LOW COST SOLID STATE RF GENERATOR

Electronics System Design-I (EC-705)Project-I

Semester – 7B.E. (E & C)

By:Group No: 1

Shraddha Sharma Ankita V. Jain (06-ECG-31) (06-ECG-41)

Internal Guide: External Guide:

Prof. M.P.Teredesai Mr.R.B. PatelH.O.D. C.E (Business Development)E & C Dept., Twin EngineersSVIT-VASAD. Vadodara

Electronics & Communication Engineering Department.Sardar Vallabhbhai Patel Institute of Technology,

VASAD.

YEAR 2009-10

Sardar Vallabhbhai Patel Institute of Technology, VASAD.

Electronics & Communication Engineering Department.

Certificate

This is to certify that project entitled “Low Cost Solid-State RF Generator” which is being submitted by

1) Shraddha Sharma (06-ECG-31)2) Ankita V. Jain (06-ECG-41)

of programme Electronics & Communication have satisfactorily completed their term work in the subject of Electronics system design-I(project-I) code EC-705, during the academic year 2009-10.

Date: /11/2009

Internal Project Guide Head of the Department

Prof. M.P.Teredesai Prof.M.P.TeredesaiH.O.D H.O.D.E & C Dept. E & C Dept.SVIT-VASAD. SVIT-VASAD

ACKNOWLEDGEMENT

Firstly, we would like to thank Gujarat University for giving us the opportunity to get industrial experience in 7th semester.

We would like to thank our HOD, Prof. M.P. Teredesai for allowing us to carry out the project work at the firm of our choice. We would specially thank Mr. Y.B. Shukla for being ever ready to guide us and clear all our queries.

We are grateful to Mr. J.T. Patel, CEO of M/S. Twin Engineers for permitting us to do our project work at his company’s premises. We thank him especially for the valuable time he gave us in which he taught us many lessons of project management.

We also heartily thank our guides, external and internal, for the precious time guiding us and clearing all of our doubts. Mr. R.B. Patel and Miss Kailash, our external guides at Twin Engineers made us familiar with the concept of constant motivation and taught us the intricacies of successfully carrying out a project.

PREFACE

The pre-final semester of B.E. in Electronics & Communications recently involved undertaking a project in an industrial environment along with the academic subjects. As a part of the requirement, we decided to do our project work with M/S. Twin Engineers, G.I.D.C. Makarpura, Vadodara.

The firm had many projects in hand and our group was fortunately assigned the most challenging one. We were required to manufacture a low cost solid-state RF Generator. We consider this project to be the most challenging on. It is a device that generates the specified value of the frequency continuously with the help of solid state power mosfet placed inside it and the combining circuitry. The generated RF frequency can be further applied inside a food dryer. The efficiency of the whole heating system depends upon this RF Generator which is comparatively much more than that obtained from the conventional or microwave heating.

In the following report, we have described how we accomplished our target of manufacturing a working prototype of a low cost solid state RF Generator. The report is divided into chapters which deal with specific areas related to the RF concepts. We hope that by the end of this report, the concept of RF energy and its generation will be clear.

TWIN ENGINEERS

HISTORY & BACKGROUND

Twin Engineers is situated at 195, Behind V.C.C.I. Complex, G.I.D.C. Makarpura, Vadodara, India. It is promoted by Mr. J.T. Patel; Ex General Manager (M/S Gujarat Communication & Electronics Ltd.) who has also worked in Physical Research Laboratory, Ahmedabad. Twin Engineers is growing in size and scope. It delivers full line of durable, reliable and highly precision professional grade equipments and RF Components.

PRODUCT RANGE

Twin Engineers manufactures a complete system for Broadcast applications. 50W/500W VHF/UHF Unattenuated low power TV Transmitters. Parabolic dish antennas for satellite reception-3.7 GHz to 4.2 GHz Tele vision receive only in S Band and C Band FM Transmitters RF Loads

TWIN ENGINEERS’-COSTUMERS

Prasar Bharti - Broadcasting Corporation of India, Doordarshan All India Radio- AIR M/S Webel Mediatronics M/S Ferno U.K. M/S INS AUSTRALIA

PERSONNEL

Highly skilled and experienced technical manpower having expertise in broadcast, RF Amplifiers, RF peripheral devices and antenna. The experience of the technical staff in the field of manufacturing professional grade electronic equipments is of more than 15 years

ASSEMBLY FACILITIES AT TWIN ENGINEERS

Antistatic work stations for precision assembly Wave soldering Aging facilities Transformer winding R-Core Assembly of PCB and Sub Modules

TESTING FACILITIES AT TWIN ENGINEERS

Twin Engineers have a highly sophisticated test set up for its products.It has a specialized testing and inspecting site for Antenna Field Strength measurement which is duly certified by their customers.

QUALITY CONTROL

Twin Engineers’ Quality slogan is “TO STRIVE CONTINUOUSLY FOR COSTUMER SATISFACTION THROUGH RELIABLE PRODUCT”.

FACILITIES AVAILABLE AT TWIN ENGINEERS

Easy availability of AL Extusions-upto 200mm diameter x 6.5 mt. long Welding facilities such as argon welding Brazing facility Precision machining capabilities. Easy availability of Al and C.I. castings of various grades Inspection and measuring facilities

PROJECTS HANDLED

Twin engineers successfully supplied TV transmitting antenna to doordarshan Twin engineers successfully supplied TV transmitters 2x50W uhf VLPT to

doordarshan Twin engineers successfully supplied TVRO(ATTENDED AND UNATTENDED) in

C band to doordarshan Twin engineers are now manufacturing 500W UHF LPT TV Transmitter for

doordarshan.

CONTACT:

Mr. R.B. PATEL, C.E. (BUSINESS DEVELOPMENT and R&D)TWIN ENGINEERS195, Behind V.C.C.I ComplexGIDC, MakarpuraVadodara-390010Gujarat (India)Tele-Fax: - 0265-2647987Email: - twinengineers@gmail.com

INDEX

1. Introduction 11.1 Radio Frequency 21.2 Comparison of RF Heating & Microwave Heating 7

2. System Specifications 9

3. Block Diagram 113.1 Block Diagram 123.2 Detailed Description 13

4. Schematic Diagram 144.1 Circuit diagrams 154.2 Circuit descriptions 184.3 Component List 21

5. Hardware Description 225.1 Power Mosfet 235.2 Balanced Feed Choke 285.3 Balun Transformer 29

6. Testing & Results 316.1 Calculations 326.2 Performance Measurements 33

7. Future Expansion 34

8. Conclusion 37

9. Applications & Limitations 39

10. Bibliography 42

11. Appendix 44

CHAPTER 1

INTRODUCTION

1. INTRODUCTION

1.1 RADIO FREQUENCY

Radio waves were first predicted by mathematical work done in 1865 by James Clerk Maxwell. Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations.

His most significant achievement was the development of the classical electromagnetic theory, synthesizing all previous unrelated observations, experiments and equations of electricity, magnetism and even optics into a consistent theory

Radio frequency (RF) is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 KHz to 300 GHz.

This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves. Since most of this range is beyond the vibration rate that most mechanical systems can respond to, RF usually refers to oscillations in electrical circuits.

Diagram of the electric (E) and magnetic (H) fields of Radio Waves emanating from a radio transmitting antenna (small dark vertical line in the center). The E and H fields are perpendicular as implied by the phase diagram in the lower right.

NAMED FREQUENCY BANDSGeneralBroadcast Frequencies:Longwave AM Radio = 148.5 - 283.5 kHz (LF) Mediumwave AM Radio = 530 kHz - 1710 kHz (MF) Shortwave AM Radio = 3 MHz - 30 MHz (HF) TV Band I (Channels 2 - 6) = 54 MHz - 88 MHz (VHF) FM Radio Band II = 88 MHz - 108 MHz (VHF) TV Band III (Channels 7 - 13) = 174 MHz - 216 MHz (VHF) TV Bands IV & V (Channels 14 - 69) = 470 MHz - 806 MHz (UHF)

Amateur radio frequencies - The range of allowed amateur radio frequencies vary between countries. The Amateur radio frequency allocations lists frequencies allocated for amateur radio use.

IEEE US

Band Frequency range Origin of name

HF band 3 to 30 MHz High Frequency

VHF band

30 to 300 MHz Very High Frequency

UHF band

300 to 1000 MHz

Ultra High Frequency Frequencies from 216 to 450 MHz were sometimes called P-band: Previous, since early British radar used this band but later switched to higher frequencies.

L band 1 to 2 GHz Long wave

S band 2 to 4 GHz Short wave

C band 4 to 8 GHz Compromise between S and X

X band 8 to 12 GHzUsed in WW II for fire control, X for cross (as in crosshair)

Ku band 12 to 18 GHz Kurz-under

K band 18 to 27 GHz German Kurz (short)

Ka band 27 to 40 GHz Kurz-above

V band 40 to 75 GHz

W band 75 to 110 GHz W follows V in the alphabet

mm   band 110 to 300 GHz

USES OF RADIO FREQUENCY

Radio Frequency (RF) refers specifically to the electromagnetic field, or radio wave, that is generated when an alternating current is input to an antenna.

This field can be used for wireless broadcasting and communications over a significant portion of the electromagnetic radiation spectrum -- from about 3 kilohertz (kHz) to thousands of gigahertz (GHz) -- referred to as the RF spectrum.

As the frequency is increased beyond the RF spectrum, electromagnetic energy takes the form of infrared, visible light, ultraviolet, X rays and gamma rays.

Many types of wireless devices make use of RF fields: radio, television, cordless phones, cell phones, satellite comm. systems, and many measuring and instrumentation systems used in manufacturing. Some wireless devices, such as remote control boxes and cordless mice, operate at infrared or visible light frequencies.

RADIO FREQUENCY HEATING

In a radio frequency heating system, the RF generator creates an alternating electric field between two electrodes. The material to be heated is conveyed between the electrodes, where an alternating energy field causes polar molecules in the material to continuously reorient them to face opposite electrodes much like the way bar magnets move to face opposite poles in an alternating magnetic field. Friction resulting from this molecular movement causes the material to rapidly heat throughout its entire mass.

The illustration below depicts a radio frequency drying system with material between the electrodes. Polar molecules within the material are represented by the spheres with plus (+) and minus (-) signs connected by bars.

The approx. Power delivered per unit length of heating pipe (W/met sq),P is given by :

P = pi (E^2) f K E A tan (a)

Where E is average peak electric field (V/m), f is the frequency of the signal (Hz), K is the dielectric constant of product to be heated, E is the permittivity of free space (8.85*10^-12 F/m), A is the cs. area(m^2) of heating pipe and tan (a) is the loss factor (typically 0.1 for water).

DIAGRAMATIC REPRESENTATION OF RF HEATING

RADIO FREQUENCY DRYING

The use of Radio Frequency (RF) drying can offer many benefits over conventional drying, including faster line speeds, more consistent moisture levels, lower drying temperature, and smaller equipment.

Conventional heating (i.e. conduction, convection, radiant) has a heat source on the outside and relies on transferring the heat to the surface of the material and then conducting the heat to the middle of the material.

Radio Frequency heating is different; it heats at the molecular level so it heats from within the material and heats the middle as well as the surface.

DIFFERENCE IN CONVENTIONAL AND RF HEATING

A conventionally dried product is hot and dry on the outside and cold and wet on the inside. Unfortunately, this is not efficient because the dry outer layer acts as an insulating barrier and reduces the conduction heat transfer to the middle of the product. This dry outer layer can cause quality problems, such as surface cracking, a skin on coatings and uneven solids dispersion through wicking of sizing and additives from the middle to the surface.

With Radio Frequency drying, the heating is from within so there is no hot, dry outer layer. The product is heated throughout so the water in the middle will be heated and will move to the surface.

In general, because of the heat losses at the surface, radio frequency dried products are hot and dry on the inside and cooler and wetter on the outside.

The combination of two technologies, using the RF heating to heat the inside and move the water to the surface where conventional methods are effective at removing it, offers some great potential benefits.

Materials have a major effect on the success of RF heating. Some materials heat very well and some do not heat well at all. The key measure of “heatability” is the loss factor of the material. The loss factor is a material property that determines how well the material absorbs the RF energy.

If the material has a high loss factor, it absorbs energy quickly and thus heats quickly. If a material has a low loss factor, it absorbs energy slowly and thus heats slowly. In general, polymers tend to have low loss factors and thus do not heat well.

Water, on the other hand, has a high loss factor so it heats rapidly. This is why RF lends itself to drying so well, it heats the water quickly but does not heat most base materials.

1.2 COMPARISON OF RF AND MICROWAVE HEATING

The unique characteristics of RF heating and drying offer many benefits over conventional heating and drying methods. There are numerous cases where microwave dryers in industrial production have been replaced by RF dryers for these fundamental reasons. The excerpts below provide a detailed explanation of scale-up differences between microwave and RF.

FASTER HEATING AND DRYING TIMESRF heat products directly and throughout the thickness of the product.

MORE EVEN HEATING AND DRYINGRF heats from within the material and does not rely on conduction. This provides uniform temperature gradient throughout the material for more consistent product quality.  Binder/sizing migration is significantly reduced.

SELF LIMITING DURING DRYINGThe heating rate is proportional to the amount of water in the material. As the material dries, less RF energy is absorbed, the heating rate decreases. Most materials will not overheat. This improves product quality.

MOISTURE LEVELING AND PROFILINGThe heating rate is proportional to the amount of moisture, wet areas heat faster than dry areas within a product. This accelerated drying continues until the moisture is uniform throughout the product & creates consistent product quality.

SELECTIVE HEATINGDifferent materials heat at different rates so it is possible to heat only one part of a composite material or to dry a coating without heating the substrate.

INSTANT START AND STOPPower and heating start and stop instantly, with a virtually no warm up or cool down time.

ENERGY EFFICIENCYEnergy usage is proportional to the amount of work being done. All energy goes into the work without losses to the environment. If the line is running at less than capacity, energy usage is lower. This can lower fuel costs.

SPEED AND UNIFORMITY Heating occurs instantly and uniformly throughout the mass of a homogeneous material. No temperature differential is required to force heat via conduction from the surface to the center as in convection or infrared heating processes

FEWER ENVIRONMENT ISSUESThere is no combustion or combustion by-products with RF. This saves on both capital and operating costs.

MOISTURE EQUILIBRATION Because wetter areas absorb more RF power than dryer areas, more water is automatically removed from wet areas resulting in a more uniform moisture distribution.

SPACE SAVING

The applicator, or electrode section, is slightly wider than the load itself. Length will be a small fraction of the length of the convection dryer required to do the same work. Special applicator designs may be used, multi-pass, multi-zone, arched, inclined, or vertical.

PHYSICAL CONTACT The load may be supported by electrodes or conveyed under or between them. Self-supporting webs or strands need not touch anything, thus avoiding surface marking and contamination.

EFFICIENCY Power is consumed primarily in the work load. There are no losses from heating masses of cast iron or huge volumes of hot air , no long warm up or cooling times are required. Power is consumed only when the load is present and only in proportion to the load.

PRECISE CONTROL Power control is accurately metered and may be recorded. A meter constantly displays the amount of power being applied to heat the product.

CHAPTER 2

SYSTEM SPECIFICATIONS

2. SYSTEM SPECIFICATIONS

The project deals with the generation of predefined value of the Radio Frequency in a circuit card. This circuit card can be installed in the heating modules for the heating of food products.

RF Generator is formed by the combination of various solid-state devices. Here, the solid state devices are the low cost Radio frequency power mosfets.

The output generated is fed to the 50 ohm load impedance line. There are some system specifications on the basis of which the system was planned to be built.

FREQUENCY:- 27.12MHz ( Radio frequencies reserved for industrial use by the Federal Communications Commission because of their higher penetration depth)

POTENTIAL AT OUTPUT:- 0.5 TO 3KVolts MATERIAL COST :- Rs.5000/kwatt REDUNDANCY (of one circuit card) :- 50% TOTAL EFFICIENCY (for entire circuit) :- >70% Power MOSFET (for each circuit card)

- Frequency :-27.12 to 30MHz- Operating DC voltage:- 30-50V(max)- Current rating (for the power MOSFET):- Up to 100amperes- Device Package Construction :-TO-220 or TO-247

- Output Power Variation:-Manual Control by POT(50W-1kW) - Operating Temperature :-0-50 deg - Configuration :- PUSH-PULL TYPE

CHAPTER 3

BLOCK DIAGRAM AND

DESCRIPTION

3. BLOCK DIAGRAM & DESCRIPTION

3.1 BLOCK DIAGRAM

push-pull amplifier

Input transformer Output transformer

MatcherDriver

Signal Generator

3.2 BLOCK DIAGRAM DESCRIPTION

GENERATOR: The first block is the generator which consists of a crystal oscillator. The crystal oscillator generates an output frequency of 27.12 MHz which is fed to driver stage. Variable coils and resistors help in attaining different gains (dB).Value of the frequency at output can be changed by changing the crystal value. The output obtained from the generator is around 8dB.

DRIVER: It consists of MRF476 power transistor which has an input of 30MHz, and is housed in a TO-220 package. It is used to drive the push-pull amplifier with the required amount of gain, which is the next stage. No adjustments or tunings are required at this stage.

The output obtained from this MRF476 is about 40 W, which is passed to higher stage. Although for much stable outputs; MRF 477 can be used as a pre-amplifier. The driver gives an output of approx. 30dB.

INPUT TRANSFORMER: The input transformer used here is a ferrite bead transformer, which is a 4:1 step down conventional transformer. It is used for impedance matching (along with matcher) and providing complementary gate drive signals.

PUSH-PULL AMPLIFIER: It is built around 3 symmetric pair of common source RF power mosfet, IRF 440.The output obtained from it is around 24dB.

OUTPUT TRANSFORMER: The output transformer used here is a conventional balun transformer, which is a 1:4 step up transformer. Here it acts as a combiner and is used to provide -180 degrees phase shift to output obtained from push-pull operation.

CHAPTER 4

SCHEMATIC DIAGRAM

4. SCHEMATIC DIAGRAM

The circuit is divided into three parts:- The Signal Generator The Driver The Push-Pull Amplifier

4.1 CIRCUIT DIAGRAMS

STAGE 1

Fig 1. The sine wave signal generator

STAGE 2

Fig 2. The Driver

STAGE 3

Fig 3. The Push-Pull Amplifier

4.2 CIRCUIT DESCRIPTION

1. THE GENERATOR

- The generator used here is a signal generator which generates sine wave to drive the driver circuitry. The supply provided to the generator circuit is of about +5V to +12V. A crystal XT1 of frequency 27.12MHz is used.

- The other supporting components are various resistors and capacitors, which can be either variable or of fixed values. The value of the gain obtained from this stage is of about 8db to 12db.

- The value of the gain can be adjusted by changing the value of the resistors and capacitors used in it.

2. THE DRIVER

- The driver is used to provide the timed signals for the higher stages. The driver helps in driving of the amplifier stage. The supply given to it is 100V. The main power RF Transistor used over here is a Motorola product i.e. MRF476.

- The driver stage active devices are operated in Class C mode. MRF476 is basically an NPN RF transistor which is designed for large signal amplifier applications to 30MHz. The driver is used to avoid the floating of the resistors in the amplifier stages into the dc potential values and for switching on and off of the solid state Radio frequency power mosfets in the push-pull amplifier stage.

- The RF Chokes present in the circuit are used for the blocking of the RF signals for the higher stage and provide a pure dc output for the amplifier stage. Output obtained from this stage is of 30db or 40watts.

3. THE PUSH-PULL AMPLIFIER STAGE

INPUT NETWORK

- The push-pull amplifier stage is the final stage of the RF Generator which generates the RF output of 27.12MHz and the power output is 1KW or a gain of 60db is obtained.

- This amplifier is formed by the chain of the solid-state devices which are power mosfets IRF440 here.

- The configuration is push-pull inverter chain in which the power mosfets are mirror images of each other to have electrical symmetry and low source-to-source impedance required for the push-pull operation.

- The resistors R4 and R5 are provided to ensure that gates of the power mosfets do not float into a dc potential thus imbalancing the amplifier bias points.

- Ferrite bead transformer T1, 4:1 step down transformer is used to couplethe output of driver stage to the amplifier stage. The secondary feeds the gates of the Mosfets through resistors R6 to R17 complementary gate drive signals to the push-pull chain of the power mosfets and for impedance matching. It is constructed using two Fair-Rite cores, μ=850 with 2 turns of #18 stranded PTFE coated wire on the primary and 1 turn of .25"copper braid on the secondary.

- The parallel resistors from R6 to R17 are connected in series with the gates of the power mosfets so that the de-Q input can be given to the power mosfets i.e. the damped oscillations are not obtained from the power mosfets. These resistors also prevent a possible VHF emitter coupled multivibrator oscillations which can occur in parallel mosfets at high frequency switching. They also reduce the gain by 5db to ensure stability.

- It is important to see to the grounding of the input components. With the gate current approaching 8A, ground loop currents make proper ground layout critical. A double sided printed circuit board with the bottom essentially intact is recommended.

- The mosfet output is coupled via inductors L5 and L6 to the output

network to cancel the effective series equivalent output capacitance and it also transforms the effective series inductance of each drain to approximately 10ohms.

OUTPUT NETWORK

- The input impedance is not constant over the range of output power. Depending on the characteristics of the driver, the output power can

appear to ‘snap on’ suddenly as the drive is increased if the input network has been previously adjusted for a good match at full output. This can be a little disconcerting, but it is the normal result of interaction between the driver and the load. The use of the matching transformer improves this effect but does not completely eliminate it.

- Balanced feed choke transformer, BFC1 serves dual purpose of compensation of output capacitance transformer and dc power decoupling. Here it uses bifilar type of winding in which one conductor starting is other conductor’s ending point. The high output impedance causes the significant voltage swing at the drain which if placed across the feed choke would cause it to saturate and become very hot.

- Push pull configuration requires a balanced circuit. The voltage to the drain of each transistor is to be fed through a bifilar choke wound on a ferrite core which provides several advantages like balanced flux in the magnetic choke & dc elimination in output balun That’s why the output is then fed to the parallel combination of the capacitors C10, C11 and C12, C13.

- The balun transformer, T3 which is a 1:4 step up transformer. Balun transformer is used for linking the balanced circuit with the unbalanced one and vice versa. The phase shift of 90 degree is provided to the input from the push and pull chains. So that a continuous one phase waveform is obtained at the output side. The balun is constructed from 24" of RG-303 50ý PTFE coax cable by forming it into a coil of 3 turns approximately 2.5 inches diameter

- Harmonic suppression coil, L7 and capacitors C17 and C18 are used to form the tuned circuit for transferring the output from the amplifier stage to 50 ohms output load impedance line.

4.3 COMPONENT LIST

Component name

Value Qty.

Signal Generator (+5v to +12v power supply)1

C1,C3 10 kpf (104) 2C2,C4 1 kpf (.001/103) 2R1, R2 1 kohm 2R3 41 ohm 1U1 Lm 555 1

Driver amplifier (100v power supply)Q2 Irf 440 1C7 100pf, 500v, chip capacitor 1L3 806nh, 2w choke 1C8 560pf, 500v, chip capacitor 1L4 105nh, 2w choke 1C9 300pf, 1000v, chip capacitor 1

Power Amplifier (1000v power supply)T2 4:1 conventional transformer; pri:

2T #18 stranded PTFE coated wire, sec:1T #14 tinned braid on two Fair-Rite #2643540002, μi=850

1

Q3 to Q8 APT 6015 LVR or (ARF 440) 6 nos or (20nos)R4, R5 1000ohm, 2watt, carbon resistor 2R6 to R17 1ohm, 2w, carbon resistor 8C10 to C15, 0.01uf, 1000v, disk ceramic 8C16, C17 0.001uf, 1000v, disk ceramic 2C18 250-480pF Mica Compression

Trimmer Capacitor, 1000v1

C19 95-230pF Mica Compression Trimmer Capacitor, 1000v

1

L 0.470uh, 5.5t #18AWG 1L5,L6,L7 0.145uh, 3.5t #14AWG, id=0.438 3RFC1 2T, #18 PTFE on a Fair-Rite

#2643665702 shield bead, μi=8501

T3 1:1 (Z) coaxial balun transformer; 24 inches RG303 PTFE Coax formed into a 3 T, 2.5” dia.

1

BFC 1 6T, #24 twisted pair enamel wire on three stacked Fair-Rite 596118021 toroids

1

CHAPTER 5

HARDWARE DESCRIPTION

5. HARDWARE DESCRIPTION

5.1 POWER MOSFET

A Power MOSFET is a specific type of metal oxide semiconductor field-effect transistor (MOSFET) designed to handle large amounts of power. Compared to the other power semiconductor devices (IGBT, Thyristor...), its main advantages are high commutation speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive.

It was made possible by the evolution of CMOS technology, developed for manufacturing Integrated circuits in the late 1970s. The power MOSFET shares its operating principle with its low-power counterpart, the lateral MOSFET.

The power MOSFET is the most widely used switch. It can be found in most power supplies, DC to DC converters, and low voltage motor controllers.

The two types of Mosfets are the depletion type and the enhancement type, and each has a n /p - channel type. The depletion type is normally on, and operates as a JFET.

The enhancement type is normally off, which means that the drain to source current increases as the voltage at the gate increases. No current flows when no voltage is supplied at the gate.

CHARACTERISTICS

1) ADVANTAGES

(i) High input impedance - Voltage controlled device - Easy to drive

To maintain the on-state, a base drive current which is 1/5th or 1/10th of collector current is required for the current controlled device (BJT). And also a larger reverse base drive current is needed for the high speed turn-off of the current controlled device (BJT). Due to these characteristics base drive circuit design becomes complicated and expensive. On the other hand, a voltage controlled MOSFET is a switching device which is driven by a channel at the semiconductor’s surface due to the field effect produced by the voltage applied to the gate electrode, which is isolated from the semiconductor surface. As the required gate current during switching transient as well as the on and off states is small, the drive circuit design is simple and less expensive.

(ii) Unipolar device - Majority carrier device - Fast switching speed As there are no delays due to storage and recombination of the minority carrier, as in the BJT, the switching speed is faster than the BJT by orders of magnitude. Hence, it has an advantage in a high frequency operation circuit where switching power loss is prevalent.

(iii) Wide SOA (safe operating area)

It has a wider SOA than the BJT because high voltage and current can be applied simultaneously for a short duration. This eliminates destructive device failure due to second breakdown.

(iv) Forward voltage drop with positive temperature coefficient - Easy to use in parallel

When the temperature increases, the forward voltage drop also increases. This causes the current to flow equally through each device when they are in parallel. Hence, the MOSFET is easier to use in parallel than the BJT, which has a forward voltage drop with negative temperature coefficient.

2) DISADVANTAGE

In high breakdown voltage devices over 200V, the conduction loss of a MOSFET is larger than that of a BJT, which has the same voltage and current rating due to the on-state voltage drop.

3) BASIC CHARACTERISTICS

(i) Vertically oriented four-layer structure (n+ p n– n+)

(ii) Parasitic BJT exists between the source and the drain.

The p-type body region becomes base, the n+ source region becomes an emitter, and the ntype drain region becomes the collector (refer to Figure 5). The breakdown voltage decreases from BVCBO to BVCEO, which is 50 ~ 60 [%] of BVCBO when the parasitic BJT is turned on. At this state, if a drain voltage higher than BVCEO is supplied, the device falls into an avalanche breakdown state. If the drain current is not limited externally, it will be destroyed by the second breakdown. So the n+ source region and the p-type body region must be shorted by metallization in order to prevent the parasitic BJT from turning on.But if the VDS rate of increase is large in the high speed turn–off state, there is a voltage drop between the base and the emitter, which causes the BJT to turn–on. This is prevented by increasing the doping density of the p - body region, which is at the bottom of the n+ source region, and by lowering the Mosfets switching speed by designing the circuit so that the gate resistance is large.

4) OUTPUT CHARACTERISTICS

Id characteristics due to VDS in many VGS conditions. It is divided into the ohmic region, the saturation (=active) region, and the cut-off region.

Ohmic region: A constant resistance region. If the drain-to-source voltage is zero, the drain current also becomes zero regardless of gate–to-source voltage.This region is at the left side of the VGS – VGS(th) = VDS boundary line (VGS – VGS (th) > VDS > 0). Even if the drain current is very large, in this region the power dissipation is maintained by minimizing VDS(on).

Saturation region: A constant current region. It is at the right side of the VGS – VGS (th) = VDS boundary line. Here, the drain current differs by the gate–to source voltage, and not by the drain-to-source voltage. Hence, thedrain current is called saturated.

Cut-off region: It is called the cut-off region, because the gate-to-source voltage is lower than the VGS(th) (threshold voltage).

5) ON-STATE CHARACTERISTICS

When the power MOSFET is in the on-state (see MOSFET for a discussion on operation modes), it exhibits a resistive behaviour between the drain and source terminals. The resistance (called RDSon for "drain to source resistance in on-state") is the sum of many elementary contributions:

RS is the source resistance. It represents all resistances between the source terminal of the package to the channel of the MOSFET: resistance of the wire bonds, of the source metallisation, and of the N+ wells;

Rch. This is the channel resistance. It is directly proportional to the channel width, and for a given die size, to the channel density. The channel resistance is one of the main contributors to the RDSon of low-voltage MOSFETs, and intensive work has been carried out to reduce their cell size in order to increase the channel density;

Ra is the access resistance. It represents the resistance of the epitaxial zone directly under the gate electrode, where the direction of the current changes from horizontal (in the channel) to vertical (to the drain contact);

RJFET is the detrimental effect of the cell size reduction mentioned above: the P implantations (see figure 1) form the gates of a parasitic JFET transistor that tend to reduce the width of the current flow.

RD is the equivalent of RS for the drain. It represents the resistance of the transistor substrate (note that the cross section in figure 1 is not at scale, the bottom N+ layer is actually the thickest) and of the package connections.

SELECTION OF IRF440 AS A POWER MOSFET

It is an N-Channel enhancement Power Mosfet designed for high voltage and high speed applications. It is available in TO-220 package and has fast switching times, excellent high voltage stability and has a rugged polysilicon gate cell structure.

5.2 BALANCED FEED CHOKE

The choke is designed to create a zero DC magnetic bias in the core when both transistors draw the same average current. With the devices operating 180 degrees out of phase, the windings present high impedance at 27.12MHz to the drain of each MOSFET.

It also acts as a 4:1 impedance transformer which greatly simplifies matching the drain impedance. The choke is constructed by winding 6 turns of #24 twisted pair (approximately 5 twists per inch) solid enamel wire around three stacked Fair-Rite toroids # 5961001801, μi=125.

This transformer uses the bifilar windings in which two wires are coiled together and starting of one wire is connected to the end of another wire.

The basic function here in the circuit is to obtain impedance matching.

The bifilar winding technique is preferred because it is the most effective and precise means of providing the most accurately balanced output from a transformer. Not only is the voltage potential on the output perfectly balanced, but the inductance, resistance and capacitance of the output are also precisely balanced.

5.3 BALUN TRANSFORMER

A Balun is a device which converts balanced impedance to unbalanced and vice versa. In addition, baluns can also provide impedance transformation, hence the name Balun Transformers. They are designed specifically to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar or coaxial cable and are similar to transmission line transformers in construction and operation.

SCHEMATIC OF A BALUN TRANSFORMER

In a balun, one pair of terminals is balanced, that is, the currents are equal in magnitude and opposite in phase. The other pair of terminals is unbalanced; one side is connected to electrical ground and the other carries the signal.Balun transformers can be used between various parts of a wireless or cable communications system. The following table denotes some common applications.

Balanced Unbalanced

Television receiver coaxial cable network

Television receiver Coaxial antenna system

FM broadcast receiver Coaxial antenna system

Dipole antenna Coaxial transmission line

Parallel-wire transmission line

Coaxial transmitter output

Parallel-wire transmission line

Coaxial receiver input

Parallel-wire transmission Coaxial transmission line

Some baluns provide impedance transformation in addition to conversion between balanced and unbalanced signal modes; others provide no impedance transformation.

For 1:1 baluns (no impedance transformation), the input and output are usually either 50 ohms or 75 ohms.

The most common impedance-transformation ratio is 1:4 (alternatively 4:1). Some baluns provide other impedance-transformation ratios, such as 1:9 (and 9:1), 1:10 (and 10:1), or 1:16 (and 16:1). Impedance-transformer baluns having a 1:4 ratio are used between systems with impedances of 50 or 75 ohms (unbalanced) and 200 or 300 ohms (balanced).

Most television and FM broadcast receivers are designed for 300-ohm balanced systems, while coaxial cables have characteristic impedances of 50 or 75 ohms. Impedance-transformer baluns with larger ratios are used to match high-impedance balanced antennas to low-impedance unbalanced wireless receivers, transmitters, or transceivers.

In order to function at optimum efficiency, a balun must be used with loads whose impedances present little or no reactance. Such impedances are called "purely resistive."  As a general rule, well-designed communications antennas present purely resistive loads of 50, 75, or 300 ohms, although a few antennas have higher resistive impedances.

The "balanced" terminals of some baluns can be connected to an unbalanced system. One terminal of the balanced pair (input or output) is connected to ground, while the other is connected to the active system element.  When this is done, the device does not operate as a true balun, because both the input and the output are unbalanced. A balun used in this way has been called an "un-un" (for "unbalanced-to-unbalanced").

Some baluns can work as an impedance transformer between two unbalanced systems if there is little or no reactance.  But certain types of baluns do not work properly when connected in this manner. It is best to check the documentation provided with the device, or contact the manufacturer, if "un-un" balun operation is contemplated.

CHAPTER 6

TESTING AND RESULTS

6. TESTING AND RESULTS

6.1 CALCULATIONS

STAGE 1: SIGNAL GENERATOR

INPUT OUTPUTPower supply: +5V Gain: 8db

STAGE 2: DRIVER AMPLIFIER

INPUT OUTPUTPower supply: +100V Gain: 30db

O/P Power: 40watts

STAGE 3: PUSH-PULL AMPLIFIER

INPUT OUTPUTPower supply: 1000V Gain: 57db

O/P Power: 1000watts

RF Frequency: 27.12MHz

TOTAL OUTPUT POWER = 1000 Watts

5.2 PERFORMANCE MEASUREMENTS

The amplifier was operated under two conditions. First the amplifier was driven with a 27.12MHz RF signal, modulated by a 1 kHz sine wave, at a 50% duty cycle, up to a peak power out of 1600W.

Then the amplifier was driven with a 27.12MHz CW RF signal up to a continuous power out of 1200W. Due to the close correlation of the modulated data and the CW data it was concluded that there is significant thermal margin.

Figure 1 show the performance data for this amplifier. Figure 1 is a plot of Pin versus Pout. The curves show the classical class C characteristics, with low gain at low power output, improving as the output power increases. The gain peaks over 30dB when the amplifier output is between 600W and 1kW.

In a class C amplifier there is a tradeoff between maximum output power and efficiency. Less drive will produce higher gain but at a reduced efficiency, approximately 60% for 20 dB gain.

FIGURE 1: Pin versus Pout

CHAPTER 7

FUTURE EXPANSION

7. FUTURE EXPANSION

8 RF GENERATOR

(1KW each)

10 HEATING STAGES

(2KW each)

6 HEATING MODULES

(20KW each)

1 HEATING UNIT(120KW each)

DESCRIPTION

The project deals with the development of a RF GENERATOR which can also be referred to as a circuit card.

The Redundancy obtained with the project is approximated to be around 50% which means that even if one circuit card fails, the system should still work.

If such 8 circuit cards or RF generators are connected, with an output of 1KWatt each, there is development of a HEATING STAGE.

With each heating stage we get an output of 1KWatt. If 10 such heating stages are combined, we get an output power of 20KWatts. This leads to the formation of the next higher stage known as the HEATING MODULE.

Combining such 6 heating modules leads to generation of a HEATING UNIT. A heating stage is the highest culminating point of the entire system and generates an extremely high power nearing about 120KWatts.

In a heating stage, a method of controlling heat transferred to a surface of a moist food product, including the step of moving the food product relative to the streams of high velocity gas by positioning the moist food product on a conveyor and moving the conveyor such that the streams of high velocity gas impinge against discrete areas on upper and lower surfaces of the food product.

Hence we see how the designed radio frequency generator can be installed in a dielectric heating unit. The use of Radio Frequency (RF) heating or drying can offer many benefits over conventional drying, including faster line speeds, more consistent moisture levels, lower drying temperature, and smaller equipment.

CHAPTER 8

CONCLUSION

CONCLUSION

As per the project definition, we have manufactured RF Generator within the stipulated time of the semester which can generate the radio frequency of 27.12MHz.

We faced problem in connecting the power mosfets in push-pull inverter configuration. The project included a lot of profoundness and keen insight. It was tested under rugged conditions and days of brainstorming and post testing modification of the design led to the final product. This product is able to sustain its capacity power without resistor failing due to high temperatures developed inside the unit at high frequencies because of power mosfets. Also the VSWR is quite satisfactory at the required frequencies.

The RF Generator which we have manufactured is useful for large scale heating units which work at radio frequencies. The performance matches with the industrial standards and the price is reasonable compared to other competitors.

Hence, we manufactured a working prototype of RF Generator.

CHAPTER 9

APPLICATIONSAND

LIMITATIONS

8. APPLICATIONS & LIMITATIONS

APPLICATIONS

RF heating is used in:

Fiberglass mats are dried with higher speeds and more consistent quality product.

Foam products are dried with cycle times reduced from 4 hours to 1 hour. Many foams are dried with a hybrid system of convection and RF.

Ceramic fiberboard and shapes are dried faster with reduced binder migration with combined convection and RF.

Ceramic powders and filter cakes are dried faster and at lower temperatures, improving overall purity and quality.

Ceramic honeycomb extrusions and metallized coatings are dried much more quickly and uniformly.

Dyed yarn packages drying time is reduced and the drying provides consistent color throughout.

The heating unit is used in:

Drying of food products, ceramic material etc. Fiberglass mats are dried with higher speeds and more consistent quality product.

Foam products are dried with cycle times reduced from 4 hours to 1 hour. Many types of foam are dried with a hybrid system of convection and RF.

Ceramic fiberboard and shapes are dried faster with reduced binder migration with combined convection and RF.

Ceramic powders and filter cakes are dried faster and at lower temperatures, improving overall purity and quality.

Ceramic honeycomb extrusions and metallized coatings are dried much more quickly and uniformly.

Dyed yarn packages drying time is reduced and the drying provides consistent color throughout.

LIMITATIONS

There is a major difference between the way RF and microwave equipment are available in the market. Unlike microwave sources, one cannot purchase an RF high power source.

Due to the high impedance nature of RF coupling, the RF source and applicator normally need to be designed and built together. Manufacturers of RF equipment develop the whole system, rather than only the power source. Therefore, developments in RF processing must involve the commercial RF manufacturers.

The most common commercial RF frequencies are 13MHz, 27MHz, and 40MHz.

The radio frequencies reserved for industrial use by the Federal Communications Commission are 13.56 MHz ±.05%, 27.12 MHz ±.60% and 40.68 MHz ±.05%. It is important that the frequency remain within tolerance or be attenuated so as not to interfere with radio communications - however, not all frequencies are equal when it comes to RF drying.

A high moisture content product offers a good "load" to an RF dryer since sufficient water molecules exist within the product to absorb the RF energy. Earlier generations of RF dryers operated at the lower frequency levels of 13 to 27 MHz, and these lower frequencies required that high voltages be applied to the product. These systems had a tendency to become unstable at low moisture contents and were prone to random arcing problems, making the lower frequency systems unpopular for applications where extremely low moisture levels were required.

CHAPTER 10

BIBLIOGRAPHY

10. BIBLIOGRAPHY

www. freepatentsonline.com

www.electronicsforyou.com

www.scienceworld.com

www.google.com

www.en.wikipedia.org

www.globalspec.com

www.radiofrequency.com

www.alldatasheets.com

www.vishay.com

www.microwavedevices.com

www.rflabs.com

www.electronicdesign.com

CHAPTER 11

APPENDIX