May 242019

Flyback Converters for Dummies

A simple flyback converter high voltage power supply for NIXIE tubes.

Ronald Dekker

Special thanks to Frans Schoofs, who really understands how flyback converters work


If you are interested in Flyback Converters you might want to
keep track of my present project: the µTracer:
a miniature radio-tube curve-tracer
Click here to read about my “low-noise” 6 to 90 V converter project which replaces the anode battery in battery tube receivers.


In the NIXIE clocks that I have built, I did not want to have the big and ugly mains transformer in the actual clock itself. Instead I use an AC adapter that fits into the mains wall plug. This means that I have to use some sort of an up-converter to generate the 180V anode supply for the NIXIEs.

This page describes a simple boost converter and a more efficient flyback converter both of which can be used as a high voltage power supply for a 6 NIXIE tube display. Frans Schoofs beautifully explained to me the working of the flyback converter and much of what he explained to me you find reflected on this page. I additionally explain the essentials of inductors and transformers that you need to know. This is just a practical guide to get you going, it is not a scientific treatise on the topic.

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What you need to know about inductors

Consider the simple circuit consisting of a battery connected to an inductor with inductance L and resistance R (Fig. 1). When the battery is connected to the inductor, the current does not immediately change from zero to its maximum value V/R. The law of electromagnetic induction, Faraday’s law prevents this. What happens instead is the following. As the current increases with time, the magnetic flux through this loop proportional to this current increases. The increasing flux induces an e.m.f. in the circuit that opposes the change in magnetic flux. By Lenz’s law, the induced electric field in the loop must therefore be opposite to the direction of the current. As the magnitude of the current increases, the rate of the increase lessens and hence the induced e.m.f. decreases. This opposing e.m.f. results in a linear increase in current at a rate I=(V/L)*t. The increase in current will finally stop when it becomes limited through the series resistance of the inductor. At that moment the amount of magnetic energy stored in the inductor amounts to E=0.5*L*I*I.

Figure 1


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