High Voltage Pulses

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Depending on your needs, this has the potential to be a very interesting subject.

For example, at one simple low-cost low-voltage extreme you have the HV pulse generator in a disposable flash camera, charging the flash capacitor to 300 to 400V. These use inductive flyback to make the high voltage. The inductor's current is charged to 50mA, etc., and the transistor switch is opened for the flyback pulse. Higher-performance flash and strobes may use an IGBT to drive a pulse transformer that triggers the flash tube.

For an example in the middle you have auto ignition coils making 20 to 30kV sparks. These conventionally also use a charged inductor with flyback to 300V, etc., but it has a secondary winding, so it's a transformer. More energy is needed, so the inductor is charged to several amps before the transistor switch ("points" in the old days) is opened.

The older tube-type color TVs and computer monitors used 20 to 30kV high-voltage flyback transformers. We won't say anything about the high-voltage pulses in stun guns. One point to make here is that high-voltage transformer design becomes a critical issue.

For an example at the high end, we can look at a Tesla coil, making MV sparks. Like the ignition coil, it's a transformer, but it has a higher turns ratio. As with most high-voltage pulse transformers, it struggles with secondary winding capacitance, which it solves with a carefully contrived LC resonance.

Going in another direction, there's a need for faster high-voltage pulses. In WW2 times the radars that helped win the war needed microsecond-long 15A 20kV pulses for their microwave magnetrons operating at 300kW power levels. HV tubes, such as the 5D21, 715B and 6C21 (to 3MW), were used with pulse-forming transformers, another interesting subject.

The 100kW 5D21 tube (at right) was widely used, and in my mind was made famous by the exciting USS Tang and USS Bergall patrol reports widely posted online. If the radar failed, the sub was vulnerable to being spotted and sunk by aircraft until it could be fixed, so a tech installing a spare 5D21 tube was of essential interest to the crew, and certainly earned a mention in the captain's war patrol report. E.g., see the Tang's May 15, 1944 report (link).

In modern times we can use specialized MOSFET gate drivers to obtain faster 5 to 25ns pulse risetimes. A pair of mosfets can be used to 1.5kV, series connected parts are used to 10kV or so. The latter require carefully-matched timings for the transformer-coupled gate pulses, and the German company Behlke, a master at this art, pots everything so you can't see how they did it.

Inexpensive off-line converter ICs, such as the IR2104, going for $1 in high quantities, can be used with inexpensive n-channel mosfets to make simple 500V 25ns pulse generators, but they do have a limited repetition rate.

What for ?

electric fence.

stun gun

spark generator

insulation tester

What ?