Multiple-Output, Single-Ended, Resonant Power Converter

The present invention relates generally to DC-to-DC power converters. More particularly, the present invention relates to a high-frequency, single-ended, resonant converter which provides a main output DC voltage, which may be constant or variable, and multiple auxiliary regulated output voltages.

A single-ended, resonant power converter is described in Steigerwald U.S. Pat. No. 4,845,605, issued July 4, 1989, which patent is assigned to the instant assignee and is incorporated herein by reference. The converter of the Steigerwald patent is capable of operating at high frequencies, e.g. 1 MHz, and achieving high power densities. Furthermore, zero-voltage switching is realized by the Steigerwald converter, resulting in highly efficient converter operation.

One way to obtain multiple output voltages from a DC-to-DC power converter, such as the aforementioned single-ended, resonant converter, is to provide additional windings on the output transformer. In order to obtain regulated output voltages, however, a high degree of coupling among all transformer windings is essential. At high frequencies, tight coupling is difficult to achieve, resulting in output voltages which do not track closely. Moreover, if the main output winding is short-circuited, or if the main output winding is used to provide a variable voltage, then the auxiliary output voltages cannot be regulated because they track the main output voltage by virtue of the transformer coupling. Hence, it is desirable to provide a regulated power supply with multiple auxiliary output voltages that are independent of the main output voltage, which power converter is sufficiently simple in construction in order to be practicable for widespread applications.

Accordingly, an object of the present invention is to provide a new and improved single-ended, high-frequency, resonant power converter which is capable of providing multiple regulated output voltages.

Another object of the present invention is to provide a single-ended, resonant power converter which is capable of providing a variable main output voltage and multiple auxiliary regulated output voltages which are independent of the main output voltage.

Still another object of the present invention is to provide a regulated power supply which has multiple outputs and is simple in construction.

Yet another object of the present invention is to provide a single-ended, resonant power converter which is capable of providing multiple regulated, auxiliary output voltages independent of the converter switching frequency.

The foregoing and other objects of the present invention are achieved in a high-frequency, single-ended, resonant power converter capable of providing multiple regulated output voltages. The single-ended, resonant converter comprises a single, primary-side power switching device coupled to a resonant circuit which includes a capacitor, an inductor, and the parasitic capacitance of the power switching device. In accordance with the present invention, an input filter inductor has a primary winding and multiple auxiliary, or secondary, windings. When the power switching device is conducting, an input DC voltage is applied to the input inductor so that the voltage across the input inductor (which is substantially equal to the input DC voltage) is transformed directly to the auxiliary windings. The voltage across each auxiliary winding is rectified by a corresponding output diode. A filter capacitor is coupled to each output diode for maintaining the respective voltage applied thereto when the power switching device is turned off. As a result, the converter provides multiple auxiliary, regulated output voltages in addition to the main converter output DC voltage. Advantageously, these auxiliary output voltages are a function of the input voltage only and are not a function of the main output voltage of the converter. Hence, the converter provides the multiple regulated, auxiliary output voltages even if the main output voltage is variable. The converter of the present invention is particularly suitable for applications wherein the input voltage is fully or semi- regulated, such as distributed power applications, e.g. satellite systems, radar systems, and distributed computing systems, because the auxiliary output voltages, which are directly proportional to the input DC voltage, are approximately as well regulated as the input voltage.

The single-ended, resonant power converter of the present invention is advantageously highly efficient. First, zero-voltage switching of the power switching device is maintained by the resonant action of the main power circuit. In addition, power to the auxiliary converter outputs is not transferred through the resonant circuit elements.

As another advantage, the high frequency, single-ended, resonant power converter of the present invention is simple in construction. In particular, to obtain multiple, regulated auxiliary output voltages from a single-ended, resonant power converter, such as the Steigerwald converter hereinabove discussed, the only additional elements required are auxiliary secondary windings on the input inductor, and an output diode and filter capacitor per auxiliary output.

The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a single-ended resonant DC-to-DC power converter of the prior art;

FIG. 2 is a schematic diagram of a multiple-output, single-ended, resonant DC-to-DC power converter in accordance with a preferred embodiment of the present invention; and

FIG. 3 is a schematic diagram of a multiple-output, single-ended, resonant DC-to-DC power converter in accordance with an alternative embodiment of the present invention.

FIG. 1 illustrates a single-ended, resonant, DC-to-DC power converter 10, such as that described in Steigerwald U.S. Pat. No. 4,845,605, cited hereinabove. An input filter inductor L.sub.1 receives an input DC voltage V.sub.IN. Inductor L.sub.1 is coupled to a single power switching device Q.sub.1. Power switching device Q.sub.1 is illustrated in FIG. 1 as comprising a power MOSFET having a drain electrode connected to inductor L.sub.1 and a source electrode connected to the circuit common. Switching device Q.sub.1 also includes a parasitic output capacitance, represented by capacitance C.sub.q, and a parasitic anti-parallel diode, represented by diode D.sub.q. A main resonant power circuit 11, including a resonant circuit 12, is coupled in parallel with switching device Q.sub.1 at the junction between inductor L.sub.1 and switching device Q.sub.1 to complete the converter. The resonant circuit 12 includes the series combination of a DC blocking capacitor C.sub.b, a resonant inductor L.sub.r, and another capacitor C.sub.d. The resonant circuit further includes parasitic capacitance C.sub.q when switching device Q.sub.1 is turned off. The primary winding 14 of an output transformer T.sub.o is connected in parallel with capacitor C.sub.d. Secondary windings 16 and 18 are connected together at a center tap terminal 20, with the remaining end of each secondary winding 16 and 20 being connected, respectively, to the anode of a rectifying diode D.sub.1 and D.sub.2, respectively. The cathodes of diodes D.sub.1 and D.sub.2 are connected to each other and further to an input terminal of an output filter inductor L.sub.o. The other terminal of output filter inductor L.sub.o is coupled to an output filter capacitor C.sub.o, the other terminal of which is connected to the transformer center tap terminal. A control circuit 22, such as the one described in the Steigerwald patent, cited hereinabove, is coupled to the gate electrode of switching device Q.sub.1 for controlling the conduction interval thereof in order to adjust and regulate the converter output voltage V.sub.OUT.

FIG. 2 illustrates the portion of a preferred embodiment of a multiple output, single-ended, resonant, DC-to-DC converter of the present invention. An input filter inductor L.sub.2 includes a primary winding 26 and multiple auxiliary, or secondary, windings 28-30. For illustrative purposes, three secondary windings are shown in the embodiment of FIG. 2. Each secondary winding 28-30 is coupled in series with a diode 31-33, respectively, and a filter capacitor 34-36, respectively. Furthermore, to obtain an even higher degree of voltage regulation, a series pass regulator of a type well-known in the art may be employed to receive the respective output voltages at the junctions between the corresponding series combination of the diode and filter capacitor. For example, series pass regulators 37 and 38 are shown in FIG. 2 as being coupled to capacitors 34 and 35, respectively. Suitable series pass regulators may comprise semiconductor devices, such as the 7800 series of monolithic voltage regulators manufactured by Motorola, Inc., National Semiconductor Corporation, and others.

In operation, when switching device Q.sub.1 is on, i.e. conducting, the DC input voltage V.sub.IN is applied directly to the input inductor L.sub.2. Hence, this voltage is transformed directly to secondary windings 28-30 and rectified by diodes 31-33. As a result, the Output voltages V.sub.01 -V.sub.03 are directly proportional to the input voltage V.sub.IN. When Q.sub.1 is turned off, filter capacitors 34-36 maintain the Output voltages V.sub.01 -V.sub.03, respectively. Advantageously, therefore, the output voltages V.sub.01 -V.sub.03 are independent of the switching frequency of device Q.sub.1, which controls the main output voltage V.sub.OUT of the converter. Moreover, if desired, one of the output voltages V.sub.01 -V.sub.03 may be coupled to control circuit 22 for providing regulated power thereto. For example, the output voltage V.sub.03 at the junction between diode 33 and capacitor 36 is shown as being coupled to control circuit 22 (FIG. 1).

As another advantage of the present invention, the output voltages V.sub.01 -V.sub.03 are approximately as well regulated as the input voltage V.sub.IN. Furthermore, as explained hereinabove, if an even higher degree of regulation of the output voltages V.sub.01 -V.sub.03 is desired, then series pass regulators may be employed, such as regulators 37 and 38 illustrated in FIG. 2. Fortunately, the power dissipation of such series regulators is generally low due to the substantially regulated input voltage V.sub.IN.

As still another advantage of the power converter of the present invention, the auxiliary output voltages V.sub.01 -V.sub.03 are not functions of the main output voltage V.sub.OUT, which is controlled by the switching frequency of switching device Q.sub.1. In particular, the auxiliary output voltages V.sub.01 -V.sub.03 are only dependent upon the input voltage V.sub.IN. Hence, efficient, regulated auxiliary output voltages V.sub.01 -V.sub.03 are maintained even if the main output voltage V.sub.OUT is variable.

Yet another advantage of the power converter of the present invention is high-efficiency operation. First of all, zero-voltage switching of switching device Q.sub.1 is maintained due to the resonant action of resonant circuit 12 as described in Steigerwald U.S. Pat. No. 4,845,605, cited hereinabove. Secondly, auxiliary output power for providing the multiple auxiliary output voltages V.sub.01 -V.sub.03 is not transferred through the resonant circuit elements.

FIG. 3 illustrates an alternative embodiment of the portion of a power converter according to the present invention wherein input inductor L.sub.3 comprises a combination differential-mode/common-mode choke having two coupled primary windings 40 and 42. In such case, the input voltage V.sub.IN is ideally divided equally between the two primary windings 40 and 42 of choke L.sub.3 when switching device Q.sub.1 is turned on. (In practice, however, the voltage does not divide exactly equally due to slight differences in the inductances of windings 40 and 42). Choke L.sub.3 further comprises multiple secondary windings, shown in FIG. 3 as secondary windings 44-48. Each secondary winding 44-48 is coupled in series with a rectifying diode 50-54, respectively, which in turn is coupled to a capacitor 56-60, respectively, for maintaining the auxiliary output voltages V.sub.01 -V.sub.05 when switching device Q.sub.1 is turned off. Advantageously, the auxiliary Output voltages V.sub.01 -V.sub.05 are independent of the main output voltage V.sub.OUT and the converter switching frequency and are approximately as well regulated as the input voltage V.sub.IN. Moreover, if desired, one of the multiple auxiliary output voltages, e.g. voltage V.sub.05, may be coupled to control circuit 22 (FIG. 2) for providing control power thereto.

While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.