Power Supplies Power supplies • Supply electric energy • Convert power input into suitable output • Either AC or DC input • Usually DC output in electronics • General block diagram: Unregulated power supplies • Only use a tranfsormer, a rectifier and a filter • Prone to voltage drops under load • Transformer • Used to transform the AC input to an AC output of a suitable amplitude • Usually the most expensive part of a power supply unit Rectifier • Convert AC input to DC • Different circuits can output different waveforms Half Wave Rectification • A single diode is used • Removes half of the AC input wave • Supplies lower voltages Full Wave rectification • Uses two diodes and a centre tapping on the transformer • More efficient, easier smoothing due to higher frequency • More expensive (tapped transformer) Bridge rectifier • Uses four diodes, no need for centre tapped transformer • Diodes only need half the reverse breakdown voltage Filter circuit • Typically consists of a reservoir capacitor and a low pass filter • Contributes to removing the AC pulses Reservoir capacitor • A (large) capacitor acts as a storage for output current • Supplies current when the rectifier circuit is not conducting Low pass filter • Removes even more of the AC ripple • Passes lower frequencies (our DC, 0Hz) • Blocks higher frequencies (the AC, e.g. 50Hz) • Either LC or RC low pass filters are used • LC filters are usually more efficient due to higher XL to XC ratio Regulated power supplies • Output voltage drops at increasing load currents in unregulated power supplies • This is usually solved by using a regulator (stabiliser) Regulators • Shunt regulators • In parallel with the load • Tries to maintain a steady total current between the load and the regulator depending on the load current • Zener diodes • Series regulators • In series with the load • Uses a negative feedback system to control its own resistance IC Regulators • The mentioned circuits are rarely used with the introduction of ICs • ICs use the same principles, with enhancements • LM78Xxx, LM79xx for negative outputs DC/DC conversion • Buck converter • Consists of a switching transistor and a flywheel circuit • L1 & C1 keep the current going during transistor OFF periods • Output voltage depends on ratio of ON/OFF states of transistor • Output voltage always lower than input voltage DC/DC conversion • Boost converter • Similar operation (switching transistor) DC/DC conversion • Flyback converter • Uses a transformer to allow isolation DC/DC conversion • Inverting regulator • The induction changes polarity • The output is inverted/negative