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