A thyristors is a three-terminal semi-controlled device, having four layers of alternating p-type and n-type semiconductor layers (i.e. three p-n junctions) comprising its main power handling section. In forward bias, thyristors turn-on can be controlled using a gate terminal. The other two terminals called anode and cathode handle the large applied potentials(often of both polarities) and conduct major currents through the thyristors.

It acts as bistable switch, conducting when a suitable gate current is applied and continuing to conduct as long as there is a sufficient forward voltage across its main terminals. They are known for their ability to handle high power levels and can be turned on and off rapidly, making them useful in controlling large amounts of electrical power. Some of the common applications of thyristors include power control, motor drives, lighting dimmers, and voltage regulation.

Types of Thyristors

Infineon offers two types of thyristors with different dimensions, technologies, and housings. These are tailored for diverse applications, including phase control, uninterruptible power supplies (UPS), and soft start applications.

Based on construction Infineon has classified two types of thyristors:

  • Disc type
  • Power Block-Module 

Based on the triggering method thyristors are classified into the following types:

  • SCR’s and GTO’s
  • MOS-controlled
  • Static Induction
  • Optically triggered
  • Bi-directional

How Does a Thyristors Work?

When a positive voltage is applied to the anode (w.r.to cathode), the thyristors is in its forward-blocking state. The center junction, J2 is reverse-biased. In this operating mode, the gate current is held to zero (open circuit). In practice, the gate electrode is biased to a small negative voltage (concerning the cathode) to reverse bias the GK-junction J3 and prevent charge carriers from being injected into thep-base.

In this condition thermally generated leakage current flows through the device and can often be approximated as zero in value (the actual value of the leakage current is typically many orders of magnitude lower than the conducted current in the on-state). As long as the forward applied voltage does not exceed the value necessary to cause excessive carrier multiplication in the depletion region around J2 (avalanche breakdown), the thyristors remains in an off-state (forward-blocking).

If the applied voltage exceeds the maximum forward-blocking voltage of the thyristors, it will switch to its on-state. However, this turn-on mode causes non-uniformity in the current flow, which is generally destructive, and should be avoided.

When a positive gate current is injected into the device, J3 becomes forward-biased and electrons are injected from the n-emitter into the p-base. Some electrons diffuse across the p-base and get collected in the n-base. This collected charge causes a change in the bias condition of J1. The change in bias of J1 causes holes to be injected from the p-emitter into the n-base. These holes diffuse across the n-base and are collected in the p-base. The addition of these collected holes in the p-base acts the same as the gate current. The entire process is regenerative and will cause the increase in charge carriers until J2 also becomes forward-biased and the thyristors is latched in its on-state (forward-conduction).

The regenerative action will take place as long as the gate current is applied in a sufficient amount and for a sufficient length of time. This mode of turn-on is considered to be the desired one as it is controlled by the gate signal.

Once the thyristors has moved into forward conduction, any applied gate current is excessive. The thyristors is latched and, for SCRs, cannot be returned to a blocking mode by using the gate terminal. Anode current must be commutated away from the SCR for a sufficient time to allow stored charge in the device to recombine. Only after this recovery time has occurred, can a forward voltage be reapplied and the SCR again be operated in a forward-blocking mode. To turn off the SCR external circuit is required.

Difference Between Power Diode and Thyristors:

The below table provides the difference between Power diode and Thyristors:

What is a Power Diode?

The power diode is a two-terminal, uncontrolled switch having one p-type and one n-type semiconductor layer. The positive terminal is called the anode and the negative terminal is called the cathode.

Power diode is specifically designed to handle high voltages and currents, making it suitable for power electronics applications. The most common type of power diode is the silicon diode, which is widely used in various electrical and electronic devices.

Types of Power Diodes

Infineon offers various types of power diode for customer systems.

  • CoolSiC Schottky diodes
  • Si Ultra soft diodes
  • Si Rapid 1 and Rapid 2 diodes
  • EC diodes
  • Welding diodes
  • Diode discs
  • Diode modules

Typical applications of Power diodes and Thyristors:

Power diode and thyristors find applications in various power electronics and control systems.

  • Input rectifier for drives
  • Low and medium voltage soft starter
  • Input rectifier and bypass for UPS
  • Crowbar for wind applications: Once the DC-bus voltage exceeds a defined protection level the crowbar is triggered and discharges the DC-bus capacitor.

These applications demonstrate the versatility of power diode and thyristors in controlling and manipulating electrical power in diverse systems, ranging from basic rectification to complex power control and regulation.

Comparison between Power diode and thyristors

The below table provides a comparative overview of the differences between power diode and thyristors, including their functions, controllability, structure, turn-on and turn-off mechanisms, applications, and common types.