Design Con 2015

1-kV power supply produces a continuous arc

-February 05, 2004

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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