Module 7: Power Electronics
Line losses, converter topology, RF reflections and thermal design for power semiconductors.
-
1calcA 1000 W load runs from a 50 V bus. What current flows before considering cable loss?
-
Correct. I = P / V = 1000 / 50 = 20 A.
-
50 A treats voltage as current; load power and voltage together set current.
-
100 A would be 1000 W from a 10 V bus.
-
Watts are not amps; power combines voltage and current.
-
-
2conceptAt fixed load power and cable resistance, what happens to cable heat if the bus voltage doubles?
-
Doubling voltage lowers current at fixed power, so heat does not rise.
-
Current changes, and wire heat depends on current squared.
-
Current halves, but heat is I²R, so the heat falls more than half.
-
Correct. Doubling voltage halves current; half current squared is one quarter heat.
-
-
3conceptWhich converter topology naturally produces an output above the input while preserving polarity?
-
A buck steps voltage down.
-
Correct. A boost converter stores energy in the inductor, then adds that energy to the input path to raise output voltage.
-
The basic buck-boost can step up or down, but it inverts polarity.
-
An LDO only regulates below its input and wastes the drop as heat.
-
-
4calcIf |Γ| = 0.2 at an RF load, what fraction of incident power reflects?
-
2% would require |Γ| around 0.14.
-
Correct. Reflected power fraction is |Γ|² = 0.2² = 0.04, or 4%.
-
|Γ| is a voltage-wave ratio; power uses the square.
-
40% doubles the voltage ratio instead of squaring it.
-
-
5scenarioA MOSFET dissipates 3 W, has θJA = 30 °C/W, and ambient is 40 °C. The simple junction-temperature estimate is 130 °C. What is the best engineering response?
-
Case temperature is not junction temperature. Silicon can be unsafe while the package feels only warm.
-
Correct. You can attack loss (lower Rds(on), lower current, lower switching loss) or the heat path (copper, vias, airflow, package), then measure the actual assembly.
-
Firmware may help, but thermal design is not a single firmware knob and layout/package resistance can dominate.
-
Higher switching frequency often raises switching loss, which can make the thermal problem worse.
-