1-kV power supply produces a continuous arc

Designing a high-voltage switching power supply that can produce a sustained arc can be challenging. This compact and efficient design delivers 1 kV at 20W and can withstand a continuous arcing, or short-circuit, condition (Figure 1). It uses standard, commercially available components. R₁ sets the LTC1871 switching-regulator controller for a nominal operating frequency of 120 kHz. The circuit operates as a discontinuous flyback structure, producing 333V across C₁. The diode/capacitor charge-pump multiplier triples this voltage to create 1000V at the output. Figure 2 shows the switching waveforms. When the primary switch, Q₁, is on, the output rectifiers are reverse-biased, and energy is stored in the transformer, T₁. When Q₁ turns off, energy transfers to the secondary winding, and C₂ and C₃ pump up the output voltage through the rectifiers. The primary voltage goes high and is clamped through the transformer and rectifier, D₁, by the voltage across C₁. The transformer is well-coupled, so the leakage inductance creates little voltage spike. A small RC snubber across the primary winding damps the ringing and reduces EMI (electromagnetic interference).

For current-limit protection, the circuit in Figure 1 contains two active circuits and one passive element. The voltage across the current-sense resistor, R₂, limits peak primary current to 7.5A. Q₂ provides secondary-side current limit. Notice the bump on the leading edge of the current ramp of Trace 2 in Figure 2. This bump coincides with the positive excursion of the voltage across R₃ in Trace 4, which is the refresh current for C₂ and C₃. When the circuit is overloaded, this slug of current becomes high enough to enhance Q₂, folding back the load current (Figure 3). A hard short circuit results in relatively low power dissipation. Omitting Q₂ for the secondary-side current limit results in substantially increased short-circuit current and internal power dissipation, resulting in failure of the primary switch Q₁. R₄ provides a load impedance for the power supply.

This load helps to limit the peak-current stress in the multiplier capacitors and diodes. Don't skimp on the power rating for R₄, because dissipation during a continuous arc can be substantial. Should R₄ fail open, the feedback circuit forces a full duty cycle with catastrophic results. Too low a value for R₄ can result in charred circuits and hours of debugging. (Yes, a hearty explosion elicits a round of applause from the lab crew.) Arcing is the most stressful condition, and the output capacitor constantly charges and discharges (Figure 4) . As a final figure of merit, the circuit is efficient (Figure 5). The efficiency reaches 87.3% at 12V input and a full load of 20W and increases to 87.7% with an overload of 24W.

So what is this circuit good for? A battery-operated bug zapper, perhaps. And, like raking a live wire across a grounded file, this is a great tool for befuddling the AM-radio listeners on the production floor. The circuit probably doesn't deliver enough energy for use as an ion generator for a plasma cutter, though one engineer I knew was willing to give it a try. A previous version of the circuit used a monolithic switcher, and with the right materials for banana jack and plug, created a bright orange glow and enough heat to raise thoughts about the fire extinguisher (plenty of ozone, too). I'd stay away from using this circuit as a cat trainer or an electric fence. The circuit does generate a lethal voltage potential, and lawsuits can be quite costly. Prototype this circuit at your own risk.

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