Thyristors and diodes

Semiconductor components used as rectifiers

Diodes are used to convert an alternating current into a direct current or to conduct the current flow in a specific direction. To do so, a diode features a blocking and a conducting direction, meaning that it blocks the current in one direction and conducts it fully in another. A diode contains a positively and a negatively doped section with the positive section lacking electrons and the negative one featuring an excess of electrons. The positively doped section acts as an isolator. If a voltage is connected in the blocking direction, the rest of the charge carriers are removed from the positively charged section; the current can no longer flow – the diode is blocking. If a voltage is connected in the conducting direction, free charge carriers surge into the positively doped section from the negatively doped section. The diode is now conductive.

This property is used to rectify voltages. If a diode is connected in series to an alternating current source, it only allows the positive or negative half-wave pass as required. If the voltage is then smoothed, a direct current is received. If more diodes are connected to a diode bridge, negative half-waves can also be used.

Capsule-type devices/discrete thyristors and diodes
  • Voltage-proof up to 7,200 V
  • Switching currents up to 8,000 A
Thyristor/diode modules
  • Voltage-proof up to 3,600 V
  • Switching currents up to 2,000 A
Diode/thyristor bridge rectifiers
  • Voltage-proof up to 1,800 V
  • Switching currents up to 200 A

A thyristor consists of four or more layers, thus producing at least three p-n/n-p junctions. In home mode, the thyristor blocks in both directions. It can be activated or triggered by the gate terminal. A trigger voltage needs to be connected in the conducting direction to do so. The thyristor now acts in the same way as a diode: The current does not flow in the conducting direction until the current flow in the thyristor stops and the thyristor changes to blocked mode. This happens every 10 ms in the case of 50 Hz alternating current applications.

There are different design options for diodes and thyristors: individually connected in capsule-type devices, individually or in pairs in modules, or 4-7 devices connected together in rectifiers.