Title of Invention

AN IMPROVED HIGH FREQUENCY INVERTER CIRCUIT ARRANGEMENT

Abstract This invention relates to an improved high frequency inverter circuit arrangement for power conversion and control having a full bridge rectifier (2) with a non- smoothing filter for supplying pulsating DC, a series resonant RLC circuit comprising a coil (L) and a pair of capacitors (C1, C2) for maximum power hauling, an inverter circuit comprising a pair of semiconductor switches (I, II), coupled to said series resonant circuit, wherein said coil (L) is replaced by a short circuited bar (M' L') and an induction heating coil (L1) is arranged between any one of said non-smooth DC supply terminal (A or B) and the corresponding capacitor (C1 or C2) for generating therein the required alternating magnetic flux.
Full Text The present invention relates to an improved high frequency
inverter circuit arrangement for power conversion and control.
More particularly, the invention relates to an improved inverter
circuit arrangement for high frequency power conversion and
control in applications like induction heating; AC/DC drives for
rolling, textile and cement mills, electric traction, cranes,
excavators, etc; uninterrupted power supply (UPS) for computers,
aircraft and space applications; metallurgical process equipment
and high voltage DC transmission.
Induction heating is used for generation of steam, contamination
free heating in distillation plants and chemical/polymer
engineering, for heating solid PVC granules etc, used in
plastic/PVC moulding.
It can also be used in domestic appliances like hot plates,
electric kettles, food warmers, espresso coffee machines, etc.
For induction heating a series resonant inverter circuit can be
used with reduced switching losses and attractive possibilities
for higher frequency operation. This results in simple, light
weight construction of the overall system, simpler inverter
control protection and low maintenance operation.
There are basically two types of inverter circuits - half bridge
inverter circuit and full bridge inverter circuit. Half bridge
inverters have less number of power semiconductor switches, are
easier to control and inexpensive. The microprocessor controlled
computation for control circuit is easier in half bridge
circuits.
Full bridge inverters have more number of semiconductor switches.
The control for gate pulses is more complicated in full bridge
inverters.
A series resonant inverter is used with pulse width modulated
inverter circuit for obtaining fixed turn off and variable turn
on periods.
In the existing half bridge series inverter circuit a capacitor,
an inductor (load coil) and a load resistance ae used in series.
Two semiconductor switches are provided for single phase AC
generation.
Increase in operative frequencies is achieved by using an
appropriate drive for the gates of semiconductor devices.
Switching losses are reduced by adopting soft switching
techniques.
For semiconductor switching devices power metal oxide semiconductor field
effect transistor MOSFETS can be used like insulated gate bipolar transistors
IGBT.
The IGBT offers low ohmic resistance and requires very little power for gate
drive. A large resonant current pulse flows through the transistor which helps
in overcoming the problem associated with current tailing and turn-off latching
in conventional pulse width modulated PWM inverters.
Use of pulse width modulation helps in controlling the output voltage without
significantly adding to the number of power circuit components of the
inverter. This also eliminates or reduces the lower order harmonic
frequencies. For reducing the switching losses of power, devices zero current
switching ZCS or zero voltage switching ZVS can be adopted.
This inverter circuit however, has some limitations. The current handling in
the inductor coil is low. Power handling capacity is also low due to less
current in the coil.
Turn-off problem in the semiconductor switches can also be encountered
because of the presence of inductance in series circuit. In order to overcome
the turn-off problem, a separate protective snubber circuit is required in each
one of the semiconductor switches.
Thus, one object of the present invention is to provide a series
resonant half-bridge inverter circuit with higher power handling
capacity at reduced switching losses.
Another object of the present invention is to do away with the
separate snubber protection circuit for the semiconductor
switches for overcoming the turn-off problem.
Yet another object of the invention is to provide a simple, less
expensive and easier to control inverter circuit for induction
heating for industrial, domestic or other applications.
These and other objects of the invention are achieved by
replacing the inductor coil (load coil) of the series resonant
half bridge inverter by a very low resistance short-circuiting
bar and connecting the inductor coil between the collector
terminal of the first semiconductor switch and the positive
terminal of the DC source.

Alternatively, the induction coil can be connected between the
negative terminal of the DC source and the emitter terminal of
the second semiconductor switch.
The short circuiting bar acts as the load as before where the
resistance tends to zero and alternating current flows through

the bar and the series resonant circuit, delivering higher
current through the inverter coil or load coil.
Thus, the present invention provides an improved high frequency
inverter circuit arrangement for power conversion and control,
comprising a full-bridge rectifier with a non-smoothing filter
for supplying pulsating DC, a series resonant RLC circuit
comprising a coil, and a pair of capacitors for maximum power
handling, an inverter circuit comprising a pair of semiconductor
switches coupled to said series resonant circuit wherein said
coil is replaced by a short circuited bar and an induction
heating coil is arranged between any one of said non-smooth DC
supply terminal and the corresponding capacitor for generating
therein the required alternating magnetic flux .
The present invention will now be described in detail with the
help of the accompanying drawings, where :
Fig. 1 shows a half-bridge series resonant inverter of the prior
art,
Fig. 2 shows the phase shifted pulses for gates of the two semi-
conductor switches of Fig. 1,
Fig. 3 shows the current through the inductor coil and pulses in
respective gate drives,

Fig.4 shows the inverter circuit arrangement of the present
invention,
Fig.5 shows details of the DC source of the circuit arrangement
of Fig. 4,
Fig.6 shows the improved inverter circuit arrangement of Fig.4
of the present invention incorporating details of the DC
source of Fig.5.
In Fig.l an existing half-bridge series inverter circuit is shown
with a DC source S, capacitor C1 and C2 and inductor (load) coil
ML with a load resistance RL used in series. Two semiconductor
switches I and II are provided for generation of single phase AC.
This inverter circuit may be provided with a protective snubber
circuit, not shown for convenience, for overcoming turn-off
problems.
Phase shifted pulses for the two gates Gl and 62 of the
semiconductor switches I and II of Fig. 1 are shown in Fig.2. In
Fig.3, the current IL through the inductor coil ML and the pulses
in the respective gate drives are shown.
In Fig.4 the inverter cirucit for the present invention is shown
with DC supply source as generally indicated by the block S.
Details of this DC source S are shown in Fig.5.


A harmonic filter circuit 1 having a pair of coils a and b, is
connected to a supply source AC. For the sake of convenience, the
supply source has been shown as single phase AC. The two coils a
and b are of equal number of turns and are connected as shown in
Fig.5.
Because of the circuit configuration of the harmonic filter 1,
the harmonic contents are not allowed to be injected into the AC
source. A full bridge rectifier unit 2 connected to AC supply
through filter 1 provides pulsating DC output to a non-smooth LC
filter circuit through a resistor R for providing zero current
switching signal at 3 for the driver circuit of the inverter.
Thus, non-smooth DC is available at the terminals A and B.
In Fig.6, the block S of Fig.4 has been replaced by the details
illustrated in Fig.5.
In an idle slot when there is no signal at the gates G1 and G2 of
the respective semiconductor switches I and II, capacitors C1 and
C2 will be charged to the voltage V available at the terminals
AB
A,B through the inductor coil L . The capacitors C1 and C2 are
of equal value and the voltages applied against capacitors C1 and
C2 are equal to VAB
When a signal from the driver circuit is applied at gate 61 of
semiconductor switch I, switch I is in conducting mode and
capacitor C1 discharges through loop QRM'L'Q in the direction M'
to L'. Simultaneously, capacitor C2 will be charged by voltage
VAB across terminals A and B and the charging current will flow
through loop AQRM' L'OB in the direction M' to L'.
The algebraic sum of the discharging current through capacitor C1
and the charging current through capacitor C2 will flow through
the short circuit bar M'L' from M' to L'.
After this slot, a no-signal state will appear, as shown in
Fig.3, when there is no signal from the driver circuit. The two
semiconductor switches I and II are now in OFF state. Capacitors
C1 and C2 will again be charged at VAB.
In the next half cycle, when a signal from the driver circuit
is applied at gate G2 of the semiconductor switch II, swtich II
is in conducting mode and capacitor C2 discharges through loop
L'M'POL' in the direction l_ ' to M' . Simultaneously, capacitor CI
will be charged through loop AQL'IVPOB by voltage V across
AB
terminals A and B in the direction L' to M'.
The algebraic sum of this discharge current of capacitor C2 and
the charging current of capacitor CI will flow through the short-
circuiting bar M'L' in the direction L' to M'.
Therefore, current flowing through the short circuiting bar M'L'
is alternating. In other words,by feeding gate triggering pulse
alternately at gates Gl and G2, there will be generation of an
alternating current in short circuiting bar M'L' and bar M'L'
will act an an alternating current source.
The frequency of this AC generation will depend on the frequency
of the alternating gate triggering pulses, which can be set in
the range of 28-33 KHz. It is interesting to note that during
free-wheeling period as the inverter tries to feed back the
stored energy into the DC supply system, this excess energy finds
a low-impedance path ( = through the capacitor C
placed between terminals A and B as shown in Fig.6. As a result,
the generated alternating current through M'L' bar will also flow
through the induction heating working coil L1 which in turn will
generate the required alternating magnetic flux.
The induction heating working coil L1 can be arranged between any
one of the non-smooth DC supply terminals A or B and the
corresponding capacitor C1 or C2. Coil L1 being in the same loop
with the AC source M'L' alternating current will flow through L1.
The shape of the working coil L1 will depend on the purpose for
which bhe high frequency inverter circuit is utilized. This may
take the form of a helical coil or a spiral coil.
WE CLAIM :
i. An improved high frequency inverter circuit arrangement
for power conversion and control, comprising :
a full-bridge recitifier (2) with a non-smoothing filter
for supplying pulsating DC;
a series resonant RLC circuit comprising a coilland a
pair of capacitors (C1,C2) for maximum power handling;
an inverter circuit comprising a pair of semiconductor
switches (I,II), coupled to said series resonant circuit,
wherein
said coil (L) is replaced by a short circuited bar M'L' and an
induction heating coil L1 is arranged between any one of said
non-smooth DC supply terminal (A or B)and the corresponding
capacitor (C1 or C2) for generating therein the required
alternating magnetic flux.
2. The high frequency inverter circuit as claimed in claim 1,
wherein a harmonic filter circuit (1) is provided connected to an

AC supply source so that the harmonic contents are not allowed to
be injected into the AC source.
3. The high frequency inverter circuit as claimed in claim 1,
wherein said semi-conductor switches (I,II) can be bi-polar
junctional transistors (BJT) or (IGBT) etc .
4. The high frequency inverter circuit as claimed in claim 1, wherein the
frequency of AC generation in said short circuited bar M' L' will depend on the
frequency of the alternating gate triggering pulses.
5. The high frequency inverter circuit as claimed in claim 4, wherein the
frequency of AC generation is preferably selected between 28 KHz and 33
KHz.
6. The high frequency inverter circuit as claimed in claim 1 wherein the
working coil may take the form of helical coil or spiral coil.
7. The high frequency inverter circuit as claimed in claim 1, wherein zero
current switching (ZCS) or zero voltage switching (ZVS) is adopted for
reducing switching losses.
8. The high frequency inverter circuit arrangement for power conversion
such as substantially herein described and illustrated in the accompanying
drawings.


This invention relates to an improved high frequency inverter circuit arrangement
for power conversion and control having a full bridge rectifier (2) with a non-
smoothing filter for supplying pulsating DC, a series resonant RLC circuit
comprising a coil (L) and a pair of capacitors (C1, C2) for maximum power
hauling, an inverter circuit comprising a pair of semiconductor switches (I, II),
coupled to said series resonant circuit, wherein said coil (L) is replaced by a short
circuited bar (M' L') and an induction heating coil (L1) is arranged between any
one of said non-smooth DC supply terminal (A or B) and the corresponding
capacitor (C1 or C2) for generating therein the required alternating magnetic
flux.

Documents:

69 cal-2001-correspondence.pdf

69 cal-2001-description (provisional).pdf

69 cal-2001-examination report.pdf

69 cal-2001-form 18.pdf

69 cal-2001-form 3.pdf

69 cal-2001-form 5.pdf

69 cal-2001-form 6.pdf

69 cal-2001-granted-drawings1.1.pdf

69 cal-2001-granted-form 2.1.pdf

69 cal-2001-granted-specification1.1.pdf

69 cal-2001-pa.pdf

69 cal-2001-petition under rulr 138.pdf

69 cal-2001-reply to examination report.pdf

69-cal-2001-form 1.pdf

69-cal-2001-granted-abstract.pdf

69-cal-2001-granted-assignment.pdf

69-cal-2001-granted-claims.pdf

69-cal-2001-granted-correspondence.pdf

69-cal-2001-granted-description (complete).pdf

69-cal-2001-granted-description (provitional).pdf

69-cal-2001-granted-drawings.pdf

69-cal-2001-granted-examination report.pdf

69-cal-2001-granted-form 1.pdf

69-cal-2001-granted-form 18.pdf

69-cal-2001-granted-form 2.pdf

69-cal-2001-granted-form 26.pdf

69-cal-2001-granted-form 3.pdf

69-cal-2001-granted-form 5.pdf

69-cal-2001-granted-form 6.pdf

69-cal-2001-granted-reply to examination report.pdf

69-cal-2001-granted-specification.pdf

69-cal-2001-others.pdf


Patent Number 244527
Indian Patent Application Number 69/CAL/2001
PG Journal Number 50/2010
Publication Date 10-Dec-2010
Grant Date 09-Dec-2010
Date of Filing 05-Feb-2001
Name of Patentee INDIAN SCHOOL OF MINES
Applicant Address DHANBAD
Inventors:
# Inventor's Name Inventor's Address
1 CHAKRABARTI R N C/O BIRLA INSTITUTE OF TECHNOLOGY MESRAM RANCHI PIN 835 215
2 SADHU PRADIP KUMAR C/O BIRLA INSTITUTE OF TECHNOLOGY MESRAM RANCHI PIN 835 215
3 CHOWDHURY S P C/O BIRLA INSTITUTE OF TECHNOLOGY MESRAM RANCHI PIN 835 215
PCT International Classification Number H03K 19/094
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA