5SHY3545L0009 ABB 可控硅的原理

5SHY3545L0009

What is a thyristor?

The 5SHY3545L0009 thyristor is a unidirectional semiconductor device made of silicon, basically a thyristor (SCR) is a three-terminal, four-layer semiconductor device consisting of alternating P-type and N-type materials.

The thyristor has three pn junctions J1, J2 and J3, and the figure below shows a thyristor with a pnpn layer. The thyristor has A terminal anode (A), a cathode (K), and a gate G), and the gate terminal (G) is connected to a p layer near the cathode (K) terminal.

Second, a thyristor is like two transistors

The 5SHY3545L0009 transistor four layers work like two transistors, they are connected together (as shown below), the output of one forms the input of the other, and the gate acts as a kind of “starter motor” to activate them.

A single thyristor (SCR) is a combination of a pnp transistor (Q1) and an npn transistor (Q2). Here, Q1’s emitter acts as the anode terminal of the SCR, while Q2’s emitter acts as its cathode. In addition, the base of Q1 is connected to the collector of Q2, the collector of Q1 is connected to the base of Q, and the gate terminal of the thyristor is also connected to the base of Q2.

Three, three states of the thyristor

5SHY3545L0009 So how do thyristors work? We can put it in three possible states, in which it is either completely off or fully on, meaning that it is essentially a binary digital device.

1. Thyristor forward blocking mode

Normally, the thyristor is turned off in the absence of current flowing into the gate: no current can flow from the anode to the cathode.

5SHY3545L0009 Here you can think of the thyristor as two diodes connected together, with both the upper and lower diodes being forward-biased. However, this means that the junction at the center is reverse-biased, so the current cannot flow from the top all the way to the bottom. This state is called forward blocking. Although it is similar to the forward bias in conventional diodes, no current flows.

5SHY3545L0009 Here, positive bias is applied to the thyristor by connecting the positive terminal (A) to the positive terminal of the battery and the negative terminal (K) to the negative terminal of the battery, as shown below. In this case, the junctions J1 and J3 get a forward bias, while the junctions J2 get a reverse bias.

Here, except for the tiny currents that flow as saturated currents, the current also cannot pass through the thyristor, as shown by the blue curve in the characteristic curve below.

2, thyristor reverse blocking mode

Now suppose we reverse the anode/cathode connection, we might see that both the upper and lower diodes are reverse-biased, so there is still no current flowing through the thyristor. This is called reverse blocking (similar to reverse bias in simple diodes).

In this mode, the thyristor is reversely biased by connecting the anode terminal (A) of the thyristor to the negative terminal of the battery and the cathode terminal (K) to the positive terminal of the battery, which causes the reverse bias of the junction J1 and J3, which in turn prohibits current from flowing through the thyristor, although the junction J2 remains in the forward bias state. In this state, the thyristor behaves like a typical diode.

In this reverse bias condition, only the reverse saturation current flows through the device, as in the case of the reverse bias diode, which is represented by the blue line on the characteristic curve. The thyristor also exhibits reverse breakdown beyond the reverse safety voltage limit, just like a diode.