Monolithically integratable control circuit for switching inductive loads comprising a Darlington-type final stage

A monolithically integratable control circuit for switching inductive loads, comprising a Darlington-type final stage, is described. The base of the Darlington control transistor is coupled to the collector of a transistor for extracting charge, the transistor conducting in phase opposition to the Darlington control transistor. The emitter of the transistor for extracting charge is coupled to the negative supply terminal and to the output terminal of the final stage, via a first and a second diode respectively.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to monolithically integratable control circuits for 
switching inductive loads and, more particularly, to control circuits of 
the kind comprising a Darlington-type final stage for use in actuating 
relays, solenoids and d.c. motors. 
2. Description of the Related Art 
Switching control circuits of the kind described herein usually comprise a 
final power transistor coupled in series with the inductive load between 
the two terminals of a supply voltage generataor and alternately switched, 
via a base control signal, from a high-voltage low-current state to a 
low-voltage high-current state. 
In the first state (the "off" state), the transistor is virtually an open 
circuit between the emitter and collector terminals. In the second or "on" 
state, a short-circuit is established. Thus, the two states respectively 
prevent or allow current to flow through the load. 
As is known, the manner of operation of a transistor closest to the 
operation of an ideal switch is that in which the transistor operates in a 
saturation condition when closed and is in a cut off condition when open. 
In the latter case, however, the maximum possible switching frequency of 
the transistor is mainly limited, during the change-over from a saturation 
condition to a cut-off condition by the effects of the storage of charge 
occurring during the conduction phase. The switching frequency limitation 
arises because the collector region of power transistors, which is dense 
and has high resistivity in order to withstand high inverse voltages, has 
a relatively long switching-off transient phase during which an increase 
in collector-emitter voltage does not correspond to a decrease in the 
collector current, the collector current remaining constant for a certain 
time. 
This phase, of course, is the phase in which the transistor dissipates most 
energy even though it is of no use in operation. When the load coupled to 
the final stage is inductive, the counter-EMF induced by the variation in 
the current through the load, due to switching of the final stage, 
abruptly increases the collector-emitter voltage of the still conducting 
transistor during the switching-off phase. The counter-EMF, in combination 
with the supply voltage, thus produces very high power dissipation in the 
transistor, sometimes with destructive effect. 
A reduction in the switching-off time, therefore, is advantageous both for 
increasing the maximum possible switching frequency and for improving the 
efficiency of the control circuit with regard to energy consumption, 
because of the reduction in time when the operation of the final power 
transistor departs from the operation of an ideal switch. 
The usual method for reducing the switching time of a power transistor 
operating in saturation during the conduction stage is to couple the base 
of the transistor to a circuit means having a low impedance, thus enabling 
the charge stored in the transistor to flow away rapidly when the 
saturated transistor is switched off. 
The circuit means can simply be a transistor which is actuated in phase 
opposition relative to the transistor to be switched off. The circuit 
means produces a current for extracting the charge from the base of the 
switched-off transistor. The phase-opposition transistor is inserted with 
its collector and emitter terminals between the load and the base of the 
power transistor to be switched off, or between the base and the supply 
voltage generator terminal to which the load is coupled. 
In the first case the efficiency with which charge is extracted is not very 
high because the collector-emitter voltage applied to the extraction 
transistor is limited. Even so, extraction continues until switching-off 
is complete. 
In the second case, however, the extraction transistor initially acts more 
efficiently since the collector-emitter voltage applied to it is higher. 
Extraction, however, is interrupted before the final transistor has been 
completely switched off, if the load is of the inductive type. The reason 
for interruption of extraction is that, during switching-off, a 
counter-EMF is induced in an inductive load that lowers the potential 
levels of the final transistor, coupled to the load, below the potential 
level of the negative supply terminal. Consequently, if the extracting 
transistor has its emitter coupled to the negative supply terminal, the 
extraction transistor is inversely polarized and stops extracting charges. 
On the contrary, a diode has to be inserted between the two transistors to 
prevent any feedback of current. 
In order therefore to obtain high switching rates in a circuit for 
controlling inductive loads, it is necessary to combine the two 
previously-mentioned systems, using two different extraction transistors, 
the emitter of one transistor being coupled to the negative supply 
terminal, whereas the other transistor is coupled to the output terminal, 
as described e.g. in Italian Patent Application No. 20213/A 82 by the 
present applicants. 
This method of extracting charge from the switched-off saturated transistor 
is very effective initially and continues until switching-off is complete. 
Clearly, however, this method involves a more complicated circuit, owing to 
the polarization and control means required, and more space is needed for 
integration of the circuit elements, thus increasing implementation 
expense. The same considerations also apply to control circuits which the 
final power transistor is kept in the active zone of its operating range, 
but is switched by a transistor which operates in saturation when 
conductive. In that case, to increase the rate of switching, the base of 
the last-mentioned transistor must be coupled to one or two extraction 
transistors as previously described, so as to accelerate the discharge 
process. 
This case applies, e.g., to the case of the control switching circuits to 
which the invention relates, such circuits comprising a Darlington circuit 
final stage made up of a final power transistor operating in the active 
zone and its control transistor operating in saturation, the two being 
interconnected at a common collector. NPN type transistors are normally 
used, owing to their switching characteristics. 
Switching control circuits of the aforementioned kind are used for special 
applications in which it is important to reduce energy consumption by the 
circuit when inoperative, since such consumption is the largest item in 
the total consumption during the various operating stages. A Darlington 
circuit final stage has low consumption when inoperative, less than that 
of other final stages, since its current gain is very high. 
Even though a Darlington circuit stage requires a minimum voltage for 
operation, equal to a base-emitter voltage plus a collector-emitter 
voltage at saturation, thus resulting in a greater loss of useful voltage, 
this voltage loss is an unimportant percentage of the supply voltages 
normally used for control circuits for switching inductive loads. 
Furthermore, a Darlington circuit final stage, particularly if made up of 
NPN transistors, has considerable advantages regarding integration and can 
be switched more quickly than a single final transistor of equal power 
operating in saturation. 
SUMMARY OF THE INVENTION 
It is an object of the invention is to construct a monolithically 
integratable control circuit for switching inductive loads comprising a 
Darlington circuit final stage, the circuit switching at higher speeds and 
being industrially cheaper than known circuits. 
The aforementioned and other objects are accomplished, according to the 
present invention by coupling a Darlington circuit to an inductive load 
impedance. An input terminal of the Darlington circuit is coupled to a 
switching signal and an collector of a control transistor. The base of the 
control transistor is coupled to the switching signal after the switching 
signal has been logically inverted. The emitter of the control transistor 
is coupled through a first diode to ground potential and through a second 
diode to the load impedance. A third diode is coupled across the load 
impedanmce to compensate for reverse potentials resulting from the 
inductive impedance. 
These and other features of the invention will be understood upon reading 
of the following description along with the drawings.

OPERATION OF THE PREFERRED EMBODIMENT 
Detailed Description of the FIGURE 
Referring to the FIGURE, a diagram of a control circuit according to the 
invention comprising a final Darlington-type stage circuit including a 
first bipolar transistor T.sub.1 and a second bipolar transistor T.sub.2, 
both of NPN type, T.sub.2 being the final power transistor and T.sub.1 
being its cotrol transistor is shown. 
The emitter of the final transistor T.sub.2, whose collector is coupled to 
the positive terminal +V.sub.cc of a supply voltage generator, constitutes 
the output terminal for the circuit, which is coupled to the inductive 
load to be switched off and on. The inductive load, represented by a 
resistance R.sub.L and an inductance L connected in series, is inserted 
between the output terminal and the negative terminal of the supply 
voltage generator, which can be the ground potential of the circuit. 
A feedback diode D.sub.E is coupled outside the circuit in parallel to 
R.sub.L and L. As is known, the feedback diode is necessary because of the 
inductive load which, after the final transistor has been switched off, 
must be supplied with the current required for the switching transient 
condition. 
The base of transistor T.sub.2 is coupled to the emitter of transistor 
T.sub.1, whose collector is also coupled to the supply voltage generator 
positive terminal +V.sub.cc. 
The base of transistor T.sub.1 is coupled, via a control circuit means 
represented by a block C in the drawing, to a source of switching signals 
represented by a block SW, transistor T.sub.1 and consequently transistor 
T.sub.2 being switched in response to the switching signals. 
The base of transistor T.sub.1 is coupled to the collector of a third NPN 
type bipolar transistor T.sub.s, whose base is also coupled to the control 
circuit means C, so that transistor T.sub.s is switched to the conductive 
state in phase opposition to transistor T.sub.1. 
In the drawing, a waveform is shown at the base of transistor T.sub.1 and 
T.sub.s represent the switching signals applied to the bases of the 
aforementioned transistors. 
The emitter of transistor T.sub.s is coupled to the anodes of a first and a 
second diode D.sub.1 and D.sub.2. The cathodes diodes D.sub.1 and D.sub.2 
are respectively coupled to the negative terminal -V.sub.cc of the supply 
voltage generator and to the emitter of the final power transistor 
T.sub.2. 
Operation of the Preferred Embodiment 
During the state when the transistors T.sub.1 and T.sub.2 of the final 
Darlington circuit stage are conductive, transistor T.sub.s is kept 
switched off by the control circuit means C. 
When, in response to a switching signal from source SW, the control circuit 
means C switches off transistor T.sub.1 and consequently transistor 
T.sub.2, it simultaneously switches on transistor T.sub.s, which 
immediately produces a current for extracting charges from the base of 
transistor T.sub.1, which is still saturated, thus accelerating its 
switching-off transient. 
As has been stated, it is easier to switch off a Darlington circuit final 
stage than a single final transistor of equal power, because the control 
transistor operating in saturation has very reduced dimensions in a 
Darlington circuit stage, thus limiting the accumulation of charges 
therein. 
Consequently transistor T.sub.s can be made smaller than the transistors 
used for discharging a single final power transistor operating in 
saturation. During the switching-off process, the current flowing from the 
base of transistor T.sub.1 via transistor T.sub.s is initially discharged 
to ground potential via diode D.sub.1. Owing however to the counter-EMF 
induced in the inductive load L during the switching process, the voltage 
level of the emitter of transistor T.sub.2 drops below the potential level 
of the negative terminal of the supply voltage generator, thus bringing 
the emitter and base potentials of T.sub.1 below the supply voltage level 
also. 
As a result, diode D.sub.1 is inversely polarized and no longer conductive. 
However, diode D.sub.2 is directly polarized so that the emitter current 
of transistor T.sub.s can flow through it to the negative terminal 
-V.sub.cc (via load L and R.sub.L) until transistor T.sub.1 has been 
completely switched off. 
The extraction current flowing through the load is not a disadvantage 
because the extraction current may only be a fraction of the current 
needed by the inductive load during its normal switching-off transient 
condition. The dimensions of the feedback diode D.sub.E can be 
advantageously reduced. 
It is therefore clear that a control circuit for switching, according to 
the invention, comprising a Darlington circuit final stage, can give a 
high switching rate and also greatly simplify the circuit and effectively 
reduce the total substrate integration area compared with known circuits. 
The control circuit can also be made up exclusively of NPN-type 
transistors, which is advantageous both technologically and with regard to 
the switching rates. 
A second, but no less important advantage of the circuit in the FIGURE, is 
it that transistor T.sub.s can also be supplied by a smaller voltage 
source than the voltage source for the final stage, with a resulting 
saving in supply energy. 
Although only one embodiment of the invention has been illustrated and 
described, numerous variations will be clear to those skilled in the art. 
For example, the described control circuits can form part of a more 
complex control circuit, e.g. a control circuit for inductive loads 
operating with a push-pull final stage. Also, the Darlington circuit final 
stage can be switched "in time", by the method and by the circuit means 
described in the patent application cited above, by the present inventors, 
according to which the charge-extracting circuit means are actuated only 
for a certain time after the switching-off of the final stage begins, to 
prevent delays in subsequent switching-on. 
The above description is included to illustrate the operation of the 
preferred embodiment and is not means to limit the scope of the invention. 
The scope of the invention is to be limited only by the following claims. 
From the above discussion, many variations will be apparent to one skilled 
in the art that would yet be encompassed by the spirit and scope of the 
invention.