This round of Ideas for Design includes power-supply overvoltage protection, a load that achieves zero ohms, and a method for determining MOSFET junction temperature and switching losses.
Systems incorporating microprocessors, FPGA, ASICs, and other expensive devices can sustain costly damage if the power supply voltages exceed the nominal values. Rather than hoping for the best, good design practice says you should add circuitry that will indicate an overvoltage (OV) condition, take corrective action, take protective action, or perform a combination of all of these functions.
The general approach to an electronic load is to use a transistor across the input terminals so that the current flows from drain (collector) to source (emitter). A resistance is affected by causing a current flow in proportion to the applied voltage in the manner of a resistor, I = V/R. A controller monitors the applied voltage and adjusts the current in response. To achieve 0 Ω, the terminal voltage must be 0 in the presence of a current.
The junction temperature of power MOSFETs is one of the major criteria used to obtain temperature derating curves for power converters. This article describes an improved technique for determining MOSFET junction temperature and switching losses more accurately based on the given thermal resistances and lead and case (package) temperature measurements.