1. Field of the Invention
The present invention relates to a switching device equipped with a controller duty-controlling a controlled variable of a controlled system to a desired amount, and in particular, to the switching device that is able to perform such a control by making a switching element turn on/off with less noise due to the switching operations of the switching device.
2. Description of the Related Art
In general, on-vehicle power electrical equipment is equipped with a switching device that uses a power switching device. This kind of power switching device is on/off-operative on, for example, pulse width modulation (PWM). Hence, controlling the on/off operations of the power switching device allows controlled systems, such as DC-DC converter on a hybrid car, to have a desired amount of controlled variable.
Especially, when the controlled variable shows a desired target amount, the power switching element is then subjected to periodical on/off operations. Such operations are likely to raise the energy of only noise components whose frequencies correspond to switching frequencies and their harmonic frequencies, which are due to intervals between start timings of the on operations and/or the off operations. Such noise components of higher energy are occasionally superposed on a broadcasting signal tuned by an on-vehicle radio receiver or other similar radio devices. In such an undesired situation, the speaker of a car radio is obliged to output bloop for users.
To resolve this drawback, Japanese Patent Laid-open Publication No, 2002-335672 and 2003-88101 disclose a countermeasure for noise removal. That is, in those documents, a switching frequency is decided such that its harmonics are related to a broadcasting frequency band by a predetermined relationship. Practically, the predetermined relationship is that the harmonics have a specific frequency difference from a tuned broadcasting frequency. This is helpful for preventing the speaker from outputting the noise.
However, in such countermeasures, if modes of the on/off operations carried out by the power switching element are changed by, for instance, the PWM control, the possibility that a switching frequency or its harmonics overlaps with a tuned broadcasting frequency cannot be denied.