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Types of high-voltage pulse power supplies

by Timo Stehmann28. August 2013 16:18

I started out writing a post about pulse transformers, but soon realised it is important to know the different types of power supplies and their applications, first. There are different ways to categorise pulse power supplies and the diagram below is just one crude way of doing it (by no means complete):

 

 

It is important to understand the difference between these power supplies. Sure, a fixed pulse energy supply must have a certain output voltage rating, but it is a long shot from an ideal voltage source. Another thing to keep in mind is the target application for each pulser type. Applications can also be classified as the type of load (as indicated by the blue fields in the diagram above). Also known as the load regimes. Let us have a closer look:

Fixed pulse energy

Loads attached to these types of pulse generators are highly non-linear, "resistive" loads. One typical application is the generation of atmospheric pressure plasmas (e.g. TEA CO2 lasers, Excimer lasers, etc.). The main design consideration is the pulse energy. As a result everything starts by designing the energy storage which will be switched. In most cases, this will be a charged capacitor and the basic building block is the C-C transfer circuit and its different permutations. More information can be found here, but here again the fundamental idea behind C-C transfer pulse generators:

The capacitor C1 is initially charged and contains the required pulse energy. When the switch is closed, a resonant transfer from C1 to C2 is obtained. The voltage over C2 rises until the discharge voltage is reached. At this point in time the energy in C2 is transferred into the discharge. A simulation of this process is shown next to the diagram above. The voltage over C1 and C2, the current through the inductor L1 and the discharge current (-i(r1)) are shown. The values of C1 and C2 are fixed by the pulse energy and operating voltage. The inductance, L1, is determiner by the transfer time or, in other words, the output voltage rise-time. Take note: For fast pulses the inductance, L1, has to become very small.

In general, it is a bad idea to use these pulse generators to drive capacitive or highly inductive loads. The main reason: there is no place for the energy to be dissipated and the energy remains in the circuit, oscillating back and forth. For pulse generators with pulse compression the oscillations can become chaotic and completely unpredictable. This can damage circuit components, especially the switching device.

Voltage sources

The aim is to generate a voltage pulse with a certain shape, i.e. amplitude, length and rise-time. The amount of pulse energy transferred into the load depends on the load and can vary greatly. Applications range from plasma lamps to Pockels cells (opto-electronic devices). In many cases, a very fast voltage rise-time is required. If a capacitive load is connected this can result in excessive pulse currents. Hence, voltage source pulse generators can handle only small capacitive loads.

These types of power supplies are technically difficult to build for very larger power values. The use of pulse transformers can complicate the matter even further. For very fast pulses, transformers cannot be utilised and novel switching devices have to be used. For example, avalanche transistors and series stacked MOSFET transistors.

Parasitic inductances and capacitances are enemy number one if they are not damped. Undamped series and/or parallel LC parasitic components can cause unwanted ringing of the output voltage and additional dissipative elements (i.e. resistors) have to be added to get rid of ringing. This is the one reason that can limit the output power and efficiency of these pulse power supplies. Undamped ringing can lead to resonant effects at higher pulse repetition rates.

Resonant supplies

These are not true pulse power supplies, since they operate in a steady-state mode. However, when it comes to large capacitive loads it is difficult to get power into the load. A typical example of a capacitive load are large DBD (dielectric barrier discharge) reactors. DBD reactors can be viewed as a capacitor with two dielectric regions: a region which can sustain high voltages (e.g. glass or ceramic) and region of gas sandwiched between two metal plates:

 

 

Another way to model a DBD reactor is by two series connected capacitors: one for the glass/ceramic and one by the air gap. When the applied voltage is high enough, the air gap will break down and a discharge is obtained. The discharge current is supplied by the capacitor's displacement current. Below the discharge threshold, the DBD reactor will be a pure capacitor and will not absorb any power from the power supply (except for very small dielectric losses and conduction losses due to the displacement current). The main advantage is that an arc discharge cannot be formed. DBDs have many applications, ranging from ozone generators to large VUV discharge lamps.

One way to get energy into the discharge, is to use a resonant circuit by connecting an inductor in series with the DBD reactor and driving the resulting series LC circuit with a half- or full-bridge inverter. The steady-state voltage over the air gap as a function of frequency is shown below:

Without a discharge, there is a strong resonant peak. At this resonant frequency a very large reactive current will flow in the LC circuit and this can destroy the power supply. Care must be taken to avoid this resonant peak. As soon as the discharge is initiated the discharge gap can be more or less modelled as a resistor. This will dampen the LC circuit and the resonant frequency will shift to a lower frequency. In this case the resonant peak will be the frequency at which maximum power is transferred into the discharge.

Summary

This was just a small introduction to the different types of pulse power supplies. It is by no means a complete picture and the zoo of pulse generators is much larger with many weird and wonderful topologies. Some of them take analogue electronics to a completely new level and can make your head spin.

In my next post, I will discuss the use of pulse transformers and it is vital to have a basic understanding of pulser types and their typical applications (read: load regimes).

 

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