DC TO AC CONVERTER

CONVERTISSEUR C.C.-C.A.

English Abstract

An electronic transformer consisting of a square wave power oscillator having
a diode
reverse capacitance serving as frequency tuning, wherein said power oscillator
connected to a dc source typicly derived from the ac outlet and said generator
is driving a transformer having substantial leakage inductance and the primary
of said
transformer is connected through a capacitor that forms a resonant circuit
with said
leakage inductance.
A second capacitor is connected to the junction of said first capacitor and
transformer primary and a resistor are in series with said second capacitor .
The voltage developed on said first resistor is proportional to the current in
the
transformer primary and after rectified it is fed to a diode with apritiable
reverse
capacitance. This capacitance then will decrease in response to the current
and will
increase the frequency of said power oscillator and in turn will limit said
current to a safe
value.

Claims

Claims:
1. An unregulated resonant converter consisting at least two switching
elements a first
and second switch driven by a square wave oscillator having a first diode with
appreciable reverse capacitance as a tuning element.
Said first and second switches connected to a dc supply and to a transformer
with
substantial leakage inductance.
A first capacitor serving as resonant capacitor connected in series with the
primary of
said transformer and this junction are connected to a second capacitor in
series with a
sensing resistor. The voltage developed across the sensing resistor is
rectified by second
diode , filtered by a third capacitor and said voltage is fed via first
resistor to change
the reverse voltage across said first diode having substantial reverse
capacitance. The
voltage developed across the sensing resistor is proportional to the current
and vill tune
the oscillator above the resonance so as to keep sqaid current from reaching
destructive
values.
2. A tunable oscillator as in claim 1. Having a first diode as the essential
tuning element.
3. A first diode as in claim 1 and 2 having a substantial reverse capacitance
4. A pair of switching element driven by said oscillator as in claim 1
connected to a dc
rail and to a transformer
5. A transformer as in claim1 and 4 having a substantial leakage inductance
6. A first capacitor as in claim 1 connected in series with said transformer
primary
serving as resonant capacitor
7. A second capacitor as in claim 1 connected to the junction of said first
capacitor and
transformer primary and said capacitor is in series with a first resistor.
8. A first resistor as in claim 1 and 7 serving as current sensing resistor
having a second diode to rectify the voltage across it and a third capacitor.
9. third capacitor as in claim 1 and 8 to filter the rectified ac.
10. Second resistor as in claim 1 feeding first diode with the voltage
developed in
proportional to the current through the primary of said transformer so as to
tune said
oscillator by reducing its reverse capacitance to increase frequency and keep
device
stresses to a minimum. A zener diode in series with said second resistor maybe
employed to set suitable threshold for the current through the primary of said
transformer.

Description

CA 02331345 2001-O1-12
Background of the Invention:
This invention relates to off line resonant converters, in particular to those
unregulated in
nature i.e. electronic transformers. More importantly those having some form
of
protection far no toad or overload condition.
Description of the Prior art:
The prior art described below is of a series resonant type. Such converters
have simple
overload protection but at the cost of much degraded transiFormer
construction. In order to
answer the need for cost reduction an overload protection consisting of two
diodes
connected to the do rail and their junction is connected to the resonant
capacitor.
The operation of the prior art will be described in reference to figure 1.
Mosfet 1 and mosfet 2 is connected to do rail 3 positive and 4 negative. The
drive oscillator 5 providing square wave pulses to said mosfet l and mosfet 2.
The
junction of said mosfet 1 and mosfet 2 is connected to transformer
6 having appreciable leakage inductance and the other side; of the primary
winding 7 is
connected to resonant capacitor 8. The other side of capacitor 8 is connected
to the
negative rail 4. The current limiting first diode 9 is connected between do
rail positive 3
and to the other side of capacitor8. Second limiting diode 10 is connected
between
negative rail of 4 and the cathode of diode 10 is connected to the anode of
diode 9.
Transformer 6 having a secondary winding 11 connected to load 12.
By decreasing the load resistance 12, the voltage across capacitor 8 will
increase until
the rail voltage. If the load 12 is a short circuit, the diode 9~ and diode '
i 0 will limit the
current to approximately twice of the nominal value.
This has two major disadvantages. Upon short circuit the flux density in the
core will rise
to double of the nominal, and the current in the transformer will do the same.
__. ___ _.-____ __

CA 02331345 2001-O1-12
2
This is self defeating -because even-though the mosfets are protected they
will
have to endure twice as much current, translating 4 times higher heat loss
then nominal,
which means more e~pens-ive mosfets. Furthermore the doubling of the
~u~derasity sill
require more then double cross sectional area for the transformer ~. The wire
dimensioning suffers similarly coupled with the need for a much increased core
diameter.
To sum it up this inexpensive overload protection leads to an n expensive
overall
solution.
This is not including applications requiring rectification that would demand
much larger
heatsinking.
The present invention is free from the shortcomings of the garevions art
yielding a truly low cost converter with low thez-n~ai gnat current stresses
under
overlo2td conditions
Description of the ~'referreri Embodiment:
The present invention will be described in relation to fig. 2
mosfet l 3 and mosfet 14 is connected to do rail l 5 positive and 1 S
negative.
The junction of the mosfet 13 andl4 is connected to transformer 28 having
primary 27
and secondary windings 17. The other side of primary 27 is connected to
resonant
capacitor 18 and the other side of capacitor 18 is connected to negative rail
l~.The
junction of said capacitor 18 and primary winding 27 is connected to capacitor
19 in
series with sensing resistor 25. The other side of resistor 2_'i is connected
to ground.
square wave oscillator 2fl having a tuning capacitance using diode 2l,having
sustentiai
reverse capacitance. The voltage across sensing resistor 2_'i is rectified by
diode 22 and
filtered by capacitor 25. This rectified voltage is fed through resistor 23 to
diode 21. The
diode 21 is connected to the oscillator via capacitor 23. ~scillator 20
outputs are driving
mosfet 13 and 14. Under nominal conditions the voltage across resistor
25 is less then a diode drop. If load 24 connected to the secondary 17 having
a short condition, then the voltage across sensing resistor 25 will be greater
then a
diode drop and start tuning the oscillator to a higher frequency thereby
reducing current .
This action will bring about a safe current limit while increasing the
converter frequency
proportionally so the magnetics will operate ~viin safe limits under overload
conditions. The amount of overload can be desyd ~y the; appropriate choice of
a ~ener
diode placed in series w~t~i the voltage developed 'lOy rcetifier diode 22.
Typical ratios of i.3 0-~~rload/n~minal is easily achieved.
The present invention en .~. resonant converter of unregulated nature
to be protected under overlap r~ion by loss-less sensing said condition and
using
this information sets a Limit to the stresses on both the semiconductor and
the magnetics
thereby yielding a high frequency high performance elecWonic transformer at a
low cost.
It is believed that the method used to obtain such a converter described in
present
invention is not obvious to those trained in the art and is of

CA 02331345 2001-O1-12
significant improvement in relation to the prior art ,therefore the following
are claimed

Representative Drawing