Motor driving circuit with filter condensers

Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components

Reexamination Certificate

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Details

C363S144000

Reexamination Certificate

active

06219260

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a motor driving circuit having a PWM driving circuit for changing an on-off ratio of a pulse signal so as to control driving currents flowing in plural phases of driving coils. Particularly, the present invention relates to a motor driving circuit which can prevent the influence of high frequency noise generated in a PWM driving circuit.
There is known a motor driving circuit having a PWM driving circuit for changing an on-off ratio of a pulse signal so as to control driving currents applied to plural phases of driving coils. In such a motor driving circuit, the frequency of the pulse signal in the PWM driving circuit is usually set to be a high frequency of dozens kHz or more. In addition, the changes of currents in the PWM driving circuit, that is, the leading and trailing edges of currents are made sharp. Thus, high frequency noise is generated from the PWM driving circuit.
When this high frequency noise gets into a circuit section other than the motor driving circuit, the driving currents flowing into the driving coils fluctuate, thereby causing a malfunction of the motor. In the related art, any of the following measures is taken to avoid such a harmful influence.
First, the PWM driving circuit is shielded to prevent high frequency noise from radiation. Second, the changes of the currents are made gentle enough to restrain high frequency components from being generated, so that high frequency noise generated in the circuit is itself lowered. Third, a snubber circuit (for example, filter condensers) for absorbing noise is added into a circuit in which a current having a sudden change flows, so that high frequency components are absorbed by this snubber circuit.
However, any of the measures has its own problem. First, in the method in which the PWM driving circuit is shielded, it costs much to provide a shield. In addition, it is impossible to give a perfect shield to the PWM driving circuit.
Second, in the method to make the changes of the currents gentle, the loss in driving transistors connected to the driving coils increases so that the efficiency deteriorates. In addition, since the loss increases, large-capacity ones must be adopted as those transistors. Such transistors are so expensive as to increase the cost of the driving circuit.
Third, in the case where a snubber circuit is provided for absorbing noise, it is necessary to add new electric/electronic elements to the driving circuit, so that the cost of the driving circuit is likewise increased. In addition, it is necessary to give careful consideration onto the characteristics of elements to be used in the snubber circuit, in order to be able to absorb high frequency noise satisfactorily.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a motor driving circuit having a novel configuration which can prevent the aforementioned problems and which can restrain the influence of high frequency noise generated in a PWM driving circuit.
In order to achieve the above object, according to the present invention, there is provided a motor driving circuit comprising:
a base board having electric conductivity;
an insulating film formed on the base board;
a PWM driving circuit formed on the insulating film for controlling driving currents flowing into plural phases of driving coils of the motor;
conductive patterns formed on the insulating film and including paths in which the driving currents flow; and
connector for connecting the base board and a low impedance portion of the conductive patterns such that filter condensers are formed between the base board and the paths.
Preferably, the capacitances of the respective filter condensers are determined by adjusting width of a part of the conductive patterns.
Preferably, the capacitances of the respective filter condensers are determined by adjusting at least one of thickness and material of the insulating film.
Preferably, the low impedance portion is either a portion having a ground potential or a portion having a power supply potential of the motor driving circuit.
Preferably, the connector is a fastening member for short-circuiting the low impedance portion and the base board.
Preferably, one of the ends of the respective film condensers are individually connected to the plural phases of the driving coils, and the other ends thereof are connected to the base board in common.
According to the present invention, there is also provided a method of preventing high frequency noise generated from a PWM driving circuit of a motor driving circuit, comprising the steps of:
preparing a base board having electric conductivity;
forming an insulating film on the base board;
forming the PWM driving circuit on the insulating film for controlling driving currents flowing into plural phases of driving coils of the motor;
forming conductive patterns including paths in which the driving currents flow on the insulating film;
selecting a portion having low impedance out of the conductive patterns; and
connecting the low impedance portion and the base board to form filter condensers between the base board and the paths.
Preferably, the capacitances of the respective filter condensers are determined by adjusting width of a part of the conductive patterns.
Preferably, the capacitances of the respective filter condensers are determined by adjusting at least one of thickness and material of the insulating film.
Preferably, the low impedance portion and the base board are connected by a fastening member.
According to the present invention, it is not necessary to provide a special shield to a PWM driving circuit in order to restrain the influence of high frequency noise. It is therefore possible to get rid of the cost of the shield. In addition, in comparison with the case where a shield is given to the PWM driving circuit, it is possible to restrain the influence of high frequency noise.
In addition, it is not necessary to make the change of currents gentle. It is therefore possible to reduce the loss in driving transistors. In accordance therewith, small-capacitance and low-cost ones can be used as those transistors.
Furthermore, according to the present invention, the characteristics of condensers can be set desirably by adjusting the film thickness of an insulating film, the shapes of wiring patterns, or the like. It is therefore possible to form filter condensers having characteristics suitable for an adopted PWM driving circuit.


REFERENCES:
patent: 5602451 (1997-02-01), Kohge et al.
patent: 5625265 (1997-04-01), Vlahu
patent: 2749514 (1998-02-01), None

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