CA1111517A - Inductive load driving amplifier - Google Patents

Abstract

ABSTRACT:
An inductive load driving amplifier is disclosed which includes two serially connected transistors, with Zener diodes limiting the collector-emitter voltage of the one and diverting the collector-base current of the other when the first one switches off. The driver is implemented as an integrated circuit. This collector isolation consti-tutes a high voltage Zener diode which contributes to limit the collector voltages. The combination of these features prevents the device from being exposed to second breakdown when switching highly inductive loads (e.g. several Henrys under SOV), thereby avoiding the harmful effects of break-downs during switching.

Description

: 15 FIELD OF THE INVENTION:
16 This invention relates to a transistorized power , .
17 amplifier and, more particularly, to an amplifier for 18 driving an inductive load which, more specifically, can 19 be a relay coil.
20 BACKGROUND OF THE INVENTION:
21 The transie~t phenomena which appear when turn-- 22 ing the power on or off for an inductive load such as a 23 relay coil, raise important problems when the driving 24 circuits are comprised of transistors. Indeed, the over-voltages caused by self-inductance effects and, more 26 specifically, those caused when turning the power off, 27 may entail the breakdown of the transistors included in 28 the circuit. It can easily be understood that these problems . 29 ' are even more critical when the circuit elements are to be implemented in integrated circuit technology. The purpose ' - ~
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1 of the subject invention, therefore, is to provide

2 structures which avoid these phenomena which cause

3 transistor brea};downs.

4 Many solutions have already been proposed to solve these problems, which are more or less complex 6 and unrelaible. In those solutions, one drawback 7 is that only comparatively low powér supplies (12 volts, 8 for instance) can be used.

9 OBJECTS OF T~ INVI~NTION:
Therefore, an object of this invention is 11 to provide for a relatively simple and reliable inductive 12 load driving circuit which can be advantageously embodied 13 in integrated circuit technology while making it possible 14 to feed inductive loads under comparatively high voltages.
SUMMARY:
16 These and other objects, features and advantages 17 of the invention are accomplished by the inductive 18 load driving amplifier invention which includes two 19 serially connected transistors, with Zener diodes limiting the collector-emitter voltage of the one 21 and diverting the collector-base current of the other 22 when the first one switches off. The driver is implemented 23 as an integrated circuit. The collector isolation 24 constitutes a high voltage Zener diode which contributes to limit the collector voltages. The combination 26 of these features prevents the device from being exposed 27 to second breakdown when switching highly inductive 28 loads (e.g. several llenrys at 50V).

- 1~1151'7 1 D~SCRl~TIO;~ OF 1`11~ FIGUR~S:
2 This invelltion can ~e further uncl~rstood 3 by way of a non-limitative example, with rcrerence 4 to the accompanying drawings, in which:
Figure 1 is a preferred embodiment of this 6 invention.
7 Figure 2 is the schematic diagram of the 8 integrated circuit corresponding to one portion of 9 the circuit shown in Figure 1.

DISCUSSIO~l OF THE PREFERRED EMBODII~NT:
11 As mentioned above, the transient phenomena 12 which appear when powering on and, more particularly, 13 when powering off an inductive load circuit, may cause 14 the breakdown of the circuit. In those situations, the elements which are most adversely affected are 16 those transistors which may be broken down owing to 17 overvoltages or to avalanche effects in the transistors, 18 i.e. the so-called secondary breakdowns. Indeed, 19 when electric current is accumulated in a limited area of the base of the transistor to be turned off, 21 this brings about a local temperature rise and the 22 thermal disturbance which creates the avalanclle effect 23 which may cause the breakdown of the base-emitter 24 junction of the transistor. These deteriorations will be avoided, here, by providing diversion paths 26 for diverting harmful electric charges and distributing 27 overvoltages over a plurality of transistors. To 28 this end, transistor characteristics will be employed, 29 wherein the hreakdown of the collector-~ase jUIlCtiOIl -l~llS~

1 of a transistor having its emitter unconnected, is 2 hic~ller than thc breakdown voltagc of tlTe collector-3 base junction of a transistor having its base unconnected.
4 The circuit of this invention is comprised of at least two transistors, which are series-connected 6 in the feeding path of the inductive charge, a means 7 for controlling the off operation of the transistor 8 the more remotely positioned from this load thereby 9 helping to turn off the following series-connected transistor in the circuit, and a means for completing 11 the turning off this last transistor through the creation 12 of a path for diverting its base current.
13 Figure 1 illustrates a preferred embodiment 14 of the circuit according to this invention. The inductive load, here is a relay coil K. It is fed through a 16 circuit comprised of two series-connected NPN transistors 17 TNl and TN2. The coil is mounted between positive 18 terminal V of the power supply and the collector 19 of TNl. The emitter of TNl is connected to the collector of TN2 the emitter of wllich is grounded. The base 21 of TN1 is fed by a current generator TP formed of 22 a PNP transistor, and a voltage divider formed of 23 two resistors Rl and R2 supplying a constant reference 24 voltage to the base of TP. ~ load resistor R3 mounted between V and the emitter of TP makes it possible 26 to limit the current to the base of TNl and to at 27 least one referenced power supply, e.g. a Zener diode 28 Z, the second electrode of which is grounded. The-29 control signal for powering on/off coil K, i.e. for FR9-75-~24 1151';' .

1 cor.trolling the conducting, non-conducting states 2 of TNl-TN2 is applied to the base of TN-2, either directly 3 or ~hroucJh amplifier (TN3, R4). Finally, two diodes 4 Dl and D2 are mounted, one between the emitter and S the base of TNl, the other one at the terminals of 6 coil K. They are not absolutely necessary, but, as 7 it-will be seen later, they are utilized to improve 8 the qualities of the circuit and make the operation 9 thereof more reliable.
At first, since the voltage applied to the 11 base of transistor TN3 is positive, the latter is 12 conducting, and the voltage at the base of TN2, then 13 is grounded; TN2 is non-conducting and coil K is not 14 fed. When causing TN3 to be non-conducting through a voltage drop at the base thereof, this very transistor 16 has its collector potential increased as well as the 17 base potential of TN2. TN2, then tends to be conducting 18 and, since the base of TNl is receiving the current 19 from generator TP, transistors TNl, TN2 are conducting and feed coil K. When the relay must not be fed any 21 more, TN3 is turned on anew upon application of a 22 positive voltage to input EN. The potential applied 23 to the base of TN2 is lowered and t~llds to block the 24 same. The potential at point A tends to go up to voltage Vz from which value the Zener diode Z becomes 26 conducting (in practice, a plurality of Zener diodes 27 may be series-connected in order to set Vz to an adequate 28 value). As the Zener diode becomes conducting, a 29 low impedance current sink is created which draws Ivr,~ , ~lllS17 1 the current from yenerator TP and from the base of 2 TNl while TN2 is opposing to the emitter current of 3 TN~ Otherwise, the current ~lowin~ from the collector 4 to the emitter of TNl would tend to be accumulated in a portion of the transistor base, thereby generatin~
6 the above-mentioned thermo-electronic avalanche effect 7 which would cause the breakdown of transistor TNl.
8 Therefore, it has been seen that series-9 connected plurality of transistors in the inductive load feeding circuit, in combination with the creation 11 of a low impedance reference voltage at the base of 12 the transistor(s) immediately adjacent to this load, 13 prevents the circuit from being broken down because 14 of eY~tra breakdown currents when the load stops being fed.
16 Other elements have been added to the circuit 17 shown in Figure 1 for improved operation. First, 18 a diode Dl limits the inverse voltage pulse in the 19 base-emitter junction of TNl which could be transmitted through a capacitive effect. This diode is useful 21 only when the circuit operating conditions, and more 22 specifically when the inverse current in the base 23 of TNl and the lateral resistance Or the same, are 24 such that the breakdown voltage in the base-emitter junction of this transistor may possibly be reached.
26 In another case, the use of diode D1 should 27 be avoided since it reduces the switching rate of the 28 circuit and increases the power dissipation of TNl.

T~ n ~1 _ 7 ~ __ n ~ A . r 1~1151'7 1 Second, a diode D2 has beell added between 2 the collector of TNl and therminal V o~ the power 3 sup~ly in order to prevent any overvoltage from appearing 4 on this collector. ~s a matter of fact, this overvoltage is already limited as an effective Zener diode Zl 6 is inherently present in the integrated circuit of 7 transistor TNl. Reference to Figure 2, it will facilitate 8 an understanding of the phenomenon causing the presence 9 of this diode Zl. This Figure is a conventional schematic illustration of an NPN transistor in a substrate SUBS
ll containing a plurality of elements isolated from one 12 another by areas ISO. Each of these elements is, 13 therefore, in an isolated pocket in the epitaxial 14 layer (it should be noted that the drawing shows also a buried N type subcollector layer highly doped (M ) 16 which is intended to reduce the resistance in the 17 collector of transistor TNl). Highly doped P type 18 isolation walls delimiting the isolation pocket of l9 TNl are formed through a diffusion process. Then, the P type slightly doped base area, the P type highly 21 doped emitter area and the contact collector area 22 are also diffused in a conventional way. The MP area 23 formed between the epitaxial layer and the subcollector, 24 on the one hand, and the substrate and the isolation wall ISO, on the other hand, act as the diode Zl.
26 Since this diode is back~.7ard biased, it operates as 27 a Zener diode.

- ~115~7 hile the invention has been particularly 2 sho~n and described with reference to the preferred 3 ~mbo~ ents thereof, it will be understood by those 4 skilled in the art that the foregoing and other changes in form and det~ils may be made therein without departing 6 from the spirit and scope of the invenLLon,

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A transistorized power amplifier intended to drive an inductive load, comprising:
a first and a second series-connected NPN type transistor in the feeding path of said load;
a current generator driving the base of said first transistor, means for driving the base of said second transistor so as to control the "on" and "off" turning operations of the series-mounted circuit intended to feed said load;
means for creating a low-impedance reference voltage source which short-circuits the base current of said first transistor when said second transistor is turned off;
a Zener diode between the collector of said first transistor and ground;
whereby an inductive load may be driven without suffering avalanche breakdown.