A switched mode DC/DC power converter for high outputs includes two parallel channels. Each channel consists of a flyback transformer (Fb-Tr1, Fb-Tr2) with a timed primary switch (S1, S2) on the primary side and a secondary switch (S3, S4) on the secondary side. The output flow within one channel from the primary supply voltage Ub through the primary power switch (S1; S2), the flyback transformer (Fb-Tr1, Fb-Tr2), and the secondary power switch (S3; S4) to the secondary output voltage (Ua1, Ua2) occurs, respectively, during the blockage phase. The two flyback transformers (Fb-Tr1, Fb-Tr2) are loosely linked with each through a link circuit (L.C.). The link circuit (L.C.) transmits energy from the transformer which is, respectively, in the forward phase to the other transformer. The power switches (S1, S3; S2, S4) are actuated by a control unit (C.U.) so as to be of correct frequency and phase.

BACKGROUND OF THE INVENTION: 
1. FIELD OF THE INVENTION: 
The invention is directed to a switched-mode DC- to -DC which combines a 
flyback-type converter with a flux-type converter, containing at least a 
first and a second flyback transformer each with at least one primary 
winding and one secondary winding, wherein a timed primary switch is wired 
in series with the primary winding and one secondary switch is wired in 
series with each secondary winding. 
2. DESCRIPTION OF THE RELATED ART 
DC/DC power converters for converting a primary DC voltage into a secondary 
DC voltage are extensively known and in general use. Basically, one 
distinguishes between two types, the forward-type converter and the 
flyback-type converter. Flyback-type converters are only known as single 
phase circuits. Forward-type converters are built as single phase circuits 
and a push-pull type circuits. 
The known flyback-type converters have a series of disadvantages. The 
calculation of the transformer requires lengthy repetitive processes, 
protective measures against transformer saturation, the effects of leakage 
inductance and exceeding of the breakdown voltage of the power switch must 
be taken, which greatly increases the quantity of the required components. 
The transformer transmits the energy only during half of each period, 
which tends to lower the overall efficiency of the circuit. 
An increased energy flow and thus an improved efficiency result if 
push-pull circuits are used. Push-pull circuits are only known 
forward-type converters. Here, two power switches operate in push-pull 
action by means of two primary windings upon one transformer. Push-pull 
circuits require, however, very good balancing in the primary circuit, so 
that no DC current magnetic biasing arises in the transformer. In 
addition, specific protective measures must be taken in order to prevent 
the power switches from being simultaneously on line which would result in 
a short circuit. 
A single-phase converter is known from the DE-pbulication 
"Elektronikpraxis", Oct. 1986, page 52 and following, which operates with 
two transformers, whose primary windings are wired in series and whose 
secondary windings are wired in parallel through each one rectifier. 
Through an appropriate polarity of the primary and secondary windings and 
the rectifiers, the one transformer works as forward-type while, the other 
transformer works as a flyback-type converter. This type of circuit has 
certain advantages compared to the basic circuits, for instance, no 
protective measures are required against exceeding the breakdown voltage 
of the power switch. In spite of that the quantity of components is still 
high because two transformers have to be used. Also, this circuit can only 
be operated as a single-phase converter. This means that each transformer 
transmits power only during one-half of each period. 
A refinement of this circuit is known from the Japanese patent application 
No. 60-170461(A). Two transistors actuated in push-pull action work upon 
four transformers, of which, respectively, two operate as single phase 
flyback-type converters and the other two as single-phase forward-type 
converters with the help of rectifier diodes switched at the secondary 
windings for generation of secondary voltage. Here, also, each transformer 
transmits power only during half of each period. 
A single-phase DC/DC power converter with a transformer and one primary 
winding is known from the U.S. Pat. No. 4,455,596. The transformer has two 
secondary windings, one of which acts as a flyback-type converter with the 
help of a rectifier diode of suitable polarity, and the other acts as a 
forward-type converter with the help of a rectifier diode of suitable 
polarity. The transformer transmits energy during both halves of each 
period. Of course, the energy transmitted during the flux phase must be 
temporarily stored in a memory choke and can only be released to a 
secondary circuit during the subsequent blocking phase. The necessity to 
utilize a memory choke and a second rectifier diode cancels the saving 
involved in utilizing a single transformer. Apart from that, both 
secondary voltages fluctuate greatly as a function of the respective load 
resistance, as this is known in flyback-type converters. 
SUMMARY OF THE INVENTION 
The present invention is based upon the task to provide a circuit 
arrangement for a DC/DC power converter, which transmits electrical power 
during the forward phase as well as during the blocking phase and operates 
with a minimum of components. 
This task is solved by loosely linking the transformers with each other 
with the help of a link circuit which transmits energy from the 
transformer which is just in the forward phase to the other transformer. 
This involves advantages in that the quantity of the components, 
particularly of the extensive and voluminous transformer components, is 
reduced to the absolute minimum and that the transformer are operated in 
an optimum fashion, since they transmit power during the flux phase as 
well as during the blocking phase. Flyback-type converters and 
forward-type converters can be switched to a secondary circuit, since they 
yield energy during differing periods. Also the voltage and current loads 
of the primary load switch are considerably reduced when compared with the 
conventional flyback converter. 
The circuit arrangement in the subject invention represents the combination 
of a push-pull forward-type converter with a push-pull flyback-type 
converter, in spite of the fact that according to the prevailing opinion, 
push-pull flyback-type converters cannot exist. In this circuit 
arrangement, the demagnetizing of the transformer cores is actively 
assisted by the magnetic field of the respective other primary coil. For 
this reason, an air gap, such as in a conventional push-pull forward-type 
converter, would not be required in the transformer cores. The 
transformers can be dimensioned exclusively for the conducting state 
operation, which considerably simplifies the calculation. 
Additionally, it is possible in this type of circuit to increase the 
demagnetizing of the transformer cores by the magnetic field respectively 
generated by the other primary winding up to an opposite magnetizing, so 
that the modulation of the iron core can be considerably increased 
compared to a conventional flyback-type converter, with the consequence of 
an improved power flow and efficiency. 
A final advantage of this circuit consists in the possibility to tap two 
output voltages which can be regulated independently of each other. The 
regulating range of the two output voltages essentially depends in what 
ratio the flux converter portion of the circuit contributes to the 
generation of the individual secondary voltages. 
The link circuit which connects the two flyback transformers with each 
other and which transmits energy from the tranformer which is forward in 
the flux phase to the other transformer, can be realized very easily in 
actual practice, and indeed either as an air gap between the two 
transformer cores, through which the magnetic lines of force can transit 
from one core to the other, or through a closed wire loop, which is wound 
over both transformers. 
In order to be able to transmit particularly high powers, several 
transformer cores are put together in such a way, that they constitute the 
edges of a parallelepiped, particularly of a cube. Herein, a primary 
winding and a secondary winding is wound around each pair of magnetic 
cores. 
The four primary windings can be supplied with current through four primary 
switches. In accordance with a refinement of the invention, two primary 
windings wound respectively upon two magnetic cores lying diagonally 
opposite each other can, however, be respectively wired in parallel, so 
that only two primary switches are required. 
This embodiment with the magnetic cores arranged in a three-dimensional 
fashion has the advantage, that only relatively small core sizes are 
required, and that the number of windings of the primary and secondary 
coils are twice as great as is the case in conventional transformer 
circuits. This facilitates the accurate observance of the computed values 
in fabrication, especially with high outputs. 
Because of the high timing frequency achievable with the circuit 
arrangement in the invention, it is advisable to design the power switches 
as semiconductor components. The primary switches are transistors; the 
secondary switches can, as required, be designed as transistors, 
thyristors or diodes. 
It was shown in a surprising manner that the circuit operates perfectly 
also in case of only one primary switch being operative.

DESCRIPTION OF THE PREFERRED EMOBIDMENTS 
FIG. 1 shows a push-pull DC/DC voltage converter as a block diagram in 
schematic illustration. Two flyback transformers Fb-Trl, Fb-Tr2 are 
provided, whose primaries are each connected through one timed primary 
power switch S1, S2 alternately with the primary DC voltage Ub. The 
secondaries of the flyback transformers Fb-Tr1, Fb-Tr2 are applied to 
secondary DC voltage circuits through secondary power switches S3, S4, at 
which the secondary voltages Ua1, Ua2 can be tapped. The primary switches 
S1. . . S4 are switched on and off by a control unit C.U. 
Both flyback tranformers Fb-Tr1, Fb-Tr2 are additionally loosely linked 
with each other by a link circuit L.C. This linkage is so that energy can 
be transmitted, respectively, from the transformer which is just in the 
forward phase to the other transformer. Both transformers transmit power 
during both halves of each period. They are being optimally utilized. The 
peak loads of the primary switches S1, S2 and lower, as far as the voltage 
and also the current is concerned, than is the case with conventional 
DC/DC voltage converters. 
FIG. 2 shows a first realization of the circuit block diagram illustrated 
in FIG. 1. The first flyback transformer Fb-Tr1 is constituted by a U-core 
1, which is wound with a primary winding L1 and a secondary winding L3. 
Both windings L1, L3 are closely linked as is known in flyback converters. 
The second flyback transformer Fb-Tr2 also consists of a U-core 1' which 
is wound with a primary winding L2 and a secondary winding L4. Both 
windings L2, L4 are closely linked. 
Both primary windings L1, L2 are alternately switched to the primary DC 
voltage Ub through timed primary power switches S1, S2. 
The secondary power switches are, in this case, designed as rectifier 
diodes D3, D4. The secondary DC voltages Ua1, Ua2 can be tapped at the 
condensers C1, C2. The level of the secondary output voltage Ua1, Ua2 can 
be controlled by the in-out ratio of the primary power switches S1, S2. 
The linkage circuit L.C. is constituted by the air gaps 2, 2' in the 
embodiment example in FIG. 2. The magnetic lines of force transit from one 
transformer to the other through these air gaps 2, 2' and indeed, 
respectively, during its forward phase, which is caused by the polarity of 
the primary windings L1, L2, the secondary windings L3,L4 and the diodes 
D3,D4. The linkage between the two transformers Fb-Tr1, Fb-Tr2 can be 
influenced by the magnitude of the air gaps 2, 2'. 
If, for instance, the primary power switch S1 is closed, then the 
transformer Fb-Tr1 is in the forward phase. The magnetic lines of force 
generated by the primary winding L1 in the transformer core 1 flows 
through the air gaps 2, 2' into the other transformer core 1' and generate 
a voltage in its secondary winding L4, which voltage is rectified by the 
diode D4 and supplied to the condenser C2, where it assists in the 
formation of the secondary voltage Ua2. 
During the consecutive blocking phase of the transformer Fb-Tr1, the 
primary power switch S1 is open. The magnetic lines of force in the 
transformer core 1 generate now an output voltage in the secondary winding 
L3, which is rectified by the diode D3, transmitted to the condenser C1, 
where it assists in the generation of the secondary output voltage Ua1. 
Analogous considerations can be made also for the transformer Fb-Tr2. Both 
flyback transformers Fb-Tr1, Fb-Tr2 thus alternately assist towards the 
generation of both output voltages Ua1, Ua2. The regulation range of the 
two output voltages Ua1, Ua2 essentially depends on the share with which 
the respective forward-type converter assists in the generation of the 
output voltages Ua1 or Ua2. 
FIG. 3 shows a circuit arrangement in which two secondary windings L3, L4 
operate through their associated rectifier diodes D3, D4 upon the same 
secondary circuit for generation of one single output voltage + Ua. 
In this embodiment example, also the transformer core consists of two 
U-cores 11, 11', however with short legs and a long base. The winding 
pairs L1, L3 or L2, L4 are arranged in the region of the air gaps 12, 12'. 
The desired loose linkage is achieved by the length of the non-wound 
portion of the transformer cores 11, 11'. Fine adjustment of the magnetic 
linkage is additionally possible by a magnetic shunt 13. 
In a circuit according to FIG. 3, with an output of approximately 1.5 
kilowatts, it was possible to measure an efficiency of 96-98%. 
In the circuits in FIGS. 2 and 3, the magnetic field built up by one 
primary coil in a transformer core is demagnetized by the magnetic field 
generated by the other primary coil L2 or L1. This demagnetizing by the 
other primary coil can be increased up to a magnetization of opposite 
polarity. The magnetic conditions in the transformer cores correspond then 
to those of a push-pull flux converter. For this reason, air gaps are 
unnecessary in the circuits in FIGS. 2 and 3. The flyback transformers can 
be dimensioned in accordance with the rules applicable for the push-pull 
forward-type converters-transformers. This applies aslso for the power 
switches. 
FIG. 4 shows an arrangement of transformer cores, which is suited for 
particularly high outputs. The transformer core 21 consists of eight 
U-cores, which form the edges of a parallelepiped, especially that of a 
cube. The primary and secondary windings are divided into, respectively, 
two partial windings L1', L1". . . L4', L4". Partial windings with the 
same designation are located on legs of the transformer core 21 which are 
respectively diagonally opposite each other. The arrangement of the air 
gaps 22 occurs in the same way as was discribed in connection with FIGS. 2 
or 3. Each partial winding has twice the number of individual windings. 
The partial windings carrying the same designations can be wired in 
parallel; it is however also possible to assign its own power switch to 
each partial winding. 
FIG. 5 shows a plan view of an arrangement of transformer cores and 
windings destined for even higher outputs. Here four arrangements 
corresponding to FIG. 4 are interlocked. Nine pairs of combined primary 
and secondary partial windings 11 . . . 19 are attached, respectively, 
where the transformer cores touch. The combination of primary and 
secondary partial winding 11, 13, 15, 17, 19 are assigned to the flyback 
transformer the combination of primary and secondary partial windigns 12, 
14, 16, 18 are assigned to the other flyback transformer, wherein, in this 
case also, the associated combination of primary and secondary partial 
windings are switched in series with a power switch either individually or 
in parallel. 
FIG. 6 shows a modified circuit arrangement in which the linking circuit is 
a closed wire loop wound over both transfomer cores. The primary switches 
include thyristors Trl' and Tr2', while the secondary switches include 
transistors Tr3 and Tr4. The primary switches Trl', Tr2' and the secondary 
switches Tr3, Tr4, have their respective control inputs connected to 
respective outputs of a control unit which controls, in a timed 
relationship, the openings and closings of the respective pairs of 
switches.