JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (January/February - 2021 - R16)

The gate characteristics of an SCR represent the relationship between the gate-to-cathode voltage (V_g) and the gate current (I_g). The characteristics consist of a region bounded by the maximum gate power dissipation curve, the minimum gate trigger voltage, and the minimum gate trigger current required to turn on the device.

Key regions include:

• Non-triggering region: Below the minimum gate voltage and current, the SCR will not turn on.
• Triggering region: The area where the gate signal is sufficient to initiate conduction.
• Maximum limits: The gate signal must remain below the maximum allowable gate voltage (V_gm), maximum gate current (I_gm), and maximum gate power dissipation (P_gm) to prevent permanent damage to the gate-cathode junction.

The V-I characteristics of an IGBT plot the collector current (I_c) against the collector-emitter voltage (V_ce) for various gate-emitter voltages (V_ge). The IGBT behaves similarly to a BJT but is controlled by a voltage-driven gate like a MOSFET.

The characteristics are divided into:

• Cut-off region: When V_ge is less than the threshold voltage, the device is in the off-state, and only a small leakage current flows.
• Active region: The collector current is controlled by the gate-emitter voltage. In this region, the IGBT acts as a voltage-controlled current source.
• Saturation region: As V_ce increases, the device enters saturation where the current becomes relatively independent of V_ce for a fixed V_ge, allowing the device to act as a switch.

The characteristics show that higher V_ge values result in higher collector currents for a given V_ce, demonstrating the device's ability to handle high power with low control power requirements.