# Some questions about a simple high voltage circuit

The schematic below occurs as an example circuit in a set of lecture notes that I'm using. It is a circuit for building up a high voltage across the capacitor $C$ using a constant voltage $V_0$.

I understand how the circuit works, but would like to know the following.

1. What is it called?

2. Is it used in practice?

3. If so, how does one operate the switch and what kind of switch is it?

For questions 3, could one for example use a diode with a high frequency sinusoidal bias voltage to achieve the rapid switching that is needed for high voltages?

simulate this circuit – Schematic created using CircuitLab

Is it used in practice?

Well, that circuit in isolation only warms up a resistor, a capacitor, a diode and an inductor. So, no, that wouldn't be useful for anything. But:

What is it called?

That is a switch-mode power supply! I'm a bit surprised you're not finding this in your lecture notes, but the schematic you show is very similar to the picture on Wikipedia's Boost Converter page. So yes, that is a Boost Converter. Your Load is connected in parallel to the capacitor – the capacitor has the job of stabilizing the output voltage.

If so, how does one operate the switch and what kind of switch is it?

Depends! You can actually even manually operate a manual switch and build a Boost converter :) But that's definitely more of a fun educational-type circuit than anything useful.

Usually, these switches are made of transistors. What kind of transistor (or transistor-based switch module) depends on the actual application – you'll find NPN BJTs, N-channel MOSFETs, optically triggered devices, IGBTs, and so on, for different voltages, currents, temperature environment.

How that transistor is then operated depends on the transistor.

In real-world applications, you'll often find ICs dedicated to the job of controlling the transistor so that the voltage at the output is just right – but that's out of scope for a lecture on basic power systems, so it doesn't matter here.

• The lecture notes cover basic circuit theory and electronics, and the circuit was used to illustrate some Laplace domain calculations. Their circuit was slightly different since they also had a spark gap across the capacitor. That's why I said it's a high voltage generator. Is it still called a boost converter then? Thanks! – Étienne Bézout Aug 8 '17 at 9:52

This circuit is an example of a "boost" converter and usually has $R$ missing or a small value so that you can measure the current.

This type of circuit is common in DC/DC converters where you want more output volts than input. One very common use is in AC/DC switch mode power supplies to generate a 380V rail or similar while ensuring the input current follows the input voltage for power factor correction. A second converter then takes this to generate the required output voltage.

The switch is some kind of transistor, typically a N-channel MOSFET, and there are several switch mode control ICs you can use to control it.

• Alright, thanks! The lecture notes cover basic circuit theory and electronics, and the circuit was used to illustrate some Laplace domain calculations. Their circuit was slightly different since they also had a spark gap across the capacitor. That's why I said it's a high voltage generator. Is it still called a boost converter then? Thanks! – Étienne Bézout Aug 8 '17 at 9:48