Semiconductor Devices and Power Supplies
Semiconductor Devices and Power Supplies
This domain tests diodes, rectifiers, zener regulation, transistors, thyristors, linear regulators and switch-mode power supplies.
Core rules
- A silicon diode drops about 0.7 V; a bridge rectifier therefore loses about 1.4 V. Schottky diodes drop less.
- A zener regulates only with a series resistor that limits current and dissipates the difference.
- A BJT needs correct bias and a base resistor; a MOSFET gate is voltage-driven but needs discharge and gate protection.
- An SCR or triac latches on; a triac needs zero-crossing or snubber design in AC loads.
- Linear regulators such as 78xx need input/output capacitors and adequate heatsinking.
- An SMPS rectifies mains, switches at high frequency, then uses a transformer, optocoupler and feedback such as TL431. The primary side is hazardous.
- Ripple in a capacitor-input supply is approximately I/(2fC) for a full-wave rectifier at 50 Hz.
Worked example
A 12 V AC secondary feeds a bridge and 1000 µF capacitor at 1 A. Peak DC is 12 × 1.414 − 1.4 = 15.6 V. Ripple is 1/(2 × 50 × 0.001) = 10 V peak-to-peak, leaving about 10.6 V average, too low for a 12 V regulator. With 4700 µF, ripple becomes 1/(100 × 0.0047) = 2.1 V, giving about 14.5 V average. That leaves enough headroom for a 12 V regulator but still needs checking at low mains.
Traps
Connecting a zener without a series resistor; probing an SMPS primary; ignoring capacitor ESR; assuming a heatsink is isolated; replacing a fuse in a switch-mode supply without finding the shorted switch device.