Series resonant converter having an actively controlled third element

A series resonant converter comprising an actively-controlled third element, or actively-controlled resonant preload. The actively-controlled third element is controlled by logic that makes it operative only when it is required by load conditions, and thus the desired characteristics of the third element are only employed when needed. The present invention incorporates the actively-controlled reactive preload and which operates on demand, and results in a converter output that is able to regulate from no load to full load. The third element is operated on demand and only at low power levels, and there is no penalty of higher RMS current in the power chain. The present invention is particularly applicable for use in charging batteries used in electrical vehicles. With the features provided by the present invention, an induction battery charger is able to use a high efficient resonant power supply, incorporating very low volume and weight coupling transformers, and which operates at high frequency and high power. The present invention may be used with all multi-element resonant converters that utilize more than two elements. The present invention improves on all resonant converters and provides for a more desirable operating characteristics, particularly in the case of load regulation.

BACKGROUND 
The present invention relates generally to resonant converters, and more 
particularly, to a resonant converter having an actively-controlled 
resonant preload. 
It is generally well known that conventional series resonant converters 
cannot be operated effectively at no load or minimal load. In an attempt 
to eliminate this problem, a third element is added to the conventional 
series resonant converter. A paper entitled "Topologies for Three Element 
Resonant Converters", by Rudy Sevems, published in APEC, 1990, page 712, 
references substantially all known resonant converter designs 
incorporating reactive third elements. This paper discusses two element 
resonant topologies, and describes how many of the limitations of the two 
element designs can be overcome by adding the third reactive element. 
A paper entitled "A Comparison of Half-Bridge resonant Converter 
Topologies, by Robert L. Steigerwald" published in IEEE Transactions on 
Power Electronics, Vol. 3, No. 2, April 1988, discusses half-bridge series 
resonant, parallel resonant and combination series-parallel resonant 
converters. This paper indicates that the combination series-parallel 
converter, which takes on the desirable characteristics of the pure series 
and the pure parallel converter, and thus removes the main disadvantages 
of these two converters. It is shown that the combination series-parallel 
converter can run over a large input voltage range and a large load range 
(no load to full load) while maintaining excellent efficiency. 
Accordingly, it is an objective of the present invention to provide for a 
resonant converter that is able to effectively regulate from no load to 
full load. 
SUMMARY OF THE INVENTION 
In order to provide for the above and other objectives, features and 
advantages, the present invention improves upon all previous resonant 
converter designs by adding an actively-controlled third element, 
comprising an actively-controlled resonant preload, to a series resonant 
converter. The actively-controlled third element provided by the present 
invention is made operative only when it is required by load conditions, 
and thus the desirable characteristics of the third element are only 
employed when they are needed. 
More specifically, the present invention may be employed in a series 
resonant converter comprising a switch, a main transformer having a 
primary and secondary windings, first and second reactive elements coupled 
in series with the primary winding of the main transformer, a load coupled 
across a secondary winding of the transformer, a third reactive element 
coupled across the primary winding of the transformer, and drive circuitry 
coupled to the switch. The improvement provided by the present invention 
comprises a current sensing transformer coupled in series with the primary 
winding of the main transformer, a switch coupled in series with the third 
reactive element, and control logic coupled between the current sensing 
transformer and the switch for determining a low load condition and for 
turning on the switch in response thereto. The control logic typically 
comprises a plurality of pairs of series coupled diodes and a resistor 
coupled in parallel with the plurality of pairs of diodes that are adapted 
to rectify the current sensed by the current sensing transformer and 
convert this current to a DC voltage, and a comparator coupled to receive 
the DC voltage and a reference voltage that is adapted to compare these 
two voltages and provide an output signal that controls the switching of 
the switch in the event of a low load condition. 
The comparator is adapted to turn off the AC switch when the input current 
is above a predetermined minimum current set by the reference signal. The 
size of the proportional current signal is determined by the turns ratio 
of the current sensing transformer. The proportionality of the current 
signal is on the order of 50:1, in that the current produced by the 
current sensing transformer is 1/50 of the current of the main 
transformer. The third reactive element may comprise an inductor. The 
switch is typically a plurality of semiconductor switches that comprise 
semiconductor power field effect transistors. 
The present invention provides for the incorporation of the 
actively-controlled reactive preload into the series resonant converter 
that operates on-demand, and results in a converter output that is able to 
regulate from no load to full load. The novel idea of the present 
invention is the on-demand feature of the reactive preload. As was 
mentioned above, series resonant converters cannot be operated effectively 
at no load or minimal load. By adding a third element to the series 
resonant converter, its operating deficiency (i.e. poor load regulation) 
can be cured. However, this is achieved with a penalty of higher RMS 
current in the power chain. However, and in accordance with the present 
invention, by adding a third element that is operated on-demand (only at 
low power levels), the RMS current penalty of the conventional third 
element design is eliminated. 
The present invention is particularly applicable for use in charging 
batteries used in electrical vehicles. With the features provided by the 
present invention, an induction battery charger is able to use a high 
efficient resonant power supply, incorporating very low volume and weight 
coupling transformers, and which operates at high frequency and high 
power. The present invention may be used with all multi-element resonant 
converters that utilize more than two elements.

DETAILED DESCRIPTION 
Referring to the drawing figures, FIG. 1 shows conventional two-element and 
three-element resonant converters 10, 10a. Without the addition of the 
third element, this circuit illustrates a classical series resonant 
converter. 
The conventional resonant converter 10, 10a comprises a switch 11, which 
may be provided by a plurality of semiconductor power switches 12, such as 
field effect transistors (FETs), for example. The switch 11 drives a 
primary winding 13 of a transformer 14. The switching of the plurality of 
semiconductor power switches 12 is controlled by a drive circuit 23. In 
the conventional two-element series resonant converter 10, first and 
second elements, comprising an inductor 15 and a capacitor 16, 
respectively, are coupled in series with the primary winding 13 of the 
transformer 14. The transformer 14 has a secondary winding 17 that is 
coupled to a load 20 by way of a plurality of diodes 18 and a filter 
capacitor 19. 
The current in the conventional two-element converter 10 is expressed as: 
##EQU1## 
where z.sub.0 =.sqroot.L.sub.1 /C.sub.1. As is illustrated from this 
equation, the main disadvantage of the two-element series resonant 
converter 10 is that the output voltage cannot be regulated for a no-load 
case. Reference is made to the paper "A Comparison of Half-Bridge Resonant 
Converter Topologies", by Robert Steigerwald, IEEE Transactions on Power 
Electronic, Volume 3, No. 2, April 1986, which discusses this deficiency. 
However, the two-element series resonant converter 10 is a highly desirable 
topology for high voltage and high power, due to its ability to provide 
sine wave current to the transformer 14, resulting in low harmonic losses, 
as compared to square wave current. Consequently, a smaller transformer 14 
may be used, resulting lower losses due to the use of less copper in the 
transformer 14. There is no core saturation at blocking capacitor caused 
by the primary winding 13. Also this converter 10 is very robust for stray 
inductance of wire and the transformer 14, and overcurrent due to the 
series inductor 15 on the primary winding 14. 
The deficiency of the series resonant converter 10 is overcome by the 
addition of a third element 21 to the topology, as is also shown in FIG. 
1, illustrated by the three-element converter 10a. The third element 21, 
comprising a second inductor 21, allows the resonant converter 10a to 
operate at substantially no load. This conventional modified converter 10a 
incorporating the third element 21 is used as a current regulator. 
For the three-element converter 10a shown in FIG. 1, 
##EQU2## 
where a=L.sub.2 /21, and f.sub.resonant =1/2.pi.L.sub.1 /C.sub.1. 
As can be seen from this equation, by adjusting the "a" term in the above 
equation, that is, the ratio of L.sub.2 to L.sub.1, I.sub.out can be 
reduced to zero, or can regulate to a no load condition. However, for this 
to occur, L.sub.2 may be up to two times the L.sub.1 value, resulting in 
larger RMS current flowing in the inductor 15 and capacitor 16 (L.sub.1, 
C.sub.1), the switches 12 and the transformer 14. Thus, the efficiency of 
the three-element converter 10a is compromised and the size of the 
transformer 14 is increased over the conventional two-element converter 
10. 
The solution to this problem is a realization that the third element 21, 
comprising the second inductor 21, is only required at low load currents. 
This is provided for in a manner as is shown in FIG. 2, which illustrates 
a three-element converter 30 comprising an activity-controlled resonant 
preload 31 in accordance with the principles of the present invention. The 
activity-controlled resonant preload 31 comprises an AC switch 32 is 
inserted in series with the third element 21 (L.sub.2). Logic 33 including 
a comparator 34 and a sensing transformer 35 (T.sub.A) is provided that 
determines when a minimal load condition occurs, and turns on the AC 
switch 32 (S.sub.1), thus coupling the third element 21 (L.sub.2) into the 
circuit 30, allowing no load operation. 
The logic 33 and comparator 34 are coupled between the sensing transformer 
35 (T.sub.A) and the AC switch 32 in a conventional manner. The logic 33 
is comprised of the current sensing transformer 35 (T.sub.A) which is 
adapted to sense the current flowing through the primary winding 13 of the 
transformer 14. The current sensing transformer 35 (T.sub.A) provides a 
current signal proportional to the current in the main transformer 14 that 
is rectified and converted to DC by four diodes 41-44 and a resistor 45. 
The size of the proportional current signal is based on the turns ratio of 
the current sensing transformer 35 (T.sub.A). Typically, the 
proportionality is on the order of 50:1, in that the current produced by 
the current sensing transformer 35 (T.sub.A) is 1/50 of the current of the 
main transformer 14. The resistor 45 is chosen to provide the correct 
voltage to be compared to the 5 volt reference voltage applied to the 
comparator 34. When the input current is above the minimum required, set 
by the 5 volt reference signal, the comparator 34 turns off the AC switch 
32. 
Thus there has been described a new and improved resonant converter having 
an actively-controlled resonant preload. The concepts of the present 
invention are global, in that the principles of the present invention may 
be applied to all third and fourth element resonant converter designs. It 
is to be understood that the above-described embodiment is merely 
illustrative of some of the many specific embodiments which represent 
applications of the principles of the present invention. Clearly, numerous 
and other arrangements can be readily devised by those skilled in the art 
without departing from the scope of the invention.