Question

In IGBT an improved Miller feedback effect is seen (compared with MOSFET) because, ratio of gate-collector capacitance to gate-emitter capacitance is

• A. \( \text{Unity} \)
• B. \( \text{Lower} \)
• C. \( \text{Higher} \)
• D. \( \text{Very large} \)

Hint:

To prevent accidental turn-on from the Miller effect during fast $dv/dt$ transitions: - Keep the feedback capacitance (\(C_{gc}\) or \(C_{gd}\)) as low as possible. - Keep the input capacitance (\(C_{ge}\) or \(C_{gs}\)) relatively high. Therefore, a lower ratio of feedback capacitance to input capacitance improves performance.

Answer 1

The Correct Option is B

Concept: The Miller effect describes an increase in the effective input capacitance of an inverting amplifier caused by feedback through the capacitance between the input and output terminals.
• In a Power MOSFET, this feedback occurs across the gate-to-drain capacitance ($C_{gd}$).
• In an IGBT, it occurs across the gate-to-collector capacitance ($C_{gc}$). An undesirable consequence of the Miller effect is slew-rate induced turn-on. When a high voltage change ($dv/dt$) appears across the output collector/drain terminal during switching, it drives a displacement current through the feedback capacitance into the gate network. If this current charges the gate-emitter capacitance above the threshold voltage, the device can accidentally turn back on, causing a short-circuit glitch.

Step 1: Identifying the capacitance ratio that controls Miller feedback susceptibility.
The voltage induced at the gate terminal ($V_g$) by an output voltage transition ($dv/dt$) is determined by the capacitive voltage divider formed by the internal parasitic capacitances: \[ V_g \approx V_{\text{output}} \times \left( \frac{C_{gc}}{C_{gc} + C_{ge}} \right) \approx V_{\text{output}} \times \left( \frac{\frac{C_{gc}}{C_{ge}}}{1 + \frac{C_{gc}}{C_{ge}}} \right) \] To minimize this unintended gate voltage rise and improve stability against the Miller effect, the capacitance ratio $\frac{C_{gc}}{C_{ge}}$ needs to be as low as possible.

Step 2: Comparing IGBT design to MOSFET design.
An Insulated Gate Bipolar Transistor (IGBT) combines a MOSFET gate structure with a bipolar junction transistor output. Its internal structure is tailored to give it a significantly larger gate-to-emitter capacitance ($C_{ge}$) relative to its gate-to-collector feedback capacitance ($C_{gc}$). Because $C_{ge}$ is larger, the ratio $\frac{C_{gc}}{C_{ge}}$ in an IGBT is lower than the corresponding $\frac{C_{gd}}{C_{gs}}$ ratio in a standard Power MOSFET.

Step 3: Conclusion.
This lower capacitance ratio reduces the amount of feedback voltage reaching the gate during fast voltage transitions, providing better protection against accidental Miller turn-on. Hence, the correct choice is option (2).