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Switching Power Supply: Junction-Path Shorts(2 of 4)
Solvability
The problem is solved by identifying paths without resistance and adding resistance or inductance in series with the path.
Solution
Find Paths. Examine the schematic. Find if there are any paths between voltage sources or voltage sources and ground or through semiconductor junctions with no discrete resistance to limit current. Integrated circuits should be considered a semiconductor junction. Capacitors large enough to store sufficient energy to damage connected circuits should be considered a voltage source.
PWM IC. One common sneak path is the transistor inside a PWM controller IC that discharges the timing capacitor. If the timing capacitor has enough energy, it can damage the integrated-circuit transistor if an external current limiting resistance is not used.
Current Limits. If no information is available on damage thresholds or surge currents for the semiconductors in the path, use the peak current on Safe Operating Area (SOA) curves, or if this is not available, limit current to less than 4 times the maximum continuous current rating. The weakest semiconductor in the path determines the resistance.
Avoiding Instability. If resistance is placed in the power lead of an integrated circuit to limit current, care must be taken that it does not cause oscillation. Often, a small decoupling capacitor may be necessary.
Radiation Hardening Correlation to Reliability. These current-limit techniques are used in circuits exposed to high levels of ionizing radiation which can short all semiconductor junctions during the period of radiation. It has been found that there is a strong positive correlation between field reliability and circuits so designed, even if they are never exposed to ionizing radiation.
Do not use this information for design without independent verification of the information.
