U.S. patent application number 09/779707 was filed with the patent office on 2001-08-30 for regulated resonant converter.
Invention is credited to Hickman, Kevin.
Application Number | 20010017782 09/779707 |
Document ID | / |
Family ID | 26243616 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017782 |
Kind Code |
A1 |
Hickman, Kevin |
August 30, 2001 |
Regulated resonant converter
Abstract
There is disclosed a parallel loaded series resonant converter
300 suitable for supplying DC to a three axis gradient amplifier in
an MRI system. In conventional resonant circuits, output voltage is
regulated by controlling the switching frequency of an H-bridge or
half bridge switching arrangement and the resonant circuit (334,
335) is under more stress at no load than at full load. The voltage
across the resonant capacitor (334) is a function of the switching
frequency, the load and the voltage applied across the resonant
circuit (V.sub.A-V.sub.B). In the present invention, the peak
current at no load is reduced to lower than the peak current at
full load by controlling the voltage applied to the resonant
circuit (V.sub.A-V.sub.B) rather than controlling the switching
frequency.
Inventors: |
Hickman, Kevin; (Witney,
GB) |
Correspondence
Address: |
EVENSON, McKEOWN, EDWARDS & LENAHAN, P.L.L.C.
Suite 700
1200 G Street, N.W.
Washington
DC
20005
US
|
Family ID: |
26243616 |
Appl. No.: |
09/779707 |
Filed: |
February 9, 2001 |
Current U.S.
Class: |
363/65 |
Current CPC
Class: |
Y02B 70/10 20130101;
H02M 7/4815 20210501; H02M 7/5236 20130101 |
Class at
Publication: |
363/65 |
International
Class: |
H02M 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2000 |
GB |
0003055.1 |
Sep 13, 2000 |
GB |
00 22363.6 |
Claims
1. A method for regulating the output voltage of a resonant power
converter, including the steps of: a) providing current to a
resonant circuit through a switching arrangement having a plurality
of switches, the switches governing an alternating flow of current
through the resonant circuit; b) providing a feedback signal; and
c) controlling the behaviour of the switches in accordance with the
feedback signal in order to vary the duration over which current
passes through the resonant circuit.
2. A method according to claim 1, further including the steps: d)
detecting a current reversal event; and e) generating a crossover
signal corresponding to the current reversal event.
3. A method according to claims 1 or 2, wherein step c) corresponds
to varying the voltage drop across the resonant circuit.
4. A method according to claims 1, 2 or 3, wherein the switching
arrangement comprises an H-bridge including: a first switch, a
second switch, a third switch and a fourth switch; the first switch
being disposed in series with the third switch and the second
switch being disposed in series with the fourth switch, a first
junction being provided between the first switch and the third
switch and a second junction being provided between the second
switch and the fourth switch, and the resonant circuit being
disposed between the first junction and the second junction.
5. A method according to claim 4, wherein step c) further includes
the generation of a repeating cycle of switching signals.
6. A method according to claim 5, wherein the generation of a
repeating cycle comprises the substeps of: i) providing the first
switch with a first switching signal having a first ON-time of
fixed duration, a first onset time, t.sub.0 and a first end time,
t.sub.2; ii) providing the second switch with a second switching
signal having a second ON-time of fixed duration, a second onset
time, t.sub.3 and a second end time, t.sub.5; iii) providing the
third switch with a third switching signal having a third ON-time
of variable duration, the third ON-time beginning at the second
onset time, t.sub.3 and ending at a third end time, t.sub.4; and
iv) providing the fourth switch with a fourth switching signal
having a fourth ON-time of variable duration, the fourth ON-time
beginning at the first onset time, to and ending at a fourth end
time, t.sub.1.
7. A method according to claim 6, wherein the first ON-time and the
second ON-time have substantially the same fixed duration.
8. A method according to claims 6 or 7, wherein the third ON-time
and the fourth ON-time have substantially the same variable
duration within each repeating cycle of switching pulses.
9. A method according to claims 6, 7 or 8, wherein step c) is
achieved by varying the third end time, t.sub.4, of the third
switching signal and the fourth end time, t.sub.1, of the fourth
switching signal in accordance with the feedback signal.
10. A power converter apparatus for supplying a regulated output
voltage, including: a resonant circuit; a switching arrangement
having a plurality of switches and governing the direction and
duration of flow of current through the resonant circuit; a
feedback means for supplying a feedback signal; and a control
circuit which controls the behaviour of the switches in the
switching arrangement in accordance with the feedback signal;
characterised in that the control circuit controls the switching
arrangement by varying the duration over which current passes
through the resonant circuit in accordance with the feedback
signal.
11. An apparatus according to claim 10, further including a
crossover detection means for detecting current reversal events in
the resonant circuit and generating a crossover signal
corresponding to detected current reversal events, the control
circuit using the crossover signal to transmit a corresponding
switching signal to at least one of the switches in the switching
arrangement.
12. An apparatus according to claim 11, wherein the crossover
detection means includes a current transformer comprising a
secondary circuit coupled to the resonant circuit and each current
reversal event in the resonant circuit induces a corresponding
current reversal event in the secondary circuit.
13. An apparatus according to claim 12, wherein the resonant
circuit includes a capacitor and a first inductor in series, the
first inductor connected to a first side of the capacitor.
14. An apparatus according to claim 13, wherein the resonant
circuit includes a second inductor connected in series to the side
of the capacitor opposite to the first side.
15. An apparatus according to claims 13 or 14, wherein the feedback
signal corresponds to an output voltage measured across the
capacitor.
16. An apparatus according to any one of claims 10 to 15, wherein
the switching arrangement comprises an H-bridge arrangement.
17. An apparatus according to claim 16, wherein the H-bridge
includes a first switch, a second switch, a third switch and a
fourth switch, the first switch being disposed in series via a
first junction with the third switch, the second switch being
disposed in series via a second junction with the fourth switch and
the resonant circuit being disposed between the first junction and
the second junction.
18. An apparatus according to claim 17, wherein the control circuit
controls the H-bridge by addressing each of the four switches
independently with a corresponding switching signal.
19. An apparatus according to claim 18, wherein a first switching
signal addressing the first switch and a second switching signal
addressing the second switch both have a fixed duration
T.sub.F.
20. An apparatus according to claim 19, wherein a third switching
signal addressing the third switch and a fourth switching signal
addressing the fourth switch both have a duration T.sub.V that
varies in accordance with the feedback signal.
21. A power converter apparatus substantially as hereinbefore
described with reference to the accompanying FIGS. 3 to 5.
Description
[0001] The present invention relates to a regulated resonant
converter. In particular to a series resonant converter for
providing a direct current (DC) power supply to electromagnets.
[0002] Regulated resonant converters are well known and are used to
convert an alternating current (AC) mains supply to a regulated
source of electrical energy. Regulated resonant converters are
widely used for various applications, for example battery chargers,
induction heating and power supply to electromagnets. The following
discussion is especially, although not exclusively, concerned with
the use of resonant power converters in magnetic resonant imaging
(MRI) systems, for providing controlled electrical energy to
gradient coils for the purpose of modifying the magnetic field of
an MRI magnet as required for imaging. Each resonant converter
functions as a DC electricity supply for a three-axis gradient
amplifier whose output is in turn applied to the gradient coils. UK
Patent Application GB-A-23 11387 discloses a regulated resonant
converter in an MRI system and is included herein by reference.
[0003] Known regulated resonant converters comprise a series
resonant circuit through which current is switched alternately in
opposite directions at a frequency which corresponds to, or which
is close to, the resonant frequency of the series resonant circuit,
by an arrangement of switching transistors fed via a rectifier from
an AC mains supply. In this way, higher frequency perturbations are
strongly attenuated. Operation of the switching transistors is
controlled by signals generated in a control circuit in dependence
upon a fed-back sample of an output voltage from the converter
which is developed in the resonant circuit, and a crossover voltage
derived in dependence upon current reversal in an inductor, which
forms a part of the resonant circuit, regulation being effected in
dependence upon modification of the fed-back sample.
[0004] A typical parallel loaded series resonant converter for an
MRI system converts the input from a 50 Hz three-phase source of
400 V AC (or a 60 Hz source at 480 V AC) into six isolated outputs
at 400 V DC. The load applied in parallel across a capacitor
component of the resonant circuit can vary from zero to 25 kW.
[0005] The transistor switches used are typically insulated gate
bipolar transistors (IGBT) but may also be power transistors or a
gate turn-off thyristors.
[0006] Conventionally, the output voltage is regulated by
controlling the switching frequency in a bridge arrangement of
transistors, for example a half-bridge, having two switches, or an
H-bridge, which has four switches. At peak line voltage and minimum
load, the peak current in the switches and in the resonant inductor
is at a maximum and power is lost as a result. The bridge
arrangement and the resonant circuit are stressed more in the
absence of a load than at full load.
[0007] It is therefore an object of the invention to obviate or at
least mitigate the aforementioned problems.
[0008] In accordance with one aspect of the present invention,
there is provided a method for regulating the output voltage of a
resonant power converter, including the steps of:
[0009] a) providing current to a resonant circuit through a
switching arrangement having a plurality of switches, the switches
governing an alternating flow of current through the resonant
circuit;
[0010] b) providing a feedback signal; and
[0011] c) controlling the behaviour of the switches in accordance
with the feedback signal in order to vary the duration over which
current passes through the resonant circuit.
[0012] The method preferably further includes the steps of:
[0013] d) detecting a current reversal event; and
[0014] e) generating a crossover signal corresponding to the
current reversal event.
[0015] Advantageously, step c) corresponds to varying the voltage
drop across the resonant circuit.
[0016] It is preferred that the switching arrangement comprises an
H-bridge including: a first switch, a second switch, a third switch
and a fourth switch; the first switch being disposed in series with
the third switch and the second switch being disposed in series
with the fourth switch, a first junction being provided between the
first switch and the third switch and a second junction being
provided between the second switch and the fourth switch, and the
resonant circuit being disposed between the first junction and the
second junction.
[0017] Step c) may comprise the generation of a repeating cycle of
switching signals.
[0018] Preferably, the generation of a repeating cycle comprises
the substeps of: providing the first switch with a first switching
signal having a first ON-time of fixed duration, a first onset
time, to and a first end time, t.sub.2; providing the second switch
with a second switching signal having a second ON-time of fixed
duration, a second onset time, t.sub.3 and a second end time,
t.sub.5; providing the third switch with a third switching signal
having a third ON-time of variable duration, the third ON-time
beginning at the second onset time, t.sub.3 and ending at a third
end time, t.sub.4; and providing the fourth switch with a fourth
switching signal having a fourth ON-time of variable duration, the
fourth ON-time beginning at the first onset time, to and ending at
a fourth end time, t.sub.1.
[0019] The first ON-time and the second ON-time may have
substantially the same fixed duration.
[0020] The third ON-time and the fourth ON-time may also have
substantially the same variable duration within each repeating
cycle of switching pulses.
[0021] Step c) can be achieved by varying the third end time,
t.sub.4, of the third switching signal and the fourth end time,
t.sub.1, of the fourth switching signal in accordance with the
feedback signal.
[0022] In accordance with a further aspect of the present
invention, there is provided a power converter apparatus for
supplying a regulated output voltage, including: a resonant
circuit; a switching arrangement having a plurality of switches and
governing the direction and duration of flow of current through the
resonant circuit; a feedback means for supplying a feedback signal;
and a control circuit which controls the behaviour of the switches
in the switching arrangement in accordance with the feedback
signal; characterised in that the control circuit controls the
switching arrangement by varying the duration over which current
passes through the resonant circuit in accordance with the feedback
signal.
[0023] The apparatus advantageously further includes a crossover
detection means for detecting current reversal events in the
resonant circuit and generating a crossover signal corresponding to
detected current reversal events, the control circuit using the
crossover signal to transmit a corresponding switching signal to at
least one of the switches in the switching arrangement.
[0024] Preferably, the crossover detection means includes a current
transformer comprising a secondary circuit coupled to the resonant
circuit and each current reversal event in the resonant circuit
induces a corresponding current reversal event in the secondary
circuit.
[0025] It is preferred that the resonant circuit includes a
capacitor and a first inductor in series, the first inductor
connected to a first side of the capacitor.
[0026] The resonant circuit may further include a second inductor
connected in series to the side of the capacitor opposite to the
first side.
[0027] Advantageously, the feedback signal corresponds to an output
voltage measured across the capacitor.
[0028] The switching arrangement preferably comprises an H-bridge
arrangement.
[0029] The H-bridge may include a first switch, a second switch, a
third switch and a fourth switch, the first switch being disposed
in series via a first junction with the third switch, the second
switch being disposed in series via a second junction with the
fourth switch and the resonant circuit being disposed between the
first junction and the second junction.
[0030] Advantageously, the control circuit controls the H-bridge by
addressing each of the four switches independently with a
corresponding switching signal.
[0031] It is preferred that a first switching signal addressing the
first switch and a second switching signal addressing the second
switch both have a fixed duration T.sub.F.
[0032] Likewise a third switching signal addressing the third
switch and a fourth switching signal addressing the fourth switch
preferably both have a duration T.sub.V that varies in accordance
with the feedback signal.
[0033] The voltage, V.sub.C, across the resonant capacitor,
C.sub.R, is a function not only of the switching frequency but also
of the load and the voltage applied across the resonant circuit
(V.sub.A-V.sub.B). Therefore, by controlling the voltage applied to
the resonant circuit (V.sub.A-V.sub.B) rather than the frequency,
the peak current at no load can be reduced to a level lower than
the peak current at full load. Variation in the ON-times of the
switching arrangement provides the control circuit with the
necessary control over the level of voltage drop
(V.sub.A-V.sub.B).
[0034] For a better understanding of the present invention,
reference will now be made, by way of example only, to the
accompanying drawings in which:
[0035] FIG. 1 shows a schematic circuit diagram of an arrangement
using a resonance converter to energise the gradient coils of an
MRI system;
[0036] FIG. 2 shows a schematic circuit diagram of a regulated
resonance converter as disclosed in UK Patent Application GB-A-23
11387;
[0037] FIG. 3 shows a schematic circuit diagram of a regulated
resonance converter which may be controlled according to the
present invention;
[0038] FIG. 4 shows the control circuit of the regulated resonance
converter in FIG. 3 in more detail;
[0039] FIG. 5 shows a timeline diagram comparing the states of the
switches to the resulting voltage output;
[0040] FIGS. 6A and 6B show alternative arrangements of the
resonant circuit which can be used with the present invention;
and
[0041] FIGS. 7A and 7B compare the resonance curves produced by the
resonance circuit using known and inventive control methods.
[0042] FIG. 1 illustrates a typical circuit arrangement for
energising X,Y and Z gradient coils 1, 2, 3 of an MRI system (not
shown). A regulated resonant converter 4 is fed from three
terminals 5, 6, 7 of a three-phase, 400 V, 50 Hz AC supply. On
output lines 8, the regulated resonant converter 4 provides a
single phase 400 V AC supply at a frequency of approximately 20
kHz. The 400 V AC 20 kHz supply is fed to three primary windings 9,
10, 11 of three transformers 12, 13, 14 respectively, which serve
for isolation purposes. Secondary windings 12a, 12b, 13a, 13b, 14a,
14b of the three transformers 12, 13, 14 respectively are arranged
to feed rectifiers 15, 16, 17, 18, 19, 20 as shown, to produce a
400 V DC supply for gradient amplifiers 21, 22, 23, 24, 25, 26
respectively. In order to provide the required rate of change of
current in the gradient coils 1, 2, 3, the gradient amplifiers are
connected in pairs so that gradient amplifiers 21 and 22 feed the
gradient coil 1, gradient amplifiers 23 and 24 feed the gradient
coil 2 and gradient amplifiers 25 and 26 feed the gradient coil
3.
[0043] In operation, the gradient amplifiers 21 to 26 are switched
to produce a waveform in 27 (as shown inset) in each of the
gradient coils 1, 2, 3, so as to produce appropriate modification
of the magnetic field produced by the magnet of an MRI system as
required for imaging. The principles of operation of the magnet,
the MRI system and the imaging system are well known to those
skilled in the art, and are not central to the present invention.
Accordingly, they will not be described in detail herein.
[0044] Referring now to FIG. 2, wherein a known regulated resonant
converter 4 is shown in greater detail, the three-phase input lines
5, 6 and 7 are arranged to feed a rectifier 28 (as shown within
broken line) thereby to provide between lines 29 and 30 a DC
voltage which is applied to a pair of serially connected switching
transistors 31 and 32, in an arrangement known as a half-bridge. A
junction 33 between the switching transistors 31 and 32 is coupled
via a series resonant circuit comprising a capacitor 34 and an
inductor 35 to a junction between two capacitors 36 and 37 which
are serially connected between the DC supply lines 29 and 30. The
transistors 31 and 32 are shunted by snubber capacitors 38 and 39
which are serially connected and coupled at a junction therebetween
to the junction 33 between the transistors 31 and 32. The snubber
capacitors 38 and 39 are arranged to be shunted by diodes 40 and 41
respectively.
[0045] The transistors 31 and 32 in operation are switched by
signals applied to their respective gate terminals via lines 42 and
43 respectively. The signals for switching the transistors 31 and
32 are derived in a control circuit 44 which is coupled via optical
links 45 and 46 (shown schematically) to the transistors 31 and 32.
Optical links here serve the same purpose as the transformers in
FIG. 1; they isolate the power converter circuit. Optical signals
are generated within the control circuit 44 in transmitters 47 and
48 and converted to corresponding electrical signals in receivers
49 and 50 which feed the transistors 31 and 32 respectively.
Switching is effected by the control circuit 44 in dependence upon
a crossover voltage applied to the control circuit 44 via lines 51
and 52, and a feedback voltage which is fed to the control circuit
44 via lines 53 and 54.
[0046] The crossover voltage is derived via a coupling transducer
55 from the line 33 and the feedback voltage comprises, in effect,
a sample of an output voltage from the regulated resonant converter
which is developed between lines 56 and 57 across the capacitor 34
which forms a part of the resonant circuit, the sample being fed
via a feedback transformer 58 and a bridge rectifier 59 to provide
a DC signal level on the lines 53 and 54 which is smoothed by a
resistor 60 and a capacitor 61 to provide the feedback voltage.
[0047] The circuit shown in FIG. 3 shares many of the features of
the prior art resonant converter in FIG. 2. The resonant converter
300 in FIG. 3 can be implemented as the regulated resonant
converter 4 of FIG. 1.
[0048] FIG. 3 shows a schematic circuit for a resonant converter
300 for connection to a three-phase source of 400 V AC at 50 Hz. It
will be understood that the same circuit can be arranged for
connection to an alternative three-phase source of 480 V AC at 60
Hz, or more generally sources at values in the range 360 V to 528
V, with little alteration. Input lines 5, 6 and 7 are arranged to
feed a diode bridge and filter circuit 320. The outputs of the
diode bridge and filter circuit 320 provide a DC voltage between
lines 329 and 330 which is applied to a first serially connected
pair (Q1, Q3) 331, 332 and a second serially connected pair (Q2,
Q4) 351, 352 of switching transistors, in an arrangement known as
an H-bridge or full-bridge, the first pair 331, 332 being in
parallel with the second pair 351, 352.
[0049] A junction 333 between the first pair of switching
transistors (Q1, Q3) 331 and 332 is coupled via a series resonant
circuit comprising a capacitor 334 and an inductor 335 to a
junction 353 between the second pair of switching transistors (Q2,
Q4) 351 and 352. Each of the switching transistors 331, 332, 351,
352 is arranged in parallel with a respective snubber capacitor
338, 339,358,359. The snubber capacitors 338, 339, 358, 359 are
arranged to be shunted by diodes 340,341, 360, 361
respectively.
[0050] The switching transistors 331, 332, 351, 352 are switched by
signals applied to their respective gate terminals via lines 342,
343, 362, 363 respectively. The signals for switching the
transistors 331, 332, 351, 352 are derived in a control circuit
310. All the switching transistors Q1, Q2, Q3, Q4, turn on when
there is zero current through them and while there is zero voltage
across them.
[0051] Switching is effected by the control circuit 310 in
dependence upon a crossover voltage applied to the control circuit
310 via lines 312 and 314, and a feedback voltage which is fed to
the control circuit 310 via lines 316 and 318. The crossover
voltage is derived via a coupling transducer 355 from the line
between junctions 333 and 353. The feedback voltage comprises a
sample of an output voltage from the regulated resonant converter
which is developed between lines 316 and 318 across the capacitor
CR 334 which forms a part of the resonant circuit.
[0052] FIG. 4 illustrates the control circuit 310 of FIG. 3 in more
detail. The sample of output voltage developed across capacitor
C.sub.R 334 is fed via a feedback transformer 412 and a bridge
rectifier 414 to provide a DC signal level to a feedback circuit
416. Likewise the voltage sensed through the current transformer CT
355 along the lines 312 and 314 is provided to a crossover circuit
410. Using the fed-back voltage level from the feedback circuit 416
and the crossover detection of the crossover circuit 410, the
processing means 418 is arranged to provide signals 342, 362, 343,
363 for controlling the switching transistors Q1, Q2, Q3 and Q4
respectively. As in FIG. 2, the control signals are insulated by an
intervening optical apparatus 420.
[0053] In conventional operation, an H-bridge arrangement of four
switches is controlled so that the switching frequency is varied
according to the output voltage feedback levels. Using the H-bridge
in FIG. 3 as an example, switch Q1 and switch Q4 are arranged to
operate in tandem. Both switches switch to the ON state
simultaneously and remain in the ON state for the same duration,
the "ON-time". Likewise the ON-times and the onset of the ON state
of switches Q2 and Q3 are respectively identical and simultaneous.
By varying the switching frequency, the output voltage level can be
regulated.
[0054] FIG. 5 illustrates the operation of a resonant converter
according to the present invention, when implemented in the circuit
of FIG. 3.
[0055] In one embodiment of the present invention, Q1 and Q2 each
have an ON-time of the same fixed duration, T.sub.F. It will be
understood that the actual ON-time is set by a potentiometer and
depends upon the desired output voltage level. By modulating the
pot, the ON-times of Q1 and Q2 can differ. In contrast, the length
of time for which Q3 and Q4 remain ON, TV, is arranged to be
variable yet within the duration of the Q1 and Q2 ON-time
respectively: that is, T.sub.V.ltoreq.T.sub.F. As for Q1 and Q2,
the ON-time T.sub.V is substantially the same for both Q3 and Q4;
strictly speaking the variable ON-time T.sub.V does vary between Q3
and Q4 ON-times within a single cycle but the variation within a
cycle is negligible. Here the ON-times are governed by feedback
from the output voltage level but crucially the switching frequency
may stay unchanged. It will be understood that both switching
frequency and ON-times for switches Q3 and Q4 can be varied
simultaneously. Simultaneous variation is particularly desirable
when a low output voltage is required.
[0056] The pulse trains a) -d) in FIG. 5 correspond to the
switching signal passed to switches Q1 - Q4 respectively. A first
half-cycle is governed by switches Q1 and Q4. At time to, both Q1
and Q4 are switched ON. After being in the ON state for a variable
duration T.sub.V, the signal to switch Q4 goes OFF at time t.sub.1.
The signal to switch Q1 goes OFF at time t.sub.2 some time later.
During the first half-cycle, the signals to switches Q2 and Q3
remain OFF. As a result, throughout the period between t.sub.0 and
t.sub.1 current can pass along line 329, across switch Q1 331 to
junction 333, through the inductor L.sub.R 335 and capacitor
C.sub.R 334 of the resonant circuit to the junction 353, over
switch Q4 352 and back along line 330--the voltage drop between
junctions 333 and 353 (V.sub.A-V.sub.B) is positive as can be seen
from pulse train e). During the period between t.sub.1 and t.sub.2
only switch Q1 is ON and the voltage drop (V.sub.A-V.sub.B) returns
to zero.
[0057] Between times t.sub.2 and t.sub.3, none of the switches is
ON but the voltage drop becomes negative as the capacitor 334 of
the resonant circuit discharges. The current, I.sub.L, in the
inductor L.sub.R 335 is shown as the waveform f) in FIG. 5.
Inductor current, I.sub.L, lags behind the voltage drop across the
resonant circuit by 90.degree..
[0058] As the inductor current, I.sub.L, reverses, the current
transformer CT 355 detects a sign change and communicates the
crossover event to the crossover circuit. On detection of the
crossover, the control circuit sets both signals to switches Q2 and
Q3 to ON, t.sub.3: a second half-cycle, governed by switches Q2 and
Q3 begins. Current now passes through the resonant circuit in the
opposite direction to the first half-cycle, running through switch
Q2 from junction 353 to junction 333 and back along line 330 via
switch Q3. At time t.sub.4 the control signal to switch Q3 goes
OFF. The voltage drop across the resonant circuit (V.sub.A-V.sub.B)
returns to zero. The fixed duration, T.sub.F, of the ON-time of
switch Q2 ends at time t.sub.5 and the capacitor C.sub.R 334 is
free to discharge. Again the crossover event in the inductor
current defines the time at which the first half-cycle of a new
cycle begins, t.sub.6.
[0059] The variable length of the ON-times, T.sub.V, is governed by
feedback of the voltage, V.sub.C, across the capacitor C.sub.R 334.
In other words, times t.sub.1 and t.sub.4 are governed by the
output voltage level. The output voltage has a waveform as
illustrated by g) in FIG. 5. The effect of increasing the ON-time
of Q3 and Q4 is to increase the voltage applied across the resonant
circuit (V.sub.A-V.sub.B), between junctions 333 and 353. In this
way the voltage V.sub.C is regulated.
[0060] It will be understood that a resonant circuit can be
constructed from many more passive components than are present the
schematic circuit in FIG. 3. The only essential being an inductor
in series with the remaining components of the resonant network.
FIG. 6A shows only the resonant circuit components of the device in
FIG. 3, a capacitor C.sub.R 334 and an inductor L.sub.R 335. A
preferred alternative arrangement has two identical inductors and a
capacitor in series, one inductor disposed on either side of a
capacitor as illustrated in FIG. 6B. A resonant circuit with a
first inductor L.sub.R1, a capacitor C.sub.R and a second inductor
L.sub.R2 in series can even out the voltage to earth behaviour of
the resonant circuit over a switching cycle. Similar resonant
circuits having non-identical first and second inductors are also
possible.
[0061] Finally, the nature of the control methods of the present
invention is illustrated in diagrams of the resonance curves
associated with the resonant circuit. The conventional control
method allows the switching frequency to be altered which
corresponds to variation in the x-direction in FIG. 7A. For
illustrative purposes only, FIG. 7A shows a typical resonant curve
700 and the range over which switching frequency is varied 720
(typical values of the control frequency range, 20-40 kHz are
given; the resonant frequency, f.sub.R, 710 is somewhat lower).
[0062] The method according to the present invention permits
variation of the voltage across the resonant circuit. The inventive
method allows variation of the voltage at a particular switching
frequency corresponding to a variation in the y-direction as
illustrated in FIG. 7B. The Figure shows more than one resonant
curve 700, 702 which define the range 730 over which the voltage
drop across the resonant circuit is varied.
[0063] When a high load is placed in parallel with the capacitor
334 of the resonant circuit 300, relatively more current is drawn,
thereby dampening the amplitude of the peak in the resonance curve.
However the opposite effect is observed when the load is very small
or zero: little current is drawn by the load and the resonance peak
is very large. To maintain a constant voltage output level, using
the conventional frequency variation method, results in significant
increases in switching frequency. The increase in switching
frequency in turn results in large peak values for the inductor
current, I.sub.L.
[0064] The inventive method avoids the large inductor current
values by altering the voltage drop across the resonant
circuit.
[0065] It will be immediately obvious to a person skilled in the
art that a regulated resonant converter according to the invention
can be implemented in a range of devices including power supply
devices for electromagnets, induction heating equipment and battery
chargers.
[0066] Furthermore it will readily be seen that further
conventional components can be introduced in the circuit of the
invention without detracting from the inventive effect, so for
example, a further capacitor (not shown) may be included between
the junctions 333 and 353 for the purposes of AC coupling.
* * * * *