AC Transistor Switch With Overcurrent Protection

The invention relates to transistor power switching circuitry for AC applications, and more particularly to overcurrent protection circuitry.

Various types of overcurrent protection circuitry are known for AC power transistor switching circuits. The higher the level of sensitivity desired, the higher the degree of sophistication required. A tradeoff is the switching speed, which decreases as the number of intermediate components increases. The present invention overcomes this tradeoff and affords both of the previously incompatable results of simplicity and fast speed in the protective circuitry.

The present invention uses the AC switching device itself for current level detection, affording both accuracy and quick direct detection. The invention eliminates auxiliary sensing devices such as shunts, current transformers and the like. Instead, the invention uses a portion of the switch already in the main AC current path for each direction current flow.

FIG. 1 is a schematic circuit diagram of an AC transistor switch with overcurrent protection constructed in accordance with the invention.

FIG. 2 is another embodiment of the circuit of FIG. 1.

FIG. 3 illustrates in further detail the comparator circuitry of FIG. 1.

FIG. 4 illustrates in further detail alternative comparator circuitry of FIG. 1.

In FIG. 1, AC transistor switch 2 includes first and second transistors, such as n channel field effect transistors FET 1 and FET 2 connected source to source, in anti-series between first and second main terminals T1 and T2. The main terminals are connectable in an AC load circuit 4 having an AC power source 6 and a load 8 to be controlled by the FETs. First and second diodes D1 and D2 are each connected in reverse or anti-parallel relation with a respective FET. FETs have an inherent diode in the reverse direction, and in preferred form diodes D1 and D2 are these inherent diodes. This well known transistor-diode configuration provides a common reference point C, the source to source connection, allowing a common control signal for the opposing transistors.

Control terminal 10 supplies a drive signal from a voltage source V.sub.G to the gates of FET 1 and FET 2 to drive them into a conductive ON state. When terminal T1 is positive with respect to terminal T2, current flows in one direction from terminal T1 through FET 1 and diode D2 to terminal T2. In the reverse direction, with terminal T2 positive with respect to terminal T1, current flows from terminal T2 through FET 2 then through diode D1 to terminal T1.

Comparator means 12 senses current flow through diodes D1 and D2 and responds to given threshold overload current to turn off the FETs. Comparator 12 senses the one direction current flow through diode D2 and senses the reverse direction current flow through diode D1. Comparator 12 is connected to each main terminal T1 and T2 through respective third and fourth diodes D3 and D4 for sensing the current-induced voltage drop across diodes D1 and D2 and removing the drive signal when the sensed voltage exceeds a given threshold. The comparator is also connected to the control terminal 10 and includes switch means 14 for removing the drive signal. Switch 14 is actuated when the potential across either diode D1 or D2 exceeds a predetermined differential from a selectable reference voltage.

Diode D3 has its cathode connected in common with the cathode of diode D1 to main terminal T1. Diode D4 has its cathode in common with the cathode of diode D2 to main terminal T2. The anodes of diodes D3 and D4 are connected together at point 16 to comparator circuitry 12. Point 16 is thus referenced to the more negative of terminals T1 and T2, and comparator circuitry 12 senses the more negative of the main terminals. The voltage on each main terminal in each respective half cycle is sensed through the respective diode D3 or D4 to thus sense current flow through each inherent diode D1 and D2 by its voltage drop to its respective main terminal T1 or T2. The sensed current level is compared against a reference level 18 by comparator circuit 12, and a given threshold differential actuates switch 14 to remove the drive signal and turn off the FETs.

The current-induced voltage across diodes D1 and D2 is instantaneous with respect to the current. Normal delays associated with other means are thus avoided.

FIG. 2 shows an alternate embodiment, and like reference numerals are used where appropriate to facilitate clarity. Bipolar transistors Q1 and Q2 are connected in anti-series with the AC load circuit. Q1 and Q2 are preferably NPN transistors connected emitter to emitter. Some Darlington transistors have inherent reverse diode characteristics, and these may be used for diodes D1 and D2. Otherwise, diodes D1 and D2 are added to the circuitry. The remainder of the protective circuitry is comparable to FIG. 1 and will now be described in greater detail.

As noted above, current level through diodes D1 and D2 is sensed by the voltage drop thereacross to their respective main terminals T1 or T2. In preferred form, comparator circuitry 12 includes voltage sensing means 20, FIG. 3, sensing the voltage on each main terminal T1 and T2 through the respective diode D3 or D4. It is preferred that the voltage sensing means be referenced to a common point C between the series connected FETs such that the sensed voltage at each main terminal is referenced to common point C. The voltage sensing means thus senses the voltage drop from common point C across each diode D1 or D2 to its respective main terminal T1 or T2, for sensing current flow through each respective diode D1 or D2.

In one form voltage sensing means 20 is a differential amplifier having one input 22 connected to the common anode point 16 of diodes D3 and D4, and having another input 24 connected to a reference voltage V.sub.ref which may be tied or referenced to common point C. Amplifier 20 generates an output when the difference in voltage levels between inputs 22 and 24 exceeds a predetermined differential selectable according to V.sub.ref which corresponds to selectable current level 18 of FIG. 1. A selectable time delay may be provided by RC means such as resistor 23 and capacitor 25. Amplifier 20 is likewise referenced to common point C. The output of amplifier 20 actuates switch means 14 of FIG. 1 as described previously or alternatively actuates switch means 26 in FIG. 3 to close the latter and remove the drive signal from the transistors by diverting the drive signal through switch 26 to common point C, whereby to turn off the transistors. Various types of fast turn-off circuitry as known in the art, for example applying a negative bias, may alternatively or additionally be incorporated for switches 14 or 26.

FIG. 4 shows alternate voltage sensing means within comparator circuitry 12. An SCR 28 (silicon controlled rectifier) has its cathode connected to common anode point 16 of diodes D3 and D4, and its anode connected to control terminal 10. Voltage division circuitry comprising resistors 30 and 32 is connected and referenced to common point C and to the SCR cathode, and supplies trigger voltage to the gate of SCR 28. If the voltage at either main terminal T1 or T2 and thus at point 16 drops below a given level relative to point C, then the divided voltage at point 34 at the gate of SCR 28 is sufficient to trigger the SCR into conduction. When SCR 28 turns ON, it diverts gate drive from gate terminal 10 away from the FET gates and instead through SCR 28 to point 16 and the more negative of the main terminals, to thus turn off the FETs. Diode 38 is connected between the SCR cathode and common point C so that as the FETs turn off the conduction path for SCR 28 is maintained through diode 38. Thus the protective turn-off remains until the drive signal is removed, allowing the SCR to reset. Capacitor 36 may be provided to afford a programmed level-dependent delay in the triggering signal.

It is recognized that various modifications are possible within the scope of the appended claims.