A single-ended, resonant power converter includes an input filter having at least one primary winding and multiple auxiliary, or secondary, windings. A regulated input DC voltage is transformed directly to the auxiliary windings. The voltages across the auxiliary windings are respectively rectified and filtered to provide multiple, auxiliary, regulated output voltages which are independent of the main output voltage and converter switching frequency.

FIELD OF THE INVENTION 
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. 
BACKGROUND OF THE INVENTION 
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. 
OBJECTS OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF THE 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.