DK148815B - TRIACS TENSION CONTROL BOARD FOR TRIACS - Google Patents

Description

148815

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a DC voltage controlled circuit for triacs having a series voltage of a resistor and a first charge capacitor connected to the mains voltage, and with a trigger diode, over which the capacitor voltage can be applied to the triacence control electrode and connected to a series capacitor in parallel. by another charging capacitor and a controllable coupling means, whose control electrode is controlled by the control voltage.

Triacs are increasingly being used for power management of AC power consumers. Triacs allow full-wave operation of the consumer with phase cuts in a simple way, since only one power building element is required. At a certain angle of each half wave, the triac must be turned on.

Couplings that produce the ignition of triac's suitable ignition pulses are known in large numbers and require various builds of building elements. A wide, very simple apparatus is shown in the figure to the right of the dotted line. Above a resistor, a capacitor is charged, and at a certain charging voltage, the trigger diode turns on the triac. The angle of ignition is set by the resistance being designed as variable resistance. In the case of couplings that are not directly operated but which must be changed by an electrical size, voltage or current, the following difficulties are shown: The continuously controllable unipolar resistors needed for resistance are not replaced by many electronic building elements. There are photo resistors or field plates. None of these can be directly controlled by a voltage or current, but one must go. The detour over light or magnetism in the form of so-called optoelectric or magnetic couplers, which unnecessarily increases the cost of the coupling. As continuous controllable polar building element, transistors in diode bridge couplings are also used as variable resistors. This means has the disadvantage that the emitter of the transistor as a reference point for the controlling size U or I is not galvanically connected to any of the other points. This leads to potential difficulties in the further coupling design. When a high voltage, e.g. the mains voltage of 220 volts must be regulated, there is a high voltage on the variable resistance. Of field plates, none at all, and of photo resistors and transistors, only a few less expensive types can be used in this case. Of course, there are also known couplings which supply ignition pulses and which can be controlled directly by an electric voltage or electric current. However, these are considerably more complicated than the known widespread couplings.

U.S. Patent No. 3,676,706 discloses a trigger circuit of this kind. In this known circuit, the controllable coupling means consists of a transistor pair - namely one transistor for each half-wave - together with a phase-reversing transistor for the alternate control of the two transistors in the pair.

It is the object of the invention to provide the design of a trigger circuit of the type indicated initially, which is substantially simplified relative to the circuit thus known.

The stated object is achieved by a trigger circuit which according to the invention is characterized by a) that the controllable coupling means is a field power transistor, b) that the circuit comprises a balanced bridge coupling, in which one longitudinal branch consists of the drain power source of the field power transistor. and the second longitudinal branch consists of a voltage divider; c) a capacitor is coupled in the transverse branch of the bridge coupling between the voltage divider's outlet and the field effect transistor's control electrode; and d) the control voltage is applied to the field effect transistor's control electrode.

In this circuit, just as in the known, there is an overlay of the control voltage with the alternating voltage. While three transistors are required for the required control function in the known circuit, a single field effect transistor can be satisfied in the circuit according to the invention, and it has further been found that this circuit can operate at a lower control voltage than the known one.

The invention will be explained in more detail below with reference to the drawing, which shows an embodiment of a trigger circuit according to the invention.

A mains voltage Un lies above two terminals 10, 11 which over a load resistor 12, or a consumer, and the coupling path of a triac 13 are connected to each other. A serial combination of a charging resistor 14 and a charging capacitor 15 bridges the triac 13. The connection point between the building elements 14, 15 is connected through a trigger diode 16 to the control electrode of the triac 13 and through a serial combination of another charging capacitor 17 and an N-channel barrier layer field transistor 18 source extension connected to terminal 11. In parallel with the channel of the field power transistor 18, one of two resistors 19, 20 comprising voltage parts is coupled. The control electrode of the field effect transistor 18 is connected, partly over a capacitor 21, to the outlet of the voltage divider 19, 20 and partly over an input resistor 22 to a terminal 23. The control DC voltage Us lies between terminals 23 and 24, where the terminal 24 is connected to the terminal 11. The voltage divider 19 , 20 together with the channel of the field effect transistor 18 forms a bridge coupling. Resistors 19, 20 are dimensioned such that a symmetrical field effect transistor with voltage-free control electrode provides a bridge coupling equal to the channel center.

, 148815 In the following, the operation of the portion to the right of the dotted line shall be described. In the blocking phase of triac 13, i.e. at the beginning of each grid half, the charging capacitor 15 is charged across the charging resistor 14. If the capacitor voltage reaches the breakthrough diode of the diode 161, it suddenly becomes conductive and delivers an ignition pulse at the triac 13 control electrode.

Through the now conducting triac, the net half-wave on the ignition circuit disappears. The capacitor 15 is first discharged first over the trigger diode 16 and then over the resistor 14 and the triac 13. At the beginning of the next AC voltage half-wave, at which the triac 13 returns to the blocking phase, the process repeats itself with reverse polarity.

In the circuit shown, the resistor 14 is constant and supplies an immutable charge current by means of the resistor. The variable charge rate, and thus the change in timing, is obtained by the second charge capacitor 17. At sufficiently small currents during the pinch-off action, the drainage current of the field effect transistor 18 constitutes a unipolar controllable ohmic resistance. With voltage-free control electrode on the field power transistor 18, this resistance is very low ohm.

The two charge capacitors 17, 15 are thus connected in parallel and the charging current through the resistor 14 distributes on the two capacitors 15, 17. This makes the charging process slower and a small current angle is obtained. At higher control electrode voltages, the channel resistance is practically infinite, whereby only the charge capacitor 15 is active. The charging speed is increased and a large current angle is obtained.

By continuously changing the control electrode voltage, the channel resistance is continuously controlled. Thereby, almost the entire current angle range between 0 ° and 180 ° can be continuously smoothed over by an appropriate dimensioning of the capacitors 15, 17.

In the case of a symmetrical field effect transistor, the channel resistance can be controlled in the same way both by a control electrode source voltage and by an equal control electrode drain voltage. In the present circuit, an equal channel resistance value in each half-wave is required when the two current angles in each whole wave of the mains voltage must be equal. Since the polarity of the source-source voltage alternates in each half-wave, a direct introduction of the control voltage between the control electrode and the source is not possible. The negative voltage of the control electrode relative to the drain source channel, especially relative to the channel center, would be significantly higher for positive drain than for negative drain. Thus, the control of the field effect transistor would be different in the half waves. The same channel resistance in the half-waves is obtained when the control voltage is applied symmetrically with respect to sinks and source.

The symmetrical insertion is obtained by passing the control current voltage Us over the resistor 22 to the control electrode and thus to the storage capacitor 21. If the time constant T = R 22 · C 21 is large in relation to the grid period time, the voltage of the control electrode is due to the input of the control voltage on it in the balanced state. bridge branch storage capacitor 21 is equal to the channel center at each moment of a grid period. Thus, the alternating polarity of the drain source voltage is without influence on the control of the field power transistor 18. The same channel resistance value and current angle are obtained at all times irrespective of the relevant grid half-wave.

However, it is also possible to use a lower value of the time constant T = R 22 »C 21, provided that one is determined to find minor variations in the timing. Moreover, it should be noted that only in the "ideal" case, where the control voltage circuit connected to terminals 23, 24 has only a vanishingly small internal resistance, the real time constant of the product R 22 · C 21 is determined, since in most normal in this case, this control voltage source has some internal resistance, the value of which must be added to the value of R 22, before the time constant is formed by multiplying the value of C 21.

In this context, it is noted that the main function of capacitor C 21 is to prevent the DC voltage from being discharged through resistor 20 to line 24-11. This DC voltage can only be discharged through the extremely high-gate port source - stretch in the field power transistor 18. Only here the considerable advantage is obtained that only a virtually vanishing small control power is needed, so that very high-frequency control voltage sources can also be used. and in that case the resistance 22, as demonstrated by practical experiments, may be very small.

If a positive control voltage is to be used instead of a negative control voltage, instead of an N-channel field effect transistor, a P-channel field effect transistor is chosen.