DC to DC Converter

Posted on Jul 25, 2012

The intended use of the 24 volts is to power an amplifier that will supply wave forms of up to a couple hundred Hz to bias a varactor or pair of varactors in an RF circuit. This meant that the current requirements would be low - probably a little over a milliamp. It also made me worry about recovery spikes from the rectifier, and harmonics from the switching itself, as they might affect the operation of the circuit the power supply is meant to power. The circuit here only has minimal filtering. Filtering to remove filter glitches will be added to another assembly. I first considered a switched capacitor power supply since this could be made to switch `softly` at a low frequency, but the parts count started looking pretty high considering I had to go from a regulated +5 volt power supply up to a regulated +24 volt output. This would have taken five half wave multiplier stages.

DC to DC Converter
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The basic converter provides an output voltage that is about 10 times the input voltage. The voltage boost occurs when the 2N4401 switches off, and the magnetic flux that had been supported by current flowing in the inductor, collapses. This rapid change in flux is accompanied by the voltage on the collector of the 2N4401 rapidly rising, limited for the most part by the turnoff time of the transistor, but ultimately being clamped to a diode drop above the voltage across the output capacitor. This type of converter is often referred to as a boost converter or a flyback converter, the later term referring to the high voltage power supply that was integrated with the horizontal deflection circuits in cathode ray tube television sets. High voltages would be developed when the horizontal scan circuit rapidly made the luminous spot return (or "fly back") to the left side of the screen. Oscillation frequency is a little under 300 kHz when under load. I had first used a 2N2222 as the NPN because they are pretty common parts, but I could not resist the improved efficiency of the faster switching 2N4401. After measuring the input voltage required to maintain the output voltage at +24 volts as the load was varied, the basic circuit was then treated as a building block and placed inside an analog regulation loop that could supply the range of voltages needed. From the graph (further below) , the analog regulator's output, the...

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