This is the simplest means of generating a high voltage impulse in a load. Practical considerations usually dictate that more sophisticated means be used (like Marx generators, Transmission line pulse formers, Impulse transformers, etc.), but the basic capacitor discharge circuit is a good place to start.
The circuit above has all the essential components:
Echg - A means of charging the capacitor. Often, either a current limited HV power supply and a switch to connect it to the capacitor, or a HV power supply and a large series resistor to limit the charging current.
Cs - A capacitor to store the energy.
S - A switch to apply the energy to the load
R1 and L Series resistance and/or inductance, either parasitic or added for pulse shape control
R2 - Load resistance
Cload - Load capacitance
Assuming the parasitic inductances are small (often, this is not a valid assumption), the output of a capacitive impulse generator can be represented by a pair of exponentials, reflecting the charging of the load capacitance and then, the discharge of the storage and load capacitance. The most common way to describe the waveform is by it's rise and fall times.
A standard waveform for lightning impulse testing would be a 2/50, where the load voltage reaches its peak in 2 microseconds, and the decay to half the peak voltage takes 50 microseconds.
In these systems, the capacitor is used to store the energy to be used for the impulse. Since fast rise times are usually desired, the capacitor should have low parasitic inductance. Resistive losses also result in lower efficiency and slower rise times. Commercial energy storage capacitors are designed to a specific capacitance. The manufacturer then tests them, and their actual characteristics (capacitance, stored energy) are marked on the label.
The switches for an impulse generator fall into two general categories. The first is those that are primarily mechanical in nature, consisting of contacts that are closed by some means such as a spring, solenoid, air cylinder, or other actuator. The second is those that have no moving parts, with the triggered spark gap being very popular, although in some applications, devices such as thyratrons or SCR's are used.
The rectified output of a high voltage transformer is probably the simplest system used for charging the capacitor. Some form of current limiting is necessary because the capacitor looks like a dead short when fully discharged. The current limiting is often in the form of a series impedance. The impedance be either inductive or resistive and can either be in the primary side of the transformer or the secondary (or be in sort of both, in the form of leakage inductance in the transformer).
A resistive current limiter is simple, but the energy dissipated in the resistor is signficant, being equal to the stored energy in the capacitor. Inductive current limiters don't have the power dissipation problem of a resistor, but are more susceptible to unwanted resonance effects, particularly with parasitic reactances. A resonant charging scheme using a diode and an inductor is very popular for capacitor discharge circuits that are fired repeatedly.
Fruengel recommends the use of a voltage multipler (Cockroft-Walton type), because it has a hyperbolic voltage/current characteristic that lends itself to capacitor charging. The disadvantage is that there is significant stored energy in the capacitor stack of the multiplier, although raising the input frequency reduces the size of capacitor required, and the stored energy. In fact, a logical outgrowth of this trend is the use of switching power supplies.
In recent years, switching power supplies have become popular for capacitor charging. The generally high (tens of kHz) switching frequency reduces the stored energy in the supply, which enhances safety and reduces the chances of a flashover arc developing. They can provide a constant charging current, reducing the power lost compared to a series resistor RC scheme. They can also detect faults and shut down the supply if an arc develops or a capacitor fails (shorted) during charging. HV power supply manufacturers such as Maxwell have power supplies designed specifically for charging capacitors.
The system for charging should take into account the voltage reversal on the storage capacitor if any. For instance, a currrent limited HV transformer feeding a bridge rectifier is a convenient way to charge a capacitor. However, if the capacitor discharge waveform has any voltage reversal, the diodes in the bridge will be forward biased in parallel with the capacitor, and the resulting high peak currents will most likely destroy the diodes.
Copyright 1998, Jim Lux / cdimp.htm / 3 May 1998 / Back to HV Home / Back to home page / Mail to Jim
