Jawaharlal Nehru Technological University Hyderabad B.Tech III Year POWER ELECTRONICS Previous Question Paper (November/December - 2020 - R16)

An Insulated Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device that combines the high input impedance and fast switching characteristics of a MOSFET with the low on-state conduction losses of a Bipolar Junction Transistor (BJT). It consists of four layers (p-n-p-n) and is controlled by a gate voltage.

The structure consists of a p+ substrate, an n- drift region, a p-base region, and n+ source regions. When a positive gate-to-emitter voltage (Vge) is applied above the threshold voltage, an n-channel inversion layer is formed in the p-base region under the gate. This allows electrons to flow from the n+ source into the n- drift region. This electron current acts as the base current for the internal p-n-p transistor, which then injects holes from the p+ substrate into the n- drift region. This conductivity modulation significantly reduces the resistance of the n- drift region, allowing the device to handle high currents with low voltage drops.

The working principle can be understood through its equivalent circuit, which is a Darlington configuration of a MOSFET driving a p-n-p transistor:

Key operational characteristics include:

• Turn-on: Initiated by applying a positive voltage to the gate, creating a conductive channel for electron flow, which triggers the p-n-p transistor.
• Turn-off: Initiated by removing the gate voltage. The channel disappears, and the device turns off. However, because of the stored minority carriers in the drift region, there is a "tail current" during turn-off, which limits the switching frequency compared to a pure MOSFET.
• Advantages: High input impedance, easy gate drive requirements, high current density, and low conduction losses.
• Applications: Widely used in motor drives, uninterruptible power supplies (UPS), and high-power DC-to-AC inverters.

The dynamic characteristics of an SCR describe its behavior during the transition between the ON state and the OFF state. These transitions are not instantaneous due to the presence of internal junction capacitances and charge carrier storage effects.

1. Turn-On Characteristics: When a gate signal is applied, the SCR does not turn on immediately. The turn-on time (t_on) is the sum of three intervals:

• Delay time (t_d): The time taken for the anode current to rise to 10% of its final value. During this time, the gate current begins to flow and charge carriers are injected into the inner layers.
• Rise time (t_r): The time taken for the anode current to rise from 10% to 90% of its maximum value. The conduction spreads across the junction area.
• Spread time (t_p): The time taken for the conduction to spread over the entire cross-section of the semiconductor layers.

2. Turn-Off Characteristics: An SCR is turned off by reducing the anode current below the holding current. The turn-off time (t_q) is the minimum time required for the device to regain its forward blocking capability. It consists of two parts:

• Reverse recovery time (t_rr): The time taken for the excess charge carriers to be removed from the junctions. The current flows in the reverse direction during this phase.
• Gate recovery time (t_gr): The time required for the recombination of charge carriers in the inner junctions to restore the forward blocking state.

Waveform Description: The turn-on waveform shows a gradual rise in anode current (i_a) and a simultaneous drop in anode-to-cathode voltage (v_ak). Conversely, the turn-off waveform shows a brief period of reverse current flow followed by the recovery of the forward voltage blocking capability as the junction depletion layers reform.