ELEMENTS OF DC CONVERTER CIRCUITS BASED ON THYRISTORS

B. A. Lapin

(Recommended by the scientific seminar of the Electromechanical Faculty)

The ideal switching properties and short recovery time of silicon controlled rectifiers determine their wide use in the technology of regulating and converting direct-current energy.

Such converters are often made as single-valve converters with pulse-width, frequency, or relay control, and are used in traction electric drives—mine electric locomotives, electric cars—for regulating the excitation of electric machines, as regulated power supplies for radio-electronic equipment [1], and so on.

The main functional elements of single-valve DC converters are the forced-commutation unit and the control circuit. In some cases, special devices for monitoring the controllability of the converter and for emergency high-speed shutdown also play an important role.

The principles by which the devices listed are constructed largely determine the technical-economic and regulating properties of the converter as a whole.

Below are described several elements of thyristor converters developed and tested in the laboratory of the Department of Electric Drive Automation, TPI.

Forced-Commutation Unit

The forced-commutation unit is intended for the periodic switching off of the power thyristor of the converter in the anode circuit of the device. The circuit of the device is shown in Fig. 1 as part of a converter operating with an inductive-active load and counter-emf. A second version of the circuit is shown in Fig. 2.

Both versions operate in two intervals: a) resonant recharging of the commutating capacitance; b) commutation of the power rectifier of the converter.

Resonant recharging of the capacitance is possible under the condition that \(KUV_1\) is open, and proceeds through the circuit \(KUV_1—KUV_2—L_k\) after \(KUV_2\) is switched on.

The commutation process of valve \(KUV_1\) begins after a half-period of the natural oscillations of the circuit \(L_k C_k\), when the current in the resonant circuit changes its direction. In this case \(KUV_2\) closes, and the commutating pulse is supplied to \(KUV_1\) through \(D_1\).

Fig. 1.

Fig. 2.

In the circuit of Fig. 2, commutation takes place through valve \(KUV_3\), which is switched on at the moment when the direction of the current in the circuit \(L_k C_k\) changes, along the chain \(R D\).

A distinctive feature of this commutation circuit is the inclusion, in the power circuit, of a choke operating in a discontinuous-current mode. With its help, periodically, at the commutation frequency, power is drawn from the load circuit into the commutating device. In this case the capacitance is charged to a voltage exceeding the supply-source voltage:

\[ U = E_n + I_n \sqrt{\frac{L_{\mathrm{dr}} + L_k}{C_k}}. \]

This functional relation between the commutating voltage and the current being turned off significantly increases the reliability of commutation and makes it possible to reduce the value of \(C_k\).

The method for calculating the commutating elements for this circuit is given in [1, 2].

The inclusion of the choke in the discontinuous-current circuit makes it possible to solve the problem of measuring the load current very simply. For this purpose, the secondary winding of the choke \(W_2\), connected through diode \(D_2\) to the clamping circuit \(R_1 C_1\), is used. The voltage at its output is determined by the expression

\[ U_{C1} \cong k \cdot I_n \sqrt{\frac{L_{\mathrm{dr}} + L_k}{C_k}}. \]

Here \(k = \dfrac{W_2}{W_1}\) is the transformation coefficient.

The voltage \(U_{C1}\) in this system is used for current-limiting purposes. The value of the cutoff current is set by selecting the Zener diode \(D_3\) and the transformation coefficient.

Control circuit

The structure of the converter control circuit depends to a considerable extent on the adopted method of controlling the converter power valve. The circuit considered below (Fig. 3) is intended for pulse-width modulation of the converter pulses. In this case the control circuit performs the role of a pulse-width modulator.

(PWM), converting the control voltage into a time interval between two pulse sequences.

The operating principle of this PWM (Fig. 3) is based on comparison

Fig. 3.

of the control and sawtooth voltages. Commands to turn on the valves of the converter are generated at the instants when these two voltages are equal and on the reverse stroke of the sawtooth (Fig. 4).

According to their functional characteristics, the control circuit can be divided into the following elements: a) a master generator; b) a source of sawtooth voltage; c) a comparing device; d) a pulse amplifier-shaper; e) a blocking trigger.

The role of the master is played by blocking generator \(BG_1\), operating in a self-oscillating mode. Its load is the input circuit of the switching valve \(KUV_2\) (Figs. 1, 2). The frequency of \(BG_1\) depends on the product \(R_1 C_1\) and is regulated by

Fig. 4.

power \(R_1\). For temperature stabilization of the generator frequency, resistance \(R_2\) and silicon diode \(D_1\) are included in the base circuit of triode \(T_1\).

The sawtooth voltage is formed by means of the integrating circuit \(R_3 C_2\). The storage capacitance \(C_2\) is discharged during the flyback of the sawtooth through diode \(D_2\) when \(BG_1\) operates.

The principal element of the PWM is a comparison device based on a balanced comparator [3].

A sawtooth voltage is applied to one of its inputs (diode \(D_4\)); to the other (diode \(D_5\)) the control voltage is applied. At the instant when the two voltages become equal, the blocking oscillator of the comparator (\(BG_3\)) operates and starts the triggered \(BG_2\), which performs the function of a shaping amplifier. When this device is used in a closed-loop system, the mismatch signal should be applied to diode \(D_5\) instead of \(U_{\mathrm{cont}}\). A voltage from the current-limiting unit is applied to the third input of the comparator (diode \(D_6\)) (Fig. 1).

A special feature of the comparator is that, after its first operation, it continues to operate in a self-oscillatory mode while the timing capacitance \(C_3\) is charged to the sawtooth amplitude. This results in diode \(D_5\) being blocked during part of the next period, while \(C_3\) is discharging through \(R_4\), and voltage comparison is impossible.

To eliminate this phenomenon, after the first operation of the comparator the capacitance is discharged through diode \(D_3\) and the left triode of the trigger, which is flipped by a pulse from winding \(W_{3-5}\). Blocking from the sawtooth-voltage source is removed at the end of the period, when a pulse from \(BG_1\) returns the trigger to its initial position.

The circuit considered makes it possible to regulate the pulse duration within the range \(0\)—\(0.95\) and is distinguished by stable operation over a wide temperature range.

Device for controllability monitoring and emergency shutdown

In emergency conditions, or in the event of faults in the control circuit, converters with forced commutation may lose controllability. In this case the entire voltage of the primary source reaches the output. In some cases, for example when operating into a load that cannot tolerate overvoltages, or when transformer-coupled to the load, this may lead to destruction of the power thyristor or to failure of the load.

To prevent these consequences, it is necessary to use special monitoring and disconnecting devices that satisfy the following requirements:

1. Detection of commutation failure and shutdown of the converter power valve with a delay of not more than one period.

2. Complete autonomy of operation, i.e., independence from the control circuit.

3. Constant readiness for action.

A protection circuit developed with these requirements in mind is shown in Fig. 5a.

The device for monitoring controllability consists of the integrating circuit \(R_1 C_1\), diodes \(D_1\), \(D_2\), and transformer \(Tr_1\).

During normal operation of the converter, capacitance \(C_1\) periodically—

…charges through \(R_1\) during the pulse and discharges through \(D_1\) at the beginning of the pause (Fig. 5b).

The constant \(R_1 C_1\) is chosen so that, at the maximum pulse length equal to \(T\), the amplitude of the voltage \(U_c\) is less than the breakdown voltage \(U_{\mathrm{pr}}\) of diode \(D_2\).

Fig. 5.

When commutation fails, the pause between pulses disappears. Therefore the voltage \(U_c\) reaches the breakdown voltage and diode \(D_2\) breaks down at the instant \(t_1\). In this case \(C_1\) discharges through the primary winding of \(Tr_1\).

From Fig. 5b it is seen that the delay \(\Delta t\) is much smaller than the period \(T\).

For emergency shutdown of the power valve \(KUV_1\), a reserve commutating device is used, consisting of \(KUV_0\) and \(C_0\) (Fig. 5a). Switching on \(KUV_0\) occurs when \(D_2\) operates, by means of transformer \(Tr_1\).

To ensure the constant readiness of the protection to operate, the capacitance \(C_0\), before the converter is switched on, is charged to the voltage \(E_p\) through contact \(L_1\), diode \(D_0\), and resistance \(R_0\).

After the linear circuit breaker is switched on, \(C_0\) is connected to the cathode of the valve \(KUV_0\). During operation of the converter it is continuously recharged through the circuit \(L_2—C_0—D_0—R_0\). The resistance \(R_0\) must be sufficiently small so that \(KUV_0\) remains in the on state after the protection has operated. Then the voltage from its cathode, equal to \(E_p\), can be used to block the control circuit in order to prevent repeated starting of \(KUV_1\), and also to switch off the linear circuit breaker.

LITERATURE

1. Report on contract work. “Development of an adjustable direct-current converter.” Tomsk, 1965.

2. Circuit of artificial commutation with choke charging of the commutating capacitance. Izvestiya TPI, vol. 153.

3. Calculation and design of pulse devices using transistors. Publishing House “Sovetskoe radio,” edited by Shterk, 1964.