INEXPENSIVE ELECTRONIC TRANSFORMER

TRANSFORMATEUR ELECTRONIQUE PEU COUTEUX

English Abstract

A Schmitt oscillator driving a pair of switching elements via a high current
inverting buffer and the pulse
train obtained at the junction of said switching elements is fed in series
with a resonant capacitor and the
primary of an isolation transformer having a primary and secondary winding and
having an appreciable
amount of leakage inductance transferred to the primary so as to achieve
resonant transitions of the voltage
waveforms at the junction of said switching elements. To ensure resonant
transitions said isolation
transformer is equipped with a gap sufficient to achieve aforesaid mode of
operation. The voltage
developed on said resonant capacitor is fed to a voltage doubler to produce a
DC voltage proportional to the
load current, if DC voltage obtained as described is greater that the lower
threshold of Schmitt oscillator a
timing resistor from the junction of said switching elements will increase the
frequency so as to limit the
current that can be obtained on the secondary.

Claims

Claims:
1. An electronic transformer converting low frequency AC line voltage to a DC
voltage source which in
turn is being converted to a train of high frequency square wave pulses in
order to produce an isolated
AC or DC voltage of a various levels comprising:
first and second switching transistor means each having a control electrode, a
power inert electrode
and a power output electrode, the power output electrode of said first
switching transistor being
connected to the power output electrode of said second switching transistor
and the power input
electrode of said first and second switching transistors being connected to
said DC voltage source and
the control electrodes of first and second switching transistors being
connected to a high current
inverting buffer, having an input and output, the input of which is connected
to a Schmitt oscillator
having an input and an output whose oscillation frequency is governed by a
timing resistor between the
inert and output of said oscillator and timing capacitor between the input of
said oscillator and ground,
and the power output electrons of said first and second switching transistors
being connected to the
primary winding of an isolation transformer, having a secondary winding
connected to a load
characterized by a resistance, being in series with a resultant leakage
inductance and a resonant
capacitor comprising a series resonant network where a high frequency
sinusoidal voltage is generated
across said resonant capacitor and said high frequency sinusoidal voltage is
being coupled through the
coupling capacitor to the junction of the cathode of the first and the anode
of the second voltage
doubling diodes providing a DC voltage across the smoothing capacitor which is
proportional to the
voltage generated on said resonant capacitor. This DC voltage is being
suitably divided by the first and
second voltage dividing resistors and thus biasing the first switching diode
off if lower threshold
voltage of said Schmitt oscillator input is less than said DC voltage across
said second voltage dividing
resistor and on if not, if said first switching diode is off then second
switching diode will be allowing
current to flow through the pull-up resistor to the timing capacitor thereby
increasing the frequency of
oscillation of said Schmitt oscillator and reducing its duty cycle resulting
in reducing the current
flowing through the resonant capacitor so as to limit the current on said load
on said secondary
winding.
2. The electronic transformer of claim 1, wherein said first and second
switching transistors are being
driven by a high frequency square wave at the control electrons through a high
current inverting
buffer.
3. The electronic transformer of claim 2, wherein said first and second
switching transistors are
producing square wave pulses of high frequency at the common junction of the
power output
electrodes of the said first and second transistors.
4. The electronic transformer of claim 1, wherein the power outfit electrodes
of the said first and second
switching transistors are joined and connected to the said primary winding of
said isolation transformer
having a secondary winding and a substantial leakage and magnetizing
inductance referred to the
primary winding.
5. The electronic transformer of claim 4, wherein the secondary winding of
said isolation transformer is
connected to a load having a substantial resistive component.
6. The electronic transformer of claim 5, wherein there is a resonant
capacitor in series with the primary
winding of said isolation transformer forming a series resonant network with
the leakage inductance of
said primary winding.

7. The electronic transformer of claim 6, wherein said high frequency
sinusoidal voltage
is coupled by the coupling capacitor to the junction of the cathode of the
first voltage
doubling diode and the anode of the second voltage doubling diode so as to
develop a
DC voltage across said smoothing capacitor and said DC voltage is proportional
the
current flowing in said load.
8. The electronic transformer of claim 7, wherein the voltage across the
smoothing
capacitor is suitably divided by the first and second voltage dividing
resistors such
that the said DC voltage across the second voltage divider resistor biases the
first
switching diode such that if the voltage across said second voltage divider
resistor is
greater than the lower threshold voltage of the Schmitt oscillator said first
switching
diode is off and where said first switching diode is biased such that if the
voltage
across said second voltage divider resistor is less than the lower threshold
voltage of
said Schmitt oscillator said first switching diode is on.
9. The electronic transformer of claim 8, wherein a second switching diode is
being
employed having an on state when said first switching diode is in the off
state and
therefore the current injected through the pull up resistor will increase the
frequency
of oscillation of said Schmitt oscillator while reducing the duty cycle of
said
oscillation at the same time resulting in a lowered current in said load which
in turn
reduces the voltage across said second voltage dividing resistor so as to
complete the
negative feed-back loop resulting in limiting and regulating the current in
said load in
order to protect the semiconductor devices from destruction.
10. The electronic transformer of claim 9, wherein the ratio of the first and
second
voltage dividing resistors will set the current limit in said load.

Description

CA 02290907 1999-11-09
Background of the Invention:
The present invention is related to switch mode power supplies of unregulated
nature and more closely
related to resonant mode sullies.
Description of the Prior Art:
Most household appliances that require voltage conversion and line isolation
are still using an iron core
transformer whereby a primary and secondary winding is isolated by a second
layer of tape or by other
means to obtain at least 1000V AC isolation between primary and secondary. The
weight and size of a
conventional iron core transformer above the 100W level is increasingly adding
to the system size and
weight parameters and the efficiencies are generally in the low 80% regions.
Furthermore, they generate a
fair amount of hum field (60Hz) that is particularly troublesome with audio
amplifiers. The resent
invention duplicates the voltage isolation and conversion parameters with a
drastic reduction in size and
weight yet offers greater than 90% efficiency. At power levels above 100W the
cost saving on a system
level becomes increasingly evident.
Summary of Invention:
The present invention provides a simple topology for converting the line
voltage into an isolated voltage of
AC or DC of different levels aixl it does it so by converting the frequency of
the incoming signal to a train
of high frequency pulses which is generated by a Schmitt oscillator and is
further amplified by a pair of
switching elements typically MOSFETS so as to drive a transformer that is
constructed such that a primary
and secondary is installed on a split bobbin having a substantial leakage
inductance to introduce a resonant
nature which is essential for EMC and loss-less operation. The isolation
transformer, having a lowered
magnetizing inductance by gapping, is ensuring that under light load or no
load conditions the transitions
on output switching devices remain soft i.e. that inductive energy stored in
the primary magnetizing
inductance is enough to overcome the capacitive energy of the capacitance of
the output switching
elements, typically MOSFETS. A current limit is being provided by taking a
signal from the resonant
capacitor which is connected in series with the primary inductance of said
resonant transformer and said
DC voltage is being provided by a voltage doubler and said DC voltage is being
fed back to the input of
Schmitt oscillator by a switching diode so as to enable another switching dio~
to come into conduction to
increase the frequency of the Schmitt oscillator with the current produced by
a resistor connected between
the output of the switching elements and the input of the Schmitt oscillator.
The current limit is then
established by an increased frequency and decreased duty cycle powering the
transformer having
substantial leakage inductance.
This simple topology that closely approximates the behaviour of an ofrline
transformer has a diode bridge
and a smoothing capacitor to produce a DC rail but it is substantially smaller
in size and weight and as per
described above has a very simple means of current limiting the secondary
current by sampling and
controlling the current in the primary, since there is no voltage feedback
from the secondary to the primary

CA 02290907 1999-11-09
side, the load regulation is primarily determined by the cancellation of the
leakage inductance via the series
resonant capacitor, furthermore this electronic transformer is provided with
an electronic shutdown facility
which makes it easy to control for thermal over-voltage or to switch it on and
off in any desired sequence
without an additional high current switching element. Thus it can eliminate
the usage of bulky transformers
in applications such as battery chargers, welding units, audio amplifiers, and
low voltage lighting or any
other application wherein very tight voltage regulation is not necessary but
short circuited and/or current
limited behaviour is highly desired.
Description of the Preferred Embodiment:
An embodiment of the present invention is described below with reference to
the attached drawing. In order
to simplify the descriptions of the various embodiments, identical reference
numerals have been assigned to
identical components in the drawing.
F'IG.1 is a schematic diagram of the present invention in accordance with the
preferred embodiment.
A Timing Capacitor 1 is connected to the ground and the other side of said
Timing Capacitor 1 is connected
to the input of Schmitt Oscillator 2 and the output of said Schmitt Oscillator
2 is fed back to the input
through the first timing resistor 3. The output of Schmitt Oscillator 2 is
connected to the High Current
Inverting Buffer 4 and the outputs of High Current Inverting Buffer 4 are
driving the gates of the three
electrode MOSFETS 5 and 6. The junction of MOSFET 5 and MOSFET 6 is connected
to the Primary
Winding 7 of Isolation Transformer 8. The Secondary Winding 20 of said
Isolation Transformer 8 is
connected to the Load 21 and the other side of said Load 21 is connected to
the other side of said
Secondary Winding 20. The remaining electrode of MOSFET 5 is connected to a DC
Source 9 and the
other side of said DC Source 9 is connected to the remaining electrode of
MOSFET 6. The other side of
said Primary Winding 7 is connected to a series Resonant Capacitor 10 and the
Coupling Capacitor 11, the
other side of said Resonant Capacitor 10 is grounded. The other side of said
Coupling Capacitor 11 is
connected to the junction of the cathode of the lg' Voltage Doubling Diode 12
and the anode of the 2"d
Voltage Doubling Diode 13, the other side of said 1" Voltage Doubling Diode 12
is grounded. The cathode
of the 2~ Voltage Doubling Diode 13 is connected to the junction of the 1~'
Voltage Dividing Resistor 15
and the Smoothing Capacitor 14, the other side of said Smoothing Capacitor 14
is grounded The other side
of said 1~ Voltage Dividing Resistor 15 is connected to the junction of the
cathode of the lg' Switching
Diode 17 and the 2"d Voltage Dividing Resistor 16, the other side of said 2~''
Voltage Dividing Resistor 16
is grounded The anode of said 1~' Switching Diode 17 is connected to the
junction of the anode of the 2nd
Switching Diode 18 and the Pull-up Resistor 19. The other side of Pull-up
Resistor 19 is connected to the
Primary Winding 7 of the Isolation Transformer 8 on the same side of said
Primary Winding 7 as the
junction of MOSFET S and 6 is connected. The cathode of said 2~ Switching
Diode 18 is connected to the
input of the Schmitt Oscillator 2.
The operation of the circuitry of the preferred embodiment is discussed in
detail with respect to FIG.1
The Schmitt Oscillator 2 produces a square wave which has a frequency defined
by the Timing Capacitor 1
and the Timing Resistor 3 said square wave signal passes through the High
Current Inverting Buffer 4
which drives the rtes of the MOSFETS 5 and 6. The MOSFETS are in turn fed by a
DC Source 9 which is
typically the rectified and filtered AC line voltage. The MOSFETS 5 and 6 feed
square wave pulses to the
Isolation Transformer 8 which has a resistance in series with the Primary
Winding 7, transferred by the
square of the turns ratio as a result of the resistance of Load 21 on the
Secondary Winding 20, and a
leakage inductance in series with the Resonant Capacitor 10 which forms a
series resonant network.
Depending on the current and the Load 21, the amplitude of the high frequency
sinusoidal voltage
appearing on the Resonant Capacitor 10 will be varied and proportional to said
load current. The Coupling
Capacitor 11 feeds the Voltage Doubling Diodes 12 and 13 to produces a DC
voltage across the Smoothing
Capacitor 14 proportional to the voltage across the Resonant Capacitor 10. The
voltage across the
Smoothing Capacitor 14 is then fed to the Voltage Dividing Resistors 15 and
16. If the voltage produced
across the 2nd Voltage Dividing Resistor 16 reaches the lower threshold of the
Schmitt Oscillator 2 the
cathode of the 1" Switching Diode 17 will be more positive than its anode and
will thus be removed from
the circuit such that all current through the Pull-up Resistor 19 and the 2"d
Switching Diode 18 will flow
to the Timing Capacitor 1 for the duration of pulse train being at the
positive rail level wherein said pulse

CA 02290907 1999-11-09
train is generated at the junction of MOSFET 5 and 6. Therefore, the current
rived from said pulse train
will increase the frequency of oscillation at the Schmitt Oscillator 2 and
reduce its duty cycle in turn
reducing the current flowing through the Resonant Capacitor 10 so as to limit
the current in the Load 21.

Representative Drawing