Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
1999-10-26
2001-04-17
Wong, Peter S. (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C363S065000, C323S283000
Reexamination Certificate
active
06218815
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to regulators, and more particularly to a multiple stage synchronous regulator, where each stage is activated in a sequential manner.
DESCRIPTION OF THE RELATED ART
DC to DC regulators are generally used to convert an unregulated DC voltage to a regulated DC voltage. Such regulators are widely used in switch mode DC power supplies for generating the appropriate DC voltage signals on the system board and option cards of a computer system. The two most common types of DC to DC regulator topologies include buck and boost topologies. In both buck and boost topologies, an oscillator type circuit, such as a pulse width modulator, turns on and off a primary power switch driving current from the unregulated DC source voltage through a choke inductor or transformer to develop an output voltage. In the buck regulator circuit, when the primary switch is turned on, the source voltage is coupled to the load through the inductor and when the primary switch is turned off, the load current flows through a free-wheeling rectifier or diode. The buck regulator is commonly used to step-down the voltage of the unregulated source. In a boost regulator circuit, the primary switch is turned on to apply power to an inductor or transformer for storing energy and is turned off to release the stored energy to the output circuit and load. The boost regulator is often used to increase the voltage level at the output. The present invention is illustrated using a buck type regulator, although it is applicable for boost and other regulator topologies.
Synchronous DC to DC regulation is often used to improve the efficiency and performance of DC to DC converters by reducing losses in the power switches or diodes. In general, two switches are synchronized so that one switch is turned on while the second is off, and vice versa. In particular, a primary switch is turned on while the second switch is turned off during a first portion of each cycle to provide power, and then the primary switch is turned off and the synchronous switch is turned on during a flux reversal portion of each cycle to free-wheel load current or to otherwise release stored energy to the output. Generally, the standard method of achieving an active switch instead of a passive diode in low voltage, high current DC to DC regulators is to use a power metal-oxide-semiconductor field-effect transistor (MOSFET). Typically, dual synchronous MOSFETs are driven by a pulse width modulated (PWM) circuit which controls each cycle based on a feedback input.
Typical regulators have a single stage switching system or linear regulator system where the average power density is on the order of ten watts per cubic inch (10 W/in
3
). Although several factors are responsible for limiting the practicable power density achievable, one of the more important factors is the frequency of operation. In fact, the amount of power loss is proportional to the frequency of the regulator. A typical single stage regulator uses a primary power inductor which is designed according to precise specifications in order to reduce power loss and heat generation as much as possible for particular frequency ranges. The inductor is required to have relatively low losses in its core and copper winding and further requires substantial shielding to achieve a high rating, thereby resulting in a relatively large and expensive part. A similar analysis applies to the switching transistors, which are almost invariably large and expensive MOSFETs at the desired frequency of operation. MOSFETs usually have relatively high switching losses due to high gate capacitance. Also, to achieve the desired drain to source resistance, a significant amount of silicon is used, resulting in a relatively large part.
Thus, the power loss for both switching FETs and power inductors for typical switching regulators is increased with increasing frequency. However, a higher frequency is desired to reduce the resulting ripple voltage at the output to achieve the desired regulation. The output load or filter capacitor increases in size with increases of output ripple voltage, which decreases with increased frequency. Therefore, a higher frequency at the output allows a smaller load capacitor since the capacitor is switched at a higher rate and thus requires less storage per cycle. Yet because of the power loss limitations of the switching FET and power inductors, the capacitor must usually be a relatively large and costly part at the practical frequency ranges of operation.
In this manner, typical single stage switching regulators use fairly large switching components and filter capacitors in order to reduce the power losses and to increase the efficiency to achieve the desired or necessary voltage regulation.
SUMMARY OF THE INVENTION
A multiple stage sequential synchronous regulator according to the present invention includes a plurality of switching stages activated sequentially to reduce the amount of stress applied to each stage. Multiple stages further reduce the effective frequency per stage, thereby reducing the power loss of each stage. The time sharing of several stages reduces the average current per stage and allows increased utilization of each of the switching parts. In this manner, the switching transistors and power inductors are replaced with several significantly smaller, lighter and cheaper components.
Furthermore, each switching part can be pushed past its rated limits due to smaller average current, thereby increasing the efficiency of the parts. The cumulative result is a smaller, lighter and cheaper voltage regulator capable of processing a much greater amount of power, which further results in a dramatic increase in power density.
A regulator according to the present invention includes a sequential drive system including a plurality of switch stages for converting an unregulated DC voltage to a regulated output voltage according to a modulated signal. A logic circuit sequentially selects the switch stages on consecutive cycles of the modulated signal. Each of the switch stages preferably includes synchronous switches coupled to an inductor, where a first switch is activated to initiate a power phase and a second switch is activated to activate a flux reversal phase of the inductor when that stage is selected. In this manner, two stages effectively decrease the frequency stress applied to each stage by a factor of two. Correspondingly, n stages decrease the stress applied to each stage by a factor of n. However, since each stage is coupled to the output, the output capacitor sees the combined frequency of all the stages.
In the preferred embodiment, a sequential logic circuit asserts sequential enable signals to corresponding synchronous switching stages. Each stage includes preferably two amplifiers for driving a power switch and a flux reversal switch, respectively, to implement the power and flux reversal stages for each cycle. Each pair of switches activates current through a corresponding output inductor, where all of the output inductors are connected together and to a single output filter capacitor. Thus, the effective frequency of the switching amplifiers, switching transistors and output inductors is proportionately reduced by the number of stages, thereby substantially reducing the power loss in each stage. Furthermore, the output capacitor is exposed to the cumulative frequency, which can be significantly higher than a single stage embodiment. Thus, the output capacitor may therefore be reduced in size.
In this manner, the overall switching regulator is smaller, less expensive and capable of providing substantially more power. In fact, use of the present invention allows a 3.3V regulator capable of supplying 20 amperes (A) to be implemented in a package less than one cubic inch, thereby resulting in a power density of at least 66 W/in
3
. This is a substantial improvement over prior art, which is typically on the order of 10 W/in
3
. Further, the cost of all of the switching components for a four stage regulator is substantially less
Kates Barry K.
Sheehan, Jr. Edward P.
Dell USA, D.P.
Skjerven Morrill & MacPherson LLP
Vu Bao Q.
Wong Peter S.
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