Overvoltage protected integrated circuit network, to control current flow through resistive or inductive loads

The main switching transistor 11 is serially connected between a load 12, 12' and a source of supply 13, R. An auxiliary transistor 15, the base of which is controlled through a voltage sensing device, for example a Zener diode 18 has its main switching path connected to the base of the main switching transistor 11 to control the main switching transistor 11 to become conductive in case of overvoltage, sensed by breakdown of the Zener diode 18. If the load is inductive, an additional inductive turn-off current bypass transistor 22, 22' can be provided (FIG. 2, 3), rendered conductive when overvoltage of an inductive kick is sensed, to bypass turn-off current around the main semiconductor switching transistor, or, in an alternative connection, to control the main switching transistor to again become conductive and itself bypass the inductive turn-off current, so that current flow due to overvoltages, or inductive turn-off current will be conducted by semiconductors operated under conditions of controlled conduction.

Cross reference to related applications assigned to the assignee of the 
present application: 
Ser. No. 823,286, filed Aug. 10, 1977, Seiler et. al. 
Ser. No. 823,285, filed Aug. 10, 1977, Seiler et. al. 
The present invention relates to a protective circuit arrangement or 
network for integrated circuits to protect the integrated circuits against 
overvoltages in which the integrated circuit is used as a control element 
or control switch to control current flow through a load. 
BACKGROUND AND PRIOR ART 
It has previously been proposed to control current flow through a load, 
particularly incandescent lamps, relay coils, motors, or the like by 
controlling conduction of a power transistor, or another power 
semiconductor element. If the load is essentially a low impedance load, 
having one terminal connected to a supply source which is subject to 
voltage surges, protecting the integrated circuit by an integrated Zener 
diode is difficult. 
Zener diodes as voltage protecting devices are well known; these diodes 
break down when the predetermined voltage is applied thereto, thus 
limiting overvoltage peaks. Such circuits cannot always be used in 
integrated circuit (IC) networks, or may be suitable therefore. If excess 
voltages occur during a period of time which is more than a sudden brief 
excess pulse, then integrated Zener diodes cannot be used since the 
thermal capacity of the housing for the IC chip must then be dimensioned 
to be capable of dissipating the power in the Zener diode upon breakdown. 
This power is substantially higher than the normal operating power, and 
hence the IC chip will heat excessively. Excessive heating of the chip 
will also damage, or destroy the power transistor, so that it will, 
thermally, break down. 
SUBJECT MATTER OF THE PRESENT INVENTION 
It is an object to provide a network and circuit to protect IC chips having 
a control transistor to control current flow through a load against 
overvoltages which can occur during operation, and in which the voltage 
limiting circuit can be integrated with the IC chip. 
Briefly, an auxiliary controlled semiconductor switch is provided having 
its switching path connected to the control terminal of the main control 
semiconductor switch. The auxiliary controlled semiconductor switch is 
connected to a voltage sensing network which senses overvoltages so that 
in case of an overvoltage, the auxiliary controlled semiconductor switch 
is rendered conductive controlling the main semiconductor switch to become 
conductive so that excessive voltages are bypassed through the main 
semiconductor switch, which is dimensioned and designed to be capable of 
carrying higher currents and dissipating a greater power level than a 
voltage sensing device, such as a Zener diode. 
The network has the advantage that by rendering the power transistor 
conductive, the semiconductor element is thermally essentially stable. A 
conductive transistor, for example, is thermally more stable than a 
transistor or semiconductor which is operated in blocked condition. 
Additionally, a conductive transistor or similar semiconductor has a much 
lower heat loss than a Zener diode, considering the current flow through 
both devices to be equal. A conductive transistor has a heat power loss 
which is almost an order of magnitude less than that of a Zener diode, 
substantially decreasing the thermal loading of the IC chip. 
If the load is an inductive load, for example a relay coil or the like, a 
free wheeling or anti-inductive kick circuit can be used which, in 
accordance with a feature of the invention, can be a transistor to conduct 
the inductive kick current. Inductive loads not only generate overvoltages 
but also turn-off currents. The free wheeling transistor can be so 
connected that the turn-off current is bypassed around the main switching 
control transistor. The free wheeling or bypassed transistor is controlled 
either directly from a terminal of the network or it can be controlled 
from the auxiliary transistor which additionally compensates for 
overvoltages being applied to the network.

An integrated circuit IC (FIG. 1) comprises a driver stage 10 and power 
stage formed by a main switching transistor 11. The driver stage 10 can be 
of any desired type and need not be described in detail. Power transistor 
11 is a npn transistor, the base of which is controlled by driver stage 
10. The emitter of the power transistor 11 is connected to ground, 
chassis, or a reference terminal. Its collector is connected to one 
terminal of a load 12, shown as a resistive load, for example an 
incandescent light bulb. The other terminal of the load 12 is connected to 
the protective terminal 13 of the supply source. Positive terminal 13 is 
connected over a current limiting resistor 14 to the IC including the 
driver stage 10 and the power transistor 11. The current limiting resistor 
14 usually is not integrated with the IC and need not necessarily be used. 
As well known, the various circuit components form diodes with respect to 
the substrate of the chip. These diodes are shown schematically in broken 
lines as diodes 110, 111, bridging the driver stage 10, and the power 
transistor 11, respectively. 
An auxiliary npn transistor 15 has its emitter is connected to the current 
limiting resistor 14 thence to terminal 3, and its collector through a 
collector resistor 16 to the base of the power transistor 11. The 
base-emitter path of transistor 15 is protected by a protective diode 17 
bridging the base-emitter junction. A voltage limiting device 18, in 
simplest form a Zener diode is connected between the base of the 
transistor 15 and reference potential. One or more Zener diodes may be 
used, and other devices may also be used. The elements 15 to 18 can be 
integrated on the IC, as shown schematically by being enclosed in the 
chain dotted box. 
Operation 
If overvoltage occurs at terminal 13, and the voltage rises above the 
threshold level of the voltage sensing device 18, voltage sensor 18 will 
limit the voltage at the junction point of the external resistor with the 
IC to the limiting voltage thereof. Since current through the voltage 
limiting circuit can flow entirely or partially through a base-emitter 
path of transistor 15, the transistor 15 will become conductive when the 
voltage limiting device responds, that is, when current will flow through 
the voltage limiting device 18. This causes transistor 15 to become 
conductive, controlling transistor 11 to likewise become conductive due to 
the connection of resistor 16 with the base of transistor 11. If the 
excess voltage agains drops below the limiting value of the voltage 
sensing device 18, transistor 15 will block which again causes blocking of 
transistor 11, unless it is commanded to be conductive by driver stage 10. 
Embodiment of FIG. 2: similar parts of functioning similarly, have been 
given the same reference numerals and will not be described again. The 
protective diode 17 has been replaced, however, by a coupling resistor 19. 
Two reverse polarity protective diodes 20, 21 are connected in the 
network, one parallel to the driver stage 10 and the other parallel to the 
switching path of the power transistor 11. They protect the IC against 
reverse polarity. 
The embodiment of FIG. 2 can advantageously be used with loads which are 
not only resistive, but additionally are inductive. For example, and as 
shown, load 12' is the solenoid, or coil of a relay. Upon interruption of 
current flow through an inductive load, a turn-off current will occur. To 
conduct a turn-off current, a free wheeling pnp transistor 22, serially 
connected with a resistor 23 is connected in parallel to the switching 
path of the main switching transistor 11. The base of free wheeling or 
anti-inductive kick transistor 22 is controlled by a Zener diode. The 
Zener diode is not strictly necessary, for it could be replaced by other 
suitable circuit elements, such as ordinary diodes, resistors, or the 
like. The base of transistor 22, in effect, is connected to the emitter of 
the auxiliary transistor 15. 
Operation 
If transistor 11 is controlled to cutoff, the emitter voltage of transistor 
22 will rise in positive direction. At instant of time when it has risen 
to such an extent that current will flow through the base-emitter junction 
of transistor 22, transistor 22 will become conductive and the 
disconnecting current from the inductive load 12' can be connected through 
the main switching path, that is, the emitter-collector path of transistor 
22 and resistor 23. The Zener diode 24, if used, compensates for voltage 
drop across resistor 14. It is preferably provided, and insures that 
transistor 22 will be conductive only at the instant when the voltage at 
its emitter has at least the voltage of the supply terminal 13. 
The collector of transistor 22 could, theoretically, be connected through 
resistor 23 not to reference or chassis or ground, but also to the base of 
transistor 11. In this case, the turn-off current would not flow through 
the transistor 22 but transistor 22 would cause transistor 11 to become 
conductive and the turn-off current would flow through transistor 11. This 
permits smaller dimensioning of transistor 22; one of the main current 
path terminals of transistor 22 are then connected to the junction between 
the load and transistor 11, and the other terminal is connected to one of 
the remaining terminals of transistor 11. 
Embodiment of FIG. 3: this circuit shows a connection in which an inductive 
load has one terminal connected to ground or reference, and the control 
switch therefore is in the positive supply line. A main switching 
transistor 11' is a pnp transistor, and the free wheeling transistor 15', 
as well as the bypass transistor 22' are of reverse conductivity type with 
respect to those previously described, and are npn transistors. The 
connection of Zener diode 18 and resistor 19 is inverted. The substrate 
diode 111 does not occur, or, rather, it is identical with the diode 110. 
Resistor 14 is connected to ground or reference potential. The base of the 
transistor 22' is connected to the base of the transistor 25 through two 
serially connected diodes 25, 26. 
The control terminal for the turn-off current of the inductive load, that 
is, the base of transistor 22' is directly connected to the voltage 
limiting circuit formed by Zener diode 18 and resistor 19, and connected 
to auxiliary transistor 15'. 
The connection of the control terminal of the transistor 22' to the base of 
transistor 15' or, rather, to the voltage limiting circuit has the 
advantage that the resistors 19 can have a dual function. It functions, as 
before, as the emitter resistor for transistor 15; additionally, it 
functions as a compensating resistor for the voltage drop across resistor 
14 when the transistor 22' has become conductive and conducts inductive 
turn-off current. The two diodes 25, 26 also are compensating diodes; use 
of one, particularly of more diodes in series results in overcompensation 
for the voltage drop across resistor 14. 
It is not a necessary feature that the voltage limiting circuit 18 is 
connected directly to the supply voltage; it could also be connected to 
separate loads, or other circuit elements in order to limit voltages 
occurring thereafter. The control of transistors 15 and 11, or 15', 11', 
respectively, will then be effected by overvoltages occurring at the 
respective loads. 
Various changes and modifications may be made and features described in 
connection with any one of the embodiments may be used with any of the 
others, within the scope of the inventive concept.