Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-07-02
2003-05-20
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S098000, C363S132000, C323S282000
Reexamination Certificate
active
06567279
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to switch-mode regulators in general and, more particularly, to low output voltage switching regulators with isolation.
BACKGROUND DISCUSSION
Switch-mode regulators are widely used to supply power to electronic devices, such as in computers, printers, telecommunication equipment, and other devices. Such switch-mode regulators are available in variety of configurations for producing the desired output voltage or current from a source voltage with or without galvanic isolation. The former are also known as an isolated power converters and the later are called a non-isolated power converters.
One of the more challenging loads for power supplies are microprocessors. Because most of the microprocessors are implemented in complementary metal-oxide-semiconductor (CMOS) technology, the power dissipation of the microprocessor generally increases linearly with the clock frequency and to the square of the power supply voltage. There are three common techniques used to reduce power dissipation: power supply voltage reduction, selective clock speed reduction and reducing capacitive loading of internal nodes within the microprocessor. The first two techniques may be used in combination and could be controlled by circuit designer. Even a small reduction in power supply voltage makes a significant reduction in power dissipation. Also, if the clock is removed or significantly slowed in portions of the microprocessor not being used at any given time, very little power is dissipated in those portions and the overall power dissipated is significantly reduced.
However, these power savings techniques come at a cost. Power supply current can swing widely—from hundreds of milli-amperes to over few tens amperes with the microprocessor unable to tolerate more than a few percent change in voltage. Further, the change in current can occur in tens of nanoseconds and may change in an order of magnitude. The power supply designed to supply the microprocessor must have a sufficient low impedance and tight regulation to supply such dynamic power consumption. With output voltages approaching 1V or even sub-volt levels and load currents approaching hundred amperes, the power supplies are very difficult to make and control and still operate efficiently.
In addition, a dedicated power supply for the microprocessor has to be placed in close proximity to the microprocessor. Thus, the power supply must be small and efficient. To meet these requirements, a small DC-to-DC switching power regulator is usually used. The widely used switching regulator to convert a higher input voltage (usually 5V or 12V) to a lower output voltage level is “buck” regulator. In applications where input voltage, often referred as bus voltage, is greater than 12V (e.g. 24V or 48V) single non-isolated switching regulator, such as “buck” regulator, becomes very difficult to make small and efficient. In addition, in these applications a galvanic isolation is very often required thus switching regulator needs to have isolation. One of the most common approaches is to use two stage conversion. First stage conversion is provided using an isolated switching regulator in order to provide galvanic isolation and to step-down high voltage input bus (typically 48V) to lower voltage bus (5V or 3.3V). The second stage is then realized using “buck” switching regulator. Obvious disadvantages of this approach are need for two switching regulators which increases overall cost and reduces overall efficiency.
Three kinds of feedback are generally used to control the operation of the regulator: voltage alone (with current limiting), voltage with peak current control, and voltage with the average current control. For reference see “Fueling the Megaprocessors—Empowering Dynamic Energy Management” by Bob Mammano, published by the Unitrode Corporation, 1996. The voltage with the average current control type of regulation is generally preferred over the other types for the described reasons. Regardless of which type of the feedback control is used, there is need for output current sensing either directly on indirectly. The most common approach is to the sensing resistor in series with the output inductor.
The circuit reconstructs the output inductor current as a differential voltage across the sensing resistor. Most integrated circuits using this approach regulates output voltage with current mode control and use the signal for output voltage feedback. The sensing resistor value must be on one side large enough to provide a sufficiently high voltage, usually tens of millivolts, to overcome input offset errors of the sense amplifier coupled to the sensing resistor and yet small enough to avoid excessive power dissipation. Since the power dissipated in the sensing resistor increases with the square of current, this approach has the obvious efficiency drawback with high output current and low output voltage. For low voltage, high current applications, the value of the sensing resistor may be close or even higher than the on resistance of the power switch and inductor which are minimized for maximum efficiency. Thus, sensed signal is relatively small and requires use of more expensive either comparators or amplifiers. Further, the circuitry implementing the average current control technique is significantly more complicated than the circuitry of the other two techniques.
Power inductors are known to have parasitic (or inherent) winding resistance, and therefore can be represented by an equivalent circuit of a series combination of an ideal inductor and a resistor. When direct (DC) current flows through the inductor (or a current having a DC component), a DC voltage drop is imposed across the inductor, which voltage is a product of the magnitude of the DC (component of the) current and the parasitic resistance of the inductor. Since such an inductor may already be present in the circuit, there is no an additional loss of efficiency in using the inductor for this purpose.
Parasitic resistance of the output inductor is used for current sensing as described in U.S. Pat. No. 5,465,201, issued to Cohen, U.S. Pat. No. 5,877,611, issued to Brkovic, U.S. Pat. No. 5,982,160, issued to Walters et al. and U.S. Pat. No. 6,127,814, issued to Goder, all of which patents are hereby incorporated herein by reference. In U.S. Pat. No. 5,877,611 and U.S. Pat. No. 5,982,160 load current dependant output voltage regulation employing inductor current sensing is proposed. Again, sensed signal is limited to product of inductor's winding resistance and inductor current and can be increased only by means of active amplification, which adds complexity, inaccuracy and mostly additional cost. In order to maximize efficiency of the converter, inductor's parasitic resistance (particular at high current applications) has to be minimized thus, the sensed signal is relatively small and requires use of more expensive either comparators or amplifiers. Very often error due to offset in comparator and/or amplifier is larger than variation in the winding resistance of the inductor (windings printed on the PCB).
Perhaps the most common approach to sensing the output inductor current indirectly in isolated topologies is to use sense resistor in series with power switches or current sense transformer. Use of sense resistor in single ended topologies, such as for example forward, flayback and others, as well as in full-bridge and push-pull topologies, allows that one end of sense resistor is coupled to GND pin of control chip, usually coupled to input return, which simplifies current sensing. On other hand, the sensing resistor value must be large enough to keep the sensed signal above the noise floor and yet small enough to avoid excessive power dissipation. In case of half-bridge converter, for example, this approach is not good since only one power switch is coupled to input return and sensed signal does not reflect current through second, floating power switch. Using sense resistor in return input path is also not good solution since sensed cu
di/dt Inc.
Lowenstein & Sandler PC
Nguyen Matthew
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