Transformer Design for Charging Defibrillator Capacitors - Datatronic - #1

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TRANSFORMER DESIGN FOR CHARGING DEFIBRILLATOR CAPACITORS ByKirby Creel Senior Design Engineer Datatronics Generating high voltage by means of flyback topology is a common approach. Using the generated voltage to charge a capacitor for a high energy pulse are used in defibrillators, photo- flashs, strobes and ignition circuits to name a few. The procedure outlined in this article is useful in the initial transformer design phase for charging a capacitor in a stated time. The procedure presented eliminates “cut and try” or over-design approaches. Selection of critical values can be made with confidence using the guidance provided. First a little background in flyback topology. Flyback topology has several advantages - 1) Simple circuit. 2) High voltage output is not dependent on large turns ratios. 3) Circuit is self limiting. Output can be shorted without damage. 4) Output can be regulated over a large range. 5) Transformer can provide voltage isolation. 6) Multiple isolated outputs are possible. One output can be used for a low ratio feedback voltage. 7) Smoothing choke not required. Some disadvantages - 1) Requires a transformer. 2) Fast switching can generate EMI problems. 3) Leakage inductance must be kept low for good efficiency. 4) Without a feedback loop, circuit may be damaged if the load is removed. 1
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