What is Breakover Voltage of a GTO Thyristor?

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Editorial Team - everything PE

Jun 11, 2024

The breakover voltage of a GTO Thyristor (VBF)is the minimum forward voltage at which the GTO thyristor switches from the off state (blocking state) to the on state (conducting state) without any gate signal. In other words, it is the voltage at which the GTO thyristor breaks over into conduction due to the application of high voltage alone, rather than due to a gate trigger pulse. This voltage is an important parameter that impacts the design and safety of electrical circuits. It specifies the maximum forward voltage the GTO can block without turning on, playing a crucial role in preventing unintended conduction, ensuring circuit protection, and maintaining reliable operation in high-voltage applications.  

 Structure of GTO Thyristor

A GTO thyristor is a fully controlled semiconductor device used to control high-power electronic devices. Unlike SCRs, a GTO can be turned off by applying a negative gate current. This capability allows for direct control of the off-state via the gate terminal. A GTO thyristor is similar in structure to a conventional thyristor, consisting of four semiconductor layers (PNPN) and three junctions. It has three terminals anode, cathode and gate. The outermost P+ layer is connected to the anode terminal. The next N-type base layer is lightly doped to handle high voltage. The middle P-type base layer plays a key role in controlling the device. The outermost N-type layer is connected to the cathode terminal.

The construction of GTO thyristor is slightly different from conventional SCR. The gate is diffused into the P-type base layer, interfacing with the N+ regions to control current effectively. The gate contact is made through multiple small regions in the N+ cathode layer for efficient turn-off. Small N+ regions are embedded in the P-type base layer to form cathode shorts. These shorts are crucial for turn-off capability, helping extract charge carriers from the P-type base layer.

When a forward voltage is applied between the anode and cathode, junctions J1 and J3 are forward biased and junction Jis reverse-biased. The device blocks current due to the high-impedance state of the Jjunction. A small leakage current flows due to minority carriers. When the forward bias voltage is further increased, a point is reached where the reverse-biased junction J2 vanishes due to avalanche breakdown. This results in a large current flowing from the anode to the cathode making the device turn on. This voltage is known as the forward breakover voltage of the GTO thyristor.

V-I Characteristics of GTO Thyristor

Thus, the forward breakover voltage is the minimum voltage required to turn on the GTO when the gate current is zero. It is typical value ranges from several hundred volts to several thousand volts, depending on the design and application of the device, and is specified in the manufacturer’s datasheet.

Key features of Breakover Voltage of GTO Thyristor

  • Intrinsic Turn-On Capability: The breakover voltage characterizes the intrinsic ability of the GTO to turn on due to the applied forward voltage alone, without any external triggering. When the voltage across the GTO exceeds this threshold, the junction breaks down, causing the device to switch to the on state.
  • Circuit Design and Protection: Understanding the breakover voltage is crucial for engineers while designing circuits.  It ensures that the GTO operates within safe voltage limits. If the applied voltage exceeds the breakover voltage, the GTO can turn on unintentionally, leading to circuit malfunction or damage.
  • Voltage Rating: The breakover voltage defines the maximum forward voltage the GTO can block while staying in the off state. This parameter ensures that the GTO does not turn on prematurely during normal operating conditions.
  • Overvoltage Protection: In certain applications, the breakover voltage serves as a form of overvoltage protection. If the voltage surpasses this threshold, the GTO can turn on to divert excess energy and safeguard other components in the circuit.

Typical breakover voltages for GTO thyristors are designed to be significantly higher than the normal operating voltages to ensure safe operation under various conditions. The exact value of the breakover voltage varies depending on the specific design and rating of the GTO thyristor. The GTO thyristor is normally turned on by applying a small positive voltage to the gate, which reduces the forward breakover voltage considerably.

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