An amateur-radio transceiver's auxiliary rail is built from a transformer with an…

PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits

Electronics for Measurement, Ch Power supplies: holding a rail steady

Problem

An amateur-radio transceiver's auxiliary rail is built from a transformer with an off-load secondary of

18.0\,\mathrm{V}

rms at

230\,\mathrm{V}

mains, a diode bridge and a

4700\,\mathrm{\mu F}

reservoir capacitor. Under the rail's

1.0\,\mathrm{A}

load, the combined diode and winding drop is a constant

2.4\,\mathrm{V}

, subtracted from the rectified peak and roughly independent of mains voltage. Mains is

50\,\mathrm{Hz}

(ripple at

100\,\mathrm{Hz}

). ((a)) Find the loaded peak, peak-to-peak ripple and trough voltage at nominal mains and at

-10\%

mains (

207\,\mathrm{V}

rms). ((b)) The rail feeds a

12\,\mathrm{V}

linear regulator with

2.2\,\mathrm{V}

dropout. Using the

-10\%

-mains trough, find the margin above dropout and decide whether the regulator stays in regulation. ((c)) The rail's mean voltage at nominal mains also feeds a buck converter delivering

5.0\,\mathrm{V}

2.0\,\mathrm{A}

to a digital board, switching at

100\,\mathrm{kHz}

with a

100\,\mathrm{\mu H}

inductor. Find the duty cycle and the inductor's peak-to-peak ripple current. ((d)) The converter's output is smoothed by

220\,\mathrm{\mu F}

with

25\,\mathrm{m\Omega}

of series resistance. Find the peak-to-peak output ripple.

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