Circuits on Higher Voltage

Almost always the conditional control circuitry (timers [555], logic, etc.) uses low voltage electronics now. This then turns ON or OFF the high voltage with some other switch. Depending on the speed of the switching this will be handled by a mechanical high voltage relay, a solid state high voltage relay (Silicon Controlled Rectifier [SCR]), high voltage MOSFETS (Metal Oxide Semi-conductor Field Effect Transistor), IGBT (Isolated Gate Bipolar Transistor) or in the case of most power grids turning ON a motor to move the bus bars of a distribution switch.

Right. And if a low-voltage supply isn't available, then it may be more practical to create one from the high-voltage, to operate your "complex" low-voltage circuitry, which can then run the high-voltage switch or amplifier, than to attempt running your "complex" circuitry directly off the high voltage.**

It's interesting to speculate about the design of the high-voltage regulator. This could be a simple zener-regulated emitter follower, but the limited voltage rating of small transistors can be a problem (TV HOT transistors have high-voltage ratings, but they have very low betas at low currents, typically 10). E.g., an mpsa44 is only rated at 400 to 500 volts. The circuit shows a series cascode connection, which can double the operating voltage; two cascode stages can triple it.

Mosfets provide a better pass element than BJTs, because they come with ratings to well above 1kV. Depletion-mode mosfets, a special type, are even more appealing, because like tubes and JFETs, they can self-bias and thereby eliminate the awkward series resistors to the high-voltage supply. IXYS makes high-voltage depletion-mode mosfets, e.g., the 1kV 100mA IXTP01N100D, in stock for $1.29 at Mouser. You can use one of these with a 3-terminal regulator, as shown at the right.

High-voltage mosfets like to oscillate at RF frequencies, when used in the linear mode at modest currents, but a gate resistor or ferrite bead usually solves that problem.

High-voltage linear regulators are usually used at low currents, to save power. At low currents the mosfets are operating near or in their "subthreshold" region, with gate voltages approaching their "cutoff" voltage. The graph below shows the characteristic curves for an IXTP3N100, a 1kV 3A depletion-mode mosfet. Note that V_GS = -3V at low currents.

In the circuit above right, that would mean the 3-terminal regulator operates with an input 3 volts above its output. Cute, huh?

If your low-voltage circuits require very much current, you'll be unhappy with the wasted V*I of a linear regulator, and that calls for a high-voltage buck switching dc-dc converter. ICs are readily available for this purpose up to 600V or so, that's the DC "bulk voltage" from an offline PFC stage, for example. But for 1kV or higher the task is a bit more difficult.

The O.P., Jay, asked about running from 10kV. Whew! At that point separate isolated power supplies and optically-isolated circuitry is in order.

** Some years ago a few of us on the usenet group sci.electronics.design spent a few weeks exploring full-voltage low-current 1kV circuits, amplifiers, timers, etc., for the fun of it. You can read the posts on Google.

In the power industry we use high voltage, high power semiconductors regularly in equipment such as HVDC converter stations. They enable us to transfer large amounts of power between AC grids which are not synchronized. Thyristors and Insulated Gate Bipolar Transistors are the most commonly used devices.