CONTROL CIRCUITS FOR STABILIZED POWER SUPPLIES OF NONSTANDARD FREQUENCY

M. A. ZHITKOV, A. K. LUZGINOV, A. Yu. ROZHDESTVENSKII

(Presented by the scientific-technical seminar of the Scientific Research Institute of AEM)

Recently there has arisen a need to develop secondary power supplies of nonstandard frequency, in which static converters are used. The widest application for such purposes has been found by autonomous inverters, which make it possible to combine the functions of inversion and voltage regulation in a single device.

In many cases it is necessary to provide regulation or, as a particular case of this, stabilization of the output voltage, which calls for the development of appropriate control circuits.

The simplest method of regulating the output voltage of an inverter is “duration control.” The requirements imposed on control circuits for such inverters are determined by the type of power section, the output power, and the nature of the load. One of the basic requirements is also the stability of the phase of the output voltage.

The implementation of the “duration control” method is associated with the use of sweep-type modulators [1]. On the basis of the requirements for stability of the phase of the inverter output voltage, the sweep function must be a symmetrical triangular or sinusoidal alternating-sign function. It should be noted that obtaining a symmetrical triangular sweep function of high linearity presents difficulties; therefore, it is preferable to obtain a sinusoidal sweep function by means of resonant circuits. In addition, the control circuit must satisfy generally accepted requirements, including a high gain, steep edges of the control signals and their strict symmetry, and a wide range of regulation of the duty cycle of the control signals.

The possible variants for constructing control circuits that satisfy these requirements can be reduced to two structural circuits, which are shown in Figs. 1 and 2, where

\[ \begin{aligned} \Phi &\text{ — sweep-function generator,}\\ \mathrm{CC},\ \mathrm{CC}_1,\ \mathrm{CC}_2 &\text{ — comparison circuits,}\\ \mathrm{BPK} &\text{ — half-wave switching unit,}\\ \mathrm{U} &\text{ — push-pull power amplifier,}\\ \mathrm{I} &\text{ — inverter.} \end{aligned} \]

The main difference between these circuits is the different number of comparison channels. With two comparison channels in Fig. 1, owing to the spread in the nominal values of the circuit elements, as well as to changes in the parameters of semiconductor devices during operation under the influence of various facto-

Fig. 1

(parameters (heat, time, etc.), an asymmetry of the signals appears, leading to the appearance of even harmonics in the inverter output voltage.

This asymmetry is further aggravated by the fact that the presence of a dc component in the transformer of a push-pull amplifier leads to the appearance of self-induction emf spikes, which cause false turn-on of the thyristors.

Fig. 2

Taking the noted shortcomings into account, a control circuit corresponding to the block diagram in Fig. 2 was developed in the converter laboratory of the Research Institute of Electrical Engineering.

The symmetry of the control signals is achieved by using a single comparison circuit and by subsequently separating the signals in time into two channels by means of a half-wave switching unit. The power section of the converter is an inverter with sequential commutation [2].

One version of the control system according to the block diagram of Fig. 2 is the circuit shown in Fig. 3.

Here the sweep-function generator is an integrating circuit \(RC_1\). The comparison circuit is assembled on transistors \(T_1\)—\(T_3\), Zener diode \(D_1\), and potentiometer \(R_2\). The half-wave switching unit includes two coincidence circuits on diodes \(D_2D_3\) and \(D_4D_5\) and a trigger with a counting input on transistors \(T_4T_5\).

The amplifier is assembled according to a conventional circuit on transistors \(T_6T_8\) and \(T_7T_9\). The circuit for forming auxiliary pulses to start the inverter commutation circuit is assembled on transistors \(T_{10}\)—\(T_{12}\).

The operating timing diagrams are shown in Fig. 4.

The circuit operates as follows. Rectangular signals from the master oscillator with frequency \(f\) are applied to \(\Phi\). The generated triangular voltage is compared with the dc voltage \(U_y\), set by potentiometer \(R_2\). The difference of these signals, isolated by Zener diode \(D_1\), is amplified by triodes \(T_1,T_2T_3\).

Rectangular pulses taken from the collector of \(T_3\) and adjustable in width are applied to two coincidence circuits together with the trigger signals. The frequency of the trigger pulses is \(f/2\). As a result of time selection, control pulses shifted in phase by \(180^\circ\) are taken from resistors \(R_{10}\), \(R_{11}\). These pulses control the push-pull power amplifier. For reliable blocking of the triodes of the final stage, a bias voltage is introduced, taken from the circuit formed by diodes \(D_8D_9\).

Fig. 3.

Fig. 4

The output pulses of the push-pull power amplifier are applied to the control electrodes of the power thyristors and to the input of an amplifier built around transistor \(T_{10}\). The amplified signals are differentiated by means of the \(C_8 R_{31}\) circuit and are fed to the input of the amplifier on transistor \(T_{11}\), which synchronizes the operation of the triggered blocking oscillator on the trailing edge of the control pulse. The increased noise immunity of the blocking oscillator is ensured by triggering at the collector and by using an additional bias taken from diodes \(D_{12}\), \(D_{13}\).

Adjustment of the converter output voltage is carried out by changing \(U_y\), set by potentiometer \(P_2\). To stabilize the converter output voltage, the feedback voltage must be connected in series with \(U_y\) (terminals 1—2 in Fig. 3).

The circuit developed was used to control a single-phase autonomous inverter.

A disadvantage of the circuit is the use in the BPC of a trigger with a counting input, which makes it difficult to use for controlling a three-phase inverter with phase-by-phase regulation. However, this drawback can be eliminated by applying to the modulator a signal from the master oscillator with frequency \(f/2\) and using it for time selection, while applying a signal of double frequency \(f\) to the comparison circuit.

Experimental investigations of control circuits assembled according to the structural diagrams in Figs. 1 and 2 from identical elements showed that the asymmetry of the output pulses of the circuit having two comparison channels is 5–8%, and the ratio of the amplitude of the spurious signal produced as a result of the increase in the self-induction emf to the amplitude of the useful signal is \(1 \div 5\)—\(1 \div 7\), whereas in the circuit,

Fig. 5.          Fig. 6.

having one comparison channel, the asymmetry of the output pulses is 0.5%, and the amplitude ratio is 1:15–1:20.

Figure 6 presents the experimental “input–output” characteristic of the control circuit, and Fig. 5 presents the experimental “input–output” characteristic of the converter.

Conclusions

1. The use of a single comparison channel and time selection of pulses makes it possible to obtain strict symmetry of the output signals.

2. The use of the output-stage transistors in a deep-saturation mode makes the circuit insensitive to load asymmetry determined by the spread in the resistances of the control junctions of the thyristors.

References

1. F. E. Temnikov. Theory of Sweep Systems. Gosenergoizdat, 1963.

2. A. I. Zaitsev, M. A. Zhitkov. Independent inverters with pole commutation in a pulse-width modulation mode. Proceedings of the VI Scientific and Technical Conference on Production Automation, Vol. 5. Tomsk, 1969.

3. O. I. Timakhov, V. K. Lyubchenko. Pulse Selectors. “Sovetskoe radio,” 1966.