Generator of Nanosecond Pulses
Abstract Generated abstract
This paper describes a nanosecond pulse generator based on rapid capacitor discharge through thyristor switching. The proposed circuit adapts a single-cycle pulse-shaping scheme by using a pulse current transformer as the load and adding a saturable inductor to shorten the pulse leading edge, while separating capacitor charging and discharge with main and auxiliary thyristors. The developed generator uses two thyristors and four transistors, synchronized master oscillators, and a triggered multivibrator to produce approximately 50 ns current pulses, with load current determined by capacitor voltage and the combined resistance of the transformer, choke, and main thyristor.
Full Text
Nanosecond Pulse Generator
M. S. Aleinikov, E. P. Koltsov, K. L. Pesterev,
V. S. Titov, V. I. Ivanov
(Presented by the scientific and technical seminar of the Department of Instruments and Devices of Automation Systems)
To obtain powerful current pulses, the principle of charging and discharging a capacitor may be used. The discharge of the capacitor must proceed at high speed. To switch the capacitor from the “charge” state to the “discharge” state, thyristors can be used as switches; they are characterized by high operating efficiency as switches in the kilo- and megawatt ranges in comparison with semiconductor triodes, since they operate at large values of supply voltage and current. A circuit of single-cycle pulse shapers with restoration of the thyristor to its initial state has been proposed [1]. The circuit proposed in [1] can be used as a nanosecond pulse generator if the following circuit changes are introduced: a) a pulsed current transformer is used as the load; b) to reduce the duration of the leading edge of the pulses, a saturable inductor must be installed. Since the thyristors must pass large currents at a very high rate of change of current, it is necessary to use powerful thyristors, which have large dimensions, weight, and cost.
Fig. 1. Block diagram of a nanosecond pulse generator
The most advantageous circuit, and the one satisfying the above requirements, is the circuit shown in Fig. 1. In this circuit thyristor \(T_1\) is the main one, and thyristor \(T_2\) is auxiliary. A pulse duration equal to \(50\cdot 10^{-9}\) s is provided by the capacitance \(C_0\), and the maximum current in the load is determined by the formula:
\[ I_{\mathrm{l}}=\frac{U_c}{R}, \]
\[ R=R_{\mathrm{tr}}+R_{\mathrm{dr}}+R_{T_1}, \]
where
\[ \begin{aligned} R_{\mathrm{tr}}&\text{— resistance of the pulse transformer;}\\ R_{\mathrm{dr}}&\text{— resistance of the saturating choke;}\\ R_{T_1}&\text{— forward-current resistance of the thyristor.} \end{aligned} \]
Fig. 2. Block diagram of the nanosecond-pulse generator
Labels in the diagram: power-supply unit; master oscillator No. 1; differentiating circuit; master oscillator No. 2; pulse transformer; circuit for charging capacitance \(C_0\); capacitance \(C_0\); circuit for discharging capacitance \(C_0\); load.
The circuit has the advantage that thyristor \(T_2\) may be chosen to be of low power, because the pulse repetition frequency is low (period \(5\cdot 10^3\) s); therefore the capacitance \(C_0\) may be charged over \(5\cdot 10^3\) s, and this requires a very small charging current flowing through thyristor \(T_2\).
The block diagram of the developed nanosecond-pulse generator is shown in Fig. 2. Master oscillator 1 produces rectangular pulses and, through a pulse transformer, applies them to the control electrode of the thyristor in the capacitance-charging circuit. The thyristor opens and charging of the capacitance begins. When charging of the capacitance is completed, the charging current becomes equal to zero, and the thyristor of the charging circuit closes automatically. The same pulses from master oscillator 1 enter the differentiating circuit, and negative pulses from the shaping circuit are applied to master oscillator 2, which is assembled according to the circuit of a multivibrator operating in standby mode. Positive pulses from master oscillator 2 are applied to the control electrode of the thyristor in the capacitance-discharge circuit; the thyristor opens, and the capacitance is discharged through the small resistance of the saturating choke and of the current pulse transformer into the secondary
winding, the load is connected. Positive current pulses are taken from the load. The pulse transformer is needed to decouple the capacitor charging circuit and the capacitor discharge circuit. For synchronous operation of two generators, master generator 2 operates in triggered mode and is triggered by pulses (from the trailing edge of the pulses) of master generator 1.
Fig. 3. Schematic diagram of the developed generator
The schematic diagram of the developed generator is shown in Figure 3. The generator circuit is simple, assembled on two thyristors and four transistors. To trigger thyristor \(T_1\), a multivibrator circuit with a single emitter capacitor is used. Such a multivibrator makes it possible to obtain short pulses with a minimum duration down to several tenths of a microsecond (on diffusion transistors) at a large duty factor [2]. By the trailing edge of the pulse of the multivibrator considered above, another multivibrator, operating in a triggered mode, is started through a differentiating circuit; through a diode, control pulses are delivered to thyristor \(T_2\).
The generator is powered from a 220-volt DC source and consumes a current of 0.2 ampere.
REFERENCES
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R. V. Bilik et al. Pulse circuits using dinistors and thyristors. Publishing House “Nauka,” 1968.
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E. F. Doronkin. Pulse generators using transistors. Publishing House “Svyaz,” 1965.