In a voltage regulator converter circuit comprising an input 20 and an output 25, an input signal can be controlled by applying a high frequency switching signal (1 KHz-30 MHz) to an input circuit 12, 13, 16 and 17. An output of the circuit is obtained via an inductor 28 and a capacitor 29. The input can be DC to 500 Hz and the output can be DC or AC. Switches 12, 13, 16 and 17 can be thyristors, bipolar transistors, field effect transistors or IGBi transistors. Diodes 11, 14, 15 and 18 are connected in series with the switches 12, 13, 16 and 17. In an alternative embodiment (FIG. 9) the four switches can be connected in series between an AC input and an output is obtained from a center point of the series connection via an inductor (106) and a capacitor (107). A further aspect (FIG. 10) relates to transformed coupled input and output switching stages.

DESCRIPTION 
The present invention relates to the field of electrical power supplies, 
and particularly although not exclusively to an AC voltage regulator.

A conventional AC variable transformed (a variac) for stepping down a mains 
voltage, for example 240 volts AC to a reduced AC voltage, comprises an AC 
input terminal, across which is connected an inductive winding, and an AC 
output terminal, which takes power from the winding at a selectable 
voltage, depending upon where a wiper blade is positioned along the 
winding. The wiper is typically a rotating wiper which rotates across the 
winding which is formed in a substantially cylindrical or ring shape. The 
wiper may be driven by a servo motor, in order to automatically move the 
wiper, thus varying the output voltage, in response to a control signal. 
However, the conventional variac has the problem of high weight, large 
size, poor response time in moving the wiper blade, and produces noise 
which is fed back onto the mains supply, and at the output terminal. 
In the field of conventional DC step down regulators an example of which is 
as shown in FIG. 1 of the accompanying drawings, a conventional DC 
regulator is provided with a DC input, a DC output, and a pair of switches 
comprising first and second switches 1,2 respectively, which operate at 
high frequency. The output voltage at the output terminal is proportional 
to the mark space ratio of the switching signal which is used to control 
the switches. For example if the pair of switches 1,2 operates to connect 
the DC output to the DC input with a 50% mark/space ratio, the output 
voltage V.sub.out provided at the output terminal, after smoothing by the 
smoothing circuit comprising a capacitor 4 and an inductor choke 3, will 
be 50% of the input voltage V.sub.in ie. related by the expression: 
EQU V.sub.out =50% V.sub.in 
Similarly, for a 20% mark/space ratio for the switches 1,2 connecting the 
DC input to the DC output, the output voltage will be 20% of the input 
voltage, ie: 
EQU V.sub.out =20% V.sub.in 
In the conventional DC regulators it is essential that the pair of switches 
1 and 2 do not close simultaneously, as this will cause short circuit of 
the DC input supply. Thus the switches 1,2 are controlled so that the 
switch 1 opens just before the switch 2 closes, and conversely the switch 
2 opens just before the switch 1 closes. In the period in each switching 
cycle, in which both the switches 1 and 2 are open is called the "dead 
time". 
During the dead time, the choke inductor 3 is open circuit, and 
consequently, the stored energy in the choke produces high voltages which 
occur across the switches 1,2. This often leads to catastrophic failure of 
the circuit elements, particularly the switches. 
The conventional circuit of FIG. 1 may be modified by the inclusion of a 
respective diode connected in parallel with each of the switches 1,2 to 
provide the arrangement as shown in FIG. 2 of the accompanying drawings. 
In the circuit of FIG. 2, during the dead time a current path is maintained 
through one or other of the diodes, depending on the direction of current, 
and current may be returned to the DC rails. 
Due to the inclusion of the first and second diodes, the voltage at a node 
X, between the switches 1,2 is unable to fluctuate outside the incoming DC 
input voltage plus the diode's forward voltage drop, so that the voltage 
occurring across either switch cannot exceed the DC supply voltage by more 
than the forward voltage drop of a conducting diode. 
In FIGS. 3 to 5, there are shown various modes of operation of the circuit 
of FIG. 2, being in FIG. 3, with the first switch 1 closed, in FIG. 4 with 
both of the first and second switches 1,2 open (corresponding to the dead 
time in the switching cycle) and in FIG. 5 showing the second switch 2 
closed. The dotted lines show the current flow in each of these 
conditions. Referring to FIG. 4, when both of the first and second 
switches are open, there exists a current path between the DC output 
terminals, via the second diode 7. Similarly, referring to FIG. 5, when 
the second switch is closed, a current path flows between the DC output 
terminals via the closed second switch. 
Referring to FIG. 3, when the first switch is closed and the second switch 
is open, current may flow directly between the DC input rails and DC 
output rails, and the voltage across the DC output is limited to be no 
more than the voltage occurring across the DC input supply. 
Specific embodiments to the present invention aim to provide an AC voltage 
regulator which has reduced size and weight compared to conventional wire 
wound variable transformers of the type including a wiper blade driven 
across a winding by manual means or by a servo motor. 
Embodiments of the present invention seek to provide an easily controlled 
AC voltage regulator capable of transforming alternating current of a 
first voltage into alternating current of a second voltage. 
According to a first aspect of the present invention there is provided a 
voltage regulator circuit comprising: 
an AC input having first and second input terminals for receiving an AC 
input signal; 
an AC output having first and second output terminals for outputting an AC 
output signal; and 
switching means for alternatively connecting and disconnecting the input 
terminals with the output terminals so as to allow or prevent transmission 
of the input signal to the output, the switching means being arranged to 
switch in response to a control signal, 
wherein the switching means comprises a first switching means and a second 
switching means, controlled by the control signal so that during one half 
cycle of the AC input the first switching means is closed and the second 
switching means is alternatively opened and closed, and during the inverse 
half cycle of the AC input the second switching means is closed and the 
first switching means is alternatively opened and closed. 
The circuit may have an advantage of being less bulky than a prior art 
variac type AC voltage transformer or prior art fixed output auto 
transformer. 
Preferably, a said control signal is of a frequency higher than a frequency 
of said AC input signal. 
Preferably, the circuit further comprises a control means for generating 
the control signal for controlling the switching means. 
By varying a parameter of the control signal, such as a duty cycle or phase 
relationship of the control signal, the proportion between the time in 
which the input is connected to the output, and consequently the 
proportion between the time in which the AC input signal is transmitted to 
the output, to appear as the output signal, can be varied. Further, as the 
rms voltage of the output signal is dependant on the proportion of the AC 
input signal transmitted to the output, the voltage of the output signal 
may be controlled by varying the control signal parameter(s). 
The circuit may have an advantage of allowing step down in voltage from an 
AC mains supply of a first, higher voltage, to an AC output of a second, 
lower voltage, in an easily controllable manner. The circuit may also be 
used to step up an AC supply of a lower voltage to an AC output voltage of 
a higher voltage. The circuit may be capable of responding to changes in 
required output voltage more rapidly than conventional variacs or 
conventional auto transformers, and of producing an adjustable, stable AC 
output from an AC input source, without producing significant frequency 
harmonics fed back onto the input. 
Preferably, the switching means comprises a first switching means and a 
second switching means. Preferably said first switching means is connected 
between said first input terminal and said first output terminal. 
Preferably, said second switching means is connected between said second 
input terminal and said first output terminal. 
Preferably, the first switching means operates to alternately connect and 
disconnect the first input terminal with the first output terminal. 
Preferably the second switching means acts to alternately connect and 
disconnect the first output terminal with at least one of the second input 
terminal and/or the second output terminal. 
Preferably, the first and second switching means are alternately switched 
during single cycle of a control signal. Preferably the frequency of the 
switching cycle is in the range 1 kHz-100 kHz. An input frequency of the 
AC input supply may lie in the range DC to 500 Hz. 
Preferably, the first and second switching means operate alternately in the 
switching cycle such that when the first switching means is 
non-connective, ie. in an OFF condition, the second switching means 
alternates at high frequency ON and OFF, and vice versa. 
The first and second switching means are preferably simultaneously each non 
connecting for a pre determined time in each said switching cycle. 
Preferably, in one said switching cycle in which the first and second 
switching means are alternately turned ON and OFF, the ratio of the 
duration in which the first switching means is turned ON, to the duration 
in which the second switching means is turned ON, being variable. 
Said control means may comprise a pulse width modulation circuit responding 
to an input from means such as a potentiometer, computer or a feedback 
circuit or a combination of these. The control means may provide a control 
signal comprising a pulse width modulated signal having a frequency in the 
range of the operating frequency of the switching means, for example 1 
kHz-100 kHz. 
Preferably, said first switching means comprises first and second 
electronic switches. Said first and second electronic switches preferably 
each comprise a transistor or other semiconductor switching device, 
operable in response to a said control signal. 
Preferably, said second switching means comprises third and fourth 
switches. Said third and fourth switches are preferably operable in 
response to said control signal. 
Preferably, said first and second switching means are connected between 
said first and second input rails, the first and second switching means 
being connected at a common connection node, which is also connected to 
said first AC output terminal. 
Preferably said circuit comprise means for dissipating current from said 
common node, when said first and second switching means are both non 
conductive, or "OFF". 
Preferably said current dissipating means comprise one or a plurality of 
diodes, preferably, current is arranged to dissipate via a single 
transistor and a single diode at any given time. 
A said switch and a said diode may be included in a single integrated 
circuit. 
Preferably, there is provided a filter between the output terminal and the 
input terminal. Said filter is preferably provided between the switching 
means and the output. The filter preferably comprises an inductor and a 
capacitor. The conductor may be connected serially between the switching 
means and the first or second output terminal. The capacitor may be 
connected in parallel with the first and second output terminal. 
According to a second aspect of the present invention, there is provided a 
transformer circuit for transforming an input signal having an input 
voltage to an output signal having an output voltage, the transformer 
circuit comprising: 
an input having first and second input terminals for receiving the input 
signal; 
an output having first and second output terminals for outputting the 
output signal; 
switching means for alternately connecting and disconnecting the input 
terminals with the output terminals so as to allow or prevent transmission 
of the input signal to the output, the switching means being arranged to 
switch in response to a control signal; and 
an isolating transformer for isolating the input from the output. 
Said input signal may comprise a signal of variable voltage. Said input 
signal may comprise an AC or a DC signal. 
Said output signal may comprise a signal of varying voltage with time. The 
output signal may comprise an AC or a DC signal. 
Preferably said switching means comprises an input switch between the input 
and a primary of said transformer; and an output switch between the output 
and a secondary of said transformer. 
Preferably said input switch is controllable in response to an input 
control signal. Preferably said output switch is controllable in response 
to an output control signal. 
Preferably said input switch is arranged to connect and disconnect said 
input signal with said transformer primary in response to said input 
switch control signal. Preferably said output switch is arranged to 
connect said secondary transformer winding with said output in response to 
said output switch control signal. 
By controlling a parameter of said input switch control signal, a time 
proportion between a period of connection of said input signal with said 
transformer primary and a period of disconnection of said input signal 
with said primary may be varied. By controlling a parameter of said output 
switching signal, a proportion between a period of connection of said 
transformer secondary with said output and a period of disconnection 
between said secondary and said output may be controllably varied. 
By varying of said parameter of said input switch control signal and/or 
said output switch control signal, an rms voltage of said output signal 
may be controllably varied. 
Suitably, the filter comprises a capacitor connected between the first and 
second output terminals, and an inductor, preferably a choke, connected in 
series with the first output terminal between the first output terminal 
and the switching means. 
The invention includes a method of transforming or regulating an input 
signal having an input voltage to produce an output signal having an 
output voltage, the method comprising the steps of alternately allowing or 
preventing transmission of the input signal by means of a switch, the 
switch being controllable to vary the ratio of the time periods in which 
the input signal is transmitted or prevented, within a switching cycle. 
For a better understanding of the invention, and to show how embodiments of 
the same may be carried into effect, reference will now be made, by way of 
example, to the accompanying diagrammatic drawings, in which: 
FIG. 6 shows a first AC voltage regulator according to a first specific 
embodiment of the present invention; 
FIG. 7 shows the first AC voltage regulator during a positive half cycle of 
the AC input signal; 
FIG. 8 shows the first AC voltage regulator during a negative half cycle of 
the AC input signal; 
FIG. 9 shows a second AC voltage regulator according to a second specific 
embodiment of the present invention; and 
FIG. 10 shows a third specific embodiment according to the present 
invention. 
Referring to FIG. 6 of the accompanying drawings, a first AC voltage 
regulator circuit comprises an alternating current input 20 comprising 
first and second input terminals 30,31 connected respectively to a first 
voltage input rail 21 and a second voltage input rail 22; connected 
between the first and second input voltage rails, first and second 
switching means 23,24 respectively, connected in series across the input 
voltage rails, the first switching means comprising a first electronic 
switch 12 connected in antiparallel with a third electronic switch 16, and 
the second switching means comprising a second electronic switch 13 
connected in antiparallel with a fourth electronic switch 17; connected in 
series on either side of the first and second switching means, first and 
second diodes 11,14 respectively, the first and second diodes arranged for 
conduction of current in a first direction between the first and second 
input rails; connected between the switching means and the input rails, on 
either side of the second switching means respectively, a third diode 15 
and a fourth diode 18, the third and fourth diodes arranged for conduction 
of a current between the first and second input rails in an opposite 
direction to the current direction permitted by the first and second 
diodes 11,14; an AC output 25 comprising first and second output terminals 
32,33 respectively, the first output terminal 32 being connected to a 
first output rail 26, and the second output terminal 33 being connected to 
a second output rail 27; a connection between the first and second 
switching means being commoned at a node X, the commoned connection being 
connected via a series inductor 28 to the first output rail 26; and 
connected between the output rails 26,27 of the AC output terminal 25, a 
capacitor 29, the capacitor 29 and the inductor 28 forming a smoothing 
filter. 
By "anti parallel" referred to above in relation to the connections between 
the first and third electronic switches 12 and 16 respectively, this may 
include the situation where two diodes are connected in series. Similarly, 
for the connection between the second electronic switch 13 and the fourth 
electronic switch 17. 
The circuit of FIG. 6 operates as follows: 
Referring to FIG. 7 of the accompanying drawings, an alternating current 
input is applied to the AC input terminals 30,31. During a positive half 
cycle of the input waveform signal, the second input rail 22 is held 
negative with respect to the first input rail 21. The third and fourth 
switches 16,17 respectively are held "ON". The third and fourth diodes 
15,18 respectively are reverse biased, and therefore non conductive, and 
prevent short circuit of the input supply rails/terminals via the third 
and fourth switches. 
The first and second switches 12,13 are operated via a control means to 
alternately connect the common node "X" to which the first output rail is 
connected via the inductor 26, either to the first input rail 21 or to the 
second input rail 22. During one switching cycle of the control means the 
average voltage at the common node "X" is substantially proportional to 
the ratios of "on" time between the first and second switches 12,13 or to 
the "Mark to Space Ratio" expressed as a percentage, times the 
instantaneous voltage across the input terminals 21,22. The operating 
frequency of the first and second switches is typically in the order of 
several tens of kHz, whereas the frequency of the AC input supply may be 
typically of the order of 50 Hz. The low pass LC filter comprising the 
inductor 28 and the capacitor 29 is included to reject the high frequency 
to prevent this appearing at the output terminals. 
In each high frequency switching cycle, between switching the first switch 
12 "OFF" and switching the second switch 13 "ON" there is a small "dead 
time" when both the first and second switches are open, (or "OFF") in 
order to prevent a condition in which the first and second switches both 
conduct. If both switches were to simultaneously conduct, this would lead 
to short circuit of the input supply, iva the first and second diodes 
11,14, which, being forward biassed, are both conductive. 
During the "dead time" when the first and second switches are "OFF" any 
current still flowing in the inductor 28 or in the load, which itself may 
be inductive, is given two paths via which it may flow, either via switch 
17 and diode 18 as shown by the dotted line in FIG. 7 or via the switch 16 
and diode 15 as shown by the solid line, thus protecting the first and 
second switches 12,13, from high voltages that could otherwise result in 
damage. 
Referring to FIG. 8 of the drawings, during a negative half cycle of the 
input supply, the second rails 22,27 and second terminals 31,33 become 
positive with respect to the first rails 21,26 and first terminals 30,32. 
The first and second switches 12,13 are held "ON". The third and fourth 
switches 16,17 are operated via the control means to alternately connect 
the common node "X" to which the first output rail is connected via the 
inductor 28, either to the first input rail 21 or to the second input rail 
22. During one switching cycle of the control means the average voltage at 
the common node "X" is substantially proportional to the ratios of the 
"ON" time between the third and fourth switches 16,17 or to the mark to 
space ratio expressed as a percentage, times the instantaneous voltage 
across the input terminals 21,22. The mark space ratio with which the 
third and fourth switches are operated in the negative input supply half 
cycle is preferably the same as the mark space ratio with which the first 
and second switches are operated during the positive input supply half 
cycle. 
During the "dead time" when the third and fourth switches are "OFF" any 
current still flowing in the inductor 28 or in the load, which itself may 
be inductive, is given two paths via which it may flow, either via second 
switch 13 and second diode 14, as shown dotted in FIG. 8, or via the first 
switch 12 and the first diode 11 as shown by the solid line, thus 
protecting the third and fourth switches 16,17 from high voltages that 
could otherwise result in damage. 
The overall result at the output terminal is an AC output which is derived 
from the signal at the node X, when smoothed by the filter circuit 
comprising the inductor 28 and the capacitor 29, comprises an output 
signal of the same fundamental frequency as the input supply signal, but 
of reduced voltage. The voltage relationship between the output signal and 
the input signal can be written as follows: 
EQU AC.sub.out AC.sub.in .times.mark space ratio 
Referring to FIG. 9 of the accompanying drawings there is shown a second AC 
voltage regulator circuit according to a second specific embodiment of the 
present invention. The second AC voltage regulator circuit comprises an AC 
input 100 having first and second input terminals connected respectively 
to a first input rail 101 and a second input rail 102; an AC output 103 
comprising first and second output terminals connected respectively to a 
first AC output rail 104 and a second AC output rail 105; a filter circuit 
comprising an inductor 106, and a capacitor 107, the inductor connected in 
series with the first output rail 104, and the capacitor connected across 
the first and second output rails 104,105; a first switching means 110 
connected between the first AC input rail 101 and the first AC output rail 
104, a second switching means 111 connected between the second AC input 
rail 102 and the first AC output rail 104, the first and second switching 
means connected to each other at a common node X, and the first AC output 
rail 104 connected to the common node X, via the inductor 106. 
The first switching means comprises a first electronic switch 113 and a 
second electronic switch 114 serially connected with a first electronic 
switch. Connected in parallel across the first switch is a first diode 
117. Connected across the second switch is a second diode 118, the second 
diode arranged for conducting current in an opposite direction to the 
first diode. 
The second switching means 111 comprises a third electronic switch 115 
connected in series with a fourth electronic switch 116, the third and 
fourth electronic switches being connected together in series, between the 
common node X and the second AC input rail 102 and the second AC output 
rail 105. Across the third electronic switch 115 is a third diode 119, and 
connected in parallel across the fourth electronic switch 116 is a fourth 
diode 120, the third and fourth diodes arranged for conducting current in 
opposite directions to each other. 
The first diode is arranged to conduct current in the same direction as the 
third diode, and the second diode is arranged to conduct current in the 
same direction as the fourth diode. 
The first, second, third and fourth electronic switches are operable in 
response to a control signal for switching a current channel ON and OFF at 
a high frequency, in order of several tens of kHz, for example in the 
range 1 kHz-100 kHz. 
Preferably the electronic switches comprise semiconductor switching devices 
for example thyristors, bipolar transistors, field effect transistors or 
insulated gate bipolar transistors. Each switching means and its 
corresponding diode connected in antiparallel, may be integrated into a 
single package, or several switches and/or diodes may be packaged 
together. 
Operation of the second AC voltage regulator circuit is as follows. 
During a positive half cycle of the AC input signal, the first AC input 
rail 101 is positive with respect to the second AC input rail 102. The 
second and fourth switching devices 114,116 respectively are controlled to 
be conductive, ie. "ON" condition. The first and third switching devices 
113,115 respectively are operated via the control means at high frequency 
to alternately connect the common node "X" to which the first output rail 
104 is connected via inductor 106, either to the first input rail 101, or 
to the second input rail 102. During one switching cycle of the control 
means the average voltage at the common node "X" is substantially 
proportional to the ratios of "on" time between the first and third 
switches 113,115 or "Mark to Space Ratio" expressed as a percentage term, 
times the instantaneous voltage across the input terminals 101,102. The 
operating frequency of the first and third switches is typically in the 
order of several tens of kHz, whereas the frequency of the AC input supply 
may be typically of the order of 50 Hz. Therefore a low pass LC filter 
comprising inductor 106 and capacitor 107 is included to reject the high 
frequency switched voltage at node "X", and pass the low frequency of the 
input supply voltage. 
During the "dead time" when the first and third switches are "OFF" any 
current flowing in the inductor or in the load which may itself be 
inductive is given two paths via which to it may flow either via switching 
116 and diode 119 as shown by the dotted line or via switch 114 and diode 
117 as shown by the solid line thus protecting the first and third 
switches 113,115 from high voltages that could otherwise result in damage. 
During a negative half cycle of the AC input supply, the first input rail 
101 becomes negative with respect to the second input rail 102. The first 
and third switching devices are turned "on" so as to conduct, and the 
second and fourth switches 118,120 are switched ON and OFF alternately at 
high frequency, to chop the negative half cycle with a predetermined mark 
space ratio, so that the signal at the node X comprises chopped negative 
voltage with respect to the lower rail 102,105. During the "dead time" 
between switching the second and fourth switching devices, the second and 
fourth diodes 118,120 respectively, which are each in parallel with their 
respective switches, provide current path to the supply rails for 
dissipation of current from the node X. 
During the "dead time", current dissipates from the node X via the third 
switch 115, and the fourth diode 120. Current may also dissipate via the 
switch 113 and the diode 118. Thus the voltage at the node X is unable to 
float outside AC input voltage range. 
The specific embodiments to the present invention may have an advantage, 
that by controlling the switching frequency of the switching means, input 
frequencies from the range DC to 500 Hz may be voltage regulated without 
the production of significant harmonic currents or other significant mains 
interference. This may have an advantage of allowing the specific 
embodiments to be particularly suited for powering of input voltage 
sensitive equipment, such as, for example, computing apparatus. 
Circuits according to the specific embodiments may also have an advantage 
of providing an AC to AC voltage regulator of reduced weight and bulk 
compared to conventional AC variable transformers (variacs) or auto 
transformers. 
Typically, specific embodiments to the present invention may be able to 
maintain a preselected output voltage to within 1% of a preselected value, 
load and line regulation being typically achieved in less than 10 AC input 
signal cycles, from changing the input voltage or the load current. 
Specific embodiments to the present invention may have an advantage of 
being completely short circuit protected, due to the operation of the 
switching means. The Embodiments may be remotely controlled by a 
potentiometer or other control means, eg. computer control, and may be 
shut down within a time scale of 50 .mu.s. 
The embodiments may have an advantage of providing an AC variable voltage 
control device including a plurality of switching field effect 
transistors, a small amount of mains filtering, low cost control 
electronics and an output choke, mounted on a single printed circuit 
board. 
Typically the embodiments may be capable of providing power at six amps 
continuous power to 10 amp peak in regulated or non regulated embodiments. 
Embodiments may be capable of providing power of frequencies as low as DC. 
Embodiments may have an advantage of being able to respond to required 
voltage changes more quickly than conventional wire wound variacs, whilst 
being lighter, being remotely controllable, and being short circuit 
protected. The embodiments may have an advantage of providing very low 
electromagnetic interference loaded back onto an AC mains input supply. 
Referring to FIG. 10 of the accompanying drawings, there is shown an 
electronic isolating transformer according to a third specific embodiment 
of the present invention. 
The isolating transformer circuit comprises an input 200 consisting of a 
first input terminal 201 connected to an input supply rail AC1 and a 
second input terminal 202 connected to a second input supply rail AC2; an 
output 250 consisting of a first output terminal 251 connected to an 
output supply rail AC3 and a second output terminal 252 connected to a 
second output supply rail AC4; an input switching arrangement 210, and an 
output switching arrangement 211, the input and output switching 
arrangements being isolated from each other by a transformer 212, the 
transformer comprising for example a small ferrite transformer capable of 
operation of several tens of kHz; the input switching arrangement 
comprising a first input switch 200 comprising the first switching 
transistor 221 and a second switching transistor 222 connected in series 
with the first switching transistor, a second input switch 223 comprising 
a third switching transistor 224 and a fourth switching transistor 225 the 
third and fourth switching transistors being connected in series with each 
other, the first input switch being connected in series with the second 
input switch between the first input terminal 201 and the second input 
terminal 202; between the first and second input switches is connected one 
end of a primary winding of the transformer 212, another end of the 
transformer primary winding being connected to the first and second input 
terminals 201,202 via respective first and second series capacitors 
226,227; each of the first to fourth switching transistors being provided 
with a respective first to fourth parallel diode 228-231, the second 
diode, connected across the second transistor being arranged for 
conducting current in a direction opposite to the first diode connected 
across the first transistor, the third diode connected across the third 
transistor being arranged for conducting current in a direction opposite 
to the fourth diode connected across the fourth transistor, the first and 
third diodes being connected for conducting current in the same direction 
as each other, and the second and fourth diodes being arranged for 
conducting current in the same direction as each other. The output 
switching arrangement 211 comprising a first output switch 260 and a 
second output switch 261, the first and second output switches being 
connected in series with each other across the first and second output 
terminals 251,252 respectively, the first output switch comprising a fifth 
switching transistor 262 and a sixth switching transistor 263 and the 
second output switch comprising a seventh switching transistor 264 and 
connected in series with an eighth switching transistor 265. Each of the 
fifth to eighth switching transistors is provided with a respective fifth 
to eighth diode 266-269 respectively, the fifth diode being connected for 
conduction of current in a direction opposite to the sixth diode and the 
seventh diode being connected for conduction of current in a direction 
opposite to that allowed by the eighth diode, the fifth and seventh diodes 
being connected for allowing current in a same direction, and the sixth 
and eighth diodes being connected for allowing current flow in the same 
direction, the direction of flow of current allowed by the fifth and 
seventh diodes being opposite to the direction of current flow allowed by 
the sixth and eighth diodes. 
One end of a secondary winding of the transformer is connected to a node 
between the first and second output switches, whereas the other end of the 
secondary winding is connected between third and fourth capacitors 270,271 
to the first output terminal 251 and the second output terminal 252. Each 
of the first to eighth switching transistors may be controlled by one or 
more switching signals provided by a control means. 
Operation of the electronic isolating transformer of FIG. 10 will now be 
described. The electronic isolating transformer circuit of FIG. 10 is 
capable of transforming a DC or an AC input voltage presented at the first 
and second input terminals in either a step up or a step down mode to 
produce an AC or DC output signal at the first and second output terminals 
251,252. 
The circuit is capable of four quadrant operation, ie. power flow in either 
direction, between the input and the output or vice versa. Secondary power 
factor may be reflected to the primary with only a small extra leading 
element apparent when the circuit is on light load. 
Operation of the circuit of FIG. 10 is similar to the operation of the 
circuit of FIG. 9 as described previously. The first to fourth switching 
transistors 221-225, which are each preferably isolated gate bipolar 
transistors, are series connected across input supply rails AC1,AC2. Each 
transistor has its respective corresponding diode connecting in anti 
parallel between the collector and emitter. During a positive half cycle, 
the first and third transistors 221,224 are switched at high frequency by 
first and second control signals SIG. A and SIG. B respectively, which are 
in anti phase, such that when the first transistor is ON the third 
transistor is OFF and vice versa. The duty cycle of the ON times of the 
first and third switching transistors 221,224 is held at near 50:50 with a 
small amount of "dead time" during which both transistors are "OFF" to 
ensure that short circuiting of the input supply does not occur during 
switching transistors. The second and fourth transistors 222 and 225 are 
held in the "ON" condition and can conduct current in both directions via 
either the appropriate transistor itself or its associated anti parallel 
second or fourth diode, 229,231 as appropriate. The first and third diodes 
connected in anti parallel with a first and third switching transistors 
provide paths for current to flow back onto the input supply rails AC1, 
AC2 from any connected reactive load. 
In FIG. 10, there is shown operation during a mains positive half cycle. 
The signal SIG. C shows the typical voltage wave form that appears on the 
primary winding of the transformer during one half cycle of mains applied 
across the input supply rails AC1 and AC2. The transformer comprises a 
small ferrite transformer capable of operation at several tens of kHz and 
as such the secondary voltage (shown as SIG. D) follows the primary 
voltage accurately even at these high frequencies. If the output switching 
arrangement 211 is connected as a second bridge of four series connected 
isolated gate bipolar transistors as shown and the gate signals are the 
same as those in the first four isolated gate by polar transistors 221-225 
the resultant output of the circuit across the output rails AC3,AC4 is 
proportionally and subtantially the same as the input signal across the 
input rails AC1, AC2. In this way, an AC low frequency to AC low frequency 
signal may be transformed via a small ferrite isolating transformer 
operating at high frequency. 
The embodiment shown will operated over a frequency range from DC to 
approximately an order of magnitude below the high switching frequency, 
used for switching the switching transistors, typically 20-80 kHz. By 
adjusting the phase angle between the control signals SIG. A and SIG. B, 
the output signal and/or between control signals SIG. AA and SIG. BB 
and/or the percentage of dead time during its switching cycle on one or 
both transistor bridges 210,211 with a possible inclusion of extra 
inductors in series with a transformer primary and/or secondary, it is 
possible to adjust the value of the output voltage across the output 
terminals 251,252 from zero to a value determined by the turns ratio of 
the ferrite transformer. By this means it is also possible to compensate 
for voltage drops within the switching devices and transformer regulation, 
thus providing load compensation. 
The reader's attention is directed to all papers and documents which are 
filed concurrently with or previous to this specification in connection 
with this application and which are open to public inspection with this 
specification, and the contents of all such papers and documents are 
incorporated herein by reference. 
All of the features disclosed in this specification (including any 
accompanying claims, abstract and drawings), and/or all of the steps of 
any method or process so disclosed, may be combined in any combination, 
except combinations where at least some of such features and/or steps are 
mutually exclusive. 
Each feature disclosed in this specification (including any accompanying 
claims, abstract and drawings), may be replaced by alternative features 
serving the same, equivalent or similar purpose, unless expressly stated 
otherwise. Thus, unless expressly stated otherwise, each feature disclosed 
is one example only of a generic series of equivalent or similar features. 
The invention is not restricted to the details of the foregoing 
embodiment(s). The invention extends to any novel one, or any novel 
combination, of the features disclosed in this specification (including 
any accompanying claims, abstract and drawings), or to any novel one, or 
any novel combination, of the steps of any method or process so disclosed.