Device for guarding mispolarization of inductive loads

A device for switching inductive loads includes a series circuit which is connected between positive and negative poles of voltage source and has a load and a switch associated with the load. A free-running diode is connected parallel to the load. A guard circuit for guarding against mispolarization of the voltage source has an electronic switch connected in series with the free-running diode. The electronic switch is made conducting through a charge pump triggered by an oscillator, given correct polarization of the voltage source.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The invention relates to a device for switching inductive loads, in 
particular in a motor vehicle, including a series circuit which is located 
between a positive pole and a negative pole of a voltage source and has a 
load and a switch associated with the load, a free-running diode connected 
parallel to the load, and a guard circuit guarding against mispolarization 
of the voltage source. 
A motor vehicle has inductive loads, such as magnet valves, with a current 
flow which must be controllable linearly (current flow.fwdarw.valve 
force.fwdarw.hydraulic pressure). As a rule, that is done by clocked 
switching of a load (such as a load L1 in FIG. 1) with a variable pulse 
width. Typically, integrated power semiconductors with MOSFET end stages 
(such as a switch Q1 in FIG. 1), which have a substrate diode that carries 
current from a source terminal to a drain terminal, are used for 
switching. A free-running diode (such as a diode D1 in FIG. 1) is 
connected parallel to the inductive load. As a rule, such a configuration 
is part of an engine or transmission control system (such as a controller 
ST outlined in dashed lines in FIG. 1), except for the inductive load 
itself. 
One general problem in motor vehicle electronics is the possibility of 
mispolarization of the voltage source (such as a voltage source Bat shown 
in FIG. 1). In that case, the free-running diode and the substrate diode 
would be connected in the current flow direction to the voltage source, 
and the resultant current would destroy the free-running diode and the 
substrate diode. Current flow must therefore be prevented by suitable 
provisions if the voltage source is mispolarized, but the free-running 
current for the load must be preserved, and no components that reduce the 
voltage to be applied to the load can be allowed in the load current 
circuit. 
For that reason, it is attractive to use a simple mispolarization guard 
diode, which is known per se, or another electronic switch, known from 
European Patent Application 0 436 778 A2, that becomes nonconductive if 
the voltage source becomes mispolarized, in the load current circuit. 
A version including a main relay having an exciter coil, with a 
mispolarization guard diode, which is in series with the voltage source 
and can be excited through the vehicle ignition switch, is generally 
known. The disadvantages of that version are high cost, large structural 
volume, limited reliability, and the finite life of the contacts of the 
main relay. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a device for 
switching inductive loads, especially in a motor vehicle, which overcomes 
the hereinafore-mentioned disadvantages of the heretofore-known devices of 
this general type, which has a guard circuit to protect against 
mispolarization of a voltage source, which neither hinders a free-running 
circuit of the load nor reduces a voltage to be applied to the load, and 
which overcomes the disadvantages of a main relay. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a device for switching inductive loads, in 
particular in a motor vehicle, comprising a voltage source having a 
positive pole, a negative pole and a polarization; a series circuit 
connected between the positive and negative poles of the voltage source, 
the series circuit including a load and a switch associated with the load; 
a free-running diode connected parallel to the load; and a guard circuit 
guarding against mispolarization of the voltage source, the guard circuit 
having an oscillator, a charge pump, and an electronic switch connected in 
a series circuit with the free-running diode, the electronic switch made 
conducting through the charge pump triggered by the oscillator, if the 
polarization of the voltage source is correct. 
In accordance with another feature of the invention, the electronic switch 
is an inverse-driven n-channel MOSFET having a source terminal connected 
to the positive pole of the voltage source. 
In accordance with a further feature of the invention, the electronic 
switch has a gate terminal, and the charge pump is connected between the 
source and gate terminals of the electronic switch. 
In accordance with an added feature of the invention, there is provided a 
voltage supply of the oscillator effected through a load switch from the 
voltage source by a mispolarization-protected, integrated voltage 
regulator. 
In accordance with a concomitant feature of the invention, there is 
provided a pulse-proof limiter diode connected parallel to the electronic 
switch; a connecting line connected from the electronic switch to the 
free-running diode; and a further pulse-proof limiter diode connected 
between the connecting line and the negative pole of the voltage source; 
the free-running diode and the limiter diodes having cathodes connected to 
one another. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
device for switching inductive loads, it is nevertheless not intended to 
be limited to the details shown, since various modifications and 
structural changes may be made therein without departing from the spirit 
of the invention and within the scope and range of equivalents of the 
claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the figures of the drawings in detail and first, 
particularly, to FIG. 1 thereof, there is seen a device which includes a 
series circuit of an inductive load L1 and an electronic switch Q1 for 
triggering the load, through a further non-illustrated circuit that is 
connected to poles "+" and "-" of a voltage source Bat (a motor vehicle 
battery). The load current circuit is shown in heavy lines. A free-running 
diode D1 is connected parallel to the load L1. 
According to the invention, a guard circuit S which is shown as a box and 
is used to protect against mispolarization of the voltage source, is 
inserted in a series circuit with the free-running diode D1. In this 
exemplary embodiment, the guard circuit S is inserted between the 
free-running diode D1 and a junction A, which is disposed between the 
positive pole +and the load L1. The circuit diagram of this guard circuit 
is shown in FIG. 2. 
The guard circuit S has an inverse-driven N-channel MOSFET (source terminal 
at positive potential in normal, non-mispolarized operation) acting as an 
electronic switch Q2. The electronic switch Q2 is connected along a 
connecting line in a series circuit with the free-running diode D1, 
between the input 1 connected to the junction A and the output 2 connected 
to the cathode of the free-running diode D1. This inverse circuit has no 
significance to the switching behavior of the switch Q2. Terminal 3 is 
connected to the negative pole of the voltage source Bat. 
The switch Q2 could intrinsically be constructed as a p-channel MOSFET 
instead, but this would involve substantially higher costs and a complex 
trigger circuit, for a comparable channel resistance. 
If the voltage source Bat is correctly polarized, a charge pump P generates 
a positive gate-to-source voltage, which makes the switch Q2 conducting, 
and as a result the free-running current can flow unhindered. 
In the event of mispolarization of the voltage source Bat, no positive 
gate-to-source voltage is generated, and the switch Q2 is nonconducting, 
so that the mispolarization protection is effective. 
Since the circuit of a charge pump and an oscillator is known per se, this 
need not be described in further detail herein and a basic description 
will suffice. 
In this exemplary embodiment, the charge pump P has three stages, including 
capacitors C1, C2, C3 and diodes D3a, D3b and D4a, D4b. Triggering the 
charge pump P is effected through a Schmitt trigger oscillator Osz, which 
is made up of an inverter U1A, a capacitor C5 and a resistor R4, at a 
frequency of 100 kHz, for instance. Downstream inverters U1B, U1C and U1D 
are used for push-pull generation and for decoupling. 
The voltage supply of the oscillator Osz is accomplished from the voltage 
source Bat through a non-illustrated ignition switch by using a 
mispolarization-protected, integrated voltage regulator (of 5 V, for 
instance), that is present in an engine controller ST. 
A Zener diode D6 which carries current toward the gate terminal, a resistor 
R3 and a capacitor C4 are connected between source and gate terminals of 
the switch Q2: 
in the event of a malfunction, the Zener diode D6 limits the gate voltage 
to a value allowable for the switch Q2; 
the three-stage charge pump generates an idling voltage of approximately 15 
V; with the loading by the resistor R3, the result is a typical value of 
12 V, which is entirely sufficient to switch the switch Q2 completely; 
the capacitor C4 acts as a storage element, and its capacitance should be 
selected to be high as compared to the capacitors C1-C3, so that the 
alternating voltage components of the charge pump are sufficiently 
filtered out; furthermore, if the voltage source is mispolarized, the 
gate-to-source voltage at the switch Q2 that occurs through the capacitive 
voltage divider C4/C1+C2+C3 remains reliably below the making threshold of 
the switch; 
the resistor R3 discharges the capacitor C4 after the engine controller is 
turned off, and as a result the switch Q2 is reliably made nonconducting, 
until the next time the engine is started (or the next time the voltage 
source Bat possibly becomes mispolarized). 
Outputs of the inverters U1C and U1D are each connected to a respective 
resistor R1, R2, which limits the charge currents of the capacitors C1, C2 
and C3 and the output currents of the inverters U1C and U1D to tolerable 
values, in the event of rapid interference pulses. 
The switch Q2, which is constructed as an n-channel MOSFET, also has a 
substrate diode D3, which enters the blocking state if the voltage source 
Bat becomes mispolarized. Since the supply voltage for the oscillator Osz 
is also lacking if the voltage source Bat is mispolarized, the charge pump 
is not activated. No positive gate-to-source voltage builds up, and the 
switch Q2 does not become conducting. 
A pulse-proof limiter substance diode D2 is connected parallel to the 
substrate diode of the switch Q2 and a pulse-proof limiter diode D5 is 
connected from the negative pole of the (correctly polarized) voltage 
source Bat to the drain terminal of the switch Q2. The limiter diodes D2 
and D5 serve to limit the voltage of positive and negative interference 
pulses. 
One of these diodes is operated in the flow direction and the other in the 
Zener direction. A screen capacitor C6 is connected between the drain 
terminal of the switch Q2 and the negative pole of the (correctly 
polarized) voltage source Bat, through terminal 3, or in other words 
parallel to the limiter diode D5. 
The guard circuit described herein is inexpensive, reliable and not subject 
to any wear. A plurality of free-running circuits can be protected 
simultaneously and jointly against mispolarization with one guard circuit. 
In a preferred exemplary embodiment of the invention, components 
dimensioned as follows are used: 
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C1, C2, C3 = 2.2 nF 
D2 = RD16EB 
C4 = 100 nF D3a, D3b = BAV99 
C5 = 1 nF D4a, D4b = BAV99 
C6 = 3.3 .mu.F D5 = RD33EB 
R1, R2 = 1 k.OMEGA. 
D6 = RD18EB 
R3 = 1M.OMEGA. U1A-U1D = 74HC14 
R4 = 10 k.OMEGA. Q2 = BUZ70/SIE 
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