Solid State Drive Control Questions and Answers

Answer: A

Conduction loss depends on the current and the on-state drop, and switching loss on the energy per transition and the frequency. Both must be computed before a heatsink can be sized, and the second is what limits the switching frequency.

Answer: B

Correct answer: carry their heat away to the surrounding air

A semiconductor die is small and its loss is concentrated, so without a path to a large surface the junction temperature would rise past its limit in seconds. The heatsink provides that path and that surface. It plays no electrical role, and where the device package is not isolated the heatsink must be insulated from it.

Answer: C

Correct answer: the devices overheat within minutes

Forced air cooling is designed on the assumption that the air is moving, and the thermal resistance without it is several times higher. At full load the junction temperature reaches its limit in a very short time, so drives monitor the fan and trip on its failure rather than waiting for the over-temperature to be detected.

Answer: D

A device switching hundreds of amperes at kilovolts dissipates a great deal per transition, so the frequency has to be kept to hundreds of hertz rather than kilohertz. Multilevel topologies recover the waveform quality that the low frequency costs.

Answer: A

The first two describe the essential difference between the two device families. PWM needs gate-controlled turn-off, so the thyristor survives only in line-commutated rectifiers and in very large load-commutated inverters.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: dead time exists to prevent shoot-through, and over-modulation buys fundamental voltage at the price of low-order harmonics. Statement 3 is the reverse of the truth - switching loss is proportional to the number of switchings per second, so a higher frequency increases it. That is the trade the designer makes against harmonic quality.

Answer: C

Correct answer: 1-R, 2-P, 3-Q

Three conversions in three directions. A rectifier goes from ac to dc and forms the drive’s input stage. An inverter goes from dc to ac and forms its output stage. And a chopper goes from dc to dc, appearing in a drive as the braking chopper. The dc link sits between the first two.

Answer: D

Each solves the medium-voltage problem differently, and each brings its own control difficulty: capacitor balancing, isolated supplies, or commutation without a link. Recognising the topology tells you which difficulty to expect.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The volt-seconds lost at each switching are fixed by the dead time and the link voltage, not by the output being produced. At high output that fixed loss is a small proportion; at the low voltage of low-speed running it is a large one, so the distortion and the torque ripple are worst there.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. An electrolytic capacitor loses electrolyte through its seal, and the rate roughly doubles for every ten degrees of temperature rise. Since the capacitance falls and the equivalent series resistance rises as it dries, the capacitor is usually the first component to reach the end of its life - and keeping it cool is what extends the drive’s.