Control circuit for eliminating a voltage and current spike

Control circuit for eliminating a voltage and current spike

The object of the invention is a control circuit for eliminating a voltage and current spike in a DC circuit, the control circuit being connected to control a solid state switch, such as a transistor (PNP or NPN) or a FET transistor (NMOS, PMOS) serially connected to a load, in such a way that when the load current is turned on, the switching current increases in a controlled manner without a detrimental voltage and current spike, the control circuit including at least one operational amplifier, or other combination of an amplifier and a comparator, which has a first and second control input and a control output, which has an effect on the base control of the solid state switch, and a matching network which is connected to the control circuit between the input terminal of the control signal and the base of the switching transistor in order to delay the ascending speed of the control signal, and correspondingly that of the load current and voltage, during turn-on of the load current.

This type of control circuit is disclosed, for example, in the patent publication EP-285417. This known circuit is suitable for eliminating current and voltage spikes during turn-on of the load, but it disregards the elimination of the current spike generated during the switching off of an inductive load.

Voltage and current spikes are mainly generated during the switching off of inductive and capacitive loads. The switching off of a coil or lamp may even generate a multiple voltage spike with respect to the input voltage, the spike destroying electronic structures and causing rapid aging of components, causing their service life to shorten markedly, and shortly rendering the entire apparatus useless. Furthermore, the EMC radiation emitted to the environment is extremely harmful and has a wide spectrum. To eliminate voltage spikes, external components (capacitors, inductances, resistors and diodes) are used. In many cases the momentarily discharging energy is so intense that it also ages the components used for protection, causing them to be damaged after a short period of use.

Both current spikes and voltage spikes become more intense if the load is located at the end of long cables, in which case the cable's inductance and capacitance enhance the effect of the current and voltage spikes. The external components are, in addition, powerful and cannot be integrated directly onto silicon at all. External components take up much space and are expensive.

The aim of the present invention is to develop a solution by means of which the effect of current and voltage spikes can be eliminated, both when turning on and switching off loads, by means of the type of control circuit in which no expensive components are required. The control circuit relating to the invention requires a small additional electronic circuit which is, however, moderately priced and can be integrated onto silicon, which means that its implementation is economical.

This aim of the invention is achieved on the basis of the characteristics described in the appended claim 1 . Preferred embodiments of the invention are disclosed in the dependent claims.

Some embodiments of the invention are described in greater detail in the following, with reference to the appended drawings, in which

FIG. 1 shows a circuit diagram of a control circuit according to the first embodiment of the invention, which is a solution controlling the turning on or switching off of a load current on the basis of a desired value.

FIG. 2 shows a modified embodiment of FIG. 1, which is functionally identical with the control circuit of FIG. 1, but the operational amplifier O 2 is not of the “rail to rail” type, which means that scaling resistors R 5 , R 6 and R 7 are required.

FIG. 3 shows a circuit diagram of the control circuit relating to the invention, which is a rapidly integrating solution realised by means of switches and comparators.

The embodiment shown in FIGS. 1 and 2 is first described. The point M 2 , that is, the DRAIN voltage of T 2 , is connected to the (+) input of operational amplifier O 1 or O 2 . The connection shown “forces”point M 2 to follow the wave form of M 1 . Point M 2 is thus programmed in accordance with M 1 . Depending on the load, an appropriate wave form can be selected for point M 1 , which wave form is created by a separate matching network T 1 , R 1 , C 1 , R 2 . The desired wave form can, therefore, be determined for point M 1 through the selection of components for the matching network. R 4 and C 2 form an integrator. T 1 and T 2 may be either ordinary transistors or FET transistors or also MOSFET transistors. In the embodiment shown in FIG. 1, operational amplifier O 1 is a so-called “rail to rail” amplifier, which means that its input area extends from GND to VCC.

The turn-on can be carried out with a low gradient at point M 2 , for example, in the manner of a sine wave, that is, not by direct gradation. This prevents the high current spike generated by the cold wire of the lamp L from occurring. When the switching transistor T 1 is switched off by means of OFF control, the voltage effective at point M 1 determines, together with the dimensioning of the integrator components C 2 , R 4 , the switching-off control time and curve form of switching transistor T 2 . If the voltage at point M 2 tends to differ from the voltage at point M 1 , the differentiating variable created will control operational amplifier O 1 , which controls switching transistor T 2 , thus compensating for the change created. During a voltage spike at point M 2 , transistor T 2 remains open, whereby the voltage spike discharges to ground GND.

The embodiment shown in FIG. 3 is described in the following.

The turning on of the switching transistor, that is, the load current FET T 2 takes place as follows.

Amplifier U 1 A raises the control (input) to the accumulator voltage level VPP, the FETs T 1 and T 2 are conductive, and FET T 2 is turned on through resistor R 9 .

Current begins to flow through the lamp L and at the T 2 DRAIN pin the voltage begins to decrease. This voltage is measured by comparator U 2 A. Once the voltage has decreased to VPP −1V, comparator U 2 A changes its state and switches off FET T 4 . The control voltage of the operating FET T 2 , therefore, now passes through the chain T 1 -RB-R 9 , thus forming a low-gradient rise in the control voltage. By means of this action, the current drawn by the lamp L is limited.

Once the switching transistor controlling the load L, that is, load current FET T 2 , is almost completely open, comparator U 2 B releases control to the normal state by opening FET T 4 .

The switching off of the switching transistor, or load current FET T 2 , takes place as follows.

The base voltage of FET T 2 is discharged via the chain R 9 -T 4 -T 1 . The transient spikes typically occurring during the switching off of the current are measured by comparator U 3 B which acts as a differentiating amplifier. U 3 B, therefore, acts as a differentiating amplifier with the purpose of measuring the difference of voltage at T 2 DRAIN/above the load L. Should the voltage at the T 2 DRAIN pin increase above the accumulator voltage VPP prevailing above the load L, the comparator U 3 B will change its state, FET T 1 will close and FET T 3 open, driving the accumulator voltage directly to the base of switching transistor T 2 , via FET T 3 and diode D 3 . The control voltage of switching transistor T 2 is locked and thus the voltage spike is directed through switching transistor T 2 to ground.

The voltage of the control circuit (e.g. at point M 1 in FIGS. 1 and 2) may be modifiable, where necessary, also through programming in such a way that a processor (not shown) program selects various components or circuit alternatives or different reference voltage levels for use.

The invention is not limited to the above embodiments, but by using circuits functioning in a similar manner, a person skilled in the art may easily vary the implementing embodiments of the invention as guided by the disclosed embodiments. The essential aspect of the invention is that by means of the voltage at the DRAIN pin of the switching transistor T 2 , the control circuit according to the invention is also able to eliminate the voltage and current spikes generated while switching off a load, in addition to being able to slow down the turn-on of the current in the desired manner and, in a preferred embodiment of the invention, make it follow the desired wave form.