Power Electronics: SPWM Voltage Source Inverters & DC-DC Converters
Converter topologies, power semiconductor switching losses (IGBT/MOSFET/SiC), and grid-tied solar inverter synchronization.
In-Depth Interview Questions & Model Solutions
Q1Explain Sinusoidal Pulse Width Modulation (SPWM) in a Single-Phase Full Bridge VSI and how Modulation Index controls output voltage.
In SPWM, a high-frequency triangular carrier wave (vc with frequency fc) is compared with a low-frequency sinusoidal reference modulating wave (vr with frequency fr). When vr > vc, the upper switch of that bridge leg turns ON. The Amplitude Modulation Index `ma = Vr_peak / Vc_peak` linearly controls the fundamental RMS output voltage (`V1_rms = ma * Vdc / √2` for linear region `0 <= ma <= 1`). The Frequency Modulation Ratio `mf = fc / fr` pushes unwanted harmonic frequencies to high frequency sidebands around mf, 2mf, allowing compact, low-cost LC low-pass output filtering.
- Linear modulation range: 0 <= ma <= 1.
- Overmodulation (ma > 1) yields higher fundamental voltage but introduces low-order harmonics.
- Square wave mode (ma -> infinity) yields fundamental V1_rms = (4*Vdc) / (π * √2).
Q2Derive the voltage conversion ratio of a DC-DC Boost Converter operating in Continuous Conduction Mode (CCM).
Using the principle of Inductor Volt-Second Balance in steady state (average voltage across inductor over one switching period T is zero): During ON time (DT): Inductor is connected across input source Vin, so `VL_on = Vin`. During OFF time ((1-D)T): Inductor discharges into load via diode, so `VL_off = Vin - Vout`. Volt-second balance: `Vin * (D * T) + (Vin - Vout) * ((1 - D) * T) = 0` => `Vin * D + Vin - Vout - Vin * D + Vout * D = 0` => `Vin = Vout * (1 - D)` => `Vout = Vin / (1 - D)`. Since duty cycle 0 < D < 1, Vout is always greater than Vin.
- Vout = Vin / (1 - D) in CCM.
- Duty cycle D is limited to ~0.85 in practice due to inductor ESR and parasitics.
- Capacitor charge balance dictates output ripple voltage.
Technical Panel Interview Strategy Tips
- Railway interviews frequently ask about 3-phase traction inverters converting 25 kV AC overhead supply to variable voltage variable frequency (VVVF) for 3-phase induction motors.