Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2000-04-07
2001-09-18
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S071000, C323S272000
Reexamination Certificate
active
06292378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This present invention relates generally to a power conversion circuit and more particularly to a multiphase switching regulator.
2. Description of the Related Art
A power conversion circuit (e.g., switching regulator) accepts a Direct Current (DC) voltage source at one level and outputs a desired DC voltage at another level. The switching regulator includes one or more switches which can be implemented by Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFETs). The switches alternate between connecting and disconnecting the voltage source to circuits that drive the output. The duty cycle of the switching determines the output voltage. The switching is typically controlled by a Pulse-Width Modulation (PWM) circuit.
Switching regulators are useful in high current applications, such as high power microprocessors, Pentium II and Pentium III based applications, notebook computers, desktop computers, network servers, large memory arrays, workstations and DC high-power distribution systems, which typically require 15 to 200 amperes. The switching regulator can have multiple parallel channels to process one or more of the voltage sources to drive a common output. Each channel is substantially identical and includes an inductor. The input terminal of the inductor is switched between the voltage source and ground.
In a multiphase switching regulator, an exemplary PWM circuit provides a variable duty cycle signal to control the switching for each channel. The PWM signals are synchronous with different phases for each channel, thereby allowing each channel to be switched on at a different time. The multiple phases increase the output ripple frequency above the fundamental channel switching frequency and reduce the input ripple current, thereby significantly reducing input and output capacitors which are large and expensive. Stress and heat on the components are also reduced because the output current is spread among the multiple channels.
The DC current through each inductor is proportional to the duty cycle of its PWM signal and the value of its voltage source. Each inductor has a current limit. Typically, more PWM circuits are used when more output current is desired. The output terminals of all the inductors are electrically connected to provide a single output of the power conversion circuit.
The output terminals of all the inductors are tied together and therefore have identical voltages. The input terminal of each inductor has a rectangular wave voltage signal, which is derived from the voltage source and ground. The duty cycles of the rectangular wave voltage signals of respective channels are affected by variations in the respective PWM circuits and switches (e.g., design tolerances, offsets, and timing variations). A slight difference in the duty cycle can produce a significant difference in the DC current through the inductor in each channel.
High efficiency power conversion circuits typically use inductors with low core loss (e.g., ferrite inductors). When the peak design current is exceeded (i.e., saturation), the inductance of ferrite core material collapses abruptly which results in an abrupt increase in inductor ripple current and output voltage ripple. Thus, it is important to keep the inductor core from saturating.
Forced current sharing is a concept that all channels contribute substantially identical currents to the output. Forced current sharing prevents an inductor in one of the channels from saturating. Prior art systems sense the current in each channel and adjust the respective duty cycles to produce the same current for each channel. Current sensing decreases the efficiency of the power conversion circuit because power is dissipated by a sensing resistor. Further, current sensing requires an undesirable ripple voltage across the sensing resistor in order to work properly. Alternatively, other prior art systems employed costly precision design and trimming in an attempt to achieve accurate current sharing without sensing resistors. Typically, phase current mismatches are on the order of 30 percent or greater when employing precision duty cycle matched converters, necessitating the use of significantly higher current MOSFET's and inductors.
SUMMARY OF THE INVENTION
The present invention solves these and other problems by providing a power efficient and reduced cost multiphase switching regulator wherein sensed voltages are provided to accurately control the output currents of respective channels. The sensed voltages are derived from respective voltage waveform is applied to inputs of respective inductors in respective channels. A respective PWM circuit controls a switch coupled to the input of each inductor. The PWM circuit alternately causes the switch to connect the input of the inductor to a voltage source and ground. As a result, the voltage waveform at the input of each inductor is a rectangular wave voltage with an amplitude approximately equal to the voltage source and a duty cycle controlled by the PWM circuit. The sensed voltage is proportional to an average value of the voltage waveform at the input of the inductor and can be derived by low-pass filtering the input of the inductor. The sensed voltage is a DC value of the voltage waveform at the input of the inductor.
In one embodiment, the sensed voltages are used to achieve forced current sharing. The output currents of respective channels are adjusted to be substantially identical by adjusting the PWM circuits of respective channels accordingly to achieve substantially identical sensed voltages in all the channels.
In one embodiment, the same voltage source is supplied to each channel of the multiphase switching voltage regulator. The sensed voltage is proportional to the duty cycle of the voltage waveform at the input of the inductor, which is the same as the duty cycle of the PWM signal being applied to the switch. Identical sensed voltages indicate that the duty cycles of the voltage waveforms at the inputs of respective inductors are substantially identical. Identical duty cycles applied to identical inductors result in identical output currents.
In an alternate embodiment, two or more voltage sources are supplied to the multiphase switching voltage regulator to drive a common output. For example, a +5VDC voltage and a +12VDC voltage can supply current to a common load. The different voltage sources are processed by different channels of the multiphase switching voltage regulator. Each voltage source is coupled to a different inductor input. The outputs of the inductors are electrically connected together to provide the common output.
Identical sensed voltages achieve forced current sharing between two or more voltage sources. In the case of two or more voltage sources, identical sensed voltages do not necessarily indicate identical duty cycles for the voltage waveforms at the inputs of respective inductors. The sensed voltage is also proportional to the value of the voltage source. For example, the duty cycle for the channel with the +12VDC voltage source is less than the duty cycle for the channel with the +5VDC voltage source when the respective sensed voltages are substantially identical. The sensed voltages represent the average voltages at the inputs of the respective inductors. Identical inductors with substantially identical average voltages result in identical output currents.
The multiphase switching regulator establishes forced current sharing by comparing the sensed voltages to a master voltage. The sensed voltage of one channel is used as the master voltage for the other channels. Offset voltages are produced based on the differences between the respective sensed voltages and the master voltage. The respective offset voltage is added to the output of a feedback amplifier to generate a control voltage which is used to adjust the duty cycle of the PWM signal being applied to the respective switch couple the input of each inductor. The additions of the offset voltages force the sensed voltages of respectiv
Brooks Steven W.
Maggiolino Louis Joseph
Han Jessica
Knobbe Martens Olson & Bear LLP
Linfinity Microelectronics
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