Gate Driver Optocouplers in Induction Cooker (4) - Finish

Summary
In this article, the half-bridge series resonant and quasi resonant induction cooker topologies along with three gate driver methods were discussed. In order to reduce the design size and audible switching noise while improving power efficiency, these resonant converters are chosen.

The discrete transistor gate driver circuit is cost effective but increases design complexity while providing no safety isolation. The gate drive transformer consumes board space due its size and requires additional work and cost to achieve higher switching duty cycle above 50%. Finally, gate drive optocoupler integrated ICs saves board space through high level feature integration while providing high voltage safe isolation and noise immunity all in one package.

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Gate Driver Optocouplers in Induction Cooker (3)

Gate Driver Circuits for IGBT Power Switches
Three types driver circuits, the discrete transistor circuit (Figure 5), gate driver optocoupler (Figure 6) and gate driver transformer (Figure 7) can be used to drive the power switches in induction cooker application. There are several issues associated with high-frequency gate drivers; the parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver.

Typically, the switching frequency of an induction cooker is between 25kHz to 40kHz. In order to rapidly charge turn on and off the power switch, the gate current inductance loop between the driver and power switch should be as low as possible. Hence it is advisable to design the layout of the circuit to reduce the parasitic inductances. Since the driver rapidly charge and discharge the gate capacitor of the IGBT, a higher peak gate current may be needed for proper operation. Due to this, the power dissipation within the gate drive circuit is important to manage the increase switching speed. The higher peak current is also desirable to increase the charging and discharging during turn on and off as it will help reduce the switching losses of the IGBT.
The discrete gate drivers are constructed using the bipolar transistors. NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver and other functions or protection operation like Under Voltage Lockout (UVLO) is not as space efficient as using integrated ICs. Moreover discrete transistor drivers do not provide sufficient safety isolation or noise immunity.
Two types of isolation method are discussed in this article; pulse transformer and gate driver optocoupler. The pulse transformer is a traditional and simple solution which suffers from saturation limitation for a given transformer size that can reduce efficiency. Normally, a transformer can only transmit AC information and have a limited duty cycle of up to 50% due to the transformer volt-second relationship. Additional capacitor and zener diode on the secondary size can be added to allow a higher duty cycle. However, this increase the design board size and parasitic inductances which in turn increases power losses in the driver circuit.
Gate driver optocoupler ic is an integration of LED for safety isolation, transistors to provide drive current and protection functions like UVLO or Desaturation Detector. Gate driver ICs are easy to design and will save PCB board space in the application. Due to the integrated design, the drive circuitry can be located very close to the power switch which not only saves PCB space but also improves the overall noise immunity of the system. However, like any integrated ICs, power dissipation is main concern observed by designers.
For the single switch resonant converter, designer has the option of the discrete gate driver topology, gate transformer or gate driver optocoupler. As discussed in the previous section, the quasi-converter resonant voltage can be higher compared to the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker design, the discrete gate driver circuit is used as there is no upper power switch and both controller and the power semiconductor are able to share the same power ground. However, in cases where safety isolation and reduction of driver losses becomes an issue, the gate drive optocoupler or transformer are excellent alternatives.
For the half-bridge converter, a floating or high-side power switch needs to be driven. A high side discrete solution would increase the component count while not providing any isolation. As shown, the pulse transformer galvanic isolation solution increases in complexity for duty cycle switching above 50%. Also, the solution size is larger because of the additional discrete components on top of the transformer size. The gate driver optocoupler IC provides a good level of protection, isolation, and common-mode noise rejection. This resolves much of the problems that are associated with transformer driver or transistor discrete solution as mentioned earlier.

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Gate Driver Optocouplers in Induction Cooker (2)

By applying the transformer equivalent circuit, designers are able to map the load pot (secondary of transformer) to the primary side of circuit where the resonant inductor, Lr and capacitor Cr are located. From this, we can obtain the equivalent circuit for half-bridge and quasi resonant circuit, shown in Figure 3 and Figure 4. From these equivalent circuits, the operation of the induction cooker, the sizing of the resonant inductor, capacitor and control algorithm can be conceived.

In order to reduce component size, minimize switching losses and reduce audible noise during operation (above 20kHz resonant frequency), induction cooker circuit typically utilizes resonant or soft switching techniques. This circuitsoft switching technique can be subcategorized into two methods: Zero-voltage switching and Zero-current switching.
Zero-voltage switching occurs when the transistor turn-on at zero voltage. Zero-current switching refers to elimination of turn-off switching loss at zero current flow. The voltage or current administered to the switching circuit can be made zero by using the resonance created by an L-C resonant circuit. This topology is named a “resonant converter.” This allows the application to utilize resonant frequency and obtain the benefits mentioned compared to conventional hard switching techniques.
The advantages of half-bridge series resonant are stable switching, and lower cost due to streamlined design. The voltage within the circuit is limited to the level of the input voltage which reduces the voltage stress across IGBT power switch. This in-turn allows the designer to lower the cost by choosing a lower rating IGBT. The disadvantage is that the overall half-bridge control is more complicated, the size of heatsink and PCB area is bigger and insulated gate driver circuits, especially on the upper IGBT (S1 in Figure 1).
The advantage of quasi-resonant converter is that it needs only 1 IGBT power switch which reduces design size PCB and heat sink. The disadvantages are that the quasi resonant switching the high resonant voltage which can be higher than the DC input voltage stressed on to IGBT power switches. This requires a higher cost and blocking voltage power components.

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Gate Driver Optocouplers in Induction Cooker (1)

What is Induction Cooking?
We start by first comparing the difference between conventional gas cooking and induction cooking. In induction cooking methods, energy is transferred directly to the pot or pan while conventional cooking first generate a fire and heat energy which is then transferred to the cooking pot. Hence due to this two step energy transfer of conventional cooking, the efficiency of the induction cooking is much better.

Figure 1 & 2 shows two circuit topologies for induction cooker, the half-bridge series resonant converter Figure 1 and quasi-resonant converter Figure 2 [2]. In both topologies, there exist the resonant elements Lr and Cr. For circuit simplification, the load pot, R is assumed to be of pure resistive element. In both topologies, an ac input supply of 220V 50Hz is converted into an uncontrolled dc voltage by a full-bridge rectifier. This DC voltage is then converted into a high frequency AC voltage by the inverter IGBT (insulated gate bipolar transistors) switches, S1 and S2 in the case of the half-bridge circuit, which can be controlled using a micro-controller. Due to the high frequency switching AC, the element coil will then produce a high frequency electromagnetic field which will penetrate the ferrous material cooking pot. From Faraday’s Law and skin effect, this generates eddy current within the cooking pot which then generates heat to cook the food inside the pot.

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IGBT Gate Drivers in High-Frequency Induction Cookers (3)

Gate driver circuits for IGBT power switches
Three types of driver circuits, using discrete transistors (Fig. 5), gate driver optocouplers (Fig. 6) or gate driver transformers (Fig. 7) can be used to drive the power switches in the induction cooker. There are several issues associated with high-frequency gate drivers: parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver, all of which are involved when selecting an appropriate driver circuit.

Typically, the switching frequency of an induction cooker is between 25 kHz and 40 kHz. In order to rapidly turn on and off the power switch, the gate current inductance loop between the driver and power switch should be as low as possible. Hence it is advisable to design the layout of the circuit to reduce the parasitic inductances. Since the driver rapidly charges and discharges the gate capacitor of the IGBT, a relatively high peak gate current may be needed for proper operation. A higher peak current is also desirable to increase the charging and discharging rates during turn-on and turn-off, to help reduce the switching losses of the IGBT. Due to this, managing the power dissipation within the gate drive circuit becomes increasingly important as the switching speeds are increased.
Discrete gate drivers are constructed using bipolar transistors, and NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver, while simultaneously incorporating necessary operational and protective functions such as under voltage lockout (UVLO), is not as space efficient as using integrated circuits. Moreover most discrete transistor driver designs do not provide sufficient safety isolation or noise immunity.
Two methods of providing electrical isolation are pulse transformers and gate driver optocouplers. The pulse transformer is a traditional and simple solution, which, however, suffers from the potential for core saturation in a reasonably-sized transformer, resulting in reduced efficiency. A pulse transformer can only transmit AC signals, and most designs have a limited duty cycle ranging up to 50 percent due to the transformer volt-second relationship.
An additional capacitor and zener diode on the transformer secondary can be added to permit a higher duty cycle. However, this increases the circuit board size and parasitic inductances, which, in turn, increases power losses in the driver circuit.
The gate driver optocoupler IC integrates an LED light source and optical receiver for safety isolation, with transistors to provide sufficient drive current, and protection functions such as UVLO or desaturation detection.
Gate driver ICs are easy to design with, and will save PCB board space. Due to the integrated design, the drive circuitry can be located very close to the power switch, which not only saves PCB space but also improves the overall noise immunity of the system. However, as with any ICs, power dissipation is a major concern.
For the single-switch resonant converter, the designer has the option of the discrete gate driver, gate transformer or gate driver optocoupler topologies. As discussed previously, the quasiconverter resonant voltage can be higher than the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker designs, the discrete gate driver circuit is used as there is no upper power switch, and both the controller and power semiconductor are able to share the same power ground. However, in cases where safety isolation and reduction of driver losses becomes an issue, the gate drive optocoupler or transformer are excellent alternatives.
For the half-bridge converter, a floating or high-side power switch needs to be driven. A high-side discrete solution would increase the component count, and not provide any isolation. As shown, the pulse transformer galvanic isolation solution becomes increasingly complicated for duty cycle switching above 50 percent. Also, the solution size is larger because of the additional discrete components on top of the transformer size. The gate driver optocoupler IC provides a good level of protection, isolation, and common-mode noise rejection. This resolves many of the problems that are associated with transformer or discrete transistor drivers.

Summary
In this article, the halfbridge series resonant and quasi resonant induction cooker topologies along with three gate driver methods were discussed. In order to reduce the design size and audible switching noise while improving power efficiency, these resonant converters are chosen. The discrete transistor gate driver circuit is cost effective but increases design complexity while providing no safety isolation. The required size of the gate drive transformer consumes board space, and requires additional work, cost and board space to achieve switching duty cycles above 50 percent. Finally, the use of gate drive optocoupler ICs saves board space through high level feature integration while providing high voltage safety isolation and noise immunity all in one package.

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IGBT Gate Drivers in High-Frequency Induction Cookers (2)

How does an induction cooker work?
Figures 1 and 2 show two circuit topologies for induction cookers: the half-bridge series resonant converter, Fig. 1, and the quasi-resonant converter, Fig. 2. In both topologies, there exist the resonant elements Lr and Cr. For circuit simplification, the load pot, R, is assumed to be a purely resistive element.

In both topologies, an AC input supply of 220V 50 Hz is converted into an unregulated DC voltage by a full-bridge rectifier. This DC voltage is then converted into a high frequency AC voltage by the inverter IGBT (insulated gate bipolar transistor) switches—S1 and S2 in the case
of the half-bridge circuit—which can be controlled using a microcontroller. Due to the high frequency switching AC, the element coil will produce a high frequency electromagnetic field which will penetrate the ferrous material of the cooking pot. From Faraday’s Law and skin effect, this generates eddy current within the cooking pot which then generates heat to cook the food inside the pot.
By applying the transformer equivalent circuit, designers are able to map the load pot (secondary of transformer) to the primary side of the circuit where the resonant inductor, Lr, and capacitor, Cr, are located. From this, we can obtain the equivalent circuit for the half-bridge and quasi resonant circuits, shown in Figs. 3 and 4. From these equivalent circuits, the operation of the induction cooker, and the values of the resonant inductor, capacitor and control algorithm
can be derived.
In order to reduce component size, minimize switching losses and reduce audible noise during operation, induction cooker circuits typically utilize resonant or soft switching techniques. Soft switching can be subcategorized into two methods: zero-voltage switching and zero-current switching. Zero-voltage switching occurs when the transistor turns-on at zero voltage. Zero-current switching refers to the elimination of turn-off switching loss at zero current flow. The voltage or current provided to the switching circuit can be made zero by using the resonance created by an L-C circuit. This topology is named a “resonant converter.”
The advantages of a half-bridge series resonant circuit are stable switching and lower cost due to simplified design. The voltage within the circuit is limited to the level of the input voltage, which reduces the voltage stress across IGBT power switch. This, in turn, allows the designer to lower the cost by choosing an IGBT with a lower voltage rating. The disadvantage of this approach is that the control of the half-bridge circuit is relatively complicated and the required size of the heatsink and PCB area is greater, because of the high side gate driver circuit required for the upper IGBT, S1 in Fig. 1)
The advantage of a quasi-resonant converter is that it needs only one IGBT power switch, which reduces the size of the PCB and heat sink. The disadvantages are that the quasi-resonant switching develops a resonant voltage which can be higher than the DC input voltage, increasing stresses on the IGBT power switches. This requires highercost components with higher blocking
voltage capabilities.

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IGBT Gate Drivers in High-Frequency Induction Cookers (1)

Efficiency of induction cookers is 84 percent
Today, with the constant demand for energy saving devices, high-frequency induction cookers, already a trend in Europe, are gaining popularity in the rest of the world. These kitchen devices offer high efficiency that reduces energy usage, reduces cooking time and, simultaneously, improves user safety, particularly around children, since all heat is localized to the pan itself.

According to the U.S. Department of Energy, the typical efficiency of induction cookers is 84% compared to the 40 percent of gas cookers. In this article, two typical induction cooker designs, the halfbridge series-resonant and the quasiresonant topology, are discussed. The merits and disadvantages of these two high-frequency inverter topologies along with three gate driver circuits, discrete transistors, optocouplers integrated circuit and transformers for high frequency
operation are also discussed.

What is induction cooking?
In an induction cooktop, a magnetic field transfers electric energy directly to the object to be heated. By inducing in electric current into the ferrous cooking utensil, heat is generated in the object, and the cooking surface only gets hot from the heat reflected from the object being heated: no heat is directly produced by the induction element. Because of this direct transfer of energy, there are fewer losses, which translates to a higher level of efficiency.
This compares with conventional cooking in which a heat source, for example an electrical resistance element or a flame, transfers heat energy to the cooking pot. The two-step energy transfer is inherently less efficient than direct inductive heating.

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Power Semiconductors: The BJT, MOSFET, and IGBT (6)

VI. CONCLUSION

Whereas all of the mentioned devices have certain advantages, the IGBT has proven to make the best use of all of these advantages while minimizing the disadvantages. As such, IGBTs have taken over in popularity and are now being even further optimized to suit the needs of an ideal power semiconductor.

REFERENCES
[1] Vrej Barkhordarian, “Power MOSFET Basics”, (International Rectifier, 2004), http://www.irf.com/technical-info/appnotes/mosfet.pdf.
[2] B. Jayant Baliga, Modern Power Devices (Krieger Publishing, Malabar, Florida, 1987), Chap. 1.
[3] “IGBT Fundamentals,” (Seimens Semiconductor Group, 2004), http://www.infineon.com/cmc_upload/migrated_files/document_files/Application_Notes/lh_ap_18.pdf.
[4] Fraidoon Mazda, Power Electronics Handbook (Newnes, Oxford, 1997), Chap. 1.
[5] D.A. Grant, “Power semiconductors-innovation and improvement continue to challenge the designer,” in New Developments in Power Semiconductor Devices, IEE Colloquium on, May 1991.
[6] “Main Applications for Power Modules,” (Mitsubishi Electronics, Sept. 1998), http://www.mitsubishichips.com/webfiles/pdf/powermos2_0.pdf.

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Power Semiconductors: The BJT, MOSFET, and IGBT (5)

V. APPLICATIONS

Because power semiconductors have very wide-ranging applications, the most desirable type for a given application comes down to several factors: the amplification, the switching speed, and the power class. Trends in particular types can be seen in applications in industry, the consumer
market, and transportation.

A. Industrial Applications
Within industry, the two main uses for power semiconductors are for motor control and power supplies. For motor drives, power semiconductors can control all sizes of motors from those found in large mills to simple machine tools. The trend in the method of controlling of these motors has been toward IGBTs. Likewise, due to their versatility, the use of the IGBT has become the trend in constructing power supplies for such applications as battery charging, welding, and induction heating.

B. Consumer Applications
In the consumer market, power semiconductors can be found in audio amplifiers, heat controls, light dimmers, and again in motor controls. Because of the low cost and high amplification of MOSFETs, they have become the preference in audio amplifier construction. On the other hand, the IGBT is dominating in heat controls, dimmers, and motor drives.
An example consumer application of the IGBT can be seen in Figure 4. Here, and IGBT modules is used to control an induction coil that is used to heat a pan for cooking. In this particular case, the IGBT acts as a switch; when voltage is applied to the gate, the current will flow through the induction coil, otherwise no current will flow. Because the IGBT can handle high currents, there is little concern that the semiconductor will overheat and/or be destroyed with the high currents that are necessary for induction heating.

C. Transportation Applications
Within transportation, motor control with the IGBT is again most prevalent. IGBTs are utilized in drives for electric cars and trains. However, BJTs can be found in simpler applications such as electronic ignition and vehicle voltage regulation.

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Power Semiconductors: The BJT, MOSFET, and IGBT (4)

IV. THE INSULATED GATE BIPOLAR TRANSISTOR (IGBT)

A. Physical Characteristics
The IGBT is the most popular power semiconductor currently used today. It combines the MOS gate structure with the bipolar current conduction to create a device that is the best of both the MOSFET and the BJT. For terminals, it is a hybrid between the BJT and the MOSFET.

It has three terminals: the collector, the gate, and the emitter. Schematically, the IGBT is basically a p-n-p BJT powered by an n-channel MOSFET.

B. Operational Advantages and Disadvantages
The IGBT brings together the advantages of a BJT and MOSFET. It has high input impedance, a low power consumption, and a large safe operating area. It also has a remarkably high power handling capability for a given chip size. And, because it is a minority carrier device on a p+ substrate, it has superior conduction to a standard MOSFET. When used in combination with power integrated circuits, one can expect a cost reduction by a factor of ten [2].
The only disadvantage comes in switching speed. While the IGBT can compete with the speed of the BJT, it cannot beat the MOSFET. The faster the switching, the greater the forward voltage drop, and it has a relatively high turn-off time due to the long lifetime of minority carriers. However, these problems recently have been overcome [4], allowing the IGBT to dominate in industrial applications.

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Power Semiconductors: The BJT, MOSFET, and IGBT (3)

III. THE METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR (MOSFET)

A. Physical Characteristics
The MOSFET was introduced in the 1970s and, unlike the BJT, is a voltage controlled device. It also has three terminals, though they differ from the BJT: the source, the gate, and the drain. The source and drain diffusions are separated by the gate.

The MOSFET has a p or n channel and can operate in depletion or enhancement mode. In enhancement, no current flows when the gate voltage is zero. In depletion mode, however, a narrow n channel is formed under the gate such that current will still flow when the gate voltage is zero.

B. Operational Advantages and Disadvantages
In comparison to the BJT, the MOSFET is far superior. It has a high input impedance, reducing complexity and cost, and a low input current drive. At low currents, it also has a higher gain than the BJT. To handle higher currents, it is sufficient to simply put several MOSFETs in parallel, and because there is only one breakdown region, the safe operating region is larger. Additionally, because they are free from minority carrier storage times, MOSFETs are faster at switching than the BJTs.
Despite its advantages, the MOSFET has low gain at high currents. Moreover, it was slow to catch on and had an overall greater cost than a BJT of the same power rating. However, in recent years, the prices have come down and MOSFETs have gained in popularity [2].

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Power Semiconductors: The BJT, MOSFET, and IGBT (2)

II. THE BIPOLAR JUNCTION TRANSISTOR (BJT)

A. Physical Characteristics
One of the first types of power semiconductors, the BJT is a three layered semiconductor consisting of a sandwich of p-n-p or n-p-n materials. In addition, it has three terminals: the emitter, the collector, and the base. The base is lightly doped, whereas the emitter is heavily doped and wider.

The emitter-base region is forward biased so that majority carriers will flow across the junction. On the other hand, the collectorbase region is reverse biased, which results in a small minority carrier flow.

B. Operational Advantages and Disadvantages
When used in a common emitter mode, as it is most often, the BJT acts as a current-controlled switch. The base current is in the input and the collector current is the output. Because it is current-controlled, it has a fairly low saturation voltage, which is desirable. In addition, BJTs are able to handle high voltages and currents with few problems.
Of course, there are many drawbacks. The BJT has low gain at high frequencies, so it is not useful for amplification under those conditions. Additionally, it does not have a very high surge rating—the peak current is only about twice the maximum continuous current rating. Unlike MOSFETs, BJTs also have a relatively slow switching speed because it takes time to charge the emitter and collector depletion capacitances, which consequently slows the turn-on time.
There are also two breakdown areas associated with the BJT that reduce its safe operating area. The first is the avalanche breakdown, which causes a rapid rise in current, and a second breakdown can be brought on by inductive loads, which can overheat and destroy the transistor.

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Power Semiconductors: The BJT, MOSFET, and IGBT (1)

I. INTRODUCTION

Although power semiconductors were first developed in the late 1940s, they have matured significantly in sixty years. In today’s society, power semiconductors can be found everywhere.

They are essentially the solid-state version of the mechanical relay or the vacuum tube. Some of most common applications include motor drives, uninterruptible power supplies, audio amplifiers, and fluorescent lighting. For the best device suited for the job, it has become a competition between the bipolar junction transistor (BJT), the metal oxide field-effect transistor (MOSFET), and the insulated gate bipolar transistor (IGBT). Each has inherent advantages over the others, but the IGBT has seemed to dominate the industry in recent years.

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Circuit Diagram of The Induction Heating Inverter

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Induction Coil Design

It is within the induction coil that the varying magnetic field required for induction heating is developed through the flow of alternating current. So coil design is one of the most important aspects of the overall system. A well-designed coil provides the proper heating pattern for your part and maximizes the efficiency of the induction heating power supply, while still allowing easy insertion and removal of the part.

Induction coils are normally made of copper tubing - an extremely good conductor of heat and electricity - with a diameter of 1/8" to 3/16"; larger copper coil assemblies are made for applications such as strip metal heating and pipe heating. Induction coils are usually cooled by circulating water, and are most often custom-made to fit the shape and size of the part to be heated. So coils can have single or multiple turns; have a helical, round or square shape; or be designed as internal (part inside coil) or external (part adjacent to coil). There is a proportional relationship between the amount of current flow and distance between the coil and part. Placing the part close to the coil increases the flow of current and the amount of heat induced in the part. This relationship is referred to as the coupling efficiency of the coil.

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IMPORTANT FACTORS TO CONSIDER

The efficiency of an induction heating system for a specific application depends on several factors: the characteristics of the part itself, the design of the induction coil, the capacity of the power supply, and the degree of temperature change required for the application.

The Characteristics of the Part
METAL OR PLASTIC
First, induction heating works directly only with conductive materials, normally metals. Plastics and other non-conductive materials can often be heated indirectly by first heating a conductive metal susceptor which transfers heat to the non-conductive material.

MAGNETIC OR NON-MAGNETIC
It is easier to heat magnetic materials. In addition to the heat induced by eddy currents, magnetic materials also produce heat through what is called the hysteresis effect. During the induction heating process, magnetics naturally offer resistance to the rapidly alternating electrical fields, and this causes enough friction to provide a secondary source of heat. This effect ceases to occur at temperatures above the "Curie" point - the temperature at which a magnetic material loses its magnetic properties. The relative resistance of magnetic materials is rated on a “permeability” scale of 100 to 500; while non-magnetics have a permeability of 1, magnetic materials can have a permeability as high
as 500.

THICK OR THIN
With conductive materials, about 80% of the heating effect occurs on the surface or "skin" of the part; the heating intensity diminishes as the distance from the surface increases. So small or thin parts generally heat more quickly than large thick parts, especially if the larger parts need to be heated all the way through.

Research has shown a relationship between the heating depth of penetration and the frequency of the alternating current. Frequencies of 100 to 400 kHz produce relatively high-energy heat, ideal for quickly heating small parts or the surface/skin of larger parts. For deep, penetrating heat, longer heating cycles at 5 to 30 kHz has been shown to be most effective.

RESISTIVITY
If you use the exact same induction process to heat two same size pieces of steel and copper, the results will be quite different. Why? Steel – along with carbon, tin and tungsten – has high electrical resistivity. Because these metals strongly resist the current flow, heat builds up quickly. Low resistivity metals such as copper, brass and aluminum take longer to heat. Resistivity increases with temperature, so a very hot piece of steel will be more receptive to induction heating than a cold piece.

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HOW INDUCTION HEATING WORKS

How exactly does induction heating work? It helps to have a basic understanding of the principles of electricity. When an alternating electrical current is applied to the primary of a transformer, an alternating magnetic field is created. According to Faraday's Law, if the secondary of the transformer is located within the magnetic field, an electric current will be induced.

In a basic induction heating setup, a solid state RF power supply sends an AC current through a copper coil, and the part to be heated is placed inside the coil. The coil serves as the transformer primary and the part to be heated becomes a short circuit secondary. When a metal part is placed within the induction coil and enters the magnetic field, circulating eddy currents are induced within the part. As shown at right, these eddty currents flow against the electrical resistivity of the metal, generating precise and localized heat without any direct contact between the part and the coil.

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What is Induction Heating?

Induction heating is a process which is used to bond, harden or soften metalsor other conductive materials. For many modern manufacturing processes, induction heating offers an attractive combination of speed, consistency and control.

The basic principles of induction heating have been understood and applied to manufacturing since the 1920s. During World War II, the technology developed rapidly to meet urgent wartime requirements for a fast, reliable process to harden metal engine parts. More recently, the focus on lean manufacturing techniques and emphasis on improved quality control have led to a rediscovery of induction technology, along with the development of precisely controlled, all solid state induction power supplies.What makes this heating method so unique? In the most common heating methods, a torch or open flame is directly applied to the metal part. But with induction heating, heat is actually "induced" within the part itself by circulating electrical currents.

Since heat is transferred to the product via electromagnetic waves, the part never comes into direct contact with any flame, the coil itself does not get hot, and there is no product contamination. When properly set up, the process becomes very repeatable and controllable.

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