High frequency induction heating

IntroductionInduction heating is a non-contact heating process. It uses high frequency electricity to heat materials that are electricallyconductive. Since it is non-contact, the heating process does not contaminate the material being heated. It is also veryefficient since the heat is actually generated inside the workpiece. This can be contrasted with other heating methodswhere heat is generated in a flame or heating element, which is then applied to the workpiece. For these reasons InductionHeating lends itself to some unique applications in industry.

How does Induction Heating work ?

A source of high frequency electricity is used to drive a large alternating current througha coil. This coil is known as the work coil. See the picture opposite.

The passage of current through this coil generates a very intense and rapidly changingmagnetic field in the space within the work coil. The workpiece to be heated is placedwithin this intense alternating magnetic field.

Depending on the nature of the workpiece material, a number of things happen...

High Frequency Induction Heating http://www.richieburnett.co.uk/indheat.html

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The alternating magnetic field induces a current flow in the conductive workpiece. Thearrangement of the work coil and the workpiece can be thought of as an electricaltransformer. The work coil is like the primary where electrical energy is fed in, and theworkpiece is like a single turn secondary that is short-circuited. This causes tremendouscurrents to flow through the workpiece. These are known as eddy currents.

In addition to this, the high frequency used in induction heating applications gives rise toa phenomenon called skin effect. This skin effect forces the alternating current to flow ina thin layer towards the surface of the workpiece. The skin effect increases the effectiveresistance of the metal to the passage of the large current. Therefore it greatly increasesthe heating effect caused by the current induced in the workpiece.

(Although the heating due to eddy currents is desirable in this application, it is interesting to note that transformermanufacturers go to great lengths to avoid this phenomenon in their transformers. Laminated transformer cores,powdered iron cores and ferrites are all used to prevent eddy currents from flowing inside transformer cores. Inside atransformer the passage of eddy currents is highly undesirable because it causes heating of the magnetic core andrepresents power that is wasted.)

And for Ferrous metals ?For ferrous metals like iron and some types of steel, there is an additional heating mechanism that takes place at the sametime as the eddy currents mentioned above. The intense alternating magnetic field inside the work coil repeatedlymagnetises and de-magnetises the iron crystals. This rapid flipping of the magnetic domains causes considerable frictionand heating inside the material. Heating due to this mechanism is known as Hysteresis loss, and is greatest for materialsthat have a large area inside their B-H curve. This can be a large contributing factor to the heat generated during inductionheating, but only takes place inside ferrous materials. For this reason ferrous materials lend themselves more easily toheating by induction than non-ferrous materials.

It is interesting to note that steel looses its magnetic properties when heated above approximately 700°C. This temperatureis known as the Curie temperature. This means that above 700°C there can be no heating of the material due to hysteresislosses. Any further heating of the material must be due to induced eddy currents alone. This makes heating steel above700°C more of a challenge for the induction heating systems. The fact that copper and Aluminium are both non-magneticand very good electrical conductors, can also make these materials a challenge to heat efficiently. (We will see that thebest course of action for these materials is to up the frequency to exaggerate losses due to the skin effect.)

What is Induction Heating used for ?Induction heating can be used for any application where we want to heat an electrically conductive material in a clean,efficient and controlled manner.

One of the most common applications is for sealing the anti-tamper seals that are stuck to the top of medicine and drinksbottles. A foil seal coated with "hot-melt glue" is inserted into the plastic cap and screwed onto the top of each bottleduring manufacture. These foil seals are then rapidly heated as the bottles pass under an induction heater on the productionline. The heat generated melts the glue and seals the foil onto the top of the bottle. When the cap is removed, the foilremains providing an airtight seal and preventing any tampering or contamination of the bottle's contents until the customerpierces the foil.

Another common application is "getter firing" to remove contamination from evacuated tubes such as TV picture tubes,vacuum tubes, and various gas discharge lamps. A ring of conductive material called a "getter" is placed inside theevacuated glass vessel. Since induction heating is a non-contact process it can be used to heat the getter that is alreadysealed inside a vessel. An induction work coil is located close to the getter on the outside of the vacuum tube and the ACsource is turned on. Within seconds of starting the induction heater, the getter is heated white hot, and chemicals in itscoating react with any gasses in the vacuum. The result is that the getter absorbs any last remaining traces of gas inside thevacuum tube and increases the purity of the vacuum.

Yet another common application for induction heating is a process called Zone purification used in the semiconductormanufacturing industry. This is a process in which silicon is purified by means of a moving zone of molten material. AnInternet Search is sure to turn up more details on this process that I know little about.


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Other applications include melting, welding and brazing or metals. Induction cooking hobs and rice cookers. Metalhardening of ammunition, gear teeth, saw blades and drive shafts, etc are also common applications because the inductionprocess heats the surface of the metal very rapidly. Therefore it can be used for surface hardening, and hardening oflocalised areas of metallic parts by "outrunning" the thermal conduction of heat deeper into the part or to surroundingareas. The non contact nature of induction heating also means that it can be used to heat materials in analyticalapplications without risk of contaminating the specimen. Similiarly, metal medical instruments may be sterilised by heatingthem to high temperatures whilst they are still sealed inside a known sterile environment, in order to kill germs.

What is required for Induction Heating ?In theory only 3 things are essential to implement induction heating:

A source of High Frequency electrical power,1.A work coil to generate the alternating magnetic field,2.An electrically conductive workpiece to be heated,3.

Having said this, practical induction heating systems are usually a little more complex. For example, an impedancematching network is often required between the High Frequency source and the work coil in order to ensure good powertransfer. Water cooling systems are also common in high power induction heaters to remove waste heat from the work coil,its matching network and the power electronics. Finally some control electronics is usually employed to control theintensity of the heating action, and time the heating cycle to ensure consistent results. The control electronics also protectsthe system from being damaged by a number of adverse operating conditions. However, the basic principle of operation ofany induction heater remains the same as described earlier.

Practical implementationIn practice the work coil is usually incorporated into a resonant tank circuit. This has a number of advantages. Firstly, itmakes either the current or the voltage waveform become sinusoidal. This minimises losses in the inverter by allowing it tobenefit from either zero-voltage-switching or zero-current-switching depending on the exact arrangement chosen. Thesinusoidal waveform at the work coil also represents a more pure signal and causes less Radio Frequency Interference tonearby equipment. This later point becoming very important in high-powered systems. We will see that there are a numberof resonant schemes that the designer of an induction heater can choose for the work coil:

Series resonant tank circuit

The work coil is made to resonate at the intended operating frequency by means of a capacitor placed in series with it. Thiscauses the current through the work coil to be sinusoidal. The series resonance also magnifies the voltage across the workcoil, far higher than the output voltage of the inverter alone. The inverter sees a sinusoidal load current but it must carrythe full current that flows in the work coil. For this reason the work coil often consists of many turns of wire with only afew amps or tens of amps flowing. Significant heating power is achieved by allowing resonant voltage rise across the workcoil in the series-resonant arrangement whilst keeping the current through the coil (and the inverter) to a sensible level.

This arrangement is commonly used in things like rice cookers where the power level is low, and the inverter is locatednext to the object to be heated. The main drawbacks of the series resonant arrangement are that the inverter must carry thesame current that flows in the work coil. In addition to this the voltage rise due to series resonance can become verypronounced if there is not a significantly sized workpiece present in the work coil to damp the circuit. This is not a problemin applications like rice cookers where the workpiece is always the same cooking vessel, and its properties are well knownat the time of designing the system.

The tank capacitor is typically rated for a high voltage because of the resonant voltage rise experienced in the series tunedresonant circuit. It must also carry the full current carried by the work coil, although this is typically not a problem in lowpower applications.

Parallel resonant tank circuit

The work coil is made to resonate at the intended operating frequency by means of a capacitor placed in parallel with it.


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This causes the current through the work coil to be sinusoidal. The parallel resonance also magnifies the current throughthe work coil, far higher than the output current capability of the inverter alone. The inverter sees a sinusoidal load current.However, in this case it only has to carry the part of the load current that actually does real work. The inverter does nothave to carry the full circulating current in the work coil. This is very significant since power factors in induction heatingapplications are typically low. This property of the parallel resonant circuit can make a tenfold reduction in the current thatmust be supported by the inverter and the wires connecting it to the work coil. Conduction losses are typically proportionalto current squared, so a tenfold reduction in load current represents a significant saving in conduction losses in the inverterand associated wiring. This means that the work coil can be placed at a location remote from the inverter without incurringmassive losses in the feed wires.

Work coils using this technique often consist of only a few turns of a thick copper conductor but with large currents ofmany hundreds or thousands of amps flowing. (This is necessary to get the required Ampere turns to do the inductionheating.) Water cooling is common for all but the smallest of systems. This is needed to remove excess heat generated bythe passage of the large high frequency current through the work coil and its associated tank capacitor.

In the parallel resonant tank circuit the work coil can be thought of as an inductive load with a "power factor correction"capacitor connected across it. The PFC capacitor provides reactive current flow equal and opposite to the large inductivecurrent drawn by the work coil. The key thing to remember is that this huge current is localised to the work coil and itscapacitor, and merely represents reactive power sloshing back-and-forth between the two. Therefore the only real currentflow from the inverter is the relatively small amount required to overcome losses in the "PFC" capacitor and the work coil.There is always some loss in this tank circuit due to dielectric loss in the capacitor and skin effect causing resistive losses inthe capacitor and work coil. Therefore a small current is always drawn from the inverter even with no workpiece present.When a lossy workpiece is inserted into the work coil, this damps the parallel resonant circuit by introducing a further lossinto the system. Therefore the current drawn by the parallel resonant tank circuit increases when a workpiece is enteredinto the coil.

Impedance matchingOr simply "Matching". This refers to the electronics that sits between the source of high frequency power and the workcoil we are using for heating. In order to heat a solid piece of metal via induction heating we need to cause aTREMENDOUS current to flow in the surface of the metal. However this can be contrasted with the inverter thatgenerates the high frequency power. The inverter generally works better (and the design is somewhat easier) if it operatesat fairly high voltage but a low current. (Typically problems are encountered in power electronics when we try to switchlarge currents on and off in very short times.) Increasing the voltage and decreasing the current allows common switchmode MOSFETs (or fast IGBTs) to be used. The comparatively low currents make the inverter less sensitive to layoutissues and stray inductance. It is the job of the matching network and the work coil itself to transform thehigh-voltage/low-current from the inverter to the low-voltage/high-current required to heat the workpiece efficiently.

We can think of the tank circuit incorporating the work coil (Lw) and itscapacitor (Cw) as a parallel resonant circuit.

This has a resistance (R) due to the lossy workpiece coupled into thework coil due to the magnetic coupling between the two conductors.

See the schematic opposite.

In practice the resistance of the work coil, the resistance of the tankcapacitor, and the reflected resistance of the workpiece all introduce aloss into the tank circuit and damp the resonance. Therefore it is useful tocombine all of these losses into a single "loss resistance." In the case of aparallel resonant circuit this loss resistance appears directly across thetank circuit in our model. This resistance represents the only componentthat can consume real power, and therefore we can think of this lossresistance as the load that we are trying to drive power into in an efficient


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manner.

When driven at resonance the current drawn by the tank capacitor and the work coil are equal in magnitude and oppositein phase and therefore cancel each other out as far as the source of power is concerned. This means that the only loadseen by the power source at the resonant frequency is the loss resistance across the tank circuit. (Note that, whendriven either side of the resonant frequency, there is an additional "out-of-phase" component to the current caused byincomplete cancellation of the work coil current and the tank capacitor current. This reactive current increases the totalmagnitude of the current being drawn from the source but does not contribute to any useful heating in the workpiece.)

The job of the matching network is simply to transform this relatively large loss resistance across the tank circuit down to alower value that better suits the inverter attempting to drive it. There are many different ways to achieve this impedancetransformation including tapping the work coil, using a ferrite transformer, a capacitive divider in place of the tankcapacitor, or a matching circuit such as an L-match network.

In the case of an L-match network it can transform the relatively highload resistance of the tank circuit down to something around 10 ohmswhich better suits the inverter. This figure is typical to allow the inverterto run from several hundred volts whilst keeping currents down to amedium level so that standard switch-mode MOSFETs can be used toperform the switching operation.

The L-match network consists of components Lm and Cm shownopposite.

The L-match network has several highly desirable properties in this application. The inductor at the input to the L-matchnetwork presents a progressively rising inductive reactance to all frequencies higher than the resonant frequency of thetank circuit. This is very important when the work coil is to be fed from a voltage-source inverter that generates asquarewave voltage output. Here is an explanation of why this is so…

The squarewave voltage generated by most half-bridge and full-bridge circuits is rich in high frequency harmonics as wellas the wanted fundamental frequency. Direct connection of such a voltage source to a parallel resonant circuit wouldcause excessive currents to flow at all harmonics of the drive frequency! This is because the tank capacitor in the parallelresonant circuit would present a progressively lower capacitive reactance to increasing frequencies. This is potentially verydamaging to a voltage-source inverter. It results in large current spikes at the switching transitions as the inverter tries torapidly charge and discharge the tank capacitor on rising and falling edges of the squarewave. The inclusion of theL-match network between the inverter and the tank circuit negates this problem. Now the output of the inverter sees theinductive reactance of Lm in the matching network first, and all harmonics of the drive waveform see a gradually risinginductive impedance. This means that maximum current flows at the intended frequency only and little harmonic currentflows, making the inverter load current into a smooth waveform.

Finally, with correct tuning the L-match network is able to provide a slight inductive load to the inverter. This slightlylagging inverter load current can facilitate Zero-Voltage-Switching (ZVS) of the MOSFETs in the inverter bridge. Thissignificantly reduces turn-on switching losses due to device output capacitance in MOSFETs operated at high voltages.The overall result is less heating in the semiconductors and increased lifetime.

In summary, the inclusion of an L-match network between the inverter and the parallel resonant tank circuit achieves twothings.

Impedance matching so that the required amount of power can be supplied from the inverter to the workpiece,1.Presentation of a rising inductive reactance to high frequency harmonics to keep the inverter safe and happy.2.

Looking at the previous schematic above we can see that the capacitor inthe matching network (Cm) and the tank capacitor (Cw) are both inparallel. In practice both of these functions are usually accomplished by asingle purpose built power capacitor. Most of its capacitance can bethought of as being in parallel resonance with the work coil, with a smallamount providing the impedance matching action with the matchinginductor (Lm.) Combing these two capacitances into one leads us toarrive at the LCLR model for the work coil arrangement, which iscommonly used in industry for induction heating.


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The LCLR work coilThis arrangement incorporates the work coil into a parallel resonant circuit and uses the L-match network between thetank circuit and the inverter. The matching network is used to make the tank circuit appear as a more suitable load to theinverter, and its derivation is discussed in the section above.

The LCLR work coil has a number of desirable properties:

A huge current flows in the work coil, but the inverter only has to supply a low current. The large circulating currentis confined to the work coil and its parallel capacitor, which are usually located very close to each other.

1.

Only comparatively low current flows along the transmission line from the inverter to the tank circuit, so this can uselighter duty cable.

2.

Any stray inductance of the transmission line simply becomes part of the matching network inductance (Lm.)Therefore the heat station can be located away from the inverter.

3.

The inverter sees a sinusoidal load current so it can benefit from ZCS or ZVS to reduce its switching losses andtherefore run cooler.

4.

The series matching inductor can be altered to cater for different loads placed inside the work coil.5.The tank circuit can be fed via several matching inductors from many inverters to reach power levels above thoseachievable with a single inverter. The matching inductors provide inherent sharing of the load current between theinverters and also make the system tolerant to some mismatching in the switching instants of the paralleled inverters.

6.

For more information about the behaviour of the LCLR resonant network see the new section below labelled"LCLR network frequency response."

Another advantage of the LCLR work coil arrangement is that it does not require a high-frequency transformer to providethe impedance matching function. Ferrite transformers capable of handling several kilowatts are large, heavy and quiteexpensive. In addition to this, the transformer must be cooled to remove excess heat generated by the high currentsflowing in its conductors. The incorporation of the L-match network into the LCLR work coil arrangement removes thenecessity of a transformer to match the inverter to the work coil, saving cost and simplifying the design. However, thedesigner should appreciate that a 1:1 isolating transformer may still be required between the inverter and the input to theLCLR work coil arrangement if electrical isolation is necessary from the mains supply. This depends whether isolation isimportant, and whether the main PSU in the induction heater already provides sufficient electrical isolation to meet thesesafety requirements.

Conceptual schematicThe system schematic belows shows the simplest inverter driving its LCLR work coil arrangement.

Note that this schematic DOES NOT SHOW the MOSFET gate-drive circuitry and control electronics!

The inverter in this demonstration prototype was a simple half-bridge consisting of two MTW14N50 MOSFETs made myOn-semiconductor (formerly Motorola.) It is fed from a smoothed DC supply with decoupling capacitor across the rails tosupport the AC current demands of the inverter. However, it should be realised that the quality and regulation of the powersupply for induction heating applications is not critical. Full-wave rectified (but un-smoothed) mains can work as well assmoothed and regulated DC when it comes to heating metal, but peak currents are higher for the same average heatingpower. There are many arguments for keeping the size of the DC bus capacitor down to a minimum. In particular itimproves the power factor of current drawn from the mains supply via a rectifier, and it also minimises stored energy incase of fault conditions within the inverter.

The DC-blocking capacitor is used merely to stop the DC output from the half-bridge inverter from causing current flow


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through the work coil. It is sized sufficiently large that it does not take part in the impedance matching, and does notadversely effect the operation of the LCLR work coil arrangement.

In high power designs it is common to use a full-bridge (H-bridge) of 4 or more switching devices. In such designs thematching inductance is usually split equally between the two bridge legs so that the drive voltage waveforms are balancedwith respect to ground. The DC-blocking capacitor can also be eliminated if current mode control is used to ensure that nonet DC flows between the bridge legs. (If both legs of the H-bridge can be controlled independently then there is scope forcontrolling power throughput using phase-shift control. See point 6 in the section below about "Power control methods" forfurther details.)

At still higher powers it is possible to use several seperate inverters effectively connected in parallel to meet the highload-current demands. However, the seperate inverters are not directly tied in parallel at the output terminals of theirH-bridges. Each of the distributed inverters is connected to the remote work coil via its own pair of matching inductorswhich ensure that the total load is spread evenly among all of the inverters.

These matching inductors also provide a number of additional benefits when inverters are paralleled in this way. Firstly,the impedance BETWEEN any two inverter outputs is equal to twice the value of the matching inductance. This inductiveimpedance limits the "shoot between" current that flows between paralleled inverters if their switching instants are notperfectly synchronised. Secondly, this same inductive reactance between inverters limits the rate at which fault currentrises if one of the inverters exhibits a device failure, potentially eliminating failure of further devices. Finally, since alldistributed inverters are already connected via inductors, any additional inductance between the inverters merely adds tothis impedance and only has the effect of slightly degrading current sharing. Therefore the distributed inverters forinduction heating need not necessarily be located physically close to each other. If isolation transformers are included inthe designs then they need not even run from the same supply!

Fault tolerance


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The LCLR work coil arrangement is very well behaved under a variety of possible fault conditions.

Open circuit work coil.1.Short circuit work coil, (or tank capacitor.)2.Shorted turn in work coil.3.Open circuit tank capacitor.4.

All of these failures result in an increase in the impedance being presented to the inverter and therefore a correspondingdrop in the current drawn from the inverter. The author has personally used a screwdriver to short-circuit between turns ofa work coil carrying several hundred amps. Despite sparks flying at the location of the applied short-circuit, the load on theinverter is reduced and the system survives this treatment with ease.

The worst thing that can happen is that the tank circuit becomes detuned such that its natural resonant frequency is justabove the operating frequency of the inverter. Since the drive frequency is still close to resonance there is still significantcurrent flow out of the inverter. But the power factor is reduced due to the detuning, and the inverter load-current beginsto lead the voltage. This situation is undesirable because the load current seen by the inverter changes direction before theapplied voltage changes. The outcome of this is that current is force-commutated between free-wheel diodes and theopposing MOSFET every time the MOSFET is turned on. This causes a forced reverse recovery of the free-wheel diodeswhilst they are already carrying significant forward current. This results in a large current surge through both the diode andthe opposing MOSFET that is turning on.

Whilst not a problem for special fast recovery rectifiers, this forced recovery can cause problems if the MOSFETs intrinsicbody diodes are used to provide the free-wheel diode function. These large current spikes still represent a significant powerloss and threat to reliability. However, it should be realised that proper control of the inverter operating frequency shouldensure that it tracks the resonant frequency of the tank circuit. Therefore the leading power factor condition should ideallynot arise, and should certainly not persist for any length of time. The resonant frequency should be tracked up to its limit,then the system shut-down if it has wandered outside of an acceptable frequency range.

Power control methodsIt is often desirable to control the amount of power processed by an induction heater. This determines the rate at whichheat energy is transferred to the workpiece. The power setting of this type of induction heater can be controlled in anumber of different ways:

1. Varying the DC link voltage.

The power processed by the inverter can be decreased by reducing the supply voltage to the inverter. This can be done byrunning the inverter from a variable voltage DC supply such as a controlled rectifier using thyristors to vary the DC supplyvoltage derived from the mains supply. The impedance presented to the inverter is largely constant with varying powerlevel, so the power throughput of the inverter is roughly proportional to the square of the supply voltage. Varying the DClink voltage allows full control of the power from 0% to 100%.

It should be noted however, that the exact power throughput in kilowatts depends not only on the DC supply voltage to theinverter, but also on the load impedence that the work coil presents to the inverter through the matching network.Therefore if precise power control is required the actual induction heating power must be measured, compared to therequested "power setting" from the operator and an error signal fed back to continually adjust the DC link voltage in aclosed-loop fashion to minimise the error. This is necessary to maintain constant power because the resistance of theworkpiece changes considerably as it heats up. (This argument for closed-loop power control also applies to all of themethods that follow below.)

2. Varying the duty ratio of the devices in the inverter.

The power processed by the inverter can be decreased by reducing the on-time of the switches in the inverter. Power isonly sourced to the work coil in the time that the devices are switched on. The load current is then left to freewheelthrough the devices body diodes during the deadtime when both devices are turned off. Varying the duty ratio of theswitches allows full control of the power from 0% to 100%. However, a significant drawback of this method is thecommutation of heavy currents between active devices and their free-wheel diodes. Forced reverse recovery of thefree-wheel diodes that can occur when the duty ratio is considerably reduced. For this reason duty ratio control is not


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usually used in high power induction heating inverters.

3. Varying the operating frequency of the inverter.

The power supplied by the inverter to the work coil can be reduced by detuning the inverter from the natural resonantfrequency of the tank circuit incorporating the work coil. As the operating frequency of the inverter is moved away fromthe resonant frquency of the tank circuit, there is less resonant rise in the tank circuit, and the current in the work coildiminishes. Therefore less circulating current is induced into the workpiece and the heating effect is reduced.

In order to reduce the power throughput the inverter is normally detuned on the high side of the tank circuits naturalresonant frequency. This causes the inductive reactance at the input of the matching circuit to become increasinglydominant as the frequency increases. Therefore the current drawn from the inverter by the matching network starts to lagin phase and diminish in amplitude. Both of these factors contribute to a reduction in the real power throughput. Inaddition to this the lagging power factor ensures that the devices in the inverter still turn on with zero voltage across them,and there are no free-wheel diode recovery problems. (This can be contrasted with the situation that would occur if theinverter were detuned on the low side of the work coil's resonant frequency. ZVS is lost, and the free-wheel diodes seeforced reverse-recovery whilst carrying significant load current.)

This method of controlling power level by detuning is very simple since most induction heaters already have control overthe operating frequency of the inverter in order to cater for different workpieces and work coils. The downside is that itonly provides a limited range of control, as there is a limit to how fast power semiconductors can be made to switch. This isparticularly true in high power applications where the devices may already be running close to maximum switching speeds.High power systems using this power control method require a detailed thermal analysis of the results of switching losses atdifferent power levels to ensure device temperatures always stay within tolerable limits.

For more detailed information about power control by detuning see the new section below labelled "LCLR networkfrequency response."

4. Varying the value of the inductor in the matching network.

The power supplied by the inverter to the work coil can be varied by altering the value of the matching networkcomponents. The L-match network between the inverter and the tank circuit technically consists of an inductive and acapacitive part. But the capacitive part is in parallel with the work coil's own tank capacitor, and in practice these areusually one and the same part. Therefore the only part of the matching network that is available to adjust is the inductor.

The matching network is responsible for transforming the load impedance of the workcoil to a suitable load impedance tobe driven by the inverter. Altering the inductance of the matching inductor adjusts the value to which the load impedanceis translated. In general, decreasing the inductance of the matching inductor causes the work coil impedance to betransformed down to a lower impedance. This lower load impedance being presented to the inverter causes more power tobe sourced from the inverter. Conversely, increasing the inductance of the matching inductor causes a higher loadimpedance to be presented to the inverter. This lighter load results in a lower power flow from the inverter to the workcoil.

The degree of power control achieveable by altering the matching inductor is moderate. There is a also a shift in theresonant frequency of the overall system - This is the price to pay for combining the L-match capacitance and tankcapacitance into one unit. The L-match network essentially borrows some of the capacitance from the tank capacitor toperform the matching operation, thus leaving the tank circuit to resonate at a higher frequency. For this reason thematching inductor is usually fixed or adjusted in coarse steps to suit the intended workpiece to be heated, rather thanprovide the user with a fully adjustable power setting.

5. Impedance matching transformer.

The power supplied by the inverter to the work coil can be varied in coarse steps by using a tapped RF power transformerto perform impedance conversion. Although most of the benefit of the LCLR arrangement is in the elimination of a bulkyand expensive ferrite power transformer, it can cater for large changes in system parameters in a way that is not frequencydependent. The ferrite power transformer can also provide electrical isolation as well as performing impedancetransformation duty to set the power throughput.

Additionally if the ferrite power transformer is placed between the inverter's output and the input to the L-match circuit its


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design constraints are relaxed in many ways. Firstly, locating the transformer in this position means that the impedances atboth windings are relatively high. i.e. voltages are high and currents are comparitively small. It is easier to design aconventional ferrite power transformer for these conditions. The massive circulating current in the work coil is kept out ofthe ferrite transformer greatly reducing cooling problems. Secondly, although the transformer sees the square-wave outputvoltage from the inverter, it's windings carry currents that are sinusoidal. The lack of high frequency harmonics reducesheating in the transformer due to skin effect and proximity effect within the conductors.

Finally the transformer design should be optimised for minimum inter-winding capacitance and good insulation at theexpense of increased leakage inductance. The reason for this is that any leakage inductance exhibited by a transformerlocated in this position merely adds to the matching inductance at the input to the L-match circuit. Therefore leakageinductance in the transformer is not as damaging to performance as inter-winding capacitance.

6. Phase-shift control of H-bridge.

When the work coil is driven by a voltage-fed full-bridge (H-bridge) inverter there is yet another method of achievingpower control. If the switching instants of both bridge legs can be controlled independently then it opens up the possibilityof controlling power throughput by adjusting the phase shift between the two bridge legs.

When both bridge legs switch exactly in phase, they both output the same voltage. This means there is no voltage acrossthe work coil arrangement and no current flows through the work coil. Conversely, when both bridge legs switch inanti-phase maximum current flows through the work coil and maximum heating is achieved. Power levels between 0% and100% can be achieved by varying the phase shift of the drive to one half of the bridge between 0 degrees and 180 degreeswhen compared to the drive of the other bridge leg.

This technique is highly effective as power control can be achieved at the lower power control side. The power factor seenby the inverter always remains good because the inverter is not detuned from the resonant frequency of the work coil,therefore reactive current flow through free-wheeling diodes is minimised.

Induction Heating CapacitorsThe requirements for capacitors used in high power induction heating are perhaps the most demanding of any type ofcapacitor. The capacitor bank used in the tank circuit of an induction heater must carry the full current that flows in thework coil for extended periods of time. This current is typically many hundreds of amps at many tens or hundreds ofkilohertz. They are also exposed to repeated 100% voltage reversal at this same frequency and see the full voltagedeveloped across the work coil. The high operating frequency causes significant losses due to dielectric heating and due toskin effect in the conductors. Finally stray inductance must be kept to an absolute minimum so that the capacitor appearsas a lumped circuit element compared to the reasonably low inductance of the work coil it is connected to.

Correct choice of dielectrics and extended foil construction techniques are used to minimise the amount of heat generatedand keep effective-series-inductance to a minimum. However, even with these techniques Induction heating capacitors stillexhibit significant power dissipation due to the enormous RF currents they must carry. Therefore an important factor intheir design is allowing the effective removal of heat from within the capacitor to extend the life of the dielectric.

The following manufacturers produce purpose built components:

High Energy Corp. (UK distributer is AMS Technologies.)

Vishay Components.

Celem Power Capacitors. based in Israel.


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Range of high power induction heating capacitors from High Energy Corp.

High power conduction cooled mica capacitor from Celem Power Capacitors.Celem(Pictures courtesy of Steve Conner)

Note the large surface area of the connection plates on the Celem conduction-cooled components and the reactive powerrating (KVAR) printed on the rating label. Higher power units pictured above in aluminium cases have connections forwater cooling hoses to remove the heat generated internally.

LCLR network frequency responseThe LCLR network is a 3rd order resonant system consisting of two inductors, one capacitor and one resistor. The bodeplot below shows the way in which some of the voltages and currents within the network change as the drive frequency isaltered. The GREEN traces represent the current passing through the matching inductor, and therefore the load currentseen by the inverter. The RED traces represent the voltage across the tank capacitor, which is the same as the voltageacross the induction heating work coil. The top graph shows the AC magnitudes of these two quantities, whilst the bottomgraph shows the relative phase of the signals relative to the AC output voltage from the inverter.


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From the amplitude part of the bode plot it can be seen that maximum voltage is developed across the work coil (top redtrace) at one frequency only. At this frequency current through the work coil is also maximum and the largest heatingeffect is developed at this frequency. It can be seen that this frequency corresponds to the maximum load current drawnfrom the inverter (top green trace.) It is worth noting that the magnitude of the inverter load current has a null at afrequency only slightly lower than that which gives maximum heating. This plot shows the importance of accurate tuningin an induction heating application. For a high Q system these two frequencies are very close together. The differencebetween maximum power and minimum power can be only a few kilohertz.

From the bottom graph we can see that for frequencies below the maximum power point, the work coil voltage (green) isin-phase with the output voltage from the inverter. As the operating frequency increases the phase angle of the work coilvoltage changes abruptly through 180 degrees (phase inversion) right at the point where maximum power is beingprocessed. The phase angle of the work coil voltage then remains shifted by 180 degrees from the inverter output voltagefor all frequencies above the maximum power point.

From the bottom graph we can also see that the load current from the inverter exhibits not one but two abrupt phasechanges as the operating frequency is progressively increased. Inverter load current initially lags the inverter's outputvoltage by 90 degrees at low frequencies. Load current abruptly slews through 180 degrees to a phase lead of 90 degreesas the operating frequency passes through the "null frequency" of the network. Inverter current remains leading by 90degrees until the maximum power point is reached, where it again abruptly slews through 180 degrees and returns to the 90degree lagging phase once again.

When we consider that only current out of the inverter that is in-phase with the output voltage contributes to real powertransfer we can see that these abrupt transitions from -90 degrees to +90 degrees clearly need a more detailedexamination...


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The bode plot above shows the area of interest around the null frequency and the maximum power point in more detail. Italso shows a family of curves depicting the behaviour of the induction heating tank circuit with a variety of differentworkpieces present. This allows us to get a feel for how the network behaves with a large lossy workpiece to having noworkpiece present at all, and all loads in between.

With no workpiece installed, losses are low and Q factor is high. This gives rise to the sharply peaking currents andvoltages in the top graph, and the abruptly changing phase shifts in the bottom graph. As a lossy workpiece is introducedthe overall Q factor of the LCLR network falls. This causes less resonant rise in the inverter load current and the voltageacross the work coil. The resonant peaks become less tall, and broader as the Q factor falls. Likewise the phase of theinverter current waveform and the work coil voltage slew less rapidly for lower Q factors.

From these graphs we can deduce a few implications for any control system that must track the resonant frequency of theLCLR arrangement and control power throughput. Firstly there is more resonant rise in the LCLR network when thereis no workpiece present. Therefore the current delivered from the inverter should be decreased to prevent the work coiland tank capacitor currents sky-rocketing in the absence of any significant loss in the system. Secondly, the inverter loadcurrent with no load must be tracked very accurately if the inverter is not to see either a leading or lagging load currentbecause it slews so quickly through zero degrees.

Conversely we can say that with a large lossy workpiece present, there will be less resonant rise inherent in theLCLR arrangement and the inverter will have to supply more load current in order to achieve the required level ofcurrent in the work coil. However, the control electronics now do not need to track the resonant frequency so closelysince the diminished Q gives a load current that shifts phase in a more leisurely manner.

Finally a number of points are worthy of consideration from the plot above when considering an automatic controlstratergy to track the resonant frequency of an LCLR induction heater. For very lossy workpiece materials, (or largevolumes of metal that introduce a significant overall loss) we can see that the inverter load current phase (bottom greenplot) sometimes fails to ever cross through zero degrees to leading phase. This means that the inverter load current withheavy workloads cannot be in-phase and is always lagging by some amount. Furthermore the inverter load current is notmonotonic as frequency is swept. Therefore direct feedback from a Current Transformer (CT) on the inverter output is nota viable option. Whilst it may appear to work fine with no workpiece fitted or only moderate heating loads, it does nottrack the resonant frequency correctly and will fail to operate satisfactorily as the workload increases and network Q falls!


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(Direct feedback from inverter output current using a CT to form a free-running power oscillator results in a designwhich oscillates at low load but falls out of self-oscillation when the workload is increased.)

In contrast we can see that the work-coil voltage (and tank capacitor voltage) phase (bottom red plot) is monotonic withincreasing frequency. Furthermore it consistently passes through the -90 degree phase-lag point exactly at the frequencywhich gives maximum power regardless of how heavily the work coil is loaded. These two merits make the tank capacitorvoltage waveform an excellent control variable. In conclusion the inverter frequency should be controlled so as toachieve a consistent 90 degree lag between the tank capacitor voltage and the inverter output voltage in order toachieve maximum power throughput. We can now label some areas of interest on the bode plot diagram below.

The white vertical line indicates the frequency at which the tank capacitor voltage (and also the work coil voltage) lag theinverter output voltage by 90 degrees. This is also the point where maximum voltage is developed across the work coil andmaximum current flows through it. The white line is where you want to be to develop the maximum possible heating effectin the workpiece. If we look at the inverter load current phase (bottom green plot) we can see that this is always between 0degrees and -90 degrees when it crosses the white line no matter how abruptly or slowly it slews. This means that theinverter always sees a load current that is either in-phase or at worst slightly lagging in power factor. Such a situation isideal for supporting ZVS soft-switching in the inverter and preventing free-wheel diode reverse-recovery problems.

Looking to the right of the white line we have the area shaded in blue labelled "Inductive Load region." As the operatingfrequency is increased above the maximum power point, the voltage across the work coil decreases and less heating effectis generated in the workpiece. The inverter load current also falls and begins to lag in phase relative to the output voltageof the inverter. These properties make the blue shaded region the ideal place to operate in order to achieve control overinduction heating power. By detuning the inverter drive frequency on the high-side of the maximum power point, powerthroughput can be reduced and the inverter always sees a lagging power factor.

Conversely, to the left of the white line we have a band of frequencies labelled "Capacitive Load region." As the operatingfrequency is decreased below the maximum power point, the work coil voltage also falls and less heating effect takesplace. However, this is accompanied by the inverter load current possibly slewing to a leading phase angle when losses inthe workpiece are low and Q factor is high. This is undesirable for many solid-state inverters as the leading load currentcauses loss of ZVS and leads to forced reverse-recovery of free-wheeling diodes incurring raised switching losses andvoltage overshoots. Therefore the capacitive load region is not recommended for achieving power throughput control.


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The vertical purple line marks the other end of the capacitive load region, where the inverter load current transitions againto lagging "Inductive" load current. This second Inductive region is of little interest since it does not achieve significantpower throughput, and cannot be reached without passing through the potentially damaging capacitive load region anyway.When the LCLR network is driven from a squarewave inverter voltage there is also risk of significant current flow at aharmonic of the drive frequency. It is marked on the diagram here merely for completeness.

Note: The phase of the tank capacitor voltage was suggested as a control variable and discussed extensively in the plotsabove. This is because this voltage can be easily sensed using a high-frequency voltage transformer and provides all thenecessary control information. Whilst it exhibits a 90 degree phase shift relative to the inverter output voltage (whichmay at first appear undesirable) it is still a better control variable than trying to sense the tank capacitor current.Although the tank capacitor current is in-phase with the inverter output this current can be many hundreds of ampsmaking closed-core ferrite CTs impractical. Furthermore the 90 degree phase shift of the tank capacitor voltagewaveform means that it's zero crossings are intentionally displaced in time away from the potentially noisy switchinginstants of the inverter. This -90 degree phase shift of the voltage feedback signal can be allowed for in the design of thecontrol electronics and is a small price to pay for the eased sensing and increased noise immunity gained.

Cooling requirements#Add comments here about water cooling#

Heating pictures


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Waveforms


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This shows the inverter output current waveform when driving the LCLR work coil arrangement close to its resonantfrequency. This point corresponds to maximum power throughput and therefore maximum heating effect. Note how theinverter load current is almost a pure sinusoid.

This shows the inverter output current waveform when driving the LCLR work coil arrangement substantially above itsnatural resonant frequency. This operating point gives reduced power throughput and diminished heating effect. Atfrequencies above the natural resonant frequency of the LCLR work coil arrangement the inductive reactance of thematching network dominates and the inverter's load current lags the applied voltage. Notice the triangular load currentcaused by the inductive load integrating the inverter's squarewave voltage output over time.

This shows the voltage across the work coil under normal operation when driven close to resonance. Notice that thevoltage waveform is a pure sinusoid in shape. This is also true for the current waveform and minimises harmonic radiationand RF interference. In this case the voltage across the work coil is also higher than the DC bus voltage supplied to theinverter. Both of these properties are attributed to the high-Q factor of the induction heating tank circuit.


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This shows the output voltage from the inverter when it is mistuned to a frequency that is below the natural resonantfrequency of the work coil. Notice the very fast rise and fall times of the squarewave accompanied by excessive voltageovershoot and ringing. These are all attributed to forced reverse-recovery of the MOSFET body diodes whilst enduring thisundesirable operating mode. (Overshoot and ringing is due to reverse recovery current spikes shock-exciting strayinductance in the inverter layout into parasitic oscillation.)

This shows the output voltage from the inverter when it is tuned very slightly above the natural resonant frequency of thework coil. Notice that the rise and fall times of the squarewave are more controlled, and there is comparatively littleovershoot or ringing. This is due to the Zero Voltage Switching (ZVS) which takes place when the inverter runs in thisfavourable operating mode.


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This shows the output voltage from the inverter when it is tuned precisely to the resonant frequency of the work coil.Although this situation actually achieves maximum power throughput, it does not quite achieve Zero Voltage Switching ofthe MOSFETs. Notice the little notches on the rising and falling edges of the voltage waveform. These occur because themid-point of the bridge leg has not been fully commutated to the opposite supply rail during the dead-time before the nextMOSFET turns on. In practice a small amount of inductive reactance presented to the inverter helps provide the requiredcommutating current and achieve ZVS. For this reason the situation described for the previous photograph is preferable tobeing precisely in tune.

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High frequency induction heating