Circuit arrangement for controlling an inductive load

Details Circuit arrangement for controlling an inductive load Circuit arrangement for controlling an inductive load

2000-05-02

2002-02-26

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Current driver

C327S531000, C363S021100, C363S021120

Reexamination Certificate

active

06351162

ABSTRACT:

TECHNICAL FIELD
The invention relates to a circuit arrangement for controlling an inductive load.
BACKGROUND OF THE INVENTION
A specific field of application of the invention is the control of an inductive load in the form of an electromagnetic driver device for a brake valve of an anti-locking brake system (ABS). The disclosed embodiments of the invention are concerned specifically with the closed-loop control of an inductive load with the aid of a PWM adjusting or control signal, i.e., a control signal in the form of a pulse train with variable pulse duty factor or duty cycle (DC=ratio of pulse duration to pulse interval). The PWM control signal (PWM=Pulse Width Modulation) opens and blocks an electronic current switching element provided in the circuit of the inductive load.
The inductive load to be controlled, which is located in a first circuit branch, usually has a second circuit branch connected in parallel thereto which contains a flyback diode. The electronic current switching element is provided in a third circuit branch in series connection “upstream” or “downstream” of the first and second terminals of the first and second circuit branches. When the electronic current switching element opens and closes in alternating manner, current is flowing through the inductive load when the current switching element is opened, whereas with closed current switching element there is a reverse current flow through the flyback diode due to the energy stored in the inductive load.
If it is desired to control, for example, a fluid pressure by means of the inductive load (for example an electromagnet), this is effected by controlling the current flow through the inductive load. To this end, the current flow through the inductive load is measured in order to obtain an actual current signal, and this actual current signal is compared to a target or desired current signal corresponding to the current desired in order to provide a PWM control signal for controlling the current switching element on the basis of the comparison result.
The determination of the current flow through the inductive load usually takes place with the aid of a measuring resistor connected directly in series with the inductive load or arranged in series with the electronic current switching element. The measuring resistor delivers a measurement voltage. This measurement voltage is compared to a reference voltage corresponding to the desired current signal. The reference voltage is delivered by a reference resistor connected in series with a variable current source. By means of the variable current source, it is possible to provide the desired current signal at the reference resistor, with the reference voltage then corresponding to the desired current intensity through the inductive load.
Circuit arrangements of the type concerned here are usually in the form of integrated circuits. The inductive load to be controlled is connected to a battery voltage terminal and an output voltage terminal. Apart from the inductive load to be controlled proper, all or almost all circuit elements are provided within the integrated circuit.
In such a circuit arrangement, there arises considerable power dissipation, depending on the duty cycle (DC) of the PWM control signal. In case of high duty cycle and correspondingly strong current flow, there is high power dissipation created, which needs to be dissipated with the aid of a correspondingly large chip area. As is known, the power dissipation is proportional to the resistance and moreover proportional to the square of the current. However, current control is indispensable due to the high temperature dependency of the ohmic resistance component of the inductive load. This holds for most, if not all, applications of the type concerned here.
The dependency of the power dissipation in the measuring resistor on the duty cycle of the PWM setting signal corresponds to a monotonously increasing curve. Usable in practical application is the range between 10% and 90%. In order to obtain relatively high accuracy in particular in the lower measuring range, it is necessary to use a high resistance value for the measuring resistor. The measurement accuracy in the lower range is of particular significance in most applications. With an about linear increase of the power dissipation as a function of the duty cycle, however, the measurement accuracy is relatively low in the lower measuring range, whereas considerable power dissipation arises in the upper measuring range.
SUMMARY OF THE INVENTION
The disclosed embodiments of the present invention make available a circuit arrangement for controlling an inductive load, which, with a preset value of the measuring resistor, has lower power dissipation in the measuring resistor in comparison with that of the prior art, and which, with preset power dissipation, can make do with comparatively little chip area. With preset maximum power dissipation, an increase in the value of the measurement resistor is possible in order to thus obtain higher measurement accuracy in the lower measuring range.
To this end, the embodiments of the invention provide a circuit arrangement for controlling an inductive load, including:
a) a first circuit branch located between a first terminal and a second terminal and including the inductive load to be controlled;
b) a second circuit branch located between the first terminal and the second terminal and including a flyback diode;
c) a third circuit branch located between the first and second circuit branches on the one hand and a third terminal on the other hand and including an electronic current switching element having a control terminal to which a PWM control signal is supplied;
d) a current sensor for sensing a current dependent on the current flowing through the inductive load, and for generating an actual current signal; and
e) a closed-loop control circuit receiving as first input signal the actual current signal and as second input signal a desired current signal in order to form thereof the PWM control signal;
f) the current sensor being arranged in the second circuit branch.
The current sensor preferably is an ohmic measuring resistor connected in series with the flyback diode in the second circuit branch. The desired current signal preferably is formed in a fourth circuit branch connected to the second circuit branch and containing a reference resistor and in particular a variable current source. The current determined by the current source generates at the reference resistor a reference voltage representing the desired current signal. This desired current signal is compared to the actual current signal in the form of the measurement voltage created at the measuring resistor, in order to obtain a deviation signal that in a preferred embodiment of the invention is created as a sign signal. This sign signal then is processed further in order to obtain the PWM control signal.
With an ohmic measuring resistor, the measurement voltage changes proportionally with the current intensity. The measurement results obtained are relatively exact.
In one specific embodiment of the invention, the current sensor is constituted by a component having non-linear characteristics, for example diode. In accordance with the diode characteristics, the reference element in the fourth circuit branch then is an element having characteristics corresponding to the current sensor, and thus in particular a reference diode in the instant case. The measuring diode and the reference diode are provided with substantially identical characteristics. Due to the fact that identical characteristics are not easy to obtain, this embodiment is suitable in particular for such applications in which a relatively high tolerance is permissible. As a measuring diode, the flyback diode proper can be employed as well.
Before describing specific embodiments of the invention, the differences between the invention on the one hand and the prior art on the other hand are to be elucidated, by way of
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