Inverter and power supply systems including same

An inverter for supplying electrical energy from a DC supply to a load, includes a flyback transformer having a primary winding circuit coupled to the DC supply, and a secondary winding circuit coupled to the load. The primary winding circuit includes a first switch for interrupting the DC supply causing energy to be stored in the transformer, and the secondary winding circuit includes a second, unidirectional switch to produce an output of one sign when closed. The primary winding circuit further includes an electrical device effective to return energy to the DC supply only when the first and second switches are open.

BACKGROUND OF THE INVENTION 
The present invention relates to an inverter for supplying electrical 
energy from a DC supply to a load. The invention may also be 
advantageously used in uninterrupted power supplies and standby power 
supplies, and is therefore described below also with respect to these 
applications. 
Many forms of inverter circuits are known for converting DC to AC. One 
known circuit, called the "flyback" type, and described for example in the 
article "A New Family of Single-Phase and Three-Phase Inverters" by 
Sayed-Amr El-Hamamsy and R. D. Middlebrook, PCI, October, 1985 
Proceedings, Pages 84-98, includes a transformer having a primary winding 
coupled to the DC supply and a switch for interrupting the DC supply, 
causing energy to be stored in the transformer, which energy is outputted 
from the secondary winding. 
In the "fly-back" type of inverter, current flows in the secondary winding 
only when no current flows in the primary winding, and no current flows in 
thee secondary winding when current flows in the primary winding. The 
"flyback" type of inverter is therefore to be distinguished from the 
"forward" (or "push-pull") type inverter, in which current flows in the 
secondary winding whenever current flows in the primary winding, and no 
current flows in thee secondary winding when no current flows in the 
primary winding. 
An object of the present invention is to provide an inverter particularly 
of the "flyback" type, but having a number of advantages, as will be 
described more particularly below. 
Other objects of the invention are to provide an uninterrupted power supply 
and also a standby power supply utilizing the novel inverter. 
According to the present invention, there is provided an inverter for 
supplying electrical energy from a DC supply to a load, comprising a 
transformer including a primary winding circuit coupled to the DC supply, 
and a secondary winding circuit coupled to the load. The primary winding 
circuit includes a first controlled switch for interrupting the DC supply; 
and the secondary winding circuit including at least a second, 
unidirectional controlled switch to produce an output of one sign when 
closed. The primary winding circuit further includes an electrical device 
effective to return energy to the DC supply only and always when the first 
and second switches are open; and a control circuit for separately and 
independently controlling the operation of the first and second switches 
to open and close them, at the same frequency, such that during one 
interval in each cycle the switch in the primary winding is closed to 
produce an excess of energy which is stored in the transformer, an during 
another interval in each cycle the switches in the primary winding and the 
secondary winding are open and the excess energy stored in the transformer 
is returned to the DC supply. Such an arrangement permits fast and stable 
control of the inverter. 
The invention is particularly useful in, and is therefore described below 
with respect to, inverters in which thee transformer is a flyback 
transformer. In such transformers, the primary and secondary winding 
circuits have polarities such that there is current flow in the secondary 
winding circuit when no current flows in the primary winding circuit, and 
no current flow in the secondary winding circuit when there is current 
flow in the primary winding circuit. 
In several preferred embodiments of the invention described below, thee 
secondary winding circuit includes a third, unidirectional switch poled in 
the opposite direction as the second switch, such as to produce, when it 
is the active switch in the secondary winding circuit instead of the 
second switch, and is closed, an output of the opposite sign as said 
second switch. 
According to another important feature in preferred embodiments of the 
invention described below, the control circuit includes: means for closing 
the first switch and opening the second or third switch to start a first 
Interval in each cycle during which energy is stored in the transformer; 
means operative at the end of the first Interval to open the first switch 
and to close the active second or third switch to start a second Interval 
in each cycle during which energy stored in the transformer is delivered 
to the load or, in the case of a reactive load, energy in the load is 
stored in the transformer; and means operative at the end of the second 
Interval to open all the switches to start a third Interval during which 
excess energy then stored in the transformer may be returned to the DC 
supply via the primary winding circuit. 
As will be described more particularly below, such an inverter is capable 
of four-quadrant operation, wherein energy is supplied to the load during 
the first and third quadrants, and excess energy stored in the transformer 
or in the load may be returned to the DC supply for recharging it during 
the second and fourth quadrants. Besides permitting four-quadrant 
operation, the inverter of the present invention also permits fast and 
stable control. 
The four-quadrant operation of the inverter provides particular advantages 
when used in an uninterrupted power supply and also in a standby power 
supply. Thus, it obviates the need for the provision of large separate 
chargers normally required in such systems in order to keep the back-up 
power supply fully charged. The invention is therefore described below 
also with respect to these applications. 
Another preferred embodiment of the invention is also disclosed involving a 
different sequence of control. According to this embodiment, the primary 
winding control subcircuit opens the first switch at the beginning of each 
cycle and closes the first switch at a subsequent point in the cycle when, 
at the end of the respective cycle, the energy stored in the transformer 
would reach a predetermined value; and the secondary winding control 
subcircuit closes the second switch at the beginning of the respective 
cycle and opens the second switch when the voltage at the output of the 
secondary winding circuit reaches a predetermined value; such that a first 
interval is started by the opening of the first switch and closing of the 
second switch, during which energy in the transformer is delivered to the 
load; a second interval is started by the opening of the second switch 
during which excess energy in the transformer is returned to the power 
supply; and a third interval is started by the closing of the first switch 
during which energy is stored in the transformer. 
It will thus be seen that in the first-described embodiment, the interval 
(therein Interval III) during which excess energy is delivered to the 
power supply is of fixed termination point; whereas in the latter 
embodiment this interval (Interval II), is of "floating" duration. That 
is, Interval II in the latter embodiment starts by the opening of the 
secondary winding switch (the "second switch"), when the voltage at the 
output of the secondary winding circuit reaches a predetermined value, and 
ends by the closing of the primary winding switch (the "first switch") at 
the point in the cycle when, at the end of the cycle, the energy stored in 
the transformer would reach a predetermined value. Such a "floating" 
arrangement for determining the interval during which excess energy stored 
in the transformer is delivered to the power supply, produces a more 
efficient and stable operation. 
Further features and advantages of the invention will be apparent from the 
description below.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Basic Construction of the Inverter (FIGS. 1 and 2) 
FIG. 1 is a block diagram illustrating the main components of an inverter 
constructed in accordance with the present invention, and FIG. 2 more 
particularly illustrates those components. 
Thus, the inverter comprises a DC battery supply 2, feeding power to a 
flyback transformer 4 via a primary winding switching circuit 6. The 
secondary winding of transformer 4 is controlled by a secondary winding 
switching circuit 8 so as to output the power to a load 10 via an output 
filter 12. A control unit 14 receives inputs from the input to the primary 
winding circuit of the transformer 4, and also from the output of the 
transformer secondary winding circuit to the load 10, and controls the 
primary winding switching circuit 6 and the secondary winding switching 
circuit 8 in response to such inputs. 
FIG. 2 more particularly illustrates the main components of the inverter of 
FIG. 1. Thus, as shown in FIG. 2, transformer 4 includes two primary 
windings N.sub.1, N.sub.2 connected to the DC supply 2, and two secondary 
windings N.sub.3, N.sub.4 for supplying power to the load 10 via filter 
12, and also via an output capacitor 16 connected to the output circuit of 
the transformer secondary windings. The load 10 may be a reactive load, so 
that it receives energy from the inverter during the first and third 
quadrants, and gives energy back to the inverter during the second and 
fourth quadrants. As shown by the polarity markings in FIG. 2, in the 
flyback transformer 4 thee current normally flows in the secondary 
windings N.sub.3, N.sub.4 when no current flows in the primary windings 
N.sub.1, N.sub.2, and no current flows in the secondary windings only when 
current flows in the primary windings. 
The primary winding switching circuit shown by block 6 in FIG. 1 includes a 
switch S.sub.1 between the DC supply and primary winding N.sub.2, which 
switch is interrupted under the control of the control unit 14 for causing 
excess energy to be stored in the transformer. The primary winding 
switching circuit further includes a unidirectional conducting device or 
diode D.sub.1, in series with the power supply and primary winding 
N.sub.1. Diode D.sub.1 is effective to deliver the stored excess energy 
back to the DC supply 2 during the second and fourth-quadrants as will be 
described more particularly below. 
The secondary switching circuit indicated by block 8 in FIG. 2 includes two 
further switches S.sub.2, S.sub.3 poled in opposite directions by their 
respective unidirectional conducting devices D.sub.2, D.sub.3. One of 
these two switches would be active during each cycle of operation, 
depending on the sign of the output supplied by the inverter to the load. 
Thus, if switch S.sub.3 is the active one, switch S.sub.2 would be 
continuously open, and switch S.sub.3 would be closed during precise 
Intervals of each cycle to output pulses on one sign; whereas if switch 
S.sub.2 is the active one, switch S.sub.3 would be continuously open and 
switch S.sub.2 would be closed at precise Intervals to output pulses of 
the opposite sign. 
Switches S.sub.1, S.sub.2, S.sub.3 are controlled by control unit 14 in 
such a manner as to achieve four-quadrant operation with fast control. The 
four-quadrant operation is permitted by the addition of the abovementioned 
diode D.sub.1 in the primary winding circuit, such that diode D.sub.1 
provides a path for charging the battery supply 2 at a certain Interval 
during each cycle in which the energy, stored in the transformer and not 
used in the load, is returned to the battery supply for charging it. 
Magnetic Flux Diagram (FIG. 3) 
The manner in which the above-described four-quadrant operation is 
accomplished will be better understood by reference to the magnetic flux 
diagram illustrated in FIG. 3, which shows what occurs during each cycle. 
Thus, each cycle is divided into three Intervals I, II, III, as follows: 
Interval I is an energy storing or accumulation Interval, in which excess 
energy is stored in the transformer. This Interval is started by closing 
switch S.sub.1 at the beginning of a cycle when switches S.sub.2 and 
S.sub.3 are both open, so that current in the primary winding N.sub.2 
starts to build up according to the following equation: 
EQU I.sub.p =I.sub.op +(V.sub.dc .multidot.t)/L.sub.p 
wherein: I.sub.p is the instantaneous current in the primary winding 
N.sub.2 ; I.sub.op is the starting current; V.sub.dc is the battery 
voltage; and L.sub.p is the inductance of primary winding N.sub.2. 
Interval II is normally an energy-delivering Interval and is started by the 
opening of switch S.sub.1 and the closing of the active switch S.sub.2 or 
S.sub.3, depending on the sign of the output current. During Quadrants 1 
and 3 of this Interval, the excess energy stored in the transformer is 
used for charging the capacitor 16 (FIG. 2) for delivery to the load (10, 
FIG. 1) via the secondary winding N.sub.3 or N.sub.4, depending on whether 
switch S.sub.2 or S.sub.3 is the active one and is closed. During 
Quadrants 1 and 3 of Interval II, the current in the secondary winding 
decreases with time, as shown by descending line IIa, as follows: 
EQU I.sub.s =I.sub.os -(V.sub.out .multidot.t)/L.sub.s 
wherein: I.sub.s is the instantaneous current in the secondary winding; 
I.sub.os is I.sub.op (N.sub.p /N.sub.s). 
On the other hand, if the load is a reactive load, energy from the load is 
stored during Quadrants 2 and 4 in the transformer. Thus, the excess 
energy from the load is stored in the transformer, as shown by ascending 
line IIIb, according to the following equation: 
EQU I.sub.s =I.sub.os -(-Vout.multidot.t)/L.sub.s 
Interval III starts by closing the active switch S.sub.2 or S.sub.3, 
whichever one had been closed in Interval II according to the sign of the 
output, switch S.sub.1 in the primary winding circuit remaining open. 
Accordingly, all the switches are open. During this Interval, the excess 
of energy not delivered to the load may return to the DC supply 2 via 
diode D.sub.1 and primary winding N.sub.1, the current through the latter 
winding decreasing as follows: 
EQU I.sub.d =I.sub.od -(V.sub.dc .multidot.t)/L.sub.p 
wherein: I.sub.d is the instantaneous current through diode D.sub.1 and 
winding N.sub.1 to the battery supply 2; I.sub.od is the starting current 
through winding N.sub.1 ; and L.sub.p is the inductance of the primary 
winding N.sub.1. 
With respect to Interval III, descending line IIIa in FIG. 3 thus describes 
the excess energy returned to the power supply, via the diode D.sub.1 and 
primary winding N.sub.1, during Quadrants 2 and 4, whereas descending line 
IIIc describes the excess energy returned to the power supply during 
Quadrants 2 and 4 in a reactive load. 
In some cases, however, there may be no need to return energy back to the 
DC power supply. In such a case the energy may merely be retained within 
the transformer, as shown by horizontal line IIIb in FIG. 3. FIG. 2a 
illustrates an arrangement of the primary winding circuit that may be used 
for effecting the operation illustrated by the horizontal line IIIc in 
FIG. 3. 
Thus, the primary winding circuit shown in FIG. 2a also includes primary 
winding N.sub.1 containing diode D.sub.1 and primary winding N.sub.2 
containing switch S.sub.1, as in FIG. 2. However, it includes an 
additional switch S.sub.4 which is closed at the beginning of Interval III 
(FIG. 3) in order to short circuit winding N.sub.1, and thereby to 
preserve in the transformer the excess energy not delivered to the load 
during Interval II. 
Control Circuit 14 (FIGS. 4-7) 
FIG. 4 illustrates the control circuit 14 which is used for controlling the 
switches S.sub.1, S.sub.2 and S.sub.3 in the primary and secondary winding 
circuits illustrated in FIG. 2, and also switch S.sub.4 in the 
modification to the primary winding circuit illustrated in FIG. 2a if that 
modification is used. FIGS. 5-7 illustrate more particulars of the various 
components used in control circuit 14. 
Briefly, control circuit 14 includes a primary winding control subcircuit 
20 (more particularly illustrated in FIG. 5) which controls switch S.sub.1 
in the primary winding circuit; a secondary winding control subcircuit 30 
(more particularly illustrated in FIG. 6) which controls switches S.sub.2 
and S.sub.3 ; a flux level reference generator 40 (more particularly 
illustrated in FIG. 7); and a reference voltage generator 50, which 
controls the secondary winding control subcircuit 30. The primary winding 
control subcircuit 20 is effective to sense the energy stored in the 
transformer and to open switch S.sub.1 when the stored energy reaches a 
predetermined value as fixed by the flux level reference generator 40; and 
the secondary winding control subcircuit 30 is effective to sense the 
output voltage in the secondary winding circuit of the transformer and to 
open the active switch S.sub.2 or S.sub.3 (depending on the polarity of 
the output) when the output voltage reaches a predetermined value as fixed 
by the reference voltage generator 50. 
FIG. 5 more particularly illustrates the primary winding control subcircuit 
20. It includes a current sensor 21 for sensing the current through switch 
S.sub.1 in the primary winding circuit, and a comparator 22 which receives 
the latter sensed current. Comparator 22 also receives a signal from the 
flux level reference generator 40 which, as described above, fixes the 
predetermined value to be reached by the stored energy when switch S.sub.1 
is to be opened to end the energy-storing Interval I and to start the 
energy-delivering Interval II. Comparator 22 compares the sensed current 
from sensor 21, and the predetermined value fixed by the flux level 
reference generator 40, and actuates a flip-flop 23 to open switch S.sub.1 
when the two values are equal. Thus, flip-flop 23 is set at the start of 
each cycle when switch S.sub.1 is closed, and is reset by the output from 
comparator 22, when the two values sensed by the comparator are equal, to 
open switch S.sub.1. 
FIG. 6 illustrates the secondary winding control subcircuit represented by 
box 30 and also by the reference voltage generator 50 in FIG. 4. As 
briefly described above, this secondary winding controlled subcircuit 
senses the output voltage in the secondary winding circuit of the 
transformer, and when the output voltage reaches a predetermined value as 
fixed by the reference voltage generator 50, opens the active 
secondary-winding, switch S.sub.2, S.sub.3 (depending on the polarity of 
the output voltage), to thereby end the energy-delivering Interval II and 
to start Interval III during the which the excess energy is delivered back 
to the DC supply via diode D.sub.1 and primary winding N.sub.1 (FIG. 2). 
The secondary winding control subcircuit illustrated in FIG. 6 includes a 
comparator 31 connected to a differential amplifier 32 which receives a 
first input A from the reference voltage generator 50, and a second input 
B from the output side 33 of the active secondary-winding switch, S.sub.2 
or S.sub.3, so as to sense the output voltage of the inverter. 
In order to improve both the stability of the inverter and its tracking 
capabilities, differential amplifier 32 connected to comparator 31 
includes two further inputs, namely: a third input C from a correction 
offset signal generator 34, which senses the output current and generates 
a correction offset signal proportional thereto; and a fourth input D from 
a bidirectional sawtooth generator 35, which generates a signal whose 
magnitude and sign are a function of the output current. Accordingly, the 
differential amplifier 32 will produce, from all the foregoing inputs, an 
output signal (.alpha..DELTA.+.beta.B+.gamma.C+.delta.D), which signal is 
applied to comparator 31. 
Comparator 31 thus produces an output signal when the output voltage from 
the inverter reaches the predetermined value fixed by the reference 
voltage generator 50. This output signal is applied to a flip-flop 36, 
which is set at the beginning of the energy-delivering Interval II, and 
reset by the output from the comparator 31 at the end of the 
energy-delivering Interval II. Ending of Interval II starts the recharging 
Interval III during which the DC power supply 2 is recharged by the excess 
energy stored in the transformer during Interval I and not delivered to 
the load during Interval II. 
The secondary winding control subcircuit 30 illustrated in FIG. 6 further 
includes an EXCLUSIVE-OR circuit 37 which operates as a gate to invert (or 
not invert) the comparator signal according to the quadrant of the output 
voltage. That is, if the output voltage is negative, it is inverted; and 
if positive, it is not inverted. Circuit 37 could be omitted if the values 
inputted into comparator 31, via its differential amplifier 32, are 
absolute values. 
As explained earlier, the speed and the stability of the inverter control 
are achieved through the existence of an Interval (namely Interval III) 
during which diode D.sub.1 is conducting. Interval III is actually the 
remainder of the cycle, after the completion of the energy-storing 
Interval I and the energy-delivering Interval II, during which all of the 
switches S.sub.1, S.sub.2 or S.sub.3 are open. The flux reference 
generating circuit 40, which controls the primary winding control 
subcircuit 20 (FIG. 4), is used for this purpose. 
The flux level reference generator 40, as illustrated in FIG. 7, includes a 
sensor 41 for sensing the absolute value of the output voltage, which is 
applied as a first input A into a differential amplifier 42 functioning as 
a subtractor circuit. Differential amplifier 42 includes a second input B 
from the reference voltage generator 50, converted to an absolute value by 
circuit 43. The latter circuit subtracts input B from input A, and thereby 
produces an output signal representing the difference between the required 
output voltage and the actual output voltage. This signal is fed to an 
integrator 44 via diode 45 which increases the flux level reference signal 
used for controlling the instant of opening of switch S.sub.1 to terminate 
the energy-storing Interval I. 
On the other hand, this latter reference signal is decreased according to 
an exponential decay during Interval III during when all of the switches 
S.sub.1, S.sub.2 or S.sub.3 are open. The flux reference signal outputted 
from integrator 44 is coupled to its input via switch 46 which is closed 
during Interval III by the control circuit 14. 
It will thus be seen that when the energy outputted from the inverter is 
too low, the absolute value of input A will be smaller than that of input 
B during the energy-storing Interval I; accordingly, the voltage at the 
output of the voltage amplifier 42 will be negative, thereby supplying a 
negative input to the integrator 44. When the input to the integrator is 
negative, its output increases positively. This increases the output of 
the flux reference signal which closes switch S.sub.1 in the primary 
winding circuit for a longer period of time, thereby accumulating more 
energy in the energy-storing Interval I. The above arrangement thus 
corrects the inverter circuit when the output voltage is too low because 
the energy stored in the transformer is too low. 
However, when the energy stored in the transformer is too high, the output 
voltage of the inverter circuit will not be affected because that is 
controlled by the comparator. However, Interval III becomes too long 
because the output capacitor 16 (FIG. 2) charges at a faster rate when the 
energy being delivered is too high. The output of the integrator 44 is 
always connected to its input during Interval III, producing an 
exponential decay. Therefore, if Interval III becomes too long, the 
exponential decay significantly reduces the flux level reference signal 
outputted at 48 to control the transformer primary winding switch S.sub.1. 
Variations in the Primary and Seconding Winding Circuits (FIGS. 2a, 8a-8c) 
FIG. 2a described above illustrates a variation that may be made in the 
transformer primary winding circuit when it is not necessary to use the 
excess energy in Interval III for recharging the power supply. FIG. 8a 
illustrates another variation that may be used in the transformer primary 
winding circuit; and FIGS. 8b and 8c illustrate variations that may be 
made in the transformer secondary winding circuit. 
Thus, FIG. 8a illustrates the use of two diodes D.sub.1a, D.sub.1b and the 
two switches S.sub.1a, S.sub.1b in the transformer primary winding 
circuit, instead of a single diode D.sub.1 and a single switch S.sub.1. 
During the normal operation, both switches S.sub.1a, S.sub.1b would be 
opened and closed together. However, if it is desired to short-circuit a 
winding in the primary winding circuit, as described above with respect to 
FIG. 2a in order to save the excess energy in the transformer and not to 
use it for recharging power supply, one of the switches (e.g., S.sub.1b) 
may be used as switch S.sub.4 in FIG. 2a for this purpose. 
FIG. 8b illustrates a variation that may be made in the transformer 
secondary winding circuit, wherein, instead of using two unidirectional 
switches S.sub.2, D.sub.2 and S.sub.3, D.sub.3, the secondary winding 
circuit includes a bridge having four arms with a unidirectional switch in 
each of the four arms. Thus, two arms include the two switches S.sub.2a, 
S.sub.2b and their diodes D.sub.2a, D.sub.2b, corresponding to switch 
S.sub.2 and diode D.sub.2 in FIG. 2; and the other two arms include 
switches S.sub.3a, S.sub.3b and diodes D.sub.3a, D.sub.3b, corresponding 
to switch S.sub.3 and diode D.sub.3 in FIG. 2. 
FIG. 8c illustrates a further variation in the secondary winding circuit 
including two unidirectional switches, comprising S.sub.2 ' and D.sub.2 ' 
in series with one secondary winding and N.sub.3 ', and switch S.sub.3 ' 
and diode D.sub.3 ' in series with the other secondary winding N.sub.4 '. 
Uninterruptable Power Supply (FIGS. 9 and 10) 
The above described inverter is particularly useful for uninterruptable 
power supplies, Thus, by merely adding a winding and a switch, the 
inverter obviates the need for a separate charger required in the 
conventional uninterruptable power supply. FIGS. 9 and 10 illustrate two 
systems which may be used for this purpose. 
The system illustrated in FIG. 9 is one wherein the input, output and 
battery are all voltaicly isolated from each other and from the line. 
In FIG. 9, the inverter as described above, and as illustrated particularly 
in FIG. 2, is included within box 60, and the elements therein 
corresponding to those in FIG. 2 are identified by the same reference 
characters to facilitate understanding. To enable the inverter 60 to be 
used as an uninterruptable power supply it is only necessary to add a 
further switch S.sub.5, and a further coil N.sub.5 in the primary winding 
circuit of the transformer in the inverter and series-connected to the 
rectifier 62 of the power supply mains 64. 
During normal operation of the circuit, the power is supplied from the 
power supply mains 64, rectifier 62, and reservoir capacitor 65, to the 
load via the output capacitor 16. During this operation, switch S.sub.5 is 
controlled, instead of switch S.sub.1. That is, switch S.sub.5 is closed 
at the beginning of the cycle in order to start the energy- storing 
Interval I, and is opened to terminate that Interval and to start Interval 
II. During this normal operation, the active secondary winding switch 
S.sub.2 or S.sub.3 (according to the polarity of the output voltage 
desired) is controlled as described above, the active switch being closed 
to start Interval II, and being opened to end that Interval and to start 
Interval III, during which the excess energy not delivered to the load may 
used for recharging the DC power supply. During Interval III, the battery 
2 of the inverter 60 may be recharged via diode D.sub.1 in the same manner 
as described above with respect to FIG. 2. 
Now, should there be an interruption in the supply mains 64, the battery 
supply 2 is now used for supplying the load in the same manner as 
described above with respect to FIG. 2. 
FIG. 10 illustrates another arrangement wherein the inverter, therein 
designated 70, is used in an uninterruptable power supply. In the system 
illustrated in FIG. 10, the battery of the inverter is voltaicly coupled 
to the supply mains 74 via its rectifier 72, so that the battery is 
continuously and controllably charged by excess energy not delivered to 
the load and returned to the battery in Interval III as described above. 
During the normal operation of the system, when the load is supplied from 
the supply mains 74 via rectifier 72, the additional switch S.sub.5, 
corresponding to switch S.sub.5 in FIG. 9, is controlled in the same 
manner as described above with respect to switch S.sub.1 ; that is, it is 
closed at the start of each cycle and is opened at the end of the 
energy-storing Interval I. During this normal operation, the active 
secondary-winding switch S.sub.2 or S.sub.3 in the secondary winding 
circuit is controlled in the same manner as described above with respect 
to FIG. 2, the active switch being closed at the start of Interval II and 
being reopened at the end of that Interval and at the start of Interval 
III when the excess energy may be returned to the battery. 
However, when the mains supply 74 is interrupted, switches S.sub.1a and 
S.sub.1b are now used for controlling the transformer primary winding 
circuit, being closed at the start of the energy-storing Interval I and 
opened at the end of that Interval and the start Interval II, in the same 
manner as described above with respect to FIG. 2. 
It will be noted that the primary winding circuit in the system illustrated 
in FIG. 2 is similar to that illustrated in FIG. 8a. It will be 
appreciated, however, that it could be of other constructions, such as 
that illustrated in FIG. 2 or 2a. 
Standby Power Supply (FIG. II) 
The four-quadrant operation of the illustrated inverter makes it 
particularly useful in a standby power supply FIG. 11 illustrates one such 
arrangement wherein the inverter, indicated by the elements within block 
80, is substantially of the same construction and operates in 
substantially the same manner as described above, and is connected to the 
load in parallel with the power supply mains 82. 
In the configuration illustrated in FIG. 11, the inverter is connected in 
parallel with the power supply mains by a thyristor network 84 and 
operates at a somewhat lower voltage than the line voltage, so that the 
battery is charged during the operation of the system in a continuous 
manner in the second and fourth quadrants. However, whenever the power 
supply mains 82 fails to delivery the required line voltage, thyristor 
network 84 ceases to conduct, whereby the inverter circuit now supplies 
the power to the load and continues to do so until normal power is 
restored to the supply mains. 
Modified Sequence of Control (FIG. 12) 
The magnetic flux diagram illustrated in FIG. 12 corresponds to the diagram 
illustrated in FIG. 3 except that, whereas in FIG. 3 the starting and 
termination points of the interval (therein Interval III) during which 
excess energy is delivered to the power supply are fixed in the cycle, in 
FIG. 12 this interval (therein designated Interval II) is not fixed for 
each cycle, but rather "floats". This requires that the control sequence 
be changed so that the cycle in the embodiment of the present application 
begins with conduction in the secondary winding circuit, rather than in 
the primary winding circuit. 
More particularly, in the control sequence illustrated in FIG. 12 , the 
primary winding control circuit opens switch S.sub.1 at the beginning of 
each cycle and closes switch S.sub.1 at a subsequent point in the cycle 
when, at the end of the cycle, the energy stored in the transformer would 
reach a predetermined value; and the secondary winding circuit control 
subcircuit closes switch S.sub.2 at the beginning of the cycle, and opens 
switch S.sub.2 when the voltage at the output of the secondary winding 
circuit reaches a predetermined value. Thus, as shown in FIG. 12, a first 
interval (Interval I) is started by the opening of switch S.sub.1 and 
closing of the active secondary winding switch (S.sub.2 or S.sub.3) during 
which energy in the transformer is delivered to the load; a second 
interval (Interval II) is started by the opening of the active switch 
S.sub.2 or S.sub.3 during which excess energy in the transformer is 
returned to the power supply; and a third interval (Interval III) excess 
energy is stored in the transformer. It will thus be seen that the excess 
energy delivered to the power supply occur during Interval II, which 
starts by the opening of switch S.sub.2 when the voltage at the output of 
secondary winding which is a predetermined value; and this interval ends 
by the closing of switch S.sub.1 at that point in the cycle when, at the 
end of the cycle, the energy stored in the transformer would reach a 
predetermined value. The latter point is thus a "predicted" point, and 
circuitry is provided, as will be described more particularly below, for 
predicting that point ending Interval II 
Thus, as seen in the diagram of FIG. 12, since during Interval I energy 
stored in the transformer is delivered to the load, the magnetic flux in 
the transformer decreases, as indicated by the descending line Ia; but if 
the load is reactive, then energy in the load is transferred to the 
transformer, as indicated by the ascending line Ib. 
Interval II, when the excess energy in the transformer is delivered to the 
power supply, is normally indicated by the descending line IIa or IIb, 
respectively. However, if the modification illustrated in FIG. 2a is used, 
including a switch which short circuits one of the coils, then the level 
of the energy in the transformer stays constant, as indicated by line IIc. 
Interval III in FIG. 12, when the energy stored in the transformer is 
delivered to the load, is indicated by the ascending line IIIa, IIIb or 
IIIc, respectively. 
Measurement of Transformer Magnetic Flux (FIG. 13) 
FIG. 13 illustrates one form of primary winding control subcircuit, 
generally designated 100, that may be used for controlling switch S.sub.1 
in the primary winding circuit in accordance with the sequence illustrated 
by the diagram of FIG. 12. This circuit corresponds to circuit 20 in FIG. 
5 , but is designed so that it opens switch S.sub.1 at the beginning of 
each cycle and closes the switch at a subsequent point in the cycle when, 
at the end of the cycle, the energy stored in the transformer would reach 
a predetermined value. This value is predetermined during each cycle by a 
flux demand estimator, generally designated 102 included in the primary 
winding control subcircuit 100, which generates a voltage corresponding to 
the required flux to satisfy the load. Preferably, the generated voltage 
corresponds to a reference flux depending on the load, but may also 
correspond to a constant reference flux. 
More particularly, circuit 100 in FIG. 13 includes a "gain block" unit 104, 
which may be an amplifier or attenuator changing scales, connected across 
the input voltage, as shown by connection 106, to generate a voltage 
corresponding to the input voltage; and a sawtooth generator 108 also 
connected to the input voltage connection 106 for generating a further 
voltage also depending on the input voltage but varying with time. The 
three voltages produced by units 102, 104 and 108 are inputted into a 
summing circuit 110 which adds the outputs of the flux-demand estimator 
102 and the sawtooth generator 108, and subtracts the output from the gain 
block circuit 104, to output a voltage which is applied to one input of a 
comparator 112. 
The other input of comparator 112 is from a magnetic flux-measuring circuit 
114, which outputs a voltage corresponding to the flux in the transformer 
T.sub.r. Comparator 112 controls a flip-flop 115, which in turn controls 
the switch S.sub.1 in the primary Winding of the transformer T.sub.r. 
Flip-flop 115 is reset at the beginning of each cycle to open switch 
S.sub.1, and thus to end the energy-storing Interval I (line Ia or Ib, 
FIG. 12) during which the active switch (S.sub.2 or S.sub.3) in the 
secondary winding circuit is closed, so that energy stored in the 
transformer is delivered to the load. Switch S.sub.1 remains open during 
Interval II (line IIa, IIb, or IIc, FIG. 12) when excess energy is 
delivered to the power supply, but closes to end Interval II at the 
subsequent point in the cycle when, at the end of the cycle, the energy 
stored in the transformer would reach a predetermined value as determined 
by comparator 112, which controls flip-flop 115 to close switch S.sub.1, 
as described above. The closing of switch S.sub.1, end Interval II, starts 
Interval III, during which energy is stored in the transformer, this 
interval being ended by the end of the cycle, when switch S.sub.1 is 
opened by the resetting of flip-flop 115. 
It will thus be seen that circuit 100 illustrated in FIG. 13 operates to 
close switch S.sub.1 according to the following approximation equation: 
##EQU1## 
wherein FLX is the instantaneous flux in the transformer; 
REF-FLUX is the flux to be established at the end of the cycle; 
V.sub.b is the source voltage at the primary side; 
N.sub.p is the number of turns of the primary winding; 
T is the period of the cycle; and 
t is the instantaneous time. 
Thus, summing circuit 110 continuously adds the REF-FLUX from the 
flux-demand estimator 102 and the sawtooth signal from the sawtooth 
generator 108 (whose output is dependent on the source voltage and varies 
with time), and subtracts a fixed value from gain block 104, which is 
dependent on the source voltage; and applies this sum to one input of 
comparator 112. This sum is continuously compared with the FLUX from the 
flux-measurement block 114; and at the instant FLUX is smaller than the 
sum from the summation circuit 110, the flip-flop 115 is set to close 
switch S.sub.1 in the primary winding, thereby ending Interval II during 
which excess energy is delivered to the power supply, and starting 
Interval III during which energy is stored in the transformer. Switch 
S.sub.1 is reopened at the beginning of the next cycle by resetting of 
flip-flop 115. 
Flux-Measurement Circuit (FIG.14) 
Several methods are commonly known for measuring the magnetic flux in the 
core of a transformer. However, the known methods are generally difficult 
for pratical implementation in the described system. 
FIG. 14 illustrates a flux-measurement system which may be used for block 
114 in FIG. 13 for measuring the magnetic flux in the transformer Tr. This 
system is based on integrating the voltage measured across one of the 
windings of the transformer, and correcting the integration constant 
during the part of each cycle when switch S.sub.1 is closed, by disabling 
the integration of the measured voltage and replacing momentarily the 
integration of the voltage by a known variable related to flux. In the 
described preferred embodiment, the voltage is measured across winding 
N.sub.D containing diode D.sub.1 ; and the known variable which is related 
to flux, and which is momentarily replaced in the integration, is the 
current flowing through that winding. It will be appreciated, however, 
that the integration can be based on the voltage measured across any 
winding of the transformer, and that the known variable which is 
momentarily used in the integration process for correcting the integration 
constant may be another known variable related to the flux, e.g., current 
in another winding. 
Thus, flux-measurement system illustrated in FIG. 14 includes an auxiliary 
winding Nd in the circuit including the diode D.sub.1, for measuring the 
voltage across the transformer in order to perform the integration. The 
circuit in FIG. 14 further includes an inverting integrator, generally 
designated 120, comprising an amplifier 122, capacitor 124, and resistor 
126, for integrating the voltage across the transformer as sensed by 
winding Nd. Circuit 120 further includes resistors 128 and 130, and switch 
S.sub.x ; switch S.sub.x is closed together with switch S.sub.1 in the 
primary winding. The circuit illustrated in FIG. 14 further includes a 
current measuring circuit 132 for measuring the current in the primary 
winding of the transformer Tr. 
It will be seen that the instantaneous output voltage (V.sub.OUT) of the 
integrator circuit 120 is given by the expression: 
##EQU2## 
When the current starts to flow in the primary winding, switch S.sub.x is 
closed, and the output voltage is forced to be proportional to the real 
flux. Thus, the integration circuit 120 integrates the measured voltage 
with respect to time; but when the primary winding switch S.sub.1 is 
closed, switch S.sub.x is also closed so as to correct the integration 
constant, by disabling the integration of the measured voltage, and 
instead forcing the output to track the real current flow, i.e., the 
output of the current measuring circuit 132. 
Regulating the Energy Level (FIG. 14) 
In order to reduce the losses in the power circuits, the flux (current 
levels) should be kept as low as possible. This regulation may be 
implemented by two methods: one is by mathematically calculating REF-FLUX; 
and the other is by utilizing an energy dependent feedback system. 
FIG. 14 illustrates an energy dependent feedback system. based on a 
mathematical calculation. For the mathematical calculation approach, the 
following three cases are distinguished: 
(1) non-continuous flux in the transformer; 
(2) continuous flux, with partial energy discharge during the respective 
interval; and 
(3) continuous flux combined with flux (and energy) retention during this 
interval. 
The mathematical expression presenting the conditions of case (3) is: 
##EQU3## 
wherein: V.sub.i is the voltage of the voltage source; 
V.sub.o is the output voltage; 
I.sub.o is the output current; 
N.sub.p is the number of turns of the primary winding; 
N.sub.s is the number of turns of the secondary winding; and 
REF-FLUX is the calculated reference flux. 
FIG. 14 illustrates an electrical circuit for measuring the REF-FLUX 
according to the above mathematical expression. 
Thus, the circuit includes two multiplier/divider circuits 140 and 142, and 
two summation circuits 144 and 146. 
The expression (k.sub.1 I.sub.o) is inputted into multiplier circuit 140 
and is multiplied by the output from summation circuit 144; the latter 
circuit sums Np multiplied by V.sub.o, and N.sub.s multiplied by V.sub.i. 
The product is divided by the quantity Np times Vi in circuit 140, and the 
output is applied to summation circuit 146. 
The expression (k.sub.2 V.sub.o) is multiplied by N.sub.s and V.sub.i in 
circuit 142 and is divided by the output from summation circuit 144; and 
the result is outputted to the summation circuit 146. Thus, the REF-FLUX 
output from the latter circuit represents the sums of the outputs of the 
multiplier/divider circuits 140, 142. 
While the invention has been described with respect to several preferred 
embodiments, it will be appreciated that many other variations, 
modifications and applications of the invention may be made.