circuit. Related pages Concept of Power Electronics.

SCR-Two Transistor analogy

In other words, it can be said, that the characteristic of thyristor is combination of characteristics of thyratron tube and transistor. If we see from the constructional and operational point of view, it is a four layer PNPN three terminals Anode, Kf, Gate semi controlled device.

It is a silicon based semiconductor device, which is used in electrical circuits for switching operation.

Similarly, the collector current for transistor Q 2 is I C2 where The current through capacitor C j2 can be expressed as. If a thyristor is in the blocking state and a rapidly rising voltage is applied anzlogy the device, high currents would flow through the junction capacitors. It must be noted that a large current through the junction capacitors may cause damage to the device.

This is a pnpn thyristor.

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If we bisect it through the dotted line then we will get two transistors i. By substituting this valyue of I k in iii we get, From this relation we can assure that with increasing the value of towards unity, corresponding anode current will increase.

Now the question is how increasing? SCRwhose full form is silicon controlled rectifieris also a well known member analgy thyristor family. The two-transistor model is shown in figure 4. If a 1 and wto 2 approach unity, the denominator of annalogy 6 approaches zero and a large value of anode current is produced causing the thyristor to turn on as a result of the application of a small gate current. This model is used to demonstrate the regenerative or latching action due to positive feedback in the thyristor.

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At the first stage when we apply a gate current I git acts as base current of T 2 transistor i. The characteristic of thyristor consists of the characteristic of thyratron tube and characteristic of transistor.

Here is the explanation using two transistor model of SCR. This continuous positive feedback effect increases towards unity and anode current tends to flow at a very large value. The emitter current of transistor Q 1 analkgy the anode current I A of the thyristor and collector current I C1 is given by.

A typical variation of current gain a with emitter current I E is shown in figure 4.

Two transistor model of Thyristor | Fundamentals of Power Electronics (Subject Code-EE)

Similarly, the collector current for transistor Q 2 is I C2 where. We can turn anlaogy ON by sending a gate current signal between second P layer and cathode. That means we have control anaolgy its turn ON, once it goes to conduction mode, we lose control over it.

If the gate current I G is increased from zero to some positive value, this will increase the anode current I A as shown by equation 6.