Solid State Drive Control Questions and Answers

Answer: A

Once latched, a thyristor ignores its gate entirely, so the main circuit must bring the current to zero and hold the device reverse biased for its turn-off time. That property is what makes line-commutated converters natural and forced-commutated ones complicated.

Answer: B

Gate-controlled turn-off is what makes pulse width modulation possible, since the device can be switched thousands of times a second at will. It is the reason the IGBT displaced the thyristor from almost all drive inverters.

Answer: C

Being a field-controlled device, it needs charge to be moved into and out of its gate capacitance and nothing in between. That makes the drive circuit simple and its power requirement proportional to the switching frequency.

Answer: D

Correct answer: low voltage and high switching frequency

A MOSFET is a majority carrier device with no stored charge to remove, so it switches fast and has low switching loss, but its on-state resistance rises steeply with voltage rating. The IGBT has the opposite balance - conductivity modulation gives a low drop at high voltage, at the cost of a current tail at turn-off.

Answer: A

Lower switching loss allows higher frequencies and smaller filters, and the higher junction temperature eases the cooling. The cost premium is what has confined them so far to applications where size or efficiency justifies it.

Answer: B

A machine winding is inductive, so its current cannot stop when a switch opens and must find a path, which the diode gives it. Those diodes also carry the current during regeneration, when power flows back into the dc link.

Answer: C

Charge stored in the diode has to be swept out before it blocks, and that charge flows through the device turning on. Fast-recovery and silicon carbide diodes exist precisely to reduce that loss and the electrical noise it generates.

Answer: D

Correct answer: conduction spreads across the junction only gradually

Turn-on begins in a small area near the gate and the conducting region spreads outward at a finite velocity. If the current rises faster than the area grows, the whole current is carried by a small part of the junction, the local power density becomes enormous and the device fails. A series inductor limits the rate of rise to what the device can accept.

Answer: A

A capacitor with a resistor and diode slows the voltage transient at turn-off, keeping it inside the device's safe operating area and reducing the noise generated. Modern devices with wide safe operating areas need less snubbing than older ones did.

Answer: B

The capacitor holds the link voltage steady while the inverter draws pulsating current from it, and the machine sees a defined voltage at each instant. A current source inverter uses a large inductor instead and behaves quite differently.