This invention relates to power supply circuits, and more specifically relates to a novel cascaded buck converter circuit for producing a relatively constant, low output voltage from an input source voltage which may vary over a large range.
Buck converter circuits are well known, and generally comprise a switching transistor connected in series with an inductor, an output capacitor and a diode connected across the series-connected inductor and capacitor circuit, with its forward conduction direction such that it permits current to continue to flow through the inductor when the switching transistor is off. The input voltage may vary over some given range, for example, between 90 volts and 450 volts, and the output voltage is intended to be maintained at a constant voltage, for example, 15 volts. Such a circuit can be used as the power supply for any other electrical circuit or device. The output voltage is maintained constant by properly adjusting or varying the duty cycle of the switching transistor. This can be accomplished by a simple control circuit connected to the control electrode of the switching transistor so that the switching transistor is turned on for a longer or shorter period of time depending upon the sensed output voltage in order to maintain a constant output voltage.
A buck converter chip or power integrated circuit is sold by the International Rectifier Corporation of El Segundo, Calif., the assignee of the present invention, under their trademark "IR2100" and consists of a switching transistor structure and its control circuit integrated on the same chip. The structure of the power section of the chip is disclosed in copending application Ser. No. 07/054,627, filed May 27, 1987, in the name of Daniel M. Kinzer, entitled "HIGH POWER MOSFET AND INTEGRATED CONTROL CIRCUIT THEREFOR FOR HIGH-SIDE SWITCH APPLICATION" and assigned to the assignee of the present invention.
The IR2100 buck converter IC is connected in appropriate electrical circuit relation with an external diode, inductor and capacitor to define the complete buck converter circuit.
Buck converter circuits generate a given power loss during their operation where components of that loss include losses in the switching transistor, the inductor and the diode. Losses associated with the switching transistor are from two sources: the ohmic losses which occur during conduction, and switching losses which occur during commutation of the switching transistor between on and off conditions.
The power conduction loss of the switching transistor can be shown to be generally proportional to the maximum input voltage to be applied to the transistor and inversely proportional to the area of the power chip which is available for current conduction. The reason for these relationships is that the switching transistor must use a higher resistivity epitaxial layer in its construction to withstand higher input voltages and, therefore, will have a higher on-resistance. Thus, the power loss is generally proportional to input voltage. The area of the chip used for the switching transistor must be sufficiently large to handle the output current of the converter circuit. Consequently, higher output converter currents require a larger device area. The on-resistance of the transistor is reduced when area is increased, all other things remaining equal. Therefore, the power loss in the switching transistor is inversely proportional to its current carrying area.