FORM 2
THE PATENTS ACT 1970
As amended by the Patents (Amendment) Act 1999
COMPLETE SPECIFICATION
(SEE SECTION 10)
TITLE
High power factor phase controlled converter
APPLICANTS
Indian Institute of Technology, Bombay, Powai, Mumbai 400076, Maharashtra, India, an autonomous educational institute established in India under the Institutes of Technology Act 1961
INVENTORS
Under Section 28(2)
Dr Victor Prince Sundarsingh, Electrical Engineering Department, Indian Institute of Technology, Bombay, Powai, Mumbai - 400076, Maharashtra, India and Vinayak Narayan Shet. Goa College of Engineering, Electronics and Telecommunications Department, Farmagudi, Ponda 403401, Goa, both Indian nationals
The following specification particularly describes and ascertains the nature of this invention and the manifer in which it is to be performed :


FIELD OF INVENTION
This invention relates to a high power factor phase controlled converter.
PRIORART
Phase controlled converters or rectifiers are used in single phase AC supplies to get controlled DC output in AC to DC conversion applications such as battery chargers, AC-DC converters, DC drives, Uninterrupted Power Supply (UPS) or Switch Mode Power Supply (SMPS). Currently available phase controlled converters comprise a rectifier bridge comprising a pair of parallel spans of Silicon Controlled Rectifiers (SCRs) and diodes or SCRs. At the input side of the rectifier bridge, the junctions of the parallel spans are connected to a single phase AC supply through a first filter inductor capacitor configuration. At the output side of the rectifier bridge a-second filter inductor capacitor configuration is connected in series with the parallel spans. A load is connected across the second capacitor A Controller Circuit comprising a dual output Trigger Generator is connected to the gates of the SCRs. The Controller Circuit also comprises a Voltage
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Comparator whose output is connected to the Trigger Generator. One input of the Voltage Comparator is connected to a single phase reference AC voltage supply through an Integrator and Level Shifter and Zero Crossing Detector (ZCD). The other input of the Voltage Comparator is connected to the output of an Error Amplifier whose one input is connected to a reference DC voltage and the other input is connected to the load. Desired DC output is achieved by controlling the firing angle of the SCRs of the rectifier bridge by the Controller Circuit. For low DC output applications firing angles are to be large and conduction angles small. Due to the conduction angles being small, the power conduction takes place for small durations. During conduction charging and power supply to the load take place simultaneously. The amplitude of the current drawn from the AC mains remains the same throughout the conduction period. Therefore, the current drawn from the AC mains is large in magnitude and not sinusoidal but square in nature.^ This gives rise to large harmonics in the input current and calls for bulky capacitors in the output side of the rectifier bridge to ensure steady output voltage. Bulky capacitors increase the size of the converters and add to the cost thereof. Due to the non-sinusoidal nature of the input current DF factor
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ie displacement between voltage and current is large, giving rise to a large number of undesirable lower harmonics in the input current which results in poor efficiency of the converters; Because of the large DF factor, power factor of the converters is poor leading to poor power transmission and under utilisation of energy and poor converter efficiency. Because of the square wave nature of the input current and bulky capacitors at the output side of the rectifier bridge, the inrush current drawn by the converters is very large leading to extra cost in transmission lines by way of increase in size of the conductors. The load current drawn by these converters is normally discontinuous at low output voltages due to the small conduction angle. Hence for low output voltages an additional transformer is necessary to ensure a continuous load current. The additional transformer increases the size and reduces the efficiency of the converters. The power factor of the converters is normally in the range of 0.5 to 0.6.
OBJECTS OF INVENTION
An object of the invention is to provide a high power factor phase controlled converter which draws sinusoidal input current from the
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AC mains at all firing angles and loads and ensures a constant output voltage to the load.
Another object of the invention is to provide a high power factor phase controlled converter which eliminates generation of large harmonics in the input current thereby eliminating bulky capacitors in the output side of the converter and reducing the overall size and cost of the converter.
Another object of the invention is to provide a high power factor phase controlled converter which rninimises DF factor and increases the efficiency of the converter.
Another object of the invention is to provide a high power factor phase controlled converter which increases power factor leading to good power transmission and utilisation of energy.
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Another object of the invention is to provide a high power factor phase controlled converter which eliminates inrush current thereby reducing the cost of transmission lines and size of the conductors.
Another object of the invention is to provide a high power factor phase controlled converter which ensures practically continuous load current at all output voltages thereby eliminating the necessity of additional transformer and reducing the size of the converter and improving efficiency of the converter.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided a high power factor phase controlled converter comprising a rectifier bridge comprising a pair of parallel spans of Silicon Controlled Rectifiers (SCRs) and diodes or SCRs, the junctions of the parallel spans being connected to a single phase AC supply at the input side of the rectifier bridge, a resonant inductor and capacitor configuration connected across the rectifier bridge at the output
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side thereof, an auxiliary Silicon Controlled Rectifier (SCR) connected to the junction of the resonant inductor and capacitor configuration, a free wheeling diode connected between the cathode of the auxiliary Silicon Controlled Rectifier and ground, a filter inductor and capacitor configuration connected in series with the auxiliary Silicon Controlled Rectifier, a load connected across the filter capacitor and a Controller Circuit comprising a dual output Trigger Generator whose outputs are connected to the gates of the Silicon Controlled Rectifiers of the rectifier bridge and inputs are connected to a Voltage Comparator and a Zero Current Detector (ZCUD) and Interlock, one input of the Voltage Comparator being connected to a DC reference voltage and the other input of the Voltage Comparator being connected to a Ramp Converter which is connected to a reference AC voltage through a Zero Crossing Detector (ZCD) and to a reference DC voltage through an Error AmpHfier which is also connected to the load, a single output Trigger Generator connected to the gate of the auxiliary Silicon Controlled Rectifier and to the Zero Current Detector (ZCUD) and Interlock, the inputs of the Zero Current Detector (ZCUD) and Interlock being connected to the resonant inductor and to a reference DC voltage.
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The following is a detailed description of the invention with reference to the accompanying drawings, in which ;
Fig 1 is a block cum schematic circuit diagram of a high power factor semi controlled yfconYerter according to an embodiment of the invention;
Figs 2 represents computer generated wave forms as obtained by a typical high power factorfsemi controlled?converter of Fig 1;
Fig 3 represents actual wave forms as obtained by a typical high power factor £emicontrollecUconverter of Fig 1; and
Fig 4 gives power factor values as obtained by a conventional semi controlled converter and a typical high power factor semicontrolled converter of Fig 1 at various applied voltages and load currents.
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Referring (o Fig 1 of the accompanying drawings, the high power factor feeml controlled ~ converter 1 comprises a rectifier bridge 2 comprising a pair of parallel spans 3 and 4 of Silicon Controlled Rectifiers (SCRs) Ti and T2 and diode Di and D2. The junctions of the parallel spans are connected to a single phase AC supply at the input side of the rectifier bridge. A resonant inductor Lj and capacitor Ci configuration is connected across the rectifier bridge at the output side thereof. An auxiliary Silicon Controlled Rectifier (SCR) T3 is connected to the junction of the resonant inductor and capacitor configuration. A free wheeling diode D3 is connected between the cathode of auxiliary SCR and ground. A filter inductor L2 and capacitor C2 configuration is connected in series with the auxiliary SCR. A load R is connected across the filter capacitor C2. 5 is a Controller Circuit comprising a dual output Trigger Generator 6 whose outputs are connected to the gates of the SCRs Ti and T2 and inputs are connected to a Voltage Comparator 7 and a Zero Current Detector (ZCUD) and Interlock 8. One Input of the Voltage Comparator is connected to a reference DC voltage Vj. The other input of the Voltage Comparator is connected to a Ramp Converter 9 which is connected to a reference AC voltage through a Zero Crossing Detector (ZCD) 10. The Ramp Converter is also connected to a
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reference DC voltage V2 through an Error Amplifier 11 which is also connected to the load voltage. A single output Trigger Generator 12 is connected to the gate of the auxiliary SCR and to the ZCUD and Interlock. The inputs of the ZCUD and Interlock are connected to the resonant inductor Li and to a reference DC voltage V3.
The reference DC voltage V2 is set having regard to the output voltage requirement of the load. In the first half cycle of operation of the AC mains in the switched on condition thereof SCR Tj is turned on at a firing angle decided by the Trigger Generator 6. At this time SCRs T2 and T3 are turned off. The resonant capacitor Q gets charged through the resonant inductor Li under resonant conditions because of which the input current is sinusoidal in nature and not square. When the voltage across the resonant inductor Li becomes zero, the SCR Tj gets turned off and the SCR T3 is turned on by the Trigger Generator 12. The filter capacitor C2 starts charging through the filter inductor L2 and power is dehvered to the load. The SCR T3 gets turned off when the voltage across and current through it becomes zero. In the next half cycle of operation of the AC mains, the Trigger Generator 6 turns on SCR T2 at the set firing angle and resonant capacitor Q
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starts charging through resonant inductor Lj. When the voltage across resonant inductor Li becomes zero SCR T2 gets turned off and SCR T3 is turned on by the Trigger Generator 12. The filter capacitor C2 starts charging through filter inductor L2 and power is delivered to the load. The input current is sinusoidal as explained earlier. The SCR T3 gets turned off when the voltage across and current through it becomes zero. The cycle is thus repeated. During the instances when SCRsJTi .an&T^and Ta_are turned off, residual power in the circuit will continue to be conducted to the load through the free wheeling diode D3 thereby making full use of _the power available in the circuit and reducing power being drawn from the AC mains. The reference AC voltage decides the natural zero crossing of the AC mains. ZCD 10 decides the instant of zero crossing from-the reference AC voltage. The Error Amplifier 11 will correct the difference between the set reference DC voltage V2 and the load voltage. The reference DC voltage Vj decides the minimum firing angle above the zero crossing to avoid jiter around the zero crossing instant. The Voltage Comparator compares the output from the Ramp Converter against the reference DC voltage to decide the firing angle by the Trigger Generator 6. The Ramp Converter generates a ramp signal based on the zero crossing instant and the corrected voltage input by the
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Error Amplifier. Reference DC voltage V3 decides the current through resonant inductor Lt . The Zero Current Detector (ZCUD) and Interlock 8 compares the voltage across inductor L{ with the reference DC voltage V3 and decides the instant of triggering of SCR T3 through Trigger Generator 12. The Zero Current Detector (ZCUD) and Interlock 8 also ensures that SCR T3 is turned off before SCR Ti or T2 is triggered.
The wave forms of Fig 2 of the accompanying drawings were simulated on computer using the software SABER of Analogy, USA. The four waves were simulated at the junction of the rectifier bridge and resonant inductor Lh the junction of span 3, junction of resonant inductor Lj and SCR T3 and the junction of filter inductor L2 and SCR T3 respectively. A comparasion of waves aj and bj clearly show that from the instance of conduction the input current follows in a sinusoidal wave pattern. A comparison of waves ct and d} shows that the voltage charged across resonant capacitor Q is delivered through the load when T3 is turned on thereby indicating that power conducted by the SCR Tj or T2 is fully transferred to the load through SCR T3 without any losses.
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The components used in the typical converter corresponding to
the wave forms of Fig 3 of the accompanying drawings were the following:
Component Value
Ti,T2 SCRBTW39-600
T3 SCRBTW39-800
DtXh. BYW 88-600
D3 BYW 88-1000
Ct 100fiF"/400 (AC capacitor)
C2 lOOOOjaF/100 V (Electrolytic)
Lj 5GmH
U 3mH
R 2.5-10 Ohms
The wave forms a2, b2, c2 and d2 correspond to the wave forms
aj, bh ci and di of Fig 2 respectively. Fig 3 clearly shows that the actual
experimental values thereof match with the simulated values of Fig 2.
In Fig 4 of the accompanying drawings curves marked with circles correspond to the conventional converter and those marked with squares correspond to the converter of the invention. Fig 4 clearly shows that
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the phase converter of the invention has liigher power factor as compared to the conventional converter at the same voltages and load currents. At full load, the power factor of the converter of the invention is high as 0.85.
The embodiment of Fig 1 of the drawings is by way of example and should not be construed to be limitative of the scope of the invention. Several variations of the invention are possible without deviating from the scope of the invention. For instance the parallel spans each may comprise one SCR and one diode. Alternatively both the spans may comprise two SCRs in each and the converter may be fully controlled. In the case of a fully controlled converter, instead of SCRs, high frequency switching devices like MOSFETS (Metal Oxide Semiconductor Field Effect Transistor), IGBT (Insulated Gate Bipolar Transistor), BJT (Bipolar Junction Transistor) or GTO's (Gate-Turn-Off Thyristor) may be employed in the Rectifier Bridge thereof with high frequency Plus width modulation. Such variations of the invention are to be construed to be within the scope of the invention.
According to the invention charging and power supply to the load do not take place simultaneously. Discharge is delayed and takes place
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only while the auxiliary SCR is conducting. The input current drawn from the AC mains is sinusoidal in nature at all firing angles and loads due to the resonant inductor and capacitor configuration thereby ensuring a constant output voltage to the load. Generation of large harmonics in the input current is eliminated thereby elimiiiating bulky capacitors in the output side of the converter. The over all size and cost of the converter is thus reduced. The input current being sinusoidal in nature in line with the input voltage DF factor is minimal. Therefore, generation of undesirable lower harmonics in the input current is minimised and the efficiency of the converter is increased. Due to the elimination of harmonics in the input current and the minimal DF factor, power factor of the converter is improved leading to good power transmission and utilisation of energy. Due to the sinusoidal nature of the input current and elimination of bulky capacitors at the output side of the rectifier bridge the inrush current drawn by the converter is eliminated thereby reducing the cost of transmission lines in tenns of size of the conductors. Because of the delayed discharge, load current drawn by the converter is practically continuous at all output voltages thereby avoiding the necessity of additional transformer and reducing size of the converter and
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improving the efficiency of the converter. Power factor of the converter of the invention is of the order of 0.85 at full load.

We Claim:
1 A high power factor phase controlled converter comprising a rectifier bridge comprising a pair of parallel spans of Sihcon Controlled Rectifiers (SCRs) and diodes or SCRs, the junctions of the parallel spans being connected to a single phase AC supply at the input side of the rectifier bridge, a resonant inductor and capacitor configuration connected across the rectifier bridge at the output side thereof, an auxiliary Sihcon Controlled Rectifier (SCR) connected to the junction of the resonant inductor and capacitor configuration, a free wheeling diode connected between the cathode of the auxiliary Sihcon Controlled Rectifier and ground, a filter inductor and capacitor configuration connected in series with the auxiliary Sihcon Controlled Rectifier, a load connected across the filter capacitor and a Controller Circuit comprising a dual output Trigger Generator whose outputs are connected to the gates of the Sihcon Controlled Rectifiers of the rectifier bridge and inputs are connected to a Voltage Comparator and a Zero Current Detector (ZCUD) and Interlock, one input of the Voltage Comparator being connected to a DC reference voltage and the other input of the Voltage Comparator being connected to a Ramp Converter which is
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connected to a reference AC voltage through a Zero Crossing Detector (ZCD) and to a reference DC voltage through an Error Amplifier which is also connected to the load, a single output Trigger Generator connected to the gate of the auxiliary Silicon Controlled Rectifier and to the Zero Current Detector (ZCUD) and Interlock, the inputs of the Zero Current Detector (ZCUD) and Interlock being connected to the resonant inductor and to a reference DC voltage.
2 A high power factor phase controlled converter substantially as
herein described particularly with reference to Fig 1 of the accompanying drawings.
Dated this 7th day of June 2001.
(M A Jose) of De PENNING & De PENNING Agent for the Applicants
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