PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE named after S. M. KIROV

Volume 153
1965

PULSE-FORMING CIRCUITS

WITH VARIABLE PARAMETERS

A. I. ZAITSEV, A. P. ZAITSEV.

In various devices of automation and telemechanics, in systems of pulse control of electric machines, circuits for forming periodic sequences of voltage pulses with parameters adjustable over wide limits are used. Below are considered dividers of the pulse repetition frequency with a variable division factor, a circuit for forming pulse bursts, and a pulse-width modulator, which make it possible to form voltage pulses synchronized with the supply alternating vol—

Fig. 1. Circuit of a pulse-frequency divider with a variable division factor

with a sinusoidal or rectangular voltage. The voltage pulses are formed by half-period rectification and selection of specified half-waves of the supply voltage. The use of silicon controlled rectifiers as active elements makes it possible to implement circuits for various output powers (from several watts to tens of kilowatts per pulse).

Fig. 1 shows a pulse repetition-frequency divider with a variable division factor, containing a charge–discharge circuit \((B_1, R_1, R_2, C_H)\) and two controlled semiconductor switches—silicon controlled rectifiers \(D_1\) and \(D_2\). When the circuit is switched on, \(C_H\) is charged by voltage pulses arriving from the secondary winding of transformer \(Tr\), and in the intervals between pulses it is discharged through resistance \(R_2\). The voltage across \(C_H\) rises

Fig. 2. Circuit of a frequency divider with three-phase input voltage.

stepwise, and when its absolute value becomes greater than the reference voltage \(U_3\), a current arises in the control-electrode circuit of \(D_1\). \(D_1\) opens, and an output-voltage pulse \(U_{\text{out}}\) is produced across load resistance \(R_6\); under its action \(D_2\) opens. During a short interval of time \(C_H\) discharges through the opened \(D_2\) into the small resistance \(R_3\), which limits the discharge current within permissible limits. After the end of the conducting half-period of the input voltage, the circuit returns to its initial state, after which the process is repeated periodically. The opening of \(D_1\) will always occur at the beginning of the conducting half-period of the voltage, since the voltage of the charging pulses is in phase opposition to the latter.

Changing the division factor is carried out by means of the setting voltage. Diodes \(B_2\) and \(B_3\) eliminate the effect of the setting voltage, which is considerable in magnitude, on the control junction \(D_1\).

Fig. 3. Circuit for forming pulse bursts,

Figure 2 shows a frequency-divider circuit with a three-phase input voltage, representing a combination of the frequency divider considered above and a trigger circuit using \(D_1\), \(D_2\), \(D_3\). The trigger circuit provides dependent operation of the controlled valves. The pulse of the output voltage in the cathode circuit of the preceding controlled valve serves as the starting pulse for the following one.

If, in the frequency divider, delayed negative feedback with respect to the output voltage is introduced into the control-electrode circuit of the discharge valve, with an adjustable time delay produced by means of a second charge-discharge circuit, then it becomes possible to form bursts of pulses with separate and independent adjustment of the number of pulses in a burst and of the pause between bursts. The circuit for forming pulse bursts is shown in Fig. 3. In contrast to the preceding circuits, the discharge valve \(D_2\) is opened not by the first output pulse, but after a certain time determined by the magnitude of the setting voltage \(U_2\) and by the parameters of the charge-discharge circuit with the storage capacitor \(C_2\). During this time \(D_1\) is open, and the voltage on \(C_1\) is limited at a certain level by the shunting circuit \(B_5 R_{10}\).

The number of pulses in a burst is adjusted by means of the setting voltage \(U_2\), and the duration of the pause between bursts by means of the setting voltage \(U_1\).

Fig. 4. Circuit diagram of a pulse-width modulator for high-power pulses.

Figure 4 shows the schematic diagram of a pulse-width modulator for high-power pulses with separate and independent control of the pulse duration and the pause. The modulator consists of a circuit for forming pulse trains and a trigger circuit based on thyratrons. A special feature of the pulse-train formation circuit is the inclusion of the charge-discharge circuit with storage capacitor \(C_1\) in the grid circuit of thyratron \(T_1\). Ignition of \(T_1\) and \(D_1\) occurs simultaneously owing to their dependent switching; the pulse train is formed across resistor \(R_{10}\). The phase-shifting RC circuits in the grid circuits of \(T_2\) and \(T_3\) provide a phase shift of \(180^\circ\) between the anode and grid voltages of the thyratrons. At the moment the preceding thyratron fires, the RC circuit is shunted and the subsequent thyratron fires. The output pulses are formed across load resistor \(R_{17}\). The parameters of the output pulses are adjusted by means of the setting voltages \(U_1\) and \(U_2\).

Fig. 5. Voltage oscillograms.
a — voltage of the storage capacitor of the frequency divider,
b — output voltage of the frequency divider, c — output voltage of the pulse-burst shaping circuit, d — output voltage of the pulse-width modulator.

As a result of analyzing the operation of the circuits considered above, the expression obtained is

\[ n=\frac{4\tau_0}{T\left[1.27\,\frac{U_0}{U_{\text{з}}}-\left(1+2\frac{r}{R}\right)\right]}, \tag{1} \]

where

\(n\) is the division factor, the number of pulses in a packet, the number of omitted pulses (pause),

\(r, R\) are the charging and discharging resistances of the storage capacitor, respectively,

\[ \tau_0=rC, \]

\(C\) is the capacitance of the storage capacitor,

\(T\) is the period of the input voltage,

\(U_0\) is the amplitude of the voltage of the charging pulses,

\(U_{\text{з}}\) is the value of the reference voltage.

The duration of the pause and of the pulses of the output voltage of the pulse-width modulator is determined from (1), taking into account the expression

\[ t=nT, \tag{2} \]

where \(t\) is the duration of the pulse or pause.