Gotchas
Each gotcha is one page on the same template: the symptom as it first shows on the bench or in a design, why it happens, how to confirm that this is the cause and not another with the same symptom, the fix, and the references behind every claim. The tags group them.
- An inductance that grows with the test frequency
A winding with many turns reads high on an LCR meter, and higher at a higher test frequency, because its own capacitance brings its self-resonance close. - A low resistance that depends on how the probes sit
A two-wire reading of a winding's or a shunt's resistance includes the test leads and their contacts; four wires keep them out. - A ceramic capacitor that loses capacitance under voltage
A class-2 ceramic capacitor (X7R, X5R) can keep only a fraction of its rated capacitance at its working dc voltage and temperature, and its code does not say how much. - A TVS judged by its breakdown voltage
At the current it actually clamps, a TVS holds a voltage above its breakdown voltage, and the switch it protects sees that voltage. - A current-output monitor that clips before its full scale
Raising a current-output monitor's output resistor raises its gain, not the largest voltage its output can reach. - A small shunt that reads high by a fixed fraction
Without a Kelvin connection, the pads and the solder in a small shunt's sense path add to its resistance and become a gain error. - A power-gating timer that switches its load off too soon
A nano-power timer that gates a load's supply needs its mode pin tied to a rail, and takes a very short pulse on its DONE input as the signal to switch the load off. - An open-loop flyback with its load removed
Without regulation and without a load, a flyback's output charges until its voltage, reflected to the primary, reaches the primary clamp; the clamp then takes all the input power. - An electrolytic capacitor in the cold
An aluminum electrolytic capacitor's ESR rises steeply at low temperature, and the ripple across it rises with it. - A gate driven too little, too much, or not at all
A MOSFET is fully on only at the gate voltage it is made to be driven with; an eGaN FET's maximum gate voltage is low, and ringing adds to the drive; and a gate that nothing drives can turn the switch on. - A fixed-duty flyback that pins its source's voltage
Fed from a current-limited source, a fixed-duty-ratio flyback presents a resistance only in DCM; once its input reaches the critical voltage, CCM holds it there. - A current reading below the meter across the shunt
When a sense chain clips on the peaks of a pulsed current, the average of its readings falls below the true average, which a DMM across the shunt still reads. - Sampling at ten times the switching frequency that misses the pulses
An ADC sampling a pulsed current at ten times the switching frequency still misreads its average; the pulses' harmonics alias, and those at multiples of the sampling rate fold onto dc, the average itself.
To write up a mistake of your own, start from the gotcha page template.
By tag
Section titled “By tag”Measurement
Current sensing
Sampling
Magnetics
Capacitors
Temperature
Protection
Gate drive
Power gating
Flyback
Harvesting
The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.