CA2290907A1

CA2290907A1 - Inexpensive electronic transformer

Info

Publication number: CA2290907A1
Application number: CA002290907A
Authority: CA
Inventors: Ivan Meszlenyi
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-11-09
Filing date: 1999-11-09
Publication date: 2001-05-09

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.

Description

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 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 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.

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.