Transformer Design For Charging Defibrillator Capacitors

By Datatronic

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, photoflashs, 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 -

• Simple circuit.
• High voltage output is not dependent on large turns ratios.
• Circuit is self limiting. Output can be shorted without damage.
• Output can be regulated over a large range.
• Transformer can provide voltage isolation.
• Multiple isolated outputs are possible. One output can be used for a low ratio feedback voltage.
• Smoothing choke not required.

Some disadvantages -

• Requires a transformer.
• Fast switching can generate EMI problems.
• Leakage inductance must be kept low for good efficiency.
• Without a feedback loop, circuit may be damaged if the load is removed.