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| Reverse conducting thyristor symbol |
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| Reverse Conducting Thyristor(RCT) Equivalent Circuit |
Full Form of RCT in Electrical - Reverse Conducting Thyristor is a specialized type of asymmetrical thyristor that incorporates an antiparallel diode on the same silicon chip. This integrated diode allows current to flow in the reverse direction, effectively bypassing the reverse blocking capability of the RCT.
The inclusion of the antiparallel diode provides several benefits. Firstly, a current pulse through the diode section of the chip can turn off the RCT. This feature simplifies the control and operation of the device.
Additionally, the integration of the asymmetrical thyristor and diode into a single device has practical advantages. The reduced component count and compactness of the converter lead to a smaller heat sink requirement, resulting in a more efficient and cost-effective design.
Furthermore, the elimination of the undesirable stray loop inductance between the asymmetrical thyristor and diode helps improve the overall performance of the RCT. By eliminating this inductance, any unwanted reverse voltage transients across the asymmetrical thyristor are avoided, leading to better turn-off behavior.
One of the advantages of the RCT is its improved turn-off behavior, as the minimized stray loop inductance reduces the generation of reverse voltage transients across the thyristor. This makes the turn-off process more predictable. Additionally, the integration of the thyristor and diode functions ensures that charge carriers present in the diode during commutation do not diffuse into the thyristor part of the chip, which could cause re triggering when forward voltage is reapplied.
The RCT has specific voltage and current ratings. The forward blocking voltage of the device can vary from 400 to 2000 V, while the current rating can go up to 500 A. On the other hand, the reverse blocking voltage is typically in the range of 30 to 40 V.
One disadvantage of the RCT is its inflexibility compared to using two discrete devices. In the RCT, the current ratio between the thyristor and diode sections of the chip is fixed for a given design. This can be a limitation in certain applications where different current ratios may be desired.
In voltage source inverter circuits, where the load current is controlled by the thyristor and flows freely in the opposite direction through the diode, it is important for the RCT to have equal current ratings for both the thyristor and diode sections.
To meet the requirements of high-performance inverter and chopper circuits, purpose-designed RCT devices are now being manufactured. These devices are optimized for their intended applications, taking into account factors such as current capability, voltage ratings, and overall performance.
Other Thyristor Devices
Field-controlled thyristor (FCT), or static induction thyristor (SIT), is a new four layer semiconductor device still under development. In FCT, a negative gate drive turns off the normally conducting thyristor, but it is essential to hold this negative gate drive to achieve the off-state. It has been reported that FCT has high voltage-blocking capability, low-on-state voltage drop and improved du/dt and di/dt values.
Gate-assisted turn-off (GAT) thyristor is a normal four-layer thyristor, but its turn-off is achieved by applying a negative gate drive across gate-cathode terminals.
The use of highly interdigitated gate-cathode junctions in GAT (Gate Assisted Turn-off) thyristors allows for more effective removal of stored charges from the base region, resulting in reduced turn-off times. This is particularly important in applications such as TV deflection circuits operating at frequencies around 20 kHz.
GAT thyristors are specifically designed to have fast turn-off times, making them suitable for high-frequency applications. In the case you mentioned, 200-V GAT thyristors are capable of achieving turn-off times as low as 2.5 µs (microseconds). This fast turn-off time is beneficial for minimizing switching losses and improving the overall efficiency of the circuit.
By employing interdigitated gate-cathode structures, the GAT thyristor can efficiently extract charges from the base region during the turn-off process. This design feature helps prevent the formation of charge carriers that could impede the switching speed. The interdigitated structure essentially provides a larger contact area between the gate and cathode, enabling better control over the turn-off process and faster removal of charges.
Gate turn off thyristor is described briefly in the next section. The latest semiconductor device to enter the family of thyristors is MOSFET-controlled thyristor (MCT),seen for ASCR.