Pulse generator provided with a duty-factor limiting function

Abstract

In a pulse generator, a sawtooth-shaped wave generator circuit generates a sawtooth-shaped wave by charging and discharging a capacitor. The sawtooth-shaped wave is fed to a comparator that performs pulse-width modulation on it in accordance with the voltage it receives via a terminal and thereby produces pulses. The comparator has its output grounded through a transistor that is turned on with appropriate timing by the sawtooth-shaped wave generator circuit. Thus, the maximum duty factor of the output pulses is made equal to the duty factor of the sawtooth-shaped wave.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a pulse generator for generating pulses with which to control the switching operation of a DC / DC converter designed to step up a direct-current voltage.[0003]1. Description of the Prior Art[0004]A DC / DC converter designed to step up a direct-current (hereafter referred to as “DC”) voltage typically has an inductance coil and a switch connected in series between a DC voltage input terminal and ground. The node between the inductance coil and the switch is connected to a rectifier circuit. The rectified output voltage is fed to an output terminal. The switch is typically realized by the use of a switching transistor, and the output voltage varies according to the ratio of the length of the period in which the transistor Tr is conducting (on) and the length of the period in which the transistor Tr is cut off (off). The transistor Tr is turned on or off in synchronism with pulses (duty pulse...

AI Technical Summary

Benefits of technology

[0014]An object of the present invention is, in a pulse generator provided with a function of limiting the duty factor of the output pulses, to minimize the variation of the maximum duty factor due to temperature variation and improper adjustment, and to reduce the circuit area and the cost thereof.

[0015]Another object of the present invention is, in a DC / DC converter, to further stabilize the output voltage thereof, to reduce the circuit area and the cost thereof, and to simplify the initial adjustments that need to be made when it is incorporated into an appliance.

[0017]For example, by keeping the output voltage constant irrespective of the input voltage while the voltage of the above-mentioned sawtooth-shaped wave is rising or falling, it is possible to keep the maximum duty factor of the output pulses equal to the duty factor of the sawtooth-shaped wave.

[0018]This circuit configuration eliminates the need to prepare a DC voltage to be used as a duty-factor limiting voltage, and makes it possible to set the maximum duty factor at a desired value irrespective of the DC level or the amplitude of the sawtooth-shaped wave.

Problems solved by technology

Therefore, in a pulse generator circuit, it is not allowed to set the duty factor above a certain limit.

This inconveniently leads to a large variation of the maximum duty factor due to temperature variation and improper adjustment.

Moreover, a terminal for receiving the duty-factor limiting voltage needs to be provided separately, and the comparator requires a complicate circuit configuration to achieve comparison between the output voltage of an error amplifier and the voltage of the sawtooth-shaped wave.

This inconveniently leads to a larger circuit area, and to a higher cost.

For example, in a pulse generator circuit of a certain type, starting synchronization with a clock while keeping the duty-factor limiting voltage at the same level as when the circuit is operating independently (i.e. when synchronization with a clock is not achieved) may make the maximum duty factor of the high level period unduly low.

Therefore, no appliance-to-appliance variation is permitted in the DC level and the amplitude of the sawtooth-shaped wave, and thus it is necessary to use a high-precision voltage generator circuit or the like to generate the sawtooth-shaped wave.

This inconveniently requires an even larger circuit area and an even higher cost.

Moreover, since a pulse generator circuit that is used to control the output voltage of a DC / DC converter on the basis of feedback suffers from problems as described above, the DC / DC converter as a whole suffers from instability of its output voltage, from an unduly large circuit area and an unduly high cost, and from the difficulty of the initial adjustments that need to be made when it is incorporated into an appliance.

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